Neuronal activity and brain-derived neurotrophic factor regulate the density of inhibitory synapses in organotypic slice cultures of postnatal hippocampus

Status epilepticus alters neurogenesis and decreases the number of GABAergic neurons in the septal dentate gyrus of 9-day-old rats at the early phase of epileptogenesis

The effects of a prolonged seizure, i.e. status epilepticus (SE), on neurogenesis of dentate granule cells (DGCs) in the immature dentate gyrus (DG) and possible changes in the phenotypes of the newborn neurons have remained incompletely characterized. We have now studied neurogenesis of DGCs in 9-day-old (postnatal, P9) rats 1 week after kainate (KA)-induced SE using 5-bromo-2-deoxyuridine (BrdU) immunostaining. The phenotype characterization of the newborn cells was carried out by immunofluorescence double labeling using doublecortin (DCX) and nestin as markers for immature cells, and glial fibrillary acid protein (GFAP) as a marker for glial cells. Newborn GABAergic neurons were further identified with antibodies for parvalbumin, glutamate decarboxylase 67 (GAD67), and the GABAA receptor α1 subunit, and mRNA expression of GABAergic and immature neurons was measured with quantitative real-time PCR (qPCR) in the DG. Our results show that the number of newborn as well as GABAergic neurons was significantly decreased after SE in the superior blade of the septal DG. The majority of the newborn BrdU-stained neurons co-expressed DCX, but neither nestin nor GFAP. In both experimental groups, newborn neurons were frequently localized in close contact, but not co-localized, with the cells positively stained for the GABAergic cell markers. Nestin and calretinin mRNA expression were significantly increased after SE. Our results suggest that SE-induced disruption of DGC neurogenesis and decreased number of GABAergic neurons could modify the connectivity between these cells and disturb the maturation of the GABAergic neurotransmission in the immature DG at the early epileptogenic phase.

BDNF deregulation in Rett syndrome

BDNF is the best-characterized neurotrophin in terms of its gene structure and modulation, secretion processing, and signaling cascades following its release. In addition to diverse features at the genetic and molecular levels, the abundant expression in several regions of the central nervous system has implicated BDNF as a potent modulator in many aspects of neuronal development, as well as synaptic transmission and plasticity. Impairments in any of these critical functions likely contribute to a wide array of neurodevelopmental, neurodegenerative, and neuropsychiatric diseases. In this review, we focus on a prevalent neurodevelopmental disorder, Rett syndrome (RTT), which afflicts 1:15,000 women world-wide. We describe the consequences of loss-of-function mutations in the gene encoding the transcription factor methyl-CpG binding protein 2 (MeCP2) in RTT, and then elaborate on the current understanding of how MeCP2 controls BDNF expression. Finally, we discuss the literature regarding alterations in BDNF levels in RTT individuals and MeCP2-based mouse models, as well as recent progress in searching for rational therapeutic interventions. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Nicotine restores Wt-like levels of reelin and GAD67 gene expression in brain of heterozygous reeler mice

Important reduction of reelin, a neural development- and plasticity-associated protein, and glutamic acid decarboxylase (GAD67) are reported in brains of schizophrenic patients. These individuals are consistently engaged in tobacco smoking and nicotine is thought to alleviate negative behavioral symptoms or cognitive alterations. In mouse brain, nicotine has been shown to reduce GAD67 promoter methylation and increase its transcription. We assessed the effects of administration of nicotine (1 mg/kg s.c.) for 6 days, in male mice heterozygous for reelin (HRM), a putative model for symptoms related to schizophrenia. Expression of reelin, GAD67 and brain-derived neurotrophic factor (BDNF) was measured in different brain areas. RNA expression analysis evidenced genotype-related changes, with a marked reduction in reelin and GAD67 gene expression in prefrontal cortex, hippocampus, cerebellum, and striatum from HRM. Nicotine treatment selectively reversed the HRM-related phenotype in most brain areas and increased BDNF gene expression in cortex and hippocampus of both genotypes. Locomotor performance in their home cage revealed that HRM subjects were characterized by general hyperactivity; with nicotine administration restoring WT-like levels of locomotion. These findings are interpreted within the hypothesis of pre-existing vulnerability (based on haploinsufficiency of reelin) to brain and behavioral disorders and regulative effects associated with nicotine exposure.

Transient epileptiform signaling during neuronal network development: regulation by external stimulation and bimodal GABAergic activity

A predominance of excitatory activity, with protracted appearance of inhibitory activity, accompanies cortical neuronal development. It is unclear whether or not inhibitory neuronal activity is solicited exclusively by excitatory neurons or whether the transient excitatory activity displayed by developing GABAergic neurons contributes to an excitatory threshold that fosters their conversion to inhibitory activity. We addressed this possibility by culturing murine embryonic neurons on multi-electrode arrays. A wave of individual 0.2-0.4 mV signals ("spikes") appeared between approx. 20-30 days in culture, then declined. A transient wave of high amplitude (>0.5 mV) epileptiform activity coincided with the developmental decline in spikes. Bursts (clusters of ≥3 low-amplitude spikes within 0.7s prior to returning to baseline) persisted following this decline. Addition of the GABAergic antagonist bicuculline initially had no effect on signaling, consistent with delayed development of GABAergic synapses. This was followed by a period in which bicuculline inhibited overall signaling, confirming that GABAergic neurons initially display excitatory activity in ex vivo networks. Following the transient developmental wave of epileptiform signaling, bicuculline induced a resurgence of epileptiform signaling, indicating that GABAergic neurons at this point displayed inhibitory activity. The appearance of transition after the developmental and decline of epileptiform activity, rather than immediately after the developmental decline in lower-amplitude spikes, suggests that the initial excitatory activity of GABAergic neurons contributes to their transition into inhibitory neurons, and that inhibitory GABAergic activity is essential for network development. Prior studies indicate that a minority (25%) of neurons in these cultures were GABAergic, suggesting that inhibitory neurons regulate multiple excitatory neurons. A similar robust increase in signaling following cessation of inhibitory activity in an artificial neural network containing 20% inhibitory neurons supported this conclusion. Even a minor perturbation in GABAergic function may therefore foster initiation and/or amplification of seizure activity, as well as perturbations in long-term potentiation.

Neural circuit development in the mammalian cochlea

The organ of Corti, the sensory epithelium of the mammalian auditory system, uses afferent and efferent synapses for encoding auditory signals and top-down modulation of cochlear function. During development, the final precisely ordered sensorineural circuit is established following excessive formation of afferent and efferent synapses and subsequent refinement. Here, we review the development of innervation of the mouse organ of Corti and its regulation.

A polymorphism associated with depressive disorders differentially regulates brain derived neurotrophic factor promoter IV activity

BACKGROUND

Changes in brain derived neurotrophic factor (BDNF) expression have been associated with mood disorders and cognitive dysfunction. Transgenic models that overexpress or underexpress BDNF demonstrate similar deficits in cognition and mood. We explored the hypothesis that BDNF expression is controlled by balancing the activity of BDNF promoter IV (BP4) with a negative regulatory region containing a polymorphism associated with cognitive dysfunction and mood disorders.

