Independent cellular and ontogenetic expression of mRNAs encoding three alpha polypeptides of the rat GABAA receptor

Role of tonic GABAergic currents during pre- and early postnatal rodent development

In the last three decades it became evident that the GABAergic system plays an essential role for the development of the central nervous system, by influencing the proliferation of neuronal precursors, neuronal migration and differentiation, as well as by controlling early activity patterns and thus formation of neuronal networks. GABA controls neuronal development via depolarizing membrane responses upon activation of ionotropic GABA receptors. However, many of these effects occur before the onset of synaptic GABAergic activity and thus require the presence of extrasynaptic tonic currents in neuronal precursors and immature neurons. This review summarizes our current knowledge about the role of tonic GABAergic currents during early brain development. In this review we compare the temporal sequence of the expression and functional relevance of different GABA receptor subunits, GABA synthesizing enzymes and GABA transporters. We also refer to other possible endogenous agonists of GABAA receptors. In addition, we describe functional consequences mediated by the GABAergic system during early developmental periods and discuss current models about the origin of extrasynaptic GABA and/or other endogenous GABAergic agonists during early developmental states. Finally, we present evidence that tonic GABAergic activity is also critically involved in the generation of physiological as well as pathophysiological activity patterns before and after the establishment of functional GABAergic synaptic connections.

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.

Taurine acts as a glycine receptor agonist in slices of rat inferior colliculus

Taurine is an important endogenous amino acid for neural development and for many physiological functions, but little is known about its functional role in the central auditory system. We investigated in young rats (P10-P14) the effects of taurine on the neuronal responses and synaptic transmissions in the central nucleus of the inferior colliculus (ICC) with a brain slice preparation and with whole-cell patch-clamp recordings. Perfusion of taurine at 1mM reliably evoked a current across the membrane and decreased the input resistance in neurons of the ICC. Taurine also depressed the spontaneous and current-evoked firing of ICC neurons. All these effects were reversible after washout and could be blocked by 3 microM strychnine, an antagonist of glycine receptors, but not by 10 microM bicuculline, an antagonist of GABA(A) receptors. When the inhibitory receptors were not pharmacologically blocked, taurine reversibly reduced the postsynaptic currents/potentials evoked by electrically stimulating the commissure of the inferior colliculus or the ipsilateral lateral lemniscus. The results demonstrate that taurine reduces the neuronal excitability and depresses the synaptic transmission in the ICC by activating glycine-gated chloride channels. Our findings suggest that taurine acts as a ligand of glycine receptors in the ICC and can be involved in the information processing of the central auditory system similarly like the neurotransmitter glycine.

Periadolescent exposure to ethanol and diazepam alters the aversive properties of ethanol in adult mice

Evidence suggests that the developing adolescent brain may be especially vulnerable to long-term neurobehavioral consequences following ethanol exposure and withdrawal. In the present study, we examined the long-term effect of adolescent ethanol withdrawal on a subsequent EtOH-induced conditioned taste aversion (CTA). Periadolescent and adult C3H mice were exposed to 64 h of continuous (single withdrawal) or intermittent (multiple withdrawal) ethanol vapor. Following each ethanol exposure, animals received either 0, 1, 2, or 3 mg/kg diazepam (DZP) in an attempt to counteract the possible effect of ethanol withdrawal. About 6 weeks following ethanol and DZP treatment, animals were tested for an EtOH-induced CTA. As expected, exposure to EtOH during adolescence attenuated the EtOH-induced CTA as compared to controls. Unexpectedly, administration of DZP during withdrawal did not spare but rather mimicked the attenuation of the EtOH-induced CTA seen in animals exposed to ethanol in adolescence. This attenuation was not evident when EtOH and/or DZP was administered in adulthood. Given the similar mode of action of EtOH and DZP on the GABA system, the principal implication of the present findings is that the intoxicating effect of ethanol on the developing brain can result in long-term changes in the aversive properties of EtOH.

MEKC-LIF of gamma-amino butyric acid in microdialysate: systematic optimization of the separation conditions by factorial analysis

Micellar electrokinetic chromatography allows the efficient separation of biogenic amines and amino acids in biological samples. Analytes of interest, sample composition, and sample matrix may vary between studies, which necessitates optimization of separations to meet the requirements and conditions of an experiment. Factorial analysis is an efficient tool to accomplish this type of optimization involving multiple interacting factors. The present study describes an optimization procedure for separation of the inhibitory neurotransmitter GABA utilizing capillary electrophoresis with laser induced fluorescence detection. Standards labeled with the flourogenic reagent 3-(2-furoyl)quinoline-2 carboxaldehyde were separated with varying concentrations of borate buffer, beta-cyclodextrin, sodium dodecyl sulfate and pH. The optimized separation method had a correlation coefficient between concentration of GABA and fluorescent signal of 0.98, and was linear in the desired concentration range of 25 nM-10 microM. Glutamic acid, aspartic acid and taurine were also quantified using this separation. When applied to microdialysate collected from the region of the suprachiasmatic nucleus, this separation was able to measure daily variations in GABA levels. The factorial design experiment has proven to be a useful tool, allowing adjustments in the separation of neurotransmitters based on individual requirements.

Maturation of respiratory reflex responses in the fetus and neonate

Respiratory control in the fetus and neonate is quite immature when compared to that of adults. This immaturity involves all facets of respiration including respiratory responses to hypoxia, hypercapnia, an exaggerated apnoeic response to laryngeal stimulation and immature responses to activation of pulmonary afferents. The net result of this immaturity of breathing responses is the vulnerability of neonates and especially preterm infants to apnoea and respiratory pauses. The mechanisms behind immature control of breathing are not fully understood, but seem to originate from a predominance of inhibitory input early in life on respiratory centres. The relative contribution of up-regulation of inhibitory pathways versus down-regulation of excitatory ones is not clear. Multiple neurotransmitters have been implicated in the regulation of breathing in mammals and some of them are discussed in this chapter.

The 5'-flanking region of the rat GABA(A) receptor alpha2-subunit gene (Gabra2)

The GABA(A) receptor alpha2-subunit gene (Gabra2) has a specific spatial and temporal pattern of expression in rat brain. As a first step towards understanding the molecular mechanism underlying this regulation, we have investigated the structural properties of the 5'- flanking region of the rat Gabra2 gene. We identified six alpha2 transcript isoforms, each of which differs only in the 5'-untranslated region (UTR). Alignment of cDNA and genomic DNA sequences revealed that six 5'-UTRs are generated from three alternative first exons by alternative splicing using internal and terminal 5'-splice donor sites present in these exons. Promoter regions containing multiple transcription initiation sites were identified in the 5' proximity of each first exon. Two of these promoters lack TATA and CCAAT sequences. Finally, we have shown that differential activation of alternative promoters is used for the expression of the alpha2 mRNA isoforms during brain development, and that the diversity at the 5'-end of these transcripts affects GABA(A) receptor expression. Taken together, these results suggest that the expression of the Gabra2 gene can be influenced at both the transcriptional and post-transcriptional levels.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Increased mRNA expression for the alpha(1) subunit of the GABA(A) receptor following nitrous oxide exposure in mice

The mechanisms by which nitrous oxide (N(2)O) produces physical dependence and withdrawal seizures are not well understood, but both N(2)O and ethanol exert some of their effects via the GABA(A) receptor and several lines of evidence indicate that withdrawal from N(2)O and ethanol may be produced through similar mechanisms. Expression levels of mRNA transcripts encoding several GABA(A) receptor subunits change with chronic ethanol exposure and, therefore, we hypothesized that N(2)O exposure would produce changes in mRNA expression for the alpha(1) subunit. Male, Swiss--Webster mice, 10--12 weeks of age, were exposed for 48 h to either room air or a 75%:25% N(2)O:O(2) environment. Brains were sectioned and mRNA for the alpha(1) subunit was detected by in situ hybridization using an 35S-labelled cRNA probe. N(2)O exposure produced a significant increase in expression levels of the alpha(1) subunit mRNA in the cingulate cortex, the CA1/2 region of the hippocampus, the dentate gyrus, the subiculum, the medial septum, and the ventral tegmental area. These results lend support to the hypothesis that N(2)O effects are produced, at least in part, through the GABA(A) receptor and that N(2)O produces these effects through actions in the cingulate cortex, hippocampus, ventral tegmental area and medial septum. These results are also further evidence that ethanol and N(2)O produce dependence and withdrawal through common mechanisms.

