Excitatory actions of GABA in developing rat hypothalamic neurones

GABA is excitatory in adult vasopressinergic neuroendocrine cells

Neuronal excitability in the adult brain is controlled by a balance between synaptic excitation and inhibition mediated by glutamate and GABA, respectively. While generally inhibitory in the adult brain, GABA(A) receptor activation is excitatory under certain conditions in which the GABA reversal potential is shifted positive due to intracellular Cl(-) accumulation, such as during early postnatal development and brain injury. However, the conditions under which GABA is excitatory are generally either transitory or pathological. Here, we reveal GABAergic synaptic inputs to be uniformly excitatory in vasopressin (VP)-secreting magnocellular neurons in the adult hypothalamus under normal conditions. The GABA reversal potential (E(GABA)) was positive to resting potential and spike threshold in VP neurons, but not in oxytocin (OT)-secreting neurons. The VP neurons lacked expression of the K(+)-Cl(-) cotransporter 2 (KCC2), the predominant Cl(-) exporter in the adult brain. The E(GABA) was unaffected by inhibition of KCC2 in VP neurons, but was shifted positive in OT neurons, which express KCC2. Alternatively, inhibition of the Na(+)-K(+)-Cl(-) cotransporter 1 (NKCC1), a Cl(-) importer expressed in most cell types mainly during postnatal development, caused a negative shift in E(GABA) in VP neurons, but had no effect on GABA currents in OT neurons. GABA(A) receptor blockade caused a decrease in the firing rate of VP neurons, but an increase in firing in OT neurons. Our findings demonstrate that GABA is excitatory in adult VP neurons, suggesting that the classical excitation/inhibition paradigm of synaptic glutamate and GABA control of neuronal excitability does not apply to VP neurons.

GABA coordinates with insulin in regulating secretory function in pancreatic INS-1 β-cells

Pancreatic islet β-cells produce large amounts of γ-aminobutyric acid (GABA), which is co-released with insulin. GABA inhibits glucagon secretion by hyperpolarizing α-cells via type-A GABA receptors (GABA(A)Rs). We and others recently reported that islet β-cells also express GABA(A)Rs and that activation of GABA(A)Rs increases insulin release. Here we investigate the effects of insulin on the GABA-GABA(A)R system in the pancreatic INS-1 cells using perforated-patch recording. The results showed that GABA produces a rapid inward current and depolarizes INS-1 cells. However, pre-treatment of the cell with regular insulin (1 µM) suppressed the GABA-induced current (I(GABA)) by 43%. Zinc-free insulin also suppressed I(GABA) to the same extent of inhibition by regular insulin. The inhibition of I(GABA) occurs within 30 seconds after application of insulin. The insulin-induced inhibition of I(GABA) persisted in the presence of PI3-kinase inhibitor, but was abolished upon inhibition of ERK, indicating that insulin suppresses GABA(A)Rs through a mechanism that involves ERK activation. Radioimmunoassay revealed that the secretion of C-peptide was enhanced by GABA, which was blocked by pre-incubating the cells with picrotoxin (50 µM, p<0.01) and insulin (1 µM, p<0.01), respectively. Together, these data suggest that autocrine GABA, via activation of GABA(A)Rs, depolarizes the pancreatic β-cells and enhances insulin secretion. On the other hand, insulin down-regulates GABA-GABA(A)R signaling presenting a feedback mechanism for fine-tuning β-cell secretion.

Chloride cotransporters, chloride homeostasis, and synaptic inhibition in the developing auditory system

The role of glycine and GABA as inhibitory neurotransmitters in the adult vertebrate nervous system has been well characterized in a variety of model systems, including the auditory, which is particularly well suited for analyzing inhibitory neurotransmission. However, a full understanding of glycinergic and GABAergic transmission requires profound knowledge of how the precise organization of such synapses emerges. Likewise, the role of glycinergic and GABAergic signaling during development, including the dynamic changes in regulation of cytosolic chloride via chloride cotransporters, needs to be thoroughly understood. Recent literature has elucidated the developmental expression of many of the molecular components that comprise the inhibitory synaptic phenotype. An equally important focus of research has revealed the critical role of glycinergic and GABAergic signaling in sculpting different developmental aspects in the auditory system. This review examines the current literature detailing the expression patterns and function (chapter 1), as well as the regulation and pharmacology of chloride cotransporters (chapter 2). Of particular importance is the ontogeny of glycinergic and GABAergic transmission (chapter 3). The review also surveys the recent work on the signaling role of these two major inhibitory neurotransmitters in the developing auditory system (chapter 4) and concludes with an overview of areas for further research (chapter 5).

Novel protective effect of mifepristone on detrimental GABAA receptor activity to immature Purkinje neurons

Immature Purkinje neurons are particularly vulnerable cells. They survive in cerebellar slice cultures under treatment by the synthetic steroid mifepristone (RU486) that depolarizes them at this age. The present study aims at understanding the mechanism underlying this neuroprotective effect. In the developing cerebellum, the role of γ-aminobutyric acid (GABA) in neuron survival is unknown. In 3-d-old mouse cerebellar slice cultures, we show that GABA(A) receptor activation is depolarizing and excitatory. Antagonists of GABA(A) receptors rescue Purkinje neurons, demonstrating that GABA is endogenously released in this preparation and is toxic. Mifepristone likely protects these neurons by reversing GABA(A) receptor-mediated chloride fluxes and reducing their driving force. Neuroprotection by mifepristone is dose-dependently decreased by the agonist of GABA(A) receptors muscimol and by caffeine, an agonist of internal calcium store release. Moreover, the survival induced by neomycin, an inhibitor of calcium release, is partially reversed by muscimol. The p38 mitogen-activated protein kinase (MAPK) inhibitor SB239063 also rescues Purkinje neurons. In summary, we propose that when GABA is depolarizing, mifepristone protects Purkinje neurons by shunting GABA responses and probably chloride fluxes, by inhibiting p38 MAPK activity and likely internal calcium store release. A new and nonhormonal effect of mifepristone is thus revealed.

Development of the chloride homeostasis in the auditory brainstem

Inhibitory neurotransmission plays a substantial role in encoding of auditory cues relevant for sound localization in vertebrates. While the anatomical organization of the respective afferent auditory brainstem circuits shows remarkable similarities between mammals and birds, the properties of inhibitory neurotransmission in these neural circuits are strikingly different. In mammals, inhibition is predominantly glycinergic and endowed with fast kinetics. In birds, inhibition is mediated by gamma-Aminobutiric acid (GABA) and too slow to convey temporal information. A further prominent difference lies in the mechanism of inhibition in the respective systems. In auditory brainstem neurons of mammals, [Cl(-)](i) undergoes a developmental shift causing the actions of GABA and glycine to gradually change from depolarization to the 'classic' hyperpolarizing-inhibition before hearing onset. Contrary to this, in the mature avian auditory brainstem Cl(-) homeostasis mechanisms accurately adjust the Cl(-) gradient to enable depolarizing, but still very efficient, shunting inhibition. The present review considers the mechanisms underlying development of the Cl(-) homeostasis in the auditory system of mammals and birds and discusses some open issues that require closer attention in future studies.