METHODS

We used comparative genomics, transgenic mouse production, and magnetofection of primary neurons with luciferase reporters and signal transduction agonist treatments to identify novel polymorphic cis-regulatory regions that control BP4 activity.

RESULTS

We show that BP4 is active in the hippocampus, the cortex, and the amygdala and responds strongly to stimuli such as potassium chloride, lithium chloride, and protein kinase C agonists. We also identified a highly conserved sequence 21 kilobase 5' of BP4 that we called BE5.2, which contains rs12273363, a polymorphism associated with decreased BDNF expression, mood disorders, and cognitive decline. BE5.2 modulated the ability of BP4 to respond to different stimuli. Intriguingly, the rarer disease associated allele, BE5.2(C), acted as a significantly stronger repressor of BP4 activity than the more common BE5.2(T) allele.

CONCLUSIONS

This study shows that the C allele of rs12273363, which is associated with mood disorder, modulates BP4 activity in an allele-specific manner following cell depolarization or the combined activity of protein kinase A and protein kinase C pathways. The relevance of these findings to the role of BDNF misexpression in mood disorders and cognitive decline is discussed.

Functional alterations in GABAergic fast-spiking interneurons in chronically injured epileptogenic neocortex

Progress toward developing effective prophylaxis and treatment of posttraumatic epilepsy depends on a detailed understanding of the basic underlying mechanisms. One important factor contributing to epileptogenesis is decreased efficacy of GABAergic inhibition. Here we tested the hypothesis that the output of neocortical fast-spiking (FS) interneurons onto postsynaptic targets would be decreased in the undercut (UC) model of chronic posttraumatic epileptogenesis. Using dual whole-cell recordings in layer IV barrel cortex, we found a marked increase in the failure rate and a very large reduction in the amplitude of unitary inhibitory postsynaptic currents (uIPSCs) from FS cells to excitatory regular spiking (RS) neurons and neighboring FS cells. Assessment of the paired pulse ratio and presumed quantal release showed that there was a significant, but relatively modest, decrease in synaptic release probability and a non-significant reduction in quantal size. A reduced density of boutons on axons of biocytin-filled UC FS cells, together with a higher coefficient of variation of uIPSC amplitude in RS cells, suggested that the number of functional synapses presynaptically formed by FS cells may be reduced. Given the marked reduction in synaptic strength, other defects in the presynaptic vesicle release machinery likely occur, as well.

Transcriptional control of Gad2

Gad2 encodes GAD65, which is present preferentially in presynaptic terminals for synthesis of GABA for vesicle release. Gad2 is a regulatory target of cell activities in various brain functions and in GABA perturbation-related neurological diseases. However, our understanding of how Gad2 is transcriptionally regulated and how Gad2 transcription responds to changing cell environment under these conditions is still limited. This review discusses recent advances in the regulatory mechanisms for Gad2 transcription and highlights the characteristics of TATA-less Gad2 promoters and regulation of Gad2 transcription by CREB and by activity-dependent epigenetic modification of the chromatin structure in regulatory elements of the Gad2 gene.

The role of BDNF in the pathophysiology and treatment of schizophrenia

Brain derived neurotrophic factor (BDNF) has been associated with the pathophysiology of schizophrenia (SCZ). However, it remains unclear whether alterations in BDNF observed in patients with SCZ are a core part of disease neurobiology or a consequence of treatment. In this manuscript we review existing knowledge relating the function of BDNF to synaptic transmission and neural plasticity and the relationship between BDNF and both pharmacological and non-pharmacological treatments for SCZ. With regards to synaptic transmission, exposure to BDNF or lack of this neurotrophin results in alteration to both excitatory and inhibitory synapses. Many authors have also evaluated the effects of both pharmacological and non-pharmacological treatments for SCZ in BDNF and despite some controversial results, it seems that medicated and non-medicated patients present with lower levels of BDNF when compared to controls. Further data suggests that typical antipsychotics may decrease BDNF expression whereas mixed results have been obtained with atypical antipsychotics. The authors found few studies reporting changes in BDNF after non-pharmacological treatments for SCZ, so the existing evidence in this area is limited. Although the study of BDNF provides some new insights into understanding of the pathophysiology and treatment of SCZ, additional work in this area is needed.

Interactions of excitatory and inhibitory feedback topologies in facilitating pattern separation and retrieval

Within the brain, the interplay between connectivity patterns of neurons and their spatiotemporal dynamics is believed to be intricately linked to the bases of behavior, such as the process of storing, consolidating, and retrieving memory traces. Memory is believed to be stored in the synaptic patterns of anatomical circuitry in the form of increased connectivity densities within subpopulations of neurons. At the same time, memory recall is thought to correspond to activation of discrete areas of the brain corresponding to those memories. Such regional subpopulations can selectively activate during memory recall or retrieval, signifying the process of accessing a single memory or concept. It has been shown previously that recovery of single memory activity patterns is mediated by global neuromodulation signifying transition into different cognitive states such as sleep or awake exploration. We examine how underlying topology can affect memory awake activation and sleep reactivation when such memories share increasing proportions of neurons. The results show that while single memory activation is diminished with increased overlap, pattern separation can be recovered by offsetting excitatory associations between two memories with targeted and heterogeneous inhibitory feedback. Such findings point to the importance of excitatory-to-inhibitory current balance at both the global and local levels in the context of memory retrieval and replay, and highlight the role of network topology in memory management processes.

The neuronal PAS domain protein 4 (Npas4) is required for new and reactivated fear memories

The Neuronal PAS domain protein 4 (Npas4) is a neuronal activity-dependent immediate early gene that has recently been identified as a transcription factor which regulates the transcription of genes that control inhibitory synapse development and synaptic plasticity. The role Npas4 in learning and memory, however, is currently unknown. Here, we systematically examine the role of Npas4 in auditory Pavlovian fear conditioning, an amygdala-dependent form of emotional learning. In our first series of experiments, we show that Npas4 mRNA and protein are regulated in the rat lateral nucleus of the amygdala (LA) in a learning-dependent manner. Further, knockdown of Npas4 protein in the LA via adeno-associated viral (AAV) mediated gene delivery of RNAi was observed to impair fear memory formation, while innate fear and the expression of fear memory were not affected. In our second series of experiments, we show that Npas4 protein is regulated in the LA by retrieval of an auditory fear memory and that knockdown of Npas4 in the LA impairs retention of a reactivated, but not a non-reactivated, fear memory. Collectively, our findings provide the first comprehensive look at the functional role of Npas4 in learning and memory.

The contribution of GABAergic dysfunction to neurodevelopmental disorders

GABA (γ-aminobutyric acid) is the major inhibitory neurotransmitter in the brain. The GABAergic system is indispensable for maintaining the balance between excitation and inhibition (E/I balance) required for normal neural circuit function. E/I imbalances that result from perturbations in the development of this system, ranging from the generation of inhibitory neurons to the formation of their synaptic connections, have been implicated in several neurodevelopmental disorders. In this review, we discuss how impairments at different stages in GABAergic development can lead to disease states. We also highlight recent studies which show that modulation of the GABAergic system can successfully reverse cognitive deficits in disease models and suggest that therapeutic strategies targeting the GABAergic system could be effective in treating neurodevelopmental disorders.

