Localization of nerve growth factor, trkA and P75 immunoreactivity in the hippocampal formation and basal forebrain of adult rats

Excess cerebellar granule neurons induced by the absence of p75NTR during development elicit social behavior deficits in mice

Introduction

Recently, the cerebellum has been implicated with non-motor functions, including cognitive and emotional behavior. Anatomical and functional studies demonstrate bidirectional cerebellar connections with brain regions involved in social cognition. Cerebellar developmental abnormalities and injury are often associated with several psychiatric and mental disorders including autism spectrum disorders and anxiety. The cerebellar granule neurons (CGN) are essential for cerebellar function since they provide sensorimotor, proprioceptive, and contextual information to Purkinje cells to modify behavior in different contexts. Therefore, alterations to the CGN population are likely to compromise cerebellar processing and function. Previously we demonstrated that the p75 neurotrophin receptor (p75NTR) was fundamental for the development of the CGN. In the absence of p75NTR, we observed increased proliferation of the granule cell precursors (GCPs), followed by increased GCP migration toward the internal granule layer. The excess granule cells were incorporated into the cerebellar network, inducing alterations in cerebellar circuit processing.

Methods

In the present study, we used two conditional mouse lines to specifically delete the expression of p75NTR in CGN. In both mouse lines, deletion of the target gene was under the control of the transcription factor Atoh-1 promotor, however, one of the lines was also tamoxifen-inducible.

Results

We observed a loss of p75NTR expression from the GCPs in all cerebellar lobes. Compared to control animals, both mouse lines exhibited a reduced preference for social interactions when presented with a choice to interact with a mouse or an object. Open-field locomotor behavior and operant reward learning were unaffected in both lines. Lack of preference for social novelty and increased anxiety-related behavior was present in mice with constitutive p75NTR deletion; however, these effects were not present in the tamoxifen-inducible mice with p75NTR deletion that more specifically targeted the GCPs.

Discussion

Our findings demonstrate that alterations to CGN development by loss of p75NTR alter social behavior, and contribute to the increasing evidence that the cerebellum plays a role in non-motor-related behaviors, including social behavior.

Neurotrophic Factors and Dendritic Spines

Dendritic spines are highly dynamic structures that play important roles in neuronal plasticity. The morphologies and the numbers of dendritic spines are highly variable, and this diversity is correlated with the different morphological and physiological features of this neuronal compartment. Dendritic spines can change their morphology and number rapidly, allowing them to adapt to plastic changes. Neurotrophic factors play important roles in the brain during development. However, these factors are also necessary for a variety of processes in the postnatal brain. Neurotrophic factors, especially members of the neurotrophin family and the ephrin family, are involved in the modulation of long-lasting effects induced by neuronal plasticity by acting on dendritic spines, either directly or indirectly. Thereby, the neurotrophic factors play important roles in processes attributed, for example, to learning and memory.

The effect of nerve growth factor on supporting spatial memory depends upon hippocampal cholinergic innervation

Nerve growth factor (NGF) gene therapy has been used in clinical trials of Alzheimer's disease. Understanding the underlying mechanisms of how NGF influences memory may help develop new strategies for treatment. Both NGF and the cholinergic system play important roles in learning and memory. NGF is essential for maintaining cholinergic innervation of the hippocampus, but it is unclear whether the supportive effect of NGF on learning and memory is specifically dependent upon intact hippocampal cholinergic innervation. Here we characterize the behavior and hippocampal measurements of volume, neurogenesis, long-term potentiation, and cholinergic innervation, in brain-specific Ngf-deficient mice. Our results show that knockout mice exhibit increased anxiety, impaired spatial learning and memory, decreased adult hippocampal volume, neurogenesis, short-term potentiation, and cholinergic innervation. Overexpression of Ngf in the hippocampus of Ngf gene knockout mice rescued spatial memory and partially restored cholinergic innervations, but not anxiety. Selective depletion of hippocampal cholinergic innervation resulted in impaired spatial memory. However, Ngf overexpression in the hippocampus failed to rescue spatial memory in mice with hippocampal-selective cholinergic fiber depletion. In conclusion, we demonstrate the impact of Ngf deficiency in the brain and provide evidence that the effect of NGF on spatial memory is reliant on intact cholinergic innervations in the hippocampus. These results suggest that adequate cholinergic targeting may be a critical requirement for successful use of NGF gene therapy of Alzheimer's disease.

Cholinergic profiles in the Goettingen miniature pig (Sus scrofa domesticus) brain

Central cholinergic structures within the brain of the even-toed hoofed Goettingen miniature domestic pig (Sus scrofa domesticus) were evaluated by immunohistochemical visualization of choline acetyltransferase (ChAT) and the low-affinity neurotrophin receptor, p75^NTR . ChAT-immunoreactive (-ir) perikarya were seen in the olfactory tubercle, striatum, medial septal nucleus, vertical and horizontal limbs of the diagonal band of Broca, and the nucleus basalis of Meynert, medial habenular nucleus, zona incerta, neurosecretory arcuate nucleus, cranial motor nuclei III and IV, Edinger-Westphal nucleus, parabigeminal nucleus, pedunculopontine nucleus, and laterodorsal tegmental nucleus. Cholinergic ChAT-ir neurons were also found within transitional cortical areas (insular, cingulate, and piriform cortices) and hippocampus proper. ChAT-ir fibers were seen throughout the dentate gyrus and hippocampus, in the mediodorsal, laterodorsal, anteroventral, and parateanial thalamic nuclei, the fasciculus retroflexus of Meynert, basolateral and basomedial amygdaloid nuclei, anterior pretectal and interpeduncular nuclei, as well as select laminae of the superior colliculus. Double immunofluorescence demonstrated that virtually all ChAT-ir basal forebrain neurons were also p75^NTR -positive. The present findings indicate that the central cholinergic system in the miniature pig is similar to other mammalian species. Therefore, the miniature pig may be an appropriate animal model for preclinical studies of neurodegenerative diseases where the cholinergic system is compromised. J. Comp. Neurol. 525:553-573, 2017. © 2016 Wiley Periodicals, Inc.

Estradiol modulation of neurotrophin receptor expression in female mouse basal forebrain cholinergic neurons in vivo

The neuroprotective effect of estradiol (E2) on basal forebrain cholinergic neurons (BFCNs) has been suggested to occur as a result of E2 modulation of the neurotrophin system on these neurons. The present study provides a comprehensive examination of the relationship between E2 and neurotrophin signaling on BFCNs by investigating the effect of E2 deficiency on the expression levels of neurotrophin receptors (NRs), TrkA, TrkB, and p75 on BFCNs. The number of TrkA receptor-expressing choline acetyltransferase-positive neurons was significantly reduced in the medial septum (MS) in the absence of E2. A significant reduction in TrkB-expressing choline acetyltransferase-positive cells was also observed in ovariectomized mice in the MS and nucleus basalis magnocellularis (NBM). p75 receptor expression was reduced in the NBM and striatum but not in the MS. We also showed that estrogen receptor (ER)-α was expressed by a small percentage of TrkA- and TrkB-positive neurons in the MS (12%) and NBM (19%) and by a high percentage of TrkB-positive neurons in the striatum (69%). Similarly, ERα was expressed at low levels by p75 neurons in the MS (6%) and NBM (9%) but was not expressed on striatal neurons. Finally, ERα knockout using neuron-specific estrogen receptor-α knockout transgenic mice abolished all E2-mediated changes in the NR expression on BFCNs. These results indicate that E2 differentially regulates NR expression on BFCNs, with effects depending on the NR type and neuroanatomical location, and also provide some evidence that alterations in the NR expression are, at least in part, mediated via ERα.

Brain nerve growth factor unbalance induced by anabolic androgenic steroids in rats

PURPOSE

Anabolic androgenic steroids (AAS) are synthetic androgen-like compounds that are abused in sport communities despite their adverse effects. Nerve growth factor (NGF) influences neuronal differentiation and survival, and it also mediates higher brain functions such as learning and memory. Changes in NGF expression have been implicated in neurodegenerative disorders, including Alzheimer disease. Hence, we decided to study the effect of chronic AAS exposure on brain NGF profile, NGF-dependent cholinergic function, and related behavioral performance.

