Induction of nerve growth factor receptor (p75NGFr) mRNA within hypoglossal motoneurons following axonal injury

Comparison of two different media for maturation rate of neural progenitor cells to neuronal and glial cells emphasizing on expression of neurotrophins and their respective receptors

Neural cells derived from embryonic stem cells (ESCs) have potential usefulness for the treatment of neurodegenerative disorders. Modulation of intrinsic growth factors expression such as neurotrophins and their respective receptors by these cells is necessary to obtain functional neural cells for transplantation. In present study, we compared neural differentiation potential of two different media, NB + 5%ES-FBS + N2B27 and Ko-DMEM + 5%ES-FBS for conversion of mESC derived neural progenitors (NPs) into mature neural cells with emphasis on effect of the these two media on neurotrophins and their respective receptors expression. Immunofluorescence staining, RT-qPCR and western blot analysis showed that the expression of neuronal specific markers, MAP2 and Tuj-1, in NB + 5%ES-FBS + N2B27 medium was significantly higher than the other medium. Western blot assay revealed that the expression of BDNF and NGF increased significantly in mature neural cells obtained from NB + 5%ES-FBS + N2B27 medium but decreased in neural cells from Ko-DMEM + 5%ES-FBS medium compared to mESCs. TrkB protein was not detectable in mESCs but its expression increased in neural cells obtained from both media although its expression in NB + 5%ES-FBS + N2B27 medium was significantly higher than the other medium. In contrast to TrkB, p75NTR protein was detectable in mESCs and is remained constant in neural cells cultured in NB + 5%ES-FBS + N2B27 medium but decreased significantly in the other medium. In conclusion, our results indicated that NB + 5%ES-FBS + N2B27 medium promoted neural differentiation process of mESCs and caused enhancement of neurotrophins protein expression in addition to their cognate receptors.

Alteration in BDNF and its receptors, full-length and truncated TrkB and p75(NTR) following penetrating traumatic brain injury

The evidence that BDNF is involved in neuroprotection, neuronal repair and recovery after traumatic brain injury (TBI) is substantial. We have previously shown that the polymorphism of the human BDNF gene predicts cognitive recovery and outcome following penetrating TBI. The distribution of expression of BDNF and its receptors after penetrating TBI has not been investigated. In this study we examined the expression of these genes in a rat model of penetrating TBI. The injury is produced by a controlled penetration of a 2mm thick needle-shaped object, which is accelerated with a pellet from an air gun. We used in situ hybridization and investigated the mRNA expression of BDNF and its receptors: the full-length and the truncated TrkB and p75(NTR), from 1 day to 8 weeks following penetrating TBI. In addition, the protein level of BDNF in frontal cortex and hippocampus was measured by reverse phase protein microarray (RPPM). The mRNA expression of BDNF and its receptors decreased in the hippocampus in the border zone ipsilateral to the injury while there was an increase in mRNA expression at the contralateral side. The increase in BDNF mRNA expression in the hippocampus was sustained for 2 weeks following injury, with the highest expression noted in the CA3 cell layer. Furthermore, the protein analysis by RPPM showed increased levels of BDNF in the frontal cortex and the hippocampus up to 2 weeks after TBI. At 8 weeks following injury there was an intense labeling of the truncated TrkB receptor and the p75(NTR) in the area surrounding the cavity. Our study is the first report on the expression of BDNF and its receptors following penetrating TBI and suggests that their expression is altered long after the acute phase of injury. Further studies are needed to investigate if the late expressions of these receptors are beneficial or deleterious. In either case it indicates the possibility to influence the recovery after brain injury during the chronic phase and the development of treatments that may improve the outcome of TBI patients.

