Different postnatal development profiles of neurons containing distinct GABAA receptor beta subunit mRNAs (beta 1, beta 2, and beta 3) in the rat forebrain

Neonatal Clonazepam Administration Induced Long-Lasting Changes in GABAA and GABAB Receptors

Benzodiazepines (BZDs) are widely used in patients of all ages. Unlike adults, neonatal animals treated with BZDs exhibit a variety of behavioral deficits later in life; however, the mechanisms underlying these deficits are poorly understood. This study aims to examine whether administration of clonazepam (CZP; 1 mg/kg/day) in 7-11-day-old rats affects Gama aminobutyric acid (GABA)ergic receptors in both the short and long terms. Using RT-PCR and quantitative autoradiography, we examined the expression of the selected GABA_A receptor subunits (α1, α2, α4, γ2, and δ) and the GABA_B B2 subunit, and GABA_A, benzodiazepine, and GABA_B receptor binding 48 h, 1 week, and 2 months after treatment discontinuation. Within one week after CZP cessation, the expression of the α2 subunit was upregulated, whereas that of the δ subunit was downregulated in both the hippocampus and cortex. In the hippocampus, the α4 subunit was downregulated after the 2-month interval. Changes in receptor binding were highly dependent on the receptor type, the interval after treatment cessation, and the brain structure. GABA_A receptor binding was increased in almost all of the brain structures after the 48-h interval. BZD-binding was decreased in many brain structures involved in the neuronal networks associated with emotional behavior, anxiety, and cognitive functions after the 2-month interval. Binding of the GABA_B receptors changed depending on the interval and brain structure. Overall, the described changes may affect both synaptic development and functioning and may potentially cause behavioral impairment.

Neurochemical organization of the ventral striatum's olfactory tubercle

The ventral striatum is a collection of brain structures, including the nucleus accumbens, ventral pallidum and the olfactory tubercle (OT). While much attention has been devoted to the nucleus accumbens, a comprehensive understanding of the ventral striatum and its contributions to neurological diseases requires an appreciation for the complex neurochemical makeup of the ventral striatum's other components. This review summarizes the rich neurochemical composition of the OT, including the neurotransmitters, neuromodulators and hormones present. We also address the receptors and transporters involved in each system as well as their putative functional roles. Finally, we end with briefly reviewing select literature regarding neurochemical changes in the OT in the context of neurological disorders, specifically neurodegenerative disorders. By overviewing the vast literature on the neurochemical composition of the OT, this review will serve to aid future research into the neurobiology of the ventral striatum.

Modulation of firing activity by endogenous GABAA receptors in the globus pallidus of MPTP-treated parkinsonian mice

The globus pallidus in rodents, equivalent to the external segment of the globus pallidus in primates, plays an important role in movement regulation. Previous studies have shown abundant γ-aminobutyric acid (GABA)ergic innervation and GABAA receptors in the globus pallidus. In this study, we investigated the effects of endogenous GABAA receptors on the spontaneous firing activity of pallidal neurons in both normal and MPTP-treated mice using multi-barrel electrodes extracellular recordings in vivo. We found that in normal mice, pressure ejection of 0.1 mmol/L gabazine, a specific GABAA receptor antagonist, increased the spontaneous firing rate of globus pallidus neurons by 27.6 ± 5.6%. Furthermore, in MPTP mice (14 days after MPTP treatment), 0.1 mmol/L gabazine increased the firing rates by 51.0 ± 7.9%, significantly greater than in normal mice. These results suggest that endogenous GABAA receptors modulate the activity of globus pallidus neurons. The present findings may provide a rationale for investigations into the potential role of GABAA receptors in Parkinson's disease.

Effects of Pharmacological Block of GABA(A) Receptors on Pallidal Neurons in Normal and Parkinsonian State

The globus pallidus plays a central integrative role in the basal ganglia circuitry. Morphological studies have revealed a high level of GABA and GABA_A receptors in the globus pallidus. To further investigate the effects of endogenous GABA_A neurotransmission in the globus pallidus of normal and parkinsonian rats, in vivo extracellular recording and behavioral tests were performed in the present studies. In normal rats, micro-pressure ejection of GABA_A receptor antagonist gabazine (0.1 mM) increased the spontaneous firing rate of pallidal neurons by 28.3%. Furthermore, in 6-hydroxydopamine parkinsonian rats, gabazine increased the firing rate by 46.0% on the lesioned side, which was significantly greater than that on the unlesioned side (21.5%, P < 0.05), as well as that in normal rats (P < 0.05). In the behaving rats, unilateral microinjection of gabazine (0.1 mM) evoked consistent contralateral rotation in normal rats, and significantly potentiated the number of apomorphine-induced contralateral rotations in parkinsonian rats. The present electrophysiological and behavioral findings may provide a rational for further investigations into the potential of pallidal endogenous GABA_A neurotransmission in the treatment of Parkinson's disease.

