Region-specific expression of messenger RNAs encoding GABAA receptor subunits in the developing rat brain

Maternal taurine as a modulator of Cl- homeostasis as well as of glycine/GABAA receptors for neocortical development

During brain and spinal cord development, GABA and glycine, the inhibitory neurotransmitters, cause depolarization instead of hyperpolarization in adults. Since glycine and GABAA receptors (GABAARs) are chloride (Cl-) ion channel receptor, the conversion of GABA/glycine actions during development is influenced by changes in the transmembrane Cl- gradient, which is regulated by Cl- transporters, NKCC1 (absorption) and KCC2 (expulsion). In immature neurons, inhibitory neurotransmitters are released in a non-vesicular/non-synaptic manner, transitioning to vesicular/synaptic release as the neuron matures. In other word, in immature neurons, neurotransmitters generally act tonically. Thus, the glycine/GABA system is a developmentally multimodal system that is required for neurogenesis, differentiation, migration, and synaptogenesis. The endogenous agonists for these receptors are not fully understood, we address taurine. In this review, we will discuss about the properties and function of taurine during development of neocortex. Taurine cannot be synthesized by fetuses or neonates, and is transferred from maternal blood through the placenta or maternal milk ingestion. In developing neocortex, taurine level is higher than GABA level, and taurine tonically activates GABAARs to control radial migration as a stop signal. In the marginal zone (MZ) of the developing neocortex, endogenous taurine modulates the spread of excitatory synaptic transmission, activating glycine receptors (GlyRs) as an endogenous agonist. Thus, taurine affects information processing and crucial developmental processes such as axonal growth, cell migration, and lamination in the developing cerebral cortex. Additionally, we also refer to the possible mechanism of taurine-regulating Cl- homeostasis. External taurine is uptake by taurine transporter (TauT) and regulates NKCC1 and KCC2 mediated by intracellular signaling pathway, with-no-lysine kinase 1 (WNK1) and its subsequent kinases STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) and oxidative stress response kinase-1 (OSR1). Through the regulation of NKCC1 and KCC2, mediated by the WNK-SPAK/OSR1 signaling pathway, taurine plays a role in maintaining Cl- homeostasis during normal brain development.

Effects of AMPA receptor antagonist, NBQX, and extrasynaptic GABAA agonist, THIP, on social behavior of adolescent and adult rats

Adolescence is characterized by high significance of social interactions, along with a propensity to exhibit social facilitating effects of ethanol while being less sensitive than adults to the inhibition of social behavior that emerges at higher doses of ethanol. Among the neural characteristics of adolescence are generally enhanced levels of glutamatergic (especially NMDA receptor) activity relative to adults, whereas the GABA system is still developmentally immature. Activation of NMDA receptors likely plays a role in modulation of social behavior in adolescent animals as well as in socially facilitating and suppressing effects of ethanol. For instance, adolescent and adult rats differ in their sensitivities to the effects of NMDA antagonists and ethanol on social behavior, with adolescents but not adults demonstrating social facilitation at lower doses of both drugs and adults being more sensitive to the socially suppressing effects evident at higher doses of each. The roles of AMPA and extrasynaptic GABA_A receptors in modulation of social behavior during adolescence and in adulthood are still unknown. The present study was designed to assess whether pharmacological blockade of AMPA receptors and/or activation of extrasynaptic GABA_A receptors results in age-dependent alterations of social behavior. Adolescent and adult male and female Sprague-Dawley rats were injected with an assigned dose of either a selective AMPA antagonist, NBQX (Experiment 1) or extrasynaptic GABA_A agonist, THIP (Experiment 2) and placed into a modified social interaction chamber for a 30-min habituation period prior to a 10-min social interaction test with a novel age- and sex-matched partner. Behaviors such as social investigation, contact behavior and play behavior were scored from video recordings of the interaction tests. In Experiment 1, NBQX produced similar social inhibition at higher doses in both age groups. In Experiment 2, THIP induced inhibition in adolescents, but not adults. No social facilitation was evident following low doses of either drug. Therefore, AMPA and extrasynaptic GABA_A receptors appear to play little role if any in modulation of peer-directed social behavior in adolescence and adulthood and not likely to contribute to previously observed age differences in the social effects of acute ethanol.

Leptin alters somatosensory thalamic networks by decreasing gaba release from reticular thalamic nucleus and action potential frequency at ventrobasal neurons

Leptin is an adipose-derived hormone that controls appetite and energy expenditure. Leptin receptors are expressed on extra-hypothalamic ventrobasal (VB) and reticular thalamic (RTN) nuclei from embryonic stages. Here, we studied the effects of pressure-puff, local application of leptin on both synaptic transmission and action potential properties of thalamic neurons in thalamocortical slices. We used whole-cell patch-clamp recordings of thalamocortical VB neurons from wild-type (WT) and leptin-deficient obese (ob/ob) mice. We observed differences in VB neurons action potentials and synaptic currents kinetics when comparing WT vs. ob/ob. Leptin reduced GABA release onto VB neurons throughout the activation of a JAK2-dependent pathway, without affecting excitatory glutamate transmission. We observed a rapid and reversible reduction by leptin of the number of action potentials of VB neurons via the activation of large conductance Ca²⁺-dependent potassium channels. These leptin effects were observed in thalamocortical slices from up to 5-week-old WT but not in leptin-deficient obese mice. Results described here suggest the existence of a leptin-mediated trophic modulation of thalamocortical excitability during postnatal development. These findings could contribute to a better understanding of leptin within the thalamocortical system and sleep deficits in obesity.

The α1, α2, α3, and γ2 subunits of GABAA receptors show characteristic spatial and temporal expression patterns in rhombencephalic structures during normal human brain development

γ-Aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in adult mammalian brain, mediating its actions chiefly via a pentameric chloride ion channel, the GABAA receptor. Nineteen different subunits (α1-6, β1-3, γ1-3, δ, ε, π, θ, ρ1-3) can give rise to multiple receptor subtypes that are the site of action of many clinically important drugs. In the developing brain, however, GABAA receptors mediate excitatory actions due to an increased chloride concentration within neurons and seem to control cell proliferation, migration, differentiation, synapse maturation, and cell death. Little is known about the distribution of single subunits in the human brain. Here we describe developmental changes in the immunohistochemical distribution of four subunits (α1, α2, α3, and γ2) in the human rhombencephalon. The γ2 was the most abundant subunit in all rhombencephalic structures during development and in adults, whereas α subunits showed a structure- and age-characteristic distribution. The α1 was expressed prenatally in the molecular and Purkinje cell layer, but only postnatally in the granule cell layer and the dentate nucleus. Expression was completely absent in the inferior olivary nucleus. The α2 gradually increased during development, showing some layer specificity in the cerebellar cortex. The α3-immunoreactivity in the cerebellar cortex was relatively weak, but it was abundantly observed in different cell populations in the subcortical cerebellar structures. Structure- and age-characteristic colocalization between subunits during development suggests differences in GABAA receptor composition. Interestingly, subunit expression in several instances differed between human and rodent brain, underlining the importance of immunohistochemical studies in humans.

Ontogenic profile of seizures evoked by the beta-carboline DMCM (methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate) in rats

The beta-carboline, methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM), is a potent chemoconvulsant. While it has been utilized in adult rodents, it has not been previously examined for effects across postnatal development. DMCM is a negative allosteric modulator of benzodiazepine-sensitive GABAA receptors, receptor subtypes that are particularly enriched in limbic brain regions. This raises the possibility that DMCM may be particularly effective at evoking forebrain seizures, which is a challenge in neonatal animals due to the relative immaturity of the forebrain seizure network. The ability to selectebrain seizures is desirable when screening for drugs to use in temporal lobe epilepsy, which is characterized by seizures within the forebrain (limbic) network. To determine the profile of DMCM action across development, we examined the dose-dependent ability of DMCM to induce seizures in rats at P7, P10, P13, P14, P21 and in adulthood. We found that the highest sensitivity to DMCM occurred in P10, P13, and P14 rats. The lowest sensitivity occurred in P21 rats. Neonatal (P7) and adult (P60+) rats displayed moderate sensitivity. With moderate (0.2-0.4 mg/kg) doses of DMCM, we were able to reliably evoke limbic motor seizures without tonic-clonic components in animals as young as P7. These data support the utility of DMCM in assessing seizure threshold during development and raise the possibility for future exploration of DMCM as an agent to screen anticonvulsant drugs during the postnatal period.

