Characterization of neurotrophin receptors by affinity crosslinking

Neurotrophic factors regulate the developmental survival and differentiation of specific neuronal populations. Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are members of the nerve growth factor (NGF) protein family, also known as the neurotrophins. Insights into the different roles of neurotrophins can be gained by studying the expression of their functional receptors. Here we report the development of procedures for their radiolabeling and efficient crosslinking to specific cell-surface receptors. BDNF and NT-3 receptors in cell lines and tissue preparations expressing receptors for the 2 neurotrophins have been identified using this affinity crosslinking procedure. Like NGF, BDNF and NT-3 crosslinking to the low affinity NGF receptor (p75^NGFR) on PC12 cells. BDNF and NT-3 also crosslinked to cells expressing p145^trkB protein, producing an approximately 160 kD neurotrophin-receptor complex. Crosslinking of the 2 neurotrophins in vivo to specific trk family members in many areas of the central nervous system also produced a 160 kD receptor complex. However, in all brain regions a complex of approx. 100 kD could also be identified, all or most of which represents crosslinking to a truncated from of trkB. The broad distribution of BDNF and NT-3 receptors throughout the CNS suggests that neurotrophins may have yet unrecognized functions on specific neuronal populations. BDNF and NT-3 receptors were also found in brain areas in which the neurotrophins themselves are also synthesized, suggesting that beyond long-range trophic effects, these proteins may also act as autocrine or short-range paracrine regulators. © 1993 Wiley-Liss, Inc.     

p75 nerve growth factor receptor modulates p140trkA kinase activity,but not ligand internalization,in PC12 cells

The biological activity of nerve growth factor (NGF) has been shown to be mediated by the p140^trkA receptor tyrosine kinase, while the role of the p75 NGF receptor (p75^NGFR) is still unresolved. Here we have investigated the relative contribution of p140^trkA and p75^NGFR to early consequences of NGF binding: ligand internalization, p140^trkA autophosphorylation, and tyrosine phosphorylation of Shc, phospholipase C_γ-1 (PLC_γ-1), and extracellular signal-regulated kinases (ERKs). It was found that NGF internalization was neither prevented by blocking p140^trkA activity using the protein kinase inhibitors methylthioadenosine, staurosporine, and K-252a, nor by inhibiting NGF binding to p75^NGFR with antibodies. However, when NGF binding to p140^trkA was reduced by the use of a synthetic peptide corresponding to amino acids 36–53 of human p140^trkA, internalization of NGF was decreased. Thus, at least in PC12 cells, internalization appears to require binding of NGF to p140^trkA, but occurs irrespective of p140^trkA kinase activity and ligand occupancy of p75^NGFR. The NGF triple mutant Lys-32/Lys-34/Glu-35 to Ala, which has been demonstrated to bind to p140^trkA, but not to p75^NGFR, induced tyrosine phosphorylation more rapidly than wild-type NGF. Likewise, NGF-induced tyrosine phosphorylation was accelerated when NGF binding to p75^NGFR was prevented with REX-IgG. These findings indicate that NGF binding by p75^NGFR may modulate NGF-induced p140^trkA kinase activity. © 1994 Wiley-Liss, Inc.     

Autocrine regulation of proliferation of cerebellar granule neurons by nerve growth factor

Premigratory cerebellar granule neurons, which highly express nerve growth factor (NGF), low (gp75^NGFR) and high (gp140^trkA) affinity NGF receptors, were used as a physiological model to investigate the effects of NGF on neuronal replication. Studies in vivo and on cultures showed that NGF stimulates DNA synthesis, mitotic activity and related cell acquisition by initiating the entry of cells into the S phase and regulating their time in the G1 and S phases. The NGF-induced effects were blocked in vivo and in vitro by both monoclonal anti-NGF and anti-gp75^NGFR antibodies. These results clearly demonstrate that NGF is essential for the crucial first step of cerebellar ontogenesis and support the idea that low affinity receptors are involved in the biological response, possibly by interacting with gp140^trkA. By comparison with a number of well known mitogens, the high affinity form could be the main transducer of the mitogenic signal pathway. The early developing cerebellum appears therefore to be the first autocrine (and/or paracrine) model of NGF action on neurogenesis in the CNS. © 1994 Wiley-Liss, Inc.     