Central GABAergic mechanisms are involved in apnea induced by SLN stimulation in piglets

Stimulation of the superior laryngeal nerve (SLN) results in apnea in animals of different species, the mechanism of which is not known. We studied the effect of the GABA(A) receptor blocker bicuculline, given intravenously and intracisternally, on apnea induced by SLN stimulation. Eighteen 5- to 10-day-old piglets were studied: bicuculline was administered intravenously to nine animals and intracisternally to nine animals. The animals were anesthetized and then decerebrated, vagotomized, ventilated, and paralyzed. The phrenic nerve responses to four levels of electrical SLN stimulation were measured before and after bicuculline. SLN stimulation caused a significant decrease in phrenic nerve amplitude, phrenic nerve frequency, minute phrenic activity, and inspiratory time (P < 0.01) that was proportional to the level of electrical stimulation. Increased levels of stimulation were more likely to induce apnea during stimulation that often persisted beyond cessation of the stimulus. Bicuculline, administered intravenously or intracisternally, decreased the SLN stimulation-induced decrease in phrenic nerve amplitude, minute phrenic activity, and phrenic nerve frequency (P < 0.05). Bicuculline also reduced SLN-induced apnea and duration of poststimulation apnea (P < 0.05). We conclude that centrally mediated GABAergic pathways are involved in laryngeal stimulation-induced apnea.

Developmental changes in the expression of GABA(A) receptor subunits (alpha(1), alpha(2), alpha(3)) in the cat visual cortex and the effects of dark rearing

The present study used Western blots and Northern slot blots to determine changes in the level of expression of GABA(A) receptor subunits alpha(1), alpha(2), and alpha(3), in relation to the "critical period" in cat visual cortex. Levels of the GABA(A) alpha(1) subunit were lowest at 1 week, increased four-fold to a maximum at 10 weeks, and declined slightly (35%) into adulthood. Levels of the GABA(A) alpha(2) and alpha(3) subunits were highest at 1 week of age, decreased two-fold by 10 weeks of age and were constant thereafter. Comparison between visual cortex from normal and dark-reared cats at 5 weeks and 20 weeks showed that alpha(1) and alpha(3) subunit expression was elevated in dark-reared animals by approximately 50% at both ages. alpha(2) expression was not affected. These results implicate the importance of a shift from putative immature to mature GABA(A) receptor subunits during the critical period of visual cortex and in conjunction with parallel analysis of NMDA receptor subunit maturation, further support the notion that a changing excitatory/inhibitory balance is critical for neuronal plasticity.

Regional distribution of benzodiazepine binding sites in the human newborn and infant hypothalamus. A quantitative autoradiographic study

Using in vitro quantitative autoradiography and [3H]flunitrazepam we examined the rostrocaudal distribution of benzodiazepine binding sites in the human neonate/infant hypothalamus. The autoradiographic analysis shows the presence of a heterogeneous distribution throughout the rostrocaudal extent of this brain structure. High [3H]flunitrazepam binding corresponds primarily to the diagonal band of Broca and the preoptic region. The labelling in the preoptic region showed a rostrocaudal increase, contrasting in that with the other hypothalamic structures. Intermediate densities were present in the septohypothalamic, suprachiasmatic, periventricular and paraventricular nuclei as well as in the mammillary complex. Low binding was observed in the other hypothalamic structures. The benzodiazepine binding sites analyzed belong mostly to type II receptors. In an attempt to unravel possible differences related to age, we compared the autoradiographic distribution in three postnatal age ranges. The topographical distribution of these binding sites was almost identical in each period analyzed. We found, however, that benzodiazepine binding is generally low in the neonatal period and a tendency in increasing densities is observed during development. Taken together, these results provide evidence for a large distribution of benzodiazepine binding sites in neonate/infant hypothalamus, suggesting their implication in the development of this brain structure and the maintenance of its various functions.

Changes in respiratory timing induced by hypercapnia in maturing rats

Premature infants respond to hypercapnia by an attenuated ventilatory response that is characterized by a decrease in respiratory frequency. We hypothesized that this impaired hypercapnic ventilatory response is of central origin and is mediated via gamma-aminobutyric acid-ergic (GABAergic) pathways. We therefore studied two groups of maturing Sprague-Dawley rats: unrestrained rats in a whole body plethysmograph at four postnatal ages (5, 16-17, 22-23, and 41-42 days); and ventilated, decerebrate, vagotomized, paralyzed rats in which phrenic nerve responses to hypercapnia were measured at 4-6 and 37-39 days of age. In the unrestrained group, the increase in minute ventilation induced by hypercapnia was significantly lower at 5 days vs. beyond 16 days. Although there was an increase in tidal volume at all ages, frequency decreased significantly from baseline at 5 days, whereas it increased significantly at 16-17, 22-23, and 41-42 days. The decrease in frequency at 5 days of age was mainly due to a significant prolongation in expiratory duration (TE). In the ventilated group, hypercapnia also caused prolongation in TE at 4-6 days but not at 37-39 days of age. Intravenous administration of bicuculline (GABA(A)-receptor blocker) abolished the prolongation of TE in response to hypercapnia in the newborn rats. We conclude that newborn rat pups exhibit a characteristic ventilatory response to CO(2) expressed as a centrally mediated prolongation of TE that appears to be mediated by GABAergic mechanisms.

Transient expression of GABAA receptor subunit mRNAs in the cellular processes of cultured cortical neurons and glia

In this study, we have studied by in situ hybridisation histochemistry the expression and intracellular distribution of the GABAA receptor subunit mRNAs in cultured neurons obtained from postnatal day 1-3 rats in order to determine how neurotransmitter receptor expression may be regulated during development of the nervous system. In postnatal cortical cells, we found that GABAA receptor subunit mRNAs coding for alpha2, alpha5, beta2, beta3 and gamma2 subunits were transiently expressed in the cellular processes and growth cones after 1-3 days in culture. These observations indicate that GABAA receptor subunit mRNAs are transported (or trafficked) into the cellular processes of early postnatal cortical cells. These selective localisations were rarely observed after 5 days in culture and only in cells which had not made cell-to-cell contact. The localisation of subunit mRNAs in the processes was more effectively maintained up to 5 days or even longer if cell-to-cell contact was avoided by culturing the cells at low density or by inhibiting neurite sprouting pharmacologically with the GABA receptor channel antagonist TBPS. Finally, immunocytochemistry revealed the expression of GABAA receptors in the growth cones of pyramidal neurons in culture. Thus, the expression of mRNA correlates to the expression of protein. These results suggest that the selective trafficking of GABAA receptor subunit mRNAs during synaptogenesis may be regulated by synapse formation and/or glial-neural communication.