METHODS

Male Wistar rats were injected for 4 wk with either nandrolone or stanozolol at daily doses (5.0 mg·kg(-1), s.c.) that are considered equivalent to those abused by humans. NGF levels and NGF receptor (TrkA and p75NTR) expression were measured in the hippocampus and in the basal forebrain. Choline acetyltransferase expression was evaluated in basal forebrain. Spatial learning and memory were assessed using the Morris water maze.

RESULTS

AAS treatment caused region-specific changes in the expression of NGF and its receptors. Both nandrolone and stanozolol increased NGF levels in the hippocampus and reduced NGF levels in the basal forebrain, reduced p75NTR expression in the hippocampus, and failed to affect TrkA expression in the basal forebrain. Finally, AAS treatment reduced the expression of choline acetyltransferase in the basal forebrain and impaired the behavioral performance in the Morris water maze.

CONCLUSION

The evidence that supraphysiological doses of AAS cause neurotrophic unbalance and related behavioral disturbances raises the concern that AAS abuse in humans may affect mechanisms that lie at the core of neuronal plasticity.

Sustained expression of brain-derived neurotrophic factor is required for maintenance of dendritic spines and normal behavior

Brain-derived neurotrophic factor (BDNF) plays important roles in the development, maintenance, and plasticity of the mammalian forebrain. These functions include regulation of neuronal maturation and survival, axonal and dendritic arborization, synaptic efficacy, and modulation of complex behaviors including depression and spatial learning. Although analysis of mutant mice has helped establish essential developmental functions for BDNF, its requirement in the adult is less well documented. We have studied late-onset forebrain-specific BDNF knockout (CaMK-BDNF(KO)) mice, in which BDNF is lost primarily from the cortex and hippocampus in early adulthood, well after BDNF expression has begun in these structures. We found that although CaMK-BDNF(KO) mice grew at a normal rate and can survive more than a year, they had smaller brains than wild-type siblings. The CaMK-BDNF(KO) mice had generally normal behavior in tests for ataxia and anxiety, but displayed reduced spatial learning ability in the Morris water task and increased depression in the Porsolt swim test. These behavioral deficits were very similar to those we previously described in an early-onset forebrain-specific BDNF knockout. To identify an anatomical correlate of the abnormal behavior, we quantified dendritic spines in cortical neurons. The spine density of CaMK-BDNF(KO) mice was normal at P35, but by P84, there was a 30% reduction in spine density. The strong similarities we find between early- and late-onset BDNF knockouts suggest that BDNF signaling is required continuously in the CNS for the maintenance of some forebrain circuitry also affected by developmental BDNF depletion.

Neuroprotection signaling pathway of nerve growth factor and brain-derived neurotrophic factor against staurosporine induced apoptosis in hippocampal H19-7/IGF-IR [corrected]

Neurotrophins protect neurons against excitotoxicity; however the signaling mechanisms for this protection remain to be fully elucidated. Here we report that activation of the phosphatidyl inositol 3 kinase (PI3K)/Akt pathway is critical for protection of hippocampal cells from staurosporine (STS) induced apoptosis, characterized by nuclear condensation and activation of the caspase cascade. Both nerve growth factor (NGF) and brain-derived growth factor (BDNF) prevent STS-induced apoptotic morphology and caspase-3 activity by upregulating phosphorylation of the tropomyosin receptor kinase (Trk) receptor. Inhibition of Trk receptor by K252a altered the neuroprotective effect of both NGF and BDNF whereas inhibition of the p75 neurotrophin receptor (p75NTR) had no effect. Impairment of the PI3K/Akt pathway or overexpression of dominant negative (DN)-Akt abolished the protective effect of both neurotrophins, while active Akt prevented cell death. Moreover, knockdown of Akt by si-RNA was able to block the survival effect of both NGF and BDNF. Thus, the survival action of NGF and BDNF against STS-induced neurotoxicity was mediated by the activation of PI3K/Akt signaling through the Trk receptor.

Oligodendrocyte-myelin glycoprotein and Nogo negatively regulate activity-dependent synaptic plasticity

In the adult mammalian CNS, the growth inhibitors oligodendrocyte-myelin glycoprotein (OMgp) and the reticulon RTN4 (Nogo) are broadly expressed in oligodendrocytes and neurons. Nogo and OMgp complex with the neuronal cell surface receptors Nogo receptor-1 (NgR1) and paired Ig-like receptor-B (PirB) to regulate neuronal morphology. In the healthy CNS, NgR1 regulates dendritic spine shape and attenuates activity-driven synaptic plasticity at Schaffer collateral-CA1 synapses. Here, we examine whether Nogo and OMgp influence functional synaptic plasticity, the efficacy by which synaptic transmission occurs. In acute hippocampal slices of adult mice, Nogo-66 and OMgp suppress NMDA receptor-dependent long-term potentiation (LTP) when locally applied to Schaffer collateral-CA1 synapses. Neither Nogo-66 nor OMgp influences basal synaptic transmission or paired-pulse facilitation, a form of short-term synaptic plasticity. PirB(-/-) and NgR1(-/-) single mutants and NgR1(-/-);PirB(-/-) double mutants show normal LTP, indistinguishable from wild-type controls. In juvenile mice, LTD in NgR1(-/-), but not PirB(-/-), slices is absent. Mechanistic studies revealed that Nogo-66 and OMgp suppress LTP in an NgR1-dependent manner. OMgp inhibits LTP in part through PirB but independently of p75. This suggests that NgR1 and PirB participate in ligand-dependent inhibition of synaptic plasticity. Loss of NgR1 leads to increased phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), signaling intermediates known to regulate neuronal growth and synaptic function. In primary cortical neurons, BDNF elicited phosphorylation of AKT and p70S6 kinase is attenuated in the presence of myelin inhibitors. Collectively, we provide evidence that mechanisms of neuronal growth inhibition and inhibition of synaptic strength are related. Thus, myelin inhibitors and their receptors may coordinate structural and functional neuronal plasticity in CNS health and disease.

The p75 neurotrophin receptor is expressed by adult mouse dentate progenitor cells and regulates neuronal and non-neuronal cell genesis

BACKGROUND

The ability to regulate neurogenesis in the adult dentate gyrus will require further identification and characterization of the receptors regulating this process. In vitro and in vivo studies have demonstrated that neurotrophins and the p75 neurotrophin receptor (p75NTR) can promote neurogenesis; therefore we tested the hypothesis that p75NTR is expressed by adult dentate gyrus progenitor cells and is required for their proliferation and differentiation.

RESULTS

In a first series of studies focusing on proliferation, mice received a single BrdU injection and were sacrificed 2, 10 and 48 hours later. Proliferating, BrdU-positive cells were found to express p75NTR. In a second series of studies, BrdU was administered by six daily injections and mice were sacrificed 1 day later. Dentate gyrus sections demonstrated a large proportion of BrdU/p75NTR co-expressing cells expressing either the NeuN neuronal or GFAP glial marker, indicating that p75NTR expression persists at least until early stages of maturation. In p75NTR (-/-) mice, there was a 59% decrease in the number of BrdU-positive cells, with decreases in the number of BrdU cells co-labeled with NeuN, GFAP or neither marker of 35%, 60% and 64%, respectively.

CONCLUSIONS

These findings demonstrate that p75NTR is expressed by adult dentate progenitor cells and point to p75NTR as an important receptor promoting the proliferation and/or early maturation of not only neural, but also glial and other cell types.

