gamma-Aminobutyric acid (GABA) induces a receptor-mediated reduction in GABAA receptor alpha subunit messenger RNAs in embryonic chick neurons in culture

Maternally derived hormones, neurosteroids and the development of behaviour

In a wide range of taxa, there is evidence that mothers adaptively shape the development of offspring behaviour by exposing them to steroids. These maternal effects have major implications for fitness because, by shaping early development, they can permanently alter how offspring interact with their environment. However, theory on parent-offspring conflict and recent physiological studies showing that embryos rapidly metabolize maternal steroids have placed doubt on the adaptive significance of these hormone-mediated maternal effects. Reconciling these disparate perspectives requires a mechanistic understanding of the pathways by which maternal steroids can influence neural development. Here, we highlight recent advances in developmental neurobiology and psychiatric pharmacology to show that maternal steroid metabolites can have direct neuro-modulatory effects potentially shaping the development of neural circuitry underlying ecologically relevant behavioural traits. The recognition that maternal steroids can act through a neurosteroid pathway has critical implications for our understanding of the ecology and evolution of steroid-based maternal effects. Overall, compared to the classic view, a neurosteroid mechanism may reduce the evolutionary lability of hormone-mediated maternal effects owing to increased pleiotropic constraints and frequently influence long-term behavioural phenotypes in offspring.

To what extent is it possible to dissociate the anxiolytic and sedative/hypnotic properties of GABAA receptors modulators?

The relatively common view indicates a possible dissociation between the anxiolytic and sedative/hypnotic properties of benzodiazepines (BZs). Indeed, GABAA receptor (GABAAR) subtypes have specific cerebral distribution in distinct neural circuits. Thus, GABAAR subtype-selective drugs may be expected to perform distinct functions. However, standard behavioral test assays provide limited direction towards highlighting new action mechanisms of ligands targeting GABAARs. Automated behavioral tests, lack sensitivity as some behavioral characteristics or subtle behavioral changes of drug effects or that are not considered in the overall analysis (Ohl et al., 2001) and observation-based analyses are not always performed. In addition, despite the use of genetically engineered mice, any possible dissociation between the anxiolytic and sedative properties of BZs remains controversial. Moreover, the involvement the different subtypes of GABAAR subtypes in the anxious behavior and the mechanism of action of anxiolytic agents remains unclear since there has been little success in the pharmacological investigations so far. This raises the question of the involvement of the different subunits in anxiolytic-like and/or sedative effects; and the actual implication of these subunits, particularly, α-subunits in the modulation of sedation and/or anxiety-related disorders. This present review was prompted by several conflicting studies on the degree of involvement of these subunits in anxiolytic-like and/or sedative effects. To this end, we explored the GABAergic system, particularly, the role of different subunits containing synaptic GABAARs. We report herein the targeting gene encoding the different subunits and their contribution in anxiolytic-like and/or sedative actions, as well as, the mechanism underlying tolerance to BZs.

Regulation of GABA(A) receptor subunit expression by pharmacological agents

The gamma-aminobutyric acid (GABA) type A receptor system, the main fast-acting inhibitory neurotransmitter system in the brain, is the pharmacological target for many drugs used clinically to treat, for example, anxiety disorders and epilepsy, and to induce and maintain sedation, sleep, and anesthesia. These drugs facilitate the function of pentameric GABA(A) receptors that exhibit widespread expression in all brain regions and large structural and pharmacological heterogeneity as a result of composition from a repertoire of 19 subunit variants. One of the main problems in clinical use of GABA(A) receptor agonists is the development of tolerance. Most drugs, in long-term use and during withdrawal, have been associated with important modulations of the receptor subunit expression in brain-region-specific manner, participating in the mechanisms of tolerance and dependence. In most cases, the molecular mechanisms of regulation of subunit expression are poorly known, partly as a result of neurobiological adaptation to altered neuronal function. More knowledge has been obtained on the mechanisms of GABA(A) receptor trafficking and cell surface expression and the processes that may contribute to tolerance, although their possible pharmacological regulation is not known. Drug development for neuropsychiatric disorders, including epilepsy, alcoholism, schizophrenia, and anxiety, has been ongoing for several years. One key step to extend drug development related to GABA(A) receptors is likely to require deeper understanding of the adaptational mechanisms of neurons, receptors themselves with interacting proteins, and finally receptor subunits during drug action and in neuropsychiatric disease processes.

The role of GABA(A) receptors in the acute and chronic effects of ethanol: a decade of progress

The past decade has brought many advances in our understanding of GABA(A) receptor-mediated ethanol action in the central nervous system. We now know that specific GABA(A) receptor subtypes are sensitive to ethanol at doses attained during social drinking while other subtypes respond to ethanol at doses attained by severe intoxication. Furthermore, ethanol increases GABAergic neurotransmission through indirect effects, including the elevation of endogenous GABAergic neuroactive steroids, presynaptic release of GABA, and dephosphorylation of GABA(A) receptors promoting increases in GABA sensitivity. Ethanol's effects on intracellular signaling also influence GABAergic transmission in multiple ways that vary across brain regions and cell types. The effects of chronic ethanol administration are influenced by adaptations in GABA(A) receptor function, expression, trafficking, and subcellular localization that contribute to ethanol tolerance, dependence, and withdrawal hyperexcitability. Adolescents exhibit altered sensitivity to ethanol actions, the tendency for higher drinking and longer lasting GABAergic adaptations to chronic ethanol administration. The elucidation of the mechanisms that underlie adaptations to ethanol exposure are leading to a better understanding of the regulation of inhibitory transmission and new targets for therapies to support recovery from ethanol withdrawal and alcoholism.

Plasticity of central autonomic neural circuits in diabetes

Regulation of energy metabolism is controlled by the brain, in which key central neuronal circuits process a variety of information reflecting nutritional state. Special sensory and gastrointestinal afferent neural signals, along with blood-borne metabolic signals, impinge on parallel central autonomic circuits located in the brainstem and hypothalamus to signal changes in metabolic balance. Specifically, neural and humoral signals converge on the brainstem vagal system and similar signals concentrate in the hypothalamus, with significant overlap between both sensory and motor components of each system and extensive cross-talk between the systems. This ultimately results in production of coordinated regulatory autonomic and neuroendocrine cues to maintain energy homeostasis. Therapeutic metabolic adjustments can be accomplished by modulating viscerosensory input or autonomic motor output, including altering parasympathetic circuitry related to GI, pancreas, and liver regulation. These alterations can include pharmacological manipulation, but surgical modification of neural signaling should also be considered. In addition, central control of visceral function is often compromised by diabetes mellitus, indicating that circuit modification should be studied in the context of its effect on neurons in the diabetic state. Diabetes has traditionally been handled as a peripheral metabolic disease, but the central nervous system plays a crucial role in regulating glucose homeostasis. This review focuses on key autonomic brain areas associated with management of energy homeostasis and functional changes in these areas associated with the development of diabetes.

