Distribution of the neurotrophins brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 in the postnatal rat brain: an immunocytochemical study

Contribution of NTRK2 to the genetic susceptibility to anorexia nervosa, harm avoidance and minimum body mass index

Anorexia nervosa (AN) and bulimia nervosa (BN) are eating disorders (ED) with complex genetic and environmental components. Genetic studies and animal models support the participation of brain-derived neurotrophic factor (BDNF) in the vulnerability to AN and BN. We investigated the genetic contribution of the BDNF-specific receptor neurotrophic tyrosine kinase receptor type 2 (NTRK2) to the susceptibility to ED. We have screened the entire NTRK2 gene in 91 patients with ED and have identified 14 single-nucleotide polymorphisms (SNPs). A population-based association study with six SNPs from the NTRK2 locus was performed in 164 ED patients and 121 controls. Significant evidence of association for markers -69C>G and IVS13+40G>A was detected. We also observed a strong association between the C-A-insC haplotype (-69/IVS13+40/2784-2785) and binge-eating/purging AN (ANP, P=0.006; OR=2.27), and a reduced frequency of haplotype G-A-delCl in BN patients (P=0.034; OR=0.6). The analysis of ED-related phenotypes revealed a clear association between NTRK2, high scores of Harm avoidance measured by the temperament and character inventory (TCI-R; P=0.003) and minimum body mass index (minBMI; P<0.001). Our data support a contribution of NTRK2 to the genetic susceptibility of ED, mainly ANP, and ED-related phenotypic traits, such as Harm avoidance and minBMI.

Fetal hippocampal CA3 cell grafts enriched with FGF-2 and BDNF exhibit robust long-term survival and integration and suppress aberrant mossy fiber sprouting in the injured middle-aged hippocampus

Cell transplants that successfully replace the lost neurons and facilitate the reconstruction of the disrupted circuitry in the injured aging hippocampus are invaluable for treating acute head injury, stroke and status epilepticus in the elderly. This is because apt graft integration has the potential to prevent the progression of the acute injury into chronic epilepsy in the elderly. However, neural transplants into the injured middle-aged or aged hippocampus exhibit poor cell survival, suggesting that apt graft augmentation strategies are critical for robust integration of grafted cells into the injured aging hippocampus. We examined the efficacy of pre-treatment and grafting of donor fetal CA3 cells with a blend of fibroblast growth factor-2 (FGF-2) and brain-derived neurotrophic factor (BDNF) for lasting survival and integration of grafted cells in the injured middle-aged (12 months old) hippocampus of F344 rats. Grafts were placed at 4 days after the kainic-acid-induced hippocampal injury and were analyzed at 6 months post-grafting. We demonstrate that 80% of grafted cells exhibit prolonged survival and 71% of grafted cells differentiate into CA3 pyramidal neurons. Grafts also receive a robust afferent input from the host mossy fibers and project efferent axons into the denervated zones of the dentate gyrus and the CA1 subfield. Consequently, the aberrant sprouting of the dentate mossy fibers, an epileptogenic change that typically ensues after the hippocampal injury, was suppressed. Thus, grafts of fetal CA3 cells enriched with FGF-2 and BDNF exhibit robust integration and dampen the abnormal mossy fiber sprouting in the injured middle-aged hippocampus. Because the aberrantly sprouted mossy fibers contribute to the generation of seizures, the results suggest that the grafting intervention using FGF-2 and BDNF is efficacious for suppressing epileptogenesis in the injured middle-aged hippocampus.

Environmental enrichment in adulthood eliminates neuronal death in experimental Parkinsonism

Idiopathic Parkinson's disease (PD) affects 2% of adults over 50 years of age. PD patients demonstrate a progressive loss of dopamine neurons in the substantia nigra pars compacta (SNpc). One model that recapitulates the pathology of PD is the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Here we show that exposure to an enriched environment (EE) (a combination of exercise, social interactions and learning) or exercise alone during adulthood, totally protects against MPTP-induced Parkinsonism. Furthermore, changes in mRNA expression would suggest that increases in glia-derived neurotrophic factors, coupled with a decrease of dopamine-related transporters (e.g. dopamine transporter, DAT; vesicular monoamine transporter, VMAT2), contribute to the observed neuroprotection of dopamine neurons in the nigrostriatal system following MPTP exposure. This non-pharmacological approach presents significant implications for the prevention and/or treatment of PD.

Activity regulates positive and negative neurotrophin-derived signals to determine axon competition

Developmental axon competition plays a key role in sculpting neural circuitry. Here, we have asked how activity and neurotrophins could interact to select one axon over another. Using compartmented cultures of sympathetic neurons, we show that, in the presence of NGF, local depolarization confers a competitive growth advantage on the depolarized axon collaterals and at the same time disadvantages the growth of unstimulated axons from the same and competing neurons. Depolarization mediates the competitive advantage by activating a CaMKII-MEK pathway, which converges to enhance local NGF-mediated downstream growth signals. Patterned electrical stimulation also acts via this pathway to enhance NGF-promoted axonal growth. In contrast, the competitive disadvantage is due to BDNF secreted from and acting on the unstimulated, competing axons through p75NTR. Thus, activity regulates both positive and negative neurotrophin-derived signaling cascades to confer a competitive growth advantage on one axon versus another, thereby providing a cellular mechanism for developmental axon selection.

CNS neurotrophins are biologically active and expressed by multiple cell types

Neutrotrophins are increasingly appreciated as potential modulators of neuronal function in the adult central nervous system (CNS). To describe the neurotrophin environment within the adult CNS, mRNA and protein expression patterns of neurotrophins-3 and -4 and of brain-derived neurotrophin were investigated in adult rat spinal cord and brain. Co-localization studies with CNS cell type-specific markers demonstrates that multiple cell types, including both neurons and glia, express these neurotrophins in the normal adult CNS. Although widely implicated in important CNS functions such as synaptic plasticity, biological activity of endogenous CNS neurotrophins has not been directly demonstrated. With a sensitive neurite outgrowth bioassay we demonstrate that CNS neurotrophins elicit neurite outgrowth and are biologically active. Moreover, antibody-blocking studies suggest that these three neurotrophins may comprise the bulk of adult CNS neurotrophic activity.

Altered expression of BDNF and its high-affinity receptor TrkB in response to complex motor learning and moderate exercise

We report that rats learning complex motor skills or exercising moderately show changes in expression of brain-derived neurotrophic factor (BDNF) and its receptor, TrkB protein, in cerebellum and motor cortex. It is now known that physical activity increases expression of some neurotrophins. We examined the time course of BDNF and TrkB expression after 1, 3, 5, 7 or 14 days in one of three conditions: (1) an "acrobatic" motor skill learning condition (AC), (2) a motor activity condition (moderately paced running on a flat track; MC) and (3) an inactive social-only control (SC) that served as a baseline group. Expression levels of BDNF and TrkB were evaluated by measuring relative optical density of the immunocytochemical reaction product. In cerebellar molecular layer, expression of BDNF correlated significantly with time spent in AC and MC over the first 7 days of training and remained elevated after 14 days of AC but not of MC. Changes in TrkB protein expression in cerebellar molecular layer mirrored those for BDNF during the first 7 days of training, but subsequently its expression subsided to the control level. In motor cortex, a significant increase in BDNF and TrkB protein expression was detected in the upper layers after 14 days in AC. Increased expression of BDNF, but not of TrkB, was observed in upper motor cortical layers after 14 days of MC. These data indicate that complex motor learning and moderate physical activity with little learning produce different effects on the expression pattern of BDNF and its receptor and may have implications for neural plasticity arising from such experiences.