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.

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.

The expression of GABA(A) receptor subunits in the substantia nigra is developmentally regulated and region-specific

The substantia nigra pars reticulata (SNR) controls the spread of seizures. GABA(A)ergic drug (agonist or antagonist) infusions into the SNR have age-specific and site-specific effects on flurothyl-induced seizures. Developmental and cell-specific regulation of GABA(A) receptor subunit expression may be responsible for these specific effects. To test this hypothesis, in situ hybridization was used to examine regional expression of alpha1 and gamma2L GABA(A) receptor subunit mRNAs in the SNR during development. Distinct temporal and spatial patterns of expression were observed. In rats at postnatal days (PN) 21-60, fewer neurons were labeled with probes directed to alpha1 and gamma2L subunits in SNRanterior compared with SNRposterior. In addition, neurons in SNRanterior contained higher amounts of hybridization grains than in SNRposterior. In PN 15 rats, the labeling of neurons was relatively diffuse throughout the anterior and posterior SNR regions with moderate amounts of hybridization grains for both subunits. The finding of age-related differential distribution of alpha1 and gamma2L subunit mRNAs in the SNR suggests that GABA(A) receptor heterogeneity may play a role in the age-specific and site-specific effects of GABA(A)ergic agents on seizures in the SNR.

GABAergic deafferentation hypothesis of brain aging and Alzheimer's disease revisited

Considering the mechanisms responsible for age- and Alzheimer's disease (AD)-related neuronal degeneration, little attention was paid to the opposing relationships between the energy-rich phosphates, mainly the availability of the adenosine triphosphate (ATP), and the activity of the glutamic acid decarboxylase (GAD), the rate-limiting enzyme synthesizing the gamma-amino butyric acid (GABA). Here, it is postulated that in all neuronal phenotypes the declining ATP-mediated negative control of GABA synthesis gradually declines and results in age- and AD-related increases of GABA synthesis. The Ca2+-independent carrier-mediated GABA release interferes with Ca2+-dependent exocytotic release of all transmitter-modulators, because the interstitial (ambient) GABA acts on axonal preterminal and terminal varicosities endowed with depolarizing GABA(A)-benzodiazepine receptors; this makes GABA the "executor" of virtually all age- and AD-related neurodegenerative processes. Such a role of GABA is diametrically opposite to that in the perinatal phase, when the carrier-mediated GABA release, acting on GABA(A)/chloride ionophore receptors, positively controls chemotactic migration of neuronal precursor cells, has trophic actions and initiates synaptogenesis, thereby enabling retrograde axonal transport of target produced factors that trigger differentiation of neuronal phenotypes. However, with advancing age, and prematurely in AD, the declining mitochondrial ATP synthesis unleashes GABA synthesis, and its carrier-mediated release blocks Ca2+-dependent exocytotic release of all transmitter-modulators, leading to dystrophy of chronically depolarized axon terminals and block of retrograde transport of target-produced trophins, causing "starvation" and death of neuronal somata. The above scenario is consistent with the following observations: 1) a 10-month daily administration to aging rats of the GABA-chloride ionophore antagonist, pentylenetetrazol, or of the BDZ antagonist, flumazenil (FL), each forestalls the age-related decline in cognitive functions and losses of hippocampal neurons; 2) the brains of aging rats, relative to young animals, and the postmortem brains of AD patients, relative to age-matched controls, show up to two-fold increases in GABA synthesis; 3) the aging humans and those showing symptoms of AD, as well as the aging nonhuman primates and rodents--all show in the forebrain dystrophic axonal varicosities, losses of transmitter vesicles, and swollen mitochondria. These markers, currently regarded as the earliest signs of aging and AD, can be reproduced in vitro cell cultures by 1 microM GABA; the development of these markers can be prevented by substituting Cl- with SO4(2-); 4) the extrasynaptic GABA suppresses the membrane Na+, K+-ATPase and ion pumping, while the resulting depolarization of soma-dendrites relieves the "protective" voltage-dependent Mg2+ control of the N-methyl-D-aspartate (NMDA) channels, thereby enabling Ca2+-dependent persistent toxic actions of the excitatory amino acids (EAA); and 5) in whole-cell patch-clamp recording from neurons of aging rats, relative to young rats, the application of 3 microM GABA, causes twofold increases in the whole-cell membrane Cl- conductances and a loss of the physiologically important neuronal ability to desensitize to repeated GABA applications. These age-related alterations in neuronal membrane functions are amplified by 150% in the presence of agonists of BDZ recognition sites located on GABA receptor. The GABA deafferentation hypothesis also accounts for the age- and AD-related degeneration in the forebrain ascending cholinergic, glutamatergic, and the ascending mesencephalic monoaminergic system, despite that the latter, to foster the distribution-utilization of locally produced trophins, evolved syncytium-like connectivities among neuronal somata, axon collaterals, and dendrites, to bidirectionally transport trophins. (ABSTRACT TRUNCATED)

Plasticity in fast synaptic inhibition of adult oxytocin neurons caused by switch in GABA(A) receptor subunit expression

We found that magnocellular oxytocin neurons in adult female rats exhibit an endogenous GABA(A) receptor subunit switch around parturition: a decrease in alpha1:alpha2 subunit mRNA ratio correlated with a decrease in allopregnanolone potentiation and increase in decay time constant of the GABA(A) receptor-mediated IPSCs in these cells. The causal relationship between changes in alpha1:alpha2 mRNA ratio and the ion channel kinetics was confirmed using in vitro antisense deletion. Further, GABA(A) receptors exhibited a tonic inhibitory influence upon oxytocin release in vivo, and allopregnanolone helped to restrain oxytocin neuron in vitro firing only before parturition, when the alpha1:alpha2 subunit mRNA ratio was still high. Such observations provide evidence for the physiological significance of GABA(A) receptor subunit heterogeneity and plasticity in the adult brain.

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.

Plasticity in GABAA receptor subunit mRNA expression by hypothalamic magnocellular neurons in the adult rat

The magnocellular hypothalamic neurons exhibit a substantial degree of structural and functional plasticity over the time of pregnancy, parturition, and lactation. This study has used in situ hybridization techniques to examine whether the content of alpha 1, alpha 2, beta 2, gamma 2 GABAA receptor subunit mRNAs expressed by these cells fluctuates over this period. A process of regional, followed by cellular and then topographical, analyses within the supraoptic (SON) and posterior paraventricular (PVN) nuclei revealed that an increase in magnocellular alpha 1 subunit mRNA content occurred during the course of pregnancy up to day 19, after which a decline in expression was detected on the day of parturition. Significant fluctuations of this nature were observed only in the oxytocin neuron-enriched regions of the SON and PVN. The expression of alpha 2, beta 2, and gamma 2 subunit mRNAs in the SON and PVN and of all subunit mRNAs in the cingulate cortex did not change over this period. During lactation, gamma 2 subunit mRNA content within the PVN increased significantly on day 14 of lactation as compared with day 7, and topographical analysis suggested that it involved principally magnocellular vasopressin neurons. These results demonstrate the cell-and subunit-specific regulation of GABAA receptor mRNA expression within the hypothalamic magnocellular system. In particular, they suggest that fluctuations in alpha 1 subunit expression may contribute to the marked variations in electrical activity exhibited by magnocellular oxytocin neurons at the time of parturition. More generally, they provide evidence in support of GABAA receptor plasticity within a physiological context in the adult rat brain.