Comparison of c-jun,junB, and junD mRNA expression and protein in the rat dorsal root ganglia following sciatic nerve transection

The present study was designed to compare the expression of the Jun family of protooncogenes following nerve injury. Adult rats were anesthetized and the sciatic nerve transected. Dorsal root ganglia (DRG) at 1, 2, 3, and 7 days after nerve transection were collected, their total RNA extracted, and Northern blots performed using ³²P-labeled oligonucleotide probes. The constitutive expression of c-jun mRNA was very low in DRG. Induction of c jun mRNA was observed by day 1 after nerve transection, with a sixfold peak at 3 stays and a twofold induction still present by day 7. The constitutive expression of junB mRNA was also low in the DRG, and sciatic nerve transection produced only a modest induction (1.7fold by day 3) in the DRG ipsilateral to the nerve cut. junD mRNA was constitutively expressed at high levels in the DRG, and its level of expression did not change after sciatic nerve transection. Immunocytochemistry studies demonstrated a pattern of c-Jun, JunB, and JunD immunoreactivity (IR) associated with the cell nuclei of DRG neurons. c-Jun IR was found at very low levels in the undamaged contralateral DRG neurons, but sciatic nerve transection dramatically increased the number of c-Jun-immunoreactive neurons. Dot blot immunoblotting assay confirmed that the DRG ipsilateral to the sciatic nerve cut contained a higher level of c-Jun protein than the contralateral control DRG. Similar to c-Jun IR, JunB IR was minimal in the undamaged contralateral DRAG. However, the DRG ipsilateral to the nerve transection did not show an increase in the number of immunoreactive neurons. JunD protein was expressed at high levels in the contralateral DRG, and this level of expression persisted after sciatic nerve transection in the ipsilateral DRG. DNA gel retardation assay experiments with an AP-1 consensus sequence showed a single DNA-protein complex. This complex was increased in ipsilateral as compared with contralateral DRG extracts. The amount of DNA protein complex was reduced byc-Jun protein antiserum but was not altered when treated with a Fos antibody. We conclude that cjun, junB and junD mRNAs and proteins are differentially regulated in the DRG after sciatic nerve transection. © 1995 Wiley-Liss, Inc.     

Attenuation and recovery of nerve growth factor receptor mRNA in dorsal root ganglion neurons following axotomy

The actions of nerve growth factor (NGF) are mediated by two receptor proteins, trk and p75. Recent evidence indicates that NGF upregulates the expression of both trk and p75 in responsive neurons including rat dorsal root ganglion (DRG) neurons. Axotomy by disconnecting the neuron from its source of target-derived NGF is predicted to lead to the downregulation of trk and p75 expression. However, previous studies of the effects of axotomy on trk and p75 mRNA expression in rat DRG have yielded discrepant results. We report that following sciatic nerve crush, trk and p75 mRNA levels in L4-L6 DRG decrease to ～50% of control levels at 4–14 days, return to control levels by 30 days, and are increased by ～30% at 60 days. Similar changes are observed following nerve transection although mRNA levels are slower in returning to normal and do not exceed control levels at later timepoints. Thus, trk and p75 expression decline early following target disconnection and later recover irrespective of target reinnervation. These observations indicate that target derived NGF is required for the maintenance of NGF receptor expression in adult rat DRG neurons and that non-target derived factors can appropriate this function following peripheral nerve injury. © 1996 Wiley-Liss, Inc.     

Different expressions of BDNF, NT3, and NT4 in muscle and nerve after various types of peripheral nerve injuries

The changes in the expression of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) in the rat neuromuscular system as a result of three different types of sciatic nerve injuries have been evaluated. The changes in mRNA and protein levels for BDNF, NT-3, and NT-4 in the soleus muscle and sciatic nerve were assessed 4-28 days after sciatic nerve transection (neurotmesis), sciatic nerve crush (axonotmesis), and mild acute compression (neurapraxia). BDNF mRNA levels increased dramatically with nerve transection in the soleus muscle and the sciatic nerve 7-14 days after injury, whereas the changes were low in other types of injury. The changes of protein levels for BDNF were also similar. The mRNA and the protein levels of NT-3 in the soleus muscle did not show any significant difference. The mRNA for NT-4 in the soleus muscle decreased from 4 to 14 days after sciatic nerve transection, and the protein level was also minimum 14 days after sciatic nerve transection. Our results indicate that the neurotrophic factors in the neuromuscular system could play a role in differentiating peripheral nerve injury.     

