Up-regulation of trkB mRNA expression in the rat striatum after seizures

Systemic delivery of recombinant brain derived neurotrophic factor (BDNF) in the R6/2 mouse model of Huntington's disease

Loss of huntingtin-mediated BDNF gene transcription has been shown to occur in HD and thus contribute to the degeneration of the striatum. Several studies have indicated that an increase in BDNF levels is associated with neuroprotection and amelioration of neurological signs in animal models of HD. In a recent study, an increase in BDNF mRNA and protein levels was recorded in mice administered recombinant BDNF peripherally. Chronic, indwelling osmotic mini-pumps containing either recombinant BDNF or saline were surgically placed in R6/2 or wild-type mice from 4 weeks of age until euthanasia. Neurological evaluation (paw clasping, rotarod performance, locomotor activity in an open field) was performed. After transcardial perfusion, histological and immunohistochemical studies were performed. We found that BDNF- treated R6/2 mice survived longer and displayed less severe signs of neurological dysfunction than the vehicle treated ones. Primary outcome measures such as brain volume, striatal atrophy, size and morphology of striatal neurons, neuronal intranuclear inclusions and microglial reaction confirmed a neuroprotective effect of the compound. BDNF was effective in increasing significantly the levels of activated CREB and of BDNF the striatal spiny neurons. Moreover, systemically administered BDNF increased the synthesis of BDNF as demonstrated by RT-PCR, and this might account for the beneficial effects observed in this model.

TrkB inhibition as a therapeutic target for CNS-related disorders

The interaction of brain-derived neurotrophic factor (BDNF) with its tropomyosin-related kinase receptor B (TrkB) is involved in fundamental cellular processes including neuronal proliferation, differentiation and survival as well as neurotransmitter release and synaptic plasticity. TrkB signaling has been widely associated with beneficial, trophic effects and many commonly used psychotropic drugs aim to increase BDNF levels in the brain. However, it is likely that a prolonged increased TrkB activation is observed in many pathological conditions, which may underlie the development and course of clinical symptoms. Interestingly, genetic and pharmacological studies aiming at decreasing TrkB activation in rodent models mimicking human pathology have demonstrated a promising therapeutic landscape for TrkB inhibitors in the treatment of various diseases, e.g. central nervous system (CNS) disorders and several types of cancer. Up to date, only a few selective and potent TrkB inhibitors have been developed. As such, the use of crystallography and in silico approaches to model BDNF-TrkB interaction and to generate relevant pharmacophores represent powerful tools to develop novel compounds targeting the TrkB receptor.

TrkB/BDNF-dependent striatal plasticity and behavior in a genetic model of epilepsy: modulation by valproic acid

In mice lacking the central domain of the presynaptic scaffold Bassoon the occurrence of repeated cortical seizures induces cell-type-specific plasticity changes resulting in a general enhancement of the feedforward inhibition within the striatal microcircuit. Early antiepileptic treatment with valproic acid (VPA) reduces epileptic attacks, inhibits the emergence of pathological form of plasticity in fast-spiking (FS) interneurons and restores physiological striatal synaptic plasticity in medium spiny (MS) neurons. Brain-derived neurotrophic factor (BDNF) is a key factor for the induction and maintenance of synaptic plasticity and it is also implicated in the mechanisms underlying epilepsy-induced adaptive changes. In this study, we explore the possibility that the TrkB/BDNF system is involved in the striatal modifications associated with the Bassoon gene (Bsn) mutation. In epileptic mice abnormal striatum-dependent learning was paralleled by higher TrkB levels and an altered distribution of BDNF. Accordingly, subchronic intrastriatal administration of k252a, an inhibitor of TrkB receptor tyrosine kinase activity, reversed behavioral alterations in Bsn mutant mice. In addition, in vitro manipulations of the TrkB/BDNF complex by k252a, prevented the emergence of pathological plasticity in FS interneurons. Chronic treatment with VPA, by reducing seizures, was able to rebalance TrkB to control levels favoring a physiological redistribution of BDNF between MS neurons and FS interneurons with a concomitant recovery of striatal plasticity. Our results provide the first indication that BDNF is involved in determining the striatal alterations occurring in the early-onset epileptic syndrome associated with the absence of presynaptic protein Bassoon.