Synaptic and extrasynaptic localization of brain-derived neurotrophic factor and the tyrosine kinase B receptor in cultured hippocampal neurons

Brain-derived neurotrophic factor (BDNF) regulates synapses, but the distribution of BDNF and its receptor TrkB relative to the location of glutamatergic and gamma-aminobutyric acidergic (GABAergic) synapses is presently unknown. Immunocytochemistry was performed in primary hippocampal neuron cultures to determine whether BDNF and TrkB are preferentially localized to excitatory or inhibitory markers at 7, 14, and 21 days in vitro (DIV). Glutamatergic sites were localized with vesicular glutamate transporter type 1 (VGLUT1) as presynaptic marker and the NR1 subunit of the NMDA receptor and the GluR1 subunit of the AMPA receptor as receptor markers. GABAergic sites were labeled with the 65-kDa isoform of glutamic acid decarboxylase (GAD-65) as presynaptic marker and the gamma2 subunit of the GABAA receptor as receptor marker. During development, <30% of BDNF punctae and TrkB clusters were localized to glutamatergic and GABAergic markers. Because their rates of colocalization did not change from 7 to 21 DIV, this study details the distribution of BDNF and TrkB at 14 DIV. BDNF was preferentially colocalized with glutamatergic markers VGLUT1 and NR1 ( approximately 30% each). TrkB was also relatively highly colocalized with VGLUT1 and NR1 ( approximately 20% each) but was additionally highly colocalized with GABAergic markers GAD-65 ( approximately 20%) and gamma2 ( approximately 30%). NR1 clusters colocalized with BDNF puncta and TrkB clusters were mostly extrasynaptic, as were gamma2 clusters colocalized with TrkB clusters. These results show that, whereas most BDNF and TrkB protein is extrasynaptic, BDNF is preferentially associated with excitatory markers and that TrkB is associated equally with excitatory and inhibitory markers.

Brain-derived neurotrophic factor mRNA and protein in the piglet brainstem and effects of Intermittent Hypercapnic Hypoxia

Brain-derived neurotrophic factor (BDNF) is a neurotrophin essential for the development of normal respiratory rhythm and ventilatory control. Chronic exposure to Intermittent Hypercapnic Hypoxia (IHH) has been shown to alter ventilatory responses of piglets. This study investigated changes in BDNF distribution and expression in seven nuclei of the caudal medulla, from piglets exposed to IHH for 1, 2, 3, or 4 days before death, using non-radioactive in situ hybridisation (for mRNA) and immunohistochemistry (for protein). Compared to controls, BDNF mRNA was markedly increased across the entire medulla of the brainstem, after all durations of IHH (1-4 days). In contrast, BDNF protein expression increased after 1 day of exposure to IHH (p=0.003), but, thereafter, was not different to controls. Amongst individual nuclei, neurons of the dorsal motor nucleus of the vagus (DMNV) showed increased BDNF mRNA (p<0.01), but decreased protein expression (p=0.05) after all durations of IHH. In the ION, both mRNA and protein for BDNF were significantly increased after 1 day IHH (p<0.01 and p=0.001, respectively), but these increases were not sustained. This study is the first to investigate changes in BDNF expression in response to environmental challenges during postnatal development in the brainstem. Implications of the wide distribution of BDNF in the piglet caudal medulla and increased expression after IHH exposure are discussed, with particular reference to roles for BDNF-dependent neurons at this stage of development.

Cerebellar grafting in the oculomotor system as a model to study target influence on adult neurons

In the last decades, there have been many efforts directed to gain a better understanding on adult neuron-target cell relationships. Embryonic grafts have been used for the study of neural circuit rewiring. Thus, using several donor neuronal tissues, such as cerebellum or striatum, developing grafted cells have been shown to have the capability of substituting neural cell populations and establishing reciprocal connections with the host. In addition, different lesion paradigms have also led to a better understanding of target dependence in neuronal cells. Thus, for example, axotomy induces profound morphofunctional changes in adult neurons, including the loss of synaptic inputs and discharge alterations. These alterations are probably due to trophic factor loss in response to target disconnection. In this review, we summarize the different strategies performed to disconnect neurons from their targets, and the effects of target substitution, performed by tissue grafting, upon neural properties. Using the oculomotor system-and more precisely the abducens internuclear neurons-as a model, we describe herein the effects of disconnecting a population of central neurons from its natural target (i.e., the medial rectus motoneurons at the mesencephalic oculomotor nucleus). We also analyze target-derived influences in the structure and physiology of these neurons by using cerebellar embryonic grafts as a new target for the axotomized abducens internuclear neurons.

Elevated glial brain-derived neurotrophic factor in Parkinson's diseased nigra

We show the cellular distribution of immunoreactivity (IR) for brain-derived-neurotrophic-factor (BDNF), neurotrophin-3 (NT-3) and tyrosine kinase receptors TRKB and TRKC in idiopathic Parkinson's disease (IPD) and controls at post-mortem. In both groups, nigral neurons, astrocytes, ramified and amoeboid microglia expressed all antigens. Caudate-putamen neurons expressed all antigens except BDNF with similar distribution between groups. In IPD nigra, increased numbers of BDNF-IR and, less frequently, NT-3-IR ramified glia surrounded fragmented neurons, accompanied by BDNF-IR in surrounding neuropil. Amoeboid microglia were abundant only in IPD nigral scars. In IPD, glia might up-regulate neurotrophins in response to signals released from failing nigral neurons.

Differential regulation of neurotrophin expression in basal forebrain astrocytes by neuronal signals

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT3) promote the function and/or survival of basal forebrain (BF) cholinergic neurons in vivo and in culture. The neurotrophin source is commonly thought to be targets of cholinergic neurons and the possibility that local glial sources support cholinergic neurons has not been well examined. These sources, however, may be critical to BF neurons before or even after they reach their targets. We investigated neurotrophin expression in BF astrocytes and its regulation by neural signals. Solution hybridization and immunocytochemical assays revealed that NGF, BDNF, and NT(3) mRNA and proteins were expressed in cultured BF astrocytes. To investigate roles of neuronal signals in neurotrophin regulation, effects of K(+), glutamate, and the cholinergic agonist carbachol were examined. These stimuli affected neurotrophin expression differentially. KCl increased BDNF mRNA but did not alter NGF or NT(3) mRNA. The effect was blocked by nifedipine, suggesting that it was mediated by L-type voltage-dependent calcium currents. Carbachol also increased BDNF mRNA levels without changing NGF or NT(3). Effects were blocked by the muscarinic antagonist, atropine. In contrast, glutamate increased both NGF and BDNF mRNA. NT(3) mRNA again was unaffected. The metabotropic agonist trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) reproduced glutamate effects, whereas kainate or N-methyl-D-aspartate (NMDA) plus glycine did not. Lack of antagonism by ionotropic antagonists and blockade of glutamate effects by metabotropic antagonists confirmed metabotropic mediation. We suggest that BF astrocytes are local sources of neurotrophins for BF cholinergic neurons during development and are regulated differentially by specific neuronal signals. Critical neuronal-glial interactions may underlie basal forebrain function.

Neurotrophic factors in Huntington's disease

Huntington's disease is a neurodegenerative disorder characterized by the selective loss of striatal neurons and, to a lesser extent, cortical neurons. The neurodegenerative process is caused by the mutation of huntingtin gene. Recent studies have established a link between mutant huntingtin, excitotoxicity and neurotrophic factors. Neurotrophic factors prevent cell death in degenerative processes but they can also enhance growth and function of neurons that are affected in Huntington's disease. The endogenous regulation of the expression of neurotrophic factors and their receptors in the striatum and its connections can be important to protect striatal cells and maintains basal ganglia connectivity. The administration of exogenous neurotrophic factors, in animal models of Huntington's disease, has been used to characterize the trophic requirements of striatal and cortical neurons. Neurotrophins, glial cell line-derived neurotrophic factor family members and ciliary neurotrophic factor have shown a potent neuroprotective effects on different neuronal populations of the striatum. Furthermore, they are also useful to maintain the integrity of the corticostriatal pathway. Thus, these neurotrophic factors may be suitable for the development of a neuroprotective therapy for neurodegenerative disorders of the basal ganglia.