Expression of mRNA for brain-derived neurotrophic factor in the dorsal root ganglion following peripheral inflammation

It is well known that the nerve growth factor (NGF) may serve as a link between inflammation and hyperalgesia. Recent experiments showed that systemic injection of NGF dramatically stimulated the expression of brain-derived neurotrophic factor (BDNF) mRNA in the dorsal root ganglion (DRG). In the present study, we evaluated the change of BDNF mRNA in the DRG following peripheral inflammation and also observed colocalization of BDNF and trkA mRNAs by means of in situ hybridization histochemistry in rats. Peripheral tissue inflammation produced by an intraplantar injection of Freund's adjuvant into the paws significantly increased BDNF mRNA levels in the DRG and many neurons expressing trkA mRNA showed increased expession of BDNF mRNA. Intraplantar injection of antibody to NGF together with Freund's adjuvant prevented the increase in BDNF mRNA. These findings suggest that peripheral inflammation induces an increased expression of BDNF mRNA which is mediated by NGF in DRG.     

Distribution of neurotrophin receptors in human palatine tonsils: an immunohistochemical study

Increasing evidence indicates that nerve growth factor (NGF) exerts effects on cells of the immune system, but the possible immunomodulatory effect of other neurotrophins (brain-derived neurotrophic factor, BDNF; neurotrophin-3, NT-3; and NT-4/5) has not been studied. Neurotrophins act on responsive cells by binding a low-affinity pan-neurotrophin receptor (p75), and more specific high-affinity receptors (gp140trkAA, gp145trkB and gp145trkC considered as preferred signaling transduction receptors for NGF, BDNF and NT-3, respectively). The expression of neurotrophin receptor proteins may be considered, therefore, as a potential indication of neurotrophin activity. In the present study we investigated the distribution of both types of neurotrophin receptors in the human palatine tonsils using immunohistochemical methods. In the follicular germinal centers both lymphocytes and follicular dendritic cells (FDC) displayed gp75 IR, but not IR for trk neurotrophin receptor proteins. gp140trkA-like IR and gp145trkC-like IR were encountered on paracortical interdigitating cells (PIC), and in the high endothelial venule cells. gp145trkB-like IR was found in a cell subpopulation which probably represented macrophages. Present results suggest that NGF, NT-3 and NT-4/5 may act in PIC and indirectly in lymphocytes, whereas BDNF and NT-4/5 could control macrophages. The role of p75 on lymphocytes and FDC and whether trk neurotrophin receptor proteins present in lymphoid tissues are functional receptors for neurotrophins remains to be elucidated.     

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 nudei 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 ofp75^NGFR, it was determined that the vast majority (> 90%) of the trkA mRNA-containing cells detected in the MS and DBB also express p75^NGFR. Likewise, the vast majority of p75^NGFR-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 p75^NGFR 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 p75^NGFR are areas where high-affinity NGF binding sites have been observed autoradio-graphically, whereas areas that contain either trkA or p75^NGFR alone are areas where no high-affinity NGF binding has been reported. Together, these findings suggest that both trkA and p75^NGFR 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.     