Nuclear localization of orphan receptor protein kinase (Ror1) is mediated through the juxtamembrane domain

Background

Several receptor tyrosine kinases (RTKs) such as EGFR, FGFR, TRK, and VEGFR are capable of localizing in the cell nucleus in addition to their usual plasma membrane localization. Recent reports also demonstrate that nuclear-localized RTKs have important cellular functions such as transcriptional activation. On the basis of preliminary bioinformatic analysis, additional RTKs, including receptor tyrosine kinase-like orphan receptor 1 (Ror1) were predicted to have the potential for nuclear subcellular localization. Ror1 is a receptor protein tyrosine kinase that modulates neurite growth in the central nervous system. Because the nuclear localization capability of the Ror1 cytoplasmic domain has not been reported, we examined the cellular expression distribution of this region.

Results

The Ror1 cytoplasmic region was amplified and cloned into reporter constructs with fluorescent tags. Following transfection, the nuclear distribution patterns of transiently expressed fusion proteins were observed. Serial deletion constructs were then used to map the juxtamembrane domain of Ror1 (aa_471-513) for this nuclear translocation activity. Further site-directed mutagenesis suggested that a KxxK-16 aa-KxxK sequence at residues 486-509 is responsible for the nuclear translocation interaction. Subsequent immunofluorescence analysis by cotransfection of Ran and Ror1 implied that the nuclear translocation event of Ror1 might be mediated through the Ran pathway.

Conclusions

We have predicted several RTKs that contain the nuclear localization signals. This is the first report to suggest that the juxtamembrane domain of the Ror1 cytoplasmic region mediates the translocation event. Ran GTPase is also implicated in this event. Our study might be beneficial in future research to understand the Ror1 biological signaling pathway.

Expression of nerve growth factor immunoreactivity and messenger RNA in ischemic urinary bladder

AIMS

The bladder contractile dysfunction resulting from acute ischemia may be attributed to nerve growth factor (NGF) overexpression. This study was conducted to evaluate the acute and mid-term effects of bladder ischemia on the temporal expression of NGF immunoreactivity and mRNA.

MATERIALS AND METHODS

Bladder ischemia was induced by ligation of bilateral vesical arteries in female rats. We examined the NGF content of bladder detrusor muscle at 1 day, 1 week and 4 weeks after artery ligation. Immunoreactivity of NGF was studied by immunofluorescent staining and Western blot. The NGF mRNA was analyzed by real-time polymerase chain reaction.

RESULTS

The immunofluorescence of NGF at 1 week and 4 weeks was significantly reduced when compared to sham-operated group (P < 0.05). This decreased tendency was also found in Western blot test. An increased expression of NGF mRNA was noted at 1 day, 1 week and 4 weeks, but had no significant change when compared to sham-operated group (P > 0.05).

CONCLUSIONS

Our study showed bilateral vesical artery ligation may cause damage of detrusor muscle and there is decreased NGF immunofluorescence and elevated NGF mRNA in bladder suggesting an expression disparity following ischemia.

An agonistic mAb directed to the TrkC receptor juxtamembrane region defines a trophic hot spot and interactions with p75 coreceptors

The D5 domain of TrkC receptors is a docking site for Neurotrophin-3 (NT-3), but other domains may be relevant for function or harmonizing signals with p75(NTR) coreceptors. We report a monoclonal antibody (mAb) 2B7 targeting the juxtamembrane domain of TrkC. mAb 2B7 binds to murine and human TrkC receptors and is a functional agonist that affords activation of TrkC, AKT, and MAPK. These signals result in cell survival but not in cellular differentiation. Monomeric 2B7 Fabs also affords cell survival. Binding of 2B7 mAb and 2B7 Fabs to TrkC are blocked by NT-3 in a dose-dependent manner but not by pro-NT-3. Expression of p75(NTR) coreceptors on the cell surface block the binding and function of mAb 2B7, whereas NT-3 binding and function are enhanced. mAb 2B7 defines a previously unknown neurotrophin receptor functional hot spot; that exclusively generates survival signals; that can be activated by non-dimeric ligands; and potentially unmasks a site for p75-TrkC interactions.

Amitriptyline is a TrkA and TrkB receptor agonist that promotes TrkA/TrkB heterodimerization and has potent neurotrophic activity

Neurotrophins, the cognate ligands for the Trk receptors, are homodimers and induce Trk dimerization through a symmetric bivalent mechanism. We report here that amitriptyline, an antidepressant drug, directly binds TrkA and TrkB and triggers their dimerization and activation. Amitriptyline, but not any other tricyclic or selective serotonin reuptake inhibitor antidepressants, promotes TrkA autophosphorylation in primary neurons and induces neurite outgrowth in PC12 cells. Amitriptyline binds the extracellular domain of both TrkA and TrkB and promotes TrkA-TrkB receptor heterodimerization. Truncation of amitriptyline binding motif on TrkA abrogates the receptor dimerization by amitriptyline. Administration of amitriptyline to mice activates both receptors and significantly reduces kainic acid-triggered neuronal cell death. Inhibition of TrkA, but not TrkB, abolishes amitriptyline's neuroprotective effect without impairing its antidepressant activity. Thus, amitriptyline acts as a TrkA and TrkB agonist and possesses marked neurotrophic activity.

Protein tyrosine phosphatase receptor type Z dephosphorylates TrkA receptors and attenuates NGF-dependent neurite outgrowth of PC12 cells

Protein tyrosine phosphatase receptor type Z (Ptprz/Ptpzeta/RPTPbeta) is a receptor-like protein tyrosine phosphatase (RPTP) which is predominantly expressed in the central nervous system. Tropomyosin-related kinases (Trks) are single-pass transmembrane molecules that are highly expressed in the developing nervous system. Upon the ligand binding of neurotrophins, Trk receptors are activated through autophosphorylation of tyrosine residues; however, the PTPs responsible for the negative regulation of Trk receptors have not been fully elucidated. Here, we identified Ptprz as a specific PTP that efficiently dephosphorylates TrkA as a substrate. Co-expression of Ptprz with Trk receptors in 293T cells showed that Ptprz suppresses the ligand-independent tyrosine phosphorylation of TrkA, but not of TrkB or TrkC, and that Ptprz attenuates TrkA activation induced by nerve growth factor (NGF). Co-expression analyses with TrkA mutants revealed that Ptprz dephosphorylates phosphotyrosine residues in the activation loop of the kinase domain, which are requisite for activation of the TrkA receptor. Consistent with these findings, forced expression of Ptprz in PC12D cells markedly inhibited neurite extension induced by a low dose of NGF. In addition, an increment in the tyrosine phosphorylation of TrkA was observed in the brain of Ptprz-deficient mice. Ptprz thus appears to be one of the PTPs which regulate the activation and signalling of TrkA receptors.

Neural cell death is induced by neutralizing antibody to nerve growth factor: an in vivo study

The central nervous system (CNS) of vertebrates originates from neuroepithelial cells located within the embryonic neural tube. Coincidental with the processes of proliferation, migration and differentiation in the developing CNS, cell death is also a major phenomenon during normal development. The investigation of neural cell death in development has focused on the role of target-derived survival factors such as nerve growth factor (NGF). In this study, the effects of anti-NGF antibody on neural cell death in the cerebral cortex have been investigated. Injection of anti-NGF antibody into the cisterna magnum of mouse pups increased the number of neural cell deaths and resulted in thinning of the cerebral cortex compared with a control group. It is concluded that endogenous NGF is essential for cortical cell survival in the cerebral cortex of the newborn mouse. Moreover, this method may be applied to the other factors and different CNS regions, allowing identification of molecules and signals involved in neural cell survival.

Eukaryotic translation initiation factor 4E (eIF4E) expression in the brain tissue is induced by infusion of nerve growth factor into the mouse cisterna magnum: an in vivo study

In many cell types translation can be regulated by an expression of the translation initiation factor. Eukaryotic translation initiation factor eIF4E, which binds to the 5' cap structure of mRNA, plays an important role in translation regulation and it has been suggested that it is implicated in increased protein synthesis promoted by growth factors. In this study the effects of nerve growth factor (NGF) infusion into the cerebrospinal fluid (CSF) on eIF4E expression and phosphorylation in mouse brain tissue have been investigated. We investigated NGF as it is one of the most important growth factors and it is an important factor in cerebral cortical development, stimulating neuronal precursor proliferation. eIF4E level is also increased in response to infusion of NGF into the CSF. The present study shows that eIF4E is phosphorylated in the brain tissues treated with NGF. It is concluded that NGF regulates protein synthesis in the nervous tissue by enhancing expression and phosphorylation of eIF4E.

