Trophic actions of GABA on neuronal development

Development of cortical GABAergic circuits and its implications for neurodevelopmental disorders

GABAergic interneurons powerfully control the function of cortical networks. In addition, they strongly regulate cortical development by modulating several cellular processes such as neuronal proliferation, migration, differentiation and connectivity. Not surprisingly, aberrant development of GABAergic circuits has been implicated in many neurodevelopmental disorders including schizophrenia, autism and Tourette's syndrome. Unfortunately, efforts directed towards the comprehension of the mechanisms regulating GABAergic circuits formation and function have been impaired by the strikingly heterogeneity, both at the morphological and functional level, of GABAergic interneurons. Recent technical advances, including the improvement of interneurons-specific labelling techniques, have started to reveal the basic principles underlying this process. This review summarizes recent findings on the mechanisms underlying the construction of GABAergic circuits in the cortex, with a particular focus on potential implications for brain diseases with neurodevelopmental origin.

Kinetic properties of Cl uptake mediated by Na+-dependent K+-2Cl cotransport in immature rat neocortical neurons

GABA, the main inhibitory neurotransmitter in the adult nervous system, evokes depolarizing membrane responses in immature neurons, which are crucial for the generation of early network activity. Although it is well accepted that depolarizing GABA actions are caused by an elevated intracellular Cl- concentration ([Cl-]i), the mechanisms of Cl- accumulation in immature neurons are still a matter of debate. Using patch-clamp, microfluorimetric, immunohistochemical, and molecular biological approaches, we studied the mechanism of Cl- uptake in Cajal-Retzius (CR) cells of immature [postnatal day 0 (P0) to P3] rat neocortex. Gramicidin-perforated patch-clamp and 6-methoxy-N-ethylquinolinium-microfluorimetric measurements revealed a steady-state [Cl-]i of approximately 30 mM that was reduced to values close to passive distribution by bumetanide or Na+-free solutions, suggesting a participation of Na+-K+-2Cl- cotransport isoform 1 (NKCC1) in maintaining elevated [Cl-]i. Expression of NKCC1 was found in CR cells on the mRNA and protein levels. To determine the contribution of NKCC1 to [Cl-]i homeostasis in detail, Cl- uptake rates were analyzed after artificial [Cl-]i depletion. Active Cl- uptake was relatively slow (47.2 +/- 5.0 microM/s) and was abolished by bumetanide or Na+-free solution. Accordingly, whole-cell patch-clamp recordings revealed a low Cl- conductance in CR cells. The low capacity of NKCC1-mediated Cl- uptake was sufficient to maintain excitatory GABAergic membrane responses, however, only at low stimulation frequencies. In summary, our results demonstrate that NKCC1 is abundant in CR cells of immature rat neocortex and that the slow Cl- uptake mediated by this transporter is sufficient to maintain high [Cl-]i required to render GABA responses excitatory.

GABA(A) receptor-mediated signaling alters the structure of spontaneous activity in the developing retina

Ambient GABA modulates firing patterns in adult neural circuits by tonically activating extrasynaptic GABA(A) receptors. Here, we demonstrate that during a developmental period when activation of GABA(A) receptors causes membrane depolarization, tonic activation of GABA(A) receptors blocks all spontaneous activity recorded in retinal ganglion cells (RGCs) and starburst amacrine cells (SACs). Bath application of the GABA(A) receptor agonist muscimol blocked spontaneous correlated increases in intracellular calcium concentration and compound postsynaptic currents in RGCs associated with retinal waves. In addition, GABA(A) receptor agonists activated a tonic current in RGCs that significantly reduced their excitability. Using a transgenic mouse in which green fluorescent protein is expressed under the metabotropic glutamate receptor subtype 2 promoter to target recordings from SACs, we found that GABA(A) receptor agonists blocked compound postsynaptic currents and also activated a tonic current. GABA(A) receptor antagonists reduced the holding current in SACs but not RGCs, indicating that ambient levels of GABA tonically activate GABA(A) receptors in SACs. GABA(A) receptor antagonists did not block retinal waves but did alter the frequency and correlation structure of spontaneous RGC firing. Interestingly, the drug aminophylline, a general adenosine receptor antagonist used to block retinal waves, induced a tonic GABA(A) receptor antagonist-sensitive current in outside-out patches excised from RGCs, indicating that aminophylline exerts its action on retinal waves by direct activation of GABA(A) receptors. These findings have implications for how various neuroactive drugs and neurohormones known to modulate extrasynaptic GABA(A) receptors may influence spontaneous firing patterns that are critical for the establishment of adult neural circuits.

Evidence against GABA release from glutamatergic mossy fiber terminals in the developing hippocampus

Hippocampal mossy fibers of young rodents have been reported to corelease inhibitory neurotransmitter GABA in addition to excitatory transmitter glutamate. In this study, we aimed at re-evaluating this corelease hypothesis of both inhibitory and excitatory transmitters in the hippocampus. Electrophysiological examination revealed that, in juvenile mice and rats of the two to 3 weeks old, stimulation at the granule cell layer of the dentate gyrus elicited monosynaptic GABAergic IPSCs in CA3 neurons in the presence of ionotropic glutamate receptor (iGluR) blockers, only when rather strong stimuli were given. The group II mGluR agonist (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclo-propyl)glycine (DCG-IV), which selectively suppresses transmission at the mossy fiber-CA3 synapse, abolished almost all postsynaptic responses elicited by the weak stimuli, whereas those by strong stimuli were inhibited only slightly. In addition, the minimal stimulation elicited GABAergic IPSCs in neonatal mice of the first postnatal week, whereas these responses are not sensitive to DCG-IV. Immunohistochemical examination revealed that mossy fiber terminals expressed GABA and the GABA-synthesizing enzyme GAD67, although the expression levels were much weaker than those in the inhibitory interneurons. Notably, the expression levels of the vesicular GABA transporter were much lower than those of GABA and GAD67, and almost below detection threshold. These results suggest that mossy fiber synapses are purely glutamatergic and apparent monosynaptic IPSCs so far reported are evoked by costimulation of inhibitory interneurons, at least in young mice and rats. Hippocampal mossy fiber terminals synthesize and store GABA, but have limited ability in vesicular release for GABA in the developing rodents.