Expression of neurotrophins and Trk receptors in the avian retina

Using the RNase protection assay, we have found that nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) are expressed in the avian retina during development. The expression peaks around embryonic days 12–15, with decreasing levels at later stages of development. Abundant levels of NGF and BDNF but low levels of NT-3 mRNA were found in the adult retina. We also found that light/darkness regulated the levels of NGF and BDNF mRNAs but not the levels of NT-3 mRNA in the 5-day-old chicken retina. It was demonstrated that NGF and BDNF mRNA levels were up-regulated by light exposure. The cellular localization of mRNA expression for the neurotrophins and neurotrophin receptors TrkA, TrkB, and TrkC in the retina was studied using in situ hybridization. The patterns of NGF and trkA mRNA expression were very similar and were localized to the external part of the inner nuclear layer on the border with the outer plexiform layer and corresponded to the localization of horizontal cells. NT-3 labeling was also found over the external part of the inner nuclear layer, whereas trkC mRNA was found over all layers in the retina. BDNF labeling was found over all layers in the retina, whereas TrkB labeling was intense over cells in the ganglion cell layer, which is in agreement with the response of ganglion cells to BDNF stimulation. Functional neurotrophin receptors were suggested by the response of retinal explants to neurotrophin stimulation. These data indicate that the neurotrophins play local roles in the retina that involve interactions between specific neuronal populations, which were identified by the localization of the Trk receptor expression. The data also suggest that NGF and BDNF expression is regulated by normal neuron usage in the retina. © 1996 Wiley-Liss, Inc.     

Neurotrophin gene expression by cell lines derived from human gliomas

The expression of neurotrophin (NGF, BDNF, and NT-3) mRNAs in 24 cell lines derived from human malignant gliomas was studied by Northern analysis. Widespread expression of neurotrophin genes was found with BDNF being the most abundantly expressed. Nearly all cell lines expressed BDNF, and about two-thirds of the cell lines expressed NGF and NT-3. Half of the cell lines analyzed expressed all three neurotrophins. Secretion of NGF into the medium of several cell lines could be detected by ELISA and a PC12 neurite outgrowth assay. Immuno- and bioactive NGF was isolated from conditioned medium of one cell line. No evidence of expression of the neurotrophin receptors trk and trkB by Northern analysis was found. Receptor crosslinking with radiolabeled cognate ligands failed to detect functional receptors in all but one cell line. In this cell line a receptor complex for BDNF was found that corresponded to truncated trkB receptors that lack the signal transducing tyrosine kinase domain. Neurotrophins did not stimulate mitosis of the glioma cultures. The findings suggest that production of neurotrophins by glioma cells is a general phenomenon, although neurotrophins made by gliomas lacking their receptors may not play an autocrine but rather a paracrine role. © 1993 Wiley-Liss, Inc.     

Electromagnetic fields influence NGF activity and levels following sciatic nerve transection

Pulsed electromagnetic fields (PEMF) have been shown to increase the rate of nerve regeneration. Transient post‐transection loss of target‐derived nerve growth factor (NGF) is one mechanism proposed to signal induction of early nerve regenerative events. We tested the hypothesis that PEMF alter levels of NGF activity and protein in injured nerve and/or dorsal root ganglia (DRG) during the first stages of regeneration (6–72 hr). Rats with a transection injury to the midthigh portion of the sciatic nerve on one side were exposed to PEMF or sham control PEMF for 4 hr/day for different time periods. NGF‐like activity was determined in DRG, in 5‐mm nerve segments proximal and distal to the transection site and in a corresponding 5‐mm segment of the contralateral nonoperated nerve. NGF‐like activity of coded tissue samples was measured in a blinded fashion using the chick DRG sensory neuron bioassay. Overall, PEMF caused a significant decrease in NGF‐like activity in nerve tissue (P < 0.02, repeated measures analysis of variance, ANOVA) with decreases evident in proximal, distal, and contralateral nonoperated nerve. Unexpectedly, transection was also found to cause a significant (P = 0.001) 2‐fold increase in DRG NGF‐like activity between 6 and 24 hr postinjury in contralateral but not ipsilateral DRG. PEMF also reduced NGF‐like activity in DRG, although this decrease did not reach statistical significance. Assessment of the same nerve and DRG samples using ELISA and NGF‐specific antibodies confirmed an overall significant (P < 0.001) decrease in NGF levels in PEMF‐treated nerve tissue, while no decrease was detected in DRG or in nerve samples harvested from PEMF‐treated uninjured rats. These findings demonstrate that PEMF can affect growth factor activity and levels, and raise the possibility that PEMF might promote nerve regeneration by amplifying the early postinjury decline in NGF activity. J. Neurosci. Res. 55:230–237, 1999. Published 1999 Wiley‐Liss, Inc.     