On the presence of neurotrophin p75 receptor on rat sympathetic cerebrovascular nerves

Although the presence of neurotrophin p75 receptor on sympathetic nerves is a well-recognised feature, there is still a scarcity of details of the distribution of the receptor on cerebrovascular nerves. This study examined the distribution of p75 receptor on perivascular sympathetic nerves of the middle cerebral artery and the basilar artery of healthy young rats using immunohistochemical methods at the laser confocal microscope and transmission electron microscope levels. Immunofluorescence methods of detection of tyrosine hydroxylase (TH) in sympathetic nerves, p75 receptor associated with the nerves, and also S-100 protein in Schwann cells were applied in conjunction with confocal microscopy, while the pre-embedding single and double immunolabelling methods (ExtrAvidin and immuno-gold-silver) were applied for the electron microscopic examination. Immunofluorescence studies revealed "punctuate" distribution of the p75 receptor on sympathetic nerves including accompanying Schwann cells. Image analysis of the nerves showed that the level of co-localization of p75 receptor and TH was low. Immunolabelling applied at the electron microscope level also showed scarce co-localization of TH (which was intra-axonal) and p75. Immunoreactivity for p75 receptor was present on the cell membrane of perivascular axons and to a greater extent on the processes of accompanying Schwann cells. Some Schwann cell processes were adjacent to each other displaying strong immunoreactivity for p75 receptor; immunoreactivity was located on the extracellular sites of the adjacent cell membranes suggesting that the receptor was involved in cross talk between these. It is likely that variability of locations of p75 receptor detected in the study reflects diverse interactions of p75 receptor with axons and Schwann cells. It might also imply a diverse role for the receptor and/or the plasticity of sympathetic cerebrovascular nerves to neurotrophin signalling.

Antagonistic effects of TrkB and p75NTRon NMDA receptor currents in post-synaptic densities transplanted into Xenopus oocytes

Brain-derived neurotrophic factor (BDNF) and its receptor TrkB are essential regulators of synaptic function in the adult CNS. A TrkB-mediated effect at excitatory synapses is enhancement of NMDA receptor (NMDA-R)-mediated currents. Recently, opposing effects of TrkB and the pan-neurotrophin receptor p75(NTR) on long-term synaptic depression and long-term potentiation have been reported in the hippocampus. To further study the regulation of NMDA-Rs by neurotrophin receptors in their native protein environment, we micro-transplanted rat forebrain post-synaptic densities (PSDs) into Xenopus oocytes. One-minute incubations of oocytes with BDNF led to dual effects on NMDA-R currents: either TrkB-dependent potentiation or TrkB-independent inhibition were observed. Pro-nerve growth factor, a ligand for p75(NTR) but not for TrkB, produced a reversible, dose-dependent, TrkB-independent and p75(NTR)-dependent inhibition of NMDA-Rs. Fractionation experiments showed that p75(NTR) is highly enriched in the PSD protein fraction. Immunoprecipitation and pull-down experiments further revealed that p75(NTR) is a core component of the PSD, where it interacts with the PDZ3 domain of the scaffolding protein SAP90/PSD-95. Our data provide striking evidence for a rapid inhibitory effect of p75(NTR) on NMDA-R currents that antagonizes TrkB-mediated NMDA-R potentiation. These opposing mechanisms might be present in a large proportion of forebrain synapses and may contribute importantly to synaptic plasticity.

Beta-amyloid toxicity in embryonic rat astrocytes

The senile plaques of Alzheimer's disease contain a high concentration of beta-amyloid (betaA) protein, which may affect the glial population in the septal nucleus, an area of increased risk in AD. BetaA toxicity was measured in septal glia, via a dose-response experiment, by quantifying the effects of three different doses (0.1, 1, and 10 microM) of betaA on cell survival. Astrocytes from embryonic day-16 rats were grown in serum-free media in a single layer culture. Cells were treated on day in vitro (DIV)1 and survival was determined on DIV3 to ascertain which concentration was most toxic. In a separate set of experiments, an attempt was made to protect glial cells from the degenerative effects of betaA, with treatments of growth factors and estrogen. BetaA (10 microM) treatment was administered on DIV1, on DIV2 the cells were treated with estrogen (EST, 10 nM), insulin-like growth factors (IGF1 and IGF2, each 10 ng/ml), basic fibroblast growth factor (bFGF, 5 ng/ml) or nerve growth factor (NGF, 100 ng/ml), and on DIV3 the cells were visualized and quantified by fluorescence microscopy with DAPI (4,6-diamidino-2-phenylindole). In addition to dose-response and glial protection, experiments were also conducted to determine whether toxic effects were due to apoptosis. Our results suggest that the survival of glial populations is significantly affected in all three concentrations (0.1, 1.0, and 10 microM) of betaA. Glial protection was evident in the presence of NGF, for it showed the significantly highest survival rate relative to the betaA treatment alone. Furthermore, toxic effects of betaA appear to be due primarily to apoptosis. Significant reversal of betaA-induced apoptosis was seen with bFGF and IGF1.

Hypertension downregulates the expression of brain-derived neurotrophic factor in the ischemia-vulnerable hippocampal CA1 and cortical areas after carotid artery occlusion

We studied the effect of hypertension on brain damage and brain-derived neurotrophic factor (BDNF) expression in the hippocampal formation and cerebral cortex after permanent occlusion of bilateral common carotid arteries (CCA). Two groups of rats were used, including normotensive Wistar-Kyoto (WKY) rat and spontaneous hypertensive rat (SHR). Each group contained sham operation, 1 week and 4 weeks after bilateral CCA occlusion (n=5-10 in each time point). The blood pressure showed a significant elevation in WKY rats from 1 h after operation to 4 weeks before sacrifice (P<0.05), but was not changed in SHR (P>0.05). However, rectal temperature showed no significant change after operation in WKY rat and SHR (P>0.05) and showed no significant difference at any time point between WKY rat and SHR (P>0.05). Hematoxylin and eosin staining showed SHR had a significantly larger necrotic volume than WKY rats (n=10 in each group, 6044+/-6895 microm(3) vs. 144+/-174 microm(3), P<0.05) at 4 weeks after ischemia. In SHR, BDNF immunoreactivity and mRNA decreased significantly from 1 week to 4 weeks in both the hippocampal CA1 and cortical areas (P<0.01) but decreased transiently in dentate gyrus. However, in WKY rats, BDNF immunoreactivity and mRNA decreased transiently at 1 week (P<0.05) and recovered at 4 weeks after cerebral ischemia. Our study demonstrates that after bilateral CCA occlusion, preexisting hypertension may aggravate the brain injury and downregulate the expression of BDNF immunoreactivity and mRNA in the ischemia-vulnerable hippocampal CA1 and cortical areas but not in ischemia-resistant dentate gyrus.

Immunohistochemical distribution of NGF, BDNF, NT-3, and NT-4 in adult rhesus monkey brains

Immunohistochemical distribution and cellular localization of neurotrophins was investigated in adult monkey brains using antisera against nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Western blot analysis showed that each antibody specifically recognized appropriate bands of approximately 14.7 kDa, 14.2 kDa, 13.6 kDa, and 14.5 kDa, for NGF, BDNF, NT-3, and NT-4, respectively. These positions coincided with the molecular masses of the neurotrophins studied. Furthermore, sections exposed to primary antiserum preadsorbed with full-length NGF, BDNF, NT-3, and NT-4 exhibited no detectable immunoreactivity, demonstrating specificities of the antibodies against the tissues prepared from rhesus monkeys. The study provided a systematic report on the distribution of NGF, BDNF, NT-3, and NT-4 in the monkey brain. Varying intensity of immunostaining was observed in the somata and processes of a wide variety of neurons and glial cells in the cerebrum, cerebellum, hippocampus, and other regions of the brain. Neurons in some regions such as the cerebral cortex and the hippocampus, which stained for neurotrophins, also expressed neurotrophic factor mRNA. In some other brain regions, there was discrepancy of protein distribution and mRNA expression reported previously, indicating a retrograde or anterograde action mode of neurotrophins. Results of this study provide a morphological basis for the elucidation of the roles of NGF, BDNF, NT-3, and NT-4 in adult primate brains.