GABA exerts protective and regenerative effects on islet beta cells and reverses diabetes

Type 1 diabetes (T1D) is an autoimmune disease characterized by insulitis and islet β-cell loss. Thus, an effective therapy may require β-cell restoration and immune suppression. Currently, there is no treatment that can achieve both goals efficiently. We report here that GABA exerts antidiabetic effects by acting on both the islet β-cells and immune system. Unlike in adult brain or islet α-cells in which GABA exerts hyperpolarizing effects, in islet β-cells, GABA produces membrane depolarization and Ca(2+) influx, leading to the activation of PI3-K/Akt-dependent growth and survival pathways. This provides a potential mechanism underlying our in vivo findings that GABA therapy preserves β-cell mass and prevents the development of T1D. Remarkably, in severely diabetic mice, GABA restores β-cell mass and reverses the disease. Furthermore, GABA suppresses insulitis and systemic inflammatory cytokine production. The β-cell regenerative and immunoinhibitory effects of GABA provide insights into the role of GABA in regulating islet cell function and glucose homeostasis, which may find clinical application.

Contribution of the potassium-chloride co-transporter KCC2 to the modulation of lumbar spinal networks in mice

Spontaneous activity is observed in most developing neuronal circuits, such as the retina, hippocampus, brainstem and spinal cord. In the spinal cord, spontaneous activity is important for generating embryonic movements critical for the proper development of motor axons, muscles and synaptic connections. A spontaneous bursting activity can be recorded in vitro from ventral roots during perinatal development. The depolarizing action of the inhibitory amino acids γ-aminobutyric acid and glycine is widely proposed to contribute to spontaneous activity in several immature systems. During development, the intracellular chloride concentration decreases, leading to a shift of equilibrium potential for Cl(-) ions towards more negative values, and thereby to a change in glycine- and γ-aminobutyric acid-evoked potentials from depolarization/excitation to hyperpolarization/inhibition. The up-regulation of the outward-directed Cl(-) pump, the neuron-specific potassium-chloride co-transporter type 2 KCC2, has been shown to underlie this shift. Here, we investigated whether spontaneous and locomotor-like activities are altered in genetically modified mice that express only 8-20% of KCC2, compared with wild-type animals. We show that a reduced amount of KCC2 leads to a depolarized equilibrium potential for Cl(-) ions in lumbar motoneurons, an increased spontaneous activity and a faster locomotor-like activity. However, the left-right and flexor-extensor alternating pattern observed during fictive locomotion was not affected. We conclude that neuronal networks within the spinal cord are more excitable in KCC2 mutant mice, which suggests that KCC2 strongly modulates the excitability of spinal cord networks.

Decreased Immunoreactivities and Functions of the Chloride Transporters, KCC2 and NKCC1, in the Lateral Superior Olive Neurons of Circling Mice

Objectives

We tested the possibility of differential expression and function of the potassium-chloride (KCC2) and sodium-potassium-2 chloride (NKCC1) co-transporters in the lateral superior olive (LSO) of heterozygous (+/cir) or homozygous (cir/cir) mice.

Methods

Mice pups aged from postnatal (P) day 9 to 16 were used. Tails from mice were cut for DNA typing. For Immunohistochemical analysis, rabbit polyclonal anti-KCC2 or rabbit polyclonal anti-NKCC1 was used and the density of immunolabelings was evaluated using the NIH image program. For functional analysis, whole cell voltage clamp technique was used in brain stem slices and the changes of reversal potentials were evaluated at various membrane potentials.

Results

Immunohistochemical analysis revealed both KCC2 and NKCC1 immunoreactivities were more prominent in heterozygous (+/cir) than homozygous (cir/cir) mice on P day 16. In P9-P12 heterozygous (+/cir) mice, the reversal potential (E_gly) of glycine-induced currents was shifted to a more negative potential by 50 µM bumetanide, a known NKCC1 blocker, and the negatively shifted E_gly was restored by additional application of 1 mM furosemide, a KCC2 blocker (-58.9±2.6 mV to -66.0±1.5 mV [bumetanide], -66.0±1.5 mV to -59.8±2.8 mV [furosemide+bumetanide], n=11). However, only bumetanide was weakly, but significantly effective (-60.1±2.9 mV to -62.7±2.6 mV [bumetanide], -62.7±2.6 mV to -62.1±2.5 mV [furosemide+bumetanide], n=7) in P9-P12 homozygous (cir/cir) mice.

Conclusion

The less prominent immunoreactivities and weak or absent responses to bumetanide or furosemide suggest impaired function or delayed development of both transporters in homozygous (cir/cir) mice.

Cloning and characterization of GABAA α subunits and GABAB subunits in Xenopus laevis during development

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the adult nervous system, acts via two classes of receptors, the ionotropic GABA(A) and metabotropic GABA(B) receptors. During the development of the nervous system, GABA acts in a depolarizing, excitatory manner and plays an important role in various neural developmental processes including cell proliferation, migration, synapse formation, and activity-dependent differentiation. Here we describe the spatial and temporal expression patterns of the GABA(A) and GABA(B) receptors during early development of Xenopus laevis. Using in situ hybridization and qRT-PCR, GABA(A) α2 was detected as a maternal mRNA. All other α-subunits were first detected by tailbud through hatching stages. Expression of the various subunits was seen in the brain, spinal cord, cranial ganglia, olfactory epithelium, pineal, and pituitary gland. Each receptor subunit showed a distinctive, unique expression pattern, suggesting these receptors have specific functions and are regulated in a precise spatial and temporal manner.

In the developing hippocampus kainate receptors control the release of GABA from mossy fiber terminals via a metabotropic type of action

Kainate receptors (KARs) are glutamate-gated ion channels assembled from various combinations of GluK1-GluK5 subunits with different physiological and pharmacological properties. In the hippocampus, KARs expressed at postsynaptic sites mediate a small component of excitatory postsynaptic currents while at presynaptic sites they exert a powerful control on transmitter release at both excitatory and inhibitory connections. KARs are developmentally regulated and play a key role in several developmental processes including neuronal migration, differentiation and synapse formation. Interestingly, they can signal through a canonical ionotropic pathway but also through a noncanonical modality involving pertussis toxin-sensitive G proteins and downstream signaling molecules.In this Chapter some of our recent data concerning the functional role of presynaptic KARs in regulation of transmitter release from immature mossy fiber terminals and in synaptic plasticity processes will be reviewed. Early in postnatal development, MFs release into their targeted neurons mainly GABA which is depolarizing and excitatory. Endogenous activation of GluK1 KARs localized on MF terminals by glutamate present in the extracellular space down regulates GABA release, leading sometimes to synapse silencing. The depressant effect of GluK1 on MF responses is mediated by a metabotropic process, sensitive to pertussis toxin and phospholipase C (PLC) along the transduction pathway downstream to G protein activation. Blocking PLC with the selective antagonist U73122, unmasks the potentiating effect of GluK1 on MF-evoked GABAergic currents, which probably depend on the ionotropic type of action of these receptors.In addition, GluK1 KARs dynamically regulate the direction of spike-time dependent plasticity, a particular form of Hebbian type of learning which consists in bidirectional modifications in synaptic strength according to the temporal order of pre and postsynaptic spiking. At immature MF-CA3 synapses pairing MF stimulation with postsynaptic spiking and vice versa induces long term depression of MF-evoked GABAergic currents. In the case of positive pairing synaptic depression can be switched into spike-time dependent potentiation by blocking GluK1 KARs with UBP 302. The depressant action exerted by GluK1 KARs on MF responses would prevent the excessive activation of the CA3 associative network by the excitatory action of GABA early in postnatal development.

