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Unraveling the Story of NGF-mediated Sensitization of Nociceptive Sensory Neurons: ON or OFF the Trks?

¹ Department of Pharmacology and Toxicology and
² Department of Anesthesia Indiana University School of Medicine, Indianapolis, Indiana 46202-5120

	SUMMARY

TOP Summary Introduction Nerve Growth Factor Mediates... Direct actions of NGF... The Neurotrophin Receptors: p75... Signaling pathways activated by... Peripheral Sensitization Interactions Between the TrkA... Musings on mystifying matters References

 
Nerve Growth Factor (NGF) is produced by and affects a number of immune and inflammatory cells. As part of the inflammatory response, NGF directly or indirectly alters the sensitivity of small diameter sensory neurons that communicate noxious information. The question remains as to the receptors and intracellular signaling cascades that mediate this sensitizing action of NGF. Although the general consensus is that NGF produces peripheral sensitization by activating TrkA, recent work suggests that p75 also contributes. Thus, both NGF receptors appear to contribute to peripheral sensitization although whether they act independently or together remains to be determined. Furthermore, controversy exists as to the downstream signaling pathways involved in NGF-induced peripheral sensitization.

	INTRODUCTION

TOP Summary Introduction Nerve Growth Factor Mediates... Direct actions of NGF... The Neurotrophin Receptors: p75... Signaling pathways activated by... Peripheral Sensitization Interactions Between the TrkA... Musings on mystifying matters References

 
Nerve Growth Factor (NGF) is a critical trophic factor required during development for the growth and survival of sympathetic neurons, sensory neurons, and neurons in the central nervous system (1, 2). The trophic actions of NGF are largely attributed to the activation of a receptor tyrosine kinase (TrkA) that is expressed on peripheral and central neurons. The NGF-TrkA complex in turn activates a number of signaling pathways in the cell and can be internalized and transported to the cell body to alter gene expression (3, 4). Transgenic mice lacking NGF (5) or TrkA (6) have sensory and sympathetic deficiencies and do not survive long after birth. In addition, genetic studies in patients with congenital insensitivity to pain with anhidrosis (CIPA) reveal mutations in TrkA that impair normal development of sensory neurons (7).

In adult animals, however, NGF is not necessary for neuronal survival (8), although it may still have a trophic role in maintaining the homeostasis of neurons or in sprouting after tissue damage (9, 10). Rather, a large body of evidence supports the notion that NGF’s primary function in adults is to mediate the inflammatory and immune response after tissue injury, especially to initiate and maintain hypersensitivity, a hallmark symptom of inflammation (11, 12). This hypersensitivity manifests as increased sensitivity to a noxious stimulus (hyperalgesia) and/or the perception of a non-noxious stimuli as noxious (allodynia). One mechanism to account for this hypersensitivity is a reduced threshold for firing and an increase in the excitability of small diameter sensory neurons that communicate noxious information to the central nervous system. This phenomenon, termed peripheral sensitization, increases the firing of small diameter sensory neurons and results in an increase in transmitter release from peripheral and central terminals of these neurons (Figure 1). The increased release of the peptide transmitters, substance P (SP) and calcitonin-gene related peptide (CGRP) from peripheral endings of sensory neurons contributes to neuro-genic inflammation (13), whereas increased release of transmitters such as SP and glutamate from endings in the dorsal spinal cord augments nociception (14). Sustained release of transmitters from central terminals also can augment synaptic function between sensory nerve endings and neurons in the dorsal spinal cord (15). This phenomenon, known as central sensitization, also contributes to hyperalgesia and allodynia and may be a major mechanism for chronic pain syndromes.

View larger version (55K): [in this window] [in a new window]   Figure 1. Release of NGF produces sensitization of the peripheral terminals of sensory neurons. Upon localized tissue trauma, mast cells, macrophages, keratinocytes, and T cells all release NGF, which can then interact with its receptors, TrkA and p75, on the nerve endings. Bindings to the NTR activates intracellular signaling cascades that can modulate the activity of different ion channels in the nerve ending or could enhance the release of the neuropeptides, substance P (SP) and/or calcitonin gene-related peptide (CGRP). Under normal conditions, a noxious stimuli might elicit a single action potential whereas after treatment with NGF or as a consequence of inflammation, the nerve now give rise to multiple action potentials. Upon binding the neurotrophin, the TrkA-NGF complex may be retrogradely transported back to the cell body where it can modify the expression of multiple genes, e.g., ion channels. It is possible that p75 lacking NGF can be transported back to the cell body where it also serves to alter gene expression. See text for details.

	NERVE GROWTH FACTOR MEDIATES PERIPHERAL SENSITIZATION

TOP Summary Introduction Nerve Growth Factor Mediates... Direct actions of NGF... The Neurotrophin Receptors: p75... Signaling pathways activated by... Peripheral Sensitization Interactions Between the TrkA... Musings on mystifying matters References

 
During an episode of inflammation, the amount of NGF released into tissues and circulation increases dramatically. In animal models of inflammation, amounts of produced NGF increase in response to administration of Complete Freund’s adjuvant (CFA) (22), carrageenan (32) or airway allergens (33). In humans, increased amounts of NGF are observed in serum from asthmatic patients (34), in tissue biopsies from patients with inflammatory bowel disease (35); in seminal plasma of patients with prostatitis/chronic pelvic pain syndrome (36), in keratinocytes of psoriatic lesions (37), and in synovial fluid from patients with inflammatory joint disease (38, 39).

Measuring behavioral responses to noxious stimuli, Lewin and co-workers first observed that acute systemic administration of NGF causes hyperalgesia to a thermal stimulus within fifteen minutes of injection (40). Other investigators also showed thermal hypersensitivity after acute systemic administration of NGF (41) and after injection directly into the hind paw (42). In contrast, hyperalgesia to a mechanical stimulus has a delayed onset (hours) after injection of NGF, but is consistently observed with chronic exposure to NGF (40). Transgenic mice overexpressing NGF show an enhanced sensitivity in response to mechanical stimuli compared to that observed in wild-type animals, whereas mice with reduced expression of NGF are less responsive to noxious stimuli (43).

Measuring neuronal activity in the rat skin-nerve preparation, Rueff and Mendell observed that acute exposure of the skin to NGF decreased the threshold for firing of small diameter sensory neurons in response to heat (44). This effect of NGF was not observed in skin taken from animals pretreated for four days with the mast-cell granule-depleting drug 48/80, suggesting that the effects of NGF were mediated by release of substances from mast cells. Acute exposure of the receptive fields of primary afferents in the urinary bladder to NGF also resulted in increased excitability of small diameter sensory neurons (45). These data support the notion that acute NGF sensitizes both cutaneous and visceral sensory nerve endings, but whether the increased excitability is caused by a direct action of NGF in the nerves was not determined.

A causal relationship between the increase in NGF amounts and hypersensitivity during inflammation is established by experiments showing that infusion of a neutralizing antibody to NGF reduces withdrawal behaviors in response to noxious thermal and mechanical stimuli (46) and prevents mechanical and thermal hyperalgesia secondary to local injection of CFA into the rat hind paw (21, 22). In an analogous manner, injection of fusion protein TrkA-Immunoglobulin G (IgG)—which binds and sequesters NGF thereby minimizing its ability to bind to its native receptors—diminishes behavioral responses to noxious stimuli and prevents thermal hyperalgesia after local injection of carrageenan (47).

Interfering with the ability of NGF to bind to its receptors also attenuates excitability of sensory neurons after inflammation. In the skin-nerve preparation obtained from rats that received a subcutaneous injection of carrageenan three hours prior to harvesting the tissue, there was an increase in the number of small diameter sensory neurons exhibiting spontaneous firing (48). There also was an increase in sensitivity of sensory neurons to heat and to bradykinin but not to mechanical stimuli. These sensitizing effects of inflammation were abolished by coadministration of a TrkA-IgG fusion protein with the carrageenan (48). In an analogous manner, injection of another TrkA-fusion protein (trkAIg2) into the hind paw of guinea pigs prevented both the increase in spontaneous activity and the decrease in action potential duration induced by intradermal injection of CFA (49). These studies clearly show that NGF released during experimental inflammation is a causative agent in peripheral sensitization, but they do not establish the site of NGF action or the receptors that mediate the action.

NGF-DEPENDENT GENE EXPRESSION CONTRIBUTES TO PERIPHERAL SENSITIZATION
Based on the rapid onset of NGF-induced thermal hyperalgesia and the slow onset of mechanical hyperalgesia, Lewin and co-workers proposed that two separate mechanisms mediated acute thermal sensitivity and delayed mechanical sensitivity (40). Indeed the acute effects on thermal stimuli could be explained by activation of signaling pathways in sensory nerve terminals by NGF binding to either of its receptors resulting in phosphorylation of channel proteins that contribute to thermal sensitivity. For example, much evidence demonstrates that acute administration of NGF augments transient receptor potential vanilloid 1 receptor (TRPV1)-mediated currents through post-translational modifications. This ligand-gated ion channel is activated by noxious thermal stimuli and by the algogenic (i.e., pain-producing) agent capsaicin (50), and thus NGF-induced increase in activity of this channel could account for thermal hypersensitivity. The fact that NGF-induced thermal sensitivity is maintained with chronic administration of the neurotrophin (42) suggests that the acute actions of NGF do not decrease with continued exposure to the neurotrophin. This is supported by the observations that acute administration of anti-NGF reverses thermal hyperalgesia induced by inflammation (22) and that sensory neurons grown in culture in the presence of NGF still are sensitized by acute administration of the neurotrophin (51).

The maintenance of NGF-induced hypersensitivity and the late onset of NGF-induced mechanical hyperalgesia also could result from alterations in expression of pro-nociceptive neurotransmitters such as SP or CGRP and/or an increased expression of ion channels that regulate excitability. Indeed, NGF regulates the expression of SP during development (52, 53). Furthermore, although NGF is not required for survival of adult sensory neurons, it still regulates expression of neuropeptides in these cells. Sensory neurons isolated from neonatal and adult rats and grown in culture in the presence of NGF exhibit increased expression of SP and CGRP compared to that observed in cells grown in the absence of NGF (54, 55). This trophic effect on peptide expression appears selective for NGF because neither brain-derived neurotrophic factor (BDNF) nor neurotrophin-3 (NT-3) affected the peptide content of adult sensory neurons in culture (56). Furthermore, chronic administration of NGF significantly augmented the electrical- or capsaicin-stimulated release of substance P from spinal cord slices (57, 58). It was not determined whether this increase in release was secondary to the ability of NGF to increase peptide content in sensory neurons after chronic treatment.

Inflammation induced by intradermal injection of CFA or carregeenan also augments peptide content in sensory neurons (22, 59, 60) and increases release of SP and CGRP from sensory nerve endings in the spinal cord (61, 62). The increase in peptide content after inflammation correlates with the development of hyperalgesia and with an increase in NGF content (22, 59). Furthermore, neutralization of NGF by injection of NGF antibodies not only reduced hypersensitivity but prevented the increase in peptide content suggesting that the inflammation-induced increase in content could account for the NGF-induced hypersensitivity (22, 59).

Long-term exposure to NGF also augments the expression of a number of ion channels that contribute to the excitability of sensory neurons. Indeed, the ability of capsaicin to excite sensory neurons is, in part, dependent on NGF. Sensory neurons isolated from adult rats grown in culture without NGF show a reduced sensitivity to capsaicin compared to cells grown in the presence of NGF (63–66). The ability of NGF to increase capsaicin sensitivity is likely caused by an acute effect of NGF on the TRPV1 channel as well as on the ability of NGF to increase the expression of the TRPV1 channel. Both inflammation and long-term exposure to NGF increase the expression of TRPV1 in sensory neurons (66–69) and this increase is blocked by administration of NGF-specific antibodies (68). The ability of NGF to increase the expression of TRPV1 mRNA is blocked by the receptor tyrosine kinase inhibitor K252A (66), by expression of a dominant-negative Ras, and by a mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor (67). These data suggest that the ability of NGF to increase TRPV1 expression is mediated by activation of TrkA.

Chronic exposure to NGF also increases sodium channel expression in sensory neurons (70, 71), and this could augment excitability in response to a noxious stimulus. The neurotrophin also increases bradykinin receptor binding (72), expression of P2X3 channels (73), and expression of acid sensing ion channels (74) in sensory neurons. Thus, these NGF-induced increases in expression can render sensory neurons more sensitive to established inflammatory mediators such as bradykinin, ATP, and/or protons (75). Taken together, the data reviewed above support the notion that an NGF-induced alteration in protein expression is one mechanism that could maintain sensory neurons in a state of hypersensitivity such that their threshold for activation is reduced and thus more responsive to mildly noxious stimuli.