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.

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

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

International Union of Pharmacology. LXX. Subtypes of gamma-aminobutyric acid(A) receptors: classification on the basis of subunit composition, pharmacology, and function. Update

In this review we attempt to summarize experimental evidence on the existence of defined native GABA(A) receptor subtypes and to produce a list of receptors that actually seem to exist according to current knowledge. This will serve to update the most recent classification of GABA(A) receptors (Pharmacol Rev 50:291-313, 1998) approved by the Nomenclature Committee of the International Union of Pharmacology. GABA(A) receptors are chloride channels that mediate the major form of fast inhibitory neurotransmission in the central nervous system. They are members of the Cys-loop pentameric ligand-gated ion channel (LGIC) superfamily and share structural and functional homology with other members of that family. GABA(A) receptors are assembled from a family of 19 homologous subunit gene products and form numerous, mostly hetero-oligomeric, pentamers. Such receptor subtypes with properties that depend on subunit composition vary in topography and ontogeny, in cellular and subcellular localization, in their role in brain circuits and behaviors, in their mechanisms of regulation, and in their pharmacology. We propose several criteria, which can be applied to all the members of the LGIC superfamily, for including a receptor subtype on a list of native hetero-oligomeric subtypes. With these criteria, we develop a working GABA(A) receptor list, which currently includes 26 members, but will undoubtedly be modified and grow as information expands. The list is divided into three categories of native receptor subtypes: "identified," "existence with high probability," and "tentative."

Hyperglycosylation and reduced GABA currents of mutated GABRB3 polypeptide in remitting childhood absence epilepsy

Childhood absence epilepsy (CAE) accounts for 10% to 12% of epilepsy in children under 16 years of age. We screened for mutations in the GABA(A) receptor (GABAR) beta 3 subunit gene (GABRB3) in 48 probands and families with remitting CAE. We found that four out of 48 families (8%) had mutations in GABRB3. One heterozygous missense mutation (P11S) in exon 1a segregated with four CAE-affected persons in one multiplex, two-generation Mexican family. P11S was also found in a singleton from Mexico. Another heterozygous missense mutation (S15F) was present in a singleton from Honduras. An exon 2 heterozygous missense mutation (G32R) was present in two CAE-affected persons and two persons affected with EEG-recorded spike and/or sharp wave in a two-generation Honduran family. All mutations were absent in 630 controls. We studied functions and possible pathogenicity by expressing mutations in HeLa cells with the use of Western blots and an in vitro translation and translocation system. Expression levels did not differ from those of controls, but all mutations showed hyperglycosylation in the in vitro translation and translocation system with canine microsomes. Functional analysis of human GABA(A) receptors (alpha 1 beta 3-v2 gamma 2S, alpha 1 beta 3-v2[P11S]gamma 2S, alpha 1 beta 3-v2[S15F]gamma 2S, and alpha 1 beta 3-v2[G32R]gamma 2S) transiently expressed in HEK293T cells with the use of rapid agonist application showed that each amino acid transversion in the beta 3-v2 subunit (P11S, S15F, and G32R) reduced GABA-evoked current density from whole cells. Mutated beta 3 subunit protein could thus cause absence seizures through a gain in glycosylation of mutated exon 1a and exon 2, affecting maturation and trafficking of GABAR from endoplasmic reticulum to cell surface and resulting in reduced GABA-evoked currents.

A field guide to the anterior olfactory nucleus (cortex)

While portions of the mammalian olfactory system have been studied extensively, the anterior olfactory nucleus (AON) has been relatively ignored. Furthermore, the existing research is dispersed and obscured by many different nomenclatures and approaches. The present review collects and assembles the relatively sparse literature regarding the portion of the brain situated between the olfactory bulb and primary olfactory (piriform) cortex. Included is an overview of the area's organization, the functional, morphological and neurochemical characteristics of its cells and a comprehensive appraisal of its efferent and afferent fiber systems. Available evidence suggests the existence of subdivisions within the AON and demonstrates that the structure influences ongoing activity in many other olfactory areas. We conclude with a discussion of the AON's mysterious but complex role in olfactory information processing.

Genetics of subthalamic nucleus in development and disease

The subthalamic nucleus (STN) is a crucial node in the basal ganglia. Clinical success in targeting the STN for deep brain stimulation in Parkinson's disease patients has prompted increased interest in understanding STN biology. In this report, we discuss recent evidence for transcription factor mediated regulation of STN development. We also review STN developmental neurobiology and known patterns of gene expression in the developing and mature STN.

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.

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.