Roles of taurine-mediated tonic GABAA receptor activation in the radial migration of neurons in the fetal mouse cerebral cortex

γ-Aminobutyric acid (GABA) depolarizes embryonic cerebrocortical neurons and continuous activation of the GABA_A receptor (GABA_AR) contributes to their tonic depolarization. Although multiple reports have demonstrated a role of GABA_AR activation in neocortical development, including in migration, most of these studies have used pharmacological blockers. Herein, we performed in utero electroporation in GABA synthesis-lacking homozygous GAD67-GFP knock-in mice (GAD67^GFP/GFP) to label neurons born in the ventricular zone. Three days after electroporation, there were no differences in the distribution of labeled cells between the genotypes. The dose–response properties of labeled cells to GABA were equivalent among genotypes. However, continuous blockade of GABA_AR with the GABA_AR antagonist SR95531 accelerated radial migration. This effect of GABA_AR blockade in GAD67^GFP/GFP mice suggested a role for alternative endogenous GABA_AR agonists. Thus, we tested the role of taurine, which is derived from maternal blood but is abundant in the fetal brain. The taurine-evoked currents in labeled cells were mediated by GABA_AR. Taurine uptake was blocked by a taurine transporter inhibitor, 2-(guanidino)ethanesulfonic acid (GES), and taurine release was blocked by a volume-sensitive anion channel blocker, 4-(2-butyl-6,7-dichlor-2-cyclopentylindan-1-on-5-yl) oxobutyric acid, as examined through high-performance liquid chromatography. GES increased the extracellular taurine concentration and induced an inward shift of the holding current, which was reversed by SR95531. In a taurine-deficient mouse model, the GABA_AR-mediated tonic currents were greatly reduced, and radial migration was accelerated. As the tonic currents were equivalent among the genotypes of GAD67-GFP knock-in mice, taurine, rather than GABA, might play a major role as an endogenous agonist of embryonic tonic GABA_AR conductance, regulating the radial migration of neurons in the developing neocortex.

GABAergic contributions to alcohol responsivity during adolescence: insights from preclinical and clinical studies

There is a considerable body of literature demonstrating that adolescence is a unique age period, which includes rapid and dramatic maturation of behavioral, cognitive, hormonal and neurobiological systems. Most notably, adolescence is also a period of unique responsiveness to alcohol effects, with both hyposensitivity and hypersensitivity observed to the various effects of alcohol. Multiple neurotransmitter systems are undergoing fine-tuning during this critical period of brain development, including those that contribute to the rewarding effects of drugs of abuse. The role of developmental maturation of the γ-amino-butyric acid (GABA) system, however, has received less attention in contributing to age-specific alcohol sensitivities. This review integrates GABA findings from human magnetic resonance spectroscopy studies as they may translate to understanding adolescent-specific responsiveness to alcohol effects. Better understanding of the vulnerability of the GABA system both during adolescent development, and in psychiatric conditions that include alcohol dependence, could point to a putative mechanism, boosting brain GABA, that may have increased effectiveness for treating alcohol use disorders.

Sensitive periods in fear learning and memory

Adolescence represents a uniquely sensitive developmental stage in the transition from childhood to adulthood. During this transition, neuronal circuits are particularly susceptible to modification by experience. In addition, adolescence is a stage in which the incidence of anxiety disorders peaks in humans and over 75% of adults with fear-related disorders met diagnostic criteria as children and adolescents. While postnatal critical periods of plasticity for primary sensory processes, such as in the visual system are well established, less is known about potential critical or sensitive periods for fear learning and memory. Here, we review the non-linear developmental aspects of fear learning and memory during a transition period into and out of adolescence. We also review the literature on the non-linear development of GABAergic neurotransmission, a key regulator of critical period plasticity. We provide a model that may inform improved treatment strategies for children and adolescents with fear-related disorders.

Activations of GABAergic signaling, HSP70 and MAPK cascades are involved in baicalin's neuroprotection against gerbil global ischemia/reperfusion injury

Baicalin, a flavonoid compound isolated from the plant Scutellaria baicalensis Georgi, is known as a protective agent against delayed neuronal cell death after ischemia/reperfusion. To investigate the neuroprotective mechanism of baicalin, the present study was conducted to explore whether the alterations of GABAergic signaling, heat shock protein 70 (HSP70) and mitogen-activated protein kinases (MAPKs) were involved in its neuroprotection on gerbils global ischemia. The bilateral carotid arteries were occluded by 5 min and baicalin at the dose of 200 mg/kg was intraperitoneally injected into the gerbils immediately after cerebral ischemia. Seven days after reperfusion, neurological deficit was scored and changes in hippocampal neuronal cell death were assessed by Nissl staining as well as NeuN immunohistochemistry. The mRNA and protein expressions of GABAergic signal molecules (GABA(A)R α1, GABA(A)R γ2, KCC2 and NKCC1) were determined in ischemic hippocampus by real-time RT-PCR and Western blot, respectively. In addition, HSP70 and MAPKs cascades (ERK, JNK and p38) were also detected using western blot assay. Our results illustrated that baicalin treatment significantly facilitated neurological function, suppressed the ischemia-induced neuronal damage. Besides, administration of baicalin also caused a striking increase of GABA(A)R α1, GABA(A)R γ2 and KCC2 together with the decrease of NKCC1 at mRNA and protein levels in gerbils hippocampus following an ischemic insult. Furthermore, the protein expressions of HSP70 and phosphorylated ERK (p-ERK) were evidently augmented while the phosphorylated JNK (p-JNK) and phosphorylated p38 (p-p38) were strikingly diminished in ischemic gerbils with baicalin treatment. These findings suggest that baicalin activates GABAergic signaling, HSP70 and MAPKs cascades in global ischemia, which may be a mechanism underlying the baicalin's neuroprotection.

Topiramate attenuates cerebral ischemia/reperfusion injury in gerbils via activating GABAergic signaling and inhibiting astrogliosis

Impaired GABAergic inhibitory synaptic transmission plays an essential role in the pathogenesis of selective neuronal cell death following transient global ischemia. GABA(A) receptor (GABA(A)R), K⁺-Cl⁻ co-transporter 2 (KCC2), Na⁺-K⁺-Cl⁻ co-transporter 1 (NKCC1) and astrocytes are of particular importance to GABAergic transmission. The present study was designed to explore whether the neuroprotective effect of topiramate (TPM) was linked with the alterations of GABAergic signaling and astrocytes. The bilateral carotid arteries were occluded, and TPM (80 mg/kg/day (divided twice daily), i.p.) was injected into gerbils. At day 1, 3 and 7 post-ischemia, neurological deficit was scored and changes in hippocampal neuronal cell death were evaluated by Nissl staining. The apoptosis-related regulatory proteins (procaspase-3, caspase-3, Bax and Bcl-2) and GABAergic signal molecules (GABA(A)R α1, GABA(A)R γ2, KCC2 and NKCC1) were also detected using western blot assay. In addition, the fluorescent intensity and protein level of glial fibrillary acidic protein (GFAP), a major component of astrocyte, were examined by confocal and immunoblot analysis. Our results showed that TPM treatment significantly decreased neurological deficit scores, attenuated the ischemia-induced neuronal loss and remarkably decreased the expression levels of procaspase-3, caspase-3 as well as the ratio of Bax/Bcl-2. Besides, treatment with TPM also resulted in the increased protein expressions of GABA(A)R α1, GABA(A)R γ2 and KCC2 together with the decreased protein level of NKCC1 in gerbils hippocampus. Furthermore, fluorescent intensity and protein level of GFAP were evidently reduced in TPM-treated gerbils. These findings suggest that the therapeutic effect of TPM on global ischemia/reperfusion injury appears to be associated with the enhancement of GABAergic signaling and the inhibition of astrogliosis in gerbils.