CELLS INVOLVED IN INFLAMMATION THAT MEDIATE NGF-INDUCED PERIPHERAL SENSITIZATION
One unresolved issue regarding the ability of NGF to produce peripheral sensitization is the contribution of various immune and inflammatory cells to the phenomenon. NGF does effect sensory neurons directly; however, when performing in situ experiments or in vitro studies using tissues, it is important to consider the effects of NGF on other cells (Figure 1). NGF is produced by mast cells (76), glial cells (77), keratinocytes (37), fibroblasts (78, 79), lymphocytes (80), and macrophages (81). Many of the cells that are sources for NGF in inflammatory disorders also express the receptors for NGF (82–84). A subset of small diameter sensory neurons co-express TrkA and neuropeptides (85, 86), suggesting that activation of TrkA mediates NGF-induced sensitization via release of these neuropeptides. Of interest, however, are the observations that a majority of small-diameter sensory neurons expressing TrkA also co-express p75 (87, 88). In addition, p75 is found in a subset of small diameter sensory neurons that do not express TrkA. Thus, the distribution of this receptor supports its potential importance in peripheral sensitization.

Localization of NGF receptors on sensory neurons suggests, but does not prove, that NGF acts directly on these neurons to produce hypersensitivity. For example, NGF degranulates mast cells (83, 89) and this effect must be mediated through activation of TrkA as there are no p75 in mast cells (83). This degranulation releases a number of inflammatory mediators that contribute to peripheral sensitization including 5-hydroxytryptamine, trypase, and prostaglandins (14). In addition, because NGF is synthesized and stored in mast cells, the neurotrophin can stimulate its own release from these cells. NGF also contributes to activation and maintenance of T and B lymphocytes, macrophages, and other lymphocytes (90). The T cells and macrophages in turn release cytokines that contribute to chronic pain by altering sensitivity of sensory neurons (23).

Thus, the question remains whether NGF-induced peripheral sensitization is mediated by a direct action of the trophin on sensory neurons or is secondary to NGF actions on other inflammatory or immune cells which in turn release substances that alter the sensitivity of sensory neurons. Pretreating animals with 48/80 attenuates the ability of acutely administered NGF to augment thermal sensitivity, suggesting that mast cells contribute to the action of the neurotrophin (21, 42). Surprisingly, the loss of mast cell activity does not prevent NGF from inducing thermal hyperalgesia hours after administration (21), supporting the notion that different mechanisms mediate the actions of NGF over time. Depletion of mast cells also significantly reduces the increase in NGF levels and mechanical hyperalgesia caused by CFA-induced inflammation (91). Sympathetic nerve endings may contribute to NGF-induced hypersensitivity as sympathectomy by either surgical denervation or chronic exposure to guanethidine significantly attenuates the thermal hyperalgesia produced by intraplantar injection of NGF (91, 92) or attenuates the increased thermal and mechanical sensitivity that is secondary to CFA-induced inflammation (91). As with mast cell granule-depletion, sympathectomy delays the onset but does not abolish CFA-induced hyperalgesia, suggesting that sympathetic nerve endings contribute to acute sensitization but that other mechanisms mediate long-term sensitization. The effects of other cell types on NGF-induced peripheral sensitization have not been examined, although it is interesting to speculate that macrophages and lymphocytes may well contribute to NGF-induced hyperalgesia. These cells produce a number of cytokines that can increase the production of NGF (13–95). Furthermore, cytokines such as Tumor Necrosis Factor– (TNF) and Interleukin-1ß (IL-1ß) contribute to peripheral sensitization and could thereby impact the actions of NGF (23).

	DIRECT ACTIONS OF NGF ON SENSORY NEURONS

TOP Summary Introduction Nerve Growth Factor Mediates... Direct actions of NGF... The Neurotrophin Receptors: p75... Signaling pathways activated by... Peripheral Sensitization Interactions Between the TrkA... Musings on mystifying matters References

 
Although NGF acts on a number of cells that participate in inflammatory and immune responses, much evidence has accumulated showing direct actions of the neurotrophin on sensory neurons. In patch-clamp recordings from small diameter sensory neurons isolated from young adult rats, treatment with either NGF or NT-4/5 augmented the amplitude of the membrane current evoked by capsaicin (96). The finding that NGF can sensitize the neuronal response to capsaicin has since been replicated in many studies using different model systems. For example, TRPV1 expression in the heterologous systems of Xenopus oocytes and HEK cells showed that the thermal sensitivity to a heat stimulus was increased after exposure to NGF (97). Similar results were reported for the increased heat sensitivity of isolated small diameter sensory neurons that coexpressed TrkA (98). In addition to sensitization of a ligand-gated current, neuronal excitability is also enhanced by acute exposure to NGF wherein the number of action potentials generated by a ramp of depolarizing current in adult sensory neurons grown in culture was increased by NGF (51). Analogous to the enhancement of TRPV1 currents, acute exposure to NGF increased the capsaicin-evoked increase in the levels of intracellular free Ca²⁺ in small-diameter sensory neurons isolated from neonatal rats [postnatal day two (P2)-P5] (99). These results are consistent with previous studies wherein NGF augmented the capsaicin-gated current in adult sensory neurons (69, 96, 100). In these neonatal neurons, Bonnington and McNaughton found that treatment with NGF had no effects on the resting levels of intracellular Ca²⁺ (99). Of interest, the increase in intracellular free Ca²⁺ produced by a high concentration of extracellular potassium was not affected by NGF, suggesting that the sensitizing effect was limited to stimuli that activate the TRPV1 channel. This observation is inconsistent with the finding that the number of action potentials evoked by depolarizing current was increased by NGF (51). Additionally, a lack of effect of NGF on resting Ca²⁺ is in contrast to the report by Lamb and Bielefeldt who showed that both NGF and BDNF elevated intracellular Ca²⁺ in visceral sensory afferents isolated from the nodose ganglia (101). Furthermore, Zhu et al. failed to observe an increase in the capsaicin-gated current after treatment with NGF in sensory neurons isolated from neonatal rats (102). These authors also found that the period between P4 and P10 was critical for the development of NGF-mediated sensitization even though these neonatal neurons were sensitized by bradykinin (102). It is not at all clear why such differences are observed in what should be very similar model preparations. These striking differences in the results obtained with neonatal sensory neurons suggest that sensory neurons at this developmental stage may not be a reliably consistent model preparation in which to determine the exact signaling mechanisms regulating cellular excitability.

	THE NEUROTROPHIN RECEPTORS: P75 AND TRKA

TOP Summary Introduction Nerve Growth Factor Mediates... Direct actions of NGF... The Neurotrophin Receptors: p75... Signaling pathways activated by... Peripheral Sensitization Interactions Between the TrkA... Musings on mystifying matters References

 
NGF binds and activates two distinct receptors, the p75 and TrkA neurotrophin receptors (NTR)s. Classically, these receptors were distinguished by their binding affinity for NGF and their rates of NGF dissociation (103). NGF binds to a low affinity site (K_d ~10^–9 M) as well as to a high affinity site (K_d ~10^–11 M). Further studies demonstrate that the low affinity site was attributed to a protein having a molecular mass of ~80 kDa, designated p75, whereas the high affinity site was a protein having a molecular weight of ~140 kDa, designated TrkA. The low affinity site is characterized by a rapid rate of dissociation of NGF (t_1/2 ~3 sec), whereas dissociation from the high affinity site is much slower (t_1/2 ~10 min). Despite the differences in affinity, NGF concentrations in tissues after inflammation or injury as well as after exogenous administration likely are sufficiently high to activate both receptors.

The p75 NTR, a member of the TNF receptor (TNFR) superfamily, is composed of an extracellular NGF-binding domain that contains the four cysteine-rich motifs that are characteristic of the TNFR family (Figure 2). There is a short transmembrane spanning domain and a short intracellular domain that is not associated with any inherent catalytic activity but does contain a FAS "death domain" also characteristic of the TNFR family. The cytoplasmic tail has sites for binding of various proteins associated with p75 signaling (Figure 2). The p75 NTR has been referred to as a "common" or "pan" NTR because of its ability to bind all neurotrophins (i.e., NGF, BDNF, NT-4/5, and NT-3) with equal affinities although the rates of association and dissociation may vary with the particular neurotrophin.

View larger version (48K): [in this window] [in a new window]   Figure 2. The p75 NTR and its associated intracellular signalling pathways. The p75 NTR is made up of four extracellular cysteine rich motifs (CRM), a single membrane spanning domain, and an intracellular signaling domain (ISD). NGF binding causes the activation of sphingomyelinases (SMase) that lead to the liberation of ceramide (Cer) and subsequently to sphingosine (Sph) and sphingosine 1-phosphate (S1P). Ceramide and sphingosine can activate a multitude of effector pathways. In small-diameter sensory neurons the generation of S1P appears to enhance the firing of action potential through the modulation of a TTX-resistant sodium current and the suppression of a delayed rectifier-like current in sensory neurons. The small G proteins Rac1/Cdc42 associate with the ISD, which results in the activation of JNK. In many cell systems, activation of JNK is critically involved in apoptosis, however, in sensory neurons, activation of JNK gives rise to hypersensitivity. The ISD, through an undefined mechanism, results in the activation of PI3K with downstream activation of Akt. The ISD can associate with the complex formed by TRAF-IRAK-p62 to activate IKKß, which then phosphorylates IB and release NF-B. In sensory neurons, activation of the NF-B pathway appears to be anti-nociceptive. Lastly, activation of p75 somehow leads to increased levels of intracellular cyclic AMP and activation of PKA. See text for details

View larger version (50K): [in this window] [in a new window]   Figure 3. The TrkA NTR and its associated intracellular signaling pathways. The TrkA NTR is made up of multiple extracellular domains that consist of two cysteine-rich motifs (CRM), three leucine rich motifs (LRM) and two immunoglobulin-like motifs (IgM). Binding of NGF results in dimerization and the cross-phosphorylation of tyrosine residues on the intracellular signalling domains. These tyrosines form the binding sites for the scaffolding proteins Shc-Grb2-SOS, which leads to the activation of the small G-protein Ras, followed by activation of the serine-threonine kinases Raf, MEK, and the MAPK, Erk. In sensory neurons, the activation of Erk has been linked to hypersensitivity. Ras can also activate PI3K and its downstream effector Akt. In some cells, PI3K can activate B-Raf which leads to phosphorylation of Erk. The phosphorylated tyrosines also form the binding site for PLC-, which consequently results in the activation of PKC. It is well established that PKC plays an important role in regulating the sensitivity of sensory neurons to noxious stimulation.

The p75 NTR may not be unique in its capacity to enhance the sensitivity of TrkA for NGF. A protein called neurotrophic receptor homologue 2 (NRH2) has a great deal of similarity to p75 (113). NRH2 has a truncated extracellular domain that is incapable of binding NGF, but the transmembrane and cytoplasmic domains are very similar to p75. Also, like p75, NHR2 enhances the binding affinity of TrkA for NGF. NRH2 has been detected in a large portion of neonatal rat sensory neurons (P2/3); however, the role of NHR2 in NTR activation and signaling efficacy has yet to be determined.

	SIGNALING PATHWAYS ACTIVATED BY NEUROTROPHIN RECEPTORS

TOP Summary Introduction Nerve Growth Factor Mediates... Direct actions of NGF... The Neurotrophin Receptors: p75... Signaling pathways activated by... Peripheral Sensitization Interactions Between the TrkA... Musings on mystifying matters References

 
THE P75 NTR
Early work suggested that p75 had no catalytic activity and thereby did not activate any downstream signaling pathways. More recent work, however, has demonstrated this not to be the case, and that activation of p75 by NGF, as well as by its other ligands such as BDNF, produces significant intracellular signaling events that mediate such processes as cell growth, apoptosis, and modulation of neuronal excitability (28, 31). For brevity, we will focus our discussion on signaling pathways where data supports the notion that they mediate peripheral sensitization. The initial studies that demonstrated a p75-mediated signaling pathway independent of TrkA activity showed that NGF produced an increase in the levels of intracellular ceramide and a corresponding decrease in the content of sphingomyelin in NIH 3T3 cells (114). Indeed, NGF binding to p75 activates a neutral sphingomyelinase that metabolizes membrane sphingomyelins that then liberates ceramide (Figure 2). These findings suggest some functional signaling similarity between p75 and TNFR1, which also is capable of activating the sphingomyelin signaling cascade.