Gamma-aminobutyric acid receptor (GABA(A)) subunits in rat nucleus tractus solitarii (NTS) revealed by polymerase chain reaction (PCR) and immunohistochemistry

Expression of mRNAs encoding seven GABA(A) receptor subunits (alpha1, alpha2, alpha3, alpha5, beta2, beta3, gamma2) in the nucleus tractus solitarii (NTS) of rat medulla oblongata was examined by reverse transcription-polymerase chain reaction (RT-PCR). All subunit mRNAs, except alpha5, were clearly detected. Band densities produced by alpha1, alpha3, beta3, and gamma2 subunits were greater than those corresponding to beta2 and alpha2 transcripts. The localization of these subunits in tissue sections through NTS was examined by immunohistochemistry. The differential patterns of immunoreactivity in neuronal somata and dendrites of NTS neurons were generally in agreement with the PCR results, confirming that mRNA expression is correlated with receptor protein synthesis. At ultrastructural level, alpha1, alpha3, beta2/3, and gamma2 subunits were localized in both cytoplasmic and subsynaptic sites, the latter often apposed to GABA immunoreactive synapses. These results suggest that ionotropic receptors comprising the alpha1, alpha3, beta2/3, and gamma2 may mediate inhibitory GABA responses in the NTS.

Anabolic steroids induce region- and subunit-specific rapid modulation of GABA(A) receptor-mediated currents in the rat forebrain

Anabolic-androgenic steroids (AAS) have become significant drugs of abuse in recent years with the highest increase reported in adolescent girls. In spite of the increased use of AAS, the CNS effects of these steroids are poorly understood. We report that in prepubertal female rats, three commonly abused AAS, 17alpha-methyltestosterone, stanozolol, and nandrolone, induced rapid and reversible modulation of GABAergic currents in neurons of two brain regions known to be critical for the expression of reproductive behaviors: the ventromedial nucleus of the hypothalamus (VMN) and the medial preoptic area (mPOA). All three AAS significantly enhanced peak synaptic current amplitudes and prolonged synaptic current decays in neurons of the VMN. Conversely all three AAS significantly diminished peak current amplitudes of synaptic currents from neurons of the mPOA. The endogenous neuroactive steroids, 3alpha-hydroxy-5alpha-pregnan-20-one and 5alpha-androstane-3alpha,17beta-diol, potentiated currents in the VMN as did the AAS. In contrast to the negative modulation induced by AAS in the mPOA, the endogenous steroids potentiated responses in this region. To determine the concentration response relationships, modulation by the AAS, 17alpha-methyltestosterone (17alpha-meT), was assessed for currents evoked by ultrafast perfusion of brief pulses of GABA to acutely isolated neurons. Half-maximal effects on currents elicited by 1 mM GABA were elicited by submicromolar concentrations of AAS for neurons from both brain regions. In addition, the efficacy of 10(-5) to 10(-2) M GABA was significantly increased by 1 microM 17alpha-meT. Previous studies have demonstrated a striking dichotomy in receptor composition between the VMN and the mPOA with regard to gamma subunit expression. To determine if the preferential expression of gamma(2) subunit-containing receptors in the VMN and of gamma(1) subunit-containing receptors in the mPOA could account for the region-specific effects of AAS in the two regions, responses elicited by ultrafast perfusion of GABA to human embryonic kidney 293 cells transfected with alpha(2), beta(3), and gamma(2) or alpha(2), beta(3), and gamma(1) subunit cDNAs were analyzed. As with native VMN neurons, positive modulation of GABA responses was elicited for alpha(2)beta(3)gamma(2) recombinant receptors, while negative modulation was induced at alpha(2)beta(3)gamma(1) receptors as in the mPOA. Our data demonstrate that AAS in doses believed to occur in steroid abusers can induce significant modulation of GABAergic transmission in brain regions essential for neuroendocrine function. In addition, the effects of these steroids can vary significantly between brain regions in a manner that appears to depend on the subunit composition of GABA(A) receptors expressed.

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.

Properties and sex-specific differences of GABAA receptors in neurons expressing gamma1 subunit mRNA in the preoptic area of the rat

Gamma-aminobutyric acid type A (GABAA) receptors expressed within the medial preoptic area (mPOA) are known to play a critical role in regulating sexual and neuroendocrine functions. In the rat brain, high levels of expression of the gamma1 subunit mRNA of the GABAA receptor are restricted to a limited number of regions that mediate sexual behaviors, including the mPOA. The biophysical and pharmacological profiles of native gamma1-containing receptors in neurons are unknown. Here, we have characterized the properties of GABAA receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) and currents elicited by fast perfusion of GABA to isolated mPOA neurons of juvenile male and female rats. No significant sex-specific differences were evident in the mean peak amplitude, distribution of event amplitudes, kinetics of current decay, or the frequency of sIPSCs. The profile of modulation of sIPSCs by diazepam, beta-CCM and zolpidem, allosteric modulators that act at the benzodiazepine (BZ) site of the GABAA receptor, support the assertion that mPOA neurons of both sexes express functional gamma1-containing receptors. The ability of zolpidem to modulate both sIPSC amplitude and currents elicited by rapid perfusion of GABA to mPOA neurons differed significantly between the sexes. Zolpidem reversibly induced negative modulation of currents in mPOA neurons isolated from male rats, but had no effect in mPOA neurons from female rats. Concentration-response analysis of responses in neurons acutely isolated from male rats indicated an IC50 of 58 nM with maximal decreases of approximately 50% of control peak current amplitude. In situ hybridization analysis demonstrated that levels of the gamma1 subunit mRNA are significantly higher in mPOA neurons from male than female rats. No significant sex-specific differences were detected in the levels of alpha1, alpha2, or alpha5 mRNAs. These results suggest that native gamma1-containing receptors are expressed in primary neurons of the mPOA and that sex-specific differences in the expression of this subunit may contribute to sexual dimorphism in GABAA receptor modulation by compounds acting at the BZ site.

