Differential effects on sensory nerve processes and behavioral alterations in the rat after treatment with antibodies to nerve growth factor

Nerve Growth Factor Signaling Modulates the Expression of Glutaminase in Dorsal Root Ganglion Neurons during Peripheral Inflammation

Glutamate functions as the major excitatory neurotransmitter for primary sensory neurons and has a crucial role in sensitizing peripheral nociceptor terminals producing sensitization. Glutaminase (GLS) is the synthetic enzyme that converts glutamine to glutamate. GLS-immunoreactivity (-ir) and enzyme activity are elevated in dorsal root ganglion (DRG) neuronal cell bodies during chronic peripheral inflammation, but the mechanism for this GLS elevation is yet to be fully characterized. It has been well established that, after nerve growth factor (NGF) binds to its high-affinity receptor tropomyosin receptor kinase A (TrkA), a retrograde signaling endosome is formed. This endosome contains the late endosomal marker Rab7GTPase and is retrogradely transported via axons to the cell soma located in the DRG. This complex is responsible for regulating the transcription of several critical nociceptive genes. Here, we show that this retrograde NGF signaling mediates the expression of GLS in DRG neurons during the process of peripheral inflammation. We disrupted the normal NGF/TrkA signaling in adjuvant-induced arthritic (AIA) Sprague Dawley rats by the pharmacological inhibition of TrkA or blockade of Rab7GTPase, which significantly attenuated the expression of GLS in DRG cell bodies. The results indicate that NGF/TrkA signaling is crucial for the production of glutamate and has a vital role in the development of neurogenic inflammation. In addition, our pain behavioral data suggest that Rab7GTPase can be a potential target for attenuating peripheral inflammatory pain.

REVIEW ■ : Neurotrophic Factors and the Specification of Neural Function

Neurotrophin molecules have been shown to play important roles in the survival of neurons during devel opment. Most early studies concentrated on the initially discovered factor—nerve growth factor. Recent work has demonstrated that nerve growth factor belongs to a family of neurotrophins that include brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5. These neurotrophins exert their action via high- affinity receptors known as trks ( trkA, trkB, and trkC). A major goal of present research is to identify the neuronal locus of different trks to permit inferences about the specificity of action of the different growth factors. However, recent evidence suggests complex relationships between neurotrophins and trk receptors. These issues are explored in the context of dorsal root ganglion cells and motor neurons. Functional studies of the effects of nerve growth factor on its target cells, the nociceptive afferents, illustrate that this neurotro phin plays a role in development and function, which goes well beyond its classical role in promoting the survival of neurons during the early phases of development. The Neuroscientist 1:26-34, 1995

NGF/TrkA Signaling as a Therapeutic Target for Pain

Nerve growth factor (NGF) was first discovered approximately 60 years ago by Rita Levi-Montalcini as a protein that induces the growth of nerves. It is now known that NGF is also associated with Alzheimer's disease and intractable pain, and hence, it, along with its high-affinity receptor, tropomyosin receptor kinase (Trk) A, is considered to be 1 of the new targets for therapies being developed to treat these diseases. Anti-NGF antibody and TrkA inhibitors are known drugs that suppress NGF/TrkA signaling, and many drugs of these classes have been developed thus far. Interestingly, local anesthetics also possess TrkA inhibitory effects. This manuscript describes the development of an analgesic that suppresses NGF/TrkA signaling, which is anticipated to be 1 of the new methods to treat intractable pain.

Neurotrophic factors in peripheral neuropathies: therapeutic implications

Neurotrophic factors are proteins which promote the survival of specific neuronal populations. Many have other physiological effects on neurons such as inducing morphological differentiation, enhancing nerve regeneration, stimulating neurotransmitter expression, and otherwise altering the physiological characteristics of neurons. These properties suggest that neurotrophic factors are highly promising as potential therapeutic agents for neurological disease. Neurotrophic factors will most likely be applied to the peripheral nervous system initially, since there are fewer problems for large proteins to gain access to peripheral neurons. Many of the most intensively studied factors are active in the peripheral nervous system. These include the neurotrophins (nerve growth factor, brain derived neurotrophic factor, neurotrophin-3, neurotrophin-4/5), the insulin like growth factors, ciliary neurotrophic factor, and glial cell derived neurotrophic factor and its related proteins. The biology of these factors and their receptors in the peripheral nervous system is reviewed here. We also review data suggesting that abnormal availability of some factors may contribute towards the pathogenesis of certain types of peripheral neuropathy. Finally, the pre-clinical data suggesting that individual factors might be effective in treating neuropathy is reviewed, along with data relating to possible side effects of neurotrophic factor therapy. Several factors have already entered clinical trials with variable success. The data from these trials is reviewed as well.