Emerging themes in GABAergic synapse development

Glutamatergic synapse development has been rigorously investigated for the past two decades at both the molecular and cell biological level yet a comparable intensity of investigation into the cellular and molecular mechanisms of GABAergic synapse development has been lacking until relatively recently. This review will provide a detailed overview of the current understanding of GABAergic synapse development with a particular emphasis on assembly of synaptic components, molecular mechanisms of synaptic development, and a subset of human disorders which manifest when GABAergic synapse development is disrupted. An unexpected and emerging theme from these studies is that glutamatergic and GABAergic synapse development share a number of overlapping molecular and cell biological mechanisms that will be emphasized in this review.

A key mechanism underlying sensory experience-dependent maturation of neocortical GABAergic circuits in vivo

Mechanisms underlying experience-dependent refinement of cortical connections, especially GABAergic inhibitory circuits, are unknown. By using a line of mutant mice that lack activity-dependent BDNF expression (bdnf-KIV), we show that experience regulation of cortical GABAergic network is mediated by activity-driven BDNF expression. Levels of endogenous BDNF protein in the barrel cortex are strongly regulated by sensory inputs from whiskers. There is a severe alteration of excitation and inhibition balance in the barrel cortex of bdnf-KIV mice as a result of reduced inhibitory but not excitatory conductance. Within the inhibitory circuits, the mutant barrel cortex exhibits significantly reduced levels of GABA release only from the parvalbumin-expressing fast-spiking (FS) interneurons, but not other interneuron subtypes. Postnatal deprivation of sensory inputs markedly decreased perisomatic inhibition selectively from FS cells in wild-type but not bdnf-KIV mice. These results suggest that postnatal experience, through activity-driven BDNF expression, controls cortical development by regulating FS cell-mediated perisomatic inhibition in vivo.

Positive feedback regulation between gamma-aminobutyric acid type A (GABA(A)) receptor signaling and brain-derived neurotrophic factor (BDNF) release in developing neurons

During the early development of the nervous system, γ-aminobutyric acid (GABA) type A receptor (GABA(A)R)-mediated signaling parallels the neurotrophin/tropomyosin-related kinase (Trk)-dependent signaling in controlling a number of processes from cell proliferation and migration, via dendritic and axonal outgrowth, to synapse formation and plasticity. Here we present the first evidence that these two signaling systems regulate each other through a complex positive feedback mechanism. We first demonstrate that GABA(A)R activation leads to an increase in the cell surface expression of these receptors in cultured embryonic cerebrocortical neurons, specifically at the stage when this activity causes depolarization of the plasma membrane and Ca(2+) influx through L-type voltage-gated Ca(2+) channels. We further demonstrate that GABA(A)R activity triggers release of the brain-derived neurotrophic factor (BDNF), which, in turn by activating TrkB receptors, mediates the observed increase in cell surface expression of GABA(A)Rs. This BDNF/TrkB-dependent increase in surface levels of GABA(A)Rs requires the activity of phosphoinositide 3-kinase (PI3K) and protein kinase C (PKC) and does not involve the extracellular signal-regulated kinase (ERK) 1/2 activity. The increase in GABA(A)R surface levels occurs due to an inhibition of the receptor endocytosis by BDNF, whereas the receptor reinsertion into the plasma membrane remains unaltered. Thus, GABA(A)R activity is a potent regulator of the BDNF release during neuronal development, and at the same time, it is strongly enhanced by the activity of the BDNF/TrkB/PI3K/PKC signaling pathway.

Latent inhibition-related dopaminergic responses in the nucleus accumbens are disrupted following neonatal transient inactivation of the ventral subiculum

Schizophrenia would result from a defective connectivity between several integrative regions as a consequence of neurodevelopmental failure. Various anomalies reminiscent of early brain development disturbances have been observed in patients' left ventral subiculum of the hippocampus (SUB). Numerous data support the hypothesis of a functional dopaminergic dysregulation in schizophrenia. The common target structure for the action of antipsychotics appears to be a subregion of the ventral striatum, the dorsomedial shell part of the nucleus accumbens. Latent inhibition, a cognitive marker of interest for schizophrenia, has been found to be disrupted in acute patients. The present study set out to investigate the consequences of a neonatal functional inactivation of the left SUB by tetrodotoxin (TTX) in 8-day-old rats for the latent inhibition-related dopaminergic responses, as monitored by in vivo voltammetry in freely moving adult animals (11 weeks) in the left core and dorsomedial shell parts of the nucleus accumbens in an olfactory aversion procedure. Results obtained during the retention session of a three-stage latent inhibition protocol showed that the postnatal unilateral functional blockade of the SUB was followed in pre-exposed TTX-conditioned adult rats by a disruption of the behavioral expression of latent inhibition and induced a total and a partial reversal of the latent inhibition-related dopaminergic responses in the dorsomedial shell and core parts of the nucleus accumbens, respectively. The present data suggest that neonatal inactivation of the SUB has more marked consequences for the dopaminergic responses recorded in the dorsomedial shell part, than in the core part of the nucleus accumbens. These findings may provide new insight into the pathophysiology of schizophrenia.

Long-term plasticity at inhibitory synapses

Experience-dependent modifications of neural circuits and function are believed to heavily depend on changes in synaptic efficacy such as LTP/LTD. Hence, much effort has been devoted to elucidating the mechanisms underlying these forms of synaptic plasticity. Although most of this work has focused on excitatory synapses, it is now clear that diverse mechanisms of long-term inhibitory plasticity have evolved to provide additional flexibility to neural circuits. By changing the excitatory/inhibitory balance, GABAergic plasticity can regulate excitability, neural circuit function and ultimately, contribute to learning and memory, and neural circuit refinement. Here we discuss recent advancements in our understanding of the mechanisms and functional relevance of GABAergic inhibitory synaptic plasticity.

Excitatory synaptic activity is associated with a rapid structural plasticity of inhibitory synapses on hippocampal CA1 pyramidal cells

Synaptic activity, such as long-term potentiation (LTP), has been shown to induce morphological plasticity of excitatory synapses on dendritic spines through the spine head and postsynaptic density (PSD) enlargement and reorganization. Much less, however, is known about activity-induced morphological modifications of inhibitory synapses. Using an in vitro model of rat organotypic hippocampal slice cultures and electron microscopy, we studied activity-related morphological changes of somatic inhibitory inputs triggered by a brief oxygen-glucose deprivation (OGD) episode, a condition associated with a synaptic enhancement referred to as anoxic LTP and a structural remodeling of excitatory synapses. Three-dimensional reconstruction of inhibitory axo-somatic synapses at different times before and after brief OGD revealed important morphological changes. The PSD area significantly and markedly increased at synapses with large and complex PSDs, but not at synapses with simple, macular PSDs. Activity-related changes of PSD size and presynaptic bouton volume developed in a strongly correlated manner. Analyses of single and serial sections further showed that the density of inhibitory synaptic contacts on the cell soma did not change within 1 h after OGD. In contrast, the proportion of the cell surface covered with inhibitory PSDs, as well as the complexity of these PSDs significantly increased, with less macular PSDs and more complex, segmented shapes. Together, these data reveal a rapid activity-related restructuring of somatic inhibitory synapses characterized by an enlargement and increased complexity of inhibitory PSDs, providing a new mechanism for a quick adjustment of the excitatory-inhibitory balance. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Regulation of inhibitory neurotransmission by the scaffolding protein ankyrin repeat-rich membrane spanning/kinase D-interacting substrate of 220 kDa