BDNF selectively regulates GABAA receptor transcription by activation of the JAK/STAT pathway

The gamma-aminobutyric acid (GABA) type A receptor (GABA(A)R) is the major inhibitory neurotransmitter receptor in the brain. Its multiple subunits show regional, developmental, and disease-related plasticity of expression; however, the regulatory networks controlling GABA(A)R subunit expression remain poorly understood. We report that the seizure-induced decrease in GABA(A)R alpha1 subunit expression associated with epilepsy is mediated by the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway regulated by brain-derived neurotrophic factor (BDNF). BDNF- and seizure-dependent phosphorylation of STAT3 cause the adenosine 3',5'-monophosphate (cAMP) response element-binding protein (CREB) family member ICER (inducible cAMP early repressor) to bind with phosphorylated CREB at the Gabra1:CRE site. JAK/STAT pathway inhibition prevents the seizure-induced decrease in GABA(A)R alpha1 abundance in vivo and, given that BDNF is known to increase the abundance of GABA(A)R alpha4 in a JAK/STAT-independent manner, indicates that BDNF acts through at least two distinct pathways to influence GABA(A)R-dependent synaptic inhibition.

GABAC receptor subunit mRNA expression in the rat superior colliculus is regulated by calcium channels, neurotrophins, and GABAC receptor activity

The distribution of mRNA for the rho2 subunit of the GABA(C) receptor is much broader in organotypic SC cultures than in vivo, suggesting that GABA(C) receptor expression is regulated by environmental factors. Electrophysiological recordings indicate that neurons in SC cultures have functional GABA(C) receptors, although these receptors exhibited smaller conductance than in vivo, probably due to increased rho2 subunit expression. Adding cortical input, treatment with various neuromodulators, and blocking neuronal activity with TTX failed to affect the expression of rho2 subunits. Electrophysiological recordings revealed the presence of spontaneous Ca(2+) currents in SC cultures and preventing these, by treatment with blockers of L-type Ca(2+) channels, caused rho2 mRNA expression to decline to in vivo levels. In contrast, rho1 subunit mRNA levels remained unchanged, indicating that the two subunits are independently regulated. Surprisingly, both tonic activation and blockade of GABA(C) receptors upregulated rho1/rho2 mRNA expression. Further, NGF and BDNF promoted such expression during an early postnatal time window. In vivo, expression of the rho2 mRNA in the SC, and the rho2/rho3 mRNA in the retina increased with age. Expression of the rho2 mRNA in the visual cortex, and the rho1 mRNA in the retina and SC was constant. Subunit mRNA expression was similar in dark-reared animals, indicating that visual experience has no influence. These experiments suggest that GABA(C) receptor expression in the SC is regulated during postnatal development. While visual experience seems to have no influence on GABA(C) receptor subunits, spontaneous calcium currents selectively promote rho2 expression and both rho1 and rho2 are autoregulated both by GABA(C) receptor activity and by neurotrophic factors.

Surface expression of GABAA receptors is transcriptionally controlled by the interplay of cAMP-response element-binding protein and its binding partner inducible cAMP early repressor

The regulated expression of type A gamma-aminobutyric acid (GABA) receptor (GABA(A)R) subunit genes plays a critical role in neuronal maturation and synaptogenesis. It is also associated with a variety of neurological diseases. Changes in GABA(A) receptor alpha1 subunit gene (GABRA1) expression have been reported in animal models of epilepsy, alcohol abuse, withdrawal, and stress. Understanding the genetic mechanism behind such changes in alpha subunit expression will lead to a better understanding of the role that signal transduction plays in control over GABA(A)R function and brings with it the promise of providing new therapeutic tools for the prevention or cure of a variety of neurological disorders. Here we show that activation of protein kinase C increases alpha1 subunit levels via phosphorylation of CREB (pCREB) that is bound to the GABRA1 promoter (GABRA1p). In contrast, activation of protein kinase A decreases levels of alpha1 even in the presence of pCREB. Decrease of alpha1 is dependent upon the inducible cAMP early repressor (ICER) as directly demonstrated by ICER-induced down-regulation of endogenous alpha1-containing GABA(A)Rs at the cell surface of cortical neurons. Taken together with the fact that there are less alpha1gamma2-containing GABA(A)Rs in neurons after protein kinase A stimulation and that activation of endogenous dopamine receptors down-regulates alpha1 subunit mRNA levels subsequent to induction of ICER, our studies identify a transcriptional mechanism for regulating the cell surface expression of alpha1-containing GABA(A)Rs that is dependent upon the formation of CREB heterodimers.

Stress, ethanol, and neuroactive steroids

Neurosteroids play a crucial role in stress, alcohol dependence and withdrawal, and other physiological and pharmacological actions by potentiating or inhibiting neurotransmitter action. This review article focuses on data showing that the interaction among stress, ethanol, and neuroactive steroids may result in plastic molecular and functional changes of GABAergic inhibitory neurotransmission. The molecular mechanisms by which stress-ethanol-neuroactive steroids interactions can produce plastic changes in GABA(A) receptors have been studied using different experimental models in vivo and in vitro in order to provide useful evidence and new insights into the mechanisms through which acute and chronic ethanol and stress exposure modulate the activity of GABAergic synapses. We show detailed data on a) the effect of acute and chronic stress on peripheral and brain neurosteroid levels and GABA(A) receptor gene expression and function; b) ethanol-stimulated brain steroidogenesis; c) plasticity of GABA(A) receptor after acute and chronic ethanol exposure. The implications of these new mechanistic insights to our understanding of the effects of ethanol during stress are also discussed. The understanding of these neurochemical and molecular mechanisms may shed new light on the physiopathology of diseases, such as anxiety, in which GABAergic transmission plays a pivotal role. These data may also lead to the need for new anxiolytic, hypnotic and anticonvulsant selective drugs devoid of side effects.