A GABAA receptor alpha1 subunit tagged with green fluorescent protein requires a beta subunit for functional surface expression

gamma-Aminobutyric acid, type A (GABAA) receptors, the major inhibitory neurotransmitter receptors in the central nervous system, are heteropentameric proteins assembled from distinct subunit classes with multiple subtypes, alpha(1-6), beta(1-4), gamma(1-3), delta(1), and epsilon(1). To examine the process of receptor assembly and targeting, we tagged the carboxyl terminus of the GABAA receptor alpha1 subunit with red-shifted enhanced green fluorescent protein (EGFP). Xenopus oocytes were injected with cRNA of this fusion protein, alpha1-EGFP, alone or in combination with cRNA of GABAA receptor beta2, gamma2, or beta2+gamma2 subunits. Within 72 h after injection, EGFP fluorescence was visible in all fusion protein-injected cells. The fluorescence was associated with the plasmalemma only when the beta2 subunit was co-injected with alpha1-EGFP. Texas Red-conjugated immunolabeling of EGFP on nonpermeabilized cells demonstrated that EGFP was localized extracellularly. Hence, the COOH terminus of the alpha1 subunit is extracellular. Two-electrode voltage clamp of alpha1-EGFPbeta2- and alpha1-EGFPbeta2 gamma2-injected oocytes demonstrates that these cells express functional receptors, with EC50 values for GABA and diazepam similar to wild-type receptors. Thus, a COOH-terminal tag of the alpha1 subunit appears to be functionally silent, providing a useful marker for studies of GABAA receptor expression, assembly, transport, targeting, and clustering. Moreover, the beta2 subunit is required for receptor assembly and surface expression.

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

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

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.

Neuronal nicotinic receptor alpha 6 subunit mRNA is selectively concentrated in catecholaminergic nuclei of the rat brain

Although the neuronal nicotinic receptor alpha 6 subunit was cloned several years ago, its functional significance remains to be investigated. Here we describe an in situ hybridization study of the mRNA for this subunit in the adult rat central nervous system using oligonucleotide probes. Specific alpha 6 mRNA labelling was restricted to a few nuclei throughout the brain; it was particularly high in several catecholaminergic nuclei [the locus coeruleus (A6), the ventral tegmental area (A10) and the substantia nigra (A9)] at levels significantly higher than those found for any other known nicotinic receptor subunit mRNA. Labelling for alpha 6 mRNA was also detected at lower levels in the reticular thalamic nucleus, the supramammillary nucleus and the mesencephalic V nucleus. Some cells of the medial habenula (medioventral part) and of the interpeduncular nucleus (central and lateral parts) were also labelled. The distribution of alpha 6 mRNA was compared with the distribution of the other known nicotinic acetylcholine receptor subunit mRNAs. In several nuclei, the expression of alpha 6 was complementary to those of other alpha subunits. Moreover, some of the cell groups (such as the substantia nigra, the ventral tegmental area and the locus coeruleus) previously thought to contain mainly alpha 3 mRNA in fact were found to contain high levels of alpha 6 mRNA. Finally, we found extensive colocalization of alpha 6 and beta 3, indicating the possible existence of nicotinic receptor hetero-oligomers containing both subunits. The present results show that alpha 6 is the major nicotinic acetylcholine receptor alpha subunit expressed in dopaminergic cell groups of the mesencephalon and noradrenergic cells of the locus coeruleus. This suggests the involvement of the alpha 6 subunit in some of the major functions of central nicotinic circuits, including the modulation of locomotor behaviour and reward.