Haploinsufficiency in trkB and/or trkC neurotrophin receptors causes structural alterations in the aged hippocampus and amygdala

Neurotrophins and their cognate trk receptors regulate key events, most notably survival and differentiation of specific neuron populations, during the development of the nervous system. Their functions in the adult and aged CNS are far less well understood. We have analysed mice aged 21-23 months with haploinsufficiencies of the trkB and/or trkC genes with regard to morphological alterations in the hippocampus and amygdala. Neuronal densities and total numbers of neurons in the dentate gyrus were significantly decreased in trkB+/-, trkC+/-, and trkB+/-/C+/- mutants. In the hippocampal area CA2/3, neuronal density and the total number of neurons were only reduced in double-heterozygous mice. Within the amygdala, neuronal densities were not altered. The lateral, basolateral and basomedial nuclei of the amygdala, as well as the dentate gyrus and area CA3, revealed significant increases in the densities of degenerated axonal fragments; the most pronounced changes were found in the double-heterozygous mice. Thus, partial impairment in trkB and/or trkC receptor expression caused region-specific neuron losses in the hippocampus and increased axonal fragmentation in both hippocampus and amygdala, which may result from degeneration of both intrinsic and extrinsic fibre systems. Together, these data indicate that endogenous ligands to the trkB and trkC receptors are essential to maintain neuronal integrity in the aged hippocampus and amygdala.

Hippocampal brain-derived neurotrophic factor but not neurotrophin-3 increases more in mice selected for increased voluntary wheel running

Voluntary wheel running in rats increases hippocampal brain-derived neurotrophic factor (BDNF) expression, a neurochemical important for neuronal survival, differentiation, connectivity and synaptic plasticity. Here, we report the effects of wheel running on BDNF and neurotrophin-3 (NT-3) protein levels in normal control mice, and in mice selectively bred (25 generations) for increased voluntary wheel running. We hypothesized that increased voluntary wheel running in selected (S) mice would increase CNS BDNF and NT-3 protein levels more than in control (C) mice. Baseline hippocampal BDNF levels (mice housed without running wheels) were similar in S and C mice. Following seven nights of running, hippocampal BDNF increased significantly more in S versus C mice, and levels were correlated with distance run (considering C and S mice together). Spinal and cerebellar BDNF and hippocampal NT-3 levels were not significantly affected by wheel running in any group, but there was a small, positive correlation between spinal C3-C6 BDNF levels and distance run (considering C and S mice together). This is the first study to demonstrate that mice which choose to run more have greater elevations in hippocampal BDNF, suggesting enhanced potential for exercise-induced hippocampal neuroplasticity.

Ethanol effects on neonatal rat cortex: comparative analyses of neurotrophic factors, apoptosis-related proteins, and oxidative processes during vulnerable and resistant periods

The developing central nervous system (CNS) is highly susceptible to ethanol, with acute or chronic exposure producing an array of anomalies and cell loss. Certain periods of vulnerability have been defined for various CNS regions, and are often followed by periods of relative ethanol resistance. In the present study, neonatal rats were acutely exposed to ethanol during a time when peak cell death is found in developing cerebral cortex (postnatal day 7; P7), and during a later neonatal period of ethanol resistance (P21). Comparisons at the two ages were made of basal levels of neurotrophic factors (NTFs), and in addition, ethanol-mediated changes in NTFs, apoptosis-related proteins, antioxidant activities, and generation of reactive oxygen species (ROS) were quantified at 0, 2, and 12 h following termination of exposure. It was found that at P21, basal levels of NTF nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) were considerably higher than at P7, possibly affording protection against ethanol neurotoxicity at this age. Following ethanol treatment at P7, approximately equal numbers of pro-apoptotic and pro-survival changes were produced, although most of the pro-apoptotic alterations occurred rapidly following termination of treatment, a critical period for initiation of apoptosis. At P21, however, the large majority of ethanol-mediated changes were adaptive, favoring survival. We speculate that the capacity of the older CNS to upregulate a number of protective elements within the cellular milieu serves to greatly mitigate ethanol neurotoxicity, while in younger animals, such adjustments are minimal, thus enhancing ethanol vulnerability within this developing region.

Exercise-induced changes in hippocampal brain-derived neurotrophic factor and neurotrophin-3: effects of rat strain

We tested the hypothesis that exercise-induced changes in hippocampal brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) differ among rat strains exhibiting a range of voluntary wheel running activity. Four strains (Sprague-Dawley, Brown Norway, Dark Agouti and PVG) were given access to running wheels (1 or 7 nights). Over 7 nights, the average distance run per night was higher in PVG versus other strains, and higher in Brown Norway versus Sprague-Dawley rats. Hippocampal BDNF concentrations in sedentary rats were higher in PVG versus Sprague-Dawley rats. When data from all strains were combined, BDNF levels increased with 7 nights of wheel running and were positively correlated to the previous night distance run. Sedentary hippocampal NT-3 levels were not different between rat strains, but decreased with 7 nights of wheel access; NT-3 was negatively correlated with previous night distance run. There were no differences between strains in the correlation between distance run and BDNF or NT-3 levels. Although exercise decreases hippocampal NT-3, strain does not alter NT-3 levels. In contrast, BDNF levels increase with exercise and basal levels differ between strains, possibly due to strain differences in spontaneous activity.

The trophic role of oligodendrocytes in the basal forebrain

Traditionally, the primary function of oligodendrocytes (OLGs) in the CNS has been considered to be myelination. Here, we investigated whether OLGs may play a trophic role, particularly during development. Neurotrophin expression was assessed in postnatal day 7 basal forebrain (BF) OLGs, using in situ hybridization and detection of myelin basic protein. Nerve growth factor, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) mRNAs were revealed in OLGs in vivo and in culture. To determine whether OLGs support nearby neurons, we examined the influence of OLGs on BF cholinergic neurons. Neuronal function was enhanced by cocultured OLGs and OLG conditioned medium. Moreover, trophic effects of OLG conditioned medium were partially blocked by K252a, a trk tyrosine kinase inhibitor, and by neutralizing anti-BDNF or anti-NT-3 antisera, indicating that neurotrophins may mediate these effects, perhaps in concert with other signals. Our studies support a novel role for OLGs in providing local trophic support for neurons in the CNS.

Grafting of a new target prevents synapse loss in abducens internuclear neurons induced by axotomy

The loss of afferent synaptic boutons is a prominent alteration induced by axotomy on adult central neurons. In this work we attempted to prove whether synapse loss could be reverted by reconnection with a new target. We severed the medial longitudinal fascicle of adult cats and then transplanted embryonic cerebellar primordia at the lesion site immediately after lesion. As previously shown, the transected axons from abducens internuclear neurons penetrate and reinnervate the graft [J Comp Neurol 444 (2002) 324]. By immunocytochemistry and electron microscopy we studied the synaptology of abducens internuclear neurons under three conditions: control, axotomy and transplant (2 months of survival time). Semithin sections of the abducens nucleus were immunostained against calretinin, to identify abducens internuclear neurons, and either synaptophysin (SF), to label synaptic terminals, or glial fibrillary acidic protein (GFAP) to detect the astrocytic reaction. Optical and linear density of SF and GFAP immunostaining were measured. Data revealed a significant decrease in the density of SF-labeled terminals with a parallel increase in GFAP-immunoreactive elements after axotomy. On the contrary, in the transplant group, the density of SF-labeled terminals was found similar to control, and the astrocytic reaction induced by lesion was significantly reduced. At the ultrastructural level, synaptic coverage and linear density of boutons were measured around the somata of abducens internuclear neurons. Whereas a significant reduction in both parameters was found after axotomy, cells of the transplant group received a normal density of synaptic endings. The ratio between F- and S-type boutons was found similar in the three groups. Therefore, these findings indicate that the grafting of a new target can prevent the loss of afferent synaptic boutons produced by the axotomy.

Inhibitory but not excitatory cortical neurons require presynaptic brain-derived neurotrophic factor for dendritic development, as revealed by chimera cell culture

To address questions of whether endogenous BDNF acts differentially on inhibitory and excitatory neurons, and through what routes, we used chimera culture of cerebral cortical neurons derived from BDNF-/- mice and another type of transgenic mice that express green fluorescence protein and BDNF. Presynaptic BDNF transferred to both types of neurons, GABA-synthesizing enzyme-positive and -negative neurons. The latter neurons were confirmed to be glutamatergic with immunocytochemistry. Dendritic development of the former inhibitory neurons was promoted by endogenous BDNF transferred from presynaptic, excitatory neurons. In contrast, dendritic development of excitatory neurons was not related to the presence or absence of presynaptic BDNF, suggesting that BDNF acts on inhibitory neurons through an anterograde, transsynaptic route so as to promote dendritic development, whereas this is not the case in excitatory neurons.

