Distinct developmental patterns of expression of rat alpha 1, alpha 5, gamma 2S, and gamma 2L gamma-aminobutyric acidA receptor subunit mRNAs in vivo and in vitro

Region and layer-specific expression of GABAA receptor isoforms and KCC2 in developing cortex

Introduction

γ-Aminobutyric acid (GABA) type A receptors (GABAARs) are ligand-gated Cl-channels that mediate the bulk of inhibitory neurotransmission in the mature CNS and are targets of many drugs. During cortical development, GABAAR-mediated signals are significantly modulated by changing subunit composition and expression of Cl-transporters as part of developmental processes and early network activity. To date, this developmental evolution has remained understudied, particularly at the level of cortical layer-specific changes. In this study, we characterized the expression of nine major GABAAR subunits and K-Cl transporter 2 (KCC2) in mouse somatosensory cortex from embryonic development to postweaning maturity.

Methods

We evaluated expression of α1-5, β2-3, γ2, and δ GABAAR subunits using immunohistochemistry and Western blot techniques, and expression of KCC2 using immunohistochemistry in cortices from E13.5 to P25 mice.

Results

We found that embryonic cortex expresses mainly α3, α5, β3, and γ2, while expression of α1, α2, α4, β2, δ, and KCC2 begins at later points in development; however, many patterns of nuanced expression can be found in specific lamina, cortical regions, and cells and structures.

Discussion

While the general pattern of expression of each subunit and KCC2 is similar to previous studies, we found a number of unique temporal, regional, and laminar patterns that were previously unknown. These findings provide much needed knowledge of the intricate developmental evolution in GABAAR composition and KCC2 expression to accommodate developmental signals that transition to mature neurotransmission.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Laboratory of molecular neurobiology (1988-1994)

While Dr. Costa was the Director of FGIN he became interested in the regulation of genes encoding various neurotransmitter receptors. More specifically, there was increasing evidence supporting a role for tetanic stimulation of the NMDA-subtype of ionotropic glutamate receptors in the development of long term potentiation and long term depression. Moreover, the protein products of the immediate early gene family, such as c-fos and c-jun, were known modulators of downstream signaling events that facilitated changes in neuronal transcriptomes in response to incoming afferent stimulation. The immediate early gene products were known transcriptional factors that activated gene expression in response to excitatory stimulation. In fact, the expression of c-fos/c-jun was often used to map neuronal circuits linked through a common initiation point such as occurs in focally-evoked seizures. Dr. Costa firmly believed that excitatory and inhibitory transmission was balanced in the central nervous system and that this might come about through changes in the expression of the genes encoding these neurotransmitter receptors. In other words, persistent stimulation of NMDA receptors would be expected to increase expression of the inhibitory GABAA receptors to accommodate the increased excitation. That this receptor crosstalk might occur through the products of the immediate early genes was testable and the focus of the Molecular Neurobiology Laboratory from 1988 to 1994. In a broader sense, stimulation of ionotropic NMDA-selective glutamate receptors has been associated with numerous downstream molecular and cellular processes. How these processes are linked to changes in gene expression has been the focus of studies in the neurosciences for many years.

A legacy of discovery: from monoamines to GABA

Seldom does a single individual have such a profound effect on the development of a scientific discipline as Erminio Costa had on neuropharmacology. During nearly sixty years of research, Costa and his collaborators helped established many of the basic principles of the pharmacodynamic actions of psychotherapeutics. His contributions range from defining basic neurochemical, physiological and behavioral properties of neurotransmitters and their receptors, to the development of novel theories for drug discovery. Outlined in this report is a portion of his work relating to the involvement of monoamines and GABA in mediating the symptoms of neuropsychiatric disorders and as targets for drug therapies. These studies were selected for review because of their influence on my own work and as an illustration of his logical and insightful approach to research and his clever use of techniques and technologies. Given the significance of his work, the legions of scientist who collaborated with him, and those inspired by his reports, his research will continue to have an impact as long as there is a search for new therapeutics to alleviate the pain and suffering associated with neurological and psychiatric disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Expression of GABA A receptor alpha1 subunit mRNA and protein in rat neocortex following photothrombotic infarction

Photothrombotic infarcts of the neocortex result in structural and functional alterations of cortical networks, including decreased GABAergic inhibition, and can generate epileptic seizures within 1 month of lesioning. In our study, we assessed the involvement and potential changes of cortical GABA A receptor (GABA AR) alpha1 subunits at 1, 3, 7, and 30 days after photothrombosis. Quantitative competitive reverse transcription-polymerase chain reaction (cRT-PCR) and semi-quantitative Western blot analysis were used to investigate GABA AR alpha1 subunit mRNA and protein levels in proximal and distal regions of perilesional cortex and in homotopic areas of young adult Sprague-Dawley rats. GABA AR alpha1 subunit mRNA levels were decreased ipsilateral and contralateral to the infarct at 7 days, but were increased bilaterally at 30 days. GABA AR alpha1 subunit protein levels revealed no significant change in neocortical areas of both hemispheres of lesioned animals compared with protein levels of sham-operated controls at 1, 3, 7, and 30 days. At 30 days, GABA AR alpha1 subunit protein expression was significantly increased in lesioned animals within proximal and distal regions of perilesional cortex compared with distal neocortical areas contralaterally (Student's t-test, p<0.05). Short- and long-term alterations of mRNA and protein levels of the GABA AR alpha1 subunit ipsilateral and contralateral to the lesion may influence alterations in cell surface receptor subtype expression and GABA AR function following ischemic infarction and may be associated with formative mechanisms of poststroke epileptogenesis.

Antagonism of neurosteroid modulation of native gamma-aminobutyric acid receptors by (3alpha,5alpha)-17-phenylandrost-16-en-3-ol

Endogenous pregnane neurosteroids are allosteric modulators at gamma-aminobutyric acid type-A (GABAA) receptors at nanomolar concentrations. There is direct evidence for multiple distinct neurosteroid binding sites on GABAA receptors, dependent upon subunit composition and stoichiometry. This view is supported by the biphasic kinetics of various neuroactive steroids, enantioselectivity of some neurosteroids, selective mutation studies of recombinantly expressed receptors and the selectivity of the neurosteroid antagonist (3alpha,5alpha)-17-phenylandrost-16-en-3-ol (17PA) on 5alpha-pregnane steroid effects on recombinant GABAA receptors expressed in Xenopus oocytes and native receptors in dissociated neurons. However, it is unclear whether this antagonist action is present in a mature mammalian system. The present study evaluated the antagonist activity of 17PA on neurosteroid agonists both in vivo and in vitro by examining the effects of 17PA on 5alpha-pregnane-induced sedation in rats, native mature GABAA receptor ion channels utilizing the chloride flux assay and further studies in recombinant alpha1beta2gamma2 receptors. The data show that 17PA preferentially inhibits 3alpha,5alpha-THP vs. alphaxalone in vivo, preferentially inhibits 3alpha,5alpha-THDOC vs. alphaxalone potentiation of GABA-mediated Cl- uptake in adult cerebral cortical synaptoneurosomes, but shows no specificity for 3alpha,5alpha-THDOC vs. alphaxalone in recombinant alpha1beta2gamma2 receptors. These data provide further evidence of the specificity of 17PA and the heterogeneity of neurosteroid recognition sites on GABAA receptors in the CNS.