Nerve growth factor alters p75 neurotrophin receptor-induced effects in mouse facial motoneurons following axotomy

The p75 neurotrophin receptor (p75(NTR)) has been implicated as being detrimental for cell survival in facial motoneurons following injury. Although facial motoneurons do not respond to nerve growth factor (NGF) under normal circumstances, this study shows that NGF can interfere with p75(NTR)-mediated cell survival effects on motoneurons following injury. Twenty-five days following injury, the proportion of surviving axotomized neurons in NGF/p75(+/+) mice, which overexpress NGF, was significantly higher compared to wild-type mice, while NGF/p75(-/-) mice, which overexpress NGF but carry two mutated alleles for p75(NTR), had fewer neurons compared to wild-type and p75(-/-) mice, which carry two mutated alleles for p75(NTR), resulting in a lack of functional expression of this receptor. Sympathetic axons sprouted into the axotomized facial nucleus of both NGF/p75(+/+) and NGF/p75(-/-) following injury, due to transgene expression of NGF in reactive astrocytes. Removal of these sympathetic axons enhanced the number of surviving axotomized neurons in NGF/p75(-/-) mice but not in NGF/p75(+/+) mice. Although motoneurons do not express trkA and should therefore be unresponsive to NGF, our results reveal that NGF can influence p75-mediated motoneuron survival following axotomy.

Activation of Rac GTPase by p75 is necessary for c-jun N-terminal kinase-mediated apoptosis

The neurotrophin receptor p75 can induce apoptosis both in vitro and in vivo. The mechanisms by which p75 induces apoptosis have remained mostly unknown. Here, we report that p75 activates Rac GTPase, which in turn activates c-jun N-terminal kinase (JNK), including an injury-specific JNK3, in an NGF-dependent manner. N17Rac blocks this JNK activation and subsequent NGF-dependent apoptosis, indicating that activation of Rac GTPase is required for JNK activation and apoptosis induced by p75. In addition, p75-mediated Rac activation is modulated by coactivation of Trk, identifying Rac GTPase as one of the key molecules whose activity is critical for cell survival and death in neurotrophin signaling. The crucial role of the JNK pathway in p75 signaling is further confirmed by the results that blocking p75 from signaling via the JNK pathway or suppressing the JNK activity itself led to inhibition of NGF-dependent death. Together, these results indicate that the apoptotic machinery of p75 comprises Rac GTPase and JNK.

GDNF gene delivery to injured adult CNS motor neurons promotes axonal growth, expression of the trophic neuropeptide CGRP, and cellular protection

Glial-cell-line--derived neurotrophic factor (GDNF) has been identified as a potent survival and differentiation factor for several neuronal populations in the central nervous system (CNS), but to date, distinct effects of GDNF on motor axon growth and regeneration in the adult have not been demonstrated. In the present study, ex vivo gene delivery was used to directly examine whether GDNF can influence axonal growth, expression of neuronal regeneration-related genes, and sustain the motor neuronal phenotype after adult CNS injury. Adult Fischer 344 rats underwent unilateral transections of the hypoglossal nerve, followed by intramedullary grafts of fibroblasts genetically modified to secrete GDNF. Control animals received lesions and grafts of cells expressing a reporter gene. Two weeks later, GDNF gene delivery (1) robustly promoted the growth of lesioned hypoglossal motor axons, (2) altered the expression and intracellular trafficking of the growth-related protein calcitonin gene-related peptide (CGRP), and (3) significantly sustained the cholinergic phenotype in 84 +/- 6% of hypoglossal neurons compared with 39 +/- 6% in control animals (P < 0.001). This is the first neurotrophic factor identified to increase the in vivo expression of the trophic peptide CGRP and the first report that GDNF promotes motor axonal growth in vivo in the adult CNS. Taken together with previous in vitro studies, these findings serve as the foundation for a model wherein GDNF and CGRP interact in a paracrine manner to regulate neuromuscular development and regeneration.

p75 neurotrophin receptor signaling: mechanisms for neurotrophic modulation of cell stress?