Identification of activity-dependent gene expression profiles reveals specific subsets of genes induced by different routes of Ca(2+) entry in cultured rat cortical neurons

Neuronal activity-dependent gene transcription is a key feature of long-lasting synaptic strengthening associated with learning and memory, as well as activity-dependent neuroprotection. To comprehensively determine the molecular alterations, we carried out genome-wide microarray analysis in cultured rat cortical neurons treated with specific pharmacological agents, a model with alterations in neuronal activity, which were monitored by multi-site electrophysiological recordings. Of the approximately 27,000 genes, the expression of 248 genes was strongly changed in response to enhanced activity. These genes encompass a large number of members of distinct families, including synaptic vesicle proteins, ion channels, signal transduction molecules, synaptic growth regulators, and others. Two subsets of these genes were further confirmed to be specifically induced by Ca(2+) influx through N-methyl-D-aspartate (NMDA) receptors and L-type voltage-gated Ca(2+) channels (VGCCs). In addition, those genes dynamically regulated by the enhanced activity were also elucidated, as well as those candidate genes associated with synaptic plasticity and neuroprotection. Our findings therefore would help define the molecular mechanisms that occur in response to neuronal activity and identify specific clusters of genes that contribute to activity-dependent and Ca(2+)-inducible modulation of brain development and function.

NKCC1 phosphorylation stimulates neurite growth of injured adult sensory neurons

Peripheral nerve section promotes regenerative, elongated neuritic growth of adult sensory neurons. Although the role of chloride homeostasis, through the regulation of ionotropic GABA receptors, in the growth status of immature neurons in the CNS begins to emerge, nothing is known of its role in the regenerative growth of injured adult neurons. To analyze the intracellular Cl- variation after a sciatic nerve section in vivo, gramicidin perforated-patch recordings were used to study muscimol-induced currents in mice dorsal root ganglion neurons isolated from control and axotomized neurons. We show that the reversal potential of muscimol-induced current, E(GABA-A), was shifted toward depolarized potentials in axotomized neurons. This was attributable to Cl- influx because removal of extracellular Cl- prevented this shift. Application of bumetanide, an inhibitor of NKCC1 cotransporter and E(GABA-A) recordings in sensory neurons from NKCC1-/- mice, identified NKCC1 as being responsible for the increase in intracellular Cl- in axotomized neurons. In addition, we demonstrate with a phospho-NKCC1 antibody that nerve injury induces an increase in the phosphorylated form of NKCC1 in dorsal root ganglia that could account for intracellular Cl- accumulation. Time-lapse recordings of the neuritic growth of axotomized neurons show a faster growth velocity compared with control. Bumetanide, the intrathecal injection of NKCC1 small interfering RNA, and the use of NKCC1-/- mice demonstrated that NKCC1 is involved in determining the velocity of elongated growth of axotomized neurons. Our results clearly show that NKCC1-induced increase in intracellular chloride concentration is a major event accompanying peripheral nerve regeneration.

Refining the roles of GABAergic signaling during neural circuit formation

Our understanding of the role of GABA signaling in circuit development is rapidly expanding. Here, we review three recent refinements in our understanding of the diverse roles that GABA plays at different stages of neural circuit formation. First, we discuss recent evidence that depolarizing GABA plays at least a permissive role in promoting both excitatory and inhibitory synaptogenesis in developing neurons (including newly generated neurons in the adult). Next, we discuss recent evidence that GABAergic circuits sculpt the temporal and spatial aspects of synaptic integration. Consequently, early developmental events affecting the establishment of GABAergic circuits will control subsequent activity-dependent refinements of information processing and circuit function. In the third section, we review recent evidence of molecular mechanisms by which GABAergic signaling plays a role in the regulation of the balance between GABAergic and glutamatergic transmission in developing circuits. Throughout the review, we concentrate on the effects of the signaling by GABA(A) receptors, as told from the point of view of the GABA-responsive cells, and do not discuss mechanisms that govern GABA release or activity of GABAergic neurons per se.

A GABAergic system in airway epithelium is essential for mucus overproduction in asthma

Gamma-aminobutyric acid (GABA) is an important neurotransmitter that, through the subtype A GABA receptor (GABAAR), induces inhibition in the adult brain. Here we show that an excitatory, rather than inhibitory, GABAergic system exists in airway epithelial cells. Both GABAARs and the GABA synthetic enzyme glutamic acid decarboxylase (GAD) are expressed in pulmonary epithelial cells. Activation of GABAARs depolarized these cells. The expression of GAD in the cytosol and GABAARs in the apical membranes of airway epithelial cells increased markedly when mice were sensitized and then challenged with ovalbumin, an approach for inducing allergic asthmatic reactions. Similarly, GAD and GABAARs in airway epithelial cells of humans with asthma increased after allergen inhalation challenge. Intranasal application of selective GABAAR inhibitors suppressed the hyperplasia of goblet cells and the overproduction of mucus induced by ovalbumin or interleukin-13 in mice. These findings show that a previously unknown epithelial GABAergic system has an essential role in asthma.

Spontaneous glutamate release controls NT-3-dependent development of hippocampal calbindin-D28k phenotype through activation of sodium channels ex vivo

Functional NMDA and AMPA ionotropic glutamate receptors are expressed in embryonic hippocampal glutamatergic pyramidal neurons prior to synapse formation but their function and mechanisms of action are still unclear. At the same time, these neurons develop their calbindin-D(28k) phenotype through an activity-dependent NT-3 autocrine loop. Using single-neuron microcultures, we show here that immature pyramidal neurons spontaneously secreted glutamate and that chronic blockade of either NMDA or AMPA receptors down-regulated the number of calbindin-D(28k)-positive pyramidal neurons without affecting neuronal survival. This antagonistic effect of glutamate ionotropic receptors was mimicked by anti-TrkC antibodies and reversed by the application of NT-3. Similar results were obtained in ex vivo embryonic hippocampal slice cultures. Moreover, glutamate receptor blockade inhibited the generation of spontaneous sodium-driven action potentials which, in turn, regulate both the endogenous secretion of NT-3 and the calbindin-D(28k) phenotype acquisition. Altogether, these results suggest an unexpected role for glutamate in the development of the physiological and biochemical properties of hippocampal pyramidal neurons and support the idea that glutamate may underlie an activity-dependent mode of differentiation prior to synapse formation.