Ceramide can be further metabolized to sphingosine by an enzyme called ceramidase, and sphingosine can be phosphorylated by sphingosine kinase to produce sphingosine 1-phosphate (115, 116). Ceramide also can be metabolized by ceramide kinase to ceramide-1-phosphate. Thus, activation of the sphingomyelin cascade by NGF can give rise to at least four intracellular signaling molecules that may contribute directly or indirectly to the inflammatory response (116). Acute exposure of isolated sensory neurons to ceramide or to sphingosine 1-phosphate increases the number of action potentials generated by a ramp of depolarizing current, augments tetrodotoxin (TTX)-resistant sodium currents, and inhibits potassium currents in isolated sensory neurons (51, 117). Liberation of ceramide can lead to the activation of mitogen-activated protein kinases (MAPKs) and NF-B depending on the cell system, and these molecules have been implicated in peripheral sensitization and enhanced pain perception. Ceramide-1-phosphate also increases phospholipase A₂ (PLA₂) activity, whereas sphingosine 1-phosphate (presumably by activation of NF-B) increases the expression of cyclooxygenase-2 (118). These actions can augment the production of prostaglandins which may contribute to peripheral sensitization.

NGF binding to p75 also activates other signaling pathways associated with sensitization of sensory neurons (Figure 2). This association suggests potential mechanisms for the involvement of p75 in NGF-induced hyperalgesia, but to date few studies have addressed this notion. In the absence of an interaction with TrkA, activation of p75 initiates apoptosis in a variety of cells (119). One signaling pathway involved in this effect is the activation of small G-proteins that leads to the activation of c-jun N-terminal kinase (JNK), which, in turn, activates various downstream apoptotic pathways such as the transcription factors c-jun, ATF2, and p53 (119). Activation of JNK also may contribute to hypersensitivity after inflammation or nerve injury. Inflammation of the rat hind paw by injection of CFA or administration of NGF produces thermal hypersensitivity associated with an increase in the levels of phosphorylated JNK in the L5 ganglion (120). Inhibition of JNK activity reduces the extent of the thermal hypersensitivity produced by either inflammatory agent. Using the neuropathic pain model involving ligation of the L5 spinal nerve, immunohistochemical detection of phosphorylated JNK was dramatically increased in both the small and medium diameter sensory neurons (121, 122). Pretreatment with an inhibitor of JNK activity blocked mechanically induced allodynia resulting from nerve injury. It is unknown whether p75, TrkA, or both mediate the activation of JNK to give rise to hypersensitivity in these experimental models. This remains an important question for future studies.

The p75 NTR associates with the tumor necrosis receptor–associated factor 6 (TRAF6) and forms a scaffolding platform for the subsequent binding of the IL-1 receptor associated kinase (IRAK) (123), which is a serine-threonine kinase involved in activation of NF-B. The p75-TRAF6-IRAK forms a complex with atypical PKC–interacting protein (also termed p62). This complex ultimately associates with IB kinase (IKK), which phosphorylates IB and releases NF-B (124). In addition, TRAF proteins can form complexes with receptor-interacting protein 2 (RIP2), which is thought to be involved in the activation of NF-B (125, 126). Activation of NF-B through p75 appears important in neuronal survival after injury (127). A blocking antibody to p75 completely suppressed NGF-mediated activation of NF-B and greatly diminished the survival of embryonic trigeminal neurons (128). NGF, through activation of p75, also increases NF-B translocation to the nucleus in Schwann cells (127). Exogenous administration of a single dose of NGF into the rat paw does not increase NF-B activity in nuclear extracts from the lumbar dorsal root ganglia (129). Inhibitors of IKK or NF-B decoy molecules attenuate hyperalgesia produced by inflammation (130, 131) and by injury to the sciatic nerve (130, 132). In light of this work, further studies seem warranted to determine whether the p75-induced activation of NF-B could directly or indirectly mediate hypersensitivity of sensory neurons after inflammation.

In a variety of cell types, p75 also promotes cell survival through its activation of Akt [also called protein kinase B (PKB)], which is dependent on the activation of phosphoinositide 3'-kinase (PI3K) (133, 134). This effect appears to be independent of TrkA, despite the fact that activation of TrkA also promotes survival through the PI3K-Akt pathway. This idea is supported by the observation that treatment of PC12 cells with antisense oligonucleotides targeted to p75 resulted in decreased levels of phosphorylated Akt produced by NGF (111). These observations raise an interesting question regarding the presumed activation of PI3K solely by TrkA in the sensitization of the current conducted by TRPV1. A possible role for activation of PI3K-Akt by p75 in the regulation of sensory neuron sensitivity or excitability has yet to be determined.

In PC-12 cells, exposure to both NGF and BDNF can lead to a rapid elevation in intracellular levels of cyclic AMP (cAMP) (135). This observation suggests that the increase in cAMP is mediated by p75, as PC12 cells do not express the TrkB NTR, and thus the actions of BDNF are likely through p75. Similar observations were reported for isolated mouse cerebellar neurons, which express the p75 but not TrkA, wherein NGF rapidly elevated intracellular cAMP (136). These authors also showed that in isolated hippocampal neurons, treatment with NGF caused an outgrowth of neurites that was blocked by the protein kinase A (PKA) inhibitor KT5720. These results suggest that an NGF-induced increase in PKA activity has a functional role in the growth of neurons. Recently, it was reported that in PC12 cells both cAMP-dependent and NGF signaling pathways produce phosphorylation of Src at Ser¹⁷ (137). This phosphorylation of Src leads to the activation of the small G-protein Rap1, which in turn can activate extracellular-regulated protein kinase (Erk, a member of the MAPK family) (138). The NGF-induced activation of Rap1 is blocked by either the non-selective serine-threonine kinase inhibitor H-89 or the Src inhibitor PP2, suggesting that NGF might somehow activate PKA. This potential cross-talk of signal pathways may play a role in NGF-induced sensitization of sensory neurons. If NGF increases cAMP in sensory neurons, the cyclic nucleotide can bind to either exchange proteins that are activated directly by cAMP (Epacs) (139) or to PKA. Epacs are guanine nucleotide exchange factors that increase activity of small G-proteins and downstream MAPKs and have been implicated in the sensitization of sensory neurons (140). Increases in cAMP and activation of PKA mediate peripheral sensitization produced by prostaglandins (141, 142), but the role of Epacs in this signaling pathway in NGF-induced sensitization has not been explored.

The cellular mechanisms mediating the capacity of NGF to increase intracellular cAMP remain to be determined. A recent study, however, has shown that NGF treatment of PC12 cells produces a rapid (<2 min) increase in cAMP; this increase was blocked when cells were pretreated with BAPTA-AM, a Ca²⁺ chelator (143). Significantly, the NGF-induced increase in cAMP was blocked by KH7, an inhibitor of soluble adenylyl cyclase. Soluble adenylyl cyclase is very different from the conventional transmembrane forms of adenylyl cyclase in that it is activated by increases in bicarbonate or Ca²⁺ but not by forskolin (144). These results suggest that NGF somehow leads to an increase in intracellular Ca²⁺, which then activates soluble adenylyl cyclase with subsequent activation of PKA. It is clear that our understanding of the specific mechanisms and signaling pathways whereby NGF stimulates intracellular cAMP, PKA, and their consequent downstream effects is quite limited.

THE TRKA NTR
In contrast to p75, TrkA-mediated signaling is initiated by the autophosphorylation of critical tyrosine residues in the intracellular domain. These residues then form the binding sites for different adapter/scaffolding proteins that serve as sites for additional effector proteins as well as effector enzymes such as phospholipase C (Figure 3). The major signaling effectors activated by TrkA are the small G-proteins Ras and Rap1, leading to activation of MAPKs, PI3K, and phospholipase C– (27, 29–31). Recent work indicates that these pathways can mediate the hypersensitivity of sensory neurons.

The binding of adapter proteins such as Shc, Grb2, and the guanine-nucleotide exchange factor SOS to TrkA leads to the activation of Ras with a subsequent increase in activity of the MAPK pathway composed of Raf, MEK, and Erk. In addition, the adapter protein CRK and the guanine-nucleotide exchange factor C3G activate Rap1, which maintains activation of Erk for an extended period of time (138). Activated Erk phosphorylates several different substrates and can migrate to the nucleus where it activates specific transcription factors. In sensory neurons grown in culture, NGF increases the levels of phosphorylated Erk (67, 69, 145). This increase can be rapid where amounts of phosphorylated Erk are increased within five minutes after NGF treatment or sustained over an extended period of time (145). NGF-induced phosphorylation of Erk is blocked by the PI3K inhibitors LY294002 or wortmannin, suggesting that this kinase activity lies upstream of Erk. As mentioned above, NGF exposure also can activate B-Raf and Ras in these neurons, and the activation of B-Raf is blocked by LY294002, whereas activation of Ras is unaffected (145). In addition, RapGAP1 (a GTPase accelerating protein for Rap) blocks the slow phase but not the rapid phase of Erk phosphorylation. These results indicate that the activation of Ras is upstream from the PI3K modulation of B-Raf activity and that there may be parallel signaling pathways that mediate the activation of Erk. Erk has been implicated in hyperalgesia and in sensitization of sensory neurons. The phosphorylation of Erk is increased in small-diameter sensory neurons by capsaicin and by noxious thermal stimuli (146). Inhibition of Erk phosphorylation by the MEK inhibitor U0126 attenuated capsaicin-evoked thermal hyperalgesia (146) and mechanical hyperalgesia caused by intradermal injection of epinephrine into the rat paw (147). Furthermore, in acutely dissociated sensory neurons, U0126 attenuates the ability of capsaicin to enhance heat-evoked currents (148). These data clearly implicate Erk activation (as indicated by phosphorylation) in sensitization of sensory neurons.

Binding of NGF to TrkA also activates PI3K (independent of Ras activity) by binding to adapter proteins linked to TrkA. In addition to the activation of B-Raf, PI3K catalyzes the phosphorylation of the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP₂) at the 3-position; various species of inositol phosphates are involved in the activation of Akt. Akt, in turn, can phosphorylate a number of downstream effectors that can modulate the activity of the transcription factor nuclear factor–B (NF-B) and other as yet unknown effectors. Although a number of recent studies suggest that PI3K mediates the sensitivity of sensory neurons, the roles of Akt and its downstream effectors have yet to be determined.

Tyrosine phosphorylation of TrkA leads to the binding and activation of PLC- (Figure 3), which liberates the second messengers inositol trisphosphate and diacylglycerol (DAG) from PIP₃. Inositol trisphosphate (IP₃) binds to its receptor in the endoplasmic reticulum and produces the release of intracellular Ca2+ stores, whereas DAG binds to a number of downstream effectors (149, 150), most notably the classic and novel isoforms of protein kinase C (PKC). Much work has shown that activation of PKCs mediate sensitization of sensory neurons. Phorbol esters, which activate novel and classic PKCs, injected into the hind paw of rats produce a thermal hyperalgesia that is blocked by co-administration of PKC inhibitors (151). A component of this hyperalgesia likely involves sensitization of sensory neurons because phorbol esters or DAG analogs increase capsaicin-induced calcium entry into isolated sensory neurons (152) and increase capsaicin-, heat-, or proton-evoked currents (153, 154). Activators of PKC also increase capsaicin-evoked release of neuropeptides from isolated sensory neurons (155). Although it is well established that activation of PKC is one mechanism to account for sensitization of sensory neurons by the inflammatory mediator bradykinin (152, 156), the role of PKC in NGF-induced sensitization remains controversial.

	PERIPHERAL SENSITIZATION

TOP Summary Introduction Nerve Growth Factor Mediates... Direct actions of NGF... The Neurotrophin Receptors: p75... Signaling pathways activated by... Peripheral Sensitization Interactions Between the TrkA... Musings on mystifying matters References

 
MEDIATED BY NGF RECEPTORS
Although there are multiple signaling pathways activated by either p75 or TrkA that mediate NGF-induced peripheral sensitization, the question remains as to which pathways are causal in this phenomenon. This question is complicated by the fact that NGF can alter sensitivity of sensory neurons acutely by initiating post-translational modifications of ion channels and chronically by altering gene expression. The relative importance of each of these components in sensitization with chronic exposure to NGF either exogenously or secondary to inflammation remains unknown.

Although the general consensus seems to focus on activation of TrkA as mediating the long-term effects of NGF on sensory neurons, the data supporting this notion are mostly correlative. It is well established that the NGF-TrkA complex can be internalized and transported to the cell body (Figure 1) where it activates gene transcription (153, 157). This transport can occur in vesicle-like structures, endosomes, and these can contain NGF, TrkA, and MAPKs (4). It also is likely that NGF signals can be transported without internalization of TrkA (158). In addition to internalization, NGF binding to TrkA activates Ras and other small G-proteins that lead to activation of the MAPKs and to altered gene expression (159). Activation of p75 also can result in alterations of gene expression. For example, increases in bradykinin binding sites in cells treated with NGF are blocked by administration of a p75-specific antibody. However, the capacity of NGF to alter bradykinin binding is lost in transgenic mice with a deletion in exon III of p75 (72). Thus, additional studies are needed to establish the contribution of each NTR subtype to the long-term changes in sensory neurons produced by NGF.