Reduced brain-derived neurotrophic factor in prefrontal cortex of patients with schizophrenia

Anatomical and molecular abnormalities of excitatory neurons in the dorsolateral prefrontal cortex (DLPFC) are found in schizophrenia. We hypothesized that brain-derived neurotrophic factor (BDNF), a protein capable of increasing pyramidal neuron spine density and augmenting synaptic efficacy of glutamate, may be abnormally expressed in the DLPFC of patients with schizophrenia. Using an RNase protection assay and Western blotting, we detected a significant reduction in BDNF mRNA (mean=23%) and protein (mean=40%) in the DLPFC of patients with schizophrenia compared to normal individuals. At the cellular level, BDNF mRNA was expressed at varying intensities in pyramidal neurons throughout layers II, III, V, and VI of DLPFC. In patients with schizophrenia; neuronal BDNF expression was decreased in layers III, V and VI. Our study demonstrates a reduction in BDNF production and availability in the DLPFC of schizophrenics, and suggests that intrinsic cortical neurons, afferent neurons, and target neurons may receive less trophic support in this disorder.

Neurotrophin secretion: current facts and future prospects

The proteins of the mammalian neurotrophin family (nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5)) were originally identified as neuronal survival factors. During the last decade, evidence has accumulated implicating them (especially BDNF) in addition in the regulation of synaptic transmission and synaptogenesis in the CNS. However, a detailed understanding of the secretion of neurotrophins from neurons is required to delineate their role in regulating synaptic function. Some crucial questions that need to be addressed include the sites of neurotrophin secretion (i.e. axonal versus dendritic; synaptic versus extrasynaptic) and the neuronal and synaptic activity patterns that trigger the release of neurotrophins. In this article, we review the current knowledge in the field of neurotrophin secretion, focussing on activity-dependent synaptic release of BDNF. The modality and the site of neurotrophin secretion are dependent on the processing and subsequent targeting of the neurotrophin precursor molecules. Therefore, the available data regarding formation and trafficking of neurotrophins in the secreting neurons are critically reviewed. In addition, we discuss existing evidence that the characteristics of neurotrophin secretion are similar (but not identical) to those of other neuropeptides. Finally, since BDNF has been proposed to play a critical role as an intercellular synaptic messenger in long-term potentiation (LTP) in the hippocampus, we try to reconcile this possible role of BDNF in LTP with the recently described features of synaptic BDNF secretion.

Interplay between brain-derived neurotrophic factor and signal transduction modulators in the regulation of the effects of exercise on synaptic-plasticity

This study was designed to identify molecular mechanisms by which exercise affects synaptic-plasticity in the hippocampus, a brain area whose function, learning and memory, depends on this capability. We have focused on the central role that brain-derived neurotrophic factor (BDNF) may play in mediating the effects of exercise on synaptic-plasticity. In fact, this impact of exercise is exemplified by our finding that BDNF regulates the mRNA levels of two end products important for neural function, i.e. cAMP-response-element binding (CREB) protein and synapsin I. CREB and synapsin I have the ability to modify neuronal function by regulating gene-transcription and affecting synaptic transmission, respectively. Furthermore, we show that BDNF is capable of concurrently increasing the mRNA levels of both itself and its tyrosine kinaseB (TrkB) receptor, suggesting that exercise may employ a feedback loop to augment the effects of BDNF on synaptic-plasticity. The use of a novel microbead injection method in our blocking experiments and Taqman reverse transcription polymerase reaction (RT-PCR) for RNA quantification, have enabled us to evaluate the contribution of different pathways to the exercise-induced increases in the mRNA levels of BDNF, TrkB, CREB, and synapsin I. We found that although BDNF mediates exercise-induced hippocampal plasticity, additional molecules, i.e. the N-methyl-D-aspartate receptor, calcium/calmodulin protein kinase II and the mitogen-activated protein kinase cascade, modulate its effects. Since these molecules have a well-described association to BDNF action, our results illustrate a basic mechanism through which exercise may promote synaptic-plasticity in the adult brain.

Distribution of brain-derived neurotrophic factor and TrkB receptor proteins in the fetal and postnatal hippocampus and cerebellum of the guinea pig

This study investigates the distribution of brain-derived neurotrophic factor protein (BDNF) and its receptor, TrkB, during the development of hippocampus and cerebellum in a long-gestation species, the guinea pig. In the granule cell populations of both structures, BDNF immunoreactivity (-IR) was exclusive to postmigratory, mature neurons. In dentate granule cells, TrkB-IR was coexpressed with BDNF-IR, suggesting that the ligand-receptor interaction could occur by means of an autocrine/paracrine mechanism. In cerebellar granule cells, TrkB-IR was detected in both pre- and postmigratory cells, indicating that immature neurons are also BDNF-responsive. With advancing gestational age an increase in the intensity of BDNF-IR in granule cells was accompanied by concomitant increases in the staining and areal growth of the associated mossy fiber layer in the hippocampus, and the molecular layer in the cerebellum. The developmental increase in BDNF- and TrkB-IR in the neuropil of both structures coincided with periods of significant growth in all strata, indicating a role for BDNF and TrkB in process outgrowth. In the hippocampus, CA2, CA3, and hilar, neurons demonstrated both BDNF- and TrkB-IR during development and maturation, whereas CA1 neurons showed TrkB-IR throughout this period but only transient BDNF-IR in early gestation. In the fetal cerebellum, Purkinje cell bodies coexpressed BDNF-IR and TrkB-IR. In the postnatal period, BDNF-IR was down-regulated but TrkB-IR persisted, indicating that mature Purkinje cells might retain their responsiveness to BDNF. Thus, we have demonstrated in both the hippocampus and cerebellum that the spatiotemporal distribution of BDNF-IR and TrkB-IR coincides with the maturation of granule cells prenatally and with significant periods of neuropil growth, both prenatally and in the immediate postnatal period.

Climbing fiber development: do neurotrophins have a part to play?

The climbing fiber input to the cerebellum is crucial for its normal function but those factors which control the development of this precisely organized pathway are not fully elucidated. The neurotrophins are a family of peptides, which have many roles during development of the nervous system, including the cerebellum. Since the cerebellum and inferior olive express neurotrophins and their receptors, we propose that neurotrophins are involved in the regulation of climbing fiber development. Here we review the temporo-spatial expression of neurotrophins and their receptors at key ages during climbing fiber development and then examine evidence linking neurotrophins to climbing fiber development, including some of the intracellular pathways involved. During prenatal development the expression of neurotrophins in the hindbrain coupled with their function in neurogenesis and migration, is consistent with a role of NT3 in inferior olivary genesis. Subsequently, cerebellar expression of two neurotrophins, NT3 and NT4, is concurrent with olivary receptor expression and the time of olivary axonal outgrowth and this continues postnatally during early climbing fiber synaptogenesis on Purkinje cells. The expression-pattern of neurotrophins changes with age, with falling NGF, NT3 and NT4 but increasing granule cell BDNF. Importantly, olivary expression of neurotrophin receptors, and therefore climbing fiber responsiveness to neurotrophins, falls specifically during maturation of the climbing fiber-Purkinje cell synapse. The function of BDNF is less certain, but experimental studies indicate that it has a role in climbing fiber innervation of Purkinje cells, particularly synaptogenesis and synaptic plasticity. Its importance is highlighted by the overlap of BDNF signalling with several cellular pathways, which regulate climbing fiber maturation. From the data presented, we propose not only that neurotrophins are involved in climbing fiber development, but also that several act in a specific temporal order.