Developmental regulation of regional functionality of substantial nigra GABAA receptors involved in seizures

GABAergic (gamma-aminobutyric acid) transmission in the substantia nigra pars reticulata is critical for seizure control. We tested the hypothesis that there is a differential regional distribution and functionality of nigral GABAA receptor sites that is developmentally regulated. In adult rats, we determined the effects on flurothyl seizures of (Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid (ZAPA, a presumed agonist of the low-affinity GABAA receptor site), bicuculline (an antagonist of the low-affinity GABAA receptor site) and gamma-vinyl-GABA (a GABA-transaminase inhibitor), infused bilaterally in anterior or posterior substantia nigra pars reticulata. ZAPA infusions (8 micrograms) were anticonvulsant in anterior substantia nigra but proconvulsant in posterior substantia nigra. Bicuculline infusions (100 ng) were proconvulsant in anterior substantia nigra but ineffective in posterior substantia nigra. An anticonvulsant dose of gamma-vinyl-GABA, when infused in anterior substantia nigra, was proconvulsant when infused in posterior substantia nigra. In 15 day old rats, the effects of ZAPA, were biphasic: 2 micrograms was anticonvulsant while 8 micrograms was proconvulsant. There was no regional specificity. The data suggest that with maturation there is functional segregation of specific GABAA receptor subtypes involved in substantia nigra-mediated seizure control.

Thyroid hormonal modulation of the binding and activity of the GABAA receptor complex of brain

Thyroid hormones, which are known to act by genomic mechanisms in peripheral tissues, were found to influence the binding and function of the GABAA receptor complex in brain membranes. Submicromolar concentrations of triiodothyronine and thyroxine stereospecifically stimulated the binding of [35S]t-butylbicyclophosphorothionate (a convulsant ligand for the GABAA receptor complex) to highly washed rat brain membranes, while higher concentrations of the hormones inhibited radioligand binding. GABA-stimulated 36Cl-flux in isolated brain membrane sacs was inhibited by L-triiodothyronine with a half-maximally inhibitory concentration (IC50) of 10(-7) M. Patch-clamp analysis of recombinant GABAA receptor subunits expressed in human embryonic kidney-293 cells showed an inhibition of chloride currents by thyroid hormones. This effect required only the alpha 1 beta 2 subunits, and was not blocked by the benzodiazepine antagonist flumazenil. Since thyroid hormones are known to be concentrated in nerve terminal preparations and subsequently released, the hormones may have non-genomic mechanisms of action as putative neurotransmitters or neuromodulators in brain and act through GABAA receptors.

Differential distribution of gamma-aminobutyric acidA receptor subunit (alpha 1, alpha 2, alpha 3, alpha 5 and beta 2 + 3) immunoreactivity in the medial prefrontal cortex of the rat

A detailed mapping of the gamma-aminobutyric acid (GABA)A receptor subunits (alpha 1, alpha 2, alpha 3, alpha 5 and beta 2 + 3) in the infralimbic/ventral prelimbic region (IL/vPL) of the rat frontal cortex was carried out using subunit-specific antibodies. The alpha 1 and beta 2 + 3 subunit antibodies immunostained all layers of the IL/vPL region. Layers II and III displayed immunostaining of cell bodies whereas I, V and VI showed predominantly neuropil staining. The size of the alpha 1-positive cell bodies corresponded to that of small interneurons (range, 20-55 microns2; mean +/- SEM, 37 +/- 5.5 microns2) as well as pyramidal cells or large interneurons (range, 87-135 microns2; mean +/- SEM, 103.4 +/- 9.7 microns2). However, beta 2 + 3 antibody immunostained only small cell bodies. Immunoreactivity for alpha 2 was restricted to layers I and II, whereas alpha 3 and alpha 5 subunit expression was seen only in layer VI. The antibody to the alpha 2 subunit immunostained small cell bodies (range, 29-63 microns2; mean +/- SEM, 32 +/- 4.5 microns2) in layer II, resembling interneurons. Conversely, both alpha 3 and alpha 5 antibodies immunostained large cell bodies (range, 94-151 microns2; mean +/- SEM, 115.7 +/- 13.4 microns2), consistent with pyramidal cell labelling in layer VI.

Developmental profile of GABAA-receptors in the marmoset monkey: expression of distinct subtypes in pre- and postnatal brain

Gamma aminobutyric acid (GABA)A-receptors are expressed in fetal mammalian brain before the onset of synaptic inhibition, suggesting their involvement in brain development. In this study, we have analyzed the maturation of the GABAA-receptor in the marmoset monkey forebrain to determine whether distinct receptor subtypes are expressed at particular stages of pre- and postnatal ontogeny. The distribution of the subunits alpha 1, alpha 2, and beta 2,3 was investigated immunohistochemically between embryonic day 100 (6 weeks before birth) and adulthood. Prenatally, the alpha 2- and beta 2,3-subunit-immunoreactivity (-IR) was prominent throughout the forebrain, whereas the alpha 1-subunit-IR appeared in selected regions shortly before birth. The alpha 2-subunit-IR disappeared gradually to become restricted to a few regions in adult forebrain. By contrast, the alpha 1-subunit-IR increased dramatically after birth and replaced the alpha 2-subunit in the basal forebrain, pallidum, thalamus, and most of the cerebral cortex. Staining for the beta 2,3-subunits was ubiquitous at every age examined, indicating their association with either the alpha 1- or the alpha 2-subunit in distinct receptor subtypes. In neocortex, the alpha 1 -subunit-IR was first located selectively to layers IV and VI of primary somatosensory and visual areas. Postnatally, it increased throughout the cortex, with the adult pattern being established only during the second year. The switch in expression of the alpha 1- and alpha 2- subunits indicates that the subunit composition of major GABAA-receptor subtypes changes during ontogeny. This change coincides with synaptogenesis, suggesting that the emergence of alpha 1- GABAA-receptors parallels the formation of inhibitory circuits. A similar pattern has been reported in rat, indicating that the developmental regulation of GABAA-receptors is conserved across species, possibly including man. However, the marmoset brain is more mature than the rat brain at the onset of alpha 1-subunit expression, suggesting that alpha 1-GABAA-receptors are largely dispensable in utero, but may be required for information processing after birth.

In vivo regulation of specific GABAA receptor subunit messenger RNAs by increased GABA concentrations in rat brain

This study has examined whether changes in endogenous GABA concentrations influence GABAA receptor subunit mRNA expression in vivo. Increased GABA concentrations were achieved by treating female rats with gamma-vinyl-GABA (15 mg/100 g), an irreversible inhibitor of the GABA transaminase, daily for three days. High performance liquid chromatography analysis of brain punches from specific brain regions showed that gamma-vinyl-GABA treatment resulted in approximately two-fold increases in brain GABA content. Using in situ hybridization techniques with specific 35S-labelled oligonucleotides, the mRNA expression of the alpha 1, alpha 2, beta 2, beta 3, gamma 1 and/or gamma 2 subunits of the GABAA receptor was quantified in various brain regions including the medial preoptic nucleus, bed nucleus of the stria terminalis, bed nucleus of the anterior commissure, supraoptic and paraventricular nuclei of the hypothalamus, globus pallidus and cingulate cortex. Silver grain density analysis showed that gamma-vinyl-GABA treatment induced a significant 35 and 49% decrease in gamma 1 mRNA expression in the medial preoptic nucleus and the principle encapsulated nucleus of the bed nucleus of the stria terminalis respectively, and a significant 20% decrease in alpha 2 mRNA expression in the cingulate cortex. Expression of alpha 2 and beta 3 in the former areas was unchanged as was alpha 1, beta 2, beta 3 and gamma 2 subunit expression in the cingulate cortex. Elevation of brain GABA levels also resulted in a specific and significant 17% increase in gamma 2 mRNA expression in the supraoptic nucleus. In the globus pallidus, gamma-vinyl-GABA treatment induced a significant 29% increase in alpha 1 mRNA expression combined with 19 and 30% decreases in beta 2 and gamma 2 mRNA expression, respectively. Levels of GABAA receptor subunits expressed in the bed nucleus of the anterior commissure (alpha 2, beta 3, gamma 1) and paraventricular nucleus (alpha 1, alpha 2, beta 2, gamma 2) were not changed by gamma-vinyl-GABA treatment. These results provide in vivo evidence for a region- and subunit-specific regulation of GABAA receptor subunit mRNA levels following the elevation of brain GABA concentrations and suggest that endogenous GABA levels influence GABAA receptor subunit mRNA expression.