GABA-induced increases in [Ca2+]i in retinal neurons of postnatal rabbits

Previous studies have indicated that gamma-aminobutyric acid (GABA) plays an important trophic role in the synapse formation between horizontal cells and photoreceptors in postnatal rabbit retina. However, the mechanism of the GABA effect has not been identified. Using fluo-3 Ca2+ imaging and confocal laser scanning microscopy we examined the effect of GABA on [Ca2+]i during postnatal retinal development. GABA (100 microM) evoked a fast and transient increase of [Ca2+]i in selected populations of freshly dissociated retinal cells from postnatal rabbits. This increase was apparent on postnatal day 1 and reached a maximum on day 5. Little increase in [Ca2+]i was observed in retinal cells isolated from adult rabbits. GABA receptor antagonists, picrotoxin and bicuculline, significantly reduced the response. The GABAB agonist, baclofen, did not evoke any [Ca2+]i changes. The GABA-induced increase in [Ca2+]i was observed in all retinal layers in neonatal retinal whole-mount explants. In the outer retina, the increase was seen in cone photoreceptors which were specifically labeled with peanut agglutinin (PNA). The GABA-induced increase in [Ca2+]i may provide an important mechanism for regulating cone synaptogenesis in the outer plexiform layer of the postnatal retina.

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.

Deficiency of the beta 3 subunit of the type A gamma-aminobutyric acid receptor causes cleft palate in mice

In addition to its function in the nervous system, gamma-aminobutyric acid (GABA) has been implicated in mouse craniofacial development by the results of both teratological, and genetic studies. We previously reported that disruption of the cleft palate 1 (cp1) locus, closely linked to the pink-eyed dilution (p) locus on mouse chromosome 7, causes a 95% penetrant, recessive, neonatally-lethal cleft palate (CP) in mice homozygous for the p(4THO-II) deletion. We proposed that the beta 3 subunit gene (Gabrb3) of the GABAA receptor might be a candidate for cp1 (ref. 4); our earlier studies had localized cp1 to an interval beginning distal to the gene for the GABAA receptor alpha 5 subunit (Gabra5) and ending within the Gabrb3 coding region. To test the hypothesis that deletion of Gabrb3, and not another gene in the interval, causes CP, we performed an experiment to rescue the CP phenotype by introducing a Gabrb3 transgene into p(4THO-II) homozygotes. We now show that such transgenic mice are phenotypically normal, indicating that Gabrb3 is indeed the cp1 locus.

Regional and cellular expression sites of the alpha 1 subunit of GABAA receptors in the rat basal forebrain: a cytochemical study with glutamic acid decarboxylase, choline acetyltransferase, calcium-binding proteins and nitric oxide synthase as second markers

Forebrain sections of adult male Wistar rats were processed for the immunohistochemical detection of the GABAA receptor alpha 1 subunit. Alternate sections were used for double-staining with antibodies to glutamic acid decarboxylase (GAD), choline acetyltransferase (ChAT), the calcium binding proteins parvalbumin (PARV), calbindin (CALB) and calretinin (CR) as well as to nitric oxide synthase (NOS). alpha 1 receptor subunit-immunoreactive neurons were found to be inhomogeneously distributed in the rat basal forebrain. Numerous alpha 1 subunit-immunostained neurons occupied the central part of medial septum and diagonal band, the whole ventral pallidum and the globus pallidus. A moderate number was found in the lateral septum, and only very few in the striatum and nucleus accumbens. Double-immunofluorescence labelling revealed an apparently complete co-expression of GAD-immunoreactivity in alpha 1 subunit-immunoreactive cells of rat basal forebrain, but only a region-dependent proportion of GAD-immunoreactive cells showed alpha 1 subunit-immunoreactivity. Co-expression of PARV-immunoreactivity characterized the vast majority of the alpha 1 subunit-immunoreactive cells in the medial septum, diagonal band, ventral pallidum and globus pallidus. Striatal alpha 1 subunit-immunopositive neurons appeared PARV-immunonegative and did also not react with the other immunoreagents used in this study, except the GAD-antibody. CR-immunoreactivity was co-expressed in alpha 1 subunit-immunopositive cells of the ventral lateral septal nucleus and only exceptionally in the ventral pallidum, where the vast majority of CR-positive cells was monolabelled. A small minority of ChAT-immunoreactive, but in no case CALB- and NOS-immunoreactive cells were found to express the alpha 1 subunit-immunoreactivity. These findings confirm the data obtained by analyses of other brain regions suggesting a preferred co-existence of this GABAA receptor subunit with PARV and to a lesser degree with CR.

Modulatory actions of gamma aminobutyric acid (GABA) on GABA type A receptor subunit expression and function

Gamma aminobutyric acid (GABA) is present in the central nervous system (CNS) during very early embryogenesis. It is therefore likely to play a role not only as a neurotransmitter but also as a signal molecule for neuronal differentiation, growth, and development. It has been firmly established that formation of synapses is strengthened by GABA, and the expression of certain subunits of the GABA type A (GABAA) receptor complex is clearly promoted by GABA. This latter effect of GABA may have profound implications for the functional activity of GABAergic synapses since the pharmacological properties of GABAA receptors are governed by the subunit composition of the receptor complex. Dynamic changes in GABAA receptor expression and diversity during development and differentiation may therefore play important roles for the inhibitory potential of the CNS during mature stages.