Regionally selective effects of GABA on hypothalamic GABAA receptor mRNA in vitro

We tested whether GABAA receptor (R) subunit mRNA levels are homeostatically influenced by short-term exposure to GABA in two adjacent regions of the posterior hypothalamus. mRNA levels for seven GABAAR subunits and GABA-synthesizing enzyme (GAD) were quantified in the perifornical (PF) and dorsomedial (DM) hypothalamus following superfusion of slices for 90 min with a drug-free medium, GABA uptake blocker with or without GABAAR antagonist, gabazine, or GABAAR agonist with tetrodotoxin. Increasing endogenous GABA decreased mRNAs for all seven GABAAR subunits in the PF, and for three also in the DM, region; gabazine antagonized these effects in the PF region only and increased GAD-65 mRNA. Stimulation of GABAARs in the presence of tetrodotoxin decreased mRNA for one GABAAR subunit (beta1). We conclude that, in the PF region where GABA facilitates sleep, increased GABA release may limit GABAAR-mediated inhibition, whereas in the DM region, GABA-induced changes are mainly mediated by non-GABAA receptors.

GABA induces activity dependent delayed-onset uncoupling of GABA/benzodiazepine site interactions in neocortical neurons

Changes in the function of type A gamma-aminobutyric acid receptors (GABA(A)Rs) are associated with neuronal development and tolerance to the sedative-hypnotic effects of GABA(A)R positive modulators. Persistent activation of GABA(A)Rs by millimolar concentrations of GABA occurs under physiological conditions as GABAergic fast-spiking neurons in neocortex and cerebellum exhibit basal firing rates of 5 to 50 Hz and intermittent rates up to 250 Hz, leaving a substantial fraction of synaptic receptors occupied persistently by GABA. Persistent exposure of neurons to GABA has been shown to cause a down-regulation of receptor number and an uncoupling of GABA/benzodiazepine (BZD) site interactions with a half-life of approximately 24 h. Here, we report that a single brief exposure of neocortical neurons in primary culture to GABA for 5-10 min (t(1/2) = 3.2 +/- 0.2 min) initiates a process that results in uncoupling hours later (t(1/2) = 12.1 +/- 2.2 h). Initiation of delayed-onset uncoupling is blocked by co-incubation with picrotoxin or alpha-amanitin but is insensitive to nifedipine, indicating that uncoupling is contingent upon receptor activation and transcription but is not dependent on voltage-gated Ca2+ influx. Delayed-onset uncoupling occurs without a change in receptor number or a change in the proportion of alpha1 subunit pharmacology, as zolpidem binding affinity is unaltered. Such activity dependent latent modulation of GABA(A)R function that manifests as delayed-onset uncoupling may be relevant to physiological, pathophysiological, and pharmacological conditions where synaptic receptors are transiently exposed to GABA agonists for several minutes.

Modulation of GABAA receptor gene expression by allopregnanolone and ethanol

Expression of specific gamma-aminobutyric acid type A (GABA(A)) receptor subunit genes in neurons is affected by endogenous modulators of receptor function such as neuroactive steroids. This effect of steroids appears to be mediated through modulation of GABA(A) receptor signalling mechanisms that control the expression of specific receptor subunit genes. Furthermore, the specific outcomes of such signalling appear to differ among neurons in different regions of the brain. Neuroactive steroids such as the progesterone metabolite allopregnanolone might thus exert differential effects on GABA(A) receptor plasticity in distinct neuronal cell populations, likely accounting for some of the physiological actions of these compounds. Here we summarise experimental data obtained both in vivo and in vitro that show how fluctuations in the concentration of allopregnanolone regulate both the expression and function of GABA(A) receptors and consequently affect behaviour. Such regulation is operative both during physiological conditions such as pregnancy and lactation as well as in pharmacologically induced states such as pseudopregnancy and long-term treatment with steroid derivatives or anxiolytic-hypnotic drugs. Accordingly, long-lasting exposure of GABA(A) receptors to ethanol, as well as its withdrawal, induces marked effects on receptor structure and function. These results suggest the possible synergic action between endogenous steroids and ethanol in modulating the functional activity of specific neuronal populations.

GABAA receptors: building the bridge between subunit mRNAs, their promoters, and cognate transcription factors

The type A gamma-aminobutyric acid (GABA(A)) receptors mediate the majority of fast inhibitory neurotransmission in the CNS, and alterations in GABA(A) receptor function is believed to be involved in the pathology of several neurological and psychiatric illnesses, such as epilepsy, anxiety, Alzheimer's disease, and schizophrenia. GABA(A) receptors can be assembled from eight distinct subunit families defined by sequence similarity: alpha(1-6), beta(1-3), gamma(1-3), delta, pi, theta, and rho(1-3). The regulation of GABA(A) receptor function in the brain is a highly compensating system, influencing both the number and the composition of receptors at the cell surface. While transcriptional and translational points of control operate in parallel, it is becoming increasingly evident that many functional changes in GABA(A) receptors reflect the differential gene regulation of its subunits. The fact that certain GABA(A) receptor subunit genes are transcribed in distinct cell types during specific periods of development strongly suggests that genetic control plays a major role in the choice of subunit variants available for receptor assembly. This review focuses on the physiological conditions that alter subunit mRNA levels, the promoters that may control such levels, and the use of a conceptual framework created by bioinformatics to study coordinate and independent GABA(A) receptor subunit gene regulation. As this exciting field moves closer to identifying the language hidden inside the chromatin of GABA(A) receptor subunit gene clusters, future experiments will be aimed at testing models generated by computational analysis with biologically relevant in vivo and in vitro assays. It is hoped that through this functional genomic approach there will be the identification of new targets for therapeutic intervention.