The recent recognition that the p75 neurotrophin receptor, p75((NTR)), can induce apoptotic signals has contributed to the perception that it acts primarily as a death receptor. Although the molecular mechanisms of p75(NTR) signaling remain to be fully characterized, many of the currently identified pathways activated by p75(NTR) may be generally characterized as stress response signals. This review describes recent advances in identifying the molecular components involved in p75(NTR) signal transduction and suggests that p75(NTR) signaling may more aptly serve as a general mechanism for the transduction and modulation of stress signals.

Postnatal development of 5-HT(1A) receptor expression in rat somatic motoneurons

Prior work has established that hypoglossal motoneurons (HMs) change postnatally in their response to serotonin (5-HT), in part as a result of a decline in expression of 5-HT(1A) receptors. In the current study, two issues were addressed. First, using in situ hybridization we found that transient expression of 5-HT(1A) receptors occurs in other populations of brainstem (facial and trigeminal) and spinal (cervical and lumbar) motoneurons. Second, the participation of motoneuronal afferent (serotonergic) and efferent (neuromuscular) innervation in inducing and maintaining this decline in expression was investigated. Serotonergic innervation of the hypoglossal nucleus (nXII) was disrupted in neonatal rats by intra-cisternal injection of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), and 5-HT(1A) receptor mRNA levels in nXII from these rats were assayed at postnatal day 21. In spite of an almost complete loss of serotonergic fibers in the region, the postnatal decrease in 5-HT(1A) receptor expression by HMs still occurred. To test for potential regulation by target-derived factors or by nerve injury, receptor mRNA levels were assayed after unilateral transection of the hypoglossal nerve in adult rats. Though this treatment resulted in re-induction of developmentally transient expression of the p75 neurotrophin receptor, 5-HT(1A) receptor expression remained low. Thus, neonatal expression of 5-HT(1A) receptors appears to be common to somatic motoneurons, but we found no evidence for changes in serotonergic innervation in influencing this expression, nor did we find evidence for its regulation by peripheral factors.

Neurotrophin receptor expression is induced in a subpopulation of trigeminal neurons that label by retrograde transport of NGF or fluoro-gold following tooth injury

Tissue responses to injury are regulated by neurotrophins and neurotrophin receptor levels and can involve both retrograde and paracrine/autocrine trophic signaling. To determine how neurotrophins may contribute to the injury response, the timing and the extent of the up-regulation of neurotrophins and their receptors was examined in a model system which is particularly well suited for the analysis of trophic signaling pathways in response to injury. Injury to the occlusal surfaces of rat molar cusps induces a localized increase in nerve growth factor (NGF) expression in the dental pulp within 4-6 h. Radiolabeled NGF was transported in a receptor-mediated fashion from the teeth to a subset of neurons in the trigeminal ganglion within 15 h, indicating that these neurons possess NGF receptors (trk A and/or p75NTR). To test for NGF responses in the tooth sensory afferent neurons, levels of expression of neurotrophins and their receptors were examined by in situ hybridization in the trigeminal ganglion at 0, 4, 12, 20, 28 and 52 h post-injury. Within the maxillary division of the trigeminal ganglion, trk A expression was elevated at 4 h post-injury, with a maximum increase (2-fold) after 52 h. p75NTR was increased by 28 h post-injury and was increased 1.35-fold by 52 h. BDNF mRNA was increased 12 h after injury (1.8-fold), and 2.5-3-fold at 52 h post-injury. The trk B expression was increased only late after injury (28 and 52 h). To determine the receptor/neurotrophin phenotype of trigeminal neurons with projections to the molar teeth, these neurons were double-labeled with the retrograde tracer fluoro-gold and probes for either BDNF or trk B. The results show that tooth-innervating trigeminal neurons express BDNF, but not trk B. The timing of mRNA expression after injury and the phenotype of identified trigeminal neurons suggests a complex signaling cascade in which NGF at the injury site regulates NGF receptor expression at the levels of the cell body as well as increases in BDNF expression. Upregulated BDNF may act in a paracrine fashion on neighboring trigeminal cells expressing trk B. This signaling cascade may be a common feature of the response to mild peripheral inflammatory injuries within nociceptive pathways.