Secretion of intrinsic cues controls repulsion of nociceptive neurons

The longstanding question of how the pattern of skin sensory innervation arises led us to investigate the behavior of rat DRG sensory axonal outgrowth. Outgrowing neurites from NGF-stimulated DRGs placed in close vicinity demonstrated repulsive behavior in the form of turning responses. In contrast, NT3-dependent neurites intermingled, as did neurites cultured without collagen embedding. These observations raise the possibility that secretion and not contact repulsion is the dermatome-building mechanism of nociceptive territories. Further experiments with functional antibodies against known secreted guidance molecules had no blocking effect. Our data provide evidence that the segmented pattern of skin nociceptive sensory maps is supported by unknown intrinsic cues released from the sensory axons themselves.

Antinerve growth factor treatment prevents intestinal dysmotility in Trichinella spiralis-infected rats

Nerve growth factor (NGF) could be involved in the development of hyperalgesia as well as in nervous remodeling consequence of inflammation. Both dysmotility and increase of visceral sensitivity have been described in functional gastrointestinal disorders such as irritable bowel syndrome. Trichinella spiralis-infected rats show an exacerbated spontaneous motility and a significant increase of the excitatory response to cholecystokinin (CCK), both associated with a reversible inflammatory process and the hypertrophy of the muscle layers. In this study we determined the intestinal expression of NGF mRNA by polymerase chain reaction and NGF by enzyme-linked immunosorbent assay. We implanted serosal strain gauge transducers on duodenum, jejunum, and ileum of anesthetized Sprague-Dawley rats to record circular muscle contractions. The experimental protocol included the evaluation of intestinal spontaneous motor activity (SMA), the response to CCK-8, and the ascending contraction induced by electrical mucosal stimulation. This protocol was performed in healthy and infected nontreated rats, in healthy rats with an NGF antibody treatment (1.6 mg/rat i.p.), and in infected rats with the same treatment applied at 0 or 3 days postinfection. NGF and NGF mRNA levels in the bowel were increased during inflammation. Although anti-NGF treatments did not prevent or reverse inflammatory response, the treatment was effective in preventing the motor alterations induced by the T. spiralis infection, i.e., inhibited increased SMA, reversed altered response to CCK, and reversed in part exacerbated response to electrical stimulation.

Nerve growth factor expression and innervation of the painful intervertebral disc

Following a previous description of nociceptive nerve fibre growth into usually aneural inner parts of painful intervertebral disc (IVD), this study has investigated whether nociceptive nerve ingrowth into painful IVD is stimulated by local production of neurotrophins. Immunohistochemistry and in situ hybridization have been used to investigate expression of the candidate neurotrophin, nerve growth factor (NGF), and its high- and low-affinity receptors trk-A and p75, respectively, in painful IVD excised for the management of low back pain. IVD from patients with back pain were of two types: those that when examined by discography reproduced the patient symptoms (pain level IVD) and those that did not (non-pain level IVD). Microvascular blood vessels accompanied nerve fibres growing into pain level IVD and these expressed NGF. The adjacent nerves expressed the high-affinity NGF receptor trk-A. These vessels entered the normally avascular IVD through the discal end plates. NGF expression was not identified in non-pain level or control IVD. Some non-pain level IVD had vessels within them, which entered through the annulus fibrosus. These did not express NGF nor did nerves accompany them. These findings show that nociceptive nerve ingrowth into painful IVD is causally linked with NGF production by blood vessels growing into the IVD, from adjacent vertebral bodies.

Classical and novel directions in neurotrophin transport and research: anterograde transport of brain-derived neurotrophic factor by sensory neurons