Control of cortical interneuron migration by neurotrophins and PI3-kinase signaling

During telencephalic development, cells from the medial ganglionic eminence (MGE) are thought to migrate to the neocortex to give rise to a majority of cortical GABAergic interneurons. By combining time-lapse video-microscopy, immunofluorescence and pharmacological perturbations in a new in vitro migration assay, we find that MGE-derived cells migrate through the entire extent of the cortex and into the CA fields of the hippocampus, but avoid the dentate gyrus. Migrating neurons initially travel within the marginal zone and intermediate zone, and can enter the cortical plate from either location. Tangential migration is strongly stimulated by BDNF and NT4 and attenuated by the Trk-family inhibitor, K252a, suggesting that migration is regulated by TrkB signaling. Furthermore, TrkB-null mice show a significant decrease in the number of calbindin-positive neurons migrating tangentially in the embryonic cortex. BDNF and NT4 cause rapid activation of PI3-kinase in MGE cells, and inhibition of PI3-kinase (but not of MAP kinase or PLCγ) dramatically attenuates tangential migration. These observations suggest that TrkB signaling, via PI3-kinase activation, plays an important role in controlling interneuron migration in the developing cerebral cortex.

Oligodendrocytes as providers of growth factors

Glial cells recently are being appreciated as supporters of brain neurons. This review addresses their role as growth factor providers. While the function of astrocytes in this capacity is known, new data indicate that oligodendrocytes, the myelinating cells of the brain, exhibit similar abilities. Oligodendrocytes provide trophic signals to nearby neurons and synthesize defined growth factors. Expression of growth factors is influenced by neural signals. The review summarizes these roles and their implications in brain function.

Combined neurotrophic supplementation and caspase inhibition enhances survival of fetal hippocampal CA3 cell grafts in lesioned CA3 region of the aging hippocampus

Fetal hippocampal CA3 cells show excellent survival when homotopically grafted into the kainic acid-lesioned CA3 region of the young adult hippocampus, a model of temporal lobe epilepsy. However, survival of these cells in the kainic acid-lesioned CA3 region of the aging hippocampus is unknown. We hypothesize that fetal CA3 grafts into the lesioned CA3 region of the middle-aged and aged hippocampus exhibit significantly diminished cell survival compared with similar grafts in the lesioned young adult hippocampus unless pre-treated and transplanted with factors that augment graft cell survival. We analyzed cell survival of 5'-bromodeoxyuridine-labeled embryonic day 19 CA3 grafts following their transplantation into the lesioned CA3 region of the middle-aged and aged rat hippocampus. Grafts were placed 4 days after an i.c.v. administration of kainic acid, and absolute cell survival of grafts was quantified 1 month after grafting using 5'-bromodeoxyuridine immunostaining of serial sections and the optical fractionator counting method. Grafts into both middle-aged and aged hippocampus exhibited analogous but significantly diminished cell survival (30% of injected cells) compared with similar grafts into the young adult hippocampus (72% cell survival). However, the extent of cell survival of CA3 grafts pre-treated and transplanted with a combination of neurotrophic factors brain-derived neurotrophic factor and neurotrophin-3 and the caspase inhibitor acetyl-tyrosinyl-valyl-alanyl-aspartyl-chloro-methylketone was significantly enhanced in both middle-aged and aged hippocampus (51-63% cell survival). These results underscore that aging impairs the conduciveness of the CA3 region for robust survival of homotopic fetal CA3 grafts after lesion. However, a combined neurotrophic supplementation and caspase inhibition significantly enhances survival of fetal CA3 cells in the lesioned aging hippocampus. Thus, pre-treatment and grafting of donor cells with a combination of factors that support growth of specific donor cells may considerably enhance survival and integration of fetal grafts into the lesioned aging CNS in clinical trials.

TrkB and TrkC are differentially regulated by excitotoxicity during development of the basal ganglia

During development neurons are protected against various insults by intrinsic properties. Here we evaluate trkB (both full-length and truncated forms) and trkC expression in the striatum, cortex, and substantia nigra after intrastriatal injection of quinolinic acid (QUIN) at different stages of postnatal (P) development, by RNase protection assay and in situ hybridization. During normal development, a region-specific regulation of trkB and trkC was observed, showing the maximal mRNA levels at P5. Excitotoxic lesion did not modify striatal trkB mRNA levels at any age examined. However, trkC decreased after QUIN injection at P5 in the striatum (52 +/- 2% of control levels). On the other hand, regulation of trkB and trkC expression was observed in cortex and substantia nigra after striatal excitotoxic lesion. Both full-length and truncated receptor isoforms of trkB were enhanced in the cortex when striatal injury was produced at P21 (268 +/- 38 and 206 +/- 35%) or P30 (174 +/- 35 and 157 +/- 13%). In situ hybridization studies localized this increase in trkB expression in layers II/III and V along the cerebral cortex. Within the substantia nigra, striatal excitotoxicity at P5 selectively decreased the truncated form of trkB (70 +/- 7%), whereas the full-length form was up-regulated at P30 (130 +/- 2%). A biphasic increase in trkC mRNA levels was observed at P5 (151 +/- 3%) and P21 (168 +/- 4%). These changes were localized in the substantia nigra pars compacta. Triple-labeling studies disclosed that all these changes were mainly located in neurons. These results demonstrate that the endogenous response to excitotoxicity includes transneuronal regulation of neurotrophin receptors, which is specific for each nucleus and depends on the developmental stage.

Some forms of cAMP-mediated long-lasting potentiation are associated with release of BDNF and nuclear translocation of phospho-MAP kinase

Long-lasting forms of synaptic plasticity like the late phase of LTP (L-LTP) typically require an elevation of cAMP, the recruitment of the cAMP-dependent protein kinase (PKA), and ultimately the activation of transcription and translation; some forms also require brain-derived neurotrophic factor (BDNF). Both cAMP and BDNF can activate mitogen-activated protein kinase (MAPK/ERK), which also plays a role in LTP. However, little is known about the mechanisms whereby cAMP, BDNF, and MAPK interact. We find that increases in cAMP can rapidly activate the BDNF receptor TrkB and induce BDNF-dependent long-lasting potentiation at the Schaffer collateral-CA1 synapse in hippocampus. Surprisingly, in these BDNF-dependent forms of potentiation, which are also MAPK dependent, TrkB activation is not critical for the activation of MAPK but instead appears to modulate the subcellular distribution and nuclear translocation of the activated MAPK.

Macrophage stimulating protein is a novel neurotrophic factor

Macrophage stimulating protein (MSP), also known as hepatocyte growth factor-like, is a soluble cytokine that belongs to the family of the plasminogen-related growth factors (PRGFs). PRGFs are alpha/beta heterodimers that bind to transmembrane tyrosine kinase receptors. MSP was originally isolated as a chemotactic factor for peritoneal macrophages. Through binding to its receptor, encoded by the RON gene, it stimulates dissociation of epithelia and works as an inflammatory mediator by repressing the production of nitric oxide (NO). Here, we identify a novel role for MSP in the central nervous system. As a paradigm to analyze this function we chose the hypoglossal system of adult mice. We demonstrate in vivo that either administration of exogenous MSP or transplantation of MSP-producing cells at the proximal stump of the resected nerve is sufficient to prevent motoneuron atrophy upon axotomy. We also show that the MSP gene is expressed in the tongue, the target of the hypoglossal nerve, and that MSP induces biosynthesis of Ron receptor in the motoneuron somata. Finally, we show that MSP suppresses NO production in the injured hypoglossal nuclei. Together, these data suggest that MSP is a novel neurotrophic factor for cranial motoneurons and, by regulating the production of NO, may have a role in brain plasticity and regeneration.