Subpopulations of GABA neurons in the dentate gyrus express high levels of the alpha 1 subunit of the GABAA receptor

The alpha 1 subunit of the gamma-aminobutyric acid (GABA)A receptor is highly expressed in a subgroup of neurons in the hippocampal formation. The distribution and chemical identities of these neurons in the dentate gyrus have been studied with double-labeling in situ hybridization and immunohistochemical methods. Double labeling for the alpha 1 subunit and glutamate decarboxylase 65 (GAD65) mRNAs indicated that virtually all neurons in the dentate gyrus that are heavily labeled for the alpha 1 subunit are GABA neurons. However, many GAD65 mRNA-labeled neurons in the hilus do not contain high levels of the alpha 1 subunit mRNA and protein. Studies were thus conducted to determine if the somatostatin neurons of the hilus were part of the alpha 1 subunit-labeled group. Double labeling for the alpha 1 subunit and pre-prosomatostatin mRNAs demonstrated virtually no co-localization of these mRNAs in hilar neurons. Thus, the strongly labeled alpha 1 mRNA-containing neurons and the somatostatin neurons constitute two distinct populations of hilar GABA neurons. Double labeling for the alpha 1 subunit polypeptide and its mRNA with immunohistochemical and in situ hybridization methods demonstrated directly that neurons of the dentate gyrus that express high levels of the alpha 1 subunit mRNA are the same neurons that show extensive labeling for the alpha 1 subunit along their somal and dendritic surfaces. The high levels of alpha 1 subunit expression in some populations of GABA neurons could be related to prominent disinhibitory functions of these neurons.

Structural and pharmacological aspects of the GABAA receptor: involvement in behavioral pathogenesis

The gamma-aminobutyric acidA (GABAA) receptor is a complex hetero-oligomeric protein. It is composed of several subunits which assemble to form a functional chloride channel. The precise molecular organization of the receptor is as yet unknown. In the first part, we review recent literature dealing with the molecular and pharmacological aspects of the GABAA receptor, the second part will review some of the pathologies probably associated with gene defects and/or quantitative differential expression of transcripts encoding GABAA receptor subunits.

Heterogeneous current responses to GABA and glycine are present in post-natally cultured hippocampal neurons

In a patch-clamp study of cultured hippocampal neurons, heterogeneous desensitization responses were observed in all cells with GABAA-gated channels, but in only 5% of cells with glycine-gated channels. GABA- and glycine-activated whole-cell currents from 'fast' and 'slow' cells had very similar amplitudes of about 2.0 nA, but different time-courses of desensitization. Single-channel main conductance states obtained from slow and fast cells both had values of about 27 +/- 1 pS for GABA, and values of 24 +/- 1.8 pS for slow and 19 +/- 1.5 pS for fast desensitizing glycine-gated channels. For GABA, the channel open or burst frequency of fast desensitizing cells was about twice that of slow desensitizing ones, whereas for glycine, the opening frequency of slow desensitizing cells was double that of fast desensitizing cells. Pronounced outward rectification was observed for all but the fast desensitizing glycine-gated cells. Dose-response curves obtained for slow and fast desensitizing cells displayed similar degrees of cooperativity and antagonist affinity, but clearly greater GABA sensitivity for fast desensitizing cells. In contrast, fast desensitizing glycine-gated cells displayed low antagonist affinity, whereas both types of cells displayed similar agonist sensitivity and cooperativity. These results indicate a mosaic-like distribution of different GABAA and glycine receptor isoforms in hippocampal neurons, with the possible existence of pre-natal-like glycine receptor subunits at this early stage of post-natal life.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Modulation of [35S]TBPS binding by ligands with preferential affinity for benzodiazepine BZ1 sites in the cerebral cortex of newborn and adult rats

The present study was designed to compare the allosteric coupling between the Cl- channel of the GABAA receptor and the different benzodiazepine recognition site subtypes (BZ sites) in the cerebral cortex of newborn (5-day-old) and adult rats (90-day-old). To this aim, we reexamined the heterogeneity of cortical GABAA receptors in self- and cross-competition binding experiments using [3H]flunitrazepam and two ligands with higher affinity for benzodiazepine BZ1 sites relative to benzodiazepine BZ2 sites, the triazolopyridazine 3-methyl-6-[3-(trifluoromethyl)phenyl]-1,2,4-triazolo [4,3-b] pyridazine (CL 218,872) and the imidazopyridine N,N,6-trimethyl-2-(4-methylphenyl)-imidazo[1,2-a]-pyridine-3-acetamide hemitartrate (zolpidem). Benzodiazepine BZ1 sites accounted for 52% of the total number of binding sites in adult rats, but were not detected in newborn rats. On the other hand, two classes of benzodiazepine BZ2 sites with high and low affinity for zolpidem were present in newborn and adult rats. These sites were designated as benzodiazepine BZ2H (high affinity for zolpidem, Kd approximately 150 nM) and benzodiazepine BZ2L (low affinity for zolpidem, Kd approximately 3000 nM). High densities of benzodiazepine BZ2H sites were measured in both newborn and adult rats (75% and 41% of the total number of [3H]flunitrazepam binding sites, respectively), whereas benzodiazepine BZ2L sites accounted for 25% and 7% of the total number of cortical sites in neonates and adults, respectively. Flunitrazepam, CL 218,872 and zolpidem inhibited in a concentration-dependent manner the binding of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) to the convulsant site of cortical GABAA receptors in newborn and adult rats. The IC50 for flunitrazepam was about 3-fold greater in adults than in neonates. This rightward shift in the concentration-response curve may be due to a decrease with age in the intrinsic efficacy of flunitrazepam. In contrast, CL 218,872 and zolpidem were 4-fold more potent at inhibiting [35S]TBPS binding in adult rats relative to neonates. The different affinities of CL 218,872 and zolpidem for benzodiazepine BZ1 and BZ2 receptors may account, at least in part, for the age-related changes in their inhibitory potencies. These results demonstrate that benzodiazepine BZ2 sites mediate the modulation of [35S]TBPS binding by benzodiazepine recognition site ligands in the cerebral cortex of newborn rats. Further, benzodiazepine BZ2 sites may be involved in the inhibition of [35S]TBPS binding by flunitrazepam, CL 218,872 and zolpidem in the cerebral cortex of adult rats.