Development of the GABAergic system from birth to adolescence

The neurotransmitter GABA (γ-aminobutyric acid), acting via inotropic GABA(A) and metabotropic GABA(B) receptors, plays an essential role in a variety of distinct neuronal processes, including regulation of neuronal excitability, determination of temporal aspects of spike trains, control of the size and propagation of neuronal assemblies, generation of oscillatory activity, and neuronal plasticity. Although the developmental switch between excitatory and inhibitory GABA(A) receptor-mediated responses is widely appreciated, the fact that the postnatal maturation of the GABAergic system lasts until late adolescence is not so persuasively promoted. This review summarizes recent knowledge of the maturation of various aspects of the GABAergic systems, like functional expression of GABA synthesizing/degrading enzymes and transporters, density of GABAergic synapses, GABAergic projection patterns, GABA receptor subunit composition, and properties of GABAergic interneurons, with an emphasis on the late developmental alterations. In addition, some aspects of the development of mental capabilities during adolescence and their relation the delayed maturation of the GABAergic system are presented.

Developmental expression and distribution of GABA(A) receptor α1-, α3- and β2-subunits in pig brain

The principal function of the γ-aminobutyric acid (GABA) system in the adult brain is inhibition; however, in the neonatal brain, GABA provides much of the excitatory drive. As the brain develops, transmembrane chloride gradients change and the inhibitory role of GABA is initiated and continues throughout juvenile and adult life. Previous studies have shown that GABA(A) receptor subunit expression is developmentally regulated, and it is thought that the change in GABA function from excitation to inhibition corresponds to the changeover in expression of 'immature' to 'mature' subunit isoforms. We examined the protein expression pattern and distribution of GABA type A (GABA(A)) receptor α1-, α3- and β2-subunits in the parietal cortex and hippocampus of the developing piglet brain. Four perinatal ages were studied; 14 days preterm (P-14), 10 days preterm (P-10), day of birth (P0) and at postnatal day 7 (P7). Animals were obtained by either caesarean section or spontaneous birth. Protein expression levels and subunit localization were analysed by Western blotting and immunohistochemistry, respectively. In the cortex and hippocampus, GABA(A) receptor α1-subunit showed greatest expression at P7 when compared to all other age groups (p < 0.05). In contrast, α3 expression in the cortex was elevated in preterm brain, peaking at P0, followed by a significant reduction by P7 (p < 0.05); a similar trend was observed in the hippocampus. GABA(A) receptor β2-subunit protein expression appeared relatively constant across all time points studied in both the cortex and hippocampus. Immunolabelling of the α1-, α3- and β2-subunits was observed throughout all cortical layers at every age. GABA(A) receptor α3-subunit appeared to show specific localization to layers V and VI whilst labelling for the β2-subunit was observed in layer IV. In the hippocampus of all animals, the α1- and β2-subunits were shown to immunolabel various cells and processes in the dentate gyrus (DG), CA1 and CA3; the α3-subunit was barely observed except at the stratum moleculare of the DG. We report for the first time the ontogenesis of GABA(A) receptor subunits α1, α3 and β2 in the perinatal pig brain.

GABAA receptor subunit profiles of tangentially migrating neurons derived from the medial ganglionic eminence

During rodent corticogenesis, a sizeable subpopulation of γ-aminobutyric acid (GABA)ergic interneurons arises extracortically from the medial ganglionic eminence (MGE). These neurons progressively acquire responsiveness to GABA in the course of corticopetal tangential migration, a process regulated by ambient GABA and mediated by GABA(A) receptors. Here, we combined patch clamp electrophysiology and single-cell reverse transcription-polymerase chain reaction to examine GABA(A) receptor expression in green fluorescent MGE-derived (eGFP+) cells in telencephalic slices from gestational day 14.5 BAC-Lhx6 embryos. GABA concentration-response curves revealed lower apparent affinity and efficacy in eGFP+ cells in and around the MGE than their counterparts in the cortex. Pharmacological tests revealed subunit-selective response profiles in the MGE and cortex consistent with differential expression of GABA(A) receptor isoforms. Profiling of GABA(A) receptor subunit transcripts (α1-5, β1-3, and γ1-3, δ) uncovered increased expression of the α1-, α2-, α5-, γ2-, and γ3-subunit messenger RNAs in the cortex. We propose that the dynamic expression of certain GABA(A) receptor subunits contributes to assembling receptor isoforms that confer functional attributes important in regulating the migration and maturation of primordial GABAergic cortical interneurons.

Ontogeny of ethanol-induced motor impairment following acute ethanol: assessment via the negative geotaxis reflex in adolescent and adult rats

Adolescent rats have been observed to be less sensitive than adults to a number of ethanol effects that may serve as feedback cues to reduce further ethanol intake. Among these findings are a few reports of attenuated sensitivities of adolescents to ethanol-induced motor impairment. The purpose of the present study was to further explore potential age-related differences in ethanol-induced motor impairment in both male and female adolescent (postnatal day [P]28-32), and adult (P68-72) Sprague-Dawley rats using an inclined plane assessment of the negative geotaxis reflex. Adult males displayed significant motor impairment at 1.5 g/kg, whereas adolescent males required higher doses, showing significant motor impairment only at doses of 2.25 g/kg ethanol or greater. Intoxicated practice did not significantly influence level of motor impairment at either age. When female rats of both ages were separately analyzed in terms of their response to ethanol, a dose of 1.5 g/kg ethanol was found to significantly impair adults, whereas adolescent females showed significant motor impairment when challenged with 2.25 g/kg but not 1.5 g/kg ethanol. Yet when the 1.5 g/kg data of females at the two ages were directly compared, no significant age difference was seen at this dose. These data document an attenuated sensitivity of adolescent relative to adult rats to the motor impairing effects of ethanol using a stationary inclined plane test, an effect particularly robust in male animals, and demonstrates the utility of this test for assessment of motor coordination in adolescent and adult rats.

Stress and GABA receptors

GABA(A) receptors are sensitive to subtle changes in the environment in both early-life and adulthood. These neurochemical responses to stress in adulthood are sex-dependent. Acute stress induces rapid changes in GABA(A) receptors in experimental animals, with the direction of the changes varying according to the sex of the animals and the stress-paradigm studied. These rapid alterations are of particular interest as they provide an example of fast neurotransmitter system plasticity that may be mediated by stress-induced increases in neurosteroids, perhaps via effects on phosphorylation and/or receptor trafficking. Interestingly, some studies have also provided evidence for long-lasting changes in GABA(A) receptors as a result of exposure to stressors in early-life. The short- and long-term stress sensitivity of the GABAergic system implicates GABA(A) receptors in the non-genetic etiology of psychiatric illnesses such as depression and schizophrenia in which stress may be an important factor.

Androgens predispose males to GABAA-mediated excitotoxicity in the developing hippocampus

Clinical evidence and animal models indicate greater brain damage in newborn males following injury. In adults, glutamate is the primary source of excitotoxic cell death and the steroid, estradiol, is neuroprotective. In neonatal brain, membrane depolarization following activation of GABAA receptors is the major source of excitation. Consequent influx of calcium via L-type channels is normally trophic, but becomes excitotoxic during periods of excessive activation of GABAA receptors, such as hypoxia-ischemia, alcohol exposure and seizures. The use of sex-specific hippocampal cultures revealed greater cell death induced by the GABAA agonist, muscimol, in male- versus female-derived cultures. Pretreatment with the androgen, dihydrotestosterone (DHT) increased muscimol-induced death in both sexes. Exploration of calcium dynamics indicated that, counter to expectation, female neurons achieved higher [Ca2+]i than male, but the calcium transient duration was shorter due to faster rise and decay. However, a second exposure to muscimol within minutes of the first, caused significant attenuation of [Ca2+]i in female neurons. In contrast, while male neurons exposed to muscimol for the first time exhibited lower maximal [Ca2+]i, when exposed to muscimol again there was no attenuation in [Ca2+]i. The latter effect was induced in females by DHT, and inversely correlated with the amount of gamma2 subunit of the GABAA receptor. This novel effect of androgen on GABA-mediated excitotoxicty suggests a unique opportunity for a sex-specific therapeutic approach involving antagonism of the androgen receptor in neonatal males at risk for brain injury.