Morphological development and maturation of the GABAergic synapses in the mouse cerebellar granular layer

In the adult central nervous system (CNS), gamma-amino butyric acid (GABA) is a predominant inhibitory neurotransmitter, which regulates glutamatergic activity. Recent studies have revealed that GABA serves as an excitatory transmitter in the immature CNS, and is involved in brain morphogenesis. To elucidate how GABA exerts its effect on immature neurons and how GABAergic synapses are formed, we examined both development of pre- and post-synaptic elements of the GABAergic synapses formed between granule and Golgi cells in the mouse cerebellar granular layer. Immunohistochemistry for glutamic acid decarboxylase (GAD) demonstrated that GABA was localized throughout the Golgi cells before postnatal day 7 (P7), and became confined to the axon terminals during the second postnatal week. Electron microscopic analysis demonstrated that GABAergic synapses were clearly detected at P10. In situ hybridization and immunohistochemistry for the GABA(A) receptor alpha1 and alpha6 subunits, which are mainly involved in inhibitory synaptic transmission, demonstrated that both subunits appeared at P7. Distribution of both subunits expanded in the granular layer with special reference to the development of GABAergic synapses. Furthermore, the majority of the subunits accumulated adjacent to the GABAergic terminals. These results suggested that in the granular layer, GABA might be non-synaptically secreted from Golgi cell axons and dendrites during the first postnatal week. From the second postnatal week, GABA is synaptically released and begins to mediate inhibitory transmission. Furthermore, it was suggested that GABAergic innervation could initiate expression and trafficking of the GABA(A) receptors containing the alpha1 and alpha6 subunits.

Tonically active GABAA receptors: modulating gain and maintaining the tone

GABAA receptors not only respond to the local release of GABA from presynaptic terminals, but can also mediate a persistent 'tonic current'. This reflects the activation of high-affinity GABAA receptors by ambient GABA concentrations. Tonic GABAA-receptor-mediated signalling occurs in different brain regions, shows cell-type-specific differences in magnitude and pharmacology, and changes during brain development. Some clues to the adaptive significance of this phenomenon are beginning to emerge: in cerebellar granule cells, it alters the gain of transmission of rate-coded sensory information; in the hippocampus, it acts in a cell-type-specific manner to regulate the excitability of the network. Because tonic conductances can be modulated by changes in GABA release and uptake, and by modulators of high-affinity GABAA receptors including neurosteroids, this phenomenon provides a potentially important new window onto neuronal information processing and pathological states such as epilepsy.

Differential effects of prenatal cocaine exposure on selected subunit mRNAs of the GABA(A) receptor in rabbit anterior cingulate cortex

We have previously shown that in the dopamine-rich anterior cingulate cortex (ACC), significant changes in gamma-aminobutyric acid (GABA) immunoreactivity occur in the offspring of rabbits given intravenous injections of cocaine (3 mg/kg) twice daily during pregnancy. In the present study, the effects of prenatal cocaine exposure on the developmental expression of specific GABA(A) receptor subunit mRNAs were investigated. We compared the distribution of the alpha1, beta2, and gamma2 subunit mRNAs in cocaine- and saline-treated offspring aged postnatal days 20 and 60 (P20, P60). At P20, prenatal cocaine exposure resulted in a significant increase in alpha1 subunit mRNA in ACC lamina III and a significant reduction in the amounts of the beta2 subunit mRNA in ACC lamina II. No differences between cocaine- and saline-treated controls were detected for gamma2 subunit mRNA levels in ACC. Although the pattern of labeling was altered in cocaine-exposed animals, Nissl sections revealed no differences in lamination, indicating that the changes in GABA(A) subunit mRNAs could not be attributed to abnormal cytoarchitectonics. In P60 brains, no significant differences were observed between cocaine- and saline-treated material, indicating that the observed differences were transient. Collectively, our data show that prenatal cocaine exposure elicits differential, lamina-specific changes in mRNA levels encoding selected subunits of the GABA(A) receptor. Since these changes occur during a critical period when fine tuning of synaptic organization is achieved by processes of selective elimination or stabilization of synapses, we suggest that specific subunit mRNAs of the GABA(A) receptor play a role in cortical development.

Neuronal activity and its influence on developmentally regulated GABA(A) receptor expression in cultured mouse cerebellar granule cells

Mouse cerebellar granule cells (CGCs) were cultured in either non-depolarising (5 mM KCl, '5K') or depolarising (25 mM KCl, '25K') media. CGCs at 5K developed an elaborate network of processes and formed compact cell aggregates, whilst at 25K, cell aggregation was rarely observed. GABA(A) receptor (GABAR) expression was significantly affected by the culture conditions. Depolarisation resulted in a approximately 55% reduction in the expression of total specific [(3)H]Ro15-4513 binding sites, largely due to a >85% loss of the flunitrazepam-insensitive (BZ-IS) subtype of binding sites. The number of flunitrazepam-sensitive (BZ-S) [(3)H]Ro15-4513 binding sites expressed and the K(D) of [(3)H]Ro15-4513 to either GABAR subtype were not significantly affected. The BZ-S subtype expressed by 5K CGCs was essentially all type I, as expected of mature CGCs, however at 25K, these were predominantly type II (approximately 70%) and zolpidem-insensitive (approximately 30%)-pharmacological finger-prints of immature CGCs. By immunoblotting we determined that CGCs cultured at 25K expressed GABAR alpha1, alpha6 and beta3 subunits at 14, 3 and 167% of 5K levels, respectively. GABAR beta2 subunits, however, were barely detectable. The changes in GABAR subunit expression were paralleled by similar changes in the steady-state levels of the subunit mRNAs. The switch from type I to type II BZ-S pharmacology upon depolarisation was mirrored by a switch in gene expression from alpha1 (12% of 5K) to alpha3 (371% of 5K). Interestingly, although depolarisation reduced beta2 subunit mRNA to 25% of 5K, beta2 protein was undetectable.Thus, we have shown that electrically active (5K) mouse CGCs differentiate in vitro to express a GABAR profile expected of adult CGCs in vivo. Chronically depolarised, electrically silent CGCs appear to be developmentally arrested, expressing some GABAR characteristics of prenatal CGCs. This model system should prove invaluable for elucidating signalling pathways orchestrating GABAR differentiation.

Anabolic androgenic steroids induce age-, sex-, and dose-dependent changes in GABA(A) receptor subunit mRNAs in the mouse forebrain

Chronic exposure to anabolic androgenic steroids (AAS) has deleterious effects on reproductive health in both human and animal subjects. Neurotransmission mediated by the gamma-aminobutyric acid type A (GABA(A)) receptor in the medial amygdala (MeA), the medial preoptic area (mPOA), and the ventromedial nucleus (VMN) of the hypothalamus plays a critical role in mediating sexual behaviors. Here we used semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) to examine levels of alpha(1), alpha(2), alpha(5), gamma(1), gamma(2), and epsilon subunit mRNAs in these three regions of the brain. Our results demonstrate that chronic exposure to either a high or a moderate dose of the AAS, 17alpha-methyltestosterone (17alpha-MeT), significantly decreased the levels of specific alpha and gamma subunit mRNAs in a manner that depended on the dose of AAS and age and sex of the animals. Specifically, the moderate dose of AAS elicited significant changes only in pubertal females and the majority of changes observed in pubertal animals with the high dose also occurred in females. In contrast, the moderate dose of AAS induced no significant changes in adult mice of either sex, while the high dose had effects in both males and females. In addition to determining the effects of chronic AAS treatment, a developmental analysis of drug-naïve animals demonstrated that GABA(A) receptor subunit mRNA levels in these regions of the forebrain undergo significant changes as animals proceed through puberty. These data demonstrate that the effects of AAS exposure on GABA(A) receptor expression are superimposed upon dynamic developmental changes that accompany the transition from puberty to adulthood.