Effects of sciatic nerve grafts on choline acetyltransferase and p75 expression in transected adult hypoglossal motoneurons

We previously reported that a permanent transection of adult rat sciatic and hypoglossal nerves resulted in distinct changes in the levels of both low-affinity nerve growth factor receptor (p75) and choline acetyltransferase in the corresponding motoneurons as determined by immunoreactivity. Permanent axotomy of hypoglossal motoneurons induced a progressive loss of choline acetyltransferase immunoreactivity and a persistent expression of p75 immunoreactivity, phenomena that were not observed in spinal motoneurons. These observations indicated that spinal and brainstem motoneurons respond to permanent axotomy with a differential immunoreactivity for p75 and choline acetyltransferase. Such differences could be ascribed to specific intrinsic properties of each population of motoneurons or, alternatively, to different factors present in the periphery (nerve stump or target muscle). The aim of the present study was to test these two possibilities by determining if a segment of sciatic nerve transplanted to a transected hypoglossal nerve may counteract or attenuate the loss of choline acetyltransferase immunoreactivity in injured hypoglossal motoneurons. In addition, as further parameter, we analysed the presence of p75 immunoreactivity. Prior to grafting, segments of sciatic nerve were prepared by one of three methods: (i) a fresh piece; (ii) a degenerated piece; and (iii) a heated piece. Seven and 30 days following the placement of grafts, hypoglossal motoneurons were analysed for choline acetyltransferase and p75 immunolabelling. The results revealed that viable sciatic grafts (fresh and degenerated) are able to partially attenuate the loss in the number of choline acetyltransferase-positive injured hypoglossal motoneurons, even if an important decrease in choline acetyltransferase still persists with respect to the contralateral nucleus. In addition, viable sciatic grafts decreased the number of p75 immunoreactive hypoglossal motoneurons both at seven and at 30 days. In conclusion, the effects of viable sciatic grafts on the number of choline acetyltransferase and p75-labelled hypoglossal motoneurons indicate that these adult neurons are able to respond to factors released from the sciatic nerve, and that the number of injured motoneurons positive for choline acetyltransferase and p75 can be influenced by the presence of factors that may reach their proximal stumps. Furthermore, we hypothesize that the differential expression patterns between hypoglossal and sciatic motoneurons may be due, at least in part, to factors released from the nerve trunks themselves.

GABAA-receptor-mediated conductance and action potential waveform in cutaneous and muscle afferent neurons of the adult rat: differential expression and response to nerve injury

1. Whole cell patch-clamp recordings were obtained from identified cutaneous and muscle afferent neurons (33-60 microns diam) in dissociated L4 and L5 dorsal root ganglia (DRGs) from normal rats and from rats 2-3 wk after sciatic nerve ligation or crush injury. gamma-Aminobutyric acid (GABA)-induced conductance was compared in normal and injured neurons from both functional classes of sensory neurons. 2. Control cutaneous afferent neurons had a peak GABA-mediated conductance of 287 +/- 27 (SE) nS compared with 457 +/- 42 nS for control muscle afferent neurons. 3. An inflection on the downslope of the action potential was observed in 47% of cutaneous afferent neurons compared with 20% of muscle afferent neurons. 4. After ligation and transection of the sciatic nerve there was no change in the GABA-mediated conductance of muscle afferent neurons or in the action potential waveform (23% inflected). However, the cutaneous afferent neurons displayed a greater than two-fold increase in their GABA-mediated conductance and displayed a prominent reduction in the number of neurons with inflected action potentials (13% inflected). Input resistance was similar in cutaneous and muscle afferent neurons and decreased after ligation in cutaneous but not muscle afferents. Resting potential averaged from -50 to -56 mV in normal and ligated groups for both cutaneous and muscle afferent neurons. 5. After crush injury in cutaneous afferent neurons where the transected axons were allowed to regenerate into the distal nerve stump, GABAA-receptor-mediated conductance was elevated compared with controls. However, action potential waveform was not altered by crush injury, suggesting a differential regulation of these two properties in cutaneous afferent neurons. 6. These data indicate that injury-induced plasticity of GABAA-receptor-mediated conductance and action potential waveform occurs in cutaneous but not muscle afferent DRG neurons. It appears that peripherally derived influences are critical in maintaining the electrophysiological phenotype of cutaneous afferent neurons but not muscle afferent neurons.