Activity-dependent modulation of the BDNF receptor TrkB: mechanisms and implications

Although brain-derived neurotrophic factor (BDNF) has emerged as a key regulator of activity-dependent synaptic plasticity, a conceptually challenging question is how this diffusible molecule achieves local and synapse-specific modulation. One hypothesis is that neuronal activity enhances BDNF signaling by selectively modulating TrkB receptors at active neurons or synapses without affecting receptors on neighboring, less-active ones. Growing evidence suggests that neuronal activity facilitates cell-surface expression of TrkB. BDNF secreted from active synapses and neurons recruits TrkB from extrasynaptic sites into lipid rafts, microdomains of membrane that are enriched at synapses. Postsynaptic rises in cAMP concentrations facilitate translocation of TrkB into the postsynaptic density. Finally, neuronal activity promotes BDNF-induced TrkB endocytosis, a signaling event important for many long-term BDNF functions. These mechanisms could collectively underlie synapse-specific regulation by BDNF.

Competition in neurite outgrowth and the development of nerve connections

During the development of the nervous system, neurons form their characteristic morphologies and become assembled into synaptically connected networks. In both neuronal morphogenesis and the development of nerve connections, competition plays an important role. Although the notion of competition is commonly used in neurobiology, there is little understanding of the nature of the competitive process and the underlying molecular and cellular mechanisms. In this chapter, we review a model of competition between outgrowing neurites, as well as various models of competition that have been proposed for the refinement of connections that takes place in the development of the neuromuscular and visual systems. We describe in detail a model that links competition in the development of nerve connections with the underlying actions and biochemistry of neurotrophic factors.

Expression and cellular distribution of high- and low-affinity neurotrophin receptors in malformations of cortical development

An increasing number of observations suggests an important and complex role for both high- (tyrosine kinase receptor, trk) and low- (p75) affinity neurotrophin receptors (NTRs) during development in human brain. In the present study, the cell-specific distribution of NTRs was studied in different developmental lesions, including focal cortical dysplasia (FCD, n = 15), ganglioglioma (GG, n = 15) and dysembryoplastic neuroepithelial tumors, (DNT, n = 10), from patients with medically intractable epilepsy. Lesional, perilesional, as well as normal brain regions were examined for the expression of trkA, trkB, trkC and p75(NTR) by immunocytochemistry. In normal postmortem human cortex, immunoreactivity (IR) for trk and p75(NTR) was mainly observed in pyramidal neurons, whereas no notable glial IR was found within the white matter. All three trk receptors were encountered in high levels in the neuronal component of the majority of FCD, GG and DNT specimens. Strong trkA, trkB and trkC IR was found in neurons of different size, including large dysplastic neurons and balloon cells in FCD cases. In contrast, p75(NTR) IR was observed in only a small number of neuronal cells, which also contain trk receptors. Glial cells with astrocytic morphology showed predominantly IR for trkA in FCD and GG specimens, whereas oligodendroglial-like cells in DNT showed predominently IR for trkB. P75(NTR) IR was observed in a population of cells of the microglial/macrophage lineage in both FCD and glioneuronal tumors. Taken together, our findings indicate that the neuronal and the glial components of malformations of cortical development express both high- and low-affinity NTRs. Further research is necessary to investigate how activation of these specific receptors could contribute to the development and the epileptogenicity of these developmental disorders.

Neurotrophin receptor immunoreactivity in the hippocampus of patients with mesial temporal lobe epilepsy