GABAergic circuits control stimulus-instructed receptive field development in the optic tectum

During the development of sensory systems, receptive fields are modified by stimuli in the environment. This is thought to rely on learning algorithms that are sensitive to correlations in spike timing between cells, but the manner in which developing circuits selectively exploit correlations that are related to sensory inputs is unknown. We recorded from neurons in the developing optic tectum of Xenopus laevis and found that repeated presentation of moving visual stimuli induced receptive field changes that reflected the properties of the stimuli and that this form of learning was disrupted when GABAergic transmission was blocked. Consistent with a role for spike timing-dependent mechanisms, GABA blockade altered spike-timing patterns in the tectum and increased correlations between cells that would affect plasticity at intratectal synapses. This is a previously unknown role for GABAergic signals in development and highlights the importance of regulating the statistics of spiking activity for learning.

Synapse-to-neuron ratio is inversely related to neuronal density in mature neuronal cultures

Synapse formation is a fundamental process in neurons that occurs throughout development, maturity, and aging. Although these stages contain disparate and fluctuating numbers of mature neurons, tactics employed by neuronal networks to modulate synapse number as a function of neuronal density are not well understood. The goal of this study was to utilize an in vitro model to assess the influence of cell density and neuronal maturity on synapse number and distribution. Specifically, cerebral cortical neurons were plated in planar culture at densities ranging from 10 to 5000 neurons/mm², and synapse number and distribution were evaluated via immunocytochemistry over 21 days in vitro (DIV). High-resolution confocal microscopy revealed an elaborate three-dimensional distribution of neurites and synapses across the heights of high-density neuronal networks by 21 DIV, which were up to 18 μm thick, demonstrating the complex degree of spatial interactions even in planar high-density cultures. At 7 DIV, the mean number of synapses per neuron was less than 5, and this did not vary as a function of neuronal density. However, by 21 DIV, the number of synapses per neuron had jumped 30- to 80-fold, and the synapse-to-neuron ratio was greatest at lower neuronal densities (< 500 neurons/mm²; mean approximately 400 synapses/neuron) compared to mid and higher neuronal densities (500-4500 neurons/mm²; mean of approximately 150 synapses/neuron) (p<0.05). These results suggest a relationship between neuronal density and synapse number that may have implications in the neurobiology of developing neuronal networks as well as processes of cell death and regeneration.

Contribution of GABAergic interneurons to the development of spontaneous activity patterns in cultured neocortical networks

Periodic synchronized events are a hallmark feature of developing neuronal networks and are assumed to be crucial for the maturation of the neuronal circuitry. In the developing neocortex, the early network oscillations coincide with an excitatory action of the neurotransmitter gamma-aminobutyric acid (GABA). A relationship between the emerging inhibitory action of GABA and the gradual disappearance of early synchronized network activity has been previously suggested. Therefore we investigate the interplay between the action of GABA and spontaneous activity in cultured networks of the lateral or dorsal embryonic rat neocortex, which show considerable difference in the content of GABAergic neurons. Here we present the results of long-term monitoring of spontaneous electrical activity of cultured networks growing on microelectrode arrays and the time course of changes in GABA action using calcium imaging. All cultures studied displayed stereotyped synchronized burst events at the end of the first week in vitro. As the GABA_A depolarizing action decreases, naturally or after bumetanide treatment, network activity in lateral cortex cultures changed from stereotypic bursting to more clustered and asynchronous activity patterns. Dorsal cortex cultures and cultures lacking GABA_A-receptor mediated synaptic transmission, retained an immature synchronous firing pattern, but developed prominent intraburst oscillations (∼3–10 Hz). Large, mostly parvalbumin positive, GABAergic neurons dominate the GABAergic population in lateral cortex cultures. These large interneurons were virtually absent in dorsal cortex cultures. Based on these results, we suggest that the richly interconnected large GABAergic neurons contribute to desynchronize and temporally differentiate the spontaneous activity of cultured cortical networks.

Neonatal exposure to estradiol in rats influences neuroactive steroid concentrations, GABAA receptor expression, and behavioral sensitivity to anxiolytic drugs

Gonadal steroids, in particular estradiol, exert important actions during pre- and perinatal periods in the regulation of sexual dimorphism and development of the nervous system. We have now examined the effects of neonatal estradiol administration in female rats on brain concentrations of the neuroactive steroids allopregnanolone and tetrahydrodeoxycorticosterone, expression of GABA(A) receptor subunits, and behavioral sensitivity to benzodiazepines and allopregnanolone. Administration of beta-estradiol 3-benzoate on the day of birth resulted in marked decreases in the concentrations of progesterone and allopregnanolone in the cerebral cortex at 21, 60, and 180 days after birth. The concentrations of tetrahydrodeoxycorticosterone, 17beta-estradiol, and dehydroepiandrosterone in the brain at 60 days were not affected by such treatment. Neonatal administration of beta-estradiol 3-benzoate also increased the cerebrocortical abundance of alpha(1), alpha(2), and gamma(2) subunits of the GABA(A) receptor without affecting that of alpha(3), alpha(4), alpha(5), or delta subunits. Diazepam induced a greater reduction in locomotor activity as well as a more pronounced anxiolytic-like effect in the elevated plus-maze test in rats subjected to neonatal treatment with beta-estradiol 3-benzoate than in vehicle-treated controls, while allopregnanolone induced a similar effect in both groups. These effects of estradiol suggest that it plays a major role in regulation both of GABAergic transmission and of the abundance of endogenous modulators of such transmission during development of the central nervous system.

Seizure sensitivity is ameliorated by targeted expression of K+-Cl- cotransporter function in the mushroom body of the Drosophila brain

The kcc(DHS1) allele of kazachoc (kcc) was identified as a seizure-enhancer mutation exacerbating the bang-sensitive (BS) paralytic behavioral phenotypes of several seizure-sensitive Drosophila mutants. On their own, young kcc(DHS1) flies also display seizure-like behavior and demonstrate a reduced threshold for seizures induced by electroconvulsive shock. The product of kcc shows substantial homology to KCC2, the mammalian neuronal K(+)-Cl(-) cotransporter. The kcc(DHS1) allele is a hypomorph, and its seizure-like phenotype reflects reduced expression of the kcc gene. We report here that kcc functions as a K(+)-Cl(-) cotransporter when expressed heterologously in Xenopus laevis oocytes: under hypotonic conditions that induce oocyte swelling, oocytes that express Drosophila kcc display robust ion transport activity observed as a Cl(-)-dependent uptake of the K(+) congener (86)Rb(+). Ectopic, spatially restricted expression of a UAS-kcc(+) transgene was used to determine where cotransporter function is required in order to rescue the kcc(DHS1) BS paralytic phenotype. Interestingly, phenotypic rescue is largely accounted for by targeted, circumscribed expression in the mushroom bodies (MBs) and the ellipsoid body (EB) of the central complex. Intriguingly, we observed that MB induction of kcc(+) functioned as a general seizure suppressor in Drosophila. Drosophila MBs have generated considerable interest especially for their role as the neural substrate for olfactory learning and memory; they have not been previously implicated in seizure susceptibility. We show that kcc(DHS1) seizure sensitivity in MB neurons acts via a weakening of chemical synaptic inhibition by GABAergic transmission and suggest that this is due to disruption of intracellular Cl(-) gradients in MB neurons.