For the acute, sensitizing actions of NGF, it seems unlikely that internalization of the NGF-TrkA complex—with subsequent alterations in gene expression—is involved because of the time required to transport the complex from the periphery to the nucleus. This does not preclude the potential importance of signaling pathways downstream of TrkA with effectors that alter phosphorylation of proteins and enhance excitability of sensory neurons. For example, intradermal injection of the PI3K inhibitor LY294002 or the MEK inhibitor PD98059 attenuates NGF-induced thermal hyperalgesia, suggesting the involvement of TrkA as a mediator of NGF effects (69). These authors suggest that the PI3K activity is upstream of Erk because LY294002 blocks NGF-induced increase in phosphorylation of Erk. It is important to note, however, that activation of PI3K also occurs with NGF binding to p75. Reducing the expression of TrkA in the saphenous nerve, by intrathecal administration of antisense oligodeoxynucleotides (AS-ODN) specific for TrkA, abolishes NGF-induced mechanical hyperalgesia (160), suggesting TrkA mediates NGF-induced hypersensitivity. In a neuropathic pain model, however, intrathecal injection of p75 AS-ODN reduced expression of this receptor in the dorsal root ganglia (DRG) and attenuated thermal hyperalgesia (161). p75 AS-ODN administration also prevents the injury-induced increase in phosphorylation of p38 (a MAPK) and TrkA. Intrathecal injection of the receptor tyrosine kinase inhibitor K252A also blocks phosphorylation of both TrkA and p38 (161) leading the authors to suggest that an interaction between the two NTRs mediates hypersensitivity after nerve injury.

TrkA might mediate NGF-induced hyperalgesia, as transgenic mice that have a deletion in exon III of p75 gene (106) exhibit a significant elevation in basal nociceptive thresholds and fewer numbers of all neuronal types in the DRG as compared to the numbers and thresholds observed in DRGs from wild-type mice (162). Despite the elevated threshold for nociception, which is likely caused by a reduction in the number of nociceptors in the animal, systemic administration of NGF still induces mechanical hyperalgesia after four hours (163). As pointed out by Bergmann et al., however, it is not clear whether this hyperalgesia is mediated by a direct action of NGF on sensory neurons or indirectly through its actions on other inflammatory cells. Because mast cells express only TrkA (83), the NGF-induced degranulation and release of inflammatory mediators would still occur in p75 knock-out animals and this could produce hyperalgesia. In addition, further examination of the studies with p75 exon III^–/– mice show that they still express a splice variant of p75 (164). There is now a complete p75 knock-out animal with a deletion of p75 exon IV and this animal shows an exaggerated loss of peripheral nerves compared to p75 exon III knock-out mice (164). To date, however, studies in the exon IV knock-out mouse have not been performed to examine the important question regarding the ability of NGF to produce hyperalgesia and/or to sensitize sensory neurons.

In isolated sensory neurons, measurements of excitability have not resolved the question of which NTR mediates sensitization. In their initial studies, Shu and Mendell were able to block the ability of NGF to augment the amplitude of the capsaicin-evoked current with the receptor tyrosine kinase inhibitor K252A (96), suggesting that the effects of NGF were mediated by TrkA. However, the selectivity of this compound to receptor tyrosine kinases is questionable because K252A also inhibits a number of other protein kinases such as PKC, PKA, and cGMP-dependent protein kinase (PKG) (165, 166). The involvement of TrkA in NGF-induced heat sensitivity is supported by the fact that sensory neurons sensitized by NGF expressed TrkA, whereas those neurons that were not sensitized by NGF were found to be TrkA negative (98). Measurements of p75 expression were not performed in this study. In oocytes and human embryonic kidney (HEK) cells coexpressing TrkA and TRPV1, NGF augments the capsaicin-gated currents, and this effect is not altered by the coexpression of both TrkA and p75 (97).

The p75 NTR also might directly mediate sensitization of sensory neurons. Patch clamp recordings from NGF-treated, capsaicin-sensitive small-diameter sensory neurons isolated from young adult rats indicated a significant increase in the number of action potentials evoked by a ramp of depolarizing current (51). NGF enhanced the TTX-resistant sodium current and suppressed a delayed rectifier-like potassium current, suggesting that modulation of these membrane currents contributed to the increased excitability produced by NGF. Indeed, when sensory neurons were treated with a blocking antibody for p75, the sensitizing action of NGF to increase the number of action potentials was inhibited (167). Consistent with the inability of NGF to augment the number of action potentials, the administration of p75 blocking antibody also prevented suppression of the potassium current by NGF.

NGF-INDUCED SIGNALING CASCADES
Controversies also exist as to the downstream signaling pathways that mediate the ability of NGF to alter the excitability of isolated sensory neurons. Indeed, results of studies with a similar design have been surprisingly inconsistent. In a follow-up study to their original work, Shu and Mendell observed that pretreating sensory neurons with PKA inhibitors H89 or PKI_14–22, suppressed the NGF-induced enhancement of the capsaicin-gated current recorded from adult sensory neurons (100). In contrast, in neonatal sensory neurons, the inhibition of PKA by another drug, KT5720, had no effect on the capacity of NGF to augment the increase in intracellular Ca²⁺ concentration evoked by capsaicin (99). The capsaicin-evoked elevation in intracellular Ca²⁺ is thought to reflect increased TRPV1 activity, although capsaicin also releases Ca²⁺ from intracellular stores (168). Similar to the role of PKA, inhibitors of PKC have exhibited variable results; in one study both staurosporine and BIM reduced the sensitization by NGF (99) whereas others report that staurosporine, Ro-31-8425, or BIM had no effect on the NGF-induced sensitization of the capsaicin-evoked current (97, 100).

The involvement of PLC- in NGF-induced sensitization is also controversial. In the HEK heterologous expression system, TrkA, TRPV1, and PLC- are thought to form a signaling complex that then results in the enhancement of TRPV1 activity by NGF (97) because a mutant TrkA incapable of coupling to PLC– fails to enhance capsaicin-evoked current when exposed to NGF. Furthermore, NGF-induced sensitization of small-diameter sensory neurons to heat is suppressed by the PLC inhibitor U73122 (98). In contrast, neomycin, an inhibitor of PLC- had no effect on the NGF-induced sensitization of the increase in Ca²⁺ concentration evoked by capsaicin in neonatal sensory neurons, whereas treatment with the other PLC inhibitor, U73122, elevated levels of resting calcium (99).

Based on the ability of NGF to activate Ras (and consequently Erk) in many model systems, it is surprising that MEK inhibitors have little effect on the ability of NGF to augment either capsaicin-evoked currents (97, 100) or increases in intracellular Ca²⁺ (99). This contrasts the recent work of Zhuang and co-workers who show that inhibition of Erk reduces the ability of NGF to augment capsaisin-evoked currents (69). Inhibition of PI3K by either wortmannin or LY294002 also blocks the NGF-induced sensitization of sensory neurons in two different preparations (69, 99). Taken together, these studies demonstrate that NGF, through the activation of various signaling cascades, can modulate the activity of TRPV1, but the specific signaling pathways involved are not clear. A recent study by Zhang et al. suggests another possible mechanism for the NGF-induced enhancement of TRPV1 activity (169). Using a heterologous expression system, these authors found that exposure to NGF appeared to initiate a rapid insertion of additional TRPV1 channels into the plasma membrane, which can account for an increase in intracellular calcium (169). This NGF-mediated sensitization of the capsaicin-evoked increase in intracellular Ca²⁺ levels was blocked by the Src inhibitor PP2. Additional experiments showed that Src phosphorylated Y²⁰⁰ of TRPV1, and that this phosphorylation event was critical for the insertion of TRPV1 into the membrane.

Recent work in our laboratories has focused on the sphingolipid signaling pathway as causal in mediating NGF-induced sensitization of sensory neurons. As mentioned above, NGF through p75 activates neutral sphingomyelinase, which in turn increases hydrolysis of membrane sphingomyelin to liberate ceramide (114). Exposing dissociated adult sensory neurons to C2-ceramide, a membrane-permeable analog of ceramide, increases the action potential–firing evoked by depolarizing current in a manner analogous to that observed with acute NGF treatment (51). Furthermore, inhibition of the neutral form of sphingomyelinase with glutathione (70, 171) blocked the capacity of NGF but not ceramide to augment the number of action potentials evoked by the ramp (51). Internal perfusion of small-diameter sensory neurons with either sphingosine or sphingosine 1-phosphate via the patch clamp recording pipette also produced an increase in the number of evoked action potentials with a time course that was very similar to that observed with NGF perfusion (117). Inhibition of sphingosine kinase by dimethylsphin-gosine blocked the sensitizing effect of both NGF and internally perfused sphingosine but did not affect the actions of sphingosine 1-phosphate (117). Taken together, these results indicate that NGF, via activation of p75, can liberate ceramide. This sphingolipid is then metabolized to sphingosine 1-phosphate which through presently unknown mechanisms, leads to the modulation of ion channels and ultimately the enhancement of neuronal excitability. At present, it is not known whether TrkA makes any contribution to the NGF-mediated signaling via p75. Also, the effector systems whereby sphingosine 1-phosphate might alter the activity of different ion channels are unknown. These are two important areas for future investigation that will lead to a better understanding of exactly how these signaling pathways lead to increased excitability.

	INTERACTIONS BETWEEN THE TRKA AND P75 SIGNALING CASCADES

TOP Summary Introduction Nerve Growth Factor Mediates... Direct actions of NGF... The Neurotrophin Receptors: p75... Signaling pathways activated by... Peripheral Sensitization Interactions Between the TrkA... Musings on mystifying matters References

 
There are several observations concerning the interaction between the p75 and TrkA NTRs with potential relevance to the NGF-mediated sensitization of sensory neurons. NGF binding to TrkA appears to suppress the capacity of p75 to activate sphingomyelinase (172). In PC12 cells, which express both p75 and TrkA, exposure to NGF failed to elicit the hydrolysis of sphingomyelin. After pretreatment with K252A, the ability of NGF to cause hydrolysis of sphingomyelin was restored, suggesting that the tyrosine kinase activity of TrkA somehow affected the ability of p75 to activate sphingomyelinase. This finding is contrary to two more recent observations. Pretreatment of PC12 cells with BNDF, which activates p75 in these cells, decreased the levels of tyrosine phosphorylation of TrkA produced by the TrkA-specific ligand NGF3T (173). C2 ceramide exposure also decreased the NGF3T-mediated phosphorylation of TrkA. These results suggest that activation of the p75 can negatively impact the activation of TrkA. In addition, the ability of TrkA to suppress sphingomyelinase is difficult to reconcile with our observations (51, 117) wherein NGF augments the excitability of small-diameter sensory neurons through a sphingomyelinase-dependent pathway. Along these same lines, TrkA appears to suppress the ability of the p75 pathway to stimulate NF-B by somehow modulating the activity of the p75-TRAF6-IRAK-p62 complex, perhaps by association of TrkA with p62 (123, 174). There also may be convergence of activation of TrkA and p75 on downstream signaling. Activation of the TrkA cascade resulted in the rapid and sustained activation of MAPKs, whereas downstream activation of Akt was delayed for approximately thirty minutes (175). Stimulation of the p75 cascade produced a rapid but transient activation of MAPK with no consequent activation of Akt. However, stimulation of both pathways produced a rapid and sustained activation of MAPK with a rapid activation of Akt not seen with activation of either pathway alone. Taken together, these observations suggest that there is a complex interaction between p75 and TrkA and their downstream signaling pathways that we are only beginning to understand.

	MUSINGS ON MYSTIFYING MATTERS

TOP Summary Introduction Nerve Growth Factor Mediates... Direct actions of NGF... The Neurotrophin Receptors: p75... Signaling pathways activated by... Peripheral Sensitization Interactions Between the TrkA... Musings on mystifying matters References

 
Overall, the experimental observations reviewed above do not provide a clear picture of which receptors or signaling cascades mediate the alterations in the sensitivity of sensory neurons produced by NGF. In these various studies, the results are complicated by the use of different endpoints to measure peripheral sensitization. The most clinically relevant endpoints are the measurement of alterations in behavioral responses to noxious stimuli after NGF exposure. These studies are complicated, however, by the fact that inflammation and exogenous administration of NGF affect a number of different inflammatory and immune cells and these in turn release inflammatory mediators that can sensitize sensory neurons. Thus, in vivo studies attempting to elucidate NGF signaling in sensory neurons must consider the impact of other inflammatory mediators and their respective signaling pathways. Studies utilizing sensory neurons grown in culture clearly provide more meaningful results in assessing the direct actions of NGF on these cells. An additional advantage in these studies is that a number of different endpoints can be measured, including: 1) alterations in membrane currents evoked by noxious stimuli; 2) increases in action potential–firing; 3) increases in calcium entry; and 4) increases in transmitter release. When confronted with a multitude of measurements, however, it is important to consider the possibility that these different endpoints evaluating the state of neuronal sensitivity may involve distinct NTRs and signaling pathways. For example, it is clear that NGF though p75 alters excitability in response to a ramp of depolarizing current (51, 167). In like manner, convincing evidence supports the idea that activated TrkA can alter capsaicin-evoked currents, but different signaling pathways may regulate calcium entry. Given that a number of different factors contribute to the overall sensitivity of sensory neurons, it will be important to ascertain how these components contribute to the functional alterations in neuronal activity and subsequent release of transmitters from these neurons.