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.

Immunocytochemical localization of the GABAA receptor in the cerebral cortex of the lizard Psammodromus algirus

This study examined the distribution and localization of the gamma-aminobutyric acid (GABA)A receptor in the brain cortex of a reptile by light and electron microscopy, to test whether cortical GABA inhibition is mainly mediated through the GABAA receptor complex. We used preembedding immunocytochemistry and a monoclonal antibody, raised against the receptor complex, that recognizes the beta 2 and beta 3 subunits of the receptor. GABAA receptors were distributed throughout the entire cerebral cortex except the dorsomedial cortex. The immunostaining consisted of fine granules restricted to the plexiform layers of the cortex as seen by light microscopy. This granular aspect of the immunoreactivity most likely corresponds to the immunopositive dendritic and axonal profiles observed under the electron microscope. Some neurons in the medial and lateral cortices displayed patches of immunoreactivity along the cell body and processes, and as a result their morphology was outlined. We discuss the possibility that these neurons were GABAergic as well. The immunocytochemical data demonstrate that the distribution and localization of GABAA receptors in discrete regions of the reptilian cerebral cortex resemble that of parts of the hippocampal formation of humans and rats, suggesting that the basic configuration of the GABA system in these regions is conserved.

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.

Mechanisms of GABA and glycine depolarization-induced calcium transients in rat dorsal horn neurons

1. The mechanisms and effects of GABA- and glycine-evoked depolarization were studied in cultured rat dorsal horn neurons using indo-1 recordings of [Ca2+]i and patch clamp recordings in conventional whole-cell or perforated-patch mode. 2. Application of GABA to unclamped neurons caused [Ca2+]i increases that were dose dependent and exhibited GABAA receptor pharmacology. Calcium entered the neurons via high-threshold voltage-gated calcium channels (conotoxin and nimodipine sensitive). 3. In perforated-patch recordings employing cation-selective ionophores, GABAA receptor activation depolarized 123 of 132 cells to membrane potentials as depolarized as -33 mV (mean -50 mV in all 132 cells, +12 mV above resting potential). The ionic basis of the depolarization was determined by extracellular ion substitution; increased anionic conductance could account fully for the results. 4. Glycine, acting at a strychnine-sensitive receptor, also caused Ca2+ entry into these neurons through voltage-gated Ca2+ channels. Glycine and GABA both evoked [Ca2+]i responses in the same cells and the responses were highly correlated in amplitude. Glycine also depolarized all five cells tested with perforated recording. Each of the five cells was also depolarized by muscimol to a value similar to that obtained for glycine. 5. Both the depolarization and the increases in [Ca2+]i caused by GABA and glycine could potentially play a role in processes of development and differentiation and sensory transmission in the spinal cord dorsal horn.

Ontogeny of GABAA receptor subunit mRNAs in rat spinal cord and dorsal root ganglia

Relatively little is known about the development of GABAA receptor subunits and their gene expression in mammalian spinal cord. The expression of mRNAs encoding 13 GABAA receptor subunits (alpha 1-6, beta 1-3, gamma 1-3, and delta) in embryonic, postnatal, and adult rat spinal cord and dorsal root ganglia (DRG) cells were studied by in situ hybridization and reverse transcription-polymerase chain reaction (RT-PCR) analysis. Both techniques revealed the presence of all subunit mRNAs originally found in the rat brain, except for alpha 6, which was not detectable, and delta, which was weakly detected only by RT-PCR. Two anatomically distinctive sets of subunit mRNAs were found by in situ hybridization within the ventricular zone (VZ) and mantle zone (MZ). The trio of alpha 4, beta 1, and gamma 1 subunit mRNAs emerged exclusively in neuroepithelial cells at embryonic day 13 (E13) and remained detectable in the VZ until E17. In the MZ, beta 3 subunit mRNA was first detected at E12, while alpha 2, alpha 3, alpha 5, beta 2, gamma 2, and gamma 3 transcripts appeared at E13. Expressions of the subunit mRNAs in the MZ rapidly increased and expanded in a ventrodorsal sequence from motoneurons to dorsal horn neurons before reaching a peak in the late embryonic/early postnatal period. The mRNA expressions declined during postnatal development, by region-selective depletion, with alpha 4, alpha 5, beta 1, beta 2, gamma 1, and gamma 3 subunit mRNAs becoming barely detectable. In contrast, alpha 2, alpha 3, beta 3, and gamma 2 transcripts persisted into adulthood with distinct anatomical distributions. RT-PCR analysis revealed unique developmental patterns in the intensities of PCR products, most of which were in good agreement with developmental changes in the densities of hybridized mRNA signals. However, RT-PCR amplified minute amounts of mRNAs for alpha 1, alpha 4, alpha 5, beta 1, beta 2, gamma 1, gamma 3, and delta subunits in adults, which were not found in film autoradiograms, but could be detected in a few grain-positive cells in emulsion-dipped sections. DRG cells expressed alpha 2, alpha 3, alpha 5, beta 2, beta 3, and gamma 2 subunit mRNAs during embryogenesis but only alpha 2, beta 3, and gamma 2 subunit mRNAs were reliably detected in the adult.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic transmission and modulation in the olfactory bulb