Gamma-aminobutyric acid and glutamate differentially regulate intracellular calcium concentrations in mouse gonadotropin-releasing hormone neurons

Multiple factors regulate the activity of the GnRH neurons responsible for controlling fertility. Foremost among neuronal inputs to GnRH neurons are those using the amino acids glutamate and gamma-aminobutyric acid (GABA). The present study used a GnRH-Pericam transgenic mouse line, enabling live cell imaging of intracellular calcium concentrations ([Ca(2+)](i)) to evaluate the effects of glutamate and GABA signaling on [Ca(2+)](i) in peripubertal and adult mouse GnRH neurons. Activation of GABA(A), N-methyl-d-aspartate, or alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate acid (AMPA) receptors was found to evoke an increase in [Ca(2+)](i), in subpopulations of GnRH neurons. Approximately 70% of GnRH neurons responded to GABA, regardless of postnatal age or sex. Many fewer (approximately 20%) GnRH neurons responded to N-methyl-d-aspartate, and this was not influenced by postnatal age or sex. In contrast, about 65% of adult male and female GnRH neurons responded to AMPA compared with about 14% of male and female peripubertal mice (P < 0.05). The mechanisms underlying the ability of GABA and AMPA to increase [Ca(2+)](i) in adult GnRH neurons were evaluated pharmacologically. Both GABA and AMPA were found to evoke [Ca(2+)](i) increases through a calcium-induced calcium release mechanism involving internal calcium stores and inositol-1,4,5-trisphosphate receptors. For GABA, the initial increase in [Ca(2+)](i) originated from GABA(A) receptor-mediated activation of L-type voltage-gated calcium channels, whereas for AMPA this appeared to involve direct calcium entry through the AMPA receptor. These observations show that all of the principal amino acid receptors are able to control [Ca(2+)](i) in GnRH neurons but that they do so in a postnatal age- and intracellular pathway-specific manner.

Decreased GABAA receptors functional regulation in the cerebral cortex and brainstem of hypoxic neonatal rats: effect of glucose and oxygen supplementation

Hypoxia in neonates can lead to biochemical and molecular alterations mediated through changes in neurotransmitters resulting in permanent damage to brain. In this study, we evaluated the changes in the receptor status of GABA(A) in the cerebral cortex and brainstem of hypoxic neonatal rats and hypoxic rats supplemented with glucose and oxygen using binding assays and gene expression of GABA(Aalpha1) and GABA(Agamma5). In the cerebral cortex and brainstem of hypoxic neonatal rats, a significant decrease in GABA(A) receptors was observed, which accounts for the respiratory inhibition. Hypoxic rats supplemented with glucose alone and with glucose and oxygen showed a reversal of the GABA(A) receptors, andGABA(Aalpha1) and GABA(Agamma5) gene expression to control. Glucose acts as an immediate energy source thereby reducing the ATP-depletion-induced increase in GABA and oxygenation, which helps in encountering anoxia. Resuscitation with oxygen alone was less effective in reversing the receptor alterations. Thus, the results of this study suggest that reduction in the GABA(A) receptors functional regulation during hypoxia plays an important role in mediating the brain damage. Glucose alone and glucose and oxygen supplementation to hypoxic neonatal rats helps in protecting the brain from severe hypoxic damage.

Enhanced excitatory input to melanin concentrating hormone neurons during developmental period of high food intake is mediated by GABA

In contrast to the local axons of GABA neurons of the cortex and hippocampus, lateral hypothalamic neurons containing melanin concentrating hormone (MCH) and GABA send long axons throughout the brain and play key roles in energy homeostasis and mental status. In adults, MCH neurons maintain a hyperpolarized membrane potential and most of the synaptic input is inhibitory. In contrast, we found that developing MCH neurons received substantially more excitatory synaptic input. Based on gramicidin-perforated patch recordings in hypothalamic slices from MCH-green fluorescent protein transgenic mice, we found that GABA was the primary excitatory synaptic transmitter in embryonic and neonatal ages up to postnatal day 10. Surprisingly, glutamate assumed only a minor excitatory role, if any. GABA plays a complex role in developing MCH neurons, with its actions conditionally dependent on a number of factors. GABA depolarization could lead to an increase in spikes either independently or in summation with other depolarizing stimuli, or alternately, depending on the relative timing of other depolarizing events, could lead to shunting inhibition. The developmental shift from depolarizing to hyperpolarizing occurred later in the dendrites than in the cell body. Early GABA depolarization was based on a Cl(-)-dependent inward current. An interesting secondary depolarization in mature neurons that followed an initial hyperpolarization was based on a bicarbonate mechanism. Thus during the early developmental period when food consumption is high, MCH neurons are more depolarized than in the adult, and an increased level of excitatory synaptic input to these orexigenic cells is mediated by GABA.

Activity-mediated plasticity of GABA equilibrium potential in rat hippocampal CA1 neurons

The equilibrium potential (E(GABA)(-PSC)) for gamma-aminobutyric acid (GABA) A receptor mediated inhibitory postsynaptic currents (PSCs) in hippocampal CA1 pyramidal neurons shifts when theta-burst stimulation (four pulses at 100 Hz in each burst in a train consisting of five bursts with an inter-burst interval of 200 ms, the train repeated thrice at 30-s intervals) is applied to the input. E(GABA)(-PSC) is regulated by K(+)/Cl(-) co-transporter (KCC2). GABA(B) receptors are implicated in modulating KCC2 levels. In the current study, the involvement of KCC2, as well as GABA(B) receptors, in theta-burst-mediated shifts in E(GABA)(-PSC) was examined. Whole-cell patch recordings were made from hippocampal CA1 pyramidal neurons (from 9 to 12 days old rats), in a slice preparation. Glutamatergic excitatory postsynaptic currents were blocked with dl-2-amino-5-phosphonovaleric acid (50 microM) and 6,7-dinitroquinoxaline-2,3-dione (20 microM). The PSC and the E(GABA)(-PSC) were stable when stimulated at 0.05 Hz. However, both changed following a 30-min stimulation at 0.5 or 1 Hz. Furosemide (500 microM) and KCC2 anti-sense in the recording pipette but not bumetanide (20 or 100 microM) or KCC2 sense, blocked the changes, suggesting KCC2 involvement. Theta-burst stimulation induced a negative shift in E(GABA)(-PSC), which was prevented by KCC2 anti-sense; however, KCC2 sense had no effect. CGP55845 (2 microM), a GABA(B) antagonist, applied in the superfusing medium, or GDP-beta-S in the recording pipette, blocked the shift in E(GABA)(-PSC). These results indicate that activity-mediated plasticity in E(GABA)(-PSC) occurs in hippocampal CA1 pyramidal neurons and theta-burst-induced negative shift in E(GABA)(-PSC) requires KCC2, GABA(B) receptors and G-protein activation.

