Neurotropic factors, retrograde axonal transport and cell signalling.

In vitro studies have recently identified receptors and signal transduction systems for many neurotrophic factors. In vivo, however, target-derived factors act over distances that are too great to be accounted for by simple diffusion of factors or classical second messengers. The active translocation of neurotrophic factors from the axon to the cell body by receptor-mediated retrograde transport provides a means by which factors presented at distal sites may influence somal signal transduction. We hypothesize that retrograde transport of receptors and other receptor-associated proteins leads to signalling at the cell body.

Selective destruction of nerve growth factor receptor-bearing cells in vitro using a hybrid toxin composed of ricin A chain and a monoclonal antibody against the nerve growth factor receptor.

A hybrid toxin composed of ricin A chain and a monoclonal antibody directed against the rat nerve growth factor (NGF) receptor (192-IgG) was prepared using the heterobifunctional cross-linking agent N-succinimidyl-3-(2-pyridyldithio)-propionate and purified by affinity chromatography. Characterization studies showed that the hybrid, 192-s-s-A, displaced bound 125I-labeled 192-IgG from rat superior cervical ganglion (SCG) membranes with an IC50 3-5 times lower than that of unconjugated 192-IgG. When incubated with cultured rat SCG neurons, 192-s-s-A inhibited protein synthesis in a concentration-dependent fashion. The effect of 192-s-s-A on these neurons was reversed by coincubation with an excess of 192-IgG. The IC50 of 192-s-s-A on protein synthesis in rat SCG neurons was 4 nM. Intact ricin and ricin A chain inhibited protein synthesis in these neurons with IC50 values of 5 pM and 500 nM, respectively. The 192-s-s-A hybrid had no effect on mouse SCG neurons or a human melanoma cell line known to have NGF receptors. This is consistent with the finding that 192-IgG recognizes only the rat NGF receptor. Also, 192-s-s-A did not inhibit protein synthesis in primary cultures of rat skeletal muscle or Vero cells, which do not have cell surface receptors for NGF. 192-s-s-A was able to inhibit protein synthesis in PC12 cells but the potency was 10-100 times less in these cells compared to rat SCG neurons. Ricin and A chain were also 10-100 times less potent in PC12 cells than neurons. Rat SCG neurons exposed to 192-s-s-A lost their refractile appearance under phase-contrast optics, showed granular degeneration of neurites, and died. Thus the decreased protein synthesis caused by the hybrid toxin correlated with the morphological destruction of the neurons. 192-s-s-A represents a potentially powerful tool by which to selectively destroy NGF receptor-bearing cells in vitro. The hybrid toxin may prove useful as an in vivo toxin.

Inhibitors of protein synthesis and RNA synthesis prevent neuronal death caused by nerve growth factor deprivation.

We have developed an experimental paradigm to study the mechanism by which nerve growth factor (NGF) allows the survival of sympathetic neurons. Dissociated sympathetic neurons from embryonic day-21 rats were grown in vitro for 7 d in the presence of NGF. Neurons were then deprived of trophic support by adding anti-NGF antiserum, causing them to die between 24 and 48 h later. Ultrastructural changes included disruption of neurites, followed by cell body changes characterized by an accumulation of lipid droplets, changes in the nuclear membrane, and dilation of the rough endoplasmic reticulum. No primary alterations of mitochondria or lysosomes were observed. The death of NGF-deprived neurons was characterized biochemically by assessing [35S]methionine incorporation into TCA precipitable protein and by measuring the release of the cytosolic enzyme adenylate kinase into the culture medium. Methionine incorporation began to decrease approximately 18 h post-deprivation and was maximally depressed by 36 h. Adenylate kinase began to appear in the culture medium approximately 30 h after deprivation, reaching a maximum by 54 h. The death of NGF-deprived neurons was entirely prevented by inhibiting protein or RNA synthesis. Cycloheximide, puromycin, anisomycin, actinomycin-D, and dichlorobenzimidazole riboside all prevented neuronal death subsequent to NGF deprivation as assessed by the above morphologic and biochemical criteria. The fact that sympathetic neurons must synthesize protein and RNA to die when deprived of NGF indicates that NGF, and presumably other neurotrophic factors, maintains neuronal survival by suppressing an endogenous, active death program.