On the epigenetic regulation of the human reelin promoter

Reln mRNA and protein levels are reduced by approximately 50% in various cortical structures of post-mortem brain from patients diagnosed with schizophrenia or bipolar illness with psychosis. To study mechanisms responsible for this down-regulation, we have analyzed the promoter of the human reelin gene. We show that the reelin promoter directs expression of a reporter construct in multiple human cell types: neuroblastoma cells (SHSY5Y), neuronal precursor cells (NT2), differentiated neurons (hNT) and hepatoma cells (HepG2). Deletion constructs confirmed the presence of multiple elements regulating Reln expression, although the promoter activity is promiscuous, i.e. activity did not correlate with expression of the endogenous gene as reflected in terms of reelin mRNA levels. Co-transfection of the -514 bp human reelin promoter with either Sp1 or Tbr1 demonstrated that these transcription factors activate reporter expression by 6- and 8.5-fold, respectively. Within 400 bp of the RNA start site there are 100 potential CpG targets for DNA methylation. Retinoic acid (RA)-induced differentiation of NT2 cells to hNT neurons was accompanied by increased reelin expression and by the appearance of three DNase I hypersensitive sites 5' to the RNA start site. RA-induced differentiation was also associated with demethylation of the reelin promoter. To test if methylation silenced reelin expression, we methylated the promoter in vitro prior to transfection. In addition, we treated NT2 cells with the methylation inhibitor aza-2'-deoxycytidine and observed a 60-fold increase in reelin mRNA levels. The histone deacetylase inhibitors trichostatin A (TSA) and valproic acid also induced expression of the endogenous reelin promoter, although TSA was considerably more potent. These findings indicate that one determinant responsible for regulating reelin expression is the methylation status of the promoter. Our data also raise the interesting possibility that the down-regulation of reelin expression documented in psychiatric patients might be the consequence of inappropriate promoter hypermethylation.

The function and the expression of forebrain GABA(A) receptors change with hormonal state in the adult mouse

Neurotransmission mediated by gamma-aminobutyric acid type A (GABA(A)) receptors in the mammalian medial preoptic area (mPOA) plays a pivotal role in the expression of hormone-sensitive behaviors. Hand in hand with GABAergic control of reproduction, hormone treatments that activate gonadal steroid signaling pathways in gonadectomized rats are known to regulate the expression of specific GABA(A) receptor subunit mRNAs. While the effects of exogenous hormone treatments have been well documented, little information is available as to how GABA(A) receptor-mediated transmission in the mPOA is altered by endogenous changes in hormonal state in gonadally-intact adult animals or if those changes can be ascribed to hormone-dependent changes in receptor subunit composition. In the present study, we found that both the peak amplitudes of GABA(A) receptor-mediated synaptic currents in the mPOA, as well as the ability of the endogenous neurosteroids to modulate those currents, varied as a function of the estrous cycle. Moreover, we found that the degree of neurosteroid modulation was also significantly different between wild-type and the androgen-insensitive testicular feminization (Tfm) mutant male mice. Semiquantitative RT-PCR analysis performed to assess levels of GABA(A) receptor subunit mRNAs indicated that levels of specific subunits varied over the course of the estrous cycle and between wild-type and Tfm male mice. The variations in GABA(A) receptor expression and function in the mPOA that are associated with differences in gonadal steroid signaling may contribute to the dynamic nature of GABAergic control of neuroendocrine pathways.

Changes in expression of NMDA receptor subunit mRNA by perinatal exposure to dioxin

Since dioxin and related compounds are suspected of affecting permanently the brain function of offspring of human and experimental animals, effects of perinatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the expression of rat NMDA receptor NR2A and NR2B subunit mRNA were examined. The mRNA quantification by competitive RT-PCR clearly revealed that TCDD inhibited NR2B mRNA expression and enhanced NR2A mRNA expression in the neocortex and hippocampus on postnatal day (PND) 49, whereas these changes in mRNA expression were not found on PND 5. The results demonstrate for the first time that the perinatal exposure to TCDD can alter the molecular basis of brain of offspring in adulthood.

Rapid PCR-mediated synthesis of competitor molecules for accurate quantification of beta(2) GABA(A) receptor subunit mRNA

We describe a fast and easy method for the synthesis of competitor molecules based on non-specific conditions of PCR. RT-competitive PCR is a sensitive technique that allows quantification of very low quantities of mRNA molecules in small tissue samples. This technique is based on the competition established between the native and standard templates for nucleotides, primers or other factors during PCR. Thus, the most critical parameter is the use of good internal standards to generate a standard curve from which the amount of native sequences can be properly estimated. At the present time different types of internal standards and methods for their synthesis have been described. Normally, most of these methods are time-consuming and require the use of different sets of primers, different rounds of PCR or specific modifications, such as site-directed mutagenesis, that need subsequent analysis of the PCR products. Using our method, we obtained in a single round of PCR and with the same primer pair, competitor molecules that were successfully used in RT-competitive PCR experiments. The principal advantage of this method is high versatility and economy. Theoretically it is possible to synthesize a specific competitor molecule for each primer pair used. Finally, using this method we have been able to quantify the increase in the expression of the beta(2) GABA(A) receptor subunit mRNA that occurs during rat hippocampus development.

Expression of GABA(A) receptor subunits in the rat central nucleus of the inferior colliculus

Expression of GABA(A) receptor (GABA(A)R) alpha(1), alpha(2), beta(2), gamma(1), gamma(2L) and gamma(2S) subunit mRNA was examined in three cell classes in the central nucleus of the rat inferior colliculus (CNIC). GABA(A)R alpha(1) and gamma(2L) subunit mRNA expression was greatest in large cells (over 25 microm long diameter), intermediate in medium sized cells (15 to 25 microm long diameter) and lowest in small cells (10 to 15 microm long diameter). GABA(A)R gamma(2S) and alpha(2) subunits had the opposite pattern, highest in the small cells, intermediate in medium cells and lowest in large cells. GABA(A)R beta(2) was significantly lower in small cells than the two other classes, while differences between large and medium cells were not significant. GABA(A)R gamma(1) subunit mRNAs expression was not above background in any of the three cell types assessed. The expression of GABA(A)R subunits suggests that cell classes in the rat CNIC may differ in their response to GABA and GABAergic drugs.

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.