Orexin-A and orexin-B during the postnatal development of the rat brain

Orexin-A and orexin-B are hypothalamic neuropeptides isolated from a small group of neurons in the hypothalamus, which project their axons to all major parts of the central nervous system. Despite the extensive information about orexin expression and function at different parts of the nervous system in adults, data about the development and maturation of the orexin system in the brain are a bit contradictory and insufficient. A previous study has found expression of orexins in the hypothalamus after postnatal day 15 only, while others report orexins detection at embryonic stages of brain formation. In the present study, we investigated the distribution of orexin-A and orexin-B neuronal cell bodies and fibers in the brain at three different postnatal stages: 1-week-, 2-week-old and adult rats. By means of immunohistochemical techniques, we demonstrated that a small subset of cells in the lateral hypothalamus, and the perifornical and periventricular areas were orexin-A and orexin-B positive not only in 2-week-old and adult rats but also in 1-week-old animals. In addition, orexin-A and orexin-B expressing neuronal varicosities were found in many other brain regions. These results suggest that orexin-A and orexin-B play an important role in the early postnatal brain development. The widespread distribution of orexinergic projections through all these stages may imply an involvement of the two neurotransmitters in a large variety of physiological and behavioral processes also including higher brain functions like learning and memory.

Type A GABA-receptor-dependent synaptic transmission sculpts dendritic arbor structure in Xenopus tadpoles in vivo

The emergence of dendritic arbor structure in vivo depends on synaptic inputs. We tested whether inhibitory GABAergic synaptic transmission regulates Xenopus optic tectal cell dendritic arbor development in vivo by expressing a peptide corresponding to an intracellular loop (ICL) of the gamma2 subunit of type A GABA receptors (GABA(A)R), which is required to anchor GABA(A) receptors to the postsynaptic scaffold. Enhanced green fluorescent protein (EGFP)-tagged ICL (EGFP-ICL) was distributed in a punctate pattern at putative inhibitory synapses, identified by vesicular GABA transporter immunoreactive puncta. ICL expression completely blocked GABA(A)R-mediated transmission in 36% of transfected neurons and significantly reduced GABA(A)R-mediated synaptic currents relative to AMPA receptor-mediated synaptic currents in the remaining transfected neurons without altering release probability or neuronal excitability. Further analysis of ICL-expressing neurons with residual GABA(A)R-mediated inputs showed that the capacity of benzodiazepine to enhance GABAergic synaptic responses was reduced in ICL-expressing neurons, indicating that they were likely depleted of gamma2 subunit-containing GABA(A)R. Neurons expressing a mutant form of ICL were comparable to controls. In vivo time-lapse images showed that ICL-expressing neurons have more sparsely branched dendritic arbors, which expand over larger neuropil areas than EGFP-expressing control neurons. Analysis of branch dynamics indicated that ICL expression affected arbor growth by reducing rates of branch addition. Furthermore, we found that decreasing GABAergic synaptic transmission with ICL expression blocked visual experience dependent dendritic arbor structural plasticity. Our findings establish an essential role for inhibitory GABAergic synaptic transmission in the regulation of dendritic structural plasticity in Xenopus in vivo.

GABAergic mechanism of propofol toxicity in immature neurons

Certain anesthetics exhibit neurotoxicity in the brains of immature but not mature animals. Gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the adult brain, is excitatory on immature neurons via its action at the GABAA receptor, due to a reversed transmembrane chloride gradient. GABAA receptor activation in immature neurons is sufficient to open L-type voltage-gated calcium channels. As propofol is a GABAA agonist, we hypothesized that it and more specific GABAA modulators would increase intracellular free calcium ([Ca2+]i), resulting in the death of neonatal rat hippocampal neurons. Neuronal [Ca2+]i was monitored using Fura2-AM fluorescence imaging. Cell death was assessed by double staining with propidium iodide and Hoechst 33258 at 1 hour (acute) and 48 hours (delayed) after 5 hours exposure of neurons to propofol or the GABAA receptor agonist, muscimol, in the presence and absence of the GABA receptor antagonist, bicuculline, or the L-type Ca2+ channel blocker, nifedipine. Fluorescent measurements of caspase-3,-7 activities were performed at 1 hour after exposure. Both muscimol and propofol induced a rapid increase in [Ca2+]i in days in vitro (DIV) 4, but not in DIV 8 neurons, that was inhibited by nifedipine and bicuculline. Caspase-3,-7 activities and cell death increased significantly in DIV 4 but not DIV 8 hippocampal neuronal cultures 1 hour after 5 hours exposure to propofol, but not muscimol, and were inhibited by the presence of bicuculline or nifedipine. We conclude that an increase in [Ca2+]i, due to activation of GABAA receptors and opening of L-type calcium channels, is necessary for propofol-induced death of immature rat hippocampal neurons but that additional mechanisms not elicited by GABAA activation alone also contribute to cell death.

Chapter 2: hypothalamic neural systems controlling the female reproductive life cycle gonadotropin-releasing hormone, glutamate, and GABA

The hypothalamic-pituitary-gonadal (HPG) axis undergoes a number of changes throughout the reproductive life cycle that are responsible for the development, puberty, adulthood, and senescence of reproductive systems. This natural progression is dictated by the neural network controlling the hypothalamus including the cells that synthesize and release gonadotropin-releasing hormone (GnRH) and their regulatory neurotransmitters. Glutamate and GABA are the primary excitatory and inhibitory neurotransmitters in the central nervous system, and as such contribute a great deal to modulating this axis throughout the lifetime via their actions on receptors in the hypothalamus, both directly on GnRH neurons as well as indirectly through other hypothalamic neural networks. Interactions among GnRH neurons, glutamate, and GABA, including the regulation of GnRH gene and protein expression, hormone release, and modulation by estrogen, are critical to age-appropriate changes in reproductive function. Here, we present evidence for the modulation of GnRH neurosecretory cells by the balance of glutamate and GABA in the hypothalamus, and the functional consequences of these interactions on reproductive physiology across the life cycle.

Selective death of newborn neurons in hippocampal dentate gyrus following moderate experimental traumatic brain injury

Memory impairment is one of the most significant residual deficits following traumatic brain injury (TBI) and is among the most frequent complaints heard from patients and their relatives. It has been reported that the hippocampus is particularly vulnerable to TBI, which results in hippocampus-dependent cognitive impairment. There are different regions in the hippocampus, and each region is composed of different cell types, which might respond differently to TBI. However, regional and cell type-specific neuronal death following TBI is not well described. Here, we examined the distribution of degenerating neurons in the hippocampus of the mouse brain following controlled cortical impact (CCI) and found that the majority of degenerating neurons observed were in the dentate gyrus after moderate (0.5 mm cortical deformation) CCI-TBI. In contrast, there were only a few degenerating neurons observed in the hilus, and we did not observe any degenerating neurons in the CA3 or CA1 regions. Among those degenerating cells in the dentate gyrus, about 80% of them were found in the inner granular neuron layer. Analysis with cell type-specific markers showed that most of the degenerating neurons in the inner granular neuron layer are newborn immature neurons. Further quantitative analysis shows that the number of newborn immature neurons in the dentate gyrus is dramatically decreased in the ipsilateral hemisphere compared with the contralateral side. Collectively, our data demonstrate the selective death of newborn immature neurons in the hippocampal dentate gyrus following moderate injury with CCI in mice. This selective vulnerability of newborn immature dentate neurons may contribute to the persistent impairment of learning and memory post-TBI and provide an innovative target for neuroprotective treatment strategies.