Recent evidence supports a critical role of neurotrophins in the regulation of both neuronal survival and synaptic transmission during epileptogenesis. We have examined the immunohistochemical expression of high- (tyrosine kinase receptors, trk) and low-affinity (p75) neurotrophin receptors (NTRs) in the hippocampal specimens from 18 patients with chronic temporal lobe epilepsy [TLE; 14 patients with hippocampal sclerosis (HS) and four with focal lesions (tumours) not involving the hippocampus proper]. Nonepileptic autopsy brains (n = 6) and surgical specimens from tumour patients without epilepsy (n = 3) were used as controls. Immunoreactivity (IR) for the trk receptors (trkA, trkB, trkC) was detected in normal human brain within the pyramidal neurones of hippocampal cornus ammoni (CA) regions and in the dentate gyrus. There were no detectable differences in the neuronal trk IR patterns in the hippocampus between control and TLE cases with HS, except for a decrease in neuronal density in regions where cell death had occurred (CA1, CA3 and CA4). In contrast, a consistent increase in trkA IR was observed in reactive astrocytes in CA1 and dentate gyrus. The low-affinity p75 neurotrophin receptor (p75(NTR)) was expressed in low levels in postnatal normal hippocampus. In contrast, neuronal p75(NTR) IR was detected in 10/14 cases of HS in spared neurones within the CA and hilar regions of the hippocampus. Double labelling revealed that p75(NTR)-positive neurones also contain trk receptor IR. In six cases with prominent glial activation strong p75(NTR) IR was observed in microglial cells within the sclerotic hippocampus. The present results indicate that changes in NTR expression are still detectable in the hippocampus of patients with chronic TLE and involve both glial and neuronal cells. Reactive astrocytes were immunoreactive for trkA, whereas activated microglia cells were reactive for p75(NTR), suggesting different functions for specific NTRs in the development of reactive gliosis. Moreover, the increased expression of p75(NTR) in hippocampal neurones of TLE patients may critically influence the neuronal survival during the epileptogenic process.

Expression of the neurotrophin receptor trkB is regulated by the cAMP/CREB pathway in neurons

trkB as receptor for neurotrophins brain-derived neurotrophic factor (BDNF)/neurotrophin (NT)-4/5 plays a crucial role during development, maintenance of the adult brain, and its adaptation to injury or pathological conditions. In spite of this, very little is known about the mechanisms that regulate its expression. Here, we show that forskolin (Fk) rapidly stimulates the expression of both the full-length and truncated trkB isoforms in primary cultures of cortical neurons. Gel shift assays and transient transfection experiments demonstrate that this activation occurs via a protein kinase A (PKA)/cyclic AMP-responsive element-binding protein (CREB)-dependent mechanism. Activated CREB binds to the second cyclic AMP (cAMP)-responsive element (CRE) of the two CRE sites located within the P2 promoter of the trkB gene, which is able to confer cAMP responsiveness to a heterologous promoter. Our results illustrate that the trkB gene is a target for CREB regulation and explain the increase of trkB expression produced in different adaptative responses of the nervous system where CREB is participating.

Persistent regional increases in brain-derived neurotrophic factor in the flurothyl model of epileptogenesis are dependent upon the kindling status of the animal

Brain-derived neurotrophic factor (BDNF) appears to be both regulated by and a regulator of epileptogenesis. In the flurothyl (HFE) model of kindling mice exposed to successive flurothyl trials over 8 days express a rapid, long-lasting reduction in generalized seizure threshold and a more slowly evolving change in seizure phenotype in response to subsequent flurothyl exposure. The BDNF genotype of particular mouse strains appears to influence the epileptogenic progression in this model. Thus, we hypothesized that BDNF signaling pathways are altered by flurothyl-induced seizures. Following HFE kindling, fully kindled (eight seizures) adult male C57BI/6J mice had significantly elevated whole brain BDNF levels through at least 28 days after their final seizure. Mice that received only four HFE seizures (not kindled) had elevated BDNF levels, but only at 1 day post-seizure (DPSz), while BDNF levels were not significantly altered in mice receiving just one HFE seizure at any time point studied. Regional expression patterns of BDNF in the hippocampus, hypothalamus, and frontal cortex were also elevated by one DPSz and returned to control values by 14 DPSz in mice that received four HFE seizures. No changes were seen in the cerebellum, striatum, or piriform cortex. In contrast, fully kindled mice had significantly elevated BDNF levels within the hippocampus, hypothalamus, neocortex, and striatum that remained elevated through at least 14 DPSz, while levels were unchanged in the cerebellum and piriform cortex. Regional results were confirmed using anti-BDNF immunohistochemistry (IHC). Despite changes in BDNF levels following HFE kindling, we were unable to demonstrate alterations either in full-length tyrosine kinase receptor B (TrkB) expression (Western blot and IHC) or in truncated TrkB (IHC) expression levels. Together, these data suggest a model of a positive feedback loop involving seizure activity and seizure number and persistently modified BDNF signaling pathways that influences seizure phenotypes within the HFE kindling paradigm. Thus, long-term elevations in BDNF may be responsible in part for epileptogenic processes and the development of human refractory epilepsies.