Released form of CNTF receptor alpha component as a soluble mediator of CNTF responses.

The alpha component of the receptor for ciliary neurotrophic factor (CNTF) differs from other known growth factor receptors in that it is anchored to cell membranes by a glycosylphosphatidylinositol linkage. One possible function of this type of linkage is to allow for the regulated release of this receptor component. Cell lines not normally responsive to CNTF responded to treatment with a combination of CNTF and a soluble form of the CNTF alpha receptor component. These findings not only demonstrate that the CNTF receptor alpha chain is a required component of the functional CNTF receptor complex but also reveal that it can function in soluble form as part of a heterodimeric ligand. Potential physiological roles for the soluble CNTF receptor are suggested by its presence in cerebrospinal fluid and by its release from skeletal muscle in response to peripheral nerve injury.

The neurotrophins BDNF, NT-3, and NGF display distinct patterns of retrograde axonal transport in peripheral and central neurons.

The pattern of retrograde axonal transport of the target-derived neurotrophic molecule, nerve growth factor (NGF), correlates with its trophic actions in adult neurons. We have determined that the NGF-related neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), are also retrogradely transported by distinct populations of peripheral and central nervous system neurons in the adult. All three 125I-labeled neurotrophins are retrogradely transported to sites previously shown to contain neurotrophin-responsive neurons as assessed in vitro, such as dorsal root ganglion and basal forebrain neurons. The patterns of transport also indicate the existence of neuronal populations that selectively transport NT-3 and/or BDNF, but not NGF, such as spinal cord motor neurons, neurons in the entorhinal cortex, thalamus, and neurons within the hippocampus itself. Our observations suggest that neurotrophins are transported by overlapping as well as distinct populations of neurons when injected into a given target field. Retrograde transport may thus be predictive of neuronal types selectively responsive to either BDNF or NT-3 in the adult, as first demonstrated for NGF.

Receptor tyrosine kinase specific for the skeletal muscle lineage: expression in embryonic muscle, at the neuromuscular junction, and after injury.

While a number of growth factors have been described that are highly specific for particular cell lineages, neither a factor nor a receptor uniquely specific to the skeletal muscle lineage has previously been described. Here we identify a receptor tyrosine kinase (RTK) specific to skeletal muscle, which we term "MuSK" for muscle-specific kinase. MuSK is expressed at low levels in proliferating myoblasts and is induced upon differentiation and fusion. In the embryo, it is specifically expressed in early myotomes and developing muscle. MuSK is then dramatically down-regulated in mature muscle, where it remains prominent only at the neuromuscular junction; MuSK is thus the only known RTK that localizes to the neuromuscular junction. Strikingly, MuSK expression is dramatically induced throughout the adult myofiber after denervation, block of electrical activity, or physical immobilization. In humans, MuSK maps to chromosome 9q31.3-32, which overlaps with the region reported to contain the Fukuyama muscular dystrophy mutation. Identification of MuSK introduces a novel receptor-factor system that seems sure to play an important and selective role in many aspects of skeletal muscle development and function.

Retrograde axonal transport of LIF is increased by peripheral nerve injury: correlation with increased LIF expression in distal nerve.

Leukemia inhibitory factor (LIF) is a cytokine that affects the survival and differentiation of certain neuronal populations in vitro. To identify LIF-responsive neurons in the adult rat, we have demonstrated retrograde axonal transport of 125I-LIF to sensory and motor neurons. The accumulation of 125I-LIF by both cell types was significantly increased by prior sciatic nerve crush. Retrograde transport of 125I-LIF was inhibited by excess unlabeled LIF but not by related cytokines, indicating a specific receptor-mediated mechanism. Northern blot analysis revealed LIF expression in peripheral nerve that was increased in distal segments after axotomy. The correlation between LIF expression and increased retrograde transport following injury suggests that LIF plays a role in peripheral nerve regeneration.