Excitatory actions of GABA in the suprachiasmatic nucleus

Neurons in the suprachiasmatic nucleus (SCN) are responsible for the generation of circadian oscillations, and understanding how these neurons communicate to form a functional circuit is a critical issue. The neurotransmitter GABA and its receptors are widely expressed in the SCN where they mediate cell-to-cell communication. Previous studies have raised the possibility that GABA can function as an excitatory transmitter in adult SCN neurons during the day, but this work is controversial. In the present study, we first tested the hypothesis that GABA can evoke excitatory responses during certain phases of the daily cycle by broadly sampling how SCN neurons respond to GABA using extracellular single-unit recording and gramicidin-perforated-patch recording techniques. We found that, although GABA inhibits most SCN neurons, some level of GABA-mediated excitation was present in both dorsal and ventral regions of the SCN, regardless of the time of day. These GABA-evoked excitatory responses were most common during the night in the dorsal SCN region. The Na(+)-K(+)-2Cl(-) cotransporter (NKCC) inhibitor, bumetanide, prevented these excitatory responses. In individual neurons, the application of bumetanide was sufficient to change GABA-evoked excitation to inhibition. Calcium-imaging experiments also indicated that GABA-elicited calcium transients in SCN cells are highly dependent on the NKCC isoform 1 (NKCC1). Finally, Western blot analysis indicated that NKCC1 expression in the dorsal SCN is higher in the night. Together, this work indicates that GABA can play an excitatory role in communication between adult SCN neurons and that this excitation is critically dependent on NKCC1.

Developmental expression of glycine receptor subunits in rat cerebellum

The distribution of the glycine receptor subunits alpha1-3 and beta in the developing rat cerebellum was studied from postnatal day 1 to adulthood by means of quantitative RT-PCR and immunohistochemistry. qRT-PCR of postnatal cerebella indicated the presence of mRNA for each subunit, with a relative expression of alpha2>alpha3>alpha1>beta. The immunohistochemistry indicated a strong alpha2 signal in the Purkinje cells, internal and external granular layers. The alpha1-3 subunits had weak signals in the Purkinje cells and molecular layer. The alpha1 subunit was expressed at a low level and was also found in the white matter. The function of these receptors in neuronal and glial plasma membranes in early postnatal development remains to be determined.

Regulation of excitation by glycine receptors

Our knowledge of glycine receptor (GlyR) regulation of excitation has advanced significantly in recent years. GlyRs are widespread in the CNS, are heterogeneous, and undergo developmental changes. Activation of GlyRs of immature neurons induces outflow of Cl( - ), membrane depolarization, neuronal excitation, calcium influx, and transmitter release, in contrast to the inhibitory effects these receptors have in mature neurons. Thus, GlyRs are important for neuronal excitability in both the developing and the mature CNS. This chapter is an overview of selective studies on the newly discovered roles of GlyRs in regulating neuronal excitation, and inhibition, particularly in the upper brain areas.

Sequential postsynaptic maturation governs the temporal order of GABAergic and glutamatergic synaptogenesis in rat embryonic cultures

Sequential formation of GABAergic and glutamatergic synapses is thought to be crucial for constructing the stereotypic neural networks during brain development. However, why GABAergic synapses are formed earlier than glutamatergic synapses in the developing brain is not well understood. We used electrophysiology and fluorescence imaging to study GABAergic and glutamatergic synaptogenesis in embryonic hypothalamic cultures, which contain approximately 40% GABAergic and approximately 60% glutamatergic neurons. The newly dissociated embryonic hypothalamic neurons contained a significant pool of functional GABA(A) receptors but a very low level of glutamate receptors. Within the first week of culture, the time course of GABAergic synaptogenesis in embryonic neurons coincided with that of presynaptic vesicle cycling, but both measurements lagged behind the detection of functional GABA(A) receptors. Remarkably, the GABA(A) receptors of newly dissociated embryonic neurons can be rapidly clustered into postsynaptic apparatus and generate functional synaptic currents within 4-6 h when cocultured with mature neurons. Consistent with earlier expression of GABA(A) receptors in immature neurons, synaptic GABAergic events were always detected before the onset of glutamatergic events in both purely embryonic and heterochronic cultures. Interestingly, overexpression of glutamate receptors in embryonic neurons not only increased whole-cell glutamate currents but also significantly increased the frequency of excitatory synaptic events. We conclude that the sequential formation of GABAergic and glutamatergic synapses in immature neurons is likely governed by a sequential expression of GABA(A) and glutamate receptors during neuronal development.

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.

GAD67-mediated GABA synthesis and signaling regulate inhibitory synaptic innervation in the visual cortex

The development of GABAergic inhibitory circuits is shaped by neural activity, but the underlying mechanisms are unclear. Here, we demonstrate a novel function of GABA in regulating GABAergic innervation in the adolescent brain, when GABA is mainly known as an inhibitory transmitter. Conditional knockdown of the rate-limiting synthetic enzyme GAD67 in basket interneurons in adolescent visual cortex resulted in cell autonomous deficits in axon branching, perisomatic synapse formation around pyramidal neurons, and complexity of the innervation fields; the same manipulation had little influence on the subsequent maintenance of perisomatic synapses. These effects of GABA deficiency were rescued by suppressing GABA reuptake and by GABA receptor agonists. Germline knockdown of GAD67 but not GAD65 showed similar deficits, suggesting a specific role of GAD67 in the maturation of perisomatic innervation. Since intracellular GABA levels are modulated by neuronal activity, our results implicate GAD67-mediated GABA synthesis in activity-dependent regulation of inhibitory innervation patterns.

Dual GABAergic synaptic response of fast excitation and slow inhibition in the medial habenula of rat epithalamus

We report here a novel action of GABAergic synapses in regulating tonic firing in the mammalian brain. By using gramicidin-perforated patch recording in rat brain slices, we show that cells of the medial habenula of the epithalamus generate tonic firing in basal conditions. The GABAergic input onto these cells at postnatal days 18-25 generates a combinatorial activation of fast excitation and slow inhibition. The fast excitation, mediated by gamma-aminobutyric acid type A receptors (GABA A Rs), is alone capable of triggering robust action potentials to increase cell firing. This excitatory influence of GABAergic input results from the Cl(-) homeostasis that maintains intracellular Cl(-) at high levels. The GABA A excitation is often followed by a slow inhibition mediated by GABA B Rs that suppresses tonic firing. Interestingly, in a subpopulation of the cells, the GABA B inhibition exhibits a remarkably low threshold for synaptic activation in that low-strength GABAergic input often activates selectively the GABA B slow inhibition, whereas the GABA A excitation requires further increases in stimulus strength. Our study demonstrates that the dual activation of GABAergic excitation and inhibition through GABA A Rs and GABA B Rs generates distinct temporal patterns of cell firing that alter the cellular output in an activity-dependent manner.