Recent work in molecular biology and synaptic physiology has significantly increased our understanding of inhibitory and excitatory mechanisms in the olfactory bulb. Multiple subtypes of amino acid receptors with different functional and neuromodulatory properties are likely to play key roles in processing odor information transduced and relayed to the olfactory bulb by the olfactory sensory neurons, and in modulating that information during olfactory learning.

A cluster of three GABAA receptor subunit genes is deleted in a neurological mutant of the mouse p locus

The mouse pink-eyed cleft-palate (p(cp)) mutation is characterized by hypopigmentation associated with cleft palate, neurological disorders and runting. Most p(cp) homozygotes are born with cleft palate and die shortly after birth, presumably as a result of feeding problems. A few exceptional p(cp) mutants live beyond this stage but display tremor and jerky gait. We report here that the genes encoding the gamma-aminobutyric acid type A (GABAA) receptor subunits alpha 5 (originally described as alpha 4; ref. 4), beta 3 and gamma 3 are disrupted by a deletion in p(cp) mice. We also show that the alpha 5 and gamma 3 genes are located between the p and beta 3 genes on mouse chromosome 7. The p(cp) deletion leads to alterations of binding properties of the GABAA receptors in the brain, providing an in vivo model system for studying GABAA receptor function. The human homologue of the region deleted in p(cp) mice is associated with Angelman syndrome. Thus, p(cp) mice may be useful in defining the region containing the gene(s) for this syndrome.

Immunocytochemical study of GABAA receptors in the cat visual cortex

The laminar distribution and morphological structures associated with GABAA receptor immunoreactivity in the cat visual cortex were studied by using two different polyclonal antibodies directed either against the purified GABAA receptor protein (antibody "967") or against a specific domain of the beta 1-subunit of the GABAA receptor (antibody "Q"). Immunoblots of cat visual cortex tissue with these antibodies revealed that antibody "Q" recognizes only one subunit, namely the beta 1-subunit of the GABAA receptor, and that antibody "967" recognizes three subunits. Both antibodies produced very similar staining patterns, indicating that the beta 1-subunit may be an essential component of the GABAA receptor in the cat visual cortex. The typical staining pattern showed a clear membrane structure around neuronal somata. Using cell body shape criteria, immunopositive neurons included both pyramidal cells in cortical layers II, III, and V, and nonpyramidal cells in all cortical layers. Immunopositive neurons were uniformly distributed in layers II to VI, whereas the density of immunopositive cells in layer I was lower. Some immunopositive neurons were also found in the white matter underlying the visual cortex. In gray matter, immunopositive structures also included dendrites, especially the proximal dendrites, and axon initial segments of pyramidal neurons. The immunopositive processes usually ran vertically toward the pial surface. Some astrocytes were also immunostained. They were localized in layer I and in the white matter. The overall pattern of immunostaining was similar in areas 17, 18, and 19.

Co-existent expression of GABAA receptor beta 2, beta 3 and gamma 2 subunit messenger RNAs during embryogenesis and early postnatal development of the rat central nervous system

The expression of beta 1, beta 2, beta 3, gamma 2 and delta subunit messenger RNAs of the GABAA receptor was followed by in situ hybridization histochemistry using radiolabeled oligodeoxynucleotide probes in sections of embryonic (E12-21) and early postnatal (P1-5) rat. beta 2, beta 3 and gamma 2 subunit messenger RNAs were first detectable at E15 in the spinal cord (ventral > dorsal) and lower central nervous system regions (e.g. pons, medulla and thalamus). beta 3 subunit messenger RNA was abundantly expressed in olfactory bulb neurons at E15. At E17, the expression pattern of these subunit messenger RNAs continued in the lower central nervous system. In the upper central nervous system, beta 2, beta 3, and gamma 2 subunit messenger RNAs were first detectable in the outer layer of the hippocampal and entire cortical neuroepithelium. The expression for both beta 3 and gamma 2 subunit messenger RNAs increased significantly over that observed at E15, whereas beta 2 subunit messenger RNA increased to a lesser extent and was more discretely expressed in inferior colliculus, cerebellar neuroepithelium and spinal cord (ventral = dorsal). By E19, messenger RNAs for beta 2, beta 3 and gamma 2 subunits a widespread and abundant co-existent distribution throughout the central nervous system. Exceptions to this co-expression were the absence of beta 2 messenger RNA in the dentate gyrus and beta 3 messenger RNA in entorhinal cortex, areas in which they are present in adult. There was also a differential distribution of subunit messenger RNAs in developing olfactory bulb at E19-20: the glomerular cells preferentially expressed beta 3 and gamma 2 subunit messenger RNAs; the mitral cells preferentially expressed beta 2 subunit messenger RNA; inner granule cells expressed moderate levels of beta 2, beta 3 and gamma 2 subunit messenger RNAs. Expression of beta 2, beta 3 and gamma 2 messenger RNAs was also anatomically co-existent at P5. In addition, significant expression of beta 1 and delta subunit messenger RNAs was apparent in hippocampus and entorhinal cortex. The identity of the gamma 2 expressed between E15 and E21 was shown to be mostly the short isoform of gamma 2 subunit messenger RNA. Expression of both forms was evident beginning around P3-5. These results indicate that during the late embryonic and early postnatal period of development, beta 2, beta 3 and gamma 2 subunit messenger RNAs are abundantly expressed and co-localized to most central nervous system regions.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal ontogenesis of neurons containing GABAA alpha 1 subunit mRNA in the rat forebrain