Effects of a 14-day period of hindpaw sensory restriction on mRNA and protein levels of NGF and BDNF in the hindpaw primary somatosensory cortex

Neurotrophins have been reported to play an important role in neuronal plasticity and to be regulated by neuronal activity and/or neurotransmitters. Recently, we have shown that hindpaw sensory restriction induces a cortical reorganisation in the hindpaw primary somatosensory cortex, and that acetylcholine plays a significant role in this process. Sensory restriction was obtained by hindlimb suspension for 14 days. In this study, we examined the effects of a long period of hindpaw sensory restriction on the NGF and BDNF mRNA and protein expressions in the hindpaw somatosensory cortex. mRNA and protein levels were assessed by RT-PCR and ELISA, respectively. First, we found that NGF and BDNF mRNA relative levels increased after hindpaw sensory restriction. Second, the level of NGF protein increased, whereas that of BDNF remained unchanged. This differential response of NGF and BDNF proteins to sensory restriction suggested different levels of gene regulation, i.e., at pretranslational or posttranslational states. Moreover, inasmuch as our results differ from other models of sensory restriction (dark rearing, whisker removal, etc.), we hypothesized that the regulation of neurotrophin expression is dependent on the type and duration of the sensory restriction. In conclusion, we argue that neuronal plasticity induced by hindpaw sensory restriction requires neurotrophin expression.

Co-expression of the P75 neurotrophin receptor and neurotrophin receptor-interacting melanoma antigen homolog in the mature rat brain

The p75 neurotrophin receptor (p75(NTR)) is involved in the regulation of neuronal survival and phenotype, but its signal transduction mechanisms are poorly understood. Recent evidence has implicated the cytoplasmic protein NRAGE (neurotrophin receptor-interacting MAGE (from Melanoma AntiGEn) homolog) in p75(NTR) signaling. To gain further insight into the role of NRAGE, we investigated the co-expression of NRAGE and p75(NTR) in mature rat brain. In all areas examined, NRAGE appeared to be confined to neurons. In the basal forebrain cholinergic complex, NRAGE immunoreactivity was evident in all p75(NTR)-positive neurons. There were many more NRAGE-positive than p75(NTR)-positive neurons in these regions, however. NRAGE was also expressed in areas of the basal forebrain that did not express p75(NTR), including the lateral septal nucleus and the nucleus accumbens. A finding in marked contrast to previous studies was the presence of p75(NTR) immunoreactivity in neuronal cell bodies in the hippocampus. Hippocampal p75(NTR) immunoreactivity was apparent in rats 6 months and older, and was localized to the dentate gyrus and stratum oriens. All p75(NTR)-positive neurons in the dentate gyrus and hippocampal formation were positive for NRAGE. The majority of granular cells of the dentate gyrus and pyramidal cells in the hippocampal formation were positive for NRAGE and negative for p75(NTR). NRAGE was also present in some neuronal populations that express p75(NTR) after injury, including striatal cholinergic interneurons, and motor neurons. A region of marked disparity was the cerebral cortex, in which NRAGE immunoreactivity was widespread whereas p75(NTR) was absent. The results are consistent with an important role for NRAGE in p75(NTR) signaling, as all cells that expressed p75(NTR) also expressed NRAGE. The wider distribution of NRAGE expression suggests that NRAGE may also participate in other signaling processes.

Repeated episodic exposure to ethanol affects neurotrophin content in the forebrain of the mature rat

Chronic exposure to ethanol can cause deficits in learning and memory. It has been suggested that withdrawal is potentially more damaging than the ethanol exposure per se. Therefore, we explored the effect of repeated episodic exposure to ethanol on key regulators of cortical activity, the neurotrophins. Rats were exposed to ethanol via a liquid diet for 3 days per week for 6-24 weeks. Control rats were pair-fed an isocaloric liquid diet or ad libitum fed chow and water. The concentrations of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) were determined using enzyme-linked immunosorbant assays (ELISAs). Five telencephalic structures were examined: parietal cortex, entorhinal cortex, hippocampus, the basal nucleus, and the septal nuclei. All five areas expressed each of the three neurotrophins; BDNF was most abundant and NGF the least. The parietal cortex was susceptible to ethanol exposure, NGF and BDNF content increased, and NT-3 content fell, whereas no changes were detectable in the entorhinal cortex. In the hippocampus, the amount all three neurotrophins increased following episodic ethanol exposure. Neurotrophin content in the two segments of the basal forebrain was affected; NGF and NT-3 content in the basal forebrain was reduced and NGF and BDNF content in the septal nuclei was increased by ethanol exposure. In many cases where ethanol had an effect, the change was transient so that by 24 weeks of episodic exposure, no significant changes were evident. Thus, the effects of ethanol are site- and time-dependent. This pattern differs from changes caused by chronic ethanol exposure, hence, neurotrophins must be vulnerable to the effects of withdrawal. Furthermore, the ethanol-induced changes do not appear to fit a model consistent with retrograde regulation, rather they suggest that neurotrophins act through autocrine/paracrine systems.

Ultrastructural evidence for mu-opioid modulation of cholinergic pathways in rat dentate gyrus

Within the rat hippocampal formation, cholinergic afferents and mu-opioid receptors (MORs) are involved in many crucial learning processes, including those associated with drug reward. Pharmacological data, and the overlapping distributions of cholinergic and mu-opioid systems, particularly in the dentate gyrus, suggest that MOR activation is a potential mechanism for endogenous opioid modulation of cholinergic activity. To date, anatomical evidence supporting this has not been reported. To delineate the relationship between cholinergic afferents and MOR-containing processes in the dentate gyrus, hippocampal sections were dually immunolabeled for vesicular acetylcholine transporter (VAChT) and MOR-1 and examined by electron microscopy. VAChT immunoreactivity was in unmyelinated axons and axon terminals, and was most often associated with small synaptic vesicles. MOR immunoreactivity was found in axons, axon terminals and, to a lesser extent, perikarya, which resembled GABAergic basket cells. Semi-quantitative ultrastructural analysis revealed that from 5% to 13% (depending on laminar location) of VAChT-immunoreactive (ir) presynaptic profiles contained MOR immunoreactivity. Additionally, 7% of VAChT-ir presynaptic profiles directly apposed MOR-ir axons and terminals, and there were almost no appositions to MOR-ir dendrites. These data suggest that opioids may directly and indirectly modulate acetylcholine release and/or reuptake. In the hilus and molecular layer, 4% of VAChT-ir terminals contacted dendritic shafts that were also contacted by MOR-ir terminals. This suggests that cholinergic afferents and MOR-containing afferents can converge on granule cell dendrites (which are restricted to the molecular layer) and on interneuron dendrites in the hilus. The results of this study provide ultrastructural evidence for direct and indirect modulation of cholinergic systems by mu-opioids in the hippocampal formation.