Neurotrophin trafficking by anterograde transport.

The ever-unfolding biology of NGF is consistent with a target-derived retrograde mode of action in peripheral and central neurons. However, another member of the neurotrophin family, brain-derived neurotrophic factor (BDNF), is present within nerve terminals in certain regions of the brain and PNS that do not contain the corresponding mRNA. Recent studies have shown that the endogenous neurotrophins, BDNF and neurotrophin-3 (NT-3), are transported anterogradely by central and peripheral neurons. The supply of BDNF by afferents is consistent with their presynaptic synthesis, vesicular storage, release and postsynaptic actions. Anterograde axonal transport provides an 'afferent supply' of BDNF and NT-3 to neurons and target tissues, where they function as trophic factors and as neurotransmitters.

Neurotrophic factors: from molecule to man.

Recent advances in the understanding of the physiological role of nerve growth factor (NGF) have raised the question of whether neurotrophic factors might have clinical potential in the treatment of neurodegenerative disease or nerve trauma. Although NGF was first characterized as a target-derived survival factor for developing sympathetic and sensory neurons, it is now clear that it plays an important role in the maintenance and regeneration of mature peripheral neurons. However, the highly restricted specificity of NGF for sympathetic neurons, subpopulations of neural-crest-derived sensory neurons, and striatal and basal forebrain cholinergic neurons has, for almost two decades, stimulated the search for other neurotrophic factors that might act on the many classes of neurons that do not respond to NGF. In this article, the biology of the recently discovered NGF-related family of neurotrophic factors and ciliary neurotrophic factor and their receptors are reviewed, especially in the context of the therapeutic potential of these factors in the treatment of neurological disorders of the CNS.

A prosurvival function for the p75 receptor death domain mediated via the caspase recruitment domain receptor-interacting protein 2.

In addition to promoting cell survival, neurotrophins also can elicit apoptosis in restricted cell types. Recent results indicate that nerve growth factor (NGF) can induce Schwann cell death via engagement of the p75 neurotrophin receptor. Here we describe a novel interaction between the p75 receptor and receptor-interacting protein 2, RIP2 (RICK/CARDIAK), that accounts for the ability of neurotrophins to choose between a survival-versus-death pathway. RIP2, an adaptor protein with a serine threonine kinase and a caspase recruitment domain (CARD), is highly expressed in dissociated Schwann cells and displays an endogenous association with p75. RIP2 binds to the death domain of p75 via its CARD domain in an NGF-dependent manner. The introduction of RIP2 into Schwann cells deficient in RIP2 conferred NGF-dependent nuclear transcription factor-kappaB (NF-kappaB) activity and decreased the cell death induced by NGF. Conversely, the expression of a dominant-negative version of RIP2 protein resulted in a loss of NGF-induced NF-kappaB induction and increased NGF-mediated cell death. These results indicate that adaptor proteins like RIP2 can provide a bifunctional switch for cell survival or cell death decisions mediated by the p75 neurotrophin receptor.

Kinetic modulation of Kv4-mediated A-current by arachidonic acid is dependent on potassium channel interacting proteins.