Normalization of glutamate decarboxylase gene expression in the entopeduncular nucleus of rats with a unilateral 6-hydroxydopamine lesion correlates with increased GABAergic input following intermittent but not continuous levodopa

The expression of mRNA encoding for the 67 kilodalton isoform of glutamate decarboxylase (GAD67) was examined by in situ hybridization histochemistry in the entopeduncular nucleus (EP) of adult rats with a 6-hydroxydopamine unilaterally lesion of dopamine neurons. Our results provide original evidence that continuous or intermittent levodopa administration is equally effective at reversing the lesion-induced increase in GAD67 mRNA expression in the EP when compared with vehicle controls. To characterize the GABAergic interactions that may mediate levodopa-induced alterations in the EP, double-labeling in situ hybridization was conducted with a combination of GAD67 radioactive and preproenkephalin or preprotachykinin digoxigenin-labeled complementary RNA probes in the striatum. Levels of GAD67 mRNA labeling were significantly increased by intermittent, but not continuous levodopa. Analysis at the cellular level in a dorsal sector of the striatum revealed that GAD67 mRNA levels increased predominantly in preproenkephalin-unlabeled neuronal profiles, presumably striatal/EP neurons (+99.3%). Saturation analyses of (3)H-flunitrazepam binding to GABA(A) receptors in the EP showed that the increase in GAD67 mRNA in preproenkephalin-unlabeled neurons by intermittent levodopa paralleled a significant decrease in number of GABA(A) receptors (Bmax) in the EP ipsilateral to the lesion. Continuous levodopa failed to alter striatal GAD67 mRNA levels, or the number or affinity of GABA(A) receptors when compared with vehicle-treated controls. These results suggest the normalization of GAD gene expression in the EP by intermittent levodopa involves an increase in GABAergic inhibition by striatonigral/EP neurons of the direct pathway. Conversely, the effects of continuous levodopa on GAD mRNA levels in the EP do not appear to be mediated by GABA.

Prolonged exposure to γ-aminobutyric acid up-regulates stably expressed recombinant α1β2γ2s GABAA receptors

The aim of this study was to better understand the mechanisms that underlie adaptive changes in GABA(A) receptors following their prolonged exposure to drugs. Exposure (48 and/or 96 h) of human embryonic kidney (HEK 293) cells stably expressing recombinant alpha1beta2gamma2s GABA(A) receptors for gamma-aminobutyric (GABA, 1 mM) and muscimol (100 microM), but not for diazepam (1 microM), enhanced the maximum number (B(max)) of [3H]flunitrazepam binding sites without affecting their affinity (K(d)). The GABA-induced enhancement in B(max) was reduced by the GABA receptor antagonist, bicuculline (100 microM), and by cycloheximide (10 microl/ml), a protein synthesis inhibitor. GABA (100 microM) enhanced the affinity of [3H]flunitrazepam binding to vehicle- and GABA-pretreated, but not to diazepam-pretreated, HEK 293 cells. The results suggest that chronic GABA treatment up-regulates stably expressed GABA(A) receptors, presumably by stimulating their synthesis. Unlike chronic diazepam, which produced functional uncoupling of GABA and benzodiazepine binding sites, chronic GABA failed to produce this effect.

Molecular mechanisms of tolerance to and withdrawal of GABA(A) receptor modulators

Here, we summarize recent data pertaining to the effects of GABA(A) receptor modulators on the receptor gene expression in order to elucidate the molecular mechanisms behind tolerance and dependence induced by these drugs. Drug selectivity and intrinsic activity seems to be important to evidence at the molecular level the GABA(A) receptor tolerance. On the contrary, we suggested that all drug tested are equally potentially prone to induce dependence. Our results demonstrate that long-lasting exposure of GABA(A) receptors to endogenous steroids, benzodiazepines and ethanol, as well as their withdrawal, induce marked effects on receptor structure and function. These results suggest the possible synergic action between endogenous steroids and these drugs in modulating the functional activity of specific neuronal populations. We report here that endogenous steroids may play a crucial role in the action of ethanol on dopaminergic neurons.

Effects of prenatal malnutrition on GABAA receptor α1, α3 and β2 mRNA levels

Exposure of pregnant rats to protein malnutrition throughout pregnancy alters the developing hippocampus, leading to increased inhibition and selective changes in hippocampal-mediated behaviors. Given that GABA mediates most inhibitory neurotransmission, we asked whether selective changes in the levels of GABA receptor subunit mRNAs might result. Quantitative RNase protection profiling of 12 GABAA and GABAB receptor subunit mRNAs show that alpha1 and beta2 decrease in the adult (P90) hippocampal formation of prenatally malnourished rats, while the levels of alpha3 are increased. Moreover, the distribution of alpha1, alpha3 and beta2 mRNAs remains unchanged in CA1 and CA3 hippocampal subfields relative to dentate gyrus. The data suggest that prenatal malnutrition produces global changes of certain GABAA, but not GABAB, receptor mRNAs in the hippocampal formation.

Bicuculline seizure susceptibility and nigral GABAA alpha1 receptor mRNA is altered in adult prenatally morphine-exposed females

Prenatal morphine exposure (5-10 mg/kg twice daily on gestation days 11-18) can adversely affect neurological development, including seizure susceptibility. The present study examines the effects of prenatal morphine exposure on seizure susceptibility to the GABA antagonist and convulsant bicuculline and GABA(A) alpha(1) receptor mRNA in the substantia nigra (SN) of female rats. The results demonstrate that prenatally morphine-exposed ovariectomized (OVX) females and OVX females with estradiol benzoate (EB) replacement have an increased latency to seizure onset compared to controls. In addition, prenatal morphine exposure decreases the area covered by grains of GABA(A) alpha(1) receptor mRNA in the anterior SN in both OVX and EB+progesterone (P)-treated groups, and decreases the number of GABA(A) alpha(1) receptor mRNA-labeled cells/field in EB females. Furthermore, prenatally morphine- and saline-exposed EB and EB+P females had decreased GABA(A) alpha(1) receptor mRNA-labeled cells/field in the anterior SN compared to OVX animals of the same prenatal exposure. These results demonstrate that the long term effects of prenatal morphine exposure in female rats is dependent on their hormonal status, and suggest that seizure susceptibility may be altered via neuropharmacological changes in the GABA system in the SN.