Enhanced gene expression for phosphatidylinositol 3-kinase in the hypoglossal motoneurons following axonal crush

We attempted to see whether or not gene expression of phosphatidylinositol 3-kinase (PI 3-kinase) is changeable in the hypoglossal nucleus of rats after hypoglossal nerve crushing by in situ hybridization histochemistry. After unilateral nerve crushing, an apparent enhancement of gene expression for PI 3-kinase was observed in individual neurons of the hypoglossal nucleus at the operated side on the first postoperative day, and it was sustained for 7 postoperative days. Thereafter the expression decreased at the operated side and no significant difference in the expression level was noticed between the operated nucleus and the contralateral, non-operated or sham-operated nucleus on postoperative day 14. The present study suggests that PI 3-kinase contributes to some important roles in morphological changes of mature neurons associated with nerve regeneration.

Central infusions of brain-derived neurotrophic factor and neurotrophin-4/5, but not nerve growth factor and neurotrophin-3, prevent loss of the cholinergic phenotype in injured adult motor neurons

Neurotrophic factors are molecules that prevent neuronal degeneration and regulate neuronal phenotype during either development or adulthood. Relatively little is known about the comparative responsiveness of injured adult central nervous system motor neurons to various neurotrophic factors. In the present study we examined the effects of four members of the neurotrophin family on injured adult motor neurons. Nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 or neurotrophin-4/5 were infused intracerebroventricularly into adult rats following transection of the motor hypoglossal nerve. Two weeks after axotomy, brain-derived neurotrophic factor and neurotrophin-4/5 completely prevented the loss of the cholinergic phenotype in hypoglossal motor neurons (97 +/- 11% and 99 +/- 5%, respectively) as assessed by choline acetyltransferase immunolabeling. In contrast, nerve growth factor and neurotrophin-3 exerted no protective effect. The low-affinity p75 neurotrophin receptor, capable of binding all four neurotrophins, was re-expressed in injured hypoglossal neurons; the majority of injured hypoglossal neurons also express trkB receptors but not trkA or trkC receptors. Thus, injury-induced responses to neurotrophins in adult motor neurons are mediated by trk receptors and their agonists, but may or may not also require low-affinity p75 neurotrophin receptors. Intracerebroventricular infusions of trkB agonists may be a useful means of targeting multiple and distantly separated populations of motor neurons for neurotrophic factor therapy.

Differential expression of alpha-CGRP and beta-CGRP genes within hypoglossal motoneurons in response to axotomy

In this study we have analysed, by in situ hybridization, the expression of the genes for both alpha-CGRP and beta-CGRP in hypoglossal motor nuclei following transection of the left hypoglossal nerve. Our results show that the gene for alpha-CGRP displays a peculiar sequence of regulation (a successive up-down-up-recovery sequence) within ipsilateral hypoglossal motoneurons in response to axotomy. It is initially up-regulated, then down-regulated (displaying mRNA levels below basal), and later again up-regulated before recovery. By contrast, the gene for beta-CGRP displays a successive and distinct up-down-recovery sequence of regulation (it does not display a second increase in mRNA production). The first up-regulation of the alpha-CGRP gene occurs just during the early period of perineuronal glial reaction and the second up-regulation just during the period of delayed astrocyte reaction and muscle reinnervation. Because alpha-CGRP is a neuron-derived factor for many types of cells, including astrocytes and skeletal myocytes, our results suggest that the pleiotropic alpha-CGRP may be a motoneuron-derived trophic signal for both glial and skeletal muscle cells in order to maintain the motoneuron itself and, in consequence, might be of therapeutic interest in treating degenerative disease of motoneurons. beta-CGRP might be redundant within the hypoglossal motoneurons.