The Kv4 subfamily of voltage-gated potassium channels is responsible for the transient A-type potassium current that operates at subthreshold membrane potentials to control membrane excitability. Arachidonic acid was shown recently to modulate both the peak amplitude and kinetics of the hippocampal A-current. However, in Xenopus oocytes, arachidonic acid only inhibited the peak amplitude of Kv4 current without modifying its kinetics. These results suggest the existence of Kv4 auxiliary subunit(s) in native cells. We report here a K-channel interacting protein (KChIP)-dependent kinetic modulation of Kv4.2 current in Chinese hamster ovary cells and Kv4.2 and Kv4.3 currents in Xenopus oocytes by arachidonic acid at physiological concentrations. This concentration-dependent effect of arachidonic acid resembled that observed in cerebellar granule neurons and was fully reversible. Other fatty acids, including a nonhydrolyzable inhibitor of both lipooxygenase and cyclooxygenase, 5,8,11,14-eicosatetraynoic acid (ETYA), also mimicked arachidonic acid in modulating Kv4.3 and Kv4.3/KChIP1 currents. Compared with another transient potassium current formed by Kv1.1/Kvbeta1, Kv4.3/KChIP1 current was much more sensitive to arachidonic acid. Association between KChIP1 and Kv4.2 or Kv4.3 was not altered in the presence of 10 microm ETYA as measured by immunoprecipitation and association-dependent growth in yeast. Our data suggest that the KChIP proteins represent a molecular entity for the observed difference between arachidonic acid effects on A-current kinetics in heterologous cells and in native cells and are consistent with the notion that KChIP proteins modulate the subthreshold A-current in neurons.

Neurotrophin-3 is required for the survival-differentiation of subsets of developing enteric neurons.

Neurotrophin-3 (NT-3) promotes enteric neuronal development in vitro; nevertheless, an enteric nervous system (ENS) is present in mice lacking NT-3 or TrkC. We thus analyzed the physiological significance of NT-3 in ENS development. Subsets of neurons developing in vitro in response to NT-3 became NT-3 dependent; NT-3 withdrawal led to apoptosis, selectively in TrkC-expressing neurons. Antibodies to NT-3, which blocked the developmental response of enteric crest-derived cells to exogenous NT-3, did not inhibit neuronal development in cultures of isolated crest-derived cells but did so in mixed cultures of crest- and non-neural crest-derived cells; therefore, the endogenous NT-3 that supports enteric neuronal development is probably obtained from noncrest-derived mesenchymal cells. In mature animals, retrograde transport of (125)I-NT-3, injected into the mucosa, labeled neurons in ganglia of the submucosal but not myenteric plexus; injections of (125)I-NT-3 into myenteric ganglia, the tertiary plexus, and muscle, labeled neurons in underlying submucosal and distant myenteric ganglia. The labeling pattern suggests that NT-3-dependent submucosal neurons may be intrinsic primary afferent and/or secretomotor, whereas NT-3-dependent myenteric neurons innervate other myenteric ganglia and/or the longitudinal muscle. Myenteric neurons were increased in number and size in transgenic mice that overexpress NT-3 directed to myenteric ganglia by the promoter for dopamine beta-hydroxylase. The numbers of neurons were regionally reduced in both plexuses in mice lacking NT-3 or TrkC. A neuropoietic cytokine (CNTF) interacted with NT-3 in vitro, and if applied sequentially, compensated for NT-3 withdrawal. These observations indicate that NT-3 is required for the normal development of the ENS.

A novel nervous system beta subunit that downregulates human large conductance calcium-dependent potassium channels.

The pore-forming alpha subunits of many ion channels are associated with auxiliary subunits that influence channel expression, targeting, and function. Several different auxiliary (beta) subunits for large conductance calcium-dependent potassium channels of the Slowpoke family have been reported, but none of these beta subunits is expressed extensively in the nervous system. We describe here the cloning and functional characterization of a novel Slowpoke beta4 auxiliary subunit in human and mouse, which exhibits only limited sequence homology with other beta subunits. This beta4 subunit coimmunoprecipitates with human and mouse Slowpoke. beta4 is expressed highly in human and monkey brain in a pattern that overlaps strikingly with Slowpoke alpha subunit, but in contrast to other Slowpoke beta subunits, it is expressed little (if at all) outside the nervous system. Also in contrast to other beta subunits, beta4 downregulates Slowpoke channel activity by shifting its activation range to more depolarized voltages and slowing its activation kinetics. beta4 may be important for the critical roles played by Slowpoke channels in the regulation of neuronal excitability and neurotransmitter release.

Aldose reductase inhibition increases CNTF-like bioactivity and protein in sciatic nerves from galactose-fed and normal rats.