Expression patterns and deprivation effects on GABAA receptor subunit and GAD mRNAs in monkey lateral geniculate nucleus

The expression and regulation of seven GABAA receptor subunit gene transcripts were examined in the adult monkey lateral geniculate nucleus. In situ hybridization histochemistry was used to localize mRNAs of the genes encoding monkey-specific alpha 1, alpha 2, alpha 4, alpha 5, beta 1, beta 2, and gamma 2 subunits. The highest levels of expression in the nucleus were for alpha 1, beta 2, and gamma 2 subunit transcripts. The levels were substantially higher in the magnocellular than in the parvocellular layers. Alpha-2, alpha 4, alpha 5, and beta 1 subunit mRNAs were expressed at much lower levels, and magno- and parvocellular layers had approximately equal levels of expression. Following 4- or 21-day periods of monocular deprivation induced by intravitreal injections of tetrodotoxin, levels of the alpha 1, alpha 2, alpha 4, alpha 5, beta 1, and beta 2 mRNAs were decreased in the deprived geniculate laminae. Adjacent sections hybridized with probes specific for 67-kDa glutamic acid decarboxylase (GAD) mRNA also showed decreased levels of expression in deprived laminae after the 21-day deprivation period. Levels of gamma 2 receptor subunit mRNA were unaffected by monocular deprivation. In our previous studies, a clearly significant downregulation is observed for this subunit transcript in the visual cortex of monkeys deprived for equivalent times. The differential expression and responses to deprivation in the lateral geniculate nucleus suggest region-specific regulation of GABAA receptor subunit genes.

Cellular localization and differential distribution of GABAA receptor subunit proteins and messenger RNAs within hypothalamic magnocellular neurons

The inhibitory neurotransmitter GABA plays an important role in regulating the activity of magnocellular oxytocin and vasopressin neurons located in the supraoptic and paraventricular nuclei through occupancy of GABAA receptors. However, the GABAA receptor is a hetero-oligomeric protein comprised of different subunits and the subunit types expressed in a given receptor complex appear critical for its sensitivity to GABA, benzodiazepines and/or steroids. Thus, in order to understand fully the GABAergic control of oxytocin and vasopressin secretion, definition of the GABAA receptors synthesized by magnocellular neurons in the supraoptic and paraventricular nuclei is required. In the supraoptic nucleus, antibodies directed against the alpha 1, alpha 2 and beta 2/3 subunits of the GABAA receptor revealed similar strong antigen distribution on all magnocellular neurons. Using sequential double-immunoperoxidase staining, immunoreactivity for all three subunits was observed on both oxytocin and vasopressin neurons of the supraoptic nucleus. In contrast, only alpha 2 subunit immunoreactivity was detected on the cell bodies of oxytocin and vasopressin neurons in the paraventricular nucleus. No sex differences were detected. In situ hybridization experiments using 35S-labelled oligonucleotides showed that all supraoptic neurons expressed alpha 1, alpha 2 and beta 2 subunit messenger RNA transcripts while magnocellular neurons in the paraventricular nucleus were only enriched in alpha 2 subunit messenger RNA. Quantitative analysis showed that the expression of alpha 1 and beta 2 subunit messenger RNAs in the paraventricular nucleus was half that observed in the supraoptic nucleus while expression of beta 3 subunit messenger RNA was very low in both nuclei. These results show that all oxytocin and vasopressin neurons located in the supraoptic nucleus synthesize and express alpha 1, alpha 2 and beta 2 subunits of the GABAA receptor while those in the paraventricular nucleus are only immunoreactive for the alpha 2 subunit. These observations suggest, therefore, that at least two pharmacologically distinct GABAA receptor isoforms exist on supraoptic neurons and that these are different to those expressed by paraventricular magnocellular cells. Thus, in addition to providing a definition of the subunits likely to form specific GABAA receptor isoforms on magnocellular neurons, this study gives direct evidence for GABAA receptor heterogeneity between supraoptic and paraventricular neurons, but not between oxytocin and vasopressin cells.

GABAA receptor subunit polypeptides increase in parallel but exhibit distinct distributions in the developing rat cerebellum

The GABAA receptor, a multisubunit ligand-gated ion channel, plays a central role in cell-cell communication in the developing and adult nervous system. Although the developmental expression of mRNAs encoding many subunit isoforms has been extensively characterized throughout the central nervous system, little is known concerning the relationship between subunit mRNA and polypeptide expression. To address this issue, we examined the developmental expression of the alpha 1, beta 2/3, and gamma 2 subunit polypeptides, subunits that are thought to coassemble in many brain regions. Western blot analysis using subunit-specific antibodies revealed that the levels of these polypeptides in both the cerebral cortex and cerebellum increased severalfold during the second postnatal week. Whereas polypeptide expression in the cerebellum paralleled that of the corresponding subunit mRNAs, increases in beta 2/3 and gamma 2 polypeptide expression in the cerebral cortex occurred in the absence of detectable changes in the mRNA levels. To determine whether the increases in subunit polypeptide expression in the cerebellum were accompanied by changes in distribution, immunohistochemistry was performed. These studies demonstrated that the subunits exhibited different but partially overlapping distributions that remained constant throughout postnatal development. Our findings suggest that although GABAA receptor subunit polypeptide expression may be regulated primarily at the level of the mRNA, additional regulatory mechanisms may play a role. Furthermore, the observation that subunit distribution remains constant in the cell bodies of cerebellar Purkinje neurons, which express the alpha 1, beta 2, beta 3, and gamma 2 subunit mRNAs exclusively, suggests that GABAA receptor subunit composition in this cell population does not change during postnatal maturation.

GABAA receptor alpha 1 subunit, an early marker for area specification in developing rat cerebral cortex

Changes in the expression of neurotransmitter receptors in developing cerebral cortex may be related to the functional maturation of distinct areas. In the present study, we have tested whether GABAA receptor expression in neonatal rats reflects the differentiation of cortical areas. Specifically, the alpha 1 subunit, one of the most prevalent GABAA receptor subunits in adult cerebral cortex, is up-regulated postnatally, suggesting a link with the establishment of inhibitory circuits. Using immunohistochemistry with a subunit-specific antiserum, we observed a striking area- and lamina-specific increase in staining for GABAA receptors containing the alpha 1 subunit (alpha 1-GABAA receptors), from low levels in neonates to an intense and uniform staining in adults. Already at birth, the alpha 1-subunit immunoreactivity selectively demarcated the boundaries of certain cortical areas. In particular, the primary somatosensory (S1) and visual (V1) areas were distinctly delineated with a band of alpha 1-subunit immunoreactivity located in the developing layers III and IV. The staining ended abruptly at the presumptive boundaries of S1 and V1, adjacent areas being unstained at this age. Around postnatal day 3, clusters of alpha 1-subunit positive cells were seen in layers III-IV of S1 and V1 extending their dendrites up to layer I, where they arborized profusely. In addition, the distribution of alpha 1-GABAA receptors in S1 revealed in detail the differentiation of the barrel field during early postnatal development. Although staining was observed in all areas by postnatal day 6, differences in the laminar distribution of alpha 1-GABAA receptors persisted for at least 1 more week. Our results provide evidence for the existence of area-specific boundaries in neocortex of newborn rats before layers III-IV are fully differentiated and innervated by cortical afferents. Furthermore, the area- and lamina-specific maturation of alpha 1-GABAA receptor staining demonstrates the value of this marker for investigating the cytoarchitectonic differentiation of cortical areas during development.