Axotomy induces a different modulation of both low-affinity nerve growth factor receptor and choline acetyltransferase between adult rat spinal and brainstem motoneurons

Adult rat spinal and brainstem motoneurons re-express low-affinity nerve growth factor receptor (p75) after their axotomy. We have previously reported and quantified the time course of this reexpression in spinal motoneurons following several types of injuries of the sciatic nerve. Other studies reported the reexpression of p75 in axotomized brainstem motoneurons. Results of these previous studies differed regarding the type of the most effective triggering injury for p75 reexpression, the relative duration of this reexpression and the decrease of choline acetyltransferase (ChAT) immunoreactivity (-IR) following a permanent axotomy of spinal or brainstem motoneurons. These differences suggest that these two populations of motoneurons respond to axotomy with a different modulation of p75 and ChAT expression. The aim of the present study was to determine whether differential modulation exists. We have analyzed and quantified the presence of p75- and ChAT-IR motoneurons in the hypoglossal nucleus following the same types of injury and the same time course we previously used for sciatic motoneurons. The results show that a nerve crush is the most effective triggering injury for p75 and that it induces similar temporal patterns of p75 and ChAT expression for sciatic and hypoglossal motoneurons. In contrast, a cut injury of the sciatic and hypoglossal nerves resulted in distinct temporal courses of both p75 and ChAT expression between these two populations of motoneurons. In fact, a permanent axotomy of the hypoglossal motoneurons induced i) a much longer maintenance phase for p75 than in sciatic motoneurons and ii) a progressive loss of ChAT-IR with a successive return to normal values in contrast to the modest decrease in the sciatic motoneurons. This evidence indicates that spinal and brainstem motoneurons respond to a permanent axotomy with a different modulation of p75 and ChAT expression. Altogether, the present data and the reported evidence of a differential post-axotomy cell death support the hypothesis that these two populations of motoneurons undergo different dynamic changes after axotomy.

Differential expression of p140trk, p75NGFR and growth-associated phosphoprotein-43 genes in nucleus basalis magnocellularis, thalamus and adjacent cortex following neocortical infarction and nerve growth factor treatment

A loss of target-derived neurotrophic factors is hypothesized to be one of the major determinants of central nervous system neuronal degeneration. In order to obtain further insight into early neuronal responses to injury, lesion-induced alterations in the expression of high- and low-affinity nerve growth factor receptors, as well as growth-associated phosphoprotein-43 genes in nucleus basalis magnocellularis, thalamic and neocortical neurons were studied. For this purpose, unilateral cortical devascularization operations were conducted on adult rats. Animals received i.c.v. infusions of vehicle or nerve growth factor (12 micrograms/day) and were killed at one, three, seven and 15 days post-lesion. In situ hybridization studies using 35S-labelled oligonucleotide probes for p75NGFR, p140trk and growth-associated phosphoprotein-43 messenger RNAs reveals that these genes were differentially regulated following the lesion. In the nucleus basalis magnocellularis ipsilateral to the lesion, p140trk gene expression significantly decreased on days 3 and 7, while p75NGFR messenger RNA initially increased on day 3 and decreased on days 7 and 15 after lesion. GAP-43 messenger RNA levels were significantly increased in the nucleus basalis magnocellularis on post-lesion days 3 and 7. Moreover, in contrast to p75NGFR or 140trk, growth-associated phosphoprotein-43 messenger RNA levels were significantly increased in pyramidal neurons located in the remaining cortex adjacent to the cortical lesion at all time points. In the lateral and ventroposterior nuclei of the thalamus, growth-associated phosphoprotein-43 messenger RNA level was slightly increased on days 1 and 3 and was dramatically decreased, significantly below the levels in sham-operated controls, on post-lesion days 7 and 15. During nerve growth factor application, the level of p140trk messenger RNA in the lesioned nucleus basalis magnocellularis returned to values observed in the contralateral nucleus basalis magnocellularis while p75NGFR messenger RNA was increased above values noted in all animals not treated with nerve growth factor. Nerve growth factor treatment did not affect the expression of growth-associated phosphoprotein-43 messenger RNA in any of the areas studied. p140trk messenger RNA was not up-regulated during the time that nerve growth factor was applied, as observed for p75NGFR, but only eight days after interrupting nerve growth factor treatment. Three cell types, nucleus basalis magnocellularis, cortical pyramidal and thalamic neurons, were probably affected in different ways by the devascularization with respect to lesion extent. Consequently, the remaining number of synaptic contacts in each of these brain areas is most likely different which may lead to a differential regulation of growth-associated phosphoprotein-43 messenger RNA.(ABSTRACT TRUNCATED AT 400 WORDS)