After the discovery of nerve growth factor, a classic model of neurotrophin action was developed. In this model, nerve endings compete for limited quantities of neurotrophic factors produced in neuronal target tissues. Neurotrophins are bound with high-affinity receptors expressed on the neuronal membrane and then endocytosed and retrogradely transported back to the cell body of responsive neurons. This classic model of target derived trophic support has been utilized to explain a wide range of trophic actions including effects on neuronal survival, terminal branching, and protein expression. However, a number of recent findings in the field of neurotrophin research cannot be explained using the classic model. In the peripheral nervous system (PNS), sensory neurons have been shown to contain mRNA for a member of the neurotrophin family, brain-derived neurotrophic factor (BDNF). Sensory neurons do not receive synaptic input so neurotrophin production by these cells does not fit into the classic target derived model. In contrast to target derived trophic support, BDNF produced by sensory neurons provides local autocrine and paracrine neurotrophic support in vitro. Furthermore, in vivo, sensory neurons transport BDNF in the anterograde direction away from the cell body, and opposite to the retrograde direction utilized in the classic model. Thus, out of necessity, a new direction for neurotrophin research has developed to study the production and anterograde transport of neurotrophins. The importance of this new mode of neurotrophin action in the PNS is indicated by results that implicate it in the response to pain, inflammation, and nerve injury.

Primitivism and plasticity of pain--implication of polymodal receptors

Bio-warning and defense mechanisms play the most fundamental roles in living organisms. From an evolutionary point of view, nociceptive systems are very primitive and are richly provided with humoral signaling mechanisms of aboriginal humoral defense systems, as reflected in the primitive nature of the polymodal receptor, a poorly differentiated sensory receptor signaling nociceptive information. Recent advances in studies on pain have made it possible to explain neural mechanisms of pain systems under physiological conditions and reveal that there is a large gap between physiological and pathological pains. Protracted nociceptive inputs under pathological conditions induce plastic, either functional or structural, alterations in the nociceptive pathways. These plastic changes lead to crosstalk among the neural networks, including circuits related to motor, autonomic, or psychological functions. These plastic changes, once established, persist even after the original pain sources disappear in a memory-like fashion. Thus, it is revealed that chronic pain cannot be treated by blocking pain pathways, which is effective against acute pain, but require treatment from a multidisciplinary perspective.

Identification of genes differentially expressed by nerve growth factor- and neurotrophin-3-dependent sensory neurons

The neurotrophins nerve growth factor (NGF) and neurotrophin-3 (NT3) support the survival of subpopulations of primary sensory neurons with defined and distinct physiological characteristics. Only a few genes have been identified as being differentially expressed in these subpopulations, and not much is known about the nature of the molecules involved in the processing of sensory information in NGF-dependent nociceptive neurons or NT3-dependent proprioceptive neurons. We devised a simple dorsal root ganglion (DRG) explant culture system, allowing the selection of neuronal populations preferentially responsive to NGF or NT3. The reliability of this assay was first monitored by the differential expression of the NGF and NT3 receptors trkA and trkC, as well as that of neuropeptides and calcium-binding proteins. We then identified four differentially expressed sodium channels, two enriched in the NGF population and two others in the NT3 population. Finally, using an optimized RNA fingerprinting protocol, we identified 20 additional genes, all differentially expressed in DRG explants cultured with NGF or NT3. This approach thus allows the identification of large number of genes expressed in subpopulations of primary sensory neurons and opens the possibility of studying the molecular mechanisms of nociception and proprioception.

Spinal cord injury and anti-NGF treatment results in changes in CGRP density and distribution in the dorsal horn in the rat

Spinal cord injury (SCI) results in chronic pain states in which the underlying mechanism is poorly understood. To begin to explore possible mechanisms, calcitonin gene-related peptide (CGRP), a neuropeptide confined to fine primary afferent terminals in laminae I and II in the dorsal horn of the spinal cord and implicated in pain transmission, was selected. Immunocytochemical techniques were used to examine the temporal and spatial distribution of CGRP in the spinal cord following T-13 spinal cord hemisection in adult male Sprague-Dawley rats compared to that seen in sham controls. Spinal cords from both hemisected and sham control groups (N = 5, per time point) were examined on postoperative day (POD) 3, 5, 7, 14, and 108 following surgery. Sham operated rats displayed CGRP immunoreaction product in laminae I and II outer, Lissauer's tract, dorsal roots, and motor neurons of the ventral horn. In the hemisected group, densiometric data demonstrated an increased deposition of reaction product that was statistically significant, in laminae III and IV, both ipsilateral and contralateral to the lesion that extended at least two segments rostral and caudal to the hemisection site by POD 14, and remained significantly elevated as long as POD 108. Since upregulation alone of CGRP would occur in an acute temporal window (by 2 to 3 days following spinal injury), these results are interpreted to be invasion of laminae III and IV by sprouting of CGRP containing fine primary afferents. Intrathecal delivery of antibodies against purified 2.5S nerve growth factor for 14 days to the hemisected group resulted in CGRP density in laminae I through IV that was significantly less than that seen in untreated or vehicle treated hemisected groups and to sham controls. These data indicate changes in density and distribution of CGRP following spinal hemisection that can be manipulated by changes in endogenous levels of NGF. These observations suggest possible strategies for intervention in the development of various pain states in human SCI.