The expression of GABAA receptor alpha 1 subunit mRNA in the postnatal rat forebrain was examined by in situ hybridization histochemistry. In most regions, including the isocortex, olfactory bulb, amygdala, septum, nucleus of the diagonal band, bed nucleus of the stria terminalis, basal ganglia, thalamus, and hypothalamus, the expression of alpha 1 subunit mRNA was low at birth but showed a dramatic increase during the early postnatal period. Adult levels of expression were reached at around the second or third week of life in these regions. However, in the caudate-putamen, and the nucleus accumbens, the expression of this subunit was only transient.

GABAA receptor subunit messenger RNAs show differential expression during cortical development in the rat brain

Developmental changes of the expression of various GABAA receptor subunits (alpha 1, alpha 3, alpha 4, beta 1-3, and gamma 2) were examined in the fetal rat cerebral cortex using in situ hybridization histochemistry. The subunits showed three main patterns of development. The alpha 1 subunit showed the first pattern, in which no expression was observed during embryonic development. The alpha 4 and beta 1 subunits showed the second pattern, in which expression was observed in both the undifferentiated neuroepithelium and the developing cortical layers. The alpha 3, beta 2, beta 3, and gamma 2 subunits showed the third pattern, in which expression was only seen in the developing cortical layers. These findings strongly suggest the following: (i) the alpha 1 subunit is involved in GABAergic transmission in the mature cerebral cortex; (ii) the alpha 4 and beta 1 subunits are involved in both the differentiation of the neuroepithelium and the development of the cortical plate, and (iii) the alpha 3, beta 2, beta 3, and gamma 2 subunits are involved in the development of the cortical plate. Subunits already expressed on embryonic day 13 (beta 1, beta 3, and gamma 2) appear especially likely to have a special role in neuronal development.

Expression patterns of gamma-aminobutyric acid type A receptor subunit mRNAs in primary cultures of granule neurons and astrocytes from neonatal rat cerebella

Using a competitive polymerase chain reaction assay, we have quantitated the absolute amounts of mRNA encoding 14 distinct subunits of the gamma-aminobutyric acid type A (GABAA) receptor in primary cultures of rat cerebellar granule neurons and cerebellar astrocytes. We found that the total amount of GABAA receptor subunit mRNA in astrocytes was 2 orders of magnitude lower than in neuronal cells. Furthermore, granule cell cultures expressed all 14 different GABAA subunit mRNAs, while the astroglial cultures contained detectable amounts of all the subunits expressed by granule cells except the alpha 6 and the gamma 2L subunits. Of the alpha subunit family members, the alpha 1, alpha 5, and alpha 6 mRNAs were prominent in granule cells, while the alpha 1 and alpha 2 mRNAs were abundant in astrocytes. Of the beta receptor subunit mRNAs, the beta 1 and beta 3 mRNAs were abundantly expressed in both cultures. The gamma 2S and gamma 2L mRNAs constituted the great majority of gamma subunit mRNAs in neurons, while the gamma 1 subunit mRNA was the most abundant gamma subunit mRNA in astrocytes. When various allosteric modulators of GABAA receptors were tested electrophysiologically, methyl 6,7-dimethoxy-4-ethyl-beta-carboline- 3-carboxylate (DMCM) was the only one to modulate chloride currents elicited by GABA in a significantly different manner in granule cells (negative modulation) compared with astrocytes (positive modulation). The latter effect was previously observed in transiently expressed recombinant GABAA receptors containing a gamma 1 instead of a gamma 2 subunit. Our quantitative mRNA results suggest that an important molecular determinant responsible for the DMCM-positive modulatory effect on astroglial native GABAA receptors is the presence of the gamma 1 subunit in the receptor assembly.