Differential patterns of nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 mRNA and protein levels in developing regions of rat brain

The present studies were undertaken to characterize the regional and temporal patterns of neurotrophin messenger RNA and protein levels for beta-nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 in the developing CNS. We have examined the levels of these neurotrophin messenger RNAs with ribonuclease protection assays and corresponding protein levels with enzyme-linked immunosorbent assays in the developing Long-Evans rat hippocampus, neocortex and cerebellum on postnatal days 1, 7, 14, 21, and 92. In addition, immunohistochemistry was used to localize the neurotrophins in these developing brain regions. Results indicated that in neocortex and hippocampus, messenger RNA for both nerve growth factor and brain-derived neurotrophic factor increased in an age-dependent manner, reaching a plateau by postnatal day 14. In the neocortex, nerve growth factor and brain-derived neurotrophic factor protein levels both peaked at postnatal day 14. In hippocampus, nerve growth factor protein peaked at postnatal day 7 while brain-derived neurotrophic factor peaked at postnatal day 14. In cerebellum, nerve growth factor messenger RNA levels were flat, while nerve growth factor protein peaked at postnatal day 7. Brain-derived neurotrophic factor messenger RNA increased in an age-dependent manner while the pattern for its protein levels was mixed. Neurotrophin-3 messeger RNA levels increased in an age-dependent manner in hippocampus, peaked at postnatal day14 in cerebellum, and no changes occurred in neocortex. Neurotrophin-3 protein was at its peak at postnatal day 1 and thereafter decreased at other postnatal days in all three brain regions. Results of neurotrophin immunohistochemistry often paralleled and complemented enzyme-linked immunosorbent assay data, demonstrating specific cell groups containing neurotrophin proteins in these regions. Within each region, patterns with regard to messenger RNA and respective protein levels for each neurotrophin were unique. No consistent relationship between patterns of neurotrophin messenger RNAs and their cognate proteins was observed between regions. The different regional patterns for neurotrophin messengerRNA and protein levels in each brain region indicate that messenger RNA studies of neurotrophin messenger RNA must be augmented by protein determination to fully characterize spatial and temporal neurotrophin distribution.

Age-related changes in levels of tyrosine kinase B receptor and fibroblast growth factor receptor 2 in the rat inferior colliculus: implications for neural senescence

Brain-derived neurotrophic factor and fibroblast growth factor 2, and their respective binding sites, tyrosine kinase B receptor and fibroblast growth factor receptor 2, are known to regulate neurite outgrowth and antioxidant enzyme activity. Several studies suggest that brain-derived neurotrophic factor and fibroblast growth factor are contained in the inferior colliculus. Previous work in our laboratories revealed dendritic and synaptic losses in the inferior colliculus of aged Fischer-344 rats, along with coincident increases in lipid peroxidation possibly linked to a decrease in activity of antioxidant enzymes. In an effort to identify potential causal mechanisms underlying age-related synaptic and dendritic losses that occur in the inferior colliculus, the present study attempted to determine if inferior colliculus levels of tyrosine kinase B receptor and fibroblast growth factor receptor 2 expression are altered with age. Immunocytochemistry was performed in the inferior colliculus, hippocampus and cerebellum of 3-month-old F344 rats to study distributions of the full-length and truncated isoforms of tyrosine kinase B receptor, and fibroblast growth factor receptor 2. The latter two brain regions served as positive controls. For all three antigens, immunolabeling was localized primarily in somata and proximal dendrites in all subdivisions of the inferior colliculus, and in the dentate gyrus and Ammon's horn of the hippocampus. In the cerebellum, the somata and dendrites of the Purkinje cells were also immunolabeled.A significant reduction in levels of the full-length form of tyrosine kinase B receptor in 18- and 25-month-old rats (respectively, approximately 20% and 30% relative to 3-month-olds) was revealed using western blot analyses. Inferior colliculus and hippocampal levels of the truncated form were modestly decreased ( approximately 7%) as well in the two older age groups. In contrast, levels of fibroblast growth factor receptor 2 in the inferior colliculus and hippocampus were elevated by approximately 35% in the two older age groups when compared to 3-month-olds. Changes in cerebellar levels of tyrosine kinase B receptor and fibroblast growth factor receptor 2, while similar to those in the inferior colliculus and hippocampus among the age groups, did not achieve statistical significance in this study. These findings give rise to the possibility that age-related reductions in tyrosine kinase B receptor levels could be a causal factor in the degenerative changes observed in the inferior colliculus of aged animals, including mitochondrial damage and dendritic regression. The observed increases in fibroblast growth factor receptor 2 levels may be compensatory to the increased oxidative stress. The effectiveness of the fibroblast growth factor receptor 2 response is questionable given the damage that occurs in the inferior colliculus and hippocampus of aged animals. However, the deficits could worsen in the absence of an increase in fibroblast growth factor receptor 2.

Regulation of Trk receptors following contusion of the rat spinal cord

Neurotrophins function through high-affinity tyrosine kinase (Trk) receptors to promote growth and survival of cells in the injured nervous system. To investigate the role of Trk receptors in the adult nervous system, we examined TrkA, TrkB, and TrkC mRNA expression in spinal cord and brain after spinal contusion. At 1 day postinjury, all Trk receptor transcripts were down regulated at and around the site of injury, a situation that persisted through the first week. By 42 days, Trk expression was absent only within the cavity. In addition, truncated TrkB expression was substantially increased in ependymal cells and astrocytes surrounding the lesion cavity of chronically injured spinal cords. Rostral and caudal to the injury site, TrkA, TrkB, and TrkC mRNA expression did not differ from that of uninjured control spinal cords. Furthermore, no changes were observed in TrkB or TrkC expression in the axotomized corticospinal and rubrospinal neurons. These studies suggest that loss of Trk receptors at the injury site may contribute to the early progressive cellular loss in injured spinal cords, while increased presence of truncated TrkB receptors in the chronic injured spinal cord may sequester and restrict BDNF availability to support axonal regeneration and neuronal survival. The persistence of Trk receptors on supraspinal neurons suggests that neurotrophin application can support growth and survival in the acute and chronic injury states.

Brain-derived neurotrophic factor in the control human brain, and in Alzheimer's disease and Parkinson's disease

Brain-derived neurotrophic factor (BDNF) is a small dimeric protein, structurally related to nerve growth factor, which is abundantly and widely expressed in the adult mammalian brain. BDNF has been found to promote survival of all major neuronal types affected in Alzheimer's disease and Parkinson's disease, like hippocampal and neocortical neurons, cholinergic septal and basal forebrain neurons, and nigral dopaminergic neurons. In this article, we summarize recent work on the molecular and cellular biology of BDNF, including current ideas about its intracellular trafficking, regulated synthesis and release, and actions at the synaptic level, which have considerably expanded our conception of BDNF actions in the central nervous system. But our primary aim is to review the literature regarding BDNF distribution in the human brain, and the modifications of BDNF expression which occur in the brain of individuals with Alzheimer's disease and Parkinson's disease. Our knowledge concerning BDNF actions on the neuronal populations affected in these pathological states is also reviewed, with an aim at understanding its pathogenic and pathophysiological relevance.

Neurotrophins induce death of hippocampal neurons via the p75 receptor

Nerve growth factor (NGF) and related neurotrophins influence neuronal survival and differentiation via interactions with the trk family of receptors. Recent studies have demonstrated that neurotrophins may also induce cell death via the p75 receptor. The importance and generality of neurotrophin-induced death in the brain have not been defined but may play a critical role during development and in disease-associated neuronal death. Here we demonstrate for the first time that all four members of the neurotrophin family directly elicit the death of hippocampal neurons via the p75 receptor. The hippocampus is a complex structure with many different neuronal subpopulations, and signals that influence neuronal death during development may have a critical impact on the mature function of this structure. In these studies we show that each neurotrophin causes the death of hippocampal neurons expressing p75 but lacking the cognate trk receptor. Neurotrophin-induced neuronal death is mediated by activation of Jun kinase. These studies demonstrate that neurotrophins can regulate death as well as survival of CNS neurons.

Developmental regulation of BDNF and NT-3 expression by quinolinic acid in the striatum and its main connections

Interactions between neurotrophic factors and neurotransmitters participate in the formation and maintenance of appropriate connections, as well as in neurodegenerative processes. Here we have measured changes in the developmental expression pattern of BDNF and NT-3 in the striatum, cortex, and substantia nigra induced by intrastriatal injection of the N-methyl-d-aspartate glutamate receptor agonist quinolinic acid (QUIN). Animals were injected at different postnatal ages, and BDNF and NT-3 mRNA levels were determined 6 h after lesion using a ribonuclease protection assay. Our results show a biphasic increase in BDNF mRNA levels in striatum and in the ipsilateral cortex at postnatal day (P)5 and P21. In contrast, although NT-3 expression did not change in the striatum, it was down-regulated in the ipsilateral cortex at P5 and P30. Intrastriatal QUIN injection did not induce changes in either BDNF or NT-3 expression in the ipsilateral substantia nigra. These findings show that neurotrophin expression is developmentally regulated after excitotoxic injury, which suggests that this endogenous response may be involved in different neuronal maturation and vulnerability during development.