Role of endogenous nicotinic signaling in guiding neuronal development

Spontaneous nicotinic cholinergic activity is widespread in the developing nervous system. One of the major components mediating this activity is the nicotinic acetylcholine receptor with alpha7 subunits (alpha7-nAChR) and high relative calcium permeability. We recently reported that alpha7-nAChRs co-localize in part with GABA(A) receptors during development, and the sites become co-innervated by cholinergic and GABAergic terminals. Patch-clamp recording either from embryonic chick ciliary ganglion neurons or from early postnatal mouse hippocampal interneurons reveals that alpha7-nAChR activation can impose a rapid and reversible decrease in GABA(A) receptor responses. The effect extends to GABAergic synaptic currents, and depends on intracellular calcium, calcium/calmodulin-dependent protein kinase II, and MAP kinase in the postsynaptic cell. Over the longer term, nicotinic activity has a more profound effect: it determines the time during development when GABAergic signaling converts from excitation to inhibition. It does this by changing the pattern of chloride transporters to establish the mature chloride gradient required for inhibitory GABAergic responses. The excitatory phase of GABAergic signaling is critical for proper development and integration of neurons into circuits. By driving the conversion of GABAergic signaling, nicotinic activity not only terminates one set of developmental instructions, but also initiates another by collaborating with GABAergic inhibition to impose new instructions. The results reveal a multi-layered pattern of activity-dependent controls in development and indicate the significance of nicotinic signaling in shaping these events.

New GABAergic interneurons supported by myelin-specific T cells are formed in intact adult spinal cord

Neural stem/progenitor cells are known to exist in the intact spinal cord, but the presence of newly formed neurons during adulthood has not been documented there to date. Here, we report the appearance of newly formed neurons under normal physiological conditions. These neurons are immature, express a GABAergic phenotype, and are primarily located in the dorsal part of the spinal cord. This localization appeared to be mediated by stromal-derived factor-1/CXC-chemokine receptor-4 signaling in the dorsal region. The extent of spinal cord neurogenesis was found to be greatly influenced by immune system integrity and in particular by myelin-specific T cells. These observations provide evidence for in vivo spinal cord neurogenesis under nonpathological conditions and introduce novel mechanisms regulating adult spinal cord plasticity.

Excitatory GABA action is essential for morphological maturation of cortical neurons in vivo

GABA exerts excitatory actions on embryonic and neonatal cortical neurons, but the in vivo function of this GABA excitation is essentially unknown. Using in utero electroporation, we eliminated the excitatory action of GABA in a subpopulation of rat ventricular progenitors and cortical neurons derived from these progenitors by premature expression of the Cl- transporter KCC2, as confirmed by the changes in the reversal potential of GABA-induced currents and the resting membrane potential after GABA(A) receptor blockade. We found that radial migration to layer II/III of the somatosensory cortex of neurons derived from the transfected progenitors was not significantly affected, but their morphological maturation was markedly impaired. Furthermore, reducing neuronal excitability of cortical neurons in vivo by overexpressing an inward-rectifying K+ channel, which lowered the resting membrane potential, mimicked the effect of premature KCC2 expression. Thus, membrane depolarization caused by early GABA excitation is critical for morphological maturation of neonatal cortical neurons in vivo.

GABAergic control of adult hippocampal neurogenesis in relation to behavior indicative of trait anxiety and depression states

Stressful experiences in early life are known risk factors for anxiety and depressive illnesses, and they inhibit hippocampal neurogenesis and the expression of GABA(A) receptors in adulthood. Conversely, deficits in GABAergic neurotransmission and reduced neurogenesis are implicated in the etiology of pathological anxiety and diverse mood disorders. Mice that are heterozygous for the gamma2 subunit of GABA(A) receptors exhibit a modest functional deficit in mainly postsynaptic GABA(A) receptors that is associated with a behavioral, cognitive, and pharmacological phenotype indicative of heightened trait anxiety. Here we used cell type-specific and developmentally controlled inactivation of the gamma2 subunit gene to further analyze the mechanism and brain substrate underlying this phenotype. Heterozygous deletion of the gamma2 subunit induced selectively in immature neurons of the embryonic and adult forebrain resulted in reduced adult hippocampal neurogenesis associated with heightened behavioral inhibition to naturally aversive situations, including stressful situations known to be sensitive to antidepressant drug treatment. Reduced adult hippocampal neurogenesis was associated with normal cell proliferation, indicating a selective vulnerability of postmitotic immature neurons to modest functional deficits in gamma2 subunit-containing GABA(A) receptors. In contrast, a comparable forebrain-specific GABA(A) receptor deficit induced selectively in mature neurons during adolescence lacked neurogenic and behavioral consequences. These results suggest that modestly reduced GABA(A) receptor function in immature neurons of the developing and adult brain can serve as a common molecular substrate for deficits in adult neurogenesis and behavior indicative of anxious and depressive-like mood states.

Increased NKCC1 expression in refractory human epilepsy

Cation-chloride co-transporters (CCTs), particularly NKCC1, may be important in epileptogenesis. We have performed a detailed histological examination of NKCC1 in large samples of patients with hippocampal sclerosis (HS) or focal cortical dysplasia (FCD), pathologies both commonly associated with pharmacoresistant epilepsy. We consistently found increased immunoreactivity for NKCC1 in HS and FCD, but not in adjacent histologically normal cortex. Our results suggest that NKCC1 might contribute to the pathogenesis or pathophysiology of HS and FCD, thereby potentially offering a new therapeutic target in the treatment of pharmacoresistant epilepsy.