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.

Co-localization of brain-derived neurotrophic factor (BDNF) and wild-type huntingtin in normal and quinolinic acid-lesioned rat brain

Loss of huntingtin-mediated brain-derived neurotrophic factor (BDNF) gene transcription has been described in Huntington's disease (HD) [Zuccato et al. (2001) Science, 293, 493-498]. It has been shown that BDNF is synthesized in the pyramidal layer of cerebral cortex and released in the striatum [Altar et al. (1997) Nature, 389, 856-860; Conner et al. (1997) J. Neurosci., 17, 2295-2313]. Here we show the cellular localization of BDNF in huntingtin-containing neurons in normal rat brain; our double-label immunofluorescence study shows that huntingtin and BDNF are co-contained in approximately 99% of pyramidal neurons of motor cortex. In the striatum, huntingtin is expressed in 75% of neurons containing BDNF. In normal striatum we also show that BDNF is contained in cholinergic and in NOS-containing interneurons, which are relatively resistant to HD degeneration. Furthermore, we show a reduction in huntingtin and in BDNF immunoreactivity in cortical neurons after striatal excitotoxic lesion. Our data are confirmed by an ELISA study of BDNF and by a Western blot analysis of huntingtin in cortex of quinolic acid (QUIN)-lesioned hemispheres. In the lesioned striatum we describe that the striatal subpopulation of cholinergic neurons, surviving degeneration, contain BDNF. The finding that BDNF is contained in nearly all neurons that contain huntingtin in the normal cortex, along with the reduced expression of BDNF after QUIN injection of the striatum, shows that huntingtin may be required for BDNF production in cortex.

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.

Regulation of X-chromosome-linked inhibitor of apoptosis protein in kainic acid-induced neuronal death in the rat hippocampus

XIAP (X-chromosome-linked inhibitor of apoptosis protein) is an antiapoptotic protein which inhibits the activity of caspases and suppresses cell death. However, little is known about the presence and function of XIAP in the nervous system. Here we report that XIAP mRNA is expressed in developing and adult rat brain. Using a specific antibody, we observed XIAP-immunoreactive cells in different brain regions, among others, in the hippocampus and cerebral cortex. Kainic acid, which induces delayed cell death of specific neurons, increased the levels of XIAP in the CA3 region of hippocampus. XIAP was, however, largely absent in cells undergoing cell death, as shown by TUNEL labeling and staining for active caspase-3. In cultured hippocampal neurons, XIAP was initially upregulated by kainic acid and then degraded in a process blocked by the caspase-3 inhibitor DEVD. Similarly, recombinant XIAP is cleaved by active caspase-3 in vitro. The results show that there is biphasic regulation of XIAP in the hippocampus following kainic acid and that XIAP becomes a target for caspase-3 activated during cell death in the hippocampus. The degradation of XIAP by kainic acid contributes to neuronal cell death observed in vulnerable neurons of the hippocampus after caspase activation.

Depolarisation causes reciprocal changes in GFR(alpha)-1 and GFR(alpha)-2 receptor expression and shifts responsiveness to GDNF and neurturin in developing neurons