Dual personality of GABA/glycine-mediated depolarizations in immature spinal cord

The inhibitory action of glycine and GABA in adult neurons consists of both shunting incoming excitations and moving the membrane potential away from the action potential (AP) threshold. By contrast, in immature neurons, inhibitory postsynaptic potentials (IPSPs) are depolarizing; it is generally accepted that, despite their depolarizing action, these IPSPs are inhibitory because of the shunting action of the Cl(-) conductance increase. Here we investigated the integration of depolarizing IPSPs (dIPSPs) with excitatory inputs in the neonatal rodent spinal cord by means of both intracellular recordings from lumbar motoneurons and a simulation using the compartment model program "Neuron." We show that the ability of IPSPs to suppress suprathreshold excitatory events depends on E(Cl) and the location of inhibitory synapses. The depolarization outlasts the conductance changes and spreads electrotonically in the somatodendritic tree, whereas the shunting effect is restricted and local. As a consequence, dIPSPs facilitated AP generation by subthreshold excitatory events in the late phase of the response. The window of facilitation became wider as E(Cl) was more depolarized and started earlier as inhibitory synapses were moved away from the excitatory input. GAD65/67 immunohistochemistry demonstrated the existence of distal inhibitory synapses on motoneurons in the neonatal rodent spinal cord. This study demonstrates that small dIPSPs can either inhibit or facilitate excitatory inputs depending on timing and location. Our results raise the possibility that inhibitory synapses exert a facilitatory action on distant excitatory inputs and slight changes of E(Cl) may have important consequences for network processing.

Shunting and hyperpolarizing GABAergic inhibition in the high-potassium model of ictogenesis in the developing rat hippocampus

Ontogenesis of GABAergic signaling may play an important role in developmental changes in seizure susceptibility in the high-potassium model of ictogenesis in vitro. The age-dependent effects of [K(+)](o) on the reversal potential of the GABA(A)-mediated responses and membrane potential in hippocampal slices in vitro were compared with the effect of GABA(A)-receptors antagonists and GABA(A) modulators on high-potassium induced seizures in the CA3 pyramidal layer of rat hippocampus in vivo. GABA(A) responses were depolarizing at P8-12 and hyperpolarizing at P17-21. In P8-12 rats, GABA(A) responses switch their polarity from depolarizing to hyperpolarizing upon elevation of extracellular potassium. At approximately 10 mM [K(+)](o), activation of GABA(A) receptors produced an isoelectric, purely shunting response characterized by no changes in the membrane potential but an increase in the membrane conductance. In P17-21 rats, the hyperpolarizing GABA(A) driving force progressively increased with elevation of [K(+)](o). In P8-12 rats in vivo, GABA(A)-receptor antagonists did not affect the occurrence of ictal discharges induced by intrahippocampal injection of 10 mM [K(+)](o), but significantly increased seizure duration. Diazepam and isoguvacine completely prevented seizures induced by 10 mM [K(+)](o). In P17-21 rats, GABA(A)-receptor antagonists strongly increased the occurrence of ictal activity induced both by 10 mM [K(+)](o). Taken together, these results suggest that anticonvulsive effects of GABA are because of the combination of shunting and hyperpolarizing actions of GABA. Although shunting GABA is already efficient in the young age group, a developmental increase in the hyperpolarizing GABA(A) driving force likely contributes to the increase in the GABAergic control of seizures upon maturation.

Molecular reconstitution of functional GABAergic synapses with expression of neuroligin-2 and GABAA receptors

Trans-synaptic cell adhesion molecules neuroligins and neurexins play an important role in promoting central synapse formation. We report here a molecular reconstruction of functional GABAergic synapses in non-neuronal cells with the coexpression of postsynaptic cell adhesion molecule neuroligin-2 (NL-2) and GABA(A) receptors. HEK 293T cells were co-transfected with GABA(A) receptors and NL-2 or its homologue neuroligin-1 (NL-1), and then cocultured with hypothalamic cultures which are enriched with GABAergic neurons. Both spontaneous and action potential-evoked GABAergic events were readily detected in HEK 293T cells coexpressing GABA(A) receptors with NL-2, but not with NL-1. Aggregating NL-2 with specific antibodies in live cells resulted in coaggregation of GABA(A) receptors. Expression of NL-2 in HEK 293T cells also induced stronger GABAergic presynaptic differentiation than that of NL-1 in neuronal cocultures. These results suggest that NL-2 may potentially serve as a central organizer for GABAergic synapse assembly by interacting with both presynaptic neurexins and postsynaptic GABA(A) receptors.

Theta-bursts induce a shift in reversal potentials for GABA-A receptor-mediated postsynaptic currents in rat hippocampal CA1 neurons

Theta-burst stimulation of the stratum radiatum induces a negative shift in the reversal potential (RP) of gamma-aminobutyric acid (GABA)-ergic postsynaptic currents (PSCs) in hippocampal CA1 neurons in brain slices from rats of age groups 3-4 days, 6-9 days and 3-4 weeks. Furosemide reversed the shift in the RP. The amplitude of the evoked PSC appeared to increase following the theta-burst stimulation but this increase was secondary to the change in the RP. These results indicate that the RP for GABA-ergic PSCs undergoes an activity-dependent plasticity in not only neonatal but also adult neurons presumably through an up-regulation of a K(+)-Cl(-) co-transporter. This plasticity can have significant implications for neuronal network activity in the central nervous system. Furthermore, these results indicate that studies on GABA-ergic synaptic efficacy require a careful, parallel monitoring of the RP.

GABA excitation in mouse hilar neuropeptide Y neurons

Neuropeptide Y-containing interneurons in the dentate hilar area play an important role in inhibiting the activity of hippocampal circuitry. Hilar cells are often among the first lost in hippocampal epilepsy. As many types of neurons are found in the hilus, we used a new transgenic mouse expressing green fluorescent protein (GFP) in a subset of neurons that colocalized neuropeptide Y (NPY), somatostatin (SST), and GABA for whole-cell, perforated, and cell-attached recording in 240 neurons. As these neurons have not previously been identifiable in live slices, they have not been the focus of physiological analysis. Hilar NPY neurons showed modest spike frequency adaptation, a large 15.6 +/- 1.0 mV afterhyperpolarization, a mean input resistance of 335 +/- 26 M Omega, and were capable of fast-firing. Muscimol-mediated excitatory actions were found in a nominally Ca(2+)-free/high-Mg(2+) bath solution using cell-attached recording. GABA(A) receptor antagonists inhibited half the recorded neurons and blocked burst firing. Gramicidin perforated-patch recording revealed a GABA reversal potential positive to both the resting membrane potential and spike threshold. Together, these data suggest GABA is excitatory to many NPY cells. NPY and SST consistently hyperpolarized and reduced spike frequency in these neurons. No hyperpolarization of NPY on membrane potential was detected in the presence of tetrodotoxin, AP5, CNQX and bicuculline, supporting an indirect effect. Under similar conditions, SST hyperpolarized the cells, suggesting a direct postsynaptic action. Depolarizing actions of GABA and GABA-dependent burst-firing may synchronize a rapid release of GABA, NPY, and SST, leading to pre- and postsynaptic inhibition of excitatory hippocampal circuits.