Neocortical neurons lacking the protein-tyrosine kinase B receptor display abnormal differentiation and process elongation in vitro and in vivo

The spatial and temporal expression of the protein-tyrosine kinase B (TrkB) receptor and its ligands has been correlated with the development of the neocortex. Activation of the receptor has been associated with neocortical neuronal survival, differentiation, connectivity and neurotransmitter release. Although such findings suggest an important role for TrkB signaling in corticogenesis, conclusive evidence from targeted gene deletion ("knockout"; TrkB -/-) mice has been limited, due in part to the neonatal lethality of most of these mutant mice and the confounding variables associated with the poor health of those few surviving slightly longer postnatally. In the present study, the effects of TrkB signaling on the survival, differentiation and integration of neocortical neurons was directly investigated in vitro and in vivo. First, we conducted a neuron-specific immunocytochemical analysis of TrkB -/- mice to determine whether early cortical structure and patterns of histogenesis were normal or perturbed. We then employed in vitro and in vivo approaches to extend the life of TrkB -/- neocortical neurons beyond the period possible in TrkB -/- mutant mice themselves: (i) dissociated cell culture to directly compare the developmental potential of TrkB -/-, +/- and +/+ neurons; and (ii) neural transplantation into homochronic wild-type recipients to investigate the cell-autonomous effects of the receptor knockout on the differentiation, growth and integration of neocortical neurons. These latter experiments allowed, for the first time, study of the survival and differentiation potential of TrkB -/- neocortical neurons beyond the initial stages of corticogenesis. Direct comparison of brains of TrkB -/-, +/- and +/+ littermates immunocytochemically labeled with antibodies to microtubule-associated protein-2, neurofilament and beta-tubulin III revealed subtle anatomical anomalies in the mutant mice. These anomalies include abnormally diffuse microtubule-associated protein-2 positive neurons just dorsal to the corpus callosum, and heterotopic aggregations of postmitotic neurons in the subventricular zones of the ganglionic eminences, both suggesting delayed neuronal migration and differentiation. Cell culture experiments revealed substantially reduced survival by TrkB -/- neocortical neurons, and a significant reduction in neurite outgrowth by surviving TrkB -/- neurons. In experiments where prelabeled embryonic or neonatal TrkB -/- neocortical neurons were transplanted into the cerebral cortices of neonatal wild-type recipients, a similar quantitatively significant defect in the formation of dendrites, as well as reduced integration of TrkB -/- neocortical neurons, was also evident. These findings demonstrate cell-autonomous abnormalities in the development of neocortical neurons from TrkB -/- mice, and the subtle, but potentially critical, role of protein-tyrosine kinase B signaling in neocortical neuronal survival, differentiation and connectivity.

Glial cells as targets and producers of neurotrophins

Glial cells fulfill important tasks within the neural network of the central and peripheral nervous systems. The synthesis and secretion of various polypeptidic factors (cytokines) and a number of receptors, with which glial cells are equipped, allow them to communicate with their environment. Evidence has accumulated during recent years that neurotrophins play an important role not only for neurons but also for glial cells. This brief update of some morphological, immunocytochemical, and biochemical characteristics of glial cell lineages conveys our present knowledge about glial cells as targets and producers of neurotrophins under normal and pathological conditions. The chapter discusses the presence of neurotrophin receptors on glial cells, glial cells as producers of neurotrophins, signaling pathways downstream Trk and p75NTR, and the significance of neurotrophins and their receptors for glial cells during development, in cell death and survival, and in neurological disorders.

Effects of neurotrophins on cortical plasticity: same or different?

Neurotrophins are important regulators of visual cortical plasticity. It is still unclear, however, whether they play similar or different roles and which are their effects on the electrical activity of cortical neurons in vivo. We therefore compared the effects of all neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT-4), and neurotrophin-3 (NT-3) on visual cortical plasticity and on cell spontaneous and visually evoked activity. Rats were monocularly deprived for 1 week at the peak of the critical period, and neurotrophins were infused intracortically. The main finding is that, with the exception of NT-3, all neurotrophins affect the outcome of monocular deprivation, but there are clear differences in their mechanisms of action. In particular, NT-4 and NGF counteract monocular deprivation effects without causing detectable alterations either in spontaneous or visually evoked neuronal activity. BDNF is less effective on ocular dominance plasticity and, in addition, strongly affects spontaneous and visually evoked activity in cortical neurons.

Expression of neurotrophin-3 in the mouse forebrain: insights from a targeted LacZ reporter

To obtain insights into the expression of neurotrophin-3 (NT-3) in the mouse, we have utilized mice in which the Escherichia coli lacZ gene is integrated into the neurotrophin-3 locus (NT-3(lacZneo), Fariñas et al. [1994] Nature 369:658-661). In this mouse strain, beta-galactosidase production is under control of the NT-3 promoter in its normal chromosomal environment, and histochemical measurement of beta-galactosidase provides a simple, sensitive method to determine which cells express NT-3. Our data correlate well with previous in situ mRNA and immunocytochemical studies reporting the localization of NT-3. For example, in adult NT-3(lacZneo)/+ mice, beta-galactosidase is expressed in high amounts in limbic areas of the cortex (cingulate, retrosplenial, piriform, and entorhinal), in the visual cortex, in the hippocampal formation (dentate granule cells, CA2 cells, fasciola cinereum, induseum griseum, tenia tecta, presubiculum, and parasubiculum), and in the septum (septohippocampal nucleus and lateral dorsal septum). In contrast with other studies, our results suggest more extensive expression of NT-3 in adult and aged mouse brains with cortical expression apparent at 4.5 months. To further define the cell populations expressing NT-3 in adult mice, we have combined immunocytochemistry with histochemical staining and found that beta-galactosidase is coexpressed with a neuronal marker (NeuN) and with parvalbumin and neuropeptides, markers for GABAergic interneurons. Our studies of embryonic beta-galactosidase expression in NT-3(lacZneo)/+ mice suggest sites of NT-3 expression not previously described, including embryonic piriform cortical cells and dentate granule cell precursors.

Ischemic brain damage in mice after selectively modifying BDNF or NT4 gene expression

The neurotrophins and the tyrosine kinase (Trk) B receptor may play a protective role in the pathophysiology of cerebral ischemia. In this study, the authors investigated whether reducing endogenous expression of TrkB-binding neurotrophins modifies the susceptibility to ischemic injury after 1-hour middle cerebral artery occlusion followed by 23 hours of reperfusion in a filament middle cerebral artery occlusion model. Mice lacking both alleles for neurotrophin-4 (nt4-/-) or deficient in a single allele for brain-derived neurotrophic factor (bdnf+/-) exhibited larger cerebral infarcts compared to wild-type inbred 129/SVjae mice (68% and 91%, respectively, compared to controls). Moreover, lesions were larger (21%) in nt4-/- mice after permanent middle cerebral artery occlusion. Hence, expression of both NT4 and BDNF, and by inference the TrkB receptor, confers resistance to ischemic injury.