GDNF and neurturin are structurally related neurotrophic factors that promote the survival of many different kinds of neurons and influence axonal and dendritic growth and synaptic function. These diverse effects are mediated via multicomponent receptors consisting of the Ret receptor tyrosine kinase plus one of two structurally related GPI-linked receptors, GFR(alpha)-1 and GFR(alpha)-2. To ascertain how the expression of these receptors is regulated during development, we cultured embryonic neurons under different experimental conditions and used competitive RT/PCR to measure the levels of the mRNAs encoding these receptors. We found that depolarising levels of KCl caused a marked increase in GFR(alpha)-1 mRNA and a marked decrease in GFR(α)-2 mRNA in sympathetic, parasympathetic and sensory neurons. These changes were accompanied by increased responsiveness to GDNF and decreased responsiveness to neurturin, and were inhibited by L-type Ca(2+) channel antagonists, suggesting that they were due to elevated intracellular free-Ca(2+). There was no consistent effect of depolarising levels of KCl on ret mRNA expression, and neither GDNF nor neurturin significantly affected receptor expression. These results show that depolarisation has marked and opposing actions on the expression of GFR(α)-1 and GFR(α)-2, which are translated into corresponding changes in neuronal responsiveness to GDNF and neurturin. This provides evidence for a mechanism of regulating the neurotrophic factor responses of neurons by neural activity that has important implications for structural and functional plasticity in the developing nervous system.

A neurotrophic model of the development of the retinogeniculocortical pathway induced by spontaneous retinal waves

The development of the retinogeniculate pathway or the geniculocortical pathway, or both, occurs either before birth or before eye opening in many species. It is widely believed that spontaneous retinal activity could drive the segregation of afferents into eye-specific laminae or columns and the refinement of initially diffuse receptive fields and the emergence of orderly, retinotopic organization. We show that a recent computational model that generates a phenomenologically accurate representation of spontaneous retinal activity can indeed drive afferent segregation and, more particularly, topographic and receptive field refinement in the retinogeniculocortical system. We use a model of anatomical synaptic plasticity based on recent data suggesting that afferents might compete for limited amounts of retrograde neurotrophic factors (NTFs). We find that afferent segregation and receptive field formation are disrupted in the presence of exogenous NTFs. We thus predict that infusion of NTFs into the lateral geniculate nucleus should disrupt normal development and that the infusion of such factors into the striate cortex should disrupt receptive field refinement in addition to the well known disruption of ocular dominance column (ODC) formation. To demonstrate that the capacity of our model of plasticity to drive normal development is not restricted just to spontaneous retinal activity, we also use a coarse representation of visually evoked activity in some simulations. We find that such simulations can exhibit the formation of ODCs followed by their disappearance, reminiscent of the New World marmoset. A decrease in interocular correlations stabilizes these ODCs. Thus we predict that divergent strabismus should render marmoset ODCs stable into adulthood.

The mouse neurotrophin receptor trkB gene is transcribed from two different promoters

We have analysed a 7-kb region upstream of the mouse trkB coding sequence. The region showed promoter activity in transient transfection experiments and conferred tissue-specific expression to a reporter gene. Deletion analysis of this region demonstrated the presence of two alternative promoters named P1 and P2 that have been mapped by RNase protection. P1 has been located to 1.8 kb and P2 to 0.5 kb upstream of the trkB translation start site. From the P1 promoter, alternative splicing generates various transcripts. Interestingly, P2 is located in an intron of the transcripts produced from the P1 promoter. This peculiar arrangement results in different mRNA species that encode the same protein(s) but differ in their 5'-untranslated regions. In addition, transcription of the trkB locus results in two different trkB isoforms (kinase and truncated receptors) originated by alternative splicing of the mRNA, that possess differential spatial and temporal expression patterns. Using RT-PCR, we demonstrated that there was no linkage between promoter usage and alternative splicing, since transcripts initiated from each promoter encoded both kinase and truncated receptor proteins.

The neurotrophin receptors trkA, trkB and trkC are differentially regulated after excitotoxic lesion in rat striatum

In the present work, we examined the time-dependent changes in trkA, trkB and trkC mRNA levels induced by the injection of glutamate receptor agonists into the striatum. Changes in trk mRNAs induced by quinolinate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate or 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) were analyzed by a ribonuclease protection assay. All high-affinity neurotrophin receptors showed differential regulation after intrastriatal injury. Up-regulation of trkA expression was observed in kainate- or ACPD-injected striata at 10 and 24 h, respectively, whereas quinolinate injection induced down-regulation between 4 and 6 h after injury. Interestingly, all the excitatory amino acid receptor agonists induced up-regulation of trkB-kinase mRNA levels. This increase was maximal between 2 and 4 h after injection except in kainate injected striata, which showed the peak of expression at 10 h. In contrast, no changes in trkC mRNA expression were observed after striatal excitotoxic injury. In conclusion, our results show that trk receptor mRNA levels are differentially regulated by excitatory amino acid receptor agonists in the striatum, suggesting that changes in the levels of neurotrophin receptors might be involved either in synaptic plasticity processes or in neuronal protection in the striatal excitotoxic paradigm.