Putrescine as an important source of GABA in the postnatal rat subventricular zone

The subventricular zone (SVZ) is a neurogenic region that continually gives rise to olfactory bulb (OB) GABAergic interneurons in mammals. The newly generated neuroblasts already express GABA while migrating to this structure along the rostral migratory stream (RMS). Here, we investigate in early postnatal rat if SVZ/RMS cells undertake the same synthetic pathway by which GABA is produced in differentiated neurons, i.e. the decarboxylation of glutamate by the glutamic acid decarboxylase (GAD), or, if an alternative pathway, the conversion of putrescine into GABA, also contributes to GABA synthesis. We show here that GAD immunoreactivity is not significantly detectable within the SVZ/RMS. However, strong immunolabeling is found within the OB. Nevertheless, low GAD enzymatic activity (as compared with OB) is detected in the SVZ/RMS. SVZ/RMS explants convert approximately 30% of all captured radiolabeled putrescine into GABA in vitro, showing that this pathway is important for GABA synthesis in the SVZ. We also show that SVZ/RMS, OB and choroid plexus explants are able to synthesize putrescine, as analyzed by ornithine decarboxylase (ODC) activity, providing neuroblasts with different sources of putrescine for GABA production. During early stages of neuroblast differentiation, in which neurotransmitter choice may still be undefined, an alternative pathway for GABA synthesis guarantees the production of GABA, necessary for neuroblast proliferation and migration in the SVZ/RMS.

Fetal Exposure to GABA-Acting Antiepileptic Drugs Generates Hippocampal and Cortical Dysplasias

PURPOSE

The management of epilepsy during pregnancy entails a number of concerns. While seizures may affect adversely maternal and fetal outcome, antiepileptic drugs (AEDs) may increase the incidence of congenital abnormalities and possibly affect postnatal cognitive development in the offspring. Experimental animal studies can aid in assessing teratogenic features associated with individual AEDs and/or with seizures, and to identify the mechanisms involved. The purpose of this study was to investigate the consequences of prenatal exposure to (a) different AEDs and (b) maternal seizures on brain maturational processes in rats.

METHODS

Pregnant rats received from embryonic days 14 to 19 intraperitoneal injections of carbamazepine (20 mg/kg/day), vigabatrin (200 mgkg/day), and valproate (100 mg/kg/day) at doses not widely different from those used clinically. Pups exposed to AEDs in utero were analyzed postnatally. Animals born to "kindled" pregnant animals that had experienced one generalized convulsive seizure per day during the same gestational period were analyzed in parallel.

RESULTS

Prenatal exposure to vigabatrin and valproate, which act on GABA signaling, induced hippocampal and cortical dysplasias, which were likely to result from a neuronal migration defect and neuronal death. By contrast, offspring of rats exposed to carbamazepine (which at the dose used produced low plasma concentrations) or to generalized convulsive seizures showed no clear-cut evidence of dysplasias.

CONCLUSIONS

We suggest that AEDs that increase the extracellular concentration of GABA might induce severe neuronal migration disorders. Drugs acting through other molecular targets would also perturb cortical maturation. The potential clinical relevance of these results should be a subject of future research.

Molecular regulation of cognitive functions and developmental plasticity: impact of GABAA receptors

By controlling spike timing and sculpting neuronal rhythms, inhibitory interneurons play a key role in regulating neuronal circuits and behavior. The pronounced diversity of GABAergic (gamma-aminobutyric acid) interneurons is paralleled by an extensive diversity of GABAA receptor subtypes. The region- and domain-specific location of these receptor subtypes offers the opportunity to gain functional insights into the role of defined neuronal circuits. These developments are reviewed with regard to the regulation of sleep, anxiety, memory, sensorimotor processing and post-natal developmental plasticity.

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.

Developmental localization of potassium chloride co-transporter 2 (KCC2) in the Purkinje cells of embryonic mouse cerebellum

Developmental shift in GABA actions from depolarization to hyperpolarization occurs as a result of decreasing the intracellular Cl(-) concentration regulated by K(+)-Cl(-) co-transporter 2 (KCC2). To clarify the time-course of the developmental shift on the Purkinje cells, we examined KCC2-localization in the embryonic mouse cerebellum. The KCC2 was first detected within the Purkinje cells in the Purkinje cell layer of the hemisphere at embryonic day 15 (E15) and the vermis at E17, but the ventricular and intermediate zones were negative. These results suggest that GABA might become inhibitory on the Purkinje cells after their settling in the Purkinje cell layer.

Molecular mechanisms of autism: a possible role for Ca2+ signaling

Autism spectrum disorders (ASDs) are a group of developmental disorders characterized by social and emotional deficits, language impairments and stereotyped behaviors that manifest in early postnatal life. The molecular mechanisms that underlie ASDs are not known, but several recent developments suggest that some forms of autism are caused by failures in activity-dependent regulation of neural development. Mutations of several voltage-gated and ligand-gated ion channels that regulate neuronal excitability and Ca2+ signaling have been associated with ASDs. In addition, Ca2+-regulated signaling proteins involved in synapse formation and dendritic growth have been implicated in ASDs. These recent advances suggest a set of signaling pathways that might have a role in generating these increasingly prevalent disorders.

Role of activity-dependent regulation of neuronal chloride homeostasis in development

The polarity of neurotransmission mediated by the gamma-amino butyric acid (GABA) type A receptor depends crucially on intracellular chloride concentration, which is largely determined by the expression and function of cation/chloride co-transporters. Recent evidence shows how both activity and neurotrophic factors can affect GABAergic transmission in the mammalian central nervous system through their effects on the neuron-specific chloride-extruding transporter KCC2. In particular, GABAergic neurotransmission early in development, sustained neuronal activity in mature networks and brain-derived neurotrophic factor each modulate the expression or function of KCC2. The resulting changes in intracellular chloride concentration alter the nature or strength of fast GABAergic neurotransmission, profoundly affecting the development and function of neuronal networks.

Potentially toxic effects of anaesthetics on the developing central nervous system

A growing body of experimental evidence suggests that anaesthetics, by influencing GABAergic and glutaminergic neural signalling, can have adverse effects on the developing central nervous system. The biological foundation for this is that gamma-aminobutyric acid and glutamate could act non-synaptically, in addition to their role in neurotransmission in the adult brain, in the regulation of neuronal development in the central nervous system. These neurotransmitters and their receptors are expressed from very early stages of central nervous system development and appear to influence neural progenitor proliferation, cell migration and neuronal differentiation. During the synaptogenetic period, pharmacological blockade of N-methyl-d-aspartate (NMDA)-type glutamate receptors as well as stimulation of GABAA receptors has been reported to be associated with increased apoptosis in the developing brain. Importantly, recent data suggest that even low, non-apoptogenic concentrations of anaesthetics can perturb neuronal dendritic development and thus could potentially lead to impairment of developing neuronal networks. The extrapolation of these experimental observations to clinical practice is of course very difficult and requires extreme caution as differences in drug concentrations and exposure times as well as interspecies variations are all important confounding variables. While clinicians should clearly not withhold anaesthesia based on current animal studies, these observations should urge more laboratory and clinical research to further elucidate this issue.