Scaffolding proteins play a critical role in the proper development and function of neural circuits. In contrast to the case for excitatory circuits, in which the role of several scaffolding proteins has been characterized, less is known about the scaffolding proteins that regulate inhibitory neurotransmission. The ankyrin repeat-rich membrane spanning (ARMS)/kinase D-interacting substrate of 220 kDa (Kidins220) scaffolding protein is expressed during the establishment of γ-aminobutyric acid (GABA) neurotransmission and is highly regulated by activity. To evaluate whether ARMS/Kidins220 expression affects GABAergic neurotransmission, we modified the ARMS/Kidins220 levels during the period of its maximum expression in culture (DIV 1-10). Whereas a decrease in ARMS/Kidins220 levels suppressed GABAergic neurotransmission, overexpression of ARMS/Kidins220 produced an increase in GABAergic neurotransmission in hippocampal neurons. In addition, we found that ARMS/Kidins220 regulates GABAergic neurotransmission by a presynaptic mechanism. Our results suggest that the ARMS/Kidins220 scaffold protein plays a critical role in the regulation of inhibitory transmission in hippocampal neurons.

Excitatory input onto hilar somatostatin interneurons is increased in a chronic model of epilepsy

The density of somatostatin (SOM)-containing GABAergic interneurons in the hilus of the dentate gyrus is significantly decreased in both human and experimental temporal lobe epilepsy. We used the pilocarpine model of status epilepticus and temporal lobe epilepsy in mice to study anatomical and electrophysiological properties of surviving somatostatin interneurons and determine whether compensatory functional changes occur that might offset loss of other inhibitory neurons. Using standard patch-clamp techniques and pipettes containing biocytin, whole cell recordings were obtained in hippocampal slices maintained in vitro. Hilar SOM cells containing enhanced green fluorescent protein (EGFP) were identified with fluorescent and infrared differential interference contrast video microscopy in epileptic and control GIN (EGFP-expressing Inhibitory Neurons) mice. Results showed that SOM cells from epileptic mice had 1) significant increases in somatic area and dendritic length; 2) changes in membrane properties, including a small but significant decrease in resting membrane potential, and increases in time constant and whole cell capacitance; 3) increased frequency of slowly rising spontaneous excitatory postsynaptic currents (sEPSCs) due primarily to increased mEPSC frequency, without changes in the probability of release; 4) increased evoked EPSC amplitude; and 5) increased spontaneous action potential generation in cell-attached recordings. Results suggest an increase in excitatory innervation, perhaps on distal dendrites, considering the slower rising EPSCs and increased output of hilar SOM cells in this model of epilepsy. In sum, these changes would be expected to increase the inhibitory output of surviving SOM interneurons and in part compensate for interneuronal loss in the epileptogenic hippocampus.

Synaptogenesis in adult-generated hippocampal granule cells is affected by behavioral experiences

Adult-generated hippocampal immature neurons play a functional role after integration in functional circuits. Previously, we found that hippocampus-dependent learning in Morris water maze affects survival of immature neurons, even before they are synaptically contacted. Beside learning, this task heavily engages animals in physical activity in form of swimming; physical activity enhances hippocampal neurogenesis. In this article, the effects of training in Morris water maze apparatus on the synapse formation onto new neurons in hippocampus dentate gyrus and on neuronal maturation were investigated in adult rats. Newborn cells were identified using retroviral GFP-expressing virus infusion. In the first week after virus infusion, rats were trained in Morris water maze apparatus in three different conditions (spatial learning, cue test, and swimming). Properties of immature neurons and their synaptic response to perforant pathway stimulation were electrophysiologically investigated early during neuronal maturation. In controls, newborn cells showing GABAergic and glutamatergic responses were found for the first time at 8 and 10 days after mitosis, respectively; no cell with glutamatergic response only was found. Twelve days after virus infusion almost all GFP-positive cells showed both synaptic responses. The main result we found was the anticipated appearance of GABAergic synapses at 6 days in learner, cued and swimmer rats, supported also by immunohistochemical result. Swimmer rats showed the highest percentage of GFP-positive neurons with glutamatergic response at 10 and 12 days postmitosis. Moreover, primary dendrites were more numerous at 7 days in learner, cued and swimmer rats and swimmer rats showed the greatest dendritic tree complexity at 10 days. Finally, voltage-dependent Ca(2+) current was found in a larger number of newborn neurons at 7 days postinfusion in learner, cued and swimmer rats. In conclusion, experiences involving physical activity contextualized in an exploring behavior affect synaptogenesis in adult-generated cells and their early stages of maturation.

MeCP2 is required for normal development of GABAergic circuits in the thalamus

Methyl-CpG binding protein 2 (MeCP2) is highly expressed in neurons in the vertebrate brain, and mutations of the gene encoding MeCP2 cause the neurodevelopmental disorder Rett syndrome. This study examines the role of MeCP2 in the development and function of thalamic GABAergic circuits. Whole cell recordings were carried out in excitatory neurons of the ventrobasal complex (VB) of the thalamus and in inhibitory neurons of the reticular thalamic nucleus (RTN) in acute brain slices from mice aged P6 through P23. At P14-P16, the number of quantal GABAergic events was decreased in VB neurons but increased in RTN neurons of Mecp2-null mice, without any change in the amplitude or kinetics of quantal events. There was no difference between mutant and wild-type mice in paired-pulse ratios of evoked GABAergic responses in the VB or the RTN. On the other hand, unitary responses evoked by minimal stimulation were decreased in the VB but increased in the RTN of mutants. Similar changes in the frequency of quantal events were observed at P21-P23 in both the VB and RTN. At P6, however, quantal GABAergic transmission was altered only in the VB not the RTN. Immunostaining of vesicular GABA transporter showed opposite changes in the number of GABAergic synaptic terminals in the VB and RTN of Mecp2-null mice at P18-P20. The loss of MeCP2 had no significant effect on intrinsic properties of RTN neurons recorded at P15-P17. Our findings suggest that MeCP2 differentially regulates the development of GABAergic synapses in excitatory and inhibitory neurons in the thalamus.

BDNF moderates early environmental risk factors for anxiety in mouse

Anxiety is known to be influenced by both adverse childhood experiences and genetic susceptibility factors. A polymorphism in the brain-derived neurotrophic factor (BDNF) gene modulates the association between adverse early experiences and risk for anxiety and depression in adulthood. An animal model of this gene-by-environment risk factor is lacking. Using two different early environmental manipulations, we found that a heterozygous null mutation in the mouse BDNF gene moderated the long-term effect of maternal care on innate anxiety behavior. Although changes in maternal care were associated with mild changes in anxiety in wild-type mice, this effect was magnified in heterozygous null BDNF mice with high- and low-maternal care associated with low and high levels, respectively, of avoidance behavior as measured in the open field and elevated plus maze tests. These data argue for an increased sensitivity to early environmental influences of mice with reduced BDNF function and support the important role of this neurotrophic factor in the developmental plasticity of brain circuits controlling anxiety.