Postnatal development of inhibitory synaptic transmission in the rostral nucleus of the solitary tract

To explore the postnatal development of inhibitory synaptic activity in the rostral (gustatory) nucleus of the solitary tract (rNST), whole cell and gramicidin perforated patch-clamp recordings were made in five age groups of rats [postnatal day 0-7 (P0-7), P8-14, P15-21, P22-30, and P >55]. The passive membrane properties of the developing rNST neurons as well as the electrophysiological and pharmacological characteristics of single and tetanic stimulus-evoked inhibitory postsynaptic potentials (IPSPs) were studied in brain slices under glutamate receptor blockade. During the first postnatal weeks, significant changes in resting membrane potential, spontaneous activity, input resistance, and neuron membrane time constant of the rNST neurons occurred. Although all the IPSPs recorded were hyperpolarizing, the rise and decay time constants of the single stimulus shock-evoked IPSPs decreased, and the inhibition response-concentration function to the gamma-aminobutyric acid (GABA) receptor antagonist bicuculline methiodide (BMI) shifted to the left during development. In P0-7 and P8-14, but not in older animals, the IPSPs had a BMI-insensitive component that was sensitive to block by picrotoxin, suggesting a transient expression of GABA(C) receptors. Tetanic stimulation resulted in both short- and long-term changes of inhibitory synaptic transmission in the rNST. For P0-7 and P8-14 animals tetanic stimulation resulted in a sustained hyperpolarization that was maintained for some time after termination of the tetanic stimulation. In contrast, tetanic stimulation of neurons in P15-21 and older animals resulted in hyperpolarization that was not sustained but decayed back to a more positive level with an exponential time course. Tetanic stimulation resulted in potentiation of single stimulus shock-evoked IPSPs in ~50% of neurons in all age groups. These developmental changes in inhibitory synaptic transmission in the rNST may play an important role in shaping synaptic activity in early development of the rat gustatory system during a time of maturation of taste preferences and aversions.

GABA(A) receptor alpha1 subunit deletion prevents developmental changes of inhibitory synaptic currents in cerebellar neurons

Developmental changes in miniature IPSC (mIPSC) kinetics have been demonstrated previously in cerebellar neurons in rodents. We report that these kinetic changes in mice are determined primarily by developmental changes in GABA(A) receptor subunit expression. mIPSCs were studied by whole-cell recordings in cerebellar slices, prepared from postnatal day 11 (P11) and P35 mice. Similar to reports in granule neurons, wild-type cerebellar stellate neuron mIPSCs at P11 had slow decay kinetics, whereas P35 mIPSCs decayed five times faster. When mIPSCs in cerebellar stellate neurons were compared between wild-type (+/+) and GABA(A) receptor alpha1 subunit-deficient (-/-) littermates at P35, we observed dramatically slower mIPSC decay rates in -/- animals. We took advantage of the greater potency of imidazopyridines for GABA current potentiation with alpha1 subunit-containing receptors to characterize the relative contribution of alpha1 subunits in native receptors on inhibitory synapses of cerebellar granule neurons. Zolpidem-induced prolongation of mIPSC decay was variable among distinct cells, but it increased during development in wild-type mice. Similarly, Zolpidem prolongation of mIPSC decay rate was significantly greater in adult +/+ mice than in knock-outs. We propose that an increased alpha1 subunit assembly in postsynaptic receptors of cerebellar inhibitory synapses is responsible for the fast inhibitory synaptic currents that are normally observed during postnatal development.

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.

Expression and function of striatal nAChRs differ in the flinders sensitive (FSL) and resistant (FRL) rat lines

Rats of Flinders Sensitive (FSL) and Flinders Resistant lines (FRL) differ in their susceptibility to physiological and associated behavioral responses elicited by nicotine. In the present study, we measured dopamine (DA) content in striatal dialysates to investigate the sensitivity of FSL and FRL rats to nicotine delivered locally through a microdialysis probe placed in the striatum. We also measured the expression density of striatal high-affinity nicotinic acetylcholine receptors (nAChRs), and that of mRNAs encoding for alpha3, alpha4, alpha7 and beta2 nAChR subunits in both lines. The DA content of dialysates was measured before and after a 1-min perfusion of nicotine (6, 10 or 20 nmoles/min) and the resulting DA increase was taken as a measure of the alkaloid's intrinsic activity for nAChRs involved in the release of DA. The nicotine-induced increase of striatal DA release was greater in FSL than in FRL rats for all concentrations of nicotine, suggesting that the intrinsic activity of nicotine was greater in the FSL than in the FRL rats. This was further supported by our finding that the density of high-affinity nAChRs in the striatum of FSL rats was 44% greater than in the FRL rats, whereas affinity (K(D)) was virtually the same in the two lines of rats. Also the expression of mRNAs encoding for alpha(4), alpha(7), and beta(2) subunits in the striatum was greater in FSL than in FRL rats (attomol/microg total RNA, alpha(4):98+/-10 vs. 77+/-7; alpha(7):279+/-16 vs. 184+/-16; beta(2):310+/-19 vs. 201+/-12). We hypothesize that the difference in nicotine-induced DA release in the striatum of FSL and FRL rats depends on the difference in nAChR subunit expression in the striatum between the two lines. The Flinders rats could be used as a model for nicotine self-administration studies to evaluate the susceptibilities of FSL and FRL rats to nicotine dependence.

In utero exposure to serotonergic drugs alters neonatal expression of 5-HT(1A) receptor transcripts: a quantitative RT-PCR study

In embryonic rat brain, serotonin (5-HT) acts as a differentiation signal for 5-HT neurons and their target cells during midgestation. Serotonin receptors expressed during this period include the 5-HT(1A) subtype, which may mediate some of these developmental effects. Using the highly sensitive method of competitive RT-PCR, we quantified the effects of maternal treatment with either p-chlorophenylalanine (pCPA; which depletes 5-HT in embryonic rat brain) or 5-methoxytryptamine (5-MT; a general 5-HT(1) /5-HT(2) agonist) from embryonic day E12-17 on expression of 5-HT(1A) receptor mRNA transcripts in brains of offspring at postnatal day 4 (PND 4). In offspring of both pCPA and 5-MT treated mothers, 5-HT(1A) transcripts were significantly reduced compared to vehicle controls, although effects of pCPA were greater than those of 5-MT. These results indicate that either under-stimulation of 5-HT(1A) receptors (due to pCPA-induced 5-HT depletion) or over-stimulation (by the agonist 5-MT) during prenatal development significantly reduced expression of 5-HT(1A) receptor transcripts in neonatal offspring. This may occur by disruption of 5-HT(1A) gene transcription or by post-transcriptional mechanisms (such as altered translation or turnover of mRNA). Whatever the mechanism, reductions in 5-HT(1A) receptor transcripts following in utero exposure to serotonergic drugs could significantly impact the number of 5-HT(1A) receptors expressed in neonatal rat brain. Whether such effects will persist into adulthood remains to be determined.