Consideration must be given to the age of the animal from which cells are harvested (see above) and the culture conditions used in the various experiments since these factors can impact the results (176). A number of contemporary studies have been performed using expression systems rather than native cells. The obvious advantage of these cell lines is the control of protein expression, and such results are valuable in demonstrating that interactions between NGF and specific signaling pathways may exist (i.e., a proof of concept). These studies are limited, however, because signaling paradigms can be regulated in different manners in these expression systems and signaling outcomes can be modified by overexpression of proteins. Thus, it is critical that additional studies be performed in native cells to determine the potential functional significance of results observed in expression systems.

Another complication is that NGF-induced peripheral sensitization involves both acute and chronic post-translational modifications of proteins as well as chronic alterations in gene expression. In a number of instances, especially in inflammatory models or with repeated injections of NGF, it is difficult to determine if changes in sensitivity are secondary to phosphorylation of effectors that could potentially increase or reduce excitability or to increased production of effectors. Given that different signaling pathways are likely involved in these processes, different results could be obtained based on the time of the measurement after experimental manipulation and/or the duration of inflammation or neurotrophin exposure.

Of greatest significance, however, is the fact that among the many signaling pathways activated by NGF, there is very likely a significant amount of cross-talk between them. Dissecting the exact sequence of events that regulate the activation of these pathways has been limited by the paucity of selective inhibitors to block enzymes in specific pathways. As mentioned above, one indicator commonly used to distinguish between NGF activation of TrkA and p75 is whether the observed effect is blocked by the receptor tyrosine kinase inhibitor K252A. Interpretation of results using this drug are limited by the concentration of drug used, becuase at higher concentrations, K252A also inhibits a number of other kinases. Furthermore, although a blocking antibody to p75 is available, no such antibody currently exists for TrkA. The use of both TrkA and p75 knock-out mice for studies of sensitization also is limited because both receptors participate in the growth and survival of both sympathetic and sensory neurons. Furthermore, a potential interaction between TrkA and p75 suggest that it is difficult to selectively block the actions of one receptor without affecting the other. Finally, it must be appreciated that activation of upstream effectors is very often associated with the amplification of downstream events so that activation of one signaling pathway often leads to activation of multiple downstream pathways. To date, few studies have attempted to examine systematically the sequence of activation for NGF. Thus, a number of studies are needed to further resolve the inconsistencies of the current literature and ascertain the mechanisms by which NGF produces peripheral sensitization.

Grant Nicol, Ph.D., is the Showalter Professor of Pharmacology and Toxicology at the Indiana University School of Medicine. His work focuses on the modulation of ion channel activity and excitability by inflammatory mediators in sensory neurons.

Michael R. Vasko, Ph.D., is Paul Stark Professor of Pharmacology and Departmental Chair of Pharmacology and Toxicology at Indiana University School of Medicine. Address correspondence to MRV. E-mail vaskom{at}iupui.edu; fax 317-274-1560.

	References

TOP Summary Introduction Nerve Growth Factor Mediates... Direct actions of NGF... The Neurotrophin Receptors: p75... Signaling pathways activated by... Peripheral Sensitization Interactions Between the TrkA... Musings on mystifying matters References

 