Behavioral and neurochemical characterization of mice deficient in the N-type Ca2+ channel alpha1B subunit

N-type voltage-dependent calcium channels (VDCCs) play an important role in neurotransmission, synaptic plasticity, and brain development. They are composed of several subunits named alpha(1), alpha(2), delta, beta and gamma. The alpha(1) subunit is essential for channel functions and determines fundamental channel properties. Since N-type VDCC are critically involved in the release of neurotransmitters and clinical relevance, we predicted that alpha(1) subunit KO mice would show several alterations in behavior. In the present study, we investigated neuronal functions in mice lacking the alpha(1B) (Ca(V)2.2) subunit of the N-type calcium channels. Ca(V)2.2(-/-) mice exhibited a significant increase in locomotion on an activity wheel during the dark phase. Furthermore, when challenged with apomorphine, mutant mice showed enhanced locomotor activity. Cognitive functions were examined using a Y-maze task for short-term memory and a passive avoidance task for long-term memory. The Y-maze revealed no differences in spontaneous alternation behavior between mutant and wild-type mice. The passive avoidance test revealed that the latency time in mutant mice was significantly decreased. The mutant mice showed prepulse inhibition deficits reminiscent of the sensorimotor gating deficits observed in a large majority of schizophrenic patients. Decreases in baseline levels of dopamine and serotonin within the striata and frontal cortices of mutant mice were also observed. These results suggest that Ca(2+) in the central nervous system modulates various neurophysiological functions, such as locomotor activity, long-term memory, and sensorimotor gating through the alpha(1B) subunit of the N-type calcium channels.

Exercise-induced motor improvement after complete spinal cord transection and its relation to expression of brain-derived neurotrophic factor and presynaptic markers

BACKGROUND

It has been postulated that exercise-induced activation of brain-derived neurotrophic factor (BDNF) may account for improvement of stepping ability in animals after complete spinal cord transection. As we have shown previously, treadmill locomotor exercise leads to up-regulation of BDNF protein and mRNA in the entire neuronal network of intact spinal cord. The questions arise: (i) how the treadmill locomotor training, supplemented with tail stimulation, affects the expression of molecular correlates of synaptic plasticity in spinal rats, and (ii) if a response is related to BDNF protein level and distribution. We investigated the effect of training in rats spinalized at low thoracic segments on the level and distribution of BDNF immunoreactivity (IR) in ventral quadrants of the lumbar segments, in conjunction with markers of presynaptic terminals, synaptophysin and synaptic zinc.

RESULTS

Training improved hindlimb stepping in spinal animals evaluated with modified Basso-Beattie-Bresnahan scale. Grades of spinal trained animals ranged between 5 and 11, whereas those of spinal were between 2 and 4. Functional improvement was associated with changes in presynaptic markers and BDNF distribution. Six weeks after transection, synaptophysin IR was reduced by 18% around the large neurons of lamina IX and training elevated its expression by over 30%. The level of synaptic zinc staining in the ventral horn was unaltered, whereas in ventral funiculi it was decreased by 26% postlesion and tended to normalize after the training. Overall BDNF IR levels in the ventral horn, which were higher by 22% postlesion, were unchanged after the training. However, training modified distribution of BDNF in the processes with its predominance in the longer and thicker ones. It also caused selective up-regulation of BDNF in two classes of cells (soma ranging between 100-400 microm2 and over 1000 microm2) of the ventrolateral and laterodorsal motor nuclei.

CONCLUSION

Our results show that it is not BDNF deficit that determines lack of functional improvement in spinal animals. They indicate selectivity of up-regulation of BDNF in distinct subpopulations of cells in the motor nuclei which leads to changes of innervation targeting motoneurons, tuned up by locomotor activity as indicated by a region-specific increase of presynaptic markers.

Slow intracellular accumulation of GABA(A) receptor delta subunit is modulated by brain-derived neurotrophic factor

GABA(A) receptors composed of the gamma2 and delta subunits have distinct properties, functions and subcellular localization, and pathological conditions differentially modulate their surface expression. Recent studies demonstrate that acute seizure activity accelerated trafficking of the gamma2 and beta2/3 subunits but not that of the delta subunit. The trafficking of the gamma2 and beta2/3 subunits is relatively well understood but that of the delta subunit has not been studied. We compared intracellular accumulation of the delta and gamma2 subunits in cultured hippocampal neurons using an antibody feeding technique. Intracellular accumulation of the delta subunit peaked between 3 and 6 h, whereas, maximum internalization of the gamma2 subunit took 30 min. In the organotypic hippocampal slice cultures internalization of the delta subunit studied using a biotinylation assay revealed highest accumulation between 3 and 5 h and that of the gamma2 subunit between 15 and 45 min. The surface half-life of the delta subunit was 171 min in cultured hippocampal neurons and 102 min in the organotypic hippocampal slice cultures. In the subsequent studies, internalization of the delta subunit was found to be dependent on network activity but independent of ligand-binding. Brain-derived neurotrophic factor (BDNF) reduced buildup of the delta subunit in the cytoplasmic compartments and increased its surface expression, and this BDNF effect was independent of network activity. BDNF effect was mediated by activation of TrkB receptors, PLCgamma and PKC. Increase in the basal PKC activity augmented cell surface stability of the delta subunit. These results suggest that rate of intracellular accumulation of the delta subunit was distinct and modulated by BDNF.

BDNF signaling in the formation, maturation and plasticity of glutamatergic and GABAergic synapses

In the past 15 years numerous reports provided strong evidence that brain-derived neurotrophic factor (BDNF) is one of the most important modulators of glutamatergic and GABAergic synapses. Remarkable progress regarding localization, kinetics, and molecular mechanisms of BDNF secretion has been achieved, and a large number of studies provided evidence that continuous extracellular supply of BDNF is important for the proper formation and functional maturation of glutamatergic and GABAergic synapses. BDNF can play a permissive role in shaping synaptic networks, making them more susceptible for the occurrence of plastic changes. In addition, BDNF appears to be also an instructive factor for activity-dependent long-term synaptic plasticity. BDNF release just in response to synaptic stimulation might be a molecular trigger to convert high-frequency synaptic activity into long-term synaptic memories. This review attempts to summarize the current knowledge in synaptic secretion and synaptic action of BDNF, including both permissive and instructive effects of BDNF in synaptic plasticity.

BDNF and NT-3 increase excitatory input connectivity in rat hippocampal cultures

The neurotrophic factors brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) have been shown to promote excitatory and inhibitory synapse development. However, a quantitative analysis of their influence on connectivity has proven in general difficult to achieve. In this work we use a novel experimental approach based on percolation concepts that provides a quantification of the average number of connections per neuron. In combination with electrophysiological measurements, we characterize the changes in network connectivity induced by BDNF and NT-3 in rat hippocampal cultures. We show that, on the one hand, BDNF and NT-3 accelerate the maturation of connectivity in the network by about 17 h. On the other hand, BDNF and NT-3 increase the number of excitatory input connections by a factor of about two, but without modifying the number of inhibitory input connections. This scenario of a dominant effect on the excitation is supported by the analysis of spontaneous population bursts in cultures treated with either BDNF or NT-3, which show burst amplitudes that are insensitive to the blockade of inhibition. A leaky integrate-and-fire model reproduces the experimental results well.