GABAA-receptor plasticity during long-term exposure to and withdrawal from progesterone

The subunit composition of native gamma-aminobutyric acid type A (GABAA) receptors is an important determinant of the role of these receptors in the physiological and pharmacological modulation of neuronal excitability and associated behavior. GABAA receptors containing the alpha 1 subunit mediate the sedative-hypnotic effects of benzodiazepines (Rudolph et al., 1999; McKernan et al., 2000), whereas the anxiolytic effects of these drugs are mediated by receptors that contain the alpha 2 subunit (Löw et al., 2000). In contrast, GABAA receptors containing the alpha 4 or alpha 6 subunits are insensitive to benzodiazepines (Barnard et al., 1998). Characterization of the functions of GABAA-receptors thus requires an understanding of the mechanisms by which the receptor subunit composition is regulated. The expression of specific GABAA-receptor subunit genes in neurons is affected by endogenous and pharmacological modulators of receptor function. The expression of GABAA-receptor subunit genes is thus regulated by neuroactive steroids both in vitro and in vivo. Such regulation occurs both during physiological conditions, such as pregnancy, and during pharmacologically induced conditions, such as pseudo-pregnancy and long-term treatment with steroid derivatives or anxiolytic-hypnotic drugs. Here, we summarize results obtained by our laboratory and by other groups pertaining to the effects of long-term exposure to, and subsequent withdrawal from, progesterone and its metabolite 3 alpha,5 alpha-tetrahydroprogesterone on both the expression of GABAA-receptor subunits and GABAA-receptor function.

Role of allopregnanolone in regulation of GABA(A) receptor plasticity during long-term exposure to and withdrawal from progesterone

Here we summarize recent data from our laboratory pertaining to the effects of fluctuations in the brain concentrations of the progesterone (PROG) metabolite allopregnanolone (3alpha,5alpha-TH PROG) on the expression and function of gamma-aminobutyric acid type A (GABA(A)) receptors. The effects of long-term exposure to progesterone and of its sudden withdrawal on the activity of GABA(A) receptors and on the abundance of receptor subunit mRNAs were examined in cultured rat cerebellar granule cells and cortical neurons. The effects of a persistent reduction in the brain concentration of 3alpha,5alpha-TH PROG on GABA(A) receptor function and gene expression were examined in vivo in rats subjected to long-term administration of oral contraceptives. Our results demonstrate that long-lasting changes in the exposure of GABA(A) receptors to this PROG metabolite induce marked effects on receptor structure and function. These effects of 3alpha,5alpha-TH PROG appear to be mediated through modulation of GABA(A) receptor signaling mechanisms that control the expression of specific receptor subunit genes. Furthermore, the specific outcomes of such signaling appear to differ among neurons derived from different regions of the brain. Neuroactive steroids such as 3alpha,5alpha-TH PROG might thus exert differential actions on GABA(A) receptor plasticity in distinct neuronal cell populations, likely accounting for some of the physiological effects induced by these compounds.

Distinct signal transduction pathways for GABA-induced GABA(A) receptor down-regulation and uncoupling in neuronal culture: a role for voltage-gated calcium channels

Changes in GABA receptor (GABA(A)R) gene expression are detected in animal models of epilepsy, anxiety and in post-mortem schizophrenic brain, suggesting a role for GABA(A)R regulation in neurological disorders. Persistent (48 h) exposure of brain neurons in culture to GABA results in down-regulation of GABA(A)R number and uncoupling of GABA and benzodiazepine (BZD) binding sites. Given the central role of GABA(A)Rs in fast inhibitory synaptic transmission, GABA(A)R down-regulation and uncoupling are potentially important mechanisms of regulating neuronal excitability, yet the molecular mechanisms remain unknown. In this report we show that treatment of brain neurons in culture with tetrodotoxin, glutamate receptor antagonists, or depolarization with 25 mM K(+) fails to alter GABA(A)R number or coupling. Changes in neuronal activity or membrane potential are therefore not sufficient to induce either GABA(A)R down-regulation or uncoupling. Nifedipine, a voltage-gated Ca(2+) channel (VGCC) blocker, inhibits both GABA-induced increases in [Ca(2+)](i) and GABA(A)R down-regulation, suggesting that VGCC activation is required for GABA(A)R down-regulation. Depolarization with 25 mM K(+) produces a sustained increase in intracellular [Ca(2+)] without causing GABA(A)R down-regulation, suggesting that activation of VGCCs is not sufficient to produce GABA(A)R down-regulation. In contrast to GABA(A)R down-regulation, nifedipine and 25 mM K(+) fail to inhibit GABA-induced uncoupling, demonstrating that GABA-induced GABA(A)R down-regulation and uncoupling are mediated by independent molecular events. Therefore, GABA(A)R activation initiates at least two distinct signal transduction pathways, one of which involves elevation of intracellular [Ca(2+)] through VGCCs.

Time-course effects of a single administration of cocaine on receptor binding and subunit mRNAs of GABA(A) receptors

We investigated the time-course effects of a single administration of cocaine (20 mg/kg) on GABA(A) receptor binding labeled by t-[(35)S]butylbicyclophophorothionate (TBPS) and on several types of GABA(A) receptor subunit mRNAs in the rat brain by in vitro quantitative receptor autoradiography and in situ hybridization. The levels of alpha 1, beta 2, and beta 3 subunit mRNAs in several brain regions such as the cortex, cerebellum, and striatum were significantly decreased within 1 h, while beta 3 subunit mRNA was increased in the dentate gyrus. All of these changes were transient, occurring within 1 h after the injection of cocaine. In the cortex and cerebellum, the reduction in alpha1 subunit mRNA was followed by a significant decrease in [(35)S]TBPS receptor binding, which occurred 4 h after cocaine injection. These findings suggest that acute cocaine administration discretely regulates GABA(A) receptor subunit mRNA levels in several brain regions through a change in transcription or turnover rates of subunit mRNAs, which may be closely related to cocaine-induced behavioral abnormalities.

Development and influence of inhibition in the lateral superior olivary nucleus

While studies of neuronal development and plasticity have focused on excitatory pathways, the inhibitory projection from the MNTB to the LSO provides a favorable model for studies of synaptic inhibition. This review covers recent studies from our laboratories indicating that inhibitory connections are quite dynamic during development. These findings suggest that there are two phases inhibitory transmission. During an initial depolarizing phase is growth and branching of pre- and postsynaptic elements in the LSO. During a second hyperpolarizing phase there is refinement of inhibitory afferent arborizations and the LSO dendrites that they innervate.