Myelin-, reactive glia-, and scar-derived CNS axon growth inhibitors: expression, receptor signaling, and correlation with axon regeneration

Axon regeneration is arrested in the injured central nervous system (CNS) by axon growth-inhibitory ligands expressed in oligodendrocytes/myelin, NG2-glia, and reactive astrocytes in the lesion and degenerating tracts, and by fibroblasts in scar tissue. Growth cone receptors (Rc) bind inhibitory ligands, activating a Rho-family GTPase intracellular signaling pathway that disrupts the actin cytoskeleton inducing growth cone collapse/repulsion. The known inhibitory ligands include the chondroitin sulfate proteoglycans (CSPG) Neurocan, Brevican, Phosphacan, Tenascin, and NG2, as either membrane-bound or secreted molecules; Ephrins expressed on astrocyte/fibroblast membranes; the myelin/oligodendrocyte-derived growth inhibitors Nogo, MAG, and OMgp; and membrane-bound semaphorins (Sema) produced by meningeal fibroblasts invading the scar. No definitive CSPG Rc have been identified, although intracellular signaling through the Rho family of G-proteins is probably common to all the inhibitory ligands. Ephrins bind to signalling Ephs. The ligand-binding Rc for all the myelin inhibitors is NgR and requires p75(NTR) for transmembrane signaling. The neuropilin (NP)/plexin (Plex) Rc complex binds Sema. Strategies for promoting axon growth after CNS injury are thwarted by the plethora of inhibitory ligands and the ligand promiscuity of some of their Rc. There is also paradoxical reciprocal expression of many of the inhibitory ligands/Rc in normal and damaged neurons, and NgR expression is restricted to a limited number of neuronal populations. All these factors, together with an incomplete understanding of the normal functions of many of these molecules in the intact CNS, presently confound interpretive acumen in regenerative studies.

Pentyl-4-yn-valproic acid reverses age-associated memory impairment in the Wistar rat

Pentyl-4-yn-valproic acid (VPA), a cognition-enhancing agent whose mode of action has been attributed to cell adhesion molecule-mediated neuritogenesis, has been shown to enhance hippocampus-dependent spatial learning. Here, we investigated its potential to reverse age-related memory impairment that relates mainly to declarative memory. Aged spatial learning deficits in the water maze paradigm were demonstrated by swim angle analysis, the angle between axes of start-to-platform and start-to-animal position, and latency to reach a submerged platform. Chronic pentyl-4-yn-VPA administration mediated a significant improvement in both search strategy and latency to find the submerged platform in aged animals. Pentyl-4-yn-VPA also facilitated task recall in aged animals as evidenced by increased time in the target quadrant during a probe trial 3 days following the final training session. The action of pentyl-4-yn-VPA on platform latency, search strategy and task recall suggests that this agent may have great benefit in the treatment of age-dependent cognitive decline.

Chronic exposure to ethanol alters neurotrophin content in the basal forebrain-cortex system in the mature rat: Effects on autocrine-paracrine mechanisms

Neurotrophins are broadly expressed in the mammalian forebrain: notably in cerebral cortex and the basal forebrain (e.g., the septal and basal nuclei). These factors promote neuronal survival and plasticity, and have been implicated as key players in learning and memory. Chronic exposure to ethanol causes learning and memory deficits. We tested the hypothesis that ethanol affects neurotrophin expression and predicted that these changes would be consistent with alterations in retrograde or autocrine/paracrine systems. Mature rats were fed a liquid diet containing ethanol daily for 8 or 24 weeks. Weight-matched controls were pair-fed an isocaloric, isonutritive diet. Proteins from five structures (parietal and entorhinal cortices, hippocampus, and the basal and septal nuclei) were studied. ELISAs were used to determine the concentration of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3). All three neurotrophins were detected in each structure examined. Ethanol treatment significantly (p < 0.05) affected neurotrophin expression in time- and space-dependent manners. NGF content was generally depressed by ethanol exposure, whereas NT-3 content increased. BDNF concentration was differentially affected by ethanol: it increased in the parietal cortex and the basal forebrain and decreased in the hippocampus. With the exception of NGF in the septohippocampal system, the ethanol-induced changes in connected structures were inconsistent with changes that would be predicted from a retrograde model. Thus, the present data (a) support the concept that neurotrophins act through a nonretrograde system (i.e., a local autocrine/paracrine system), and (b) that chronic exposure to ethanol disrupts these regulatory mechanisms.

Differential enantioselective effects of pentyl-4-yn-valproate on spatial learning in the rat, and neurite outgrowth and cyclin D3 expression in vitro

Previously, we demonstrated the racemic form of the valproate (VPA) analogue, 2-n-pentyl-4-pentynoic acid ([+/-]pentyl-4-yn-VPA), to be neuritogenic in vitro and to enhance cognition in vivo. To determine the enantioselectivity of these effects, the racemate and purified enantiomers of [+/-]pentyl-4-yn-VPA (84 mg/kg, i.p.) were administered to rodents 20 min prior to multi-session water maze training. The racemate and R-enantiomer significantly reduced escape latencies during water maze learning and enhanced its recall in a probe trial 3 days later. In contrast, S-pentyl-4-yn-VPA did not influence these behavioural parameters. The enantiomer-specific effects of [+/-]pentyl-4-yn-VPA were further discriminated in vitro using neuro 2A neuroblastoma and C6 glioma cell lines. In neuro 2A, the S-enantiomer induced profound neurite outgrowth at concentrations up to 0.5 mm, with the R-enantiomer and racemate being less neuritogenic. Immunoblot analysis of cyclin D3 expression in C6 glioma indicated the racemate and S-pentyl-4-yn-VPA to induce dose-dependent up-regulation of this protein, similar to that associated with G1-phase cell cycle arrest mediated by VPA, whereas R-pentyl-4-yn-VPA was without effect. These results indicate that the cognition-enhancing effects of pentyl-4-yn-VPA are due to the actions of the R-enantiomer, and that cyclin D3 up-regulation and associated anti-proliferative and pro-differentiative actions are predominantly associated with the S-enantiomer.

Subcellular relationships between cholinergic terminals and estrogen receptor-alpha in the dorsal hippocampus

Cholinergic septohippocampal neurons are affected by circulating estrogens. Previously, we found that extranuclear estrogen receptor-alpha (ERalpha) immunoreactivity in presynaptic profiles had an overlapping distribution with cholinergic afferents in the rat hippocampal formation. To determine the subcellular relationships between cholinergic presynaptic profiles and ERalpha, hippocampal sections were dually immunolabeled for vesicular acetylcholine transporter (VAChT) and ERalpha and examined by electron microscopy. Within the hippocampal formation, immunoreactivities for VAChT and ERalpha both were presynaptic, although their subcellular targeting was distinct. VAChT immunoreactivity was found exclusively within presynaptic profiles and was associated with small synaptic vesicles, which usually filled axon terminals. VAChT-labeled presynaptic profiles were most concentrated in stratum oriens of the hippocampal CA1 region and dentate inner molecular layer and hilus. In contrast, ERalpha immunoreactivity was found in clusters affiliated either with select vesicles or with the plasmalemma within preterminal axons and axon terminals. ERalpha-immunoreactive (IR) presynaptic profiles were more evenly distributed between hippocampal lamina than VAChT-IR profiles. Quantitative ultrastructural analysis revealed that VAChT-IR presynaptic profiles contained ERalpha immunoreactivity (ranging from 3% to 17%, depending on the lamina). Additionally, VAChT-IR presynaptic profiles apposed ERalpha-IR dendritic spines, presynaptic profiles, and glial profiles; many of the latter two types of profiles abutted unlabeled dendritic spines that received asymmetric (excitatory-type) synapses from unlabeled terminals. The presence of ERalpha immunoreactivity in cholinergic terminals suggests that estrogen could rapidly and directly affect the local release and/or uptake of acetylcholine. The affiliation of cholinergic terminals with excitatory terminals near ERalpha-labeled dendritic spines or glial profiles suggests that alterations in acetylcholine release could indirectly affect estrogen-modulated structural plasticity.