The impact of exaggerated polyol pathway flux on ciliary neurotrophic factor (CNTF)-like bioactivity and expression of CNTF in rat sciatic nerve was examined after 2 months of galactose intoxication. Polyol content was elevated (P < 0.001) and motor nerve conduction velocity reduced (P < 0.05) in galactose-fed rats compared with control animals or control and galactose-fed rats treated with the aldose reductase inhibitor (ARI) Ponalrestat. CNTF-like bioactivity in the galactose-fed group was reduced to 30% of that assayed in the control group (P < 0.001). ARI treatment significantly increased CNTF-like bioactivity by 60% compared with the untreated galactose group (P < 0.05) but did not restore it to control levels. Unexpectedly, bioactivity in ARI-treated control animals was increased by nearly 250% compared with untreated controls (P < 0.005). In addition to the deficit in CNTF bioactivity in untreated galactose rats, the expression of protein, but not of mRNA, was reduced (P < 0.05). In ARI-treated control and galactose-fed rats, the expression of CNTF peptide was significantly enhanced above control levels (both P < 0.05). Concomitant with the reduction in CNTF levels, there was a shift in the axonal size-frequency distribution of myelinated fibers toward smaller axons in galactose-fed rats that was prevented by ARI treatment. Since galactose feeding has little impact on levels of CNTF mRNA, these observations suggest that deficits in CNTF-like bioactivity may result from a posttranscriptional modification of neurotrophic protein expression or turnover. Unlike other functional and structural disorders in galactose neuropathy, factors other than polyol accumulation may contribute to the deficit in CNTF-like bioactivity.

NT-3 attenuates functional and structural disorders in sensory nerves of galactose-fed rats.

The present study investigated the effect of NT-3, a neurotrophin expressed in nerve and skeletal muscle, on myelinated fiber disorders of galactose-fed rats. Adult, female Sprague-Dawley rats were fed diets containing complete micronutrient supplements and either 0% D-galactose (control) or 40% D-galactose. Treated controls received 20 mg/kg NT-3 and treated galactose-fed rats received 1, 5, or 20 mg/kg NT-3 three times per week by subcutaneous injections. After 2 months, sciatic and saphenous sensory nerve conduction velocity (SNCV) and sciatic motor nerve conduction velocity (MNCV) were measured and the sciatic, sural, peroneal and saphenous nerves and dorsal and ventral roots processed for light microscopy. Treatment of control animals with NT-3 had no effect on any functional or structural parameter. Compared to control values, galactose feeding induced a sensory and motor nerve conduction deficit and a reduction in axonal caliber. Treatment with 5 and 20 mg/kg NT-3 ameliorated deficits in sciatic and saphenous SNCV in galactose-fed rats but had no effect on the MNCV deficit. NT-3 treatment also attenuated the decrease in mean axonal caliber in the dorsal root and sural nerve but not in the saphenous nerve, ventral root and peroneal nerve. These observations show that NT-3 can selectively attenuate the sensory conduction deficit of galactose neuropathy in a dose-dependent manner that depends only in part on restoration of axonal caliber of large-fiber sensory neurons.

BDNF attenuates functional and structural disorders in nerves of galactose-fed rats.

Galactose intoxication of rats was used to disrupt metabolism of Schwann cells and skeletal muscle, two sites that contain the polyol-forming enzyme aldose reductase (AR). Galactose-fed rats develop a neuropathy characterized by nerve conduction deficits and axonal atrophy. To investigate the possibility that galactose metabolism by AR influences axonal function and structure by altering production of neurotrophic factors, the impact of galactose intoxication on nerve and muscle BDNF levels and the effects of exogenous BDNF treatment on galactose neuropathy were examined using biochemical, electrophysiologic and morphometric techniques. Galactose feeding increased BDNF protein in peripheral nerve and muscle. Exogenous BDNF treatment attenuated motor nerve conduction velocity deficits in the sciatic nerve of galactose-fed animals and myelin splitting of motor axons in the ventral root. In contrast, sensory nerve conduction velocity (SNCV) deficits in the sciatic nerve and myelin splitting in the central projections of sensory neurons were not prevented by BDNF treatment. BDNF treatment did not attenuate reduced axonal caliber in the sciatic nerve, but did ameliorate the diminution of the caliber of central sensory projections in the dorsal root. These findings point to the potential use of BDNF in the treatment of peripheral neuropathies.