GODZ-mediated palmitoylation of GABA(A) receptors is required for normal assembly and function of GABAergic inhibitory synapses

Golgi-specific DHHC (Asp-His-His-Cys) zinc finger protein (GODZ) is a DHHC family palmitoyl acyltransferase that is implicated in palmitoylation and regulated trafficking of diverse substrates that function either at inhibitory or excitatory synapses. Of particular interest is the gamma2 subunit of GABA(A) receptors, which is required for targeting these receptors to inhibitory synapses. Here, we report that GODZ and, to a lesser extent, its close paralog sertoli cell gene with a zinc finger domain-beta (SERZ-beta) are the main members of the DHHC family of enzymes that are able to palmitoylate the gamma2 subunit in heterologous cells. Yeast two-hybrid and colocalization assays in human embryonic kidney 293T (HEK293T) cells indicate that GODZ and SERZ-beta show indistinguishable palmitoylation-dependent interaction with the gamma2 subunit. After coexpression in HEK293T cells, they form homomultimers and heteromultimers, as shown by coimmunoprecipitation and in vivo cross-linking experiments. Analyses in neurons transfected with dominant-negative GODZ (GODZ(C157S)) or plasmid-based GODZ-specific RNAi indicate that GODZ is required for normal accumulation of GABA(A) receptors at synapses, for normal whole-cell and synaptic GABAergic inhibitory function and, indirectly, for GABAergic innervation. Unexpectedly, GODZ was found to be dispensable for normal postsynaptic AMPA receptor-mediated glutamatergic transmission. We conclude that GODZ-mediated palmitoylation of GABA(A) receptors and possibly other substrates contributes selectively to the formation and normal function of GABAergic inhibitory synapses.

Maternal oxytocin triggers a transient inhibitory switch in GABA signaling in the fetal brain during delivery

We report a signaling mechanism in rats between mother and fetus aimed at preparing fetal neurons for delivery. In immature neurons, gamma-aminobutyric acid (GABA) is the primary excitatory neurotransmitter. We found that, shortly before delivery, there is a transient reduction in the intracellular chloride concentration and an excitatory-to-inhibitory switch of GABA actions. These events were triggered by oxytocin, an essential maternal hormone for labor. In vivo administration of an oxytocin receptor antagonist before delivery prevented the switch of GABA actions in fetal neurons and aggravated the severity of anoxic episodes. Thus, maternal oxytocin inhibits fetal neurons and increases their resistance to insults during delivery.

Inhibitory postsynaptic potentials in lumbar motoneurons remain depolarizing after neonatal spinal cord transection in the rat

GABA and glycine are excitatory in the immature spinal cord and become inhibitory during development. The shift from depolarizing to hyperpolarizing inhibitory postsynaptic potentials (IPSPs) occurs during the perinatal period in the rat, a time window during which the projections from the brain stem reach the lumbar enlargement. In this study, we investigated the effects of suppressing influences of the brain on lumbar motoneurons during this critical period for the negative shift of the reversal potential of IPSPs (E(IPSP)). The spinal cord was transected at the thoracic level on the day of birth [postnatal day 0 (P0)]. E(IPSP), at P4-P7, was significantly more depolarized in cord-transected than in cord-intact animals (E(IPSP) above and below resting potential, respectively). E(IPSP) at P4-P7 in cord-transected animals was close to E(IPSP) at P0-P2. K-Cl cotransporter KCC2 immunohistochemistry revealed a developmental increase of staining in the area of lumbar motoneurons between P0 and P7 in cord-intact animals; this increase was not observed after spinal cord transection. The motoneurons recorded from cord-transected animals were less sensitive to the experimental manipulations aimed at testing the functionality of the KCC2 system, which is sensitive to [K(+)](o) and blocked by bumetanide. Although bumetanide significantly depolarized E(IPSP), the shift was less pronounced than in cord-intact animals. In addition, a reduction of [K(+)](o) affected E(IPSP) significantly only in cord-intact animals. Therefore influences from the brain stem may play an essential role in the maturation of inhibitory synaptic transmission, possibly by upregulating KCC2 and its functionality.

Excitatory and inhibitory actions of isoflurane in bovine chromaffin cells

Isoflurane, a halogenated volatile anesthetic, is thought to produce anesthesia by depressing CNS function. Many anesthetics, including isoflurane, are thought to modulate and/or directly activate GABA(A) receptors. Chromaffin cells are known to express functional GABA(A) receptors. We previously showed that activation of the GABA(A) receptors, with specific agonists, leads to cellular excitation resulting from the depolarized anion equilibrium potential. In this study, our goal was to determine whether isoflurane mimicked this response and to explore the functional consequences of this activation. Furthermore, we sought to study the actions of isoflurane on nicotinic acetylcholine receptors (nAChRs) as they mediate the "sympathetic drive" in these cells. For these studies the Ca(2+)-indicator dye fura-2 was used to assay [Ca(2+)](i). Amperometric measurements were used to assay catecholamine release. We show that bovine adrenal chromaffin cells were excited by isoflurane at clinically relevant concentrations. Isoflurane directly activated GABA(A) receptors found in chromaffin cells, which depolarized the cells and elevated [Ca(2+)](i). Application of isoflurane directly to the chromaffin cells elicited catecholamine secretion from these cells. At the same time, isoflurane suppressed activation of nAChRs, which presumably blocks "sympathetic drive" to the chromaffin cells. These latter results may help explain why isoflurane produces the hypotension observed clinically.

Mutations in the K+/Cl- cotransporter gene kazachoc (kcc) increase seizure susceptibility in Drosophila

During a critical period in the developing mammalian brain, there is a major switch in the nature of GABAergic transmission from depolarizing and excitatory, the pattern of the neonatal brain, to hyperpolarizing and inhibitory, the pattern of the mature brain. This switch is believed to play a major role in determining neuronal connectivity via activity-dependent mechanisms. The GABAergic developmental switch may also be particularly vulnerable to dysfunction leading to seizure disorders. The developmental GABA switch is mediated primarily by KCC2, a neuronal K+/Cl- cotransporter that determines the intracellular concentration of Cl- and, hence, the reversal potential for GABA. Here, we report that kazachoc (kcc) mutations that reduce the level of the sole K+/Cl- cotransporter in the fruitfly Drosophila melanogaster render flies susceptible to epileptic-like seizures. Drosophila kcc protein is widely expressed in brain neuropil, and its level rises with developmental age. Young kcc mutant flies with low kcc levels display behavioral seizures and demonstrate a reduced threshold for seizures induced by electroconvulsive shock. The kcc mutation enhances a series of other Drosophila epilepsy mutations indicating functional interactions leading to seizure disorder. Both genetic and pharmacological experiments suggest that the increased seizure susceptibility of kcc flies occurs via excitatory GABAergic signaling. The kcc mutants provide an excellent model system in which to investigate how modulation of GABAergic signaling influences neuronal excitability and epileptogenesis.