TrkA-immunoreactive profiles in the central nervous system: colocalization with neurons containing p75 nerve growth factor receptor, choline acetyltransferase, and serotonin

The present investigation used an antibody directed against the extracellular domain of the signal transducing nerve growth factor receptor, trkA, to reveal immunoreactive perikarya or fibers within the olfactory bulb and tubercle, cingulate cortex, nucleus accumbens, striatum, endopiriform nucleus, septal/diagonal band complex, nucleus basalis, hippocampal complex, thalamic paraventricular and reuniens nuclei, periventricular hypothalamus, interpeduncular nucleus, mesencephalic nucleus of the fifth nerve, dorsal nucleus of the lateral lemniscus, prepositus hypoglossal nucleus, ventral cochlear nucleus, ventral lateral tegmentum, medial vestibular nucleus, spinal trigeminal nucleus oralis, nucleus of the solitary tract, raphe nuclei, and spinal cord. Colocalization experiments revealed that virtually all striatal trkA-immunoreactive neurons (> 99%) coexpressed choline acetyltransferase (ChAT) but not p75 nerve growth factor receptor (NGFR). Within the septal/diagonal band complex virtually all trkA neurons (> 95%) coexpressed both ChAT and p75 NGFR. More caudally, dual stained sections revealed numerous trkA/ChAT (> 80%) and trkA/p75 NGFR (> 95%) immunoreactive neurons within the nucleus basalis. In the brainstem, raphe serotonergic neurons (45%) coexpressed trkA. Sections stained with a pan-trk antibody that recognizes primarily trkA, as well as trkB and trkC, labeled neurons within all of these regions as well as within the hypothalamic arcuate, supramammilary, and supraoptic nuclei, hippocampus, inferior and superior colliculus, substantia nigra, ventral tegmental area of T'sai, and cerebellular Purkinje cells. Virtually all of these other regions with the exception of the cerebellum also expressed pan-trk immunoreactivity in the monkey. The widespread expression of trkA throughout the central neural axis suggests that this receptor may play a role in signal transduction mechanisms linked to NGF-related substances in cholinergic basal forebrain and noncholinergic systems. These findings suggest that pharmacological use of ligands for trkA could have beneficial effects on the multiple neuronal systems that are affected in such disorders as Alzheimer's disease.