Upregulation of bradykinin B2 receptor expression by neurotrophic factors and nerve injury in mouse sensory neurons

Bradykinin B2 receptor mRNA was detected at low levels, both by RT-PCR and by in situ hybridization, in freshly isolated dorsal root ganglia (DRG) and in ganglia cultured in the absence of neurotrophic factors, but was strongly upregulated by culture in the presence of nerve growth factor (NGF). The effect of NGF is mediated via TrkA receptors. The related neurotrophins, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4, were ineffective in upregulating B2 mRNA, but a small upregulation was seen with the unrelated neurotrophin glial cell line-derived neurotrophic factor (GDNF). Surface membrane B2 receptor expression, detected by immunofluorescence using a B2-specific antibody, was low in outgrowing axons cultured in the absence of neurotrophic factors, but was elevated by addition of NGF or GDNF. Conditioned media prepared by incubating injured nerve, skin, or muscle had a similar effect to NGF in upregulating B2 mRNA and protein expression, and the activity was largely removed by neutralization of NGF in the conditioned medium with an anti-NGF antibody. After nerve crush injury in vivo an enhancement in B2 mRNA expression was seen, peaking after 7 days and returning to precrush levels after 14 days. In all conditions tested, the proportion of neurons expressing B2 mRNA remained the same at around 23% of small neurons, suggesting that upregulation only occurs in the B2-positive neurons. These experiments show that NGF, and to a lesser extent GDNF, upregulates the expression of bradykinin B2 mRNA and B2 receptor protein in the surface membrane of DRG neurons and that NGF is an important factor responsible for upregulating bradykinin B2 receptor expression after nerve crush injury in vivo.     

Study on molecular mechanism for improving neural regeneration after repair of sciatic nerve defect in rat by acellular nerve allograft

Objective: Discuss the molecular mechanism for improving neural regeneration after repair of sciatic nerve defect in rat by acellular nerve allograft (ANA). Methods: Randomly divide 36 Wistar rats into six groups as normal control group, autografting group, and bridging groups of 2, 4, 8, 12 weeks, six rats for each group. Observe the expression of brain‐derived neurotrophic factor (BDNF) in L₄ spinal cord and anterior tibial muscle at the injury site, calcitonin gene‐related peptide (CGRP) protein as well as mRNA, respectively. 12w after operation, histopathological observation was performed. Results: 2w after ANA bridging the sciatic nerve defect in rats, it was observed that the expression level of BDNF in spinal cord at the injury site and CGRP protein increased, reaching the peak level at 4w, lasting till 8w, then decreased but still significantly higher than that in normal control group at 12w, and was not significantly different compared with that in autografting group. However, the expression level of BDNF in anterior tibial muscle decreased gradually within the initial 4w, then increased progressively, reaching normal level at 12w, and was not significantly different compared with that in autografting group. The expression of BDNF mRNA and CGRPmRNA was essentially the same. 12w after operation, there was nerve regeneration in bridging group of 12w and autografting group. Conclusions: ANA possessed fine histocompatibility, and might substitute autograft to repair long‐segment defect of sciatic nerve in rats. This action might be related to upregulation of protein and mRNA expression for BDNF and CGRP in spinal cord. Synapse, 2012. © 2011 Wiley Periodicals, Inc.     

Further characterization of the effects of brain-derived neurotrophic factor and ciliary neurotrophic factor on axotomized neonatal and adult mammalian motor neurons