1. Levi-Montalcini, R. and Angeletti, P.U. Nerve growth factor. Physiol. Rev. 48, 534–69 (1968).[Free Full Text]
2. Ritter, A.M., Lewin, G.R., Kremer, N.E., Mendell, L.M. Requirement for nerve growth factor in the development of myelinated nociceptors in vivo. Nature 350, 500–502 (1991).[CrossRef][Medline]
3. Goedert, M., Stoeckel, K., and Otten, U. Biological importance of the retrograde axonal transport of nerve growth factor in sensory neurons. Proc. Natl. Acad. Sci. USA 78, 5895–5898 (1981).[Abstract/Free Full Text]
4. Delcroix, J.D., Valletta, J.S., Wu, C., Hunt, S.J., Kowal, A.S., and Mobley, W.C. NGF signaling in sensory neurons: Evidence that early endosomes carry NGF retrograde signals. Neuron Jul 3; 39, 69–84 (2003).[CrossRef][Medline]
5. Crowley, C., Spencer, S.D., Nishimura, M.C. et al. Mice lacking nerve growth factor display perinatal loss of sensory and sympathetic neurons yet develop basal forebrain cholinergic neurons. Cell 76, 1001–1011 (1994).[CrossRef][Medline]
6. Smeyne, R.J., Klein, R., Schnapp, A., Long, L.K., Bryant, S., Lewin, A. Lira, S.A., and Barbacid, M. Severe sensory and sympathetic neuropathies in mice carrying a disrupted Trk/NGF receptor gene. Nature 368, 246–249 (1994).[CrossRef][Medline]
7. Indo, Y. Molecular basis of congenital insensitivity to pain with anhidrosis (CIPA): mutations and polymorphisms in TRKA (NTRK1) gene encoding the receptor tyrosine kinase for nerve growth factor. Hum. Mutat. 18, 462–471 (2001).[CrossRef][Medline]
8. Lindsay, R.M. Nerve growth factors (NGF, BDNF) enhance axonal regeneration but are not required for survival of adult sensory neurons. J. Neurosci. 8, 2394–2405 (1988).[Abstract]
9. Diamond, J., Holmes, M., and Coughlin, M. Endogenous NGF and nerve impulses regulate the collateral sprouting of sensory axons in the skin of the adult rat. J. Neurosci. 12, 1454–1466 (1992).[Abstract]
10. Diamond, J., Foerster, A., Holmes, M., and Coughlin, M. Sensory nerves in adult rats regenerate and restore sensory function to the skin independently of endogenous NGF. J. Neurosci. 12, 1467–1476 (1992).[Abstract]
11. Levi-Montalcini, R. and Skaper, S.D., Dal Toso, R., Petrelli, L., Leon, A. Nerve growth factor: From neurotrophin to neurokine. Trends Neurosci. 19, 514–520 (1996).[CrossRef][Medline]
12. Pezet, S. and McMahon, S.B. Neurotrophins: Mediators and modulators of pain. Annu. Rev. Neurosci. 29, 507–538 (2006).[CrossRef][Medline]
13. Richardson, J.D. and Vasko, M.R. Cellular mechanisms of neurogenic inflammation. J. Pharmacol. Exp. Therapeutics. 302, 839–845 (2002).[Abstract/Free Full Text]
14. Treede, R.D., Meyer, R.A., Raja, S.N., and Campbell, J.N. Peripheral and central mechanisms of cutaneous hyperalgesia. Prog. Neurobiol. 38, 397–421 (1992).[CrossRef][Medline]
15. Woolf, C.J. Evidence for a central component of post-injury pain hypersensitivity. Nature 306, 686–688 (1983).[CrossRef][Medline]
16. Sah, D.W.Y., Ossipov, M.H., and Porreca, F. Neurotrophic factors as novel therapeutics for neuropathic pain. Nature Rev. 2, 460–472 (2003).
17. Hefti, F.F., Rosenthal, A., Walicke, P.A., Wyatt, S., Vergara, G., Shelton, D.L., and Davies, A.M. Novel class of pain drugs based on antagonisms of NGF. Trends in Pharmacol. Sci. 27, 85–91 (2006).
18. Petty, B.G., Cornblath, D.R., Adornato, B.T., Chaudhry, V., Flexner, C., Wachsman, M., Sinicropi, D., Burton, L.E., and Peroutka, S.J. The effect of systemically administered recombinant human nerve growth factor in healthy human subjects. Ann. Neurol. 36, 244–246 (1994).[CrossRef][Medline]
19. Dyck, P.J., Peroutka, S., Rask, C., Burton, E., Baker, M.K., Lehman, K.A., Gillen, D.A., Hokanson, J.L., and O’Brien, P.C. Intradermal recombinant human nerve growth factor induces pressure allodynia and lowered heat-pain threshold in humans. Neurology 48, 501–505 (1997).[Abstract/Free Full Text]
20. Svensson, P., Cairns, B.E., Wang, K., and Arendt-Nielsen, L. Injection of nerve growth factor into human masseter muscle evokes long-lasting mechanical allodynia and hyperalgesia. Pain 104, 241–247 (2003).[CrossRef][Medline]
21. Lewin, G.R., Rueff, A., and Mendell, L.M. Peripheral and central mechanisms of NGF-induced hyperalgesia. Eur. J. Neurosci. 6, 1903–1912 (1994).[CrossRef][Medline]
22. Woolf, C.J., Safieh-Garabedian, B., Ma, Q.P., Crilly, P., and Winter, J. Nerve growth factor contributes to the generation of inflammatory sensory hypersensitivity. Neuroscience 62, 327–331 (1994).[CrossRef][Medline]
23. McMahon, S.B., Cafferty, W.B., and Marchand, F. Immune and glial cell factors as pain mediators and modulators. Exp. Neurol. 192, 444–462 (1995).
24. Korsching, S. The neurotrophic factor concept: A reexamination. J. Neurosci. 13, 2739–2748 (1993).[Abstract]
25. McMahon, S.B. NGF as a mediator of inflammatory pain. Phil. Trans. R. Soc. B. 351, 431–440 (1996).[Medline]
26. Dechant, G. Molecular interactions between neurotrophin receptors. Cell Tissue Res. 305, 229–238 (2001).[CrossRef][Medline]
27. Kaplan, D.R. and Miller, F.D. Neurotrophin signal transduction in the nervous system. Curr. Opin. Neurobiol. 10, 381–91 (2000).[CrossRef][Medline]
28. Roux, P.P. and Barker, P.A. Neurotrophin signaling through the p75 neurotrophin receptor. Prog. Neurobiol. 67, 203–233 (2002).[CrossRef][Medline]
29. Chao, M.V. Neurotrophins and their receptors: A convergence point for many signaling pathways. Nat. Rev. Neurosci. 4, 299–309 (2003).[CrossRef][Medline]
30. Huang, E.J. and Reichardt, L.F. Trk receptors: Roles in neuronal signal transduction. Annu. Rev. Biochem. 72, 609–642 (2003).[CrossRef][Medline]
31. Reichardt, L.F. Neurotrophin-regulated signaling pathways. Phil. Trans. R. Soc. B., 361, 1545–1564 (2006).[Medline]
32. Weskamp, G. and Otten, U. An enzyme-linked immunoassay for nerve growth factor (NGF): A tool for studying regulatory mechanisms involved in NGF production in brain and in peripheral tissues. J. Neurochem. 48, 1779–1786 1987.[Medline]
33. Braun, A., Appel, E., Baruch, R., Herz, U., Botchkarev, V., Paul, R., Brodie, C., and Renz, H. Role of nerve growth factor in a mouse model of allergic airway inflammation and asthma. Eur. J. Immunol. 28, 3240–3251 (1998).[CrossRef][Medline]
34. Bonini, S., Lambiase, A., Bonini. S., Angelucci, F., Magrini, L., Manni, L., and Aloe, L. Circulating nerve growth factor levels are increased in humans with allergic diseases and asthma. Proc. Natl. Acad. Sci. USA 93, 10955–10960 (1996).[Abstract/Free Full Text]
35. diMola, F.F., Friess, H., Zhu, Z.W., Koliopanos, A., Bley, T., DiSebastiano, P., Innocenti, P., Zimmermann, A., and Büchler, M.W. Nerve growth factor and Trk high-affinity receptor (TrkA) gene expression in inflammatory bowel disease. Gut 46, 670–678 (2000).[Abstract/Free Full Text]
36. Miller, L.J., Fischer, K.A., Goralnick, S.J., Litt, M., Burleson, J.A., Albertsen, P., and Kreutzer, D.L. Nerve growth factor and chronic prostatitis/chronic pelvic pain syndrome. Urology 59, 603–608 (2002).[CrossRef][Medline]
37. Raychaudhuri, S.P., Jiang, W.Y., and Farber, E.M. Psoriatic keratinocytes express high levels of nerve growth factor. Acta Derm. Venereol. 78, 84–86 (1998).[CrossRef][Medline]
38. Aloe, L., Tuveri, M.A., Carcassi, U., and Levi-Montalcini, R. Nerve growth factor in the synovial fluid of patients with chronic arthritis. Arthritis Rheum. 35, 351–355 (1992).[Medline]
39. Halliday, D.A., Zettler, C., Rush, R.A., Scicchitano, R., and McNeil, J.D. Elevated nerve growth factor levels in the synovial fluid of patients with inflammatory joint disease. Neurochem. Res. 23, 919–922 (2004).[CrossRef]
40. Lewin, G.R., Ritter, A.M., and Mendell, L.M. Nerve growth factor-induced hyperalgesia in the neonatal and adult rat. J. Neurosci. 13, 2136–2148 (1993).[Abstract]
41. Della Seta, D., de Acetis, L., Aloe, L., and Alleva, E. NGF effects on hot plate behaviors in mice. Pharmacol. Biochem. Behav. 49, 701–705 (1994).[CrossRef][Medline]
42. Amann, R., Schuligoi, R., Herzeg, G., and Donnerer, J. Intraplantar injection of nerve growth factor into the rat hind paw: Local edema and effects on thermal nociceptive threshold. PainPain 64, 323–329 (1995).
43. Davis, B.M., Lewin, G.R., Mendell, L.M., Jones, M.E., and Albers, K.M. Altered expression of nerve growth factor in the skin of transgenic mice leads to changes in response to mechanical stimuli. Neuroscience 56, 789–792 (1993).[CrossRef][Medline]
44. Rueff, A. and Mendell, L.M. Nerve growth factor NT-5 induce increased thermal sensitivity of cutaneous nociceptors in vitro. J. Neurophysiol. 76, 3593–3596 (1996).[Abstract/Free Full Text]
45. Dmitrieva, N and McMahon, S.B. Sensitization of visceral afferents by nerve growth factor in the adult rat. Pain 66, 87–97 (1996).[CrossRef][Medline]
46. Urschel, B.A., Brown, P.N., and Hulsebosch, C.E. Differential effects on sensory nerve processes and behavioral alterations in the rat after treatment with antibodies to nerve growth factor. Exp. Neurol. 114, 44–52 (1991).[CrossRef][Medline]
47. McMahon, S.B., Bennett, D.L., Priestley, J.V., and Shelton, D.L. The biological effects of endogenous nerve growth factor on adult sensory neurons revealed by a trkA-IgG fusion molecule. Nat. Med. 1, 774–780 (1995).[CrossRef][Medline]
48. Koltzenburg, M., Bennett, D.L., Shelton, D.L., and McMahon, S.B. Neutralization of endogenous NGF prevents the sensitization of nociceptors supplying inflamed skin. Eur. J. Neurosci. 11, 1698–1704 (1999).[CrossRef][Medline]
49. Djouhri, L., Dawbard, D., Robertson, A., Newton, R., and Lawson, S.N. Time course and nerve growth factor dependence of inflammation-induced alterations in electrophysiological membrane properties in nociceptive primary afferent neurons. J. Neurosci. 21, 8722–8733 (2001).[Abstract/Free Full Text]
50. Caterina, M.J., Schumacher, M.A., Tominaga, M., Rosen, T.A., Levine, J.D., and Julius, D. The capsaicin receptor: A heat-activated ion channel in the pain pathway. Nature 389, 816–824 (1997).[CrossRef][Medline]
51. Zhang ,Y.-H., Vasko, M.R., and Nicol, G.D. Ceramide, a putative second messenger for nerve growth factor, modulates the TTX-resistant Na⁺ current and delayed rectifier K⁺ current in rat sensory neurons. J. Physiol. 544, 385–402 (2002).[Abstract/Free Full Text]
52. Kessler, J.A. and Black, I.B. Nerve growth factor stimulates the development of substance P in sensory ganglia. Proc. Natl. Acad. Sci. USA 77 649–652 (1980).[Abstract/Free Full Text]
53. Otten, U., Goedert, M., Mayer, N., and Lembeck, F. Requirement of nerve growth factor for development of substance P–containing sensory neurons. Nature 287, 158–159 (1980).[CrossRef][Medline]
54. Lindsay, R.M. and Harmar, A.J. Nerve growth factor regulates expression of neuropeptide genes in adult sensory neurons. Nature 337, 362–364 (1989).[CrossRef][Medline]
55. Lindsay, R.M., Lockett, C., Sternberg, J., and Winter, J. Neuropeptide expression in cultures of adult sensory neurons: Modulation of substance P and calcitonin gene-related peptide levels by nerve growth factor. Neuroscience 33, 53–65 (1989).[CrossRef][Medline]
56. Mulderry, P.K. Neuropeptide expression by newborn and adult rat sensory neurons in culture: Effects of nerve growth factor and other neurotrophic factors. Neuroscience 59, 673–688 (1994).[CrossRef][Medline]
57. Malcangio, M., Ramer, M.S., Boucher, T.J., and McMahon, S.B. Intrathecally injected neurotrophins and the release of substance P from the rat isolated spinal cord. Eur. J. Neurosci. 12, 139–144 (2000).[CrossRef][Medline]
58. Bowles, W.R., Sabino, M., Harding-Rose, C., and Hargreaves, K.M. Chronic nerve growth factor administration increases the peripheral exocytotic activity of capsaicin-sensitive cutaneous neurons. Neurosci. Lett. 403, 305–308 (2006).[CrossRef][Medline]
59. Donnerer, J., Schuligoi, R., and Stein, C. Increased content and transport of substance P and calcitonin gene-related peptide in sensory nerves innervating inflamed tissue: Evidence for a regulatory function of nerve growth factor in vivo. Neuroscience 49, 693–698 (1992).[CrossRef][Medline]
60. Galeazza, M.T., Garry, M.G., Yost, H.J., Strait, K.A., Hargreaves, K.M., and Seybold, V.S. Plasticity in the synthesis and storage of substance P and calcitonin gene-related peptide in primary afferent neurons during peripheral inflammation. Neuroscience 66, 443–458 (1995).[CrossRef][Medline]
61. Garry, M.G. and Hargreaves, K.M. Enhanced release of immunoreactive CGRP and substance P from spinal cord slices occurs during carrageenan inflammation. Brain Res. 582, 139–142 (1992)[CrossRef][Medline]
62. Southall, M.D., Michael, R.L., and Vasko, M.R. Intrathecal NSAIDS attenuate inflammation-induced neuropeptide release from rat spinal cord slices. Pain 78, 39–48 (1998).[CrossRef][Medline]
63. Winter, J., Forbes, C.A., Sternberb, J., and Lindsay, R.M. Nerve growth factor (NGF) regulates adult rat cultured dorsal root ganglion neuron responses to the excitotoxin capsaicin. Neuron 1, 973–981 (1988).[CrossRef][Medline]
64. Aguayo, L.G. and White, G. Effects of nerve growth factor on TTX- and capsaicin-sensitivity in adult rat sensory neurons. Brain Res. 570, 61–67 (1992).[CrossRef][Medline]