GAP-43 and p75NGFR immunoreactivity in presynaptic cells following neuromuscular blockade by botulinum toxin in rat

Peripheral nerve lesion results in changes in protein expression by neurons and denervated Schwann cells. In the present study we have addressed the question whether similar changes take place following functional denervation. Using immunohistochemistry and immunoelectron microscopy we examined changes in growth-associated protein (GAP-43) and low-affinity nerve growth factor receptor (p75NGFR) in rat gastrocnemius muscle following botulinum toxin-induced paralysis. GAP-43 and p75NGFR were selected because they are not expressed by mature intact motor neurons or Schwann cells, but are expressed following nerve lesion in both motor neurons and denervated Schwann cells. In control muscle, GAP-43 and p75NGFR immunoreactivity was seen only in nerve fibres near blood vessels. Two weeks after toxin injection, GAP-43 immunoreactivity could be seen at the motor endplates and in axons. Intensity of staining increased with longer survival and reached a peak between 4 and 8 weeks post-injection. Ultrastructurally, GAP-43 immunoreactivity was confined to nerve terminals and axons, whereas Schwann cells remained negative. Immunostaining for p75NGFR also increased following toxin injection and was detected in some terminal Schwann cells and in perineurial cells of small nerve fascicles near the paralyzed target cells, but not in axons. These results show that changes in expression of GAP-43 in motor neurons following functional denervation closely resemble the changes following anatomical interruption of nerve-muscle contact. GAP-43 was not expressed in Schwann cells, indicating that its upregulation in these cells is induced by loss of axonal contact or nerve degeneration products. There is no support for a role of p75NGFR in incorporation of neurotrophins in axons. The restriction of p75NGFR expression to terminal Schwann cells and perineurial cells in close proximity to the paralyzed target suggests a role for a target-derived signal or, alternatively, macrophages in eliciting this expression.

In situ hybridization detection of trkA mRNA in brain: distribution, colocalization with p75NGFR and up-regulation by nerve growth factor

In situ hybridization techniques were used to examine the distribution and the nerve growth factor (NGF) regulation of trkA mRNA in the adult rat brain in order to identify neurons in discrete regions of the brain that may be NGF responsive. In agreement with previous studies, trkA mRNA was detected within cells located in the medial septum (MS), diagonal band of Broca (DBB), and caudate. trkA mRNA was also detected in many other regions of the brain, including the nucleus basalis of Meynert, substantia innominata, paraventricular nucleus of the thalamus, interpeduncular nucleus, prepositus hypoglossal nucleus, vestibular nuclei, raphe obscuris, cochlear nucleus, sensory trigeminal nuclei, and gigantocellular as well as perigigantocellular neurons in the medullary reticular formation. By combining in situ hybridization detection of trkA mRNA with immunocytochemical detection of p75NGFR, it was determined that the vast majority (> 90%) of the trkA mRNA-containing cells detected in the MS and DBB also express p75NGFR. Likewise, the vast majority of p75NGFR-IR cells detected in the MS and DBB expressed trkA mRNA. Intracerebroventricular infusions of NGF into the third ventricle adjacent to the preoptic area resulted in a 58% increase in relative cellular levels of trkA mRNA in the horizontal limb of the DBB. These data provide evidence that both p75NGFR and trkA are expressed by NGF-responsive neurons in the MS and DBB. In addition, we note that areas that contained trkA mRNA and that also have been reported to contain p75NGFR are areas where high-affinity NGF binding sites have been observed autoradiographically, whereas areas that contain either trkA or p75NGFR alone are areas where no high-affinity NGF binding has been reported. Together, these findings suggest that both trkA and p75NGFR play an important role in the formation of high-affinity NGF receptors in brain and, furthermore, suggest that NGF may have physiological effects within many regions of the brain outside of the basal forebrain.

A continuing signal maintains NGF receptor expression in hypoglossal motor neurons after crush injury

Inhibition of axonal transport by vincristine applied to hypoglossal nerves 7 days after crush injury turns off the usual injured-induced expression of low affinity nerve growth factor receptor (p75NGFr). Vincristine applied proximal but not distal to the crush prevents p75NGFr induction. These results indicate that a continuing signal is axonally transported from the crush site that induces and maintains p75NGFr expression by injured motor neurons.