GABA regulates dendritic growth by stabilizing lamellipodia in newly generated interneurons of the olfactory bulb

The initial formation and growth of dendrites is a critical step leading to the integration of newly generated neurons into postnatal functional networks. However, the cellular mechanisms and extracellular signals regulating this process remain mostly unknown. By directly observing newborn neurons derived from the subventricular zone in culture as well as in olfactory bulb slices, we show that ambient GABA acting through GABA(A) receptors is essential for the temporal stability of lamellipodial protrusions in dendritic growth cones but did not interfere with filopodia dynamics. Furthermore, we provide direct evidence that ambient GABA is required for the proper initiation and elongation of dendrites by promoting the rapid stabilization of new dendritic segments after their extension. The effects of GABA on the initial formation of dendrites depend on depolarization and Ca2+ influx and are associated with a higher stability of microtubules. Together, our results indicate that ambient GABA is a key regulator of dendritic initiation in postnatally generated olfactory interneurons and offer a mechanism by which this neurotransmitter drives early dendritic growth.

GABA-induced intracellular Ca2+ elevation in the embryonic chick brainstem slice

We examined the intracellular Ca2+ ([Ca2+]i) elevation evoked by GABA in an 8-day embryonic chick brainstem slice using a Ca imaging technique with Ca green-1 AM. When small quantities of GABA were pressure-ejected on the surface of the slice, the [Ca2+]i elevation was clearly detected. The GABA-induced [Ca2+]i elevation was eliminated in a Ca2+-free solution, whereas the previously reported GABA-induced light-scattering change was independent of extracellular Ca2+. Although, micro-application of glycine or glutamate also induced [Ca2+]i elevation, these changes were smaller than that by GABA. These results suggest that the GABA-induced [Ca2+]i elevation is due to Ca2+ entry resulting from membrane depolarization and may play an important role in the development of the central nervous system (CNS).

The cellular, molecular and ionic basis of GABA(A) receptor signalling

GABA(A) receptors mediate fast synaptic inhibition in the CNS. Whilst this is undoubtedly true, it is a gross oversimplification of their actions. The receptors themselves are diverse, being formed from a variety of subunits, each with a different temporal and spatial pattern of expression. This diversity is reflected in differences in subcellular targetting and in the subtleties of their response to GABA. While activation of the receptors leads to an inevitable increase in membrane conductance, the voltage response is dictated by the distribution of the permeant Cl(-) and HCO(3)(-) ions, which is established by anion transporters. Similar to GABA(A) receptors, the expression of these transporters is not only developmentally regulated but shows cell-specific and subcellular variation. Untangling all these complexities allows us to appreciate the variety of GABA-mediated signalling, a diverse set of phenomena encompassing both synaptic and non-synaptic functions that can be overtly excitatory as well as inhibitory.

Sequential interplay of nicotinic and GABAergic signaling guides neuronal development

GABA (gamma-aminobutyric acid), the major inhibitory transmitter in the brain, goes through a transitory phase of excitation during development. The excitatory phase promotes neuronal growth and integration into circuits. We show here that spontaneous nicotinic cholinergic activity is responsible for terminating GABAergic excitation and initiating inhibition. It does so by changing chloride transporter levels, shifting the driving force on GABA-induced currents. The timing of the transition is critical, because the two phases of GABAergic signaling provide contrasting developmental instructions. Synergistic with nicotinic excitation, GABAergic inhibition constrains neuronal morphology and innervation. The results reveal a multitiered activity-dependent strategy controlling neuronal development.

Developmental localization of potassium chloride co-transporter 2 in granule cells of the early postnatal mouse cerebellum with special reference to the synapse formation

In the adult CNS, GABA is the predominant inhibitory neurotransmitter, mediating the hyperpolarization of membrane potential and regulating the glutamatergic activity. In the immature CNS, on the other hand, GABA mediates depolarization and is involved in controlling morphogenesis. This developmental shift in GABA actions from depolarization to hyperpolarization occurs as a result of decreasing the intracellular chloride ion (Cl(-)) concentration ([Cl(-)](i)) which is regulated by the potassium (K(+))-Cl(-) co-transporter 2 (KCC2). To clarify the time-course of changes in the GABA actions during development, we examined the developmental localization of the KCC2 in the granule cells of the postnatal mouse cerebellum using specific antibodies against KCC2. The granule cell precursors and migrating granule cells were devoid of immunoreactivity against KCC2 antibodies. At postnatal day 3 (P3), the KCC2-immunolabeling was negative in the internal granular layer, although synaptophysin-positive mossy fiber terminals were detected. At P5, we first detected the KCC2-immunolabeling at the somata of granule cells and their dendrites before granule cells received inhibitory input from Golgi cells. Almost all KCC2-positive dendrites (more than 98%) attached to and formed synapses with mossy fiber terminals. As development proceeded, the number of KCC2-positive granule cells increased, and all granule cells became positive by P21. These results suggested that GABAergic transmission on granule cells might shift from excitation to inhibition after the synapse formation, and the excitatory synapse-formation and related factors might be the triggers for the expression and localization of the KCC2 in the granule cells. Furthermore, it was also suggested that formation of the GABAergic synapses and GABAergic transmission were not necessary for the KCC2-expression in the mouse cerebellar granule cells in vivo.

Seizures and Antiepileptic Drugs: Does Exposure Alter Normal Brain Development?

Seizures and antiepileptic drugs (AEDs) affect brain development and have long-term neurological consequences. The specific molecular and cellular changes, the precise timing of their influence during brain development, and the full extent of the long-term consequences of seizures and AEDs exposure have not been established. This review critically assesses both the basic and clinical science literature on the effects of seizures and AEDs on the developing brain and finds that evidence exists to support the hypothesis that both seizures and antiepileptic drugs influence a variety of biological process, at specific times during development, which alter long-term cognition and epilepsy susceptibility. More research, both clinical and experimental, is needed before changes in current clinical practice, based on the scientific data, can be recommended.