Neurotrophins and neural cytokines are two broad classes of neurotrophic factors. It has been reported that ciliary neurotrophic factor (CNTF) and brain-derived neurotrophic factor (BDNF) prevent the degeneration of axotomized neonatal motor neurons. In addition, BDNF is transported retrogradely to α-motor neurons following injection into the muscle, and patterns of BDNF expressed in spinal cord and muscle suggest a physiological role for this factor in motor neurons. In the present study, we characterize the effects of BDNF on axotomized neonatal facial motor neurons and extend these observations to adult models of motor neuron injury (axotomy-induced phenotypic injury of lumbar motor neurons). BDNF reduces axotomy-induced degeneration of neonatal neurons by 55% as determined by Nissl staining (percentage of surviving neurons in vehicle-treated cases, 25%; in BDNF-treated cases, 80%). Rescued neurons have an intact organelle structure but appear smaller and slightly chromatolytic on electron microscopic analysis. As demonstrated by intense retrograde labeling with horseradish peroxidase (HRP) applied to the proximal stump of the facial nerve, neurons rescued by BDNF have intact mechanisms of fast axonal transport. CNTF did not appear to have significant effects on neonatal motor neurons, but the lack of efficacy of this factor may be caused by its rapid degradation at the application site. BDNF is not capable of reversing the axotomy-induced reduction in transmitter markers [i.e., the acetylcholine-synthesizing enzyme choline acetyl-transferase (ChAT) or the degrading enzyme acetylcholinesterase (AChE)] in neonatal or adult animals or the axotomy-induced up-regulation of the low-affinity neurotrophin receptor p^75NGFR (nerve growth factor receptor) in adult motor neurons. However, BDNF appears to promote the expression of p^75NGFR in injured neonatal motor neurons. In concert, the findings of the present study suggest that BDNF can significantly prevent cell death in injured motor neurons. However, this neurotrophin may not be a retrograde signal associated with the induction and/or maintenance of some mature features of motor neurons, particularly their transmitter phenotype. © 1994 Wiley-Liss, Inc.     

Developmental expression of neurotrophins and their receptors in postnatal rat ventral midbrain

Neurotrophins are a group of structurally related polypeptides that support the survival, differentiation, and maintenance of neuronal populations that express the appropriate high-affinity neurotrophin receptors. Two members of the neurotrophin family, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), have been shown to increase the survival of dopaminergic neurons from the ventral midbrain in vitro. Evidence suggests that ventral midbrain neurons might be able to derive support from these trophic factors in vivo through paracrine or autocrine interactions. Both BDNF and NT-3 mRNAs and their receptor mRNAs, trkB and trkC mRNAs, respectively, have been localized to the ventral mesencephalon. However, the relative expression levels of the neurotrophins and their receptor mRNAs throughout ontogeny and in adulthood have not been elucidated. In the present study, the postnatal developmental expression of BDNF, NT-3, trkB, and trkC mRNAs was analyzed via in situ hybridization to gain insight into the possible role of these factors in vivo. We found that there was a developmental decline in the expression of BDNF and NT-3 mRNAs in the ventral mesencephalon. In contrast, no alterations in the expression of midbrain trkB or trkC mRNAs could be discerned. The present results suggest a role for BDNF and NT-3 in the earlier postnatal developmental events of responsive populations. The continued, albeit lower, expression of the neurotrophins in the ventral mesencephalon in adulthood also suggests a role for these factors in mature neuronal systems.     

Cell death of adult pyramidal CA1 neurons after intraventricular injection of a novel peptide derived from trkA

Members of the nerve growth factor (NGF) family of neurotrophins bind to the second leucine-rich motif (LRM₂) within the extracellular domains of their respective receptors (trkA, trkB, trkC). Small LRM₂ peptides have been recently demonstrated to selectively bind the neurotrophins revealing similar complex binding characteristics as full-length receptors. We extend our recent findings, showing that the peptides (A and C) do not block nerve fiber outgrowth through high affinity trk receptors in a ganglia bioassay. Since the highest concentration of neurotrophins [NGF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3)] is found in the hippocampus, the peptides were injected into the 3rd ventricle of anesthetized adult rats. The (NGF binding) LRM₂-A peptide, but not the (BDNF binding) LRM₂-B or the (NT-3 binding) LRM₂-C peptides, caused severe apoptotic neurodegeneration of hippocampal pyramidal CA1 neurons as revealed by cresyl violet staining and the TUNEL reaction. The degeneration was protected by intrahippocampal injection of NGF-beta and by the non-N-methyl-D-aspartate (NMDA) antagonist CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), indicating a glutamatergic mechanism. In situ hybridization revealed that pyramidal CA1 neurons did not express trkA and p75 receptor mRNA in sham and LRM₂-A-lesioned animals. It is concluded that the LRM₂-A peptide represents a novel peptide with properties to induce apoptotic cell death of pyramidal CA1 neurons and may be useful as an experimental agent. J. Neurosci. Res. 50:402–412, 1997. © 1997 Wiley-Liss, Inc.