Differences in the social consequences of ethanol emerge during the course of adolescence in rats: Social facilitation, social inhibition, and anxiolysis

The present experiments explored social consequences of ethanol during adolescence by examining dose-dependent ethanol-induced social facilitation and inhibition in a non-anxiogenic (familiar) environment, and ethanol-related anxiolysis in an anxiogenic (unfamiliar) environment in early (P28) and late (P42) adolescent rats. Pronounced age-related differences in the social consequences of ethanol emerged during the course of adolescence, with early adolescents being uniquely sensitive to activating effects of low doses of ethanol when tested in the familiar context in terms of play fighting-an adolescent-characteristic form of social interactions, but conversely less sensitive than late adolescents to ethanol-associated social suppression when tested at higher ethanol doses in this context. Early adolescents were also less sensitive than late adolescents to the anxiolytic effects of ethanol revealed in the unfamiliar test situation, when indexed in terms of increases in social investigation and the ethanol-induced transformation of social avoidance into social preference. Anti-anxiety properties of ethanol were found to be sex-dependent in older animals, with late adolescent females being more sensitive to ethanol anxiolysis than their male counterparts. Considerable ontogenetic differences in the social consequences of ethanol are evident even within the adolescent period, with early adolescence being a time of particularly pronounced adolescent-typical sensitivities to ethanol.

GABAA receptor kinetics in the cerebellar nuclei: evidence for detection of transmitter from distant release sites

Neurons of the cerebellar nuclei receive GABAergic input from Purkinje cells. Purkinje boutons have several closely spaced presynaptic densities without GABA transporters, raising the possibility that neurotransmitter released by one presynaptic site diffuses to multiple postsynaptic sites. To test whether such local spillover may contribute to transmission, we studied gating of GABA(A) receptors at 31-33 degrees C in cerebellar nuclear neurons acutely dissociated from mice. Currents were evoked by rapid application of long steps, brief pulses, and high-frequency trains of GABA to outside-out patches. Receptors desensitized and deactivated rapidly, and dose-response measurements estimated an EC(50) of approximately 30 microM. From these data, a kinetic scheme was developed that replicated the recorded currents. Next, we simulated diffusion of GABA in the synaptic cleft, constrained by previous electron microscopic data, and drove the kinetic GABA(A) receptor model with modeled concentration transients. Simulations predicted receptor occupancies of approximately 100% directly opposite the release site and approximately 50% at distant postsynaptic densities, such that receptors up to 700 nm from a release site opened on the timescale of the inhibitory postsynaptic currents before desensitizing. Further simulations of probabilistic release from multiple-site boutons suggested that local spillover-mediated transmission slows the onset and limits the extent of depression during high-frequency signaling.

Identification of gamma1 subunit of GABA(A) receptor in rat testis

The isoform type of gamma subunits of GABA(A) receptor is a molecular determinant of its pharmacological characteristics. At present, the existence of GABA(A) receptor in mammalian sperm is still a controversy. By using degenerate primers designed according to highly conserved region in all three gamma (gamma1, gamma2 and gamma3) subunits cloned in rat brain, we performed reverse transcription polymerase chain reaction (RT-PCR) to examine the expression pattern of gamma subunits of GABA(A) receptor in rat testis. Only one 370 bp fragment was obtained from RT-PCR in rat testis and sequencing results showed that it represented gamma1 subunit, but not gamma2 or gamma3 subunit. Using the cloned fragment as probe, a 3.8 kb transcript which in size as same as gamma1 subunit in rat brain was detected in rat testis mRNA by performing Northern blot assay. Furthermore, results of in situ hybridization assay confirmed that gamma1 subunit was expressed in round spermatids and spermatozoa, maybe also in secondary spermatocyte. These evidences proved that gamma1 subunit of GABA(A) receptor is exclusively expressed in rat testis and this feature may be the structural basis of the specific function of GABA(A) receptors in sperm acrosome reaction.

Developmental alterations of ethanol sensitivity in selectively bred high and low alcohol sensitive rats

Initial sensitivity and acute tolerance to ethanol have been implicated as risk factors in the development of alcoholism in humans. These behaviors were investigated in rats selectively bred for differences in hypnotic sensitivity following their first dose of ethanol in two different experiments. In Experiment 1, developmental profiles of the association between initial sensitivity and acute tolerance induced by a single exposure to ethanol were examined using male and female high, low, and control alcohol sensitive (HAS, LAS, and CAS) rats. Dose-response curves were constructed for duration of the loss of the righting reflex and for blood ethanol concentration (BEC) at the regain of the righting reflex. Animals were tested with a single ethanol dose ranging from 1.5 to 5.0 g/kg at either 15, 25, 40, 70, 120, or 180 days of age (DOA). For each group, acute tolerance to ethanol was estimated by the slope of the regression line using dose of ethanol and mean BEC at regain. In general, all rat lines showed an increase in hypnotic sensitivity to ethanol with age. To a large degree, the lower sensitivity observed in 15 and 25 DOA HAS and LAS rats was associated with an increase in the development of acute ethanol tolerance relative to older rats. Divergence of the LAS and CAS lines was evident by 25 DOA and remained stable with advancing age. However, HAS rats did not differ significantly from CAS rats until 40 DOA, after which the magnitude of the difference continued to increase with age. In Experiment 2, rats were treated with alcohol at 25, 70, or 180 DOA. Rats at 70 or 180 DOA required less ethanol to disrupt their motor coordination on a rotating dowel (rotarod). Blood ethanol levels were determined at the loss and subsequent regain of the ability to negotiate the rotarod. Total duration of inability to negotiate the rotarod also was recorded. HAS rats were less able to remain on a rotarod while under the influence of alcohol relative to LAS and CAS rats regardless of age. However, no evidence of acute tolerance was observed in this experiment and, in fact, there was evidence of reverse tolerance in that all animals had lower BEC values at regain of ability than they did at loss.

Redistribution and increased specificity of GABA(B) receptors during development of the rostral nucleus of the solitary tract

Recent results show that there is an abundance of gamma-aminobutyric acid (GABA) before GABAergic synapses have formed in the gustatory zone of the nucleus of the solitary tract. These results suggest that a non-synaptic, developmental function may exist for GABA prior to synaptogenesis. However, GABA exerts its physiological effect via its receptors, the development of which is a largely unknown process. The developmental expression of one of the GABA receptors in the young nucleus of the solitary tract is the focus of this study. The development of GABA(B) receptors was investigated by light and electron microscopy. The results suggest that before the development of GABAergic synapses, GABA(B) receptors are diffusely distributed. When GABAergic synapses form, the receptors become clustered. Quantitative postembedding immunohistochemical studies at the electron microscopic level show that extrasynaptic labeling for GABA(B) receptors decreases during development, but synaptic labeling increases. Increased specificity of neurotransmitter receptors at synapses has been shown in other systems during development, including other central nervous system structures, but this may be the first demonstration of the phenomenon using quantitative electron microscopy.

Effect of embryonic knock-down of GABAA receptors on the levels of monoamines and their metabolites in the CNS of the mouse

In vitro evidence indicates that gamma-aminobutyric acid (GABA), acting at GABA(A) receptors, exerts a positive trophic effect on monoaminergic neurons during embryogenesis. To determine whether in vivo antagonism of GABA(A) receptors during embryogenesis interferes with the development of monoaminergic neurons, we used mice in which the number of GABA(A) receptors was decreased by 50% by targeted deletion of the beta(3) subunit gene of the GABA(A) receptor. Levels of serotonin, dopamine, norepinephrine, and the metabolites 3,4-deoxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid were measured in the brainstem, cortex, striatum and spinal cord of female adult homozygous null (beta3-/-) and wild-type (beta3+/+) mice, as well as progenitor C57BL/6J and Strain 129/SvJ mice. The level of norepinephrine in the spinal cord of beta3-/- mice was 44% less than that of beta3+/+ mice, and did not differ in the brainstem, cortex or striatum. This finding suggests that beta3 subunit-containing GABA(A) receptors mediate the trophic effects of GABA on a subpopulation of spinally-projecting noradrenergic neurons. In contrast, the levels of serotonin, dopamine or their metabolites were unaffected, suggesting that the development of serotonergic and dopaminergic neurons may require activation of only a small fraction of GABA(A) receptors or may not be dependent on beta3 subunit-containing GABA(A) receptors. Finally, Strain 129/SvJ and C57BL/6J mice differed with respect to the levels of dopamine and its metabolites in the brainstem, spinal cord and cortex. These differences may need to be considered when assessing the phenotype of gene-targeted mice for which these mice serve as progenitor strains.

gamma-Aminobutyric acid(A) receptor subunit expression predicts functional changes in hippocampal dentate granule cells during postnatal development

Profound alterations in the function of GABA occur over the course of postnatal development. Changes in GABA(A) receptor expression are thought to contribute to these differences in GABAergic function, but how subunit changes correlate with receptor function in individual developing neurons has not been defined precisely. In the current study, we correlate expression of 14 different GABA(A) receptor subunit mRNAs with changes in the pharmacological properties of the receptor in individual hippocampal dentate granule cells over the course of postnatal development in rat. We demonstrate significant developmental differences in GABA(A) receptor subunit mRNA expression, including greater than two-fold lower expression of alpha1-, alpha4- and gamma2-subunit mRNAs and 10-fold higher expression of alpha5-mRNA in immature compared with adult neurons. These differences correlate both with regional changes in subunit protein level and with alterations in GABA(A) receptor function in immature dentate granule cells, including two-fold higher blockade by zinc and three-fold lower augmentation by type-I benzodiazepine site modulators. Further, we find an inverse correlation between changes in GABA(A) receptor zinc sensitivity and abundance of vesicular zinc in dentate gyrus during postnatal development. These findings suggest that developmental differences in subunit expression contribute to alterations in GABA(A) receptor function during postnatal development.