Maturation and segregation of brain networks that modify seizures

The mature brain is less susceptible to seizures than the immature brain. We demonstrate that in the mature substantia nigra (SN) there are two topographically discrete GABAA-sensitive regions which differ in the amount of mRNA expression of the GABAA receptor alpha 1 subunit. These two regions mediate separate anticonvulsant and proconvulsant effects and use divergent projection networks. By contrast, in the immature SN there is no special topography of mRNA expression of the alpha 1 subunit and only the proconvulsant network is present. The decreased seizure susceptibility of the mature brain may be related to postnatal segregation of GABAA-sensitive networks.

Changes in gamma-aminobutyrate type A receptor subunit mRNAs, translation product expression, and receptor function during neuronal maturation in vitro

The amounts of mRNAs encoding alpha 1, alpha 6, beta 2, beta 3, gamma 2, and delta subunits of gamma-aminobutyrate type A (GABAA) receptors and the gold immunolabeling density of their translation products were monitored during the growth of neonatal rat granule cells in primary culture. We investigated possible correlations (i) between temporal changes in mRNA content and expression density of their respective translation products and (ii) between the quantitative changes of receptor subunit expression, the GABA EC50 for Cl- channel activation, and diazepam efficacy in modulating GABA action on the Cl- channels. At 3 days in vitro, the amount of GABAA receptor subunit mRNAs and the expression of their respective translation products were very low. During the next 2 weeks both parameters for every subunit studied increased asynchronously; moreover, at 14 days in vitro the sum of gamma 2 and delta subunit expression was smaller than the expression of the alpha 1 or alpha 6 or beta 2/beta 3 subunits. This suggests that during in vitro maturation each subunit may be regulated independently and invites speculation as to possible changes in specific GABAA receptor subtype abundance during development in vitro. The maximal current intensity elicited by GABA failed to increase from 5 to 14 days in vitro, though the amount of mRNA encoding various subunits and the expression density of their respective translation products increased. Thus, qualitative changes in the GABAA receptor subtypes expressed and/or abnormalities in the subunit assembly very likely account for the uniformity of the maximal current intensity elicited by GABA during in vitro development. Also, during maturation of neuronal cultures from 5 to 20 days in vitro the extent of the positive modulation of GABA action by diazepam decreased dramatically. This finding might be related to an increase in the abundance of GABAA receptors including the alpha 6 subunit and/or to the expression, during granule cell maturation in vitro, of GABAA receptors devoid of gamma 2 subunits.

Characteristics of GABAA channels in rat dentate gyrus

Single channel currents were activated by GABA (0.5 to 5 microM) in cell-attached and inside-out patches from cells in the dentate gyrus of rat hippocampal slices. The currents reversed at the chloride equilibrium potential and were blocked by bicuculline (100 microM). Several different kinds of channel were seen: high conductance and low conductance, rectifying and "nonrectifying." Channels had multiple conductance states. The open probability (Po) of channels was greater at depolarized than at hyperpolarized potentials and the relationship between Po and potential could be fitted with a Boltzmann equation with equivalent valency (z) of 1. The combination of outward rectification and potential-dependent open probability gave very little chloride current at hyperpolarized potentials but steeply increasing current with depolarization, useful properties for a tonic inhibitory mechanism.

Experimental models of epilepsy in young animals

Seizures occur more frequently early in life. Some of these early seizures may eventually become epilepsy. Others are reactive seizures due to excessive environmental stimuli that, in any other age group, might not have elicited a similar response. To understand the developmental aspects of seizures and epilepsy in humans, it is important to study these processes in animals of equivalent ages. In this paper, we describe several animal models of developmental seizures, including their electroclinical manifestations and their validity in respect to human epileptic syndromes. There are several factors that may account for the increased seizure susceptibility of the immature brain, including the delayed development of effective systems or synaptic networks that are involved in the suppression of seizures. A better insight of the basic pathophysiology of seizures as a function of age in animal models will lead to the development of new therapeutic approaches for the treatment of age-specific epileptic disorders in humans.

Development pattern of the GABAA-benzodiazepine receptor ionophore complex in primary cultures of cortical neurons

The development pattern of the GABAA/benzodiazepine (BZ) receptor ionophore complex was characterized in mammalian cortical cultured neurons utilizing radioligand binding and GABA-induced 36Cl-influx. The specific binding of [3H]flunitrazepam, [3H]Ro15-1788 and [3H]Ro15-4513 increased with time from day 3 to day 21. The EC50 value and Emax of the GABA, muscimol and pentobarbital to enhance [3H]-flunitrazepam binding did not change during the development. Furthermore, GABA-induced 36Cl- influx also did not change during development from day 3 to day 21. However, the potency of ligands that bind to type-1 BZ receptors (alprazolam, zolpidem and C1218,872) to inhibit [3H]flunitrazepam binding increased from day 7 to day 21. Taken together, these data suggest the presence of type-II BZ receptors in early stages of the development which appears to change to type-I BZ receptors with age in the cortical neurons in culture.

Allosteric modulation of [35S]TBPS-binding in the cerebral cortex of the rat during postnatal development

The ontogenesis of the GABA-gated Cl- channel was investigated in the cerebral cortex of the rat by monitoring the binding parameters of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) at intervals after birth (1-90 days). To investigate the influence of the developmental changes in the content of GABA on [35S]TBPS-binding, the assays were carried out in unwashed membranes, in which the concentration of GABA was dependent on its content in vivo, and in repeatedly washed membranes in the presence of defined concentrations of exogenous GABA. At birth, the density (Bmax) of [35S]TBPS-binding sites in unwashed membranes was similar to that found in well-washed membranes. However, in unwashed membranes, the number of [35S]TBPS-binding sites increased by two-fold within 10 days after birth whereas in washed membranes it increased by four-fold during the same period. The higher density of [35S]TBPS-binding sites in washed membranes as compared with the unwashed counterparts persisted throughout development. In unwashed membranes, the apparent Kd for [35S]TBPS-binding increased with age whereas in washed membranes the affinity of [35S]TBPS for its binding sites remained constant throughout development. The binding of [35S]TBPS to the GABA-gated Cl- channel is allosterically modulated by drugs acting on different sites of the GABAA receptor complex. Thus, GABA and diazepam decrease [35S]TBPS-binding whereas the GABAA receptor antagonist, bicuculline, and the inverse agonist for benzodiazepine receptors, 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylic acid methyl ester, increase it.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential expression of the short and long forms of the gamma 2 subunit of the GABAA/benzodiazepine receptors

The distribution of the mRNAs encoding the gamma 2S and gamma 2L subunits of the GABAA receptor in the rat brain has been revealed by in situ hybridization, northern blot and dot blot analysis using specific antisense oligonucleotides. In addition, the quantitative distribution of the gamma 2S and gamma 2L subunit peptides participating in the fully assembled GABAA receptors/benzodiazepine receptors has been mapped by immunoprecipitation with specific anti-gamma 2S and anti-gamma 2L antibodies. Several neuronal types and brain regions are enriched in gamma 2L such as neurons of the layer II of striate cortex and cerebellar Purkinje cells as well as the inferior colliculus, superior colliculus, deep cerebellar nuclei, medulla and pons. Other neuronal types and regions are enriched in gamma 2S such as the mitral cells of the olfactory bulb, pyramidal neurons of the pyriform cortex, layer VI of the neocortex, granule cells of the dentate gyrus and pyramidal cells of the hippocampus. Other cortical areas and cerebellar granule cells express both gamma 2S and gamma 2L in comparable amounts. There is a good correlation between the relative expression of gamma 2S and gamma 2L mRNAs and the relative presence of these protein subunits in fully assembled and mature receptors in the studied brain regions. The differential distribution of gamma 2S and gamma 2L might result in differential ethanol sensitivity of the neurons expressing these GABAA receptor subunits.