Neurotrophin sensitivity of prevertebral and paravertebral rat sympathetic autonomic ganglia.

Prevertebral and paravertebral sympathetic autonomic ganglia respond differently to a large number of experimental and clinical insults. The selective involvement of subpopulations of sympathetic neurons may reflect differences in their response to neurotrophic substances. To test this hypothesis, we investigated the response of prevertebral and paravertebral rat sympathetic ganglia to selected neurotrophic substances in vivo and in vitro and identified the ganglionic distribution of neurons expressing high affinity neurotrophin receptor mRNAs. Dissociated cultures of embryonic prevertebral and paravertebral ganglionic neurons showed comparable responses to NGF deprivation and only small differences in their response to rescue with other trophic substances. In situ hybridization studies of adult rat sympathetic ganglia using probes specific for the high-affinity neurotrophin receptor transcripts trks A, B, and C demonstrated that neurons in both prevertebral and paravertebral sympathetic ganglia express predominantly trkA receptors in vivo. In addition, increased tyrosine hydroxylase (TOH) activity was induced only by doses of neurotrophic substances that activate trkA and showed only small differences between neonatal prevertebral and paravertebral ganglia. Although small differences in the sensitivity of pre- and paravertebral sympathetic neurons to various neurotrophins have been identified in our studies, they are unlikely, in isolation, to explain major differences in the sensitivity of these ganglia to neuropathologic processes.

Increased expression of CNTF receptor alpha in denervated human skeletal muscle.

The functional receptor for ciliary neurotrophic factor (CNTF) is comprised of a CNTF binding entity termed CNTF receptor alpha (CNTFRalpha), and 2 signaling molecules called LIF receptor beta and gp130. CNTFRalpha can be released from the cell surface; the soluble form can confer CNTF responsiveness to cells. CNTFRalpha has recently been localized to several nonneuronal cell types including rat skeletal muscle fibers. In this study we examined the expression pattern of CNTFRalpha in normal, denervated and dystrophic human muscle. In muscle biopsies from 12 normal subjects, 16 cases of neurogenic muscular atrophy, 4 cases of Duchenne muscular dystrophy, and 4 cases of limb girdle dystrophy, CNTFRalpha mRNA levels were determined by Northern blotting. Transcript levels were significantly increased in cases of neurogenic atrophy compared to normal controls and dystrophic muscle. By nonradioactive in situ hybridization, CNTFRalpha transcripts were detected in the sarcoplasm of both normal sized and atrophic muscle fibers. In addition, soluble CNTFRalpha was elevated 4.4-fold in the urine of ALS patients compared to normal adults. These results suggest that the expression of CNTFRalpha in human skeletal muscle fibers is regulated by innervation. This regulation appears to be selective, because CNTFRalpha mRNA was not increased in dystrophic human muscle. Increased CNTFRalpha could confer higher sensitivity to CNTF during neurodegeneration or nerve fiber regeneration.

Differential distribution of exogenous BDNF, NGF, and NT-3 in the brain corresponds to the relative abundance and distribution of high-affinity and low-affinity neurotrophin receptors.