Early expression of GABA(A) receptor delta subunit in the neonatal rat hippocampus

The cDNA library screening strategy was used to identify the genes encoding for GABA(A) receptor subunits in the rat hippocampus during development. With this technique, genes encoding eleven GABA(A) receptor subunits were identified. The alpha5 subunit was by far the most highly expressed, followed by the gamma2, alpha2 and alpha4 subunits respectively. The expression of the beta2, alpha1, gamma1, beta1 and beta3 subunits was moderate, although that of the alpha3 and delta subunits was weak. In situ hybridization experiments, using digoxigenin-labeled cRNA probes, confirmed that the delta subunit was expressed in the neonatal as well as in the adult hippocampus, and is likely to form functional receptors in association with other subunits of the GABA(A) receptor. When the more sensitive RT-PCR approach was used, the gamma3 subunit was also detected, suggesting that this subunit is present in the hippocampus during development but at low levels of expression. The insertion of the delta subunit into functional GABA(A) receptors may enhance the efficacy of GABA in the immediate postnatal period when this amino acid is still exerting a depolarizing and excitatory action.

Developmental change of GABA(A) receptor-mediated current in rat hippocampus

Developmental change of GABA(A)ergic inhibitory postsynaptic current in rat hippocampal CA3 region was examined using patch-clamp recording method. Spontaneous and evoked inhibitory postsynaptic currents were recorded from acute hippocampal slices of neonates (postnatal days 2-4) and adults (days 18-38). Decay kinetics of the spontaneous inhibitory postsynaptic current was slower in neonates than in adults. Application of 500 nM zolpidem increased decay time-constants of the inhibitory postsynaptic currents in both groups with a stronger effect on adults. Zinc (50 microM) inhibited the neonatal inhibitory postsynaptic currents but the inhibition was weaker in adults. Modification of the GABA(A)ergic inhibitory postsynaptic currents by furosemide (0.6 mM) or diazepam (100 nM) did not cause marked differences between the neonate and adult groups. These results demonstrate that GABA(A)ergic inhibitory postsynaptic currents in hippocampal CA3 pyramidal cells change developmentally and indicate that different receptor isoforms are functionally expressed between neonates and adults.

Synchronous postnatal increase in alpha1 and gamma2L GABA(A) receptor mRNAs and high affinity zolpidem binding across three regions of rat brain

The objective of this study was to correlate postnatal changes in levels of mRNAs encoding predominant GABA(A) receptor subunits with a functional index of receptor development. This study is the first to quantify the temporal relationship between postnatal changes in predominant GABA(A) receptor mRNAs and zolpidem-sensitive GABA(A) receptor subtypes. In Experiment 1, we measured zolpidem displacement of 3H-flunitrazepam from rat cerebral cortex, hippocampus, and cerebellum at 0, 6, 14, 21, 29, and 90 postnatal days. Three independent 3H-flunitrazepam sites with high (K(i)=2. 7+/-0.6 nM), low (K(i)=67+/-4.8 nM), and very low (K(i)=4.1+/-0.9 mM) affinities for zolpidem varied in regional and developmental expression. In Experiment 2, we used RNAse protection assays to quantify levels of alpha1, alpha2, beta1, beta2, gamma2S and gamma2L mRNAs in the above regions at the same postnatal ages. Although there was a high degree of regional variation in the developmental expression of zolpidem-sensitive GABA(A) receptors and subunit mRNAs, a dramatic increase in high affinity zolpidem binding sites and alpha1 mRNA levels occurred within all three regions during the second postnatal week. Furthermore, a temporal overlap was observed between the rise in alpha1 mRNA and high affinity zolpidem binding and a more prolonged increase in gamma2L in each region. These results point to the inclusion of the alpha1 and gamma2L subunits in a GABA(A) receptor subtype with a high zolpidem affinity and suggest that a global signal may influence the emergence of this subtype in early postnatal life.

Nucleus-specific differences in GABA(A)-receptor-mediated inhibition are enhanced during thalamic development

Inhibitory postsynaptic currents (IPSCs) mediated by GABA(A) receptors are much slower in neurons of the thalamic reticular nucleus (RTN) versus those in the ventrobasal complex (VB) of young rats. Here we confirm and extend those findings regarding GABA(A) response heterogeneity especially in relation to development. Whole cell patch-clamp recordings were used to investigate GABA(A) spontaneous and electrically evoked IPSCs (sIPSCs/eIPSCs) in RTN and VB cells of different aged rats. Consistent with earlier findings, sIPSC duration at P8-12 was considerably longer in RTN (weighted decay time constant: tau(D,W) = 56.2 +/- 4.9 ms; mean +/- SE) than in VB (tau(D,W) = 15.8 +/- 1.0 ms) neurons. Decay kinetics in RTN neurons did not differ at P21-30 (45.5 +/- 4.7 ms) or P42-60 (51.6 +/- 10.6 ms). In contrast, VB sIPSCs were significantly faster at both P21-30 (tau(D,W) = 10.8 +/- 0.9 ms) and P42-60 (tau(D,W) = 9.2 +/- 0.4 ms) compared with P8-12 animals. IPSCs displayed differential outward rectification and temperature dependence, providing further support for nucleus-specific responses. tau(D,W) increased with membrane depolarization but with a net larger effect in VB. By contrast, tau(D,W) was always smaller at higher temperatures but with relatively greater difference observed in RTN. Thus nuclear differences in GABA(A) IPSCs are not only maintained, but enhanced in the mature rodent under physiological conditions. These findings support our hypothesis that unique GABA(A) receptors mediate slowly decaying RTN IPSCs that are a critical and enduring feature of the thalamic circuit. This promotes powerful intranuclear inhibition and likely prevents epileptiform thalamocortical hypersynchrony.

Temporal modulation of GABA(A) receptor subunit gene expression in developing monkey cerebral cortex

In situ hybridization histochemistry was used to examine the expression of 10 GABA(A) receptor messenger RNAs corresponding to the alpha1-alpha5, beta1-beta3, gamma1 and gamma2 subunits in primary somatosensory and visual areas of macaque monkey cerebral cortex from embryonic day (E) 125 to postnatal day (P) 125. Results were compared with expression patterns in adults. In the sensorimotor cortex at E125, overall levels of all subunit transcripts were low. At E137, there was a major lamina-specific increase in all subunit messenger RNAs except gamma1. For alpha1, alpha2, alpha4, beta2, beta3 and gamma2 subunit transcripts, this increase was highest in areas 3a and 3b, particularly in layers III/IV and VI. Postnatally, there were significant decreases in all transcripts. Alpha1, alpha5, beta2 and gamma2 subunit transcripts, while still at significantly lower levels than at E137, remained expressed at levels higher than other transcripts. Unlike in rodents, there was no obvious "switch" in the major subunits expressed in fetal and adult cortex, alpha1, alpha5, beta2 and gamma2 remaining highest throughout. In area 17, the most prominently expressed subunits at earliest ages were alpha2, alpha5, beta1, beta2, beta3 and gamma2, especially in layers II/III and VI. At E150, expression for alpha2, alpha3, beta1 and beta3 subunit transcripts in these layers decreased, but levels for alpha1, alpha4, alpha5, beta2, gamma1 and gamma2 transcripts increased, particularly within layer IV. The increase at E150 was particularly marked for alpha5 transcripts, which were expressed at levels more than four times those of other transcripts. Alpha1, beta2 and gamma2 remain highest into aduthood. Fetal area 17 displayed lamina-specific patterns of expression not found in adult animals. In particular, alpha3 messenger RNAs were present in layer IVA and gamma1 transcripts were present in layer IVC at E150, despite a lack of expression in these layers in the adult. These data demonstrate increased expression of GABA(A) receptors during the period of establishment of thalamocortical and intracortical connections, and a temporal regulation that may be associated with the period of developmental plasticity.