Early neural activity and dendritic growth in turtle retinal ganglion cells

Early neural activity, both prenatal spontaneous bursts and early visual experience, is believed to be important for dendritic proliferation and for the maturation of neural circuitry in the developing retina. In this study, we have investigated the possible role of early neural activity in shaping developing turtle retinal ganglion cell (RGC) dendritic arbors. RGCs were back-labelled from the optic nerve with horseradish peroxidase (HRP). Changes in dendritic growth patterns were examined across development and following chronic blockade or modification of spontaneous activity and/or visual experience. Dendrites reach peak proliferation at embryonic stage 25 (S25, one week before hatching), followed by pruning in large field RGCs around the time of hatching. When spontaneous activity is chronically blocked in vivo from early embryonic stages (S22) with curare, a cholinergic nicotinic antagonist, RGC dendritic growth is inhibited. On the other hand, enhancement of spontaneous activity by dark-rearing (Sernagor & Grzywacz (1996)Curr. Biol., 6, 1503-1508) promotes dendritic proliferation in large-field RGCs, an effect that is counteracted by exposure to curare from hatching. We also recorded spontaneous activity from individual RGCs labelled with lucifer yellow (LY). We found a tendency of RGCs with large dendritic fields to be spontaneously more active than small-field cells. From all these observations, we conclude that immature spontaneous activity promotes dendritic growth in developing RGCs.

Enigmatic GABAergic networks in adult neurogenic zones

The discovery and description of complex GABAergic networks in adult neurogenic zones suggest the intriguing possibility of information transfer from neuronal activity to immature cells. New questions also emerge regarding the mode of GABAergic signaling and the temporal pattern of receptor activation. Non-synaptic (paracrine) signaling communicates information on population size to control the proliferation and migration of progenitor cells in the subventricular zone. How this signaling relates to olfactory bulb network activity, however, remains largely unknown. This review argues that paracrine signaling precedes and then co-exists with synaptic GABAergic signaling, which provides the timing and instruction for cells to properly differentiate and synaptically integrate into an existing network. The evidence examined in this review indicates that the commonly cited mechanism of GABA's action (i.e., depolarization leading to voltage-gated calcium channel activation and calcium entry) needs to be re-examined in the context of the unique cellular properties and organization of the adult neurogenic regions.

GABA(A) receptor diversity and pharmacology

Because of its control of spike-timing and oscillatory network activity, gamma-aminobutyric acid (GABA)-ergic inhibition is a key element in the central regulation of somatic and mental functions. The recognition of GABA(A) receptor diversity has provided molecular tags for the analysis of distinct neuronal networks in the control of specific pharmacological and physiological brain functions. Neurons expressing alpha(1)GABA(A) receptors have been found to mediate sedation, whereas those expressing alpha(2)GABA(A) receptors mediate anxiolysis. Furthermore, associative temporal and spatial memory can be regulated by modulating the activity of hippocampal pyramidal cells via extrasynaptic alpha(5)GABA(A) receptors. In addition, neurons expressing alpha(3)GABA(A) receptors are instrumental in the processing of sensory motor information related to a schizophrenia endophenotype. Finally, during the postnatal development of the brain, the maturation of GABAergic interneurons seems to provide the trigger for the experience-dependent plasticity of neurons in the visual cortex, with alpha(1)GABA(A) receptors setting the time of onset of a critical period of plasticity. Thus, particular neuronal networks defined by respective GABA(A) receptor subtypes can now be linked to the regulation of various clearly defined behavioural patterns. These achievements are of obvious relevance for the pharmacotherapy of certain brain disorders, in particular sleep dysfunctions, anxiety disorders, schizophrenia and diseases associated with memory deficits.

Transcriptional regulation of neuronal phenotype in mammals

Transcription factors (TFs) play pivotal roles in directing the formation of neurons and glia. Here I will review the recent genome-scale analysis of the expression of TFs in the developing mouse nervous system and discuss the logic by which TFs control the establishment of neuronal phenotype. Accumulating evidence suggests that while combinatorial action of TFs is able to define the basic framework of the nervous system, other control mechanisms, such as stochastic and epigenetic regulation of gene expression, also contribute to the generation of nerve cell diversity.

Thyroid hormone and γ-aminobutyric acid (GABA) interactions in neuroendocrine systems

Thyroid hormones (THs) have critical roles in brain development and normal brain function in vertebrates. Clinical evidence suggests that some human nervous disorders involving GABA(gamma-aminobutyric acid)-ergic systems are related to thyroid dysfunction (i.e. hyperthyroidism or hypothyroidism). There is experimental evidence from in vivo and in vitro studies on rats and mice indicating that THs have effects on multiple components of the GABA system. These include effects on enzyme activities responsible for synthesis and degradation of GABA, levels of glutamate and GABA, GABA release and reuptake, and GABA(A) receptor expression and function. In developing brain, hypothyroidism generally decreases enzyme activities and GABA levels whereas in adult brain, hypothyroidism generally increases enzyme activities and GABA levels. Hyperthyroidism does not always have the opposite effect. In vitro studies on adult brain have shown that THs enhance GABA release and inhibit GABA-reuptake by rapid, extranuclear actions, suggesting that presence of THs in the synapse could prolong the action of GABA after release. There are conflicting results on effects of long term changes in TH levels on GABA reuptake. Increasing and decreasing circulating TH levels experimentally in vivo alter density of GABA(A) receptor-binding sites for GABA and benzodiazepines in brain, but results vary from study to study, which may reflect important regional differences in the brain. There is substantial evidence that THs also have an extranuclear effect to inhibit GABA-stimulated Cl(-) currents by a non-competitive mechanism in vitro. The thyroid gland exhibits GABA transport mechanisms as well as enzyme activities for GABA synthesis and degradation, all of which are sensitive to thyroidal state. In rats and humans, GABA inhibits thyroid stimulating hormone (TSH) release from the pituitary, possibly by action directly on the pituitary or on hypothalamic thyrotropin-releasing hormone neurons. In mice, GABA inhibits TSH-stimulated TH release from the thyroid gland. Taken together, these studies provide strong support for the hypothesis that there is reciprocal regulation of the thyroid and GABA systems in vertebrates.

The neuronal excitatory amino acid transporter EAAC1/EAAT3: does it represent a major actor at the brain excitatory synapse?