Quantification of GABA(A) receptor subunit mRNAs by non-radioisotopic competitive RT-PCR utilizing plate-based EIA methodology

We developed a non-radioisotopic quantitative competitive RT-PCR method for the measurement of gamma-aminobutyric acid (GABA) type A receptor subunit mRNA levels. The specificity of the method was optimized by the use of four subunit-specific oligonucleotides in the sequential steps: reverse transcription, polymerase chain reaction (PCR), and detection. The biotinylated PCR products were bound on streptavidin-coated microtiter plates allowing detection of the products using dinitrophenyl (DNP)-labeled probes and anti-DNP alkaline phosphatase conjugate. The method was set up for the six major cerebellar GABA(A) receptor subunits: alpha1; alpha6; beta2; beta3; gamma2 and delta. The method is quantitative and rapid. With a large dynamic range from 10 fg to 1 ng of subunit mRNA, the accuracy was 12 and 19% (intra- and interassay coefficients of variation, respectively), which might be improved by using a smaller range of standards. The use of a double logarithmic standard curve [log (standard to competitor signal) vs. log (standard mRNA originally present)] requires only one reaction from each sample, allowing the analysis of a large number of samples in one experiment.

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.

Neurosteroids: biosynthesis and function of these novel neuromodulators

Over the past decade, it has become clear that the brain is a steroidogenic organ. The steroids synthesized by the brain and nervous system, given the name neurosteroids, have a wide variety of diverse functions. In general, they mediate their actions, not through classic steroid hormone nuclear receptors, but through ion-gated neurotransmitter receptors. This paper summarizes what is known about the biosynthesis of neurosteroids, the enzymes mediating these reactions, their localization during development and in the adult, and their function and mechanisms of action in the developing and adult central and peripheral nervous systems. The expression of the steroidogenic enzymes is developmentally regulated, with some enzymes being expressed only during development, while others are expressed during development and in the adult. These enzymes are expressed in both neurons and glia, suggesting that these two cell types must work in concert to produce the appropriate active neurosteroid. The functions attributed to specific neurosteroids include modulation of GABA(A) and NMDA function, modulation of sigma receptor function, regulation of myelinization, neuroprotection, and growth of axons and dendrites. Neurosteroids have also been shown to modulate expression of particular subunits of GABA(A) and NMDA receptors, providing additional sites at which these compounds can regulate neural function. The pharmacological properties of specific neurosteroids are described, and potential uses of neurosteroids in specific neuropathologies and during normal aging in humans are also discussed.

Effect of ethanol upon respiratory-related hypoglossal nerve output of neonatal rat brain stem slices

The actions of ethanol (EtOH) on the respiratory output of the neonatal rat brain stem slice preparation in vitro are described. Ethanol inhibited respiratory-related hypoglossal nerve activity in a dose-dependent manner. The effect of EtOH was evident within 5 min and was reversible on EtOH washout. The actions of EtOH were qualitatively similar to those of two other alcohols, methanol and octanol. We investigated the dose-response relationship for each alcohol and determined that the order of potency was methanol < EtOH << octanol, with EC(50) values of 291 mM, 39.7 mM, and 49.2 microM respectively. Application of either strychnine (5 microM) or bicuculline (5 microM) alone, partially but not significantly, reversed the inhibition of respiratory-related hypoglossal nerve activity produced by 50 mM EtOH. Preincubation of rhythmic slices with a combination of both strychnine and bicuculline (both 5 microM) partially, but significantly, blocked the inhibitory actions of EtOH, suggesting that other mechanisms also play a role in the action of EtOH. Preincubation of the slices with 25 microM APV reduced the relative degree of inhibition caused by EtOH suggesting that N-methyl-D-aspartate (NMDA)-receptor-mediated events can be affected by EtOH. Furthermore inhibition of protein kinase C by incubation with 100 nM staurosporine also reduced the efficacy of EtOH. These results suggest that the actions of EtOH may be mediated via glycine, GABA(A), and NMDA receptors and that activation of protein kinase C is involved in the EtOH-induced inhibition of respiratory-related hypoglossal nerve activity.

Evidence of two populations of GABA(A) receptors in cerebellar granule cells in culture: different desensitization kinetics, pharmacology, serine/threonine kinase sensitivity, and localization

GABA(A) receptors of rat cerebellar granule cells in culture have been studied by the whole cell patch clamp technique. The biphasic desensitization kinetic observed could be due either to different desensitization mechanisms of a single receptor population or to different receptor populations. The overall data indicate that the latter hypothesis is most probably the correct one. In fact, the fast desensitizing component was selectively potentiated by a benzodiazepine agonist and preferentially down-regulated by activation of the protein serine/threonine kinases A and G, as a consequence of the latter characteristic that receptor population was preferentially down-regulated by previous activation of N-methyl-d-aspartate glutamate receptors, via production of nitric oxide and PKG activation, most probably in dendrites. The other population is benzodiazepine insensitive and not influenced by activation of PKA or PKG. This slowly desensitizing population may correspond to the extrasynaptic delta subunit containing GABA(A) receptors described by other authors. Instead, the rapidly desensitizing population appears to represent dendritic synaptic GABA(A) receptors.

Progesterone derivatives are able to influence peripheral myelin protein 22 and P0 gene expression: possible mechanisms of action

The present study has analyzed the effect of progesterone and its derivatives (dihydroprogesterone and tetrahydroprogesterone) on the gene expression of the peripheral myelin protein 22 utilizing in vivo and in vitro models. The data obtained indicate that tetrahydroprogesterone is able to stimulate the gene expression of peripheral myelin protein 22 both in vivo (in adult but not in old animals) and in Schwann cell cultures. An effect of this steroid, which is known to interact with the GABA(A) receptor, would not be surprising, since in the present study we show the presence in Schwann cells and in the sciatic nerve of the messengers for several subunits (alpha2, alpha3, beta1, beta2, and beta3) of the GABA(A) receptor. An effect of tetrahydroprogesterone is also evident on the gene expression of another myelin protein, the peripheral myelin protein zero. However, in this case also dihydroprogesterone, which is able to bind the progesterone receptor, is involved, both in old and adult animals, in the stimulation of messengers levels of this myelin protein. In conclusion, the present data show that the gene expression of two important peripheral myelin proteins can be influenced by progesterone derivatives. The hypothesis has been put forward that part of their effects might occur not through the classical progesterone receptor, but rather via an interaction with the GABA(A) receptor.

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.

New perspectives in the functional role of GABA(A) channel heterogeneity

Gamma-aminobutyric acid A (GABA(A)) channels responsible for inhibitory synaptic transmission possess a consistent heterogeneity of structure in terms of distinct constitutive subunits. During the past 10 years, considerable progress has been made in understanding the magnitude of this large diversity. Structural requirements for clinically important drugs such as benzodiazepines and barbiturates have been elucidated, and the anatomical distribution in distinct neuronal populations and the developmental profiles of individual subunits have been elucidated with various techniques. However, the relevance of subunit heterogeneity to synaptic transmission is still largely lacking. Recently, substantial progress has been achieved in understanding the crucial role of desensitization as a molecular determinant in defining the duration and frequency responses of inhibitory synaptic transmission. This development, together with a combination of different experimental approaches, including patch-clamp recordings and ultrafast agonist applications in brain slices and mammalian cells expressing recombinant GABA(A) receptor, has begun to shed light on a possible role for subunit composition of synaptic receptors in shaping the physiological characteristics of synaptic transmission. Nowhere else in the central nervous system is the anatomical and developmental profile of GABA receptor heterogeneity as well understood as it is in the cerebellum. This review summarizes advances in the understanding of functional correlates to subunit heterogeneity in the cerebellum relevant for inhibitory synaptic function.