Ultrastructural localization of full-length trkB immunoreactivity in rat hippocampus suggests multiple roles in modulating activity-dependent synaptic plasticity

Neurotrophins acting at the trkB receptor have been shown to be important modulators of activity-dependent plasticity in the hippocampus, but the mechanisms underlying these effects are not yet well understood. To identify the cellular and subcellular targets of trkB ligands in the adult rat hippocampal formation, full-length trkB receptor immunoreactivity (trkB-IR) was localized using electron microscopy. trkB-IR was present in the glutamatergic pyramidal and granule cells. Labeling in these neurons appeared as discrete clusters and was primarily in axons, excitatory-type axon terminals, and dendritic spines and to a lesser extent in somata and dendritic shafts. trkB-IR was commonly found on the plasma membrane of dendritic spines, whereas in other subcellular regions trkB-IR was often intracellular. Labeling was strikingly dense within axon initial segments, suggesting extensive receptor trafficking. trkB-IR was not confined to pyramidal and granule cells. Dense trkB-IR was found in occasional interneuron axon initial segments, some axon terminals forming inhibitory-type synapses onto somata and dendritic shafts, and excitatory-type terminals likely to originate extrahippocampally. This suggests that trkB is contained in some GABAergic interneurons, neuromodulatory (e.g., cholinergic, dopaminergic, and noradrenergic) afferents, and/or glutamatergic afferents. These data indicate that full-length trkB receptor activation may modulate glutamatergic pathways of the trisynaptic circuit both presynaptically at axon terminals and initial segments and postsynaptically at dendritic spines and shafts. Signaling via catalytic trkB may also presynaptically affect inhibitory and modulatory neurons. A pan-trkB antibody labeled the same neuronal populations as the full-length-specific trkB antiserum, but the labels differed in density at various subcellular sites. These findings provide an ultrastructural foundation for further examining the mechanisms through which neurotrophins acting at trkB receptors contribute to synaptic plasticity.

Cellular localization of dopamine-releasing protein (DARP) in rat C6 glioma and primary mesencephalic cell cultures

Dopamine-releasing protein (DARP) is a multisubunit protein shown to have dramatic effects on development, recovery, and function of the rat catecholaminergic (CA) system. This study details efforts to determine if glial cells are responsible for the production of DARP in the central nervous system (CNS). Enzyme-linked immunosorbent assays (ELISA), Western blotting, and immunocytochemical techniques were employed to measure DARP levels and identify DARP immunoreactive proteins in rat C6 glioma cells and medium, respectively. ELISA analysis of serum-free C6 culture media revealed a maximal concentration of DARP by culture day 1. However, ELISA analysis of C6 cultures grown in F-12K/serum medium revealed that maximal levels of DARP were detected on culture day 6 with a 108% increase in DARP immunoreactivity from culture day 1. These values were determined using a polyclonal antibody generated against DARP-36aa (anti-DARP-36aa), a synthetic peptide with dopamine (DA) releasing activity, and anti-DARP B9-B10, a monoclonal antibody generated against partially purified DARP. Western blot analysis revealed that anti-DARP B9-B10 recognized proteins of approximately 60, 50, and 45 kDa in C6 cell homogenates while anti-DARP-36aa had immunoreactivity with the 60-kDa protein alone. Immunocytochemical studies demonstrated that anti-DARP-36aa and anti-DARP B9-B10 had strong immunoreactivity with proteins throughout the cytosol and in several processes of C6 cells. These results reveal that DARP is detected in glioma cells and secreted in a time-dependent fashion during culture. Primary rat mesencephalic cultures were also examined using immunocytochemistry. Incubation with DARP antibodies and antisera against glial fibrillary acidic protein (GFAP) revealed that DARP and GFAP immunoreactivity co-localized in primary mesencephalic cultures. However, the majority of DARP immunoreactivity was localized to cells without GFAP staining. These findings reveal that DARP is detected in astrocytes although the majority of DARP immunoreactivity is found in non-astrocyte type cells.

Opposing roles for neurotrophin-3 in targeting and collateral formation of distinct sets of developing cortical neurons

Neurotrophin-3 and its receptor TrkC are expressed during the development of the mammalian cerebral cortex. To examine whether neurotrophin-3 might play a role in the elaboration of layer-specific cortical circuits, slices of layer 6 and layers 2/3 neurons were cultured in the presence of exogenously applied neurotrophin-3. Results indicate that neurotrophin-3 promotes axonal branching of layer 6 axons, which target neurotrophin-3-expressing layers in vivo, and that it inhibits branching of layers 2/3 axons, which avoid neurotrophin-3-expressing layers. Such opposing effects of neurotrophin-3 on axonal branching were also observed with embryonic cortical neurons, indicating that the response to neurotrophin-3 is specified at early developmental stages, prior to cell migration. In addition to its effects on fiber branching, axonal guidance assays also indicate that neurotrophin-3 is an attractive signal for layer 6 axons and a repellent guidance cue for layers 2/3 axons. Experiments with specific antibodies to neutralize neurotrophin-3 in cortical membranes revealed that endogenous levels of neurotrophin-3 are sufficient to regulate branching and targeting of cortical axons. These opposing effects of neurotrophin-3 on specific populations of axons demonstrate that it could serve as one of the signals for the elaboration of local cortical circuits.

Neurotrophic factors and neuro-muscular disease: II. GDNF, other neurotrophic factors, and future directions

This is the second of two reviews in which we discuss the essential aspects of neurotrophic factor neurobiology, the characteristics of each neurotrophic factor, and their clinical relevance to neuromuscular diseases. The previous paper reviewed the neurotrophin family and neuropoietic cytokines. In the present article, we focus on the GDNF family and other neurotrophic factors and then consider future approaches that may be utilized in neurotrophic factor treatment.

Immunohistochemical distribution of the prohormone convertase PC5-A in rat brain

Prohormone convertase 5 is an endoprotease of the kexin/subtilisin-like family, which has been postulated to play a role in the proteolytic maturation of a variety of pro-peptides in the mammalian brain. In order to gain insight into the functional role of prohormone convertase 5 in the central nervous system, the regional, cellular and subcellular distributions of the enzyme were investigated by immunohistochemistry in rat brain using an N-terminal-directed specific antibody shown previously to recognize both the mature and unprocessed forms of the enzyme. Throughout the brain, prohormone convertase 5 immunoreactivity was concentrated within nerve cell bodies and proximal dendrites. No prohormone convertase 5 immunoreactivity was associated with astrocytes, as confirmed by the absence of prohormone convertase 5 immunolabeling in cells immunopositive for the glial protein S-100alpha. Within neurons, prohormone convertase 5 immunoreactivity was concentrated within the Golgi apparatus, as revealed immunohistochemically within the same sections using antibodies against the medial cisternae protein MG-160. It was also present within small vesicular-like elements distributed throughout the cytoplasm of perikarya and dendrites, but not of axons, as confirmed by its lack of co-localization with the synaptic terminal marker Dynamin-1. These results suggest that prohormone convertase 5 is active within early compartments of the neuronal regulated secretory pathway and that it is unlikely to be released with its processed substrates. At the regional level, prohormone convertase 5-immunoreactive perikarya were distributed extensively throughout the forebrain. The most numerous and intensely labeled were detected in the olfactory bulb, cerebral cortex, globus pallidus, endopeduncular and subthalamic nuclei, septum, diagonal band of Broca, magnocellular and medial preoptic areas, supraoptic and arcuate nuclei of the hypothalamus, and anterodorsal, laterodorsal, paraventricular and reticular nuclei of the thalamus. Moderate to dense neuronal labeling was also evident in the olfactory tubercle, caudate-putamen, claustrum, bed nucleus of the stria terminalis, substantia innominata, hippocampus, amygdala, and remaining thalamic and hypothalamic nuclei. This widespread distribution suggests that prohormone convertase 5 is involved in the processing of a variety of neuropeptide and/or neurotrophin precursors in mammalian brain.

Stimulation of neurotrophic factor secretion from 1321N1 human astrocytoma cells by novel diterpenoids, scabronines A and G

Glial cells release neurotrophic factors that maintain neurons functionally. When rat pheochromocytoma cells (PC-12) were cultivated with the conditioned medium of human astrocytoma cells (1321N1) incubated with the new diterpenoids, scabronines A and G, isolated from Sarcodon scabrosus, they changed their morphology and there was neurite outgrowth. The scabronines increased the expression of mRNA for nerve growth factor (NGF), and the secretion of NGF from 1321N1 cells in a concentration-dependent manner. However, the enhanced neurite outgrowth produced by the conditioned media was slightly inhibited by NGF neutralizing antibody, and the concentration of NGF released in response to the scabronines was insufficient to cause differentiation. These results suggest that scabronines cause the secretion of other factors together with NGF from 1321N1 cells. The diterpenoids are useful drugs to clarify the mechanism of synthesis and secretion of neurotrophic factors.