Nerve growth factor survival signaling in cultured hippocampal neurons is mediated through TrkA and requires the common neurotrophin receptor P75

The role of the common neurotrophin receptor p75 (p75NTR) in neuronal survival and cell death remains controversial. On the one hand, p75NTR provides a positive modulatory influence on nerve growth factor (NGF) signaling through the high affinity neurotrophin receptor TrkA, and hence increases NGF survival signaling. However, p75NTR may also signal independently of TrkA, causing cell death or cell survival, depending on the cell type and stage of development. Here we demonstrate that TrkA is expressed in primary cultures of hippocampal neurons and is activated by NGF within 10 min of exposure. In primary hippocampal cultures neuroprotection by NGF against glutamate toxicity was mediated by NF-kappaB and accompanied by an increased expression of neuroprotective NF-kappaB target genes Bcl-2 and Bcl-xl. In mouse hippocampal cells lacking p75NTR (p75NTR-/-) activation of TrkA by NGF was not detectable. Moreover, neuroprotection by NGF against glutamate toxicity was abolished in p75NTR-/- neurons, and the expression of bcl-2 and bcl-xl was markedly reduced as compared to wildtype cells. NGF increased TrkA phosphorylation in hippocampal neurons and provided protection that required phosphoinositol-3-phosphate (PI3)-kinase activity and Akt phosphorylation, whereas the mitogen-activated protein kinases (MAPK), extracellular-regulated kinases (Erk) 1/2, were not involved. P75NTR signaling independent of TrkA, such as increased neutral sphingomyelinase (NSMase) activity causing enhanced levels of ceramide, were not detected after exposure of hippocampal neurons to NGF. Interestingly, inhibition of sphingosine-kinase blocked the neuroprotective effect of NGF, suggesting that sphingosine-1-phosphate was also involved in NGF-mediated survival in our cultured hippocampal neurons. Overall, our results indicate an essential role for p75NTR in supporting NGF-triggered TrkA signaling pathways mediating neuronal survival in hippocampal neurons.

The effects of chronic ethanol consumption on neurotrophins and their receptors in the rat hippocampus and basal forebrain

Damage to the basal forebrain frequently results in deficits in learning and memory. Mnenonic dysfunction also occurs following prolonged ethanol consumption in humans and in animal models of chronic ethanol intake, accompanied by specific abnormalities in synaptic transmission between the basal forebrain and hippocampus. The integrity of at least some of the reciprocal neuronal connections between these brain regions is influenced by target-derived neurotrophic factors. We used a semiquantitative reverse transcription polymerase chain reaction technique to measure the messenger RNA for neurotrophins BDNF and NGF, and for their receptors trkB, trkA, and the low affinity receptor, p75(NTR) in the hippocampus and basal forebrain of rats after 28 weeks of alcohol consumption without malnutrition. This chronic ethanol treatment (CET) resulted in a marked and selective reduction in basal forebrain trkA mRNA. Western blotting revealed a similar reduction of basal forebrain trkA protein. CET effects on basal forebrain trkA may reflect impaired NGF signaling that could compromise septohippocampal synaptic connections, cholinergic differentiation, and emergent functional abilities dependent on these properties.

Synaptic and nuclear localization of brain-enriched guanylate kinase-associated protein

Brain-enriched guanylate kinase-associated protein (BEGAIN) interacts with postsynaptic density (PSD)-95/synapse-associated protein (SAP) 90. In immunohistochemistry and immunocytochemistry, BEGAIN was detected in nuclei and at synapses in neurons. Nuclear localization was also confirmed through subcellular fractionation. BEGAIN was localized exclusively in nuclei when expressed in epithelial cells. These findings led us to analyze the mechanism to determine the subcellular localization of BEGAIN in neurons. Green fluorescent protein (GFP)-tagged BEGAIN appeared first in nuclei and subsequently accumulated at dendrites. Approximately 75 and 90% of GFP-BEGAIN clusters were colocalized with synaptophysin and PSD-95/SAP90, respectively. GFP-protein containing only the N-terminal region also formed foci in nuclei and clusters at dendrites. The N-terminal BEGAIN was not precisely targeted to synapses, although it was partially localized at synapses, possibly through dimer formation with endogenous BEGAIN. The truncated form of PSD-95/SAP90 containing the guanylate kinase domain blocked synaptic targeting of BEGAIN but did not affect cluster formation at dendrites. NMDA receptor antagonists blocked localization of GFP-BEGAIN at synapses but did not affect recruitment to dendrites. These results suggest that BEGAIN is recruited to dendrites by the N-terminal region independently of NMDA receptor activity and that synaptic targeting of BEGAIN depends on NMDA receptor activity and may be mediated by interaction with PSD-95/SAP90.

Pentyl-4-yn-valproic acid enhances both spatial and avoidance learning, and attenuates age-related NCAM-mediated neuroplastic decline within the rat medial temporal lobe

2-N-Pentyl-4-pentynoic acid [pentyl-4-yn-valproic acid (VPA)] is an analogue of valproic acid that induces neuritogenesis and increases neural cell adhesion molecule (NCAM) prevalence in cultured neural cells. As memory consolidation involves synapse growth, aided by cell adhesion molecule function, we determined whether or not pentyl-4-yn-VPA had cognition-enhancing properties. Pentyl-4-yn-VPA (16-85 mg/kg) significantly improved water maze learning and task retention when given prior to each training session. Acute administration of pentyl-4-yn-VPA also influenced memory consolidation processes as, when given at 3 h post-passive avoidance training, the amnesia induced by scopolamine given 6 h post-training was prevented in a dose-dependent manner. Chronic administration of pentyl-4-yn-VPA (16.8 or 50.4 mg/kg) also significantly reduced escape latencies in the water maze task, 24 h following the last drug administration. This improved spatial learning was accompanied by enhanced neuroplasticity as the expression of NCAM polysialylated neurons in the infragranular zone of the dentate gyrus and in layer II of the perirhinal and piriform cortex was increased significantly following chronic drug treatment. The cognition-enhancing qualities of pentyl-4-yn-VPA, combined with its ability to attenuate the age-related loss of the NCAM polysialylation state, suggest that it may effectively slow the onset of cognitive decline.

Behavioral and biochemical effects of glutathione depletion in the rat brain

Glutathione serves the function of providing reducing equivalents for the maintenance of oxidant homeostasis, and besides it plays roles in intra- and intercellular signaling in the brain. Our purpose was to test the effects of depleting tissue glutathione by diethylmaleate (5.3 mmol/kg, intraperitoneal) on brain antioxidant metabolism, nerve growth factor levels, and cognitive performance in rats. Six hours after the treatment, glutathione level in the hippocampus dropped down to 30% of the mean value of vehicle-treated animals and glutathione peroxidase activity also declined. Twenty-four hours after the injection the values had been partially restored. Moreover, the hippocampal and cortical levels of nerve growth factor protein did not change in response to diethylmaleate treatment. Glutathione depletion did not influence the performance of animals in the step-through passive avoidance test, but impairs acquisition in the Morris water maze when given before training. However, when diethylmaleate was administered after acquisition in the same paradigm, it did not affect the retention tested at the following day. Our results suggest that glutathione status is important during acquisition, but not for retention, of spatial memory in maze tasks and they support the hypothesis of the oxidant/antioxidant equilibrium as a key piece acting in the regulation of brain function.

Hippocampal tyrosine kinase A receptors are restricted primarily to presynaptic vesicle clusters

Adult septohippocampal cholinergic neurons are dependent on trophic support for normal functioning and survival; these effects are largely mediated by the tyrosine kinase A receptor (TrkA), which binds its ligand, nerve growth factor (NGF), with high affinity. To determine the subcellular localization of TrkA within septohippocampal terminal fields, two rabbit polyclonal antisera to the extracellular domain of TrkA were localized immunocytochemically in rat dentate gyrus by light and electron microscopy. By light microscopy, TrkA immunoreactivity was found mostly in fine, varicose fibers primarily in the hilus and, to a lesser extent, in the granule cell and molecular layers. By electron microscopy, the central and infragranular regions of the hilus contained the highest densities of TrkA-immunoreactive profiles. Most TrkA-labeled profiles were axons (31% of 3,473), axon terminals (20%), and glia (38%); fewer were dendrites (6%), dendritic spines (5%), and granule cell and interneuron somata (<1%). TrkA immunolabeling in axons and axon terminals was discrete, often concentrated in patches of small synaptic vesicles that were adjacent to somatic and dendritic profiles. TrkA-labeled terminals formed both asymmetric and symmetric synapses, primarily with dendritic shafts and spines. TrkA-immunoreactive glial profiles frequently apposed terminals contacting dendritic spines. The findings that presynaptic profiles contain TrkA immunolabeling in sites of vesicle accumulation suggest that NGF binding to TrkA may influence transmitter release. The presence of TrkA immunoreactivity in somata, dendrites, and glia further suggests that cells within the dentate gyrus may take up NGF.