To evaluate effective means for delivering exogenous neurotrophins to neuron populations in the brain, we compared the distribution and transport of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3) following intracerebral delivery. Rats received an injection of radioiodinated or unlabeled neurotrophin into the lateral ventricle and were killed humanely after 1.5-24 hours. Other rats received continuous infusion of unlabeled neurotrophin into the lateral ventricle, the striatum, or the hippocampus for 3-14 days. The neurotrophins were detected by autoradiography or immunohistochemistry. There were striking differences between BDNF, NGF, and NT-3 in their penetration through brain tissue. These differences occurred regardless of the site or method of delivery, but were most pronounced following a bolus intracerebroventricular (ICV) injection. After ICV injection, NGF was widely distributed in tissues around the ventricles and at the surface of the brain, whereas the penetration of BDNF into brain tissue was distinctly less than that of NGF, and the penetration of NT-3 was intermediate. An ICV injection of NGF produced prominent but transient labeling of cells that contain the low-affinity NGF receptor, whereas an injection of BDNF prominently labeled the ventricular ependyma. During ICV infusion (12 micrograms/day), the distribution of BDNF was no greater than that observed after a 12-micrograms bolus injection. A sixfold increase in the amount of BDNF infused (72 micrograms/day) produced a more widespread distribution in the brain and doubled the depth of penetration into periventricular tissues near the cannula. Corresponding to their differences in penetration, NGF was retrogradely transported by basal forebrain cholinergic neurons after ICV or intrastriatal delivery, whereas NT-3 was transported by a few basal forebrain neurons after ICV delivery, and BDNF was rarely detected in neurons after ICV delivery. Delivery of BDNF directly to the striatum or the hippocampus labeled numerous neurons in nuclei afferent to these structures. In situ hybridization studies confirmed that the high-affinity BDNF receptor (TrkB) was much more widely expressed in neurons than was the high-affinity NGF receptor (TrkA). Moreover, mRNA for truncated forms of TrkB was expressed at high levels in the ependyma, the choroid epithelium, and the gray matter. It is likely that binding of BDNF to TrkB, which appears to be more abundant and ubiquitous than TrkA, restricts the diffusion of BDNF relative to that of NGF.

Axonal transport of neurotrophins by visceral afferent and efferent neurons of the vagus nerve of the rat.

The receptor-mediated axonal transport of [125I]-labeled neurotrophins by afferent and efferent neurons of the vagus nerve was determined to predict the responsiveness of these neurons to neurotrophins in vivo. [125I]-labeled neurotrophins were administered to the proximal stump of the transected cervical vagus nerve of adult rats. Vagal afferent neurons retrogradely transported [125I]neurotrophin-3 (NT-3), [125I]nerve growth factor (NGF), and [125I]neurotrophin-4 (NT-4) to perikarya in the ipsilateral nodose ganglion, and transganglionically transported [125I]NT-3, [125I]NGF, and [125I]NT-4 to the central terminal field, the nucleus tractus solitarius (NTS). Vagal afferent neurons showed minimal accumulation of [125I]brain-derived neurotrophic factor (BDNF). In contrast, efferent (parasympathetic and motor) neurons located in the dorsal motor nucleus of the vagus and nucleus ambiguus retrogradely transported [125I]BDNF, [125I]NT-3, and [125I]NT-4, but not [125I]NGF. The receptor specificity of neurotrophin transport was examined by applying [125I]-labeled neurotrophins with an excess of unlabeled neurotrophins. The retrograde transport of [125I]NT-3 to the nodose ganglion was reduced by NT-3 and by NGF, and the transport of [125I]NGF was reduced only by NGF, whereas the transport of [125I]NT-4 was significantly reduced by each of the neurotrophins. The competition profiles for the transport of NT-3 and NGF are consistent with the presence of TrkA and TrkC and the absence of TrkB in the nodose ganglion, whereas the profile for NT-4 suggests a p75 receptor-mediated transport mechanism. The transport profiles of neurotrophins by efferent vagal neurons in the dorsal motor nucleus of the vagus and nucleus ambiguus are consistent with the presence of TrkB and TrkC, but not TrkA, in these nuclei. These observations describe the unique receptor-mediated axonal transport of neurotrophins in adult vagal afferent and efferent neurons and thus serve as a template to discern the role of specific neurotrophins in the functions of these visceral sensory and motor neurons in vivo.