Compartmental expression of trkB receptor protein in the developing striatum

To investigate the role of neurotrophins in the initial formation of striatal patch versus matrix, the spatial and temporal expression of trkB receptors was examined using immunohistochemistry. Polyclonal antibodies, against the C-terminus or the tyrosine kinase domain, revealed trkB-immunoreactive cells and fibers localized to patches beginning on embryonic day 19 in the rat, which co-localized with patchy dopamine fibers, substance P-immunoreactive neurons and glutamate receptors. Patchy striatal trkB expression was maintained after lesioning the nigrostriatal dopamine system. The patchy trkB distribution persisted through postnatal day 14, then became more homogeneous at the same time that nigrostriatal afferents become homogeneous. Later in development, trkB immunoreactivity was most intense in a subpopulation of large striatal cells that were similar in size and frequency to those immunoreactive for choline acetyltransferase. The spatiotemporal expression of trkB receptor in phenotypically distinct striatal patches, as well as evidence that neurotrophins regulate expression of neuronal phenotypic markers during development, may indicate a convergence of neurotrophins and afferent innervation on to future patch cells that may regulate the establishment of striatal compartmentalization.

Competition for neurotrophic factors: ocular dominance columns

Activity-dependent competition between afferents in the primary visual cortex of many mammals is a quintessential feature of neuronal development. From both experimental and theoretical perspectives, understanding the mechanisms underlying competition is a significant challenge. Recent experimental work suggests that geniculocortical afferents might compete for retrograde neurotrophic factors. We show that a mathematically well-characterized model of retrograde neurotrophic interactions, in which the afferent uptake of neurotrophic factors is activity-dependent and in which the average level of uptake determines the complexity of the axonal arbors of afferents, permits the anatomical segregation of geniculocortical afferents into ocular dominance columns. The model induces segregation provided that the levels of neurotrophic factors available either by activity-independent release from cortical cells or by exogenous cortical infusion are not too high; otherwise segregation breaks down. We show that the model exhibits changes in ocular dominance column periodicity in response to changes in interocular image correlations and that the model predicts that changes in intraocular image correlations should also affect columnar periodicity.

Apoptosis of hippocampal neurons after amygdala kindled seizures

Seizure-induced neuronal damage may involve both excitotoxic and apoptotic (programmed cell death) mechanisms. In the present study, we used an amygdala kindled seizure model to study whether apoptotic cell death occurs. To evaluate apoptosis, we counted the numbers of cells that had DNA fragments labeled at the 3' end with digoxigenin using terminal transferase (ApopTag, Oncor). Additionally, the expression of Bax and Bcl-2, two genes associated with apoptotic cell death, was also measured following kindled seizures. We found that the number of ApopTag-positive cells in the hippocampus increased 30.4% after one kindled seizure and 82.5% after 20 seizures compared to sham controls. The ApopTag-labeled cells could be mainly interneurons of the hippocampal formation, although additional studies are required. Preferential vulnerability of inhibitory interneurons is consistent with previous studies on seizure-induced cell loss. These results, coupled with our findings that the ratio of Bax/Bcl-2 expression is increased in the hippocampus by seizures, suggest that apoptosis of hippocampal interneurons may lead to dysinhibition in the hippocampus and increased seizure susceptibility.