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.

The expression of plasma membrane Ca2+ pump isoforms in cerebellar granule neurons is modulated by Ca2+

Plasma membrane Ca2+ ATPase (PMCA) pump isoforms 2, 3, and 1CII are expressed in large amounts in the cerebellum of adult rats but only minimally in neonatal cerebellum. These isoforms were almost undetectable in rat neonatal cerebellar granule cells 1-3 days after plating, but they became highly expressed after 7-9 days of culturing under membrane depolarizing conditions (25 mM KCl). The behavior of isoform 4 was different: it was clearly detectable in adult cerebellum but was down-regulated by the depolarizing conditions in cultured cells. 25 mM KCl-activated L-type Ca2+ channels, significantly increasing cytosolic Ca2+. Changes in the concentration of Ca2+ in the culturing medium affected the expression of the pumps. L-type Ca2+ channel blockers abolished both the up-regulation of the PMCA1CII, 2, and 3 isoforms and the down-regulation of PMCA4 isoform. When granule cells were cultured in high concentrations of N-methyl-D-aspartic acid, a condition that increased cytosolic Ca2+ through the activation of glutamate-operated Ca2+ channels, up-regulation of PMCA1CII, 2, and 3 and down-regulation of PMCA4 was also observed. The activity of the isoforms was estimated by measuring the phosphoenzyme intermediate of their reaction cycle: the up-regulated isoforms, the activity of which was barely detectable at plating time, accounted for a large portion of the total PMCA activity of the cells. No up-regulation of the sarcoplasmic/endoplasmic reticulum calcium pump was induced by the depolarizing conditions.

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.

Phosphorylation of the GABAA receptor gamma2L subunit in rat sensory neurons may not be necessary for ethanol sensitivity

The effect of ethanol on the current activated by 2.5 to 40 microM gamma-aminobutyric acid (GABA) was studied in freshly isolated rat dorsal root ganglion (DRG) neurons under voltage clamp in the whole-cell and perforated-patch recording configurations. Our results confirmed that GABAA-activated current in these neurons was insensitive to ethanol at concentrations from 2.5 to 100 mM [G. White, D.M. Lovinger, F.F. Weight, Ethanol inhibits NMDA-activated current but does not alter GABA-activated current in an isolated adult mammalian neuron, Brain Res. 507 (1990) 332-336.]. In addition, the ethanol sensitivity of GABA receptors was studied under conditions that promote phosphorylation of the PKC site on the gamma2L subunit. The presence of the gamma2L and other subunit mRNAs was detected by reverse transcription (RT) of total RNA purified from adult DRG followed by polymerase chain reaction (PCR) using subunit specific primer sets. We found that the GABA response remained insensitive to 2.5-100 mM ethanol despite: (i) the extracellular preapplication of 5, 20 or 500 nM phorbol 12-myristate 13-acetate (PMA); (ii) raising free intracellular Ca2+ ([Ca2+]i) from 7 to 100 or 600 nM by altering the intracellular Ca2+/EGTA ratio; (iii) intracellular application of PKC (0.247 U ml-1 ); and (iv) combining the intracellular application of 1 microM okadaic acid and 30 microM peptide 3 with the extracellular application of 20 nM PMA. These results suggest that phosphorylation of the gamma2L subunit is not the only requirement for ethanol sensitivity of GABAA receptors.

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)

Regulation of GABA(A) receptor subunit mRNA expression by the pesticide dieldrin in embryonic brainstem cultures: a quantitative, competitive reverse transcription-polymerase chain reaction study

Cyclodiene organochlorine pesticides, such as dieldrin, inhibit gamma-aminobutyric acid (GABA)ergic neurotransmission by blocking the Cl- channel of GABA(A) receptors. This action may make the developing nervous system especially vulnerable to these neurotoxins, which could interfere with the trophic actions of GABA on developing neurons and alter expression of GABA(A) receptors. We have used an in vitro model to determine whether exposure to dieldrin alters developmental expression of GABA(A) receptor subunit mRNA transcripts. Dissociated cell cultures were prepared from embryonic day 14 (E14) brainstem and cultured in serum-containing medium for 1 day in vitro (DIV), then treated for 2 DIV with 10 microM dieldrin in serum-free medium. This dose was based on preliminary experiments and previous studies (Nagata et al.: Brain Res 645:19-26, 1994; Pomes et al.: J Pharmacol Exp Ther 271:1616-1623, 1994). Absolute amounts of alpha1, beta3, gamma1, gamma2S and gamma2L mRNA transcripts were quantified in these cultures by quantitative, competitive reverse transcription-polymerase chain reaction (RT-PCR) using subunit-selective internal standards. The most abundant GABA(A) subunit transcript was beta3, which was much more highly expressed than gamma2S, gamma1, gamma2L, or alpha1 subunit mRNAs. Dieldrin differentially regulated expression of these transcripts. Levels of beta3 subunit transcripts were significantly increased (by 300%) by dieldrin, whereas expression of gamma2S and gamma2L transcripts were decreased (by 50% and 40%, respectively). However, dieldrin did not alter the ratio of gamma2S to gamma2L transcripts, indicating that it did not affect alternative splicing of gamma2 transcripts. Dieldrin appeared to increase expression of alpha1 subunit transcripts, but this effect was not statistically significant. Dieldrin did not significantly alter expression of gamma1 subunit transcripts. These results support the hypothesis that in utero exposure to cyclodiene pesticides could pose a risk to the developing brain by virtue of their ability to alter gene expression of GABA(A) receptor subunits, which could produce GABA(A) receptors with altered functional properties.

Developmental expression of chick cortical GABA(A) receptor alpha1 subunits in vivo and in vitro

In order to examine the expression of the GABA(A) receptor alpha1 subunit during chick cortical development in vivo and in vitro, we have utilized a polyclonal antibody (RP4) directed against an alpha1(331-381) fusion protein. This antibody exhibits a high titer for precipitation of [3H]flunitrazepam binding sites in chick cortical extracts, no significant cross-reactivity with GABA(A) receptor beta2- or beta4-subunit fusion proteins, and a robust reaction with a single 51-kDa polypeptide on immunoblots of cortical membranes. This indicates monospecificity of the RP4 antiserum for the GABA(A) receptor alpha1 subunit. The alpha1-subunit antibody also showed strong immunocytochemical reactions with neurons in the embryonic mediodorsal cortex and Purkinje cells of the chick cerebellum. The ontogeny of the alpha1 subunit in chick cortex and in derived neuronal cultures was examined by quantitative Western blotting. The level of the alpha1 polypeptide increased from day 2 to day 6 in culture, acquiring 50% of the maximum expression at day 4. Expression of the cortical GABA(A) receptor alpha1 subunit increased in vivo from embryonic day 8 (E8) to day 7 post-hatching, reaching 50% of adult levels at E16. Levels of the corresponding alpha1-subunit mRNA, analyzed from E8 to E20 by quantitative reverse-transcriptase polymerase chain reaction (RT-PCR), showed a corresponding incline. These findings correlated well with previous developmental studies of GABA(A) receptor ligand binding sites both in vivo and in vitro. The parallel increase of the alpha1 subunit transcript and polypeptide with [3H]flunitrazepam binding sites suggests that this subunit may be an important component of GABA(A) receptors early in cortical ontogeny. This was investigated further by quantitative immunoprecipitation. At saturation, the RP4 antiserum consistently precipitated 50-65% of the central [3H]flunitrazepam binding sites in the developing cortex from E12 through P7, despite a 5-fold increase in the binding level. The data suggest that during cortical development the fraction of GABA(A) receptors containing alpha1 subunits remains relatively constant. Furthermore, the alpha1 polypeptide appears to be a major component of GABA(A) receptor oligomers at all stages of cortical maturation.