An initiator element mediates autologous downregulation of the human type A gamma -aminobutyric acid receptor beta 1 subunit gene

The regulated expression of type A gamma-aminobutyric acid receptor (GABA(A)R) subunit genes is postulated to play a role in neuronal maturation, synaptogenesis, and predisposition to neurological disease. Increases in GABA levels and changes in GABA(A)R subunit gene expression, including decreased beta1 mRNA levels, have been observed in animal models of epilepsy. Persistent exposure to GABA down-regulates GABA(A)R number in primary cultures of neocortical neurons, but the regulatory mechanisms remain unknown. Here, we report the identification of a TATA-less minimal promoter of 296 bp for the human GABA(A)R beta1 subunit gene that is neuron specific and autologously down-regulated by GABA. beta1 promoter activity, mRNA levels, and subunit protein are decreased by persistent GABA(A)R activation. The core promoter, 270 bp, contains an initiator element (Inr) at the major transcriptional start site. Three concatenated copies of the 10-bp Inr and its immediate 3' flanking sequence produce full neural specific activity that is down-regulated by GABA in transiently transfected neocortical neurons. Taking these results together with those of DNase I footprinting, electrophoretic mobility shift analysis, and 2-bp mutagenesis, we conclude that GABA-induced down-regulation of beta1 subunit mRNAs involves the differential binding of a sequence-specific basal transcription factor(s) to the Inr. The results support a transcriptional mechanism for the down-regulation of beta1 subunit GABA(A)R gene expression and raises the possibility that altered levels of sequence-specific basal transcription factors may contribute to neurological disorders such as epilepsy.

Chronic blockade of N-methyl-D-aspartate receptors alters gamma-aminobutyric acid type A receptor peptide expression and function in the rat

Chronic in vivo or in vitro application of GABA(A) receptor agonists alters GABA(A) receptor peptide expression and function. Furthermore, chronic in vitro application of N-methyl-D-aspartate (NMDA) agonists and antagonists alters GABA(A) receptor function and mRNA expression. However, it is unknown if chronic in vivo blockade of NMDA receptors alters GABA(A) receptor function and peptide expression in brain. Male Sprague-Dawley rats were chronically administered the noncompetitive NMDA receptor antagonist MK-801 (0.40 mg/kg, twice daily) for 14 days. Chronic blockade of NMDA receptors significantly increased hippocampal GABA(A) receptor alpha4 and gamma2 subunit expression while significantly decreasing hippocampal GABA(A) receptor alpha2 and beta2/3 subunit expression. Hippocampal GABA(A) receptor alpha1 subunit peptide expression was not altered. In contrast, no significant alterations in GABA(A) receptor subunit expression were found in cerebral cortex. Chronic MK-801 administration also significantly decreased GABA(A) receptor-mediated hippocampal Cl- uptake, whereas no change was found in GABA(A) receptor-mediated cerebral cortical Cl- uptake. Finally, chronic MK-801 administration did not alter NMDA receptor NR1, NR2A, or NR2B subunit peptide expression in either the cerebral cortex or the hippocampus. These data demonstrate heterogeneous regulation of GABA(A) receptors by glutamatergic activity in rat hippocampus but not cerebral cortex, suggesting a new mechanism of GABA(A) receptor regulation in brain.

Turnover and down-regulation of GABA(A) receptor alpha1, beta2S, and gamma1 subunit mRNAs by neurons in culture

Benzodiazepines (BZDs), barbiturates, ethanol, and general anesthetics potentiate the action of gamma-aminobutyric acid (GABA) at the type A GABA receptor (GABA(A)R) and have profound effects on mood, arousal, and susceptibility to seizures. GABA(A)R number and subunit mRNA levels change in animal models of epilepsy and anxiety and following exposure to GABA(A)R agonists and positive modulators, but the mechanism of receptor down-regulation remains unknown. Persistent exposure (48 h) of brain neurons in primary culture to GABA results in a 30% decrease in the levels of mRNA encoding the alpha1, beta2S, and gamma1 GABA(A)R subunit isoforms, which form a receptor enhanced by nonselective BZDs. Down-regulation of alpha1 mRNA (t1/2 = 8 h) precedes down-regulation of receptor number (t1/2 = 25 h), suggesting that GABA-induced GABA(A)R down-regulation is a consequence of decreased mRNA levels. The apparent half-life of the alpha1 mRNA in the presence of alpha-amanitin (9 h) is consistent with the time course of alpha1 mRNA down-regulation. Moreover, the stability of the alpha1, beta2S, and gamma1 subunit mRNAs is not altered by chronic GABA exposure. The results demonstrate that GABA(A)R subunit mRNA down-regulation is not a consequence of accelerated mRNA degradation and argue that GABA-induced GABA(A)R down-regulation is due to inhibition of transcription.

Aberrant expression of GABAA receptor subunits in the tottering mouse: an animal model for absence seizures

The single-locus mutant mouse tottering (tg) is an established model for absence seizures. We have previously reported an impairment in GABA-induced chloride uptake in tg brain [Tehrani and Barnes, Epilepsy Res. 1995;22:13-21]. In order to determine if this alteration in GABAA receptor function can be related to specific receptor isoforms, we examined the radioligand binding properties of GABAA receptors and the expression of GABAA receptor subunit mRNAs in the cerebral cortex. Saturation binding of [3H]flunitrazepam revealed a significantly lower Kd value in tg cortical tissues (1.77 +/- 0.05 nM) in comparison to that for the background C57BL/6J strain (3.23 +/- 0.23 nM), while the Bmax values were indistinguishable. Biphasic displacement of [3H]flunitrazepam binding by 2-oxoquazepam showed that low affinity binding sites account for 36 +/- 7.6 and 51 +/- 7.5% of the total in control and tg, respectively. The level of [35S]-t-butylbicyclophosphorothionate (TBPS) binding to tg cortical membranes was 73.6 +/- 5.8% of that in controls. Paired measurements by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) revealed no significant differences in the levels of GABAA receptor alpha 1, alpha 3, alpha 5, beta 2, beta 3, gamma 2 or gamma 3 subunit mRNAs between tg and control cortex. However, tg tissues showed elevated levels of alpha 2- and beta 1-subunit mRNAs, representing 256 and 177%, respectively, those of controls. For the tg cortex, the enhanced expression of GABAA receptor alpha 2 and beta 1 subunits correlates with recombinant subtypes known to have low affinity for 2-oxoquazepam and impaired binding of TBPS. These aberrant properties of GABAA receptors could influence the development or propagation of phenotypic seizures in the tottering mouse.

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.