Nerve growth factor depletion by autoimmunization produces thermal hypoalgesia in adult rats

Nerve growth factor (NGF) plays a role in mechanisms of inflammation and hyperalgesia in adult animals. We sought to determine if NGF depletion produced by autoimmunization of adult rats altered their thermal sensitivity to an acute noxious thermal stimulus. Anti-NGF IgG was not detected in the cerebrospinal fluid of any tested samples. Only those rats with the highest anti-NGF serum titers showed significant (P < 0.05) thermal hypoalgesia measured using the hot plate test (52 degrees C): the mean (+/-S.D.) hind paw lick latency of rats in the high anti-NGF titer group was 18.0 +/- 4.6 s compared to means of 10.8 +/- 4.3 s, 9.2 +/- 2.6 s and 10.1 +/- 3.0 s in the medium, low and control groups, respectively. Thus, NGF depletion by autoimmunization is a useful model for investigating the role of NGF in behavioral responses of adult rats to noxious stimuli, providing high titers of antibody are present.

aFGF, bFGF and NGF differentially regulate neuropeptide expression in dorsal root ganglia after axotomy and induce autotomy

Using immunohistochemistry and in situ hybridization the in vivo effects of acidic and basic fibroblast growth factor (aFGF, bFGF), and of nerve growth factor (NGF) on the expression of galanin, neuropeptide Y (NPY) and substance P in axotomized dorsal root ganglia (DRGs) were examined. Self-mutilation (autotomy), a supposed pain-related behavior, was investigated after growth factor treatment. One microgram of aFGF, bFGF or NGF was applied directly to the transected sciatic nerve via a capsule. In normal rats 3.2%, 0% and 17.5% of the neuron profiles in the DRGs contained galanin-, NPY- and substance P-like immunoreactivity (LI), respectively. Sciatic nerve transection induced a distinct increase in galanin- and NPY-LIs, but a downregulation of substance P-LI. Thus three days after axotomy 23.5%, 26.9% and 9.8% of the DRG neuron profiles showed immunoreactivity for galanin-, NPY- and substance P-LI, respectively. In vivo administration of aFGF counteracted the axotomy-induced increase in galanin and NPY, whereas bFGF only suppressed NPY upregulation. NGF reversed in the injury-induced decrease in substance P-LI, but had no significant effect on galanin- and NPY-LIs. These results were confirmed by monitoring the mRNA levels for these neuropeptides. Moreover, aFGF was found to induce autotomy in 60% of the rats 3 days after axotomy. NGF produced autotomy in about 30% of the rats. Taken together, the present results suggest (1) that aFGF, bFGF and NGF differentially regulate neuropeptide expression in vivo; (2) that FGFs can inhibit neuropeptide upregulation of some peptides after nerve injury; and (3) that aFGF and NGF may induce pain-related behavior.

Are the neurons in the dorsal root ganglion pseudounipolar? A comparison of the number of neurons and number of myelinated and unmyelinated fibres in the dorsal root

The neurons in the dorsal root ganglion have classically been described as pseudounipolar. Previous studies have questioned this simple organisation because an equality between the number of fibres in the dorsal root and neurons could not be established. In this study the number of neurons in the fifth lumbar dorsal root ganglion of the adult rat is compared to the number of fibres in the dorsal root. The methods used are founded on unbiased stereological principles and includes the optical disector, the Cavalieri principle, unbiased counting rules in two and three dimensions, and systematic random sampling. The number of A- and B-cells is estimated with light microscopy, and the number of myelinated and unmyelinated fibres is estimated with electron microscopy. The present study demonstrates that there is a 1:1 ratio (mean: 0.98, CV: 0.12, 95% confidence interval: 0.90-1.07) of fibres in the dorsal root to neurons in the dorsal root ganglion, as the classical theory predicts. Furthermore, the study of the two neuron subtypes supports the hypothesis that myelinated fibres originate from the A-cells and the unmyelinated fibres from the B-cells.