Developmental study of miniature IPSCs of CA3 hippocampal cells: modulation by midazolam

Maturation of GABAA/benzodiazepine receptors is associated with changes in their subunit composition. We have investigated whether these changes are accompanied by a developmental modification in the kinetic properties of miniature IPSCs (mIPSCs) and sensitivity to midazolam, a benzodiazepine agonist. In the presence of TTX (10 microM) and excitatory amino acid antagonists, AP5 (50 microM) and CNQX (50 microM), we whole-cell recorded mIPSCs in CA3 cells of hippocampal slices of adult and young (4-8 days) rats. mIPSCs were mediated by GABAA receptors as they were suppressed by bicuculline (10 microM). In both the adult and young rats, mIPSCs were similar in amplitude and kinetic properties. However, the mIPSCs frequency markedly increased with age from 4+/-3 Hz in the young rats to 20+/-9 Hz in the adult rats. In both age groups, midazolam (0.01 microM(-10) microM) and pentobarbital (30 microM) did not affect the amplitude, frequency and rise time of the mIPSCs but they increased to a similar extent their decay time constant. The current responses to isoguvacine, a GABAA agonist, were potentiated by 0.1 microM midazolam in both age groups. It is concluded that in immature and adult rats, synaptic GABAA receptors of CA3 were not different in their kinetic properties and sensitivity to midazolam.

Functional GABAA receptor heterogeneity of acutely dissociated hippocampal CA1 pyramidal cells

CA1 pyramidal cells were voltage clamped, and GABA was applied to individual cells with a modified U-tube, rapid drug application system. With Vh = -50 mV, inward currents elicited by 10 microM GABA were inhibited by GABAA receptor (GABAR) antagonists and were baclofen insensitive, suggesting that GABA actions on isolated CA1 pyramidal cells were GABAR mediated. GABA concentration-response curves averaged from all cells were fitted best with a two-site equation, indicating the presence of at least two GABA binding sites, a higher-affinity site (EC50-1 = 11.0 microM) and a lower-affinity site (EC50-2 = 334.2 microM), on two or more populations of cells. The effects of GABAR allosteric modulators on peak concentration-dependent GABAR currents were complex and included monophasic (loreclezole) or multiphasic (diazepam) enhancement, mixed enhancement/inhibition (DMCM, zolpidem) or multiphasic inhibition (zinc). Monophasic (70% of cells) or biphasic (30% of cells) enhancement of GABAR currents by diazepam suggested three different sites on GABARs (EC50-1 =1.8 nM; EC50-2 = 75.8 nM; EC50-3 = 275.9 nM) revealing GABAR heterogeneity. The imidazopyridine zolpidem enhanced GABAR currents in 70% of cells with an EC50 = 222.5 nM, suggesting a predominance of moderate affinity alpha2 (or alpha3-) subtype-containing BZ Type IIA receptors. A small fraction of cells (10%) had a high affinity for zolpidem, something that is suggestive of alpha1 subtype-containing BZ Type I receptors. The remaining 30% of cells were insensitive to or inhibited by zolpidem, suggesting the presence of alpha5 subtype-containing BZ Type IIB receptors. Whether BZ Type I and Type II receptors coexist could not be determined. The beta-carboline methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) inhibited GABAR currents in all cells at midnanomolar concentrations, but in addition, potentiated GABAR currents in some cells at low nanomolar concentrations, characterizing two groups of cells, the latter likely due to functional assembly of alpha5betaxgamma2GABARs. In all cells, GABAR currents were moderately sensitive (EC50 = 9 microM) to loreclezole, consistent with a relatively greater beta3 subtype, than beta1 subtype, subunit mRNA expression. Two populations of cells were identified based on their sensitivities to zinc(IC50 = 28 and 182 microM), suggesting the presence of at least two GABAR isoforms including alpha5beta3gamma2 GABARs. Consistent with the heterogeneity of expression of GABAR subunit mRNA and protein in the hippocampus and based on their differential responses to GABA and to allosteric modulators, distinct populations of CA1 pyramidal cells likely express multiple, functional GABAR isoforms.

Neurosteroid modulation of the GABAA receptor in the developing guinea pig cerebral cortex

Developmental changes in 5alpha-pregnan-3alpha-ol-20-one (allopregnanolone; 5alpha-3alpha-P) potentiation of muscimol and benzodiazepine binding to the GABAA receptor were studied in the guinea pig cerebral cortex at three prenatal ages (gestational day (GD) 40, GD 50, GD 62), and three postnatal ages (postnatal day (PD) 11, PD 21, PD 61) (term, about GD 68). The number and affinity of [3H]flunitrazepam binding sites, and 5alpha-3alpha-P potentiation of [3H]muscimol and [3H]flunitrazepam binding to the GABAA receptor were determined at each age. There was no age effect on the affinity (Kd) for [3H]flunitrazepam. However, the number (Bmax) of [3H]flunitrazepam binding sites doubled between GD 40 and GD 62, and then declined slightly to reach adult levels by PD 11. 5alpha-3alpha-P produced a concentration-dependent potentiation of [3H]muscimol and [3H]flunitrazepam binding at each developmental age examined. The potency (high-affinity) for 5alpha-3alpha-P potentiation of both [3H]muscimol and [3H]flunitrazepam binding was lowest at GD 40, and increased to adult levels by GD 62. In contrast, the efficacy for 5alpha-3alpha-P potentiation of both [3H]muscimol and [3H]flunitrazepam binding was greatest at GD 40, and decreased to adult levels between GD 50 and GD 62. The percentage of high-affinity zolpidem binding sites increased in an age-dependent manner from 34.2+/-2.2% at GD 40, to reach adult levels by GD 62 (59. 4+/-2.5%). These data suggest that 5alpha-3alpha-P can modulate GABAA receptors in the immature cerebral cortex, and that changes in 5alpha-3alpha-P action are temporally related to changes in GABAA receptor benzodiazepine pharmacology late in gestation in the guinea pig.

GABAA receptors mediate trophic effects of GABA on embryonic brainstem monoamine neurons in vitro

The inhibitory neurotransmitter GABA may act as a trophic signal for developing monoamine neurons in embryonic rat brain, because GABA neurons and their receptors appear in brainstem during generation of monoamine neurons. To test this hypothesis, we used dissociated cell cultures from embryonic day 14 rat brainstem, which contains developing serotonin (5-HT), noradrenaline (tyrosine hydroxylase; TH), and GABA neurons. Immunocytochemistry and reverse transcription-PCR (RT-PCR) revealed the presence of multiple alpha, beta, gamma, and delta subunits in these cultures. Competitive RT-PCR demonstrated high levels of beta3 subunit transcripts. Expression of functional GABAA receptors was demonstrated using 36Cl- flux assays. To investigate GABAergic regulation of neuronal survival and growth, cultures were treated for 1-3 d in vitro with 10 microM GABA and/or GABAA antagonist (bicuculline or the pesticide dieldrin). The effects of treatments were quantified by analysis of immunoreactive 5-HT, TH, and GABA neurons. GABAA receptor ligands differentially regulated neuronal survival and growth depending on neurotransmitter phenotype. GABA exerted positive effects on monoamine neurons, which were countered by bicuculline (and dieldrin, 5-HT neurons only). By itself, bicuculline produced inhibitory effects on both 5-HT and TH neurons, whereas dieldrin potently inhibited 5-HT neurons only. GABA neurons responded positively to both antagonists, but more strongly to bicuculline. Taken together, these results demonstrate that the activation/inhibition of GABAA receptors produces opposite effects on the development of embryonic monoamine and GABA neurons. This suggests that these neurotransmitter phenotypes may express GABAA receptors that differ in fundamental ways, and these differences determine the developmental responses of these cells to GABAergic stimuli.