Tolerance to diazepam and changes in GABA(A) receptor subunit expression in rat neocortical areas

Long-term treatment with diazepam, a full allosteric modulator of the GABA(A) receptor, results in tolerance to its anticonvulsant effects, whereas an equipotent treatment with the partial allosteric modulator imidazenil does not produce tolerance. Use of subunit-specific antibodies linked to gold particles allowed an immunocytochemical estimation of the expression density of the alpha1, alpha2, alpha3, alpha5, gamma(2L&S) and beta(2/3) subunits of the GABA(A) receptor in the frontoparietal motor and frontoparietal somatosensory cortices of rats that received long-term treatment with vehicle, diazepam (three times daily for 14 days, doses increasing from 17.6 to 70.4 micromol/kg), or imidazenil (three times daily for 14 days, doses increasing from 2.5 to 10.0 micromol/kg). In this study, tolerance to diazepam was associated with a selective decrease (37%) in the expression of the alpha1 subunit in layers III-IV of the frontoparietal motor cortex, and a concomitant increase in the expression of the alpha5 (150%), gamma(2L&S) and beta(2/3) subunits (48%); an increase in alpha5 subunits was measured in all cortical layers. In the frontoparietal somatosensory cortex, diazepam-tolerant rats had a 221% increase in the expression of alpha5 subunits in all cortical layers, as well as a 35% increase in the expression of alpha3 subunits restricted to layers V-VI. Western blot analysis substantiated that these diazepam-induced changes reflected the expression of full subunit molecules. Rats that received equipotent treatment with imidazenil did not become tolerant to its anticonvulsant properties, and did not show significant changes in the expression of any of the GABA(A) receptor subunits studied, with the exception of a small decrease in alpha2 subunits in cortical layers V-VI of the frontoparietal somatosensory cortex. The results of this study suggest that tolerance to benzodiazepines may be associated with select changes in subunit abundance, leading to the expression of different GABA(A) receptor subtypes in specific brain areas. These changes might be mediated by a unique homeostatic mechanism regulating the expression of GABA(A) receptor subtypes that maintain specific functional features of GABAergic function in cortical cell layers.

Repression of gamma-aminobutyric acid type A receptor alpha1 polypeptide biosynthesis requires chronic agonist exposure

Although it is well established that the number of gamma-aminobutyric acid type A (GABAA) receptors declines in cortical neurons exposed to GABAA receptor agonists, the mechanisms responsible for this use-dependent down-regulation remain unclear. Two hypotheses have been proposed: (i) agonist-evoked sequestration and degradation of surface GABAA receptors and (ii) repression of receptor subunit biosynthesis. We have addressed this problem using [35S]Met/Cys pulse-chase labeling of chick cortical neurons in culture and immunoprecipitation and immunoblotting with an antibody (RP4) directed against a GABAA receptor alpha1-(331-381) fusion protein. Exposure of the cells to GABA or isoguvacine for 2 h to 4 days had no effect on the initial rate of 35S incorporation into the GABAA receptor 51-kDa alpha1 polypeptide, but this rate declined by 33% after a 7-day treatment. This is consistent with a previous report (Baumgartner, B. J., Harvey, R. J., Darlison, M. G., and Barnes, E. M. (1994) Mol. Brain Res. 26, 9-17) that a 7-day GABA treatment of this preparation produced a 45% reduction in the alpha1 subunit mRNA level, while a 4-day exposure had no detectable effect. On the other hand, after a 4-day exposure to these agonists, a 30% reduction in the level of the alpha1 polypeptide was observed on immunoblots, similar to that found previously for down-regulation of GABAA receptor ligand-binding sites. Thus, the de novo synthesis of GABAA receptor alpha1 subunits is subject to a delayed use-dependent repression that was observed after, rather than before, the decline in neuronal levels of the polypeptide. Pulse-chase experiments showed a monophasic degradation of the GABAA receptor 35S-alpha1 subunit with a t1/2 = 7.7 h, a process that was unaffected by the addition of GABA to neurons during the chase period. These nascent 35S-labeled polypeptides are presumably diluted into the neuronal pool of unlabeled unassembled alpha1 subunits, which was found to exceed by a 4:1 molar ratio the amount assembled into [3H]flunitrazepam binding sites. Thus, the data reveal an alternative scheme for degradation of GABAA receptor polypeptides: a pathway that may participate in the agonist-independent degradation of unassembled receptor subunits. This differs from another pathway for the agonist-dependent degradation of mature GABAA receptors derived from the neuronal surface (Calkin, P. A., and Barnes, E. M., Jr. (1994) J. Biol. Chem. 269, 1548-1553).

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.

Alcohol, alcoholic brain damage, and GABAA receptor isoform gene expression

Selective variations in cerebral GABAA receptor pharmacology and function are observed in experimental animals subjected to a number of alcohol-treatment and -withdrawal paradigms, and where human alcoholics with and without a range of concomitant diseases are compared with non-alcoholic cases. Recombination studies have shown that variations in GABAA receptor pharmacology and function can result from altering its subunit isoform composition. This commentary examines the rôle of subunit isoform expression in the response to long-term alcohol administration in animals, and in the pathogenesis of alcoholism-related brain damage in human cases.

The effects of single and repeated phencyclidine administration on [125I] iomazenil binding in the rat brain

We measured [125I] iomazenil binding, labeling the central-type benzodiazepine receptor in 37 discrete rat brain areas following single (7.5 mg/kg, i.p.) and repeated (7.5 mg/kg/day x 14 days, i.p.) treatment with phencyclidine (PCP), a non-competitive antagonist of the N-methyl-D-aspartate(NMDA)-type glutamate receptor, using in vitro quantitative autoradiographic receptor binding assay. Both single and repeated PCP treatment produced heterogeneous changes in the rat brain in a similar manner, the magnitude of change in [125I] iomazenil binding being generally greater in the repeated treatment group than in the single treatment group. A significant increase in [125I] iomazenil binding was observed in the superficial layer (layer I-IV) of the parietal cortex in both of the PCP treatment groups and the CA1 of the hippocampus of the repeated PCP-treated group. There was a significant decrease in [125I] iomazenil binding in the piriform cortex of the repeated PCP-treated group. These results suggest that the blockade of NMDA receptor-mediated glutamatergic neurotransmission by PCP produces the compensational alterations in the central-type benzodiazepine receptor antagonist binding, and that the observed diversity may be due to dissimilar modes of organizations between glutamatergic and the GABA(gamma-aminobutyric acid)-benzodiazepine receptor complex.