The effects of anti-nerve growth factor on retrograde labelling of superior cervical ganglion neurones projecting to the molar pulp in the rat

The aims were to demonstrate sympathetic ganglion neurones projecting to the rat molar pulp and to determine whether deprivation of nerve growth factor (NGF) in neonatal rats eliminates this source of pulpal innervation. Newborn Sprague-Dawley rats were given subcutaneous injections of rabbit anti-mouse-NGF serum for 1 month. Control animals included litter mates treated with preimmune serum and untreated, age-matched rats. AT 4 months of age, Fluoro-gold (FG) was applied to the pulp chamber of the right first maxillary molar. One week later, the animals were perfusion fixed, and the superior cervical ganglia (SCG) were removed, embedded in paraffin, and serially sectioned at 10 microns. FG-labelled cells were detected by epifluorescence microscopy with a u.v. filter set. Control animals had 5-10 FG-labelled neurones widely distributed throughout the SCG ipsilateral to the injection site and no labelled cells in the contralateral SCG. NGF-deprived animals had either no FG-labelled cells or a single labelled cell in the ipsilateral SCG. These results indicate that, in rats, (1) the number of SCG neurones projecting to the molar pulp is rather low, (2) SCG neurones that innervate the dental pulp of the maxillary molar pulp are dispersed throughout the ganglion, (3) the projection from SCG to the molar is exclusively ipsilateral, and (4) neonatal NGF deprivation induces a permanent, almost total, loss of sympathetic neurones projecting to the dental pulp.

Effects of postnatal anti-nerve growth factor serum exposure on development of apical nerves of the rat molar

The present study was undertaken to test the hypothesis that development of the pulpal innervation is dependent on nerve growth factor (NGF). Newborn rats were given subcutaneous injections of a rabbit anti-mouse NGF serum on alternate days for the first 24 days postnatally. Control animals were untreated and normal rabbit serum-treated litter mates. The animals were deeply anesthetized on postnatal day 26, perfused with fixative and the first mandibular molars were processed for transmission electron microscopy to obtain a complete census of axons entering the four roots. The composition of the mental nerve was also examined. Compared to control animals, the apical innervation of molars from anti-NGF-treated rats had only 62% as many myelinated fibers and 41% as many unmyelinated axons. Those myelinated fibers present in antiserum-treated animals were slightly, but significantly, smaller in average diameter than controls. In teeth of control animals, about 20% of all unmyelinated axons were located in fibers coursing outside of nerve fascicles; these isolated fibers were disproportionately rare after antiserum exposure. The average number of unmyelinated axons per Schwann cell unit was also significantly lower. Postnatal exposure to anti-NGF had milder effects on mental nerve composition compared to the tooth innervation. Numbers of myelinated fibers were 83% of controls, unmyelinated axons were 74% of controls and there was no change in the average number of unmyelinated axons per Schwann cell unit. We conclude that development of dental innervation is highly susceptible to postnatal NGF deprivation. This may be a consequence of the mostly nociceptive composition of dental nerves and their late development.

Altered expression of nerve growth factor in the skin of transgenic mice leads to changes in response to mechanical stimuli

It has recently become clear that the neurotrophic factor, nerve growth factor, interacts specifically with nociceptive sensory neurons during development and maturity. Indeed, it may serve as a critical link between inflammation and the hyperalgesia that ensues in adult animals. Nerve growth factor is normally expressed in limiting amounts in target tissues of sensory and postganglionic sympathetic neurons. In the present study we have altered the basal level of nerve growth factor expression in the skin by producing transgenic mice that express a fusion gene construct containing either a sense or antisense nerve growth factor complementary DNA linked to the K14 keratin promoter. The K14-nerve growth factor transgene (sense or antisense) is abundantly expressed in skin from approximately embryonic day 15 and is then constitutively expressed throughout the life of the animal. In light of the fact that systemic administration of nerve growth factor to neonatal or adult rats leads to hyperalgesia, we have asked whether mice expressing the sense K14-nerve growth factor transgene exhibit similar sensory abnormalities and whether mice expressing the antisense nerve growth factor complementary DNA were hypoalgesic. Here we show that mice over-expressing nerve growth factor in skin display a profound hyperalgesia to noxious mechanical stimulation. Additionally, K14-nerve growth factor antisense mice displayed a profound hypoalgesia to the same stimuli.