Glutamate receptors in the olfactory bulb synaptic circuitry: heterogeneity and synaptic localization of N-methyl-D-aspartate receptor subunit 1 and AMPA receptor subunit 1

In this study, we analysed the molecular heterogeneity and synaptic localization of the N-methyl-D-aspartate receptor subunit 1 and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunit 1 in the olfactory bulb glomerular synaptic circuitry. Semiquantitative reverse transcriptase polymerase chain reaction showed that approximately 40% of the N-methyl-D-aspartate receptor subunit 1 messenger RNA splice variants contain the N1 exon, which conveys specific functional properties on the channel. In other forebrain and hindbrain regions that we examined, the ratio of the N1-containing (receptor subunit 1(1XX)) to N1-lacking (receptor subunit 1(0XX)) N-methyl-D-aspartate receptor subunit 1 messenger RNAs varied considerably. The cellular and subcellular distribution of N-methyl-D-aspartate receptor subunit 1 and AMPA receptor subunit 1 was investigated with antibodies generated against the C-terminal domain of the individual subunits [Petralia R. S. et al. (1994) J. Neurosci. 14, 667 696; Wenthold R. J. et al. (1992) J. biol Chem. 267, 501 507]. Both N-methyl-D-aspartate receptor subunit 1 and AMPA receptor subunit 1 were localized to the postsynaptic density of asymmetric synapses established by olfactory receptor neuron terminals with the dendrites of mitral and tufted cells. Not all of these synapses, however, were labelled. These results are consistent with the notion that glutamate is the neurotransmitter at the olfactory nerve to mitral and tufted cell synapses, and suggest a high heterogeneity in the expression of the postsynaptic glutamate receptors.

The effect of GABA stimulation on GABAA receptor subunit protein and mRNA expression in rat cultured cerebellar granule cells

1. After 8 days in vitro, rat cerebellar granule cells were exposed to 1 mM gamma-aminobutyric acid (GABA) for periods of 1, 2, 4, 6, 8 and 10 days. The effect of the GABA exposure on GABAA receptor alpha 1, alpha 6 and beta 2,3 subunit protein expression and alpha 1 and alpha 6 subunit steady-state mRNA levels, was examined using Western blotting and reverse transcriptase-polymerase chain reaction (RT-PCR), respectively. 2. GABA exposure for 2 days decreased alpha 1 (35 +/- 10%, mean +/- s.e.mean), beta 2,3 (21 +/- 9%) and alpha 6 (28 +/- 10%) subunit protein expression compared to control levels. The GABA-mediated reduction in alpha 1 subunit expression after 2 days treatment was abolished in the presence of the GABAA receptor antagonist, Ru 5135 (10 microM). 3. GABA exposure for 8 days increased alpha 1 (26 +/- 10%, mean +/- s.e.mean) and beta 2,3 (56 +/- 23%) subunit protein expression over control levels, whereas alpha 6 subunit protein expression remained below control levels (by 38 +/- 10%). However, after 10 days GABA exposure, alpha 6 subunit protein expression was also increased over control levels by 65 +/- 29% (mean +/- s.e.mean). 4. GABA exposure did not change the alpha 1 or alpha 6 subunit steady-state mRNA levels over and 8 day period, nor did it alter the expression of cyclophilin mRNA over 1-8 days. 5. These results suggest that chronic GABA exposure of rat cerebellar granule cells has a bi-phasic effect on GABAA receptor subunit expression that is independent of changes to mRNA levels. Therefore, the regulation of the GABAA receptor expression by chronic agonist treatment appears to involve post-transcriptional and/or post-translational processes.

Delta subunit inhibits neurosteroid modulation of GABAA receptors

Neurosteroid modulation of GABAA receptors has been observed with all subunit combinations investigated; however, hetero-oligomeric GABAA receptors containing delta subunits were not studied previously. We describe the effect of delta subunit expression on 3alpha,21-dihydroxy-5alpha-pregnan-20-1 (THDOC)-induced potentiation of GABA-gated currents in transfected HEK 293 cells and in cerebellar granule cells in vitro. THDOC (100 nM) significantly potentiated GABA-gated currents in cells transfected with combinations of alpha1, alpha6, beta3, and gamma2 subunit cDNAs, whereas cotransfection of delta subunit cDNA inhibited this potentiation. In contrast, the direct Cl- channel activation by THDOC at higher concentrations (1-10 microM) was not significantly dependent on delta subunit cotransfection. These results suggest that the presence of the delta subunit inhibits GABAA receptor modulation but not the direct activation by neurosteroids. Cotransfection with delta subunit also affected the negative allosteric modulation by pregnenolone sulfate. THDOC potentiation of GABA-gated currents was greater in cerebellar granule cell cultures at 4 d in vitro (DIV) compared with those at 14 DIV. Single-cell reverse transcription-PCR analysis of the mRNAs expressed in cultured cerebellar granule cells shows that an increased number of granule cells at 14 DIV express delta subunit mRNAs as compared with 4 DIV granule cells. The presence of delta subunit mRNAs detected in individual cells correlated well with the lack of sensitivity to THDOC. These results suggest that developmental expression of GABAA receptor delta subunits may play an important role in determining the region-specific neurosteroid-induced modification of fast inhibitory synaptic function.

Astroglia-released factor with negative allosteric modulatory properties at the GABA A receptor

We have previously shown, using whole-cell patch-clamp techniques, that astrocytes release a negative allosteric modulator of the gamma-aminobutyric acid type A receptor (GABAA receptor) with beta-carboline-like properties, thus, likely to act at the benzodiazepine site. Here, using patch-clamp and binding techniques, we confirm that the low-molecular-weight fraction of astroglia-conditioned medium (ACM lmf) contains a factor(s) that negatively modulates GABAA-receptor function. This factor, like beta-carbolines, enhances the specific binding of [35S]t-butyl bicyclophosphorothionate (TBPS) to adult rat cortical membranes in the presence of GABA. However, it fails to interact with various ligands of the benzodiazepine (BZD) site of the GABAA receptor ([3H]flunitrazepam, [3H]Ro 15-1788 and [3H]Ro 15-4513). The question of the actual binding site of the astroglia-derived factor on the GABAA receptor, thus, remains open and can be addressed only after the purification of the active molecule(s) of ACM Imf has been completed, and a labeled form of the endogenous ligand becomes available. Taken together, however, the data suggest that type 1 astrocytes are able to modulate the effects of the main inhibitory neurotransmission in the central nervous system.