EAAC1/EAAT3 is a transporter of glutamate (Glu) present at the post-synaptic neuronal element, in opposition to the two other main transporters, GLAST/EAAT1 and GLT1/EAAT2, expressed at the excitatory amino acid (EAA) synapse by surrounding astrocytes. Although, in the adult, EAAC1/EAAT3 exhibits a rather low expression level and is considered to make a minor contribution to Glu removal from the synapse, its early expression during brain development, before the astrocytes are functional, suggests that such a neuronal transporter is involved in the developmental effects of EAA and, possibly, in the biosynthesis and trophic role of GABA, which is excitatory in nature in different brain regions during the earlier stages of brain development. This neuronal Glu transporter is considered to have a dual action as it is apparently involved in the neuronal uptake of cysteine, which acts as a key substrate for the synthesis of glutathione, a major anti-oxidant, because the neurones do not express the Xc(-) transport system in the mature brain. Interestingly, EAAC1/EAAT3 activity/expression was shown to be highly regulated by neuronal activity as well as by intracellular signalling pathways involving primarily alpha protein kinase C (alphaPKC) and phosphatidylinositol-3-kinase (PI3K). Such regulatory processes could act either at the post-traductional level or at the transcriptional level. It is worth noting that EAAC1/EAAT3 exhibits specificity, compared with other EAA transporters, because it is present mainly in the intracellular compartment and only for about 20% at the plasma membrane. Variations in neuronal Glu uptake were shown to be associated with rapid changes in the trafficking of the transporter protein altering the membranar location of the transporter. More recent data show that astrocyte-secreted factors such as cholesterol could also influence rapid changes in the location of EAAC1/EAAT3 between the plasma membrane and the cytoplasmic compartment. Such a highly regulated process of EAAC1/EAAT3 activity/expression may have implications in the physiopathology of major diseases affecting EAA brain signalling, which is further supported by data obtained in animal models of hypoxia-anoxia, for example.

A shared vesicular carrier allows synaptic corelease of GABA and glycine

The type of vesicular transporter expressed by a neuron is thought to determine its neurotransmitter phenotype. We show that inactivation of the vesicular inhibitory amino acid transporter (Viaat, VGAT) leads to embryonic lethality, an abdominal defect known as omphalocele, and a cleft palate. Loss of Viaat causes a drastic reduction of neurotransmitter release in both GABAergic and glycinergic neurons, indicating that glycinergic neurons do not express a separate vesicular glycine transporter. This loss of GABAergic and glycinergic synaptic transmission does not impair the development of inhibitory synapses or the expression of KCC2, the K+ -Cl- cotransporter known to be essential for the establishment of inhibitory neurotransmission. In the absence of Viaat, GABA-synthesizing enzymes are partially lost from presynaptic terminals. Since GABA and glycine compete for vesicular uptake, these data point to a close association of Viaat with GABA-synthesizing enzymes as a key factor in specifying GABAergic neuronal phenotypes.

Ambient GABA constrains the strength of GABAergic synapses at Cajal-Retzius cells in the developing visual cortex

At early stages of brain development, GABA plays a dual role. It fulfills important trophic functions and provides a major excitatory drive for the immature neuronal network. Here, we investigated whether GABA itself can limit the strength of excitatory GABAergic synapses on Cajal-Retzius (CR) cells in sagittal slices from the mouse visual cortex. (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl)phosphinic acid (CGP55845), a specific GABAB receptor (GABABR) blocker, increased the frequency of spontaneous Ca2+ transients and spontaneous and miniature IPSCs (mIPSCs) but did not affect mIPSC amplitudes or kinetics. CGP55845 significantly increased evoked IPSC (eIPSC) amplitudes and decreased the paired-pulse ratio (PPR). Baclofen, a specific GABABR agonist, produced opposite effects. The size of the readily releasable pool was not affected by these GABABR modulators. The same CGP55845 actions were observed at physiological temperatures, but they were abolished after glutamate decarboxylase block with 3-mercaptopropionic acid (3-MP). These results indicate that presynaptic GABABRs dynamically regulate GABA release probability. SNAP-5114, a specific GABA transporter-2/3 (GAT-2/3) blocker, enhanced mIPSC frequencies, decreased PPR, and increased eIPSC amplitudes without changing eIPSC kinetics. These effects were blocked by CGP55845 and 3-MP. NO-711, a specific GAT-1 blocker, prolonged eIPSC decay and decreased eIPSC/mIPSC amplitudes. These NO-711-mediated effects were not sensitive to CGP55845 and 3-MP. We conclude that the strength of GABAergic inputs to CR cells is constrained by GABABRs that are persistently activated by ambient GABA. The latter is also provided by GAT-2/3 operating in the reversed mode. Presynaptic GAT-1 functions in the uptake mode and possibly provides GABA for presynaptic vesicle filling.

GABAergic signaling in young granule cells in the adult rat and mouse dentate gyrus

Throughout most of the developing brain, including the hippocampus, GABAergic synapses are the first to become functional. Several features of GABAergic signaling change across development, suggesting that this signaling in the immature brain may play important roles in the growth of young neurons and the establishment of networks. To determine whether GABA(A) receptor (GABA(A)R)-containing synapses in new neurons born in the adult dentate gyrus have similar immature features, we examined spontaneous and evoked GABA(A)R-mediated synaptic currents in young (POMC-EGFP or doublecortin-immunostained) granule cells in acute slice preparations from adult mice and rats. Spontaneous inhibitory postsynaptic currents (IPSCs) were observed in nearly all immature granule cells, but their frequency was considerably lower and their decay time constant was nearly two times longer than in neighboring mature (doublecortin-non-immunoreactive or EGFP-non-expressing) granule cells within the sub-granular zone. Evoked IPSCs (eIPSCs) in mature granule cells, but not immature granule cells, were sensitive to zolpidem, suggesting a maturational increase in GABA(A)R alpha1-subunit expression. Perforated-patch recording revealed that eIPSCs depolarized young neurons, but hyperpolarized mature neurons. The early establishment of synaptic GABAergic inputs slow IPSC decay time, and depolarizing action of eIPSCs are remarkably similar to features previously seen in neurons during development, suggesting that they are intrinsic features of immature neurons and not functions of the surrounding circuitry. These developmental features in adult-born granule cells could play a role in maturational processes such as developmental cell death. However, treatment of adult mice with GABA(A)R agonists and an inverse agonist did not significantly alter the number of 4- to 14-day-old BrdU-labeled cells.