Cation-chloride cotransporters and neuronal function

Recent years have witnessed a steep increase in studies on the diverse roles of neuronal cation-chloride cotransporters (CCCs). The versatility of CCC gene transcription, posttranslational modification, and trafficking are on par with what is known about ion channels. The cell-specific and subcellular expression patterns of different CCC isoforms have a key role in modifying a neuron's electrophysiological phenotype during development, synaptic plasticity, and disease. While having a major role in controlling responses mediated by GABA(A) and glycine receptors, CCCs also show close interactions with glutamatergic signaling. A cross-talk among CCCs and trophic factors is important in short-term and long-term modification of neuronal properties. CCCs appear to be multifunctional proteins that are also involved in shaping neuronal structure at various stages of development, from stem cells to synaptogenesis. The rapidly expanding work on CCCs promotes our understanding of fundamental mechanisms that control brain development and functions under normal and pathophysiological conditions.

Activity-dependent dendritic release of BDNF and biological consequences

Network construction and reorganization is modulated by the level and pattern of synaptic activity generated in the nervous system. During the past decades, neurotrophins, and in particular brain-derived neurotrophic factor (BDNF), have emerged as attractive candidates for linking synaptic activity and brain plasticity. Thus, neurotrophin expression and secretion are under the control of activity-dependent mechanisms and, besides their classical role in supporting neuronal survival neurotrophins, modulate nearly all key steps of network construction from neuronal migration to experience-dependent refinement of local connections. In this paper, we provide an overview of recent findings showing that BDNF can serve as a target-derived messenger for activity-dependent synaptic plasticity and development at the single cell level.

Activity-dependent regulation of inhibitory synapse development by Npas4

Neuronal activity regulates the development and maturation of excitatory and inhibitory synapses in the mammalian brain. Several recent studies have identified signalling networks within neurons that control excitatory synapse development. However, less is known about the molecular mechanisms that regulate the activity-dependent development of GABA (gamma-aminobutyric acid)-releasing inhibitory synapses. Here we report the identification of a transcription factor, Npas4, that plays a role in the development of inhibitory synapses by regulating the expression of activity-dependent genes, which in turn control the number of GABA-releasing synapses that form on excitatory neurons. These findings demonstrate that the activity-dependent gene program regulates inhibitory synapse development, and suggest a new role for this program in controlling the homeostatic balance between synaptic excitation and inhibition.

Spontaneous glutamatergic activity induces a BDNF-dependent potentiation of GABAergic synapses in the newborn rat hippocampus

Spontaneous ongoing synaptic activity is thought to play an instructive role in the maturation of the neuronal circuits. However the type of synaptic activity involved and how this activity is translated into structural and functional changes is not fully understood. Here we show that ongoing glutamatergic synaptic activity triggers a long-lasting potentiation of gamma-aminobutyric acid (GABA) mediated synaptic activity (LLP(GABA-A)) in the developing rat hippocampus. LLP(GABA-A) induction requires (i) the activation of AMPA receptors and L-type voltage-dependent calcium channels, (ii) the release of endogenous brain-derived neurotrophic factor (BDNF), and (iii) the activation of postsynaptic tropomyosin-related kinase receptors B (TrkB). We found that spontaneous glutamatergic activity is required to maintain a high level of native BDNF in the newborn rat hippocampus and that application of exogenous BDNF induced LLP(GABA-A) in the absence of glutamatergic activity. These results suggest that ongoing glutamatergic synaptic activity plays a pivotal role in the functional maturation of hippocampal GABAergic synapses by means of a cascade involving BDNF release and downstream signalling through postsynaptic TrkB receptor activation.

Synapse formation and clustering of neuroligin-2 in the absence of GABAA receptors

GABAergic synapses are crucial for brain function, but the mechanisms underlying inhibitory synaptogenesis are unclear. Here, we show that postnatal Purkinje cells (PCs) of GABA(A)alpha1 knockout (KO) mice express transiently the alpha3 subunit, leading to the assembly of functional GABA(A) receptors and initial normal formation of inhibitory synapses, that are retained until adulthood. Subsequently, down-regulation of the alpha3 subunit causes a complete loss of GABAergic postsynaptic currents, resulting in a decreased rate of inhibitory synaptogenesis and formation of mismatched synapses between GABAergic axons and PC spines. Notably, the postsynaptic adhesion molecule neuroligin-2 (NL2) is correctly targeted to inhibitory synapses lacking GABA(A) receptors and the scaffold molecule gephyrin, but is absent from mismatched synapses, despite innervation by GABAergic axons. Our data indicate that GABA(A) receptors are dispensable for synapse formation and maintenance and for targeting NL2 to inhibitory synapses. However, GABAergic signaling appears to be crucial for activity-dependent regulation of synapse density during neuronal maturation.

Differential activity-dependent, homeostatic plasticity of two neocortical inhibitory circuits

Chronic changes in neuronal activity homeostatically regulate excitatory circuitry. However, little is known about how activity regulates inhibitory circuits or specific inhibitory neuron types. Here, we examined the activity-dependent regulation of two neocortical inhibitory circuits--parvalbumin-positive (Parv+) and somatostatin-positive (Som+)--using paired recordings of synaptically coupled neurons. Action potentials were blocked for 5 days in slice culture, and unitary synaptic connections among inhibitory/excitatory neuron pairs were examined. Chronic activity blockade caused similar and distinct changes between the two inhibitory circuits. First, increases in intrinsic membrane excitability and excitatory synaptic drive in both inhibitory subtypes were consistent with the homeostatic regulation of firing rate of these neurons. On the other hand, inhibitory synapses originating from these two subtypes were differentially regulated by activity blockade. Parv+ unitary inhibitory postsynaptic current (uIPSC) strength was decreased while Som+ uIPSC strength was unchanged. Using short-duration stimulus trains, short-term plasticity for both unitary excitatory postsynaptic current (uEPSCs) and uIPSCs was unchanged in Parv+ circuitry while distinctively altered in Som+ circuitry--uEPSCs became less facilitating and uIPSCs became more depressing. In the context of recurrent inhibition, these changes would result in a frequency-dependent shift in the relative influence of each circuit. The functional changes at both types of inhibitory connections appear to be mediated by increases in presynaptic release probability and decreases in synapse number. Interestingly, these opposing changes result in decreased Parv+-mediated uIPSCs but balance out to maintain normal Som+-mediated uIPSCs. In summary, these results reveal that inhibitory circuitry is not uniformly regulated by activity levels and may provide insight into the mechanisms of both normal and pathological neocortical plasticity.