Expression of the GABAA receptor delta subunit is selectively modulated by depolarization in cultured rat cerebellar granule neurons

The levels of several GABAA receptor subunit mRNAs increase as cerebellar granule neurons migrate to their adult positions and receive excitatory mossy fiber input. Despite the temporal similarity of these increases in transcript expression in vivo, studies in cultured granule neurons demonstrated that the subunit mRNAs are differentially regulated. To address the possibility that neuronal activity regulates transcript expression, GABAA receptor subunit mRNA levels were assessed in cultured granule neurons grown in chemically defined, serum-free medium containing either nondepolarizing (5 mM) or depolarizing (25 mM) KCl concentrations. Whereas the delta subunit mRNA was almost undetectable in cultures maintained in nondepolarizing medium, an eightfold increase occurred between days 2 and 4 in cultures grown in depolarizing medium. Furthermore, delta subunit transcript expression was reduced by 76 +/- 6% when neurons in depolarizing medium were switched into nondepolarizing medium. The importance of depolarization in the initiation and maintenance of subunit transcript expression in granule neurons was selective for the GABAA receptor delta subunit. These changes in transcript expression involved calcium entry through L-type calcium channels. Nifedipine treatment (1 microM) both reduced intracellular calcium and decreased delta subunit mRNA expression by 79 +/- 4%. Furthermore, inhibition of Ca2+/calmodulin-dependent protein kinases (CaM kinases) by KN-62 (1 microM) also reduced delta subunit transcript expression. These studies demonstrate that KCl-induced depolarization, a condition that mimics the effects of neuronal activity, selectively modulates GABAA receptor delta subunit mRNA expression through a pathway involving calcium entry and activation of a CaM kinase.

Altered ratios of alternatively spliced long and short gamma2 subunit mRNAs of the gamma-amino butyrate type A receptor in prefrontal cortex of schizophrenics

The relative abundance of alternatively spliced long (gamma2L) and short (gamma2S) mRNAs of the gamma2 subunit of the gamma-amino butyrate type A (GABAA) receptor was examined in dorsolateral prefrontal cortex of schizophrenics and matched controls by using in situ hybridization histochemistry and semiquantitative reverse transcription-PCR (RT-PCR) amplification. A cRNA probe identifying both mRNAs showed that the transcripts are normally expressed at moderately high levels in the prefrontal cortex. Consistent with previous studies, overall levels of gamma2 transcripts in prefrontal cortex of brains from schizophrenics were reduced by 28.0%, although this reduction did not reach statistical significance. RT-PCR, performed under nonsaturating conditions on total RNA from the same blocks of tissue used for in situ hybridization histochemistry, revealed a marked reduction in the relative proportion of gamma2S transcripts in schizophrenic brains compared with controls. In schizophrenics, gamma2S transcripts had fallen to 51.7% (+/-7.9% SE; P < 0.0001) relative to control levels. Levels of gamma2L transcripts showed only a small and nonsignificant reduction of 16. 9% (+/-12.0% SE, P > 0.05). These findings indicate differential transcriptional regulation of two functionally distinct isoforms of one of the major GABAA receptor subunits in the prefrontal cortex of schizophrenics. The specific reduction in relative abundance of gamma2S mRNAs and the associated relative increase in gamma2L mRNAs should result in functionally less active GABAA receptors and have severe consequences for cortical integrative function.

Regulation of ion channel expression in neural cells by hormones and growth factors

Voltage-and ligand-gated ion channels are key players in synaptic transmission and neuron-glia communication in the nervous system. Expression of these proteins can be regulated at several levels (transcriptional, translational, or posttranslational) and by multiple extracellular factors in the developing and mature nervous system. A wide variety of hormones and growth factors have been identified as important in neural cell differentiation, which is a complex process involving the acquisition of cell-type-specific ion channel phenotypes. Much literature has already accumulated describing the structural and functional characteristics of ion channels, but relatively little is known about the factors that influence their synthesis and cell surface expression, although this area has generated considerable interest in the context of neural cell development. This article reviews several examples of regulated expression of these channels by cellular factors, namely peptide growth factors and steroid hormones, and discusses, where applicable, current understanding of molecular mechanisms underlying such regulation of voltage-and neurotransmitter-gated ion channels.

Expression of alpha3, beta3 and gamma1 GABA(A) receptor subunit messenger RNAs in visual cortex and lateral geniculate nucleus of normal and monocularly deprived monkeys

Complementary RNA probes derived from complementary DNA specifically subcloned from monkey tissue were used to localize, by in situ hybridization histochemistry, the relatively rare alpha3, beta3 and gamma1 subunit transcripts of the GABA(A) receptor in visual cortex and lateral geniculate nucleus of normal monkeys and in monkeys that had been deprived of vision in one eye. Overall, levels of alpha3, beta3 and gamma1 subunit transcripts were very low. In the primary visual cortex (area 17) they were concentrated in layers II and VI and in a stratum of white matter subjacent to layer VI. The localization and density of the three messenger RNAs closely resembled those of other rare (alpha2, alpha5 and beta1) transcripts but their distribution also overlapped that of the predominant alpha1, beta2 and gamma2 subunit transcripts. In area 18, alpha3 and beta3 transcript distribution resembled that in area 17, with the addition of a third band of hybridization in layer IV for beta3. Gamma1 subunit transcript localization in area 18 differed significantly from that in area 17, with increased expression restricted to layer IV. In the dorsal lateral geniculate nucleus, beta3 and gamma1 transcripts were expressed at low levels across all layers while alpha3 transcripts were restricted to the magnocellular layers. Following 15 and 18 day periods of monocular deprivation, induced by intravitreal injections of tetrodotoxin, levels of alpha3 receptor subunit transcripts showed modest reductions in layer VI of area 17 and in deprived geniculate laminae of adult animals. Reductions in alpha3 transcript levels were much more pronounced in layer IVCbeta of a five-month-old monkey deprived for the same time. Levels of beta3 and gamma1 transcripts were unaffected by monocular deprivation in cortex and geniculate at any age. Taken together with studies of other GABA(A) receptor transcripts, these results demonstrate the heterogeneity of GABA(A) receptor messenger RNA expression in the monkey geniculo-striate pathway and the varied response to reduced neuronal activity.

Effects of pentobarbital on the expression of GABAA receptor beta 1 mRNA in the hippocampus: differential responses of CA1 and CA3

Effects of barbiturates have been linked to the inhibitory GABAA receptor in the brain. The present study examines changes in the expression of GABAA receptor in the hippocampus of pentobarbital treated rat. Intraperitoneal pentobarbital injections were administered once daily for 9 days at an increasing dose schedule, 30 mg/kg at day 1-3, 60 mg/kg day 4-6, and 90 mg/kg day 7-9. Within each of the three dosage periods, the duration of sleep and extent of reduction in body temperature of the rats decreased with time. Two hours after the 9th injection, 3H-muscimol binding of the hippocampal homogenates of the animals showed that the maximal number of binding sites (Bmax), 10.2 +/- 1.6 pmol/mg protein, was not significantly greater than 9.5 +/- 1.2 of saline control, but strikingly about 7-fold control level of beta 1 mRNA was seen in the pyramidal cells of CA1 and CA2, as revealed by in situ hybridization analysis with digoxigenin-cRNA probes. However, when the rats were withdrawn from pentobarbital injection for 24 hours and 7 days, the Bmax of the hippocampi was lowered to 7.3 +/- 1.0 and 5.1 +/- 0.7, respectively, and the expression of beta 1 mRNA in CA1-2 returned toward control. The pentobarbital treatment did not significantly alter the affinity of the radioligand to the receptor in the hippocampus and the expression of beta 1 mRNA in CA3 and CA4. The results suggest the plasticity of the beta 1 mRNA in CA1-2 as well as differential involvement of CA1-2 and CA3-4 in response to the pentobarbital perturbation.

GABA inhibits migration of luteinizing hormone-releasing hormone neurons in embryonic olfactory explants

During development, a subpopulation of olfactory neurons transiently expresses GABA. The spatiotemporal pattern of GABAergic expression coincides with migration of luteinizing hormone-releasing hormone (LHRH) neurons from the olfactory pit to the CNS. In this investigation, we evaluated the role of GABAergic input on LHRH neuronal migration using olfactory explants, previously shown to exhibit outgrowth of olfactory axons, migration of LHRH neurons in association with a subset of these axons, and the presence of the olfactory-derived GABAergic neuronal population. GABAA receptor antagonists bicuculline (10(-5) M) or picrotoxin (10(-4) M) had no effect on the length of peripherin-immunoreactive olfactory fibers or LHRH cell number. However, LHRH cell migration, as determined by the distance immunopositive cells migrated from olfactory pits, was significantly increased by these perturbations. Addition of tetrodotoxin (10(-6) M), to inhibit Na+-transduced electrical activity, also significantly enhanced LHRH migration. The most robust effect observed was dramatic inhibition of LHRH cell migration in explants cultured in the presence of the GABAA receptor agonist muscimol (10(-4) M). This study demonstrates that GABAergic activity in nasal regions can have profound effects on migration of LHRH neurons and suggests that GABA participates in appropriate timing of LHRH neuronal migration into the developing brain.