Absence of p75(NTR) expression reduces nerve growth factor immunolocalization in cholinergic septal neurons

Septal axons provide a cholinergic innervation to the nerve growth factor (NGF)-producing neurons of the mammalian hippocampus. These cholinergic septal afferents are capable of responding to target-derived NGF because they possess trkA and p75(NTR), the two transmembrane receptors that bind NGF and activate ligand-mediated intracellular signaling. To assess the relative importance of p75(NTR) expression for the responsiveness of cholinergic septal neurons to hippocampally derived NGF, we used three lines of mutant and/or transgenic mice: p75(-/-) mice (having two mutated alleles of the p75(NTR) gene), NGF/p75(+/+) mice (transgenic animals overexpressing NGF within central glial cells and having two normal alleles of the p75(NTR) gene), and NGF/p75(-/-) mice (NGF transgenic animals having two mutated alleles of the p75(NTR) gene). BALB/c and C57B1/6 mice (background strains for the mutant and transgenic lines of mice) were used as controls. Both lines of NGF transgenic mice possess elevated levels of NGF protein in the hippocampus and septal region, irrespective of p75(NTR) expression. BALB/c and C57Bl/6 mice display comparably lower levels of NGF protein in both tissues. Despite differing levels of NGF protein, the ratios of hippocampal to septal NGF levels are similar among BALB/c, C57B1/6, and NGF/p75(+/+) mice. Both p75(-/-) and NGF/p75(-/-) mice, on the other hand, have markedly elevated ratios of NGF protein between these two tissues. The lack of p75(NTR) expression also results in a pronounced absence of NGF immunoreactivity in cholinergic septal neurons of p75(-/-) and NGF/p75(-/-) mice. BALB/c, C57B1/6, and NGF/p75(+/+) mice, on the other hand, display NGF immunoreactivity that appears as discrete granules scattered through the cytoplasm of cholinergic septal neurons. Elevated levels of NGF in the hippocampus and septal region coincide with hypertrophy of cholinergic septal neurons of NGF/p75(+/+) mice but not of NGF/p75(-/-) mice. Levels of choline acetyltransferase (ChAT) enzyme activity are, however, elevated in the septal region and hippocampus of both NGF/p75(+/+) and NGF/p75(-/-) mice, compared with control mice. These data indicate that an absence of functional p75(NTR) expression disrupts the normal cellular immunolocalization of NGF by cholinergic septal neurons but does not affect the ability of these neurons to respond to elevated levels of NGF, as determined by ChAT activity.

p75 neurotrophin receptor expression is induced in apoptotic neurons after seizure

Seizure causes neuronal cell loss in both animal models and human epilepsy. To determine the contribution of apoptotic mechanisms to seizure-induced neuronal cell death, rat brains were examined for the occurrence of terminal deoxynucleotidyl transferase-mediated UTP nick end labeling (TUNEL)-positive nuclei after pilocarpine-induced seizure. Numerous TUNEL-positive cells were observed throughout the postseizure hippocampus, piriform cortex, and entorhinal cortex. Combined TUNEL/NeuN immunocytochemistry demonstrated that the vast majority of TUNEL-positive cells were neurons. To identify components of the signal transduction cascade promoting postseizure apoptosis, the expression of the p75 neurotrophin receptor (p75NTR) was examined. Seizure-induced increases in p75NTR protein and mRNA were detected in hippocampus, piriform cortex, and entorhinal cortex. Immunohistochemical double labeling revealed almost complete correspondence between TUNEL-positive and p75NTR-expressing cells, suggesting that seizure-induced neuronal loss within the CNS occurs through apoptotic signaling cascades involving p75NTR.

Neurotrophin-like immunoreactivity in the human hippocampal formation

The immunohistochemical occurrence and localisation of the neurotrophins nerve growth factor, brain-derived neurotrophic factor (BDNF), neurotrophin-4, and neurotrophin-3 is described in the human post-mortem hippocampal formation from subjects aged 23 weeks of gestation to 68 years. Labeled neuronal cell bodies and processes were detectable for each neurotrophin at all examined ages with age-related changes in their distribution pattern. As a general rule, a higher number of immunoreactive perikarya was found in subjects at pre- and perinatal ages than in adults. At variance with the other neurotrophins, the BDNF antiserum labelled also extensive nerve fibre systems, whose occurrence and distribution widened with age. The results obtained provide a morphological ground in support to the concept that the neurotrophins play a functional role in the human hippocampal circuitry throughout life.

Selective inhibition of kindling development by intraventricular administration of TrkB receptor body

Recent work has shown that neurotrophin gene expression is increased after seizures evoked in the kindling model of epilepsy, but whether neurotrophins regulate kindling development is as yet unclear. In this study, we attempted to block selectively the activation of distinct neurotrophin receptors throughout kindling development in the rat via chronic intracerebroventricular administration of trk receptor bodies. The efficacy and selectivity of the trk receptor bodies were established by inhibition of neurotrophin-induced trk receptor phosphorylation in pheochromocytoma (PC12) cells and primary cultures of cortical neurons. The intracerebroventricular infusion of trkB receptor body (trkB-Fc) inhibited development of kindling in comparison with that seen with saline or human IgG controls, trkA-Fc, or trkC-Fc. These results imply that activation of trkB receptors contributes to the development of kindling, a form of activity-dependent behavioral plasticity in the adult mammalian brain.

Expression of nerve growth factor and trkA after transient focal cerebral ischemia in rats

BACKGROUND AND PURPOSE

In vitro studies have shown that nerve growth factor (NGF) is protective to cortical neurons against various insults. However, the role of NGF in relation to its high-affinity trkA receptor in the cortical neurons has not been well discussed. In this experiment, we studied the possible involvement of the NGF/receptor system in the ischemic injury of cortical neurons after focal cerebral ischemia in rats.

METHODS

Male Wistar rats received right middle cerebral artery occlusion of 90 minutes' duration. The rats were decapitated at different reperfusion time points: hour 4 and days 1, 3, 7, and 14 of recirculation. Brain sections at the level of striatum were immunostained against NGF, trkA, glial fibrillary acidic protein (GFAP), and stress protein HSP70. Double immunostaining against NGF and GFAP was also performed. Optical density of NGF immunoreactivity in the ischemic and nonischemic cortexes was compared between sham-control and ischemic animals.

RESULTS

In the sham-control rats, NGF immunoreactivity was present in the cortical and striatal neurons. However, beginning at hour 4 after recirculation, there was a significant decrease of NGF in the ischemic cortex and striatum. Beginning at day 1, NGF was absent completely in the infarcted striatum and cortex. However, in the peri-infarct penumbra area, despite a decrease in NGF at hour 4 and day 1, NGF recovered beginning at day 3 and returned almost to the sham-control level at day 14. In the nonischemic cortex, NGF increased beginning at hour 4, peaked at day 7, and returned almost to the sham-control level at day 14. The trkA and HSP70 immunoreactivities were not present in the sham-control cortex. However, trkA was induced at hour 4 in the ischemic cortex and at days 1 and 3 in the peri-infarct penumbra cortex. The HSP70 was induced at days 1 and 3 in the peri-infarct penumbra area. Double immunostaining showed that the number of GFAP-positive cells increased gradually, and NGF immunoreactivity in the GFAP-positive cells became gradually intense after ischemia.

CONCLUSIONS

Our study demonstrated a temporal profile of NGF and trkA in the ischemic cortex and NGF expression by reactive astrocytes. Our data suggest that the NGF/receptor system may play a role in the astrocyte/neuron interaction under certain pathological conditions, such as focal cerebral ischemia.