65. Bevan, S. and Winter, J. Nerve growth factor (NGF) differentially regulates the chemosensitivity of adult rat cultured sensory neurons. J. Neurosci. 15, 4918–4926 (1995).[Abstract]
66. Winston, J., Toma, H., Shenoy, M., and Pasricha, P.J. Nerve growth factor regulates VR-1 mRNA levels in cultures of adult dorsal root ganglion neurons. Pain 89, 181–186 (2001).[CrossRef][Medline]
67. Bron, R., Klesse, L.J., Shah, K., Parada, L.F., and Winter, J. Activation of Ras is necessary and sufficient for upregulation of vanilloid receptor type 1 in sensory neurons by neurotrophic factors. Mol. Cell. Neurosci. 22, 118–132 (2003).[CrossRef][Medline]
68. Ji, R.-R., Samad, T.A., Jin, S.-X., Schmoll, R., and Woolf, C.J. p38 MAPK Activation by NGF in primary sensory neurons after inflammation increases TRPV1 levels and maintains heat hyperalgesia. Neuron 36, 57–68 (2002).[CrossRef][Medline]
69. Zhuang, Z.Y., Xu, H., Clapham, D.E., and Ji, R.R. Phosphatidylinositol 3-kinase activates ERK in primary sensory neurons and mediates inflammatory heat hyperalgesia through TRPV1 sensitization. J. Neurosci. 24, 8300–8309 (2004).[Abstract/Free Full Text]
70. Fjell, J., Cummins, T.R., Davis, B.M., Albers, K.M., Fried, K., Waxman, S.G., and Black, J.A. Sodium channel expression in NGF-overexpressing transgenic mice. J. Neurosci. Res. 57, 39–47 (1999).[CrossRef][Medline]
71. Gould, H.J. III, Gould, T.N., England. J.D., Paul, D., Liu, Z.P., and Levinson, S.R. A possible role for nerve growth factor in the augmentation of sodium channels in models of chronic pain. Brain Res. 854, 19–29 (2000).[CrossRef][Medline]
72. Petersen, M., von Banchet, G.S., Heppelmann, B., and Koltzenburg, M. Nerve growth factor regulates the expression of bradykinin binding sites on adult sensory neurons via the neurotrophin receptor p75. Neuroscience 83, 161–168 (1998).[CrossRef][Medline]
73. Ramer, M.S., Bradbury, E.J., and McMahon, S.B. Nerve growth factor induces P2X₃ expression in sensory neurons. J. Neurochem. 77, 864–875 (2001).[CrossRef][Medline]
74. Mamet, J., Baron, A., Lazdunski, M., and Voilley, N. Proinflammatory mediators, stimulators of sensory neuron excitability via the expression of acid-sensing ion channels. J. Neurosci. 22, 10662–10670 (2002).[Abstract/Free Full Text]
75. Kasai, M. and Mizumura, K. Endogenous nerve growth factor increases the sensitivity to bradykinin in small dorsal root ganglion neurons of adjuvant inflamed rats. Neurosci. Lett. 272, 41– 44 (1999).[CrossRef][Medline]
76. Leon, A., Buriani, A., Dal Toso, R., Fabris, M., Romanello, S., Aloe, L., and Levi-Montalcini, R. Mast cells synthesize, store, and release nerve growth factor. Proc. Natl. Acad. Sci. USA 91, 3739–3749 (1994).[Abstract/Free Full Text]
77. Matsuoka, I., Meyer, M., and Thoenen, H. Cell-type-specific regulation of nerve growth factor (NGF) synthesis in non-neuronal cells: Comparison of Schwann cells with other cell types. J. Neurosci. 11, 3165–3177 (1991).[Abstract]
78. Bandtlow, C.E., Heumann, R., Schwab, M.E., and Thoenen, H. Cellular localization of nerve growth factor synthesis by in situ hybridization. EMBO J. 6, 891–899 (1987).[Medline]
79. Heumann, R., Korsching, S., Bandtlow, C., and Thoenen, H. Changes of nerve growth factor synthesis in nonneuronal cells in response to sciatic nerve transection. J. Cell Biol. 104, 1623–1631 (1987).[Abstract/Free Full Text]
80. Santambrogio, L., Benedetti, M., Chao, M.V., Muzaffar, R., Kulig, K., Gabellini, N., and Hochwald, G. Nerve growth factor production by lymphocytes. J. Immunol. 153, 4488–4495 (1994).[Abstract]
81. Heumann, R., Lindholm, D., Bandtlow, C., Meyer, M., Radeke, M.J., Misko, T.P., Shooter, E., and Thoenen, H. Differential regulation of mRNA encoding nerve growth factor and its receptor in rat sciatic nerve during development, degeneration, and regeneration: Role of macrophages. Proc. Natl. Acad. Sci. USA 84, 8735–8739 (1987).[Abstract/Free Full Text]
82. DiMarco, E., Mathor, M., Bondanza, S., Cutuli, N., Marchisio, P.C., Cancedda, R., and DeLuca, M. Nerve growth factor binds to normal human keratinocytes through high- and low-affinity receptors and stimulates their growth by a novel autocrine loop. J. Biol. Chem. 268, 22838–46 (1993).[Abstract/Free Full Text]
83. Horigome, K., Pryor, J.C., Bullock, E.D., and Johnson, E.M. Jr., Mediator release from mast cells by nerve growth factor. J. Biol. Chem. 268, 14881–14887 (1993).[Abstract/Free Full Text]
84. Nilsson, G., Forsberg-Nilsson, K., Xiang, Z., Hallbrook, F., Nilsson, K, and Metcalfe, D.D. Human mast cells express functional TrkA and are a source of nerve growth factor. Eur. J. Immunol. 27, 2295–2301 (1997).[Medline]
85. Averill, S., McMahon, S.B., Clary, D.O., Reichardt, L.F., and Priestley, J.V. Immunohistochemical localization of TrkA receptors in chemically identified subgroups of adult rat sensory neurons. Eur. J. Neurosci. 7, 1484–1494 (1995).[CrossRef][Medline]
86. Verge, V.M., Richardson, P.M., Benoit, R., and Riopelle, R.J. Histochemical characterization of sensory neurons with high-affinity receptors for nerve growth factor. J. Neurocytol. 18, 583–591 (1989).[CrossRef][Medline]
87. Verge, V.M., Merlio, J.P., Grondin, J., Ernfors, P., Persson, H., Riopelle, R.J., Hokfelt, T., and Richardson, P.M. Colocalization of NGF binding sites, trk mRNA, and low-affinity NGF receptor mRNA in primary sensory neurons: Responses of injury and infusion of NGF. J. Neurosci. 12, 4011–4022 (1992).[Abstract]
88. Wright, D.E and Snider, W.D. Neurotrophin receptor mRNA expression defines distinct populations of neurons in rat dorsal root ganglia. J. Comp. Neurol. 351, 329–338 (1995).[CrossRef][Medline]
89. Tal, M. and Liberman, R. Local injection of nerve growth factor (NGF) triggers degranulation of mast cells in rat paw. Neurosci. Lett. 221, 129–132 (1997).[CrossRef][Medline]
90. Simone, M.D., De Santis, S., Vigneti, E., Papa, G., Amadori, S., and Aloe, L. Nerve growth factor: A survey of activity on immune and hematopoietic cells. Hematol. Oncol. 17, 1–10 (1999).[CrossRef][Medline]
91. Woolf, C.J., Ma, Q.-P., Allchorne, A., and Poole, S. Peripheral cell types contributing to the hyperalgesic action of nerve growth factor in inflammation. J. Neurosci. 16, 2716–2723 (1996).[Abstract/Free Full Text]
92. Andreev, N.Y., Dimitrieva, N., Koltzenburg, M., and McMahon, S.B. Peripheral administration of nerve growth factor in the adult rat produces a thermal hyperalgesia that requires the presence of sympathetic post-ganglionic neurones. Pain 63, 109–115, (1995).[CrossRef][Medline]
93. Hattori, A., Tanaka, E., Murase, K., Ishida, N., Chatani, Y., Tsujimoto, M., Hayashi, K., and Kohno, M. Tumor necrosis factor stimulates the synthesis and secretion of biologically active nerve growth factor in non-neuronal cells. J. Biol. Chem. 268, 2577–2582 (1993).[Abstract/Free Full Text]
94. Safieh-Garabedian, B., Poole, S., Allchorne, A., Winter, J., and Woolf, C.J. Contribution of interleukin-1ß to the inflammation-induced increase in nerve growth factor levels and inflammatory hyperalgesia. Br. J. Pharmacol. 115, 1265–1275 (1995).[Medline]
95. Woolf, C.J., Allchorne, A., Safieh-Garabedian, B., and Poole, S. Cytokines, nerve growth factor and inflammatory hyperalgesia: The contribution of tumour necrosis factor alpha. Brit. J. Pharmacol. 121, 417–424 (1997).[CrossRef][Medline]
96. Shu, X.-Q. and Mendell, L.M. Nerve growth factor acutely sensitizes the response of adult rat sensory neurons to capsaicin. Neurosci. Lett. 274, 159–162 (1999).[CrossRef][Medline]
97. Chuang, H-H., Prescott, E.D., Kong, H., Shields, S., Jordt, S.E., Basbaum, A.L., Chao, M.V., and Julius, D. Bradykinin and nerve growth factor release the capsaicin receptor from Ptdlns(4,5)P₂-mediated inhibition. Nature 411, 957–962 (2001).[CrossRef][Medline]
98. Galoyan, S.M., Petruska, J.C., and Mendell, L.M. Mechanisms of sensitization of the response of single dorsal root ganglion cells from adult rat to noxious heat. Eur. J. Neurosci. 18, 535–541 (2003).[CrossRef][Medline]
99. Bonnington, J.K. and McNaughton, P.A. Signalling pathways involved in the sensitization of mouse nociceptive neurons by nerve growth factor. J. Physiol. 551, 433–446 (2003).[Abstract/Free Full Text]
100. Shu, X.-Q. and Mendell, L.M. Acute sensitization by NGF of the response of small-diameter sensory neurons to capsaicin. J. Neurophysiol. 86, 2931–2938 (2001).[Abstract/Free Full Text]
101. Lamb, K. and Bielefeldt, K. Rapid effects of neurotrophic factors on calcium homeostasis in rat visceral afferent neurons. Neurosci. Lett. 336, 9–12 (2003).[CrossRef][Medline]
102. Zhu, W., Galoyan, S.M., Petruska, J.C., Oxford, G.S., and Mendell, L.M. A developmental switch in acute sensitization of small dorsal root ganglion (DRG) neurons to capsaicin or noxious heating by NGF. J. Neurophysiol. 92, 3148–3152 (2004).[Abstract/Free Full Text]
103. Meakin, S.O. and Shooter, E.M. The nerve growth factor family of receptors. Trends Neurosci. 15, 323–331 (1992).[CrossRef][Medline]
104. Barbacid, M. Structural and functional properties of the TRK family of neurotrophin receptors. Ann. N.Y. Acad. Sci. 766, 442–458 (1995).[Medline]
105. Hempstead, B.L., Martin-Zanca, D., Kaplan, D.R,. Parada, L.F., and Chao, M.V. High-affinity NGF binding requires coexpression of the trk proto-oncogene and the low-affinity NGF receptor. Nature 350, 678–683 (1991).[CrossRef][Medline]
106. Lee, K.F., Li, E., Huber, L.J., Landis, S.C., Sharpe, A.H., Chao, M.V., J and aenisch, R. Targeted mutation of the gene encoding the low affinity NGF receptor p75 leads to deficits in the peripheral sensory nervous system. Cell 69, 737–749 (1992).[CrossRef][Medline]
107. Davies, A.M., Lee, K.F., and Jaenisch, R. p75-deficient trigeminal sensory neurons have an altered response to NGF but not to other neurotrophins. Neuron 11, 565–574 (1993).[CrossRef][Medline]
108. Verdi, J.M., Birren, S.J., Ibanez, C.F., Persson, H., Kaplan, D.R., Benedetti, M., Chao, M.V., and Anderson, D.J. p75LNGFR regulates Trk signal transduction and NGF-induced neuronal differentiation in MAH cells. Neuron 12, 733–745 (1994).[CrossRef][Medline]
109. Barker, P.A. and Shooter, E.M. Disruption of NGF binding to the low-affinity neurotrophin receptor p75LNTR reduces NGF binding to TrkA on PC12 cells. Neuron 3, 203–215. (1994).
110. Lachyankar, M.B., Condon, P.J., Daou, M.C., De, A.K., Levine, L.B., Oberbeier, A. and Ross, A.H. Novel functional interactions between Trk kinase and p75 neurotrophin receptor in neuroblastoma cells. J. Neurosci. Res. 71, 157–172 (2003).[CrossRef][Medline]
111. Epa, W.R., Markovska, K., and Barrett, G.L. The p75 neurotrophin receptor enhances TrkA signaling by binding to Shc and augmenting its phosphorylation. J. Neurochem. 89, 344–53 (2004).[CrossRef][Medline]
112. Wehrman, T., He, X., Raab, B., Dukipatti, A., Blau, H., and Garcia, K.C. Structural and mechanistic insights into nerve growth factor interactions with the TrkA and p75 receptors. Neuron 53, 25–38 (2007).[CrossRef][Medline]
113. Murray, S.S., Perez, P., Lee, R., Hempstead, B.L., and Chao, M.V. A novel p75 neurotrophin receptor-related protein, NRH2, regulates nerve growth factor binding to the TrkA receptor. J. Neurosci. 24, 2742–2749 (2004).[Abstract/Free Full Text]
114. Dobrowsky, R.T., Werner, M.H., Castellino, A.M., Chao, M.V., and Hannun, Y.A. Activation of the sphingomyelin cycle through the low-affinity neurotrophin receptor. Science 265, 1596–1599 (1994).[Abstract/Free Full Text]
115. Liu, G., Kleine, L., and Hébert R.L. Advances in the signal transduction of ceramide and related sphingolipids. Crit. Rev. Clin. Lab. Sci. 36, 511–573 (1999).[CrossRef][Medline]
116. Pettus, B.J., Chalfant, C.E., and Hannun, Y.A. Sphingolipids in inflammation: Roles and implications. Curr. Mol. Med. 4, 405–418 (2004).[CrossRef][Medline]
117. Zhang, Y.-H., Vasko, M.R., and Nicol, G.D. Intracellular sphingosine 1-phosphate mediates the increased excitability produced by nerve growth factor in rat sensory neurons. J. Physiol. 575, 101–113 (2006).[Abstract/Free Full Text]
118. Pettus, B.J., Kitatani, K., Chalfant, C.E., Taha, T.A., Kawamori, T., Bielawski, J., Obeid, L.M., and Hannun, Y.A. The coordination of prostaglandin E₂ production by sphingosine-1-phosphate and ceramide-1-phosphate. Mol. Pharmacol. 68, 330–335 (2005).[Abstract/Free Full Text]
119. Gentry, J.J., Barker, P.A., and Carter, B.D. The p75 neurotrophin receptor: Multiple interactors and numerous functions. Prog. Brain Res. 146, 25–39 (2004).[Medline]
120. Doya, H., Ohtori, S., Fujitani, M., Saito, T., Hata, K., Ino, H., Takahashi, K., Moriya, H., and Yamashita, T. c-Jun N-terminal kinase activation in dorsal root ganglion contributes to pain hypersensitivity. Biochem. Biophys. Res. Commun. 335,132–138 (2005).[CrossRef][Medline]
121. Obata, K., Katsura, H., Sakurai, J., Kobayashi, K., Yamanaka, H., Dai, Y., Fukuoka, T., and Noguchi, K. Suppression of the p75 neurotrophin receptor in uninjured sensory neurons reduces neuropathic pain after nerve injury. J. Neurosci. 26, 11974–11986 (2006).[Abstract/Free Full Text]