Changes in glutamate receptor and proenkephalin gene expression after kindled seizures

Changes in gene expression after kindled seizures were examined using microdissection of discrete brain areas and Northern and slot blot analyses. Experimental animals were kindled with either of two protocols: (1) a paradigm in which 50 Hz/10 s stimulus trains were delivered every 30 min through hippocampal electrodes (12 stimulations every other day for 4 days) and (2) a traditional approach in which 50 Hz/10 s stimulus trains were given to the hippocampus three times daily for 16 days. Rats were sacrificed 24 h or 30 days after the last kindled seizure. We first examined the possibility that kindling may affect transcription of mRNA for neurotransmitter receptors. We found significant decreases (22-58%) in AMPA/kainate activated glutamate receptor mRNAs (GluR1, -2, -3 mRNAs) in hippocampus, amygdala/entorhinal cortex and in frontoparietal cortex 24 h but not 30 days after rapidly kindled seizures. However, changes in GABA receptor alpha 1, alpha 2, alpha 4 or beta 1 mRNAs were not observed in any brain region 30 days after traditional kindling or 24 h after rapidly kindled seizures. In addition, we tested whether changes in the expression of proenkephalin could be detected after kindling. We found significant increases (1.7-10 fold) in proenkephalin mRNA in the frontoparietal cortex, hippocampus and in the amygdala/entorhinal cortex 24 h but not 30 days after rapidly kindled seizures. Our findings suggest that changes in glutamate receptor and proenkephalin gene expression are robust, acute sequelae to kindled seizures and may be involved in kindling.

Coincidental appearance of the alpha 1 subunit of the GABA-A receptor and the type I benzodiazepine receptor near birth in macaque monkey visual cortex

The expression of subtypes of the GABA-A/benzodiazepine receptor complex has been studied during pre- and postnatal development of Macaca monkey visual cortex using complementary radioligand and immunocytochemical labeling. Type I benzodiazepine receptors were labeled directly by [3H]zolpidem. Type II receptors were determined by the amount of binding for [3H]flunitrazepam (FZ) persisting in the presence of the type I-specific ligand CL218872. Monoclonal antibody bd24 was used to label alpha 1 subunits and bd17 to label beta 2 and beta 3 subunits of the GABA-A receptor. Radioligand binding data and bd17 immunoreactivity indicated that type II benzodiazepine receptors were present by fetal day (Fd) 74 (44% of gestation). Immunoreactivity for the beta 2/beta 3 subunits increased until 3-6 weeks after birth, and then declined somewhat into adulthood. Neither radioligand labeling for type I receptors nor immunocytochemical staining for the alpha 1 subunit were apparent until mid-gestation. Both markers appeared shortly before birth in layer 4C, and then in other cortical layers after birth. Immunoreactivity for the alpha 1 subunit increased steadily after birth until it became more intense than that for beta 2/3 subunits in the adult. Quantitative densitometry of CL218872 competition for [3H]FZ binding showed that type I/II distribution was 22%/78% at Fd103; 42%/58% at Fd131; 67%/33% at 9 months; and 61%/39% in adult visual cortex. This "switch" between benzodiazepine receptor subtypes overlaps the postnatal critical period for geniculostriate development, suggesting that the change from type II to type I receptors and the appearance of alpha 1 subunits may play a decisive role in the maturation of geniculocortical axon terminations and cortical response properties. It remains to be shown whether this "switch" is dependent on functional visual input.

Multiphasic desensitization of the GABAA receptor in outside-out patches

GABAA receptor function was studied in outside-out patches from guinea pig hippocampal neurons using a drug application system with an exchange time of under 1.5 ms. Application of GABA to these patches induced a Cl- conductance that desensitized with prolonged exposure. Increasing GABA concentrations induced larger conductance increases that were associated with more complex patterns of desensitization. Smaller GABA responses desensitized with monophasic kinetics, whereas large responses displayed bi- and triphasic kinetics. Desensitization of the response to 1 mM GABA was triphasic in about 70% of the patches (tau = 15.4, 207, and 1370 ms) and biphasic in about 30% of the patches (tau = 44 and 725 ms). All phases of desensitization reversed at the Cl- equilibrium potential. Over the concentration range from 3 microM to 3 mM, both the rate and the extent of desensitization increased; however, complete desensitization was rarely observed. The increase in desensitization rate was due to an increase in the relative contribution of the faster phases with increasing GABA. The time constants of the three phases were independent of concentration. The different phases are not mediated by separate receptor populations, because double pulse experiments demonstrated interconversion among the fastest phase and the two slower phases. We demonstrate the plausibility of a model in which multiphasic desensitization is a consequence of the faster association rate at higher GABA concentrations.

Phenotypic consequences of deletion of the gamma 3, alpha 5, or beta 3 subunit of the type A gamma-aminobutyric acid receptor in mice

Three genes (Gabrg3, Gabra5, and Gabrb3) encoding the gamma 3, alpha 5, and beta 3 subunits of the type A gamma-aminobutyric acid receptor, respectively, are known to map near the pink-eyed dilution (p) locus in mouse chromosome 7. This region shares homology with a segment of human chromosome 15 that is implicated in Angelman syndrome, an inherited neurobehavioral disorder. By mapping Gabrg3 on a panel of p-locus deletions, we have determined that the order of genes within this cluster is centromere-p(D15S12h)-Gabrg3-Gabra5-Gabrb3-telom ere. Like Gabrb3, neither the Gabra5 nor Gabrg3 gene is functionally imprinted in adult mouse brain. Mice deleted for all three subunits die at birth with a cleft palate, although there are rare survivors (approximately 5%) that do not have a cleft palate but do exhibit a neurological abnormality characterized by tremor, jerky gait, and runtiness. We have previously suggested that deficiency of the beta 3 subunit may be responsible for the clefting defect. Most notably, however, in this report we describe mice carrying two overlapping, complementing p deletions that fail to express the gamma 3 transcript, as well as mice from another line that express neither the gamma 3 nor alpha 5 transcripts. Surprisingly, mice from both of these lines are phenotypically normal and do not exhibit any of the neurological symptoms characteristic of the rare survivors that are deleted for all three (gamma 3, alpha 5, and beta 3) subunits. These mice therefore provide a whole-organism type A gamma-aminobutyric-acid receptor background that is devoid of any receptor subtypes that normally contain the gamma 3 and/or alpha 5 subunits. The absence of an overt neurological phenotype in mice lacking the gamma 3 and/or alpha 5 subunits also suggests that mutations in these genes are unlikely to provide useful animal models for Angelman syndrome in humans.

Neurons derived from embryonal carcinoma (P19) cells express multiple GABAA receptor subunits and fully functional GABAA receptors

The embryonal carcinoma (EC) cell line P19 can be induced to differentiate into neurons and glia by treatment with retinoic acid. Neuronal cells derived from P19 stem cells were found to express messenger RNAs for alpha, beta, and gamma 2 subunits of the GABAA receptor-chloride channel complex. Whole-cell voltage-clamp recording in differentiated P19 cells revealed that these cells possess GABA receptor-activated chloride currents which are blocked by bicuculline and potentiated by flurazepam. P19 EC cells thus represent a stable neuronal cell line which expresses functional GABAA receptors with all of the characteristics of native GABAA receptors.