A critical importance of polyamine site in NMDA receptors for neurite outgrowth and fasciculation at early stages of P19 neuronal differentiation

We have investigated the role of N-methyl-d-aspartate receptors (NMDARs) and gamma-aminobutyric acid receptors type A (GABA(A)Rs) at an early stage of P19 neuronal differentiation. The subunit expression was profiled in 24-hour intervals with RT-PCR and functionality of the receptors was verified via fluo-3 imaging of Ca(2+) dynamics in the immature P19 neurons showing that both NMDA and GABA excite neuronal bodies, but only polyamine-site sensitive NMDAR stimulation leads to enhanced Ca(2+) signaling in the growth cones. Inhibition of NR1/NR2B NMDARs by 1 muM ifenprodil severely impaired P19 neurite extension and fasciculation, and this negative effect was fully reversible by polyamine addition. In contrast, GABA(A)R antagonism by a high dose of 200 microM bicuculline had no observable effect on P19 neuronal differentiation and fasciculation. Except for the differential NMDAR and GABA(A)R profiles of Ca(2+) signaling within the immature P19 neurons, we have also shown that inhibition of NR1/NR2B NMDARs strongly decreased mRNA level of NCAM-180, which has been previously implicated as a regulator of neuronal growth cone protrusion and neurite extension. Our data thus suggest a critical role of NR1/NR2B NMDARs during the process of neuritogenesis and fasciculation of P19 neurons via differential control of local growth cone Ca(2+) surges and NCAM-180 signaling.

GABAergic control of CA3-driven network events in the developing hippocampus

Endogenous activity is a characteristic feature of developing neuronal networks. In the neonatal rat hippocampus, spontaneously occurring network events known as "Giant Depolarizing Potentials" (GDPs) are seen in vitro at a stage when GABAergic transmission is depolarizing. GDPs are triggered by the CA3 region and they are seen as brief recurrent events in field-potential recordings, paralleled by depolarization and spiking of pyramidal neurons. In the light of current data, GDPs are triggered by the glutamatergic pyramidal neurons which act as conditional pacemakers, while the depolarizing action of GABA plays a permissive role for the generation of these events in in vitro preparations. From an in vivo perspective, GDPs appear to be an immature form of hippocampal sharp waves.

Early defects of GABAergic synapses in the brain stem of a MeCP2 mouse model of Rett syndrome

Rett syndrome is a neurodevelopmental disorder caused by mutations in the transcriptional repressor methyl-CpG-binding protein 2 (MeCP2) and represents the leading genetic cause for mental retardation in girls. MeCP2-mutant mice have been generated to study the molecular mechanisms of the disease. It was suggested that an imbalance between excitatory and inhibitory neurotransmission is responsible for the behavioral abnormalities, although it remained largely unclear which synaptic components are affected and how cellular impairments relate to the time course of the disease. Here, we report that MeCP2 KO mice present an imbalance between inhibitory and excitatory synaptic transmission in the ventrolateral medulla already at postnatal day 7. Focusing on the inhibitory synaptic transmission we show that GABAergic, but not glycinergic, synaptic transmission is strongly depressed in MeCP2 KO mice. These alterations are presumably due to both decreased presynaptic gamma-aminobutyric acid (GABA) release with reduced levels of the vesicular inhibitory transmitter transporter and reduced levels of postsynaptic GABA(A)-receptor subunits alpha2 and alpha4. Our data indicate that in the MeCP2 -/y mice specific synaptic molecules and signaling pathways are impaired in the brain stem during early postnatal development. These observations mandate the search for more refined diagnostic tools and may provide a rationale for the timing of future therapeutic interventions in Rett patients.

GABA: a pioneer transmitter that excites immature neurons and generates primitive oscillations

Developing networks follow common rules to shift from silent cells to coactive networks that operate via thousands of synapses. This review deals with some of these rules and in particular those concerning the crucial role of the neurotransmitter gamma-aminobuytric acid (GABA), which operates primarily via chloride-permeable GABA(A) receptor channels. In all developing animal species and brain structures investigated, neurons have a higher intracellular chloride concentration at an early stage leading to an efflux of chloride and excitatory actions of GABA in immature neurons. This triggers sodium spikes, activates voltage-gated calcium channels, and acts in synergy with NMDA channels by removing the voltage-dependent magnesium block. GABA signaling is also established before glutamatergic transmission, suggesting that GABA is the principal excitatory transmitter during early development. In fact, even before synapse formation, GABA signaling can modulate the cell cycle and migration. The consequence of these rules is that developing networks generate primitive patterns of network activity, notably the giant depolarizing potentials (GDPs), largely through the excitatory actions of GABA and its synergistic interactions with glutamate signaling. These early types of network activity are likely required for neurons to fire together and thus to "wire together" so that functional units within cortical networks are formed. In addition, depolarizing GABA has a strong impact on synaptic plasticity and pathological insults, notably seizures of the immature brain. In conclusion, it is suggested that an evolutionary preserved role for excitatory GABA in immature cells provides an important mechanism in the formation of synapses and activity in neuronal networks.

Genetic analysis of the TrkB gene and schizophrenia in the Japanese population: Juntendo University Schizophrenia Projects (JUSP)

The presence of cognitive and social impairments during childhood and adolescence in patients with schizophrenia has lead to the widespread hypothesis that schizophrenia may be a neurodevelopmental disorder, which suggests that risk genes for schizophrenia may act through the disruption of normal neurodevelopmental processes. Moreover, recent studies indicate that TrkB, which is receptor of neurotrophins including BDNF, might be involved in schizophrenia. The aim of this study is to evaluate the role of sequence variation at the TrkB locus on schizophrenia. We genotyped 12 single nucleotide polymorphisms (SNPs) across TrkB in 276 subjects with schizophrenia and 274 control subjects from the Japanese population and assessed whether TrkB SNPs are associated with a diagnosis of schizophrenia. In addition, we also investigated if any association exists between the TrkB SNPs and the premorbid functioning as measured by M-PAS using 62 subjects with schizophrenia. The TrkB SNPs were not significantly associated with a diagnosis of schizophrenia. Although one TrkB SNP (rs920776) showed weak association with premorbid functioning (p=0.025), the significance did not remain after Bonferroni correction. Thus, these results do not support a significant role for TrkB sequence variation in the etiology of schizophrenia.

Conserved role of brain-derived neurotrophic factor in Val66Met: target-selective reinforcement of GABAergic synapses

Brain-derived neurotrophic factor has been implicated in higher cognitive functions, and several neurological and psychiatric disorders. Recently, a variant brain-derived neurotrophic factor (BDNFMet), having a substitution referred to as Val66Met, was reported as a product of a bdnf allele with a common single nucleotide polymorphism. It has been reported that BDNFMet is impaired in its potential for activity-dependent release. We sparsely transfected cultured hippocampal neurons with BDNFMet or wild-type BDNFVal cDNAs and examined the amount of GABA-synthetic enzyme glutamic acid decarboxylase 65 (GAD65) in the adjacent region, probably in the GABAergic synapses. BDNFMet transfection increased the GAD65 level to the same extent as transfection with BDNFVal. Our findings suggest that the activity-independent secretion of brain-derived neurotrophic factor may be sufficient to induce inhibitory regulation.