122. Zhuang, Z.Y., Wen, Y.R., Zhang, D.R., Borsello, T., Bonny, C., Strichartz, G.R., Decosterd, I., and Ji, R.R. A peptide c-Jun N-terminal kinase (JNK) inhibitor blocks mechanical allodynia after spinal nerve ligation: Respective roles of JNK activation in primary sensory neurons and spinal astrocytes for neuropathic pain development and maintenance. J. Neurosci. 26, 3551–3560 (2006).[Abstract/Free Full Text]
123. Mamidipudi, V., Li, X., and Wooten, M.W. Identification of interleukin 1 receptor-associated kinase as a conserved component in the p75-neurotrophin receptor activation of nuclear factor-kappa B. J. Biol. Chem. 277, 28010–28018 (2002).[Abstract/Free Full Text]
124. Wooten, M.W., Seibenhener, M.L., Mamidipudi, V., Diaz-Meco, M.T., Barker, P.A., and Moscat, J. The atypical protein kinase C-interacting protein p62 is a scaffold for NF-kappaB activation by nerve growth factor. J. Biol. Chem. 276, 7709–7712 (2001).[Abstract/Free Full Text]
125. McCarthy, J.V., Ni ,J., and Dixit, V.M. RIP2 is a novel NF-kappaB-activating and cell death–inducing kinase. J. Biol. Chem. 273, 16968–16975 (1998).[Abstract/Free Full Text]
126. Khursigara, G., Bertin, J., Yano, H., Moffett, H., DiStefano, P.S., and Chao, M.V. A prosurvival function for the p75 receptor death domain mediated via the caspase recruitment domain receptor-interacting protein 2. J. Neurosci. 21, 5854–5863 (2001).[Abstract/Free Full Text]
127. Carter, B.D., Kaltschmidt, C., Kaltschmidt, B., Offenhauser, N., Bohm-Matthaei, R., Baeuerle, P.A., and Barde, Y.A. Selective activation of NF-kappa B by nerve growth factor through the neurotrophin receptor p75. Science 272, 542–545 (1996).[Abstract]
128. Hamanoue, M., Middleton, G., Wyatt, S., Jaffray, E., Hay, R.T., Davies, A.M. p75-mediated NF-kappaB activation enhances the survival response of developing sensory neurons to nerve growth factor. Mol. Cell Neurosci. 14, 28–40 (1999).[CrossRef][Medline]
129. Wood, J.N. Regulation of NF-kappa B activity in rat dorsal root ganglia and PC12 cells by tumour necrosis factor and nerve growth factor. Neurosci. Lett. 192, 41–44 (1995).[CrossRef][Medline]
130. Tegeder, I., Niederberger, E., Schmidt, R., Kunz, S., Guhring, H., Ritzeler, O., Michaelis, M., and Geisslinger, G. Specific inhibition of IkappaB kinase reduces hyperalgesia in inflammatory and neuropathic pain models in rats. J. Neurosci. 24, 1637–1645 (2004).[Abstract/Free Full Text]
131. Inoue, G., Ochiai, N., Ohtori, S. et al. Injection of nuclear factor-kappa B decoy into the sciatic nerve suppresses mechanical allodynia and thermal hyperalgesia in a rat inflammatory pain model. Spine 31, 2904–2908. 2006[CrossRef][Medline]
132. Sakaue, G., Shimaoka, M., Fukuoka, T. et al. NF-kappa B decoy suppresses cytokine expression and thermal hyperalgesia in a rat neuropathic pain model. Neuroreport 12, 2079–2084 (2001).[CrossRef][Medline]
133. Roux, P.P., Bhakar, A.L., Kennedy, T.E., and Barker, P.A. The p75 neurotrophin receptor activates Akt (protein kinase B) through a phosphatidylinositol 3-kinase-dependent pathway. J. Biol. Chem. 276, 23097–23104 (2001).[Abstract/Free Full Text]
134. Bui, N.T., Konig, H.G., Culmsee, C., Bauerbach, E., Poppe, M., Krieglstein, J., and Prehn, J.H. p75 neurotrophin receptor is required for constitutive and NGF-induced survival signaling in PC12 cells and rat hippocampal neurons. J. Neurochem. 81, 594–605 (2002).[CrossRef][Medline]
135. Knipper, M., Beck, A., Rylett, J., and Breer, H. Neurotrophin induced cAMP and IP3 responses in PC12 cells. Different pathways. FEBS Lett. 324, 147–152 (1993).[CrossRef][Medline]
136. Higuchi, H., Yamashita, T., Yoshikawa, H., and Tohyama, M. PKA phosphorylates the p75 receptor and regulates its localization to lipid rafts. EMBO J. 22, 1790–1800 (2003).[CrossRef][Medline]
137. Obara,Y., Labudda, K., Dillon, T.J., and Stork, P.J. PKA phosphorylation of Src mediates Rap1 activation in NGF and cAMP signaling in PC12 cells. J. Cell Sci. 1, 6085–6094 (2004).
138. York, R.D., Yao, H., Dillon, T., Ellig, C.L., Eckert, S.P., McCleskey, E.W., and Stork, P.J. Rap1 mediates sustained MAP kinase activation induced by nerve growth factor. Nature 392, 622–626 (1998).[CrossRef][Medline]
139. de Rooij, J., Zwartkruis, F.J., Verheijen, M.H., Cool, R.H., Nijman, S.M., Wittinghofer, A., and Bos, J.L. Epac is a Rap1 guanine-nucleotide-exchange factor directly activated by cyclic AMP. Nature 396, 474–477 (1998).[CrossRef][Medline]
140. Hucho, T.B., Dina, O.A., and Levine, J.D. Epac mediates a cAMP-to-PKC signaling in inflammatory pain: An isolectin B4(+) neuron-specific mechanism. J. Neurosci. 25, 6119–6126 (2005).[Abstract/Free Full Text]
141. Hingtgen, C.M., Waite, K.J., and Vasko, M.R. Prostaglandins facilitate peptide release from rat sensory neurons by activating the adenosine 3',5'-cyclic monophosphate transduction cascade. J. Neurosci. 15, 5411–5419 (1995).[Abstract]
142. Lopshire, J.C. and Nicol, G.D. The cAMP transduction cascade mediates the prostaglandin E2 enhancement of the capsaicin-elicited current in rat sensory neurons: Whole-cell and single-channel studies. J Neurosci. 18, 6081–6092 (1998).[Abstract/Free Full Text]
143. Stessin, A.M., Zippin, J.H., Kamenetsky, M., Hess, K.C., Buck, J., and Levin, L.R. Soluble adenylyl cyclase mediates nerve growth factor–induced activation of Rap1. J. Biol. Chem. 281, 17253–17258 (2006).[Abstract/Free Full Text]
144. Buck, J., Sinclair, M.L., Schapal, L., Cann, M.J., and Levin, L.R. Cytosolic adenylyl cyclase defines a unique signaling molecule in mammals. Proc. Natl. Acad. Sci. U A 96, 79–84 (1999).[Abstract/Free Full Text]
145. York, R.D., Molliver, D.C., Grewal, S.S., Stenberg, P.E., McCleskey, E.W., and Stork, P.J. Role of phosphoinositide 3-kinase and endocytosis in nerve growth factor-induced extracellular signal-regulated kinase activation via Ras and Rap1. Mol. Cell. Biol. 20, 8069–8083 (2000).[Abstract/Free Full Text]
146. Dai, Y., Iwata, K., Fukuoka, T., Kondo, E., Tokunaga, A., Yamanaka, H., Tachibana, T., Liu, Y., and Noguchi, K. Phosphorylation of extracellular signal-regulated kinase in primary afferent neurons by noxious stimuli and its involvement in peripheral sensitization. J Neurosci. 22, 7737–7745 (2002).[Abstract/Free Full Text]
147. Aley, K.O., Martin, A., McMahon, T., Mok, J., Levine, J.D., and Messing, R.O. Nociceptor sensitization by extracellular signal-regulated kinases. J. Neurosci. 21, 6933–6939 (2001).[Abstract/Free Full Text]
148. Firner, M., Greffrath, W., and Treede, R.D. Phosphorylation of extracellular signal-related protein kinase is required for rapid facilitation of heat-induced currents in rat dorsal root ganglion neurons. Neuroscience 143, 253–263 (2006).[CrossRef][Medline]
149. Brose, N. and Rosenmund, C. Move over protein kinase C, you’ve got company: Alternative cellular effectors of diacylglycerol and phorbol esters. J. Cell Sci. 115, 4399–4411 (2002).[Abstract/Free Full Text]
150. Yang, C. and Kazanietz, M.G. Divergence and complexities in DAG signaling: Looking beyond PKC. Trends Pharmacol. Sci. 24, 602–608 (2003).[CrossRef][Medline]
151. Souza, A.L., Moreira, F.A., Almeida, K.R., Bertollo, C.M., Costa, K.A., and Coelho, M.M. In vivo evidence for a role of protein kinase C in peripheral nociceptive processing.Br. J. Pharmacol. 135, 239–247 (2002).[CrossRef][Medline]
152. Burgess, G.M., Mullaney, I., McNeill, M., Dunn, P.M., and Rang H.P. Second messengers involved in the mechanism of action of bradykinin in sensory neurons in culture. J. Neurosci. 9, 3314–3325 (1989).[Abstract]
153. Premkumar, L.S. and Ahern, G.P. Induction of vanilloid receptor channel activity by protein kinase C. Nature 408, 985–990 (2000).[CrossRef][Medline]
154. Vellani,V., Mapplebeck, S., Moriondo, A., Davis, J.B., and McNaughton, P.A. Protein kinase C activation potentiates gating of the vanilloid receptor VR1 by capsaicin, protons, heat and anandamide. J. Physiol. 534, 813–825 (2001).[Abstract/Free Full Text]
155. Barber, L.A. and Vasko, M.R. Activation of protein kinase C augments peptide release from rat sensory neurons. J. Neurochem. 67, 72–80 (1996).[Medline]
156. Cesare, P., Dekker, L.V., Sardini, A., Parker, P.J., and McNaughton, P.A. Specific involvement of PKC-epsilon in sensitization of the neuronal response to painful heat. Neuron 23, 617–624, (1999).[CrossRef][Medline]
157. Stoeckel, K., Schwab, M., Thoenen, H. Specificity of retrograde transport of nerve growth factor (NGF) in sensory neurons: A biochemical and morphological study. Brain Res. 89, 1–14 (1975).[CrossRef][Medline]
158. MacInnis, B.L., Campenot, R.B. Retrograde support of neuronal survival without retrograde transport of nerve growth factor. Science 295, 1536–1539 (2002).[Abstract/Free Full Text]
159. Averill, S., Delcroix, J.D., Michael, G.J., Tomlinson, D.R., Fernyhough, P., and Priestley, J.V. Nerve growth factor modulates the activation status and fast axonal transport of ERK 1/2 in adult nociceptive neurons. Mol. Cell. Neurosci. 18, 183–96 (2001).[CrossRef][Medline]
160. Malik-Hall, M., Dina, O.A., and Levine, J.D. Primary afferent nociceptor mechanisms mediating NGF-induced mechanical hyperalgesia. Eur. J. Neurosci. 21, 3387–3394 (2005).[CrossRef][Medline]
161. Obata, K., Yamanaka, H., Kobayashi, K., Dai, Y., Mizushima, T., Katsura, H., Fukuoka, T., Tokunaga, A., and Noguchi, K. Role of mitogen-activated protein kinase activation in injured and intact primary afferent neurons for mechanical and heat hypersensitivity after spinal nerve ligation. J. Neurosci. 24,10211–10222 (2004).[Abstract/Free Full Text]
162. Bergmann. I., Priestley. J.V., McMahon. S.B., Brocker, E.B., Toyka, K.V., and Koltzenburg, M. Analysis of cutaneous sensory neurons in transgenic mice lacking the low affinity neurotrophin receptor p75. Eur. J. Neurosci. 9, 18–28 (1997).[Medline]
163. Bergmann, I., Reiter, R., Toyka, K.V., and Koltzenburg, M. Nerve growth factor evokes hyperalgesia in mice lacking the low-affinity neurotrophin receptor p75. Neurosci. Lett. 255, 87–90(1998).[CrossRef][Medline]
164. von Schack, D., Casademunt, E., Schweigreiter, R., Meyer, M., Bibel, M., and Dechant, G. Complete ablation of the neurotrophin receptor p75NTR causes defects both in the nervous and the vascular system. Nat. Neurosci. 4, 977–978 (2001).[CrossRef][Medline]
165. Kase, H., Iwahashi, K., Nakanishi, S., Matsuda, Y., Yamada, K., Takahashi, M., Murakata, C., Sato, A., and Kaneko, M. K-252 compounds, novel and potent inhibitors of protein kinase C and cyclic nucleotide-dependent protein kinases. Biochem. Biophys. Res. Commun. 142, 436–440 (1987).[CrossRef][Medline]
166. Smith, D.S., King, C.S., Pearson, E., Gittinger, C.K., and Landreth, G.E. Selective inhibition of nerve growth factor-stimulated protein kinases by K-252a and 5'-S-methyladenosine in PC12 cells. J. Neurochem. 53, 800–806 (1989).[Medline]
167. Zhang, Y.-H. and Nicol, G.D. NGF-mediated sensitization of the excitability of rat sensory neurons is prevented by a blocking antibody to the p75 neurotrophin receptor. Neurosci. Lett. 366, 187–192 (2004).[CrossRef][Medline]
168. Olah, Z., Szabo, T., Karai, L., Hough, C., Fields, R.D., Caudle, R.M., Blumberg, P.M., and Iadarola, M.J. Ligand-induced dynamic membrane changes and cell deletion conferred by vanilloid receptor 1. J. Biol. Chem. 276, 1021–1030 (2001).
169. Zhang, X., Huang, J., and McNaughton, P.A. NGF rapidly increases membrane expression of TRPV1 heat-gated ion channels. EMBO J. 24, 4211–4223 (2005).[CrossRef][Medline]
170. Liu, B. and Hannun, Y.A. Inhibition of the neutral magnesium-dependent sphingomyelinase by glutathione. J. Biol. Chem. 272, 16281–16287 (1997).[Abstract/Free Full Text]
171. Liu, B., Andrieu-Abadie, N., Levade, T., Zhang, P., Obeid, L.M., and Hannun, Y.A. Glutathione regulation of neutral sphingomyelinase in tumor necrosis factor-alpha-induced cell death. J. Biol. Chem. 273, 11313–11320 (1998).[Abstract/Free Full Text]
172. Dobrowsky, R.T., Jenkins, G.M., and Hannun, Y.A. Neurotrophins induce sphingomyelin hydrolysis: Modulation by co-expression of p75NTR with Trk receptors. J. Biol. Chem. 270, 22135–22142 (1995).[Abstract/Free Full Text]
173. MacPhee, I.J. and Barker, P.A. Brain-derived neurotrophic factor binding to the p75 neurotrophin receptor reduces TrkA signaling while increasing serine phosphorylation in the TrkA intracellular domain. J. Biol. Chem. 272, 23547–23551 (1997).[Abstract/Free Full Text]
174. Wooten, M.W., Geetha, T., Seibenhener, M.L., Babu, J.R., Diaz-Meco, M.T., and Moscat, J. The p62 scaffold regulates nerve growth factor-induced NF-kappaB activation by influencing TRAF6 polyubiquitination. J. Biol. Chem. 280, 35625–35629 (2005).[Abstract/Free Full Text]
175. Lad, S.P., Peterson, D.A., Bradshaw, R.A., and Neet, K.E. Individual and combined effects of TrkA and p75NTR nerve growth factor receptors. A role for the high-affinity receptor site. J. Biol. Chem. 278, 24808–24817 (2003).[Abstract/Free Full Text]
176. Yamaguchi, K. Enhancement of the Ca²⁺-current by a serum factor in cultured dorsal root ganglia neurons of the adult guinea pig. Brain Res. 529, 286–293 (1990).[CrossRef][Medline]

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