GABAergic miniature postsynaptic currents in septal neurons show differential allosteric sensitivity after binge-like ethanol exposure

Binge-like ethanol treatment of septal neurons blunts GABAAR-mediated miniature postsynaptic currents (mPSCs), suggesting it arrests synaptic development. Ethanol may disrupt postsynaptic maturation by blunting feedback signaling through immature GABAARs. Here, the impact of ethanol on the sensitivity of mPSCs to zolpidem, zinc and 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha-OH-DHP) was tested. The decay phase of mPSCs showed concentration-dependent potentiation by zolpidem (0.03-100 microM), which was substantially blunted after ethanol exposure. Since zolpidem potentiation exhibited a substantial age-dependent increase in untreated neurons, this finding supported the idea that ethanol arrests synaptic development. GABAAR alpha1 subunit protein also increased with age in untreated neurons, paralleling enhanced sensitivity to zolpidem. Surprisingly, alpha1 levels were not reduced by binge ethanol even though mPSCs were relatively zolpidem-insensitive. Zinc (3-30 microM) decreased mPSC parameters in a concentration- and age-related manner with older untreated cells showing less inhibition. However, there was no increase in mPSC zinc sensitivity after binge ethanol as would be expected if a general arrest of synaptic maturation had occurred. 3alpha-OH-DHP (3-1000 nM) induced concentration-dependent potentiation of mPSC decay. Although potentiation was age-independent, binge ethanol treatment exaggerated sensitivity to this neurosteroid. Finally, chronic picrotoxin pretreatment (100 microM) intended to mimic GABAAR inhibition from ethanol pretreatment did not significantly change mPSC modulation by zolpidem, zinc or 3alpha-OH-DHP. These results suggest that binge ethanol treatment selectively arrests a subset of processes important for maturation of postsynaptic GABAA Rs. However, it is unlikely that ethanol causes a broad arrest of postsynaptic development through a direct inhibition of GABAAR signaling.

A noncanonical release of GABA and glutamate modulates neuronal migration

Immature neurons express GABA and glutamate receptors before synapse formation, and both transmitters are released at an early developmental stage. We have now tested the hypothesis that the ongoing release of GABA and glutamate modulates neuronal migration. Using 5-bromo-2'-deoxyuridine labeling and cocultures of hippocampal slices obtained from naive and green fluorescent protein-transgenic mice, we report that migration is severely affected by GABA(A) or NMDA receptor antagonist treatments. These effects were also present in munc18-1 knock-out slices in which soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent vesicular secretion of transmitters has been deleted. GABA(A) antagonists were more efficient than NMDA antagonists to reduce cell migration, in keeping with the earlier maturation of GABAergic mechanisms. We conclude that GABA and, to a lesser degree, glutamate released in a SNARE-independent mechanism exert a paracrine action on neuronal migration.

Ambient GABA promotes cortical entry of tangentially migrating cells derived from the medial ganglionic eminence

During corticogenesis, cells from the medial ganglionic eminence (MGE) migrate tangentially into the neocortical anlage. Here we report that gamma-aminobutyric acid (GABA), via GABAA receptors, regulates tangential migration. In embryonic telencephalic slices, bicuculline produced an outward current in migrating MGE-derived cells in the neocortex, suggesting the presence of and tonic activation by ambient GABA. Ambient GABA was also present in the MGE, although this required demonstration using as bioassay HEK293 cells expressing high-affinity alpha6/beta2/gamma2s recombinant GABAA receptors. The concentration of ambient GABA was 0.5+/-0.1 microM in both regions. MGE-derived cells before the corticostriate juncture (CSJ) were less responsive to GABA than those in the neocortex, and profiling of GABAA receptor subunit transcripts revealed different expression patterns in the MGE vis-à-vis the neocortex. These findings suggest a dynamic expression of GABAA receptor number or isoform as MGE-derived cells enter the neocortex and become tonically influenced by ambient GABA. Treatment with bicuculline or antibody against GABA did not affect migration of MGE-derived cells before the CSJ but decreased "crossing index," reflecting impeded migration past the CSJ into the neocortex. Treatment with diazepam or addition of exogenous GABA increased crossing index. We conclude that ambient GABA promotes cortical entry of tangentially migrating MGE-derived cells.

Primary Vagal Projection to the Contralateral Non-NTS Region in the Embryonic Chick Brainstem Revealed by Optical Recording

Using multiple-site optical recording with the voltage-sensitive dye, NK2761, we found that vagus nerve stimulation in the embryonic chick brainstem elicits postsynaptic responses in an undefined region on the contralateral side. The characteristics of the contralateral optical signals suggested that they correspond to the monosynaptic response that is related to the vagal afferent fibers. The location of the contralateral response was different from the vagal motor nucleus (the dorsal motor nucleus of the vagus nerve) and sensory nucleus (the nucleus of the tractus solitarius), and other brainstem nuclei that receive primary vagal projection. These results show that the vagus nerve innervates and makes functional synaptic connections in a previously unreported region of the brainstem, and suggest that sensory information processing mediated by the vagus nerve is more complex than expected.

Early expression of KCC2 in rat hippocampal cultures augments expression of functional GABA synapses

The development of GABAergic synapses is associated with an excitatory to inhibitory shift of the actions of GABA because of a reduction of [Cl-]i. This is due to a delayed postnatal expression of the K+ -Cl- cotransporter KCC2, which has low levels at birth and peaks during the first few postnatal weeks. Whether the expression of the cotransporter and the excitatory to inhibitory shift have other consequences on the operation of GABA(A) receptors and synapses is not yet known. We have now expressed KCC2 in immature neurones at an early developmental stage and determined the consequences on the formation of GABA and glutamate synapses. We report that early expression of the cotransporter selectively enhances GABAergic synapses: there is a significant increase of the density of GABA(A) receptors and synapses and an increase of the frequency of GABAergic miniature postsynaptic currents. The density of glutamate synapses and frequency of AMPA miniature postsynaptic currents are not affected. We conclude that the expression of KCC2 and the reduction of [Cl-]i play a critical role in the construction of GABAergic networks that extends beyond the excitatory to inhibitory shift of the actions of GABA.