Neuronal expression of zinc finger transcription factor REST/NRSF/XBR gene

The identification of a common cis-acting silencer element, a neuron-restrictive silencer element (NRSE), in multiple neuron-specific genes, together with the finding that zinc finger transcription factor REST/NRSF/XBR could confer NRSE-mediated silencing in non-neuronal cells, suggested that REST/NRSF/XBR is a master negative regulator of neurogenesis. Here we show that, although REST/NRSF/XBR expression decreases during neuronal development, it proceeds in the adult nervous system. In situ hybridization analysis revealed neuronal expression of rat REST/NRSF/XBR mRNA in adult brain, with the highest levels in the neurons of hippocampus, pons/medulla, and midbrain. The glutamate analog kainic acid increased REST/NRSF/XBR mRNA levels in various hippocampal and cortical neurons in vivo, suggesting that REST/NRSF/XBR has a role in neuronal activity-implied processes. Several alternatively spliced REST/NRSF/XBR mRNAs encoding proteins with nine, five, or four zinc finger motifs are transcribed from REST/NRSF/XBR gene. Two of these transcripts are generated by neuron-specific splicing of a 28-bp-long exon. Rat REST/NRSF/XBR protein isoforms differ in their DNA binding specificities; however, all mediate repression in transient expression assays. Our data suggest that REST/NRSF/XBR is a negative regulator rather than a transcriptional silencer of neuronal gene expression and counteracts with positive regulators to modulate target gene expression quantitatively in different cell types, including neurons.

Dynamic changes of brain-derived neurotrophic factor protein levels in the rat forebrain after single and recurring kindling-induced seizures

Regional levels of brain-derived neurotrophic factor protein were measured in the rat brain using enzyme immunoassay following seizures evoked by hippocampal kindling stimulations. One stimulation, which induced a brief, single episode of epileptiform activity in hippocampus and piriform cortex but not in parietal cortex or striatum, gave rise to a transient increase of brain-derived neurotrophic factor levels in dentate gyrus and CA3 region and a decrease in piriform cortex. After 40 rapidly recurring seizures, with epileptiform activity also involving parietal cortex and striatum, increases were observed in dentate gyrus, CA3 and CA1 regions, piriform cortex and striatum. Maximum levels were reached at 2-24 h and brain-derived neurotrophic factor then returned to baseline except in dentate gyrus, where elevated protein content was sustained for four days. The differential regulation of brain-derived neurotrophic factor protein levels in various forebrain structures, which only partly correlates to messenger RNA changes, could indicate regional differences in protein release, antero- or retrograde transport, or brain-derived neurotrophic factor promotor activation. The dynamic changes of brain-derived neurotrophic factor levels in regions involved in the generation and spread of seizure activity may regulate excitability and trigger plastic responses in the post-seizure period.

Increased expression of trkB mRNA in rat caudate--putamen following 6-OHDA lesions of the nigrostriatal pathway

The tyrosine kinase receptors trkB and trkC are essential components of the high-affinity receptors for members of the neurotrophin family, including brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3). Both neurotrophin receptor mRNAs are broadly distributed throughout the caudate-putamen. In animal models of Parkinson's disease, loss of the ventral mesencephalic dopamine projection to the striatum has been shown to alter the expression of several striatal peptides, neurotransmitter-synthesizing enzymes and receptors. To determine if expression of trkB and/or trkC striatal mRNAs is also regulated by the integrity of the dopaminergic afferents, adult rats were given unilateral injections of 6-hydroxydopamine (6-OHDA), a selective catecholamine neurotoxin, or vehicle into the right ascending medial forebrain bundle. Following a 2 week survival period, in situ hybridization with 35S-labelled cRNA probes for the kinase-specific, full-length form of trkB mRNA and all forms of trkC mRNA was performed in striatal sections. A significant increase in the hybridization density for trkB mRNA was observed in the caudate-putamen ipsilateral to the 6-OHDA injection, compared with the uninjected control side (P < 0.001). In contrast, no alteration in the hybridization density for trkC mRNA was observed in the striatum of 6-OHDA-treated rats. No alterations in trkB or trkC mRNA levels were observed in the striata of vehicle-treated animals. These data suggest that midbrain dopaminergic afferents regulate the expression of trkB mRNA in the caudate-putamen. Alternatively, since dopaminergic neurons of the ventral mesencephalon express BDNF mRNA, the up-regulation of striatal trkB mRNA may reflect a compensatory response by striatal neurons due to a loss of anterogradely and/or retrogradely derived trophic support from the ventral midbrain.