Regulation of glycinergic and GABAergic synaptogenesis by brain-derived neurotrophic factor in developing spinal neurons

Brain-derived neurotrophic factor (BDNF) effects on the establishment of glycinergic and GABAergic transmissions in mouse spinal neurons were examined using combined electrophysiological and calcium imaging techniques. BDNF (10 ng/ml) caused a significant acceleration in the onset of synaptogenesis without large effects on the survival of these neurons. Amplitude and frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) and miniature inhibitory postsynaptic currents (mIPSCs) associated to activation of glycine and GABA(A) receptors were augmented in neurons cultured with BDNF. The neurotrophin effect was blocked by long term tetrodotoxin (TTX) addition suggesting a dependence on neuronal activity. In addition, BDNF caused a significant increase in glycine- and GABA-evoked current densities that partly explains the increase in synaptic transmission. Presynaptic mechanisms were also involved in BDNF effects since triethylammonium(propyl)-4-(2-(4-dibutylamino-phenyl)vinyl)pyridinium (FM1-43) destaining with high K(+) was augmented in neurons incubated with the neurotrophin. The effects of BDNF were mediated by receptor tyrosine kinase B (TrkB) and mitogen-activated protein kinase kinase (MEK) activation since culturing neurons with either (9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'- kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester (K252a) or 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one (PD98059) blocked the augmentation in synaptic activity induced by the neurotrophin.

The development of hippocampal interneurons in rodents

Interneurons are GABAergic neurons responsible for inhibitory activity in the adult hippocampus, thereby controlling the activity of principal excitatory cells through the activation of postsynaptic GABAA receptors. Subgroups of GABAergic neurons innervate specific parts of excitatory neurons. This specificity indicates that particular interneuron subgroups are able to recognize molecules segregated on the membrane of the pyramidal neuron. Once these specific connections are established, a quantitative regulation of their strength must be performed to achieve the proper balance of excitation and inhibition. We will review when and where interneurons are generated. We will then detail their migration toward and within the hippocampus, and the maturation of their morphological and neurochemical characteristics. We will finally review potential mechanisms underlying the development of GABAergic interneurons.

Deciphering the Disease Process of Schizophrenia: The Contribution of Cortical Gaba Neurons

Schizophrenia is a devastating illness that is manifest through a variety of clinical signs and symptoms. Among these, impairments in certain critical cognitive functions, such as working memory, appear to represent the core features of the disorder. In this chapter, we review the evidence indicating that disturbances in neurotransmission by a subset of GABA neurons in the dorsolateral prefrontal cortex are commonly present in schizophrenia. Despite both pre- and postsynaptic compensatory responses, the resulting pathophysiological process, alterations in the perisomatic inhibitory regulation of pyramidal neurons, underlies a reduced capacity for the synchronization of neuronal activity at gamma frequencies that is required for working memory function. We also discuss several pathogenetic mechanisms that could rise to the alterations in GABA neurotransmission and consider the implication of these findings for therapeutic interventions to improve cognitive function in individuals with schizophrenia.

Estrogen and brain-derived neurotrophic factor (BDNF) in hippocampus: complexity of steroid hormone-growth factor interactions in the adult CNS

In the CNS, there are widespread and diverse interactions between growth factors and estrogen. Here we examine the interactions of estrogen and brain-derived neurotrophic factor (BDNF), two molecules that have historically been studied separately, despite the fact that they seem to share common targets, effects, and mechanisms of action. The demonstration of an estrogen-sensitive response element on the BDNF gene provided an impetus to explore a direct relationship between estrogen and BDNF, and predicted that the effects of estrogen, at least in part, might be due to the induction of BDNF. This hypothesis is discussed with respect to the hippocampus, where substantial evidence has accumulated in favor of it, but alternate hypotheses are also raised. It is suggested that some of the interactions between estrogen and BDNF, as well as the controversies and implications associated with their respective actions, may be best appreciated in light of the ability of BDNF to induce neuropeptide Y (NPY) synthesis in hippocampal neurons. Taken together, this tri-molecular cascade, estrogen-BDNF-NPY, may be important in understanding the hormonal regulation of hippocampal function. It may also be relevant to other regions of the CNS where estrogen is known to exert profound effects, such as amygdala and hypothalamus; and may provide greater insight into neurological disorders and psychiatric illness, including Alzheimer's disease, depression and epilepsy.

Anti-homeostatic synaptic plasticity of glycine receptor function after chronic strychnine in developing cultured mouse spinal neurons

In this study, we describe a novel form of anti-homeostatic plasticity produced after culturing spinal neurons with strychnine, but not bicuculline or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Strychnine caused a large increase in network excitability, detected as spontaneous synaptic currents and calcium transients. The calcium transients were associated with action potential firing and activation of gamma-aminobutyric acid (GABA(A)) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors as they were blocked by tetrodotoxin (TTX), bicuculline, and CNQX. After chronic blockade of glycine receptors (GlyRs), the frequency of synaptic transmission showed a significant enhancement demonstrating the phenomenon of anti-homeostatic plasticity. Spontaneous inhibitory glycinergic currents in treated cells showed a fourfold increase in frequency (from 0.55 to 2.4 Hz) and a 184% increase in average peak amplitude compared with control. Furthermore, the augmentation in excitability accelerated the decay time constant of miniature inhibitory post-synaptic currents. Strychnine caused an increase in GlyR current density, without changes in the apparent affinity. These findings support the idea of a post-synaptic action that partly explains the increase in synaptic transmission. This phenomenon of synaptic plasticity was blocked by TTX, an antibody against brain-derived neurotrophic factor (BDNF) and K252a suggesting the involvement of the neuronal activity-dependent BDNF-TrkB signaling pathway. These results show that the properties of GlyRs are regulated by the degree of neuronal activity in the developing network.

BDNF occludes GABABreceptor-mediated inhibition of GABA release in rat hippocampal CA1 pyramidal neurons

During the development of the rat hippocampus, both gamma-aminobutyric acid (GABA)(B) autoreceptors and brain-derived neurotrophic factor (BDNF) play important roles in the formation of GABAergic synapses as well as in the GABA(A) receptor-mediated transmissions. While a number of studies have reported rapid effects of BDNF on GABA(A) receptor-mediated responses, the interactions between GABA(B) autoreceptors and BDNF are less clear. Using conventional whole-cell patch-clamp recordings, we demonstrated here that BDNF significantly occludes baclofen-induced suppression of GABA(A) receptor-mediated transmissions in each of the preparations including hippocampal slices prepared from P14 rats, hippocampal CA1 pyramidal neurons isolated from P14 and P21 rats, and cultured hippocampal pyramidal neurons. This effect of BDNF was rapid and reversible, and was mediated via the activation of presynaptic TrkB receptor tyrosine kinases, and subsequent activation of phospholipase C and protein kinase C. On the contrary, in hippocampal CA1 pyramidal neurons isolated from P7 rats, BDNF failed to occlude the GABA(B) receptor-mediated inhibition of GABA release. Thus, the ability of BDNF to occlude the GABA(B) receptor-mediated inhibition of GABA release develops between P7 and P14. This demonstrates a novel aspect of the effects of BDNF on inhibitory transmissions in rat hippocampus, which may have some functional roles in the induction of developmental plasticity and/or pathophysiology of epilepsy.