Developmental changes of inhibitory synaptic currents in cerebellar granule neurons: role of GABA(A) receptor alpha 6 subunit

Eye opening and increased motor activity after the second postnatal week in rats imply an extensive development of motor control and coordination. We show a parallel development change in spontaneous IPSC (sIPSC) kinetics in cerebellar granule neurons. sIPSCs were studied by whole-cell recordings in cerebellar slices, prepared from 7-30 postnatal day old rats. Early in development, sIPSCs had slow decay kinetics whereas in older rats faster decaying sIPSCs were found in larger proportion. Currents elicited by 1 mM GABA pulses (GABACs) in nucleated patches excised from cerebellar granule neurons revealed that GABACs kinetics better approximate sIPSC decay in young but not in more developed rats. The expression of alpha 6 subunit of GABAA receptors, unique in cerebellar granule neurons, has been shown to increase during development. Therefore, we took advantage of the recently reported selective inhibition of GABAA receptors by furosemide to characterize the relative contribution of alpha 6 subunits to native receptors in inhibitory synapses of cerebellar granule neurons. Although furosemide inhibition of sIPSCs amplitude was highly variable among distinct granule cells, it increased during development. At the same time, furosemide failed to inhibit sIPSCs recorded from Purkinje neurons. From the comparison of furosemide inhibition and kinetics of sIPSCs with GABACs recorded from mammalian HEK293 cells transfected with combinations of alpha 1 and alpha 6 GABAA receptor subunits together with beta 2 gamma 2 subunits, we propose that an increased alpha 6 subunit contribution in the molecular assembly of postsynaptic receptors in cerebellar glomeruli is responsible for the developmental changes observed.

GABAA receptor alpha 1-subunit-immunopositive neurons in the rat striatum

The morphological characteristics and distribution of striatal neurons expressing the GABAA receptor alpha 1-subunit were investigated following immunolabeling with an alpha 1-subunit-specific antibody. alpha 1-Immunopositive striatal neurons are relatively scarce. Those located in the dorsal striatum are small and have rounded somata and numerous dendrite varicosities. alpha 1-Immunoreactive neurons with these characteristics were likewise observed in the ventral striatum, which also contained large pyramidal neurons with smooth dendrites and polygonal neurons with spiny dendrites. A fourth neuron type (oval neurons with dendrites oriented in bipolar fashion) was found in the lateral striatum. All four neuron types were observed in the rostral, central and caudal striatum. Double-labeling experiments using an antibody specific for DARPP-32 (a cell-level indicator of dopamine D1-like receptors) and the alpha 1 antibody showed a complete lack of colocalization of these two markers in striatal neurons.

Benzodiazepines on trial: a research strategy for their rehabilitation

Ataxia, sedation, amnesia, ethanol and barbiturate potentiation, tolerance, dependence, and the potential for drug abuse plague the clinical use of anxiolytic benzodiazepines. Benzodiazepine and non-benzodiazepine ligands that are in current clinical use act as full allosteric modulators of GABA-gated Cl- channels, and on chronic administration trigger compensatory changes in the subunit expression of GABAA receptors. In these putative abnormal receptors, full allosteric modulators have low intrinsic activity and potency, and tolerance and dependence ensue. In this review, Erminio Costa and Alessandro Guidotti discuss the development of partial allosteric modulators, such as imidazenil, which have high potency and low intrinsic activity at GABA-gated Cl- channels. Since in animals tolerant to full allosteric modulators imidazenil also fails to show cross-tolerance, it is an example of a new type of anxiolytic and anticonvulsant drug acting at GABAA receptors via benzodiazepine recognition sites.

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.

Reversible modification of GABAA receptor subunit mRNA expression during tolerance to diazepam-induced cognition dysfunction

Benzodiazepines (BZs) that are endowed with full positive allosteric modulatory (FAM) activity on GABAA receptors cause anterograde amnesia in both animals and humans. In rats subjected to a delayed object recognition test, diazepam, endowed with FAM activity, exerted an amnesic action, whereas BZs endowed with partial allosteric modulatory (PAM) activity on GABAA receptors, such as imidazenil, failed to induce amnesia, even if administered at doses five times higher than those equipotent to a standard anticonvulsant dose of diazepam (17.6 mumol/kg/os). After discontinuation of 14 days' treatment with vehicle, diazepam, or imidazenil (three times daily with increasing doses starting from 17.6 mumol/kg/os for diazepam and 2.5 mumol/kg/os for imidazenil), we compared the persistence of tolerance to the amnesic effect of diazepam with the persistence of the changes in the context of four (alpha 1, alpha 5, gamma 2L, gamma 2S) GABAA receptor subunit mRNAs in the fronto-parietal motor (FrPaM) cortex and the hippocampus. Rats receiving the long-term treatment with diazepam developed a tolerance to the amnesic effect of this drug and showed a decrease (30-50%) in the expression of mRNAs encoding for alpha 1 gamma 2L, gamma 2S GABAA receptor subunits, an increase, by approximately 30%, of the expression of mRNA of the alpha 5 subunit in the FrPaM cortex and a decrease, by approximately 25%, in the expression of mRNA, of the alpha 1 subunit in the hippocampus. These changes of subunit mRNA expression and the tolerance to the amnesic effect of diazepam returned to control values 72 hr after termination of the long-term treatment with diazepam. No tolerance to the amnesic effect of diazepam and no changes in GABAA receptor subunit mRNA expression were found in rats undergoing long-term treatment with imidazenil.

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.

Intrauterine cocaine exposure of rabbits: persistent elevation of GABA-immunoreactive neurons in anterior cingulate cortex but not visual cortex

The effects of prenatal cocaine exposure on the development of the rabbit cerebral cortex were studied. Two cortical areas were compared: primary visual cortex (VC) and anterior cingulate cortex (ACC). ACC was selected because behavioral deficits observed in cocaine-exposed infants suggest the involvement of ACC. In addition, ACC receives dense dopaminergic innervation and cocaine's action in inhibiting the re-uptake of dopamine is believed to underly the rewarding properties of cocaine. VC was selected as a control area because there is no evidence of behavioral deficits associated with visual perception in cocaine-exposed infants, and because VC receives minimal dopaminergic innervation. Two aspects of cortical development were studied: (i) cortical morphology, growth and cytoarchitectonic organization; and (ii) the development of the GABAergic neurotransmitter system. Measures of postnatal cortical growth, including cortical lamination, cell number and soma size, were compared in cocaine-exposed or control (saline) rabbits aged P5-P60. There was no difference between cocaine and saline animals in any of these parameters, and cortical cytoarchitecture appeared normal. However, despite the absence of major abnormalities in cortical development, we found that the number of GABA-immunoreactive neurons in cocaine-exposed animals was significantly higher than normal in ACC. This effect was highly consistent, was present in all laminae and at all ages studied, and persisted into maturity (P60). In contrast, in VC, the number of GABA-immunoreactive neurons in cocaine-exposed animals did not differ from normal. We suggest that increased GABA immunoreactivity may reflect a compensatory response to excessive excitatory input to ACC. A change in the balance of excitation and inhibition in ACC, reflecting 'noisy' or dysfunctional intracortical circuitry, may underly the emotional lability and attentional deficits characteristically described in infants exposed in utero to cocaine.