Developmental regulation of mossy fiber afferent interactions with target granule cells

In an in vitro model system based on purified target cerebellar granule neurons and explants of afferents, pontine mossy fiber afferents stop growing through contact-mediated mechanisms when they encounter granule neurons. Here we studied the developmental regulation of the stop signal posed by granule cells and the response of mossy fibers to the stop signal in two culture systems. Granule neurons presented in slices or as dissociated cells from postnatal day (P) 4 and P7 cerebellum were more potent in the arrest of P0 pontine neurites than younger (P0-P2) or older (up to P14) granule neurons. In contrast, pontine neurites at embryonic day (E) 18, during their period of normal growth toward the cerebellum, grew extensively on both cerebellar slices of all ages from P0 to P10 and dissociated P4 granule neurons. When E18 explants were maintained for 2 days before plating in medium conditioned by neonatal cerebellar cells, E18 pontine explants were rendered more responsive to the stop signal from P4 granule cells. These results indicate that the stop signal, and the response of afferents to it, are developmentally regulated. Moreover, factors within the target region may initiate these interactions.

Developmental changes of GABA(A) receptor-chloride channels in rat Meynert neurons

The developmental changes of GABA(A) receptors were investigated in Meynert neurons freshly dissociated from day 0, 2 week-, and 6 month-old rats using both nystatin and gramicidin perforated patch recording modes under voltage-clamp conditions. The age-related changes in the current amplitude and threshold concentration in the concentration-response relationships for GABA indicated the developmental alteration of the GABA(A) receptor subunits and the channel density. The GABA-induced E(Cl-) measured by the gramicidin perforated patch mode shifted to more negative with development. The decay time constant of GABAergic inhibitory postsynaptic spontaneous currents (sIPSCs) in the synaptic active zone accelerated with aging. The GABA-induced currents were potentiated in a concentration dependent manner in the presence of benzodiazepine (BZP) agonists, diazepam (DZP) and zolpidem (ZPM). The potentiation rate of DZP on the GABA(A) response decreased with aging, but not in the case of ZPM, which demonstrated a stronger action in the aging rat neurons. These results suggested that the GABA(A) receptor x Cl- channel complexes may thus change both the assembly and interaction of subunits as well as their functional roles with aging.

Dynamics of GABA(A) receptor binding in the ventromedial hypothalamus during postnatal development in the rat

In vitro quantitative autoradiography of [3H]muscimol binding was used to examine the developmental pattern of GABA(A) receptor expression in the ventromedial hypothalamus (VMN) of Long-Evans rats. Peak levels of [3H]muscimol binding were observed in the VMN on postnatal day 14 (PN14), binding levels were significantly lower in the VMN at earlier (PN0 and PN7) and later (PN70) ages. The developmental pattern of [3H]muscimol binding observed in the VMN differed from that seen in the hippocampus where binding was found to increase progressively from PN0 to PN14 and was maintained at high levels in adulthood. Although the profile of GABA(A) receptor expression in the developing VMN undergoes considerable modification during the critical period for sexual differentiation, no significant differences in [3H]muscimol binding were observed between male and female animals at any developmental age.

Expression of cerebellar specific glutamate and GABAA receptor subunits in heterotopic cerebellar grafts

In the cerebellum, up- and downregulation of specific GABAA and NMDA receptor subunits coincide with granule cell migration and differentiation. In this study, in situ hybridization techniques with GABAA and NMDA receptor subunit specific probes were employed to assess whether the molecular phenotype of heterotopically grafted cerebellar granule cells corresponds to that of normal cerebellum. The cerebellar anlage of rat fetuses was stereotactically grafted into the rostral striatum of adult rats. Eight weeks after transplantation, analysis demonstrated acquisition of an adult differentiation status reflected by abundant GABAA alpha 6 and NR2C mRNA expression in granule cells. Complete lack of NR2B transcripts, molecular markers of immature granule cells, argues against persistence of undifferentiated cells. The data suggest that intrinsic cell-autonomous factors largely determine the molecular commitment of granule cells and that a restricted specific environment is not necessary to promote granule cell differentiation.

Developmental expression of GABA(A) receptor subunit and GAD genes in mouse somatosensory barrel cortex

In situ hybridization histochemistry with radioactive cRNA probes was used to study patterns of gene expression for alpha1, alpha2, alpha4, alpha5, beta1, beta2, and gamma2 subunit mRNAs of typeAgamma aminobutyric acid (GABA(A)) receptors and for 67-kDa glutamic acid decarboxylase (GAD67) mRNA in mouse barrel cortex during the period (postnatal days 1-12; P1-P12) when thalamocortical innervation of layer IV barrels is occurring. The alpha1, beta2, and gamma2 subunit mRNAs increased substantially with age, especially in layers V and VI, and throughout the period studied, invariably had the same laminar-specific patterns of expression. All three mRNAs were highly expressed in the dense cortical plate at P1. In layer IV after differentiation of barrels, they were expressed in cells of both barrel walls and hollows but especially in the walls. The alpha2, alpha4, alpha5, and beta1 subunit mRNAs were expressed at lower levels and had different laminar patterns of distribution; alpha2 and alpha4 showed switches between layers over time; alpha5 was invariably associated with the subplate or its derivative, beta1 with layer IV. Levels of alpha2 mRNA did not change over time; alpha4 and beta1 mRNAs increased and alpha5 decreased. GAD67 mRNA was highest in layer I at P1 and progressively increased in other layers. These results suggest that postnatal development of GABA(A) receptors is mainly directed at the production of receptors assembled from alpha1, beta2, and gamma2 subunits, with beta1 contributing in layer IV. Other subunits may be associated with receptors involved in trophic actions of GABA during development and may give GABA(A) receptor-mediated responses in the developing cortex their particular physiological profile.

The GABAA receptor alpha 6 subunit gene (Gabra6) is tightly linked to the alpha 1-gamma 2 subunit cluster on mouse chromosome 11

We have established that the GABAA receptor alpha 6 (Gabra6) and alpha 1 (Gabra 1) subunit genes are tightly linked on mouse chromosome 11 by analysing the strain distribution patterns of RFLPs for the two genes and microsatellite markers flanking these genes in 26 BXD recombinant inbred strains. These results further demonstrate clustering of the GABAA receptor subunit genes on mouse chromosomes and the synteny for these clusters between the mouse and human genomes.

GABA-activated conductance in cultured rat inferior colliculus neurons

With the use of a whole cell voltage-clamp technique and fura-2 fluorescence measurements, the actions of gamma-aminobutyric acid (GABA) on cultured neurons from rat inferior colliculus were investigated. GABA (10-1,000 microM) induced currents in neurons held under voltage clamp that were inhibited by bicuculline (20 microM). Muscimol (100 microM) also evoked the currents, whereas baclofen (100 microM) affected neither the holding currents nor K+ conductance due to depolarizing pulses. The current density-voltage relation of GABA-induced currents, with equal concentrations of Cl- in the internal and external solutions, reversed near 0 mV. Reduction of the internal Cl- concentration shifted the-reversal potential in the negative direction as predicted from the Cl- equilibrium potential. Baclofen did not affect Ca2+ conductance due to depolarizing pulses. The extracellular application of 150 mM.KCl or 1.0 mM glutamate increased the intracellular Ca2+ concentration ([Ca2+]i) of cultured inferior colliculus neurons only when neurons were bathed in a Ca(2+)-containing external solution. However, GABA (1.0 mM) failed to increase [Ca2+]i at all concentrations of external Ca2+ used, indicating that GABA neither depolarized the cultured inferior colliculus neurons sufficiently to activate the voltage-dependent Ca2+ conductances nor evoked Ca2+ release from intracellular stores. These results suggest that in cultured rat inferior colliculus neurons, GABAA receptor channels may be predominantly responsible for the membrane conductance evoked by GABA and subsequent hyperpolarization of the neurons.