Nerve growth factor and nociception

Nerve growth factor (NGF) is thought of as a target-derived factor responsible for the survival and maintaining the phenotype of specific sets of peripheral and central neurons during development and maturation. Recently, using physiological techniques, we have shown that specific functional types of nociceptive sensory neurons require NGF, first for survival during development in utero and then for their normal phenotypic development (but not survival) in the early postnatal period. In adulthood, the physiological role of NGF changes dramatically and here it may serve as a link between inflammation and hyperalgesia. Despite apparent changes in NGF's mode of action as the animal matures, it always interacts specifically with nociceptive sensory neurons.

Selective sparing of later-born ganglion cells after neonatal transection of the infraorbital nerve

A combination of [3H]thymidine labelling and retrograde tracing with either horseradish peroxidase (HRP) or true blue (TB) was used to determine whether V primary afferent neurons born on different embryonic (E) days were differentially susceptible to neonatal transection of the infraorbital nerve (ION). In one experiment, rat fetuses were exposed to [3H]thymidine on E-8.5, 9.5, 10.5, 11.5, 12.5, 13.5, 14.5, or 15.5, the left infraorbital nerve (ION) was transected on the day of birth, and both the regenerate and intact IONs were labelled with HRP when the animals reached adulthood. The percentage of HRP labelled cells that were also heavily labelled by [3H]thymidine was calculated for both the intact ganglion and that ipsilateral to the damaged nerve for each animal. A consistently higher percentage of double labelled cells on the lesioned rather than on the intact side for a given E-day was taken as an indication that cells born on the day in question had an increased probability of survival relative to the entire population of V ganglion cells that contributed axons to the ION. Cells born late in gestation on E-12.5 through 14.5 were significantly more likely than early born (E-9.5 through 11.5) cells to survive neonatal axotomy. In a second experiment, fetuses were exposed to [3H]thymidine on either E-9.5, E-10.5, or E-14.5, the vibrissa pads on both sides of the face were injected with TB within 6 hours of birth, and the ION was transected 6-8 hours later.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution and relative density of p75 nerve growth factor receptors in the rat spinal cord as a function of age and treatment with antibodies to nerve growth factor

It has been postulated that nerve growth factor (NGF) binding to the low-affinity fast-dissociating NGF receptor (p75 NGFR) on Schwann cells and growing neurites is involved with the molecular feedback necessary for continued neurite extension during development and regeneration. Since central projections of somatosensory fibers sprout into the spinal cord after daily neonatal injections of antibodies to NGF (ANTI-NGF) for a one month period, it is of interest to determine if the distribution of p75 NGFR correlates with the occurrence of sprouting. Spinal cords from three groups of rats: untreated, preimmune sera treated and ANTI-NGF treated were examined on postnatal days (PD) 0, 14 and 30. The p75 NGFR distribution was determined using the monoclonal antibody 192 with standard immunohistochemical techniques and the optical density of the immunoreaction product was quantified using an Amersham image analysis system. The 192 immunoreaction product was localized to laminae I-IV, the dorsal columns, the dorsolateral funiculus, Lissauer's tract (LT) and the ventral horn on PD 0; to laminae I-III and medial IV and LT on PD 14; and laminae I-II and LT on PD 30. The untreated and preimmune sera treated groups show no difference in distribution. In the ANTI-NGF treated group, the 192 immunoreaction product was localized to laminae I-V and LT on PD 14 and to laminae I-III and medial IV and LT on PD 30. Similarly, the optical density of the ANTI-NGF treated group was significantly greater than same aged untreated and preimmune sera treated groups, but was not statistically different from these two groups examined 14 days earlier. Thus, ANTI-NGF treatment interferes with the postnatal downregulation of p75 NGFR in the dorsal horn and may provide for continued neurite growth.

Selective dependence of mammalian dorsal root ganglion neurons on nerve growth factor during embryonic development

We have investigated the NGF dependence of dorsal root ganglion (DRG) neurons in mammals using a paradigm of multiple in utero injections of a high titer anti-NGF antiserum. We have determined the specificity of our antiserum in relation to other members of the NGF neurotrophin family and found no cross-reactivity with brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3). To identify various classes of DRG neurons, we have stained their characteristic central projections with Dil. We show here that the NGF dependence of DRG neurons is strikingly selective. Although a majority of DRG neurons are lost after NGF deprivation during embryonic life, these are almost exclusively small diameter neurons that project to laminae I and II of the dorsal horn and presumably subserve nociception and thermoreception. Larger neurons that project to more ventral spinal laminae and subserve other sensory modalities do not require NGF for survival. These NGF-independent DRG neurons likely require one of the more recently identified neurotrophins, BDNF or NT-3.