Molecular mechanisms of benzodiazepine-induced down-regulation of GABAA receptor alpha 1 subunit protein in rat cerebellar granule cells

1. Chronic benzodiazepine treatment of rat cerebellar granule cells induced a transient down-regulation of the gamma-aminobutyric acidA (GABAA) receptor alpha 1 subunit protein, that was dose-dependent (1 nM-1 microM) and prevented by the benzodiazepine antagonist flumazenil (1 microM). After 2 days of treatment with 1 microM flunitrazepam the alpha 1 subunit protein was reduced by 41% compared to untreated cells, which returned to, and remained at, control cell levels from 4-12 days of treatment. Chronic flunitrazepam treatment did not significantly alter the GABAA receptor alpha 6 subunit protein over the 2-12 day period. 2. GABA treatment for 2 days down-regulates the alpha 1 subunit protein in a dose-dependent (10 microM-1 mM) manner that was prevented by the selective GABAA receptor antagonist bicuculline (10 microM). At 10 microM and 1 mM GABA the reduction in alpha 1 subunit expression compared to controls was 31% and 66%, respectively. 3. The flunitrazepam-induced decrease in alpha 1 subunit protein is independent of GABA, which suggests that it involves a mechanism distinct from the GABA-dependent action of benzodiazepines on GABAA receptor channel activity. 4. Simultaneous treatment with flunitrazepam and GABA did not produce an additive down-regulation of alpha 1 subunit protein, but produced an effect of the same magnitude as that of flunitrazepam alone. This down-regulation induced by the combination of flunitrazepam and GABA was inhibited by flumazenil (78%), but unaffected by bicuculline. 5. The flunitrazepam-induced down-regulation of alpha 1 subunit protein at 2 days was completely reversed by the protein kinase inhibitor staurosporine (0.3 microM). 6. This study has shown that both flunitrazepam and GABA treatment, via their respective binding sites, caused a reduction in the expression of the GABAA receptor alpha 1 subunit protein; an effect mediated through the same neurochemical mechanism. The results also imply that the benzodiazepine effect is independent of GABA, and that the benzodiazepine and GABA sites may not be equally coupled to the down-regulation process, with the benzodiazepine site being the more dominant. The biochemical mechanism underlying the benzodiazepine-mediated down-regulation of the alpha 1 subunit protein seems to involve the activity of staurosporine-sensitive protein kinases.

Treatment with an antisense oligodeoxynucleotide to the GABAA receptor gamma 2 subunit increases convulsive threshold for beta-CCM, a benzodiazepine "inverse agonist', in rats

The gamma 2 subunit of the gamma-aminobutyric acid type-A (GABAA) receptor is associated with the actions of benzodiazepines and related drugs. A phosphorothioate-modified antisense oligodeoxynucleotide directed against the gamma 2 subunit was given by i.c.v. injection (18 micrograms in 2 microliters saline) to male Sprague-Dawley rats every 12 h for 3 days. Controls received the corresponding sense oligodeoxynucleotide. 4-6 h after the last i.c.v. treatment, rats were given methyl-beta-carboline-3-carboxylate (beta-CCM), a benzodiazepine "inverse agonist', by slow i.v. infusion. Compared to naive rats, the beta-CCM threshold dose was not affected by the sense oligodeoxynucleotide, but was increased 87% in antisense oligodeoxynucleotide-treated rats. The treatment had no effect on the seizure threshold for picrotoxin. Both antisense and sense oligodeoxynucleotide treatments slightly increased the threshold for strychnine seizures. The results suggest that antisense oligodeoxynucleotide treatment altered GABAA receptor composition and interfered with the actions of a benzodiazepine receptor ligand in vivo, and may provide a tool for studying regulation of receptor structure and function.

Pharmacology of barbiturate tolerance/dependence: GABAA receptors and molecular aspects

Barbiturates are central nervous system depressants that are used as sedatives, hypnotics, anesthetics and anticonvulsants. However, prolonged use of the drugs produces physical dependence, and the drugs have a high abuse liability. The gamma-aminobutyric acidA (GABAA) receptor is one of barbiturates' main sites of action, and therefore it is thought to play a pivotal role in the development of tolerance to and dependence on barbiturates. Recent advances in the study of the GABAA receptor/chloride channel complex allow us to examine possible mechanisms that underlie barbiturate tolerance/dependence in a new light. In this minireview, we mainly focus on molecular and cellular aspects of the action of barbiturates and the possible mechanisms that contribute to development of tolerance to and dependence on barbiturates.

Down-regulation of benzodiazepine receptors by ethyl beta-carboline-3-carboxylate in cerebrocortical neurons

Effect of exposure of primary cultured cerebral cortical neurons to ethyl beta-carboline-3-carboxylate (beta-CCE) on the function of benzodiazepine receptors was studied. Exposure of neurons to beta-CCE (0.1-10 microM) decreased the binding of [3H]flunitrazepam to extensively washed membrane fractions in a dose- and time-dependent manner, whereas the binding of [3H]flunitrazepam to the cytosolic fractions increased (180%) under the same conditions as described above. Ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a] [1,4] benzodiazepine-3-carboxylate (Ro15-1788), an antagonist of the central type of benzodiazepine receptors, completely abolished the beta-CCE-induced decrease in [3H]flunitrazepam binding and the IC50 value for [3H]flunitrazepam binding to the extensively washed membrane fractions prepared from beta-CCE-treated neurons was similar to that from non-treated neurons. Scatchard analysis revealed that only the Bmax value for [3H]flunitrazepam binding decreased after the exposure to beta-CCE (1 microM) for 12 h, although the Kd value was not altered. These results indicate that beta-CCE induces the down-regulation of benzodiazepine receptors by an increase in benzodiazepine receptor internalization.

Chronic pentobarbital administration alters gamma-aminobutyric acidA receptor alpha 6-subunit mRNA levels and diazepam-insensitive [3H]Ro15-4513 binding

In order to study the chronic effects of pentobarbital, a positive GABAA receptor modulator, on the inverse agonist binding of the benzodiazepine site, binding of [3H]Ro15-4513 and levels of GABAA receptor alpha 6-subunit mRNA were investigated in the brains of pentobarbital-tolerant/dependent animals, using receptor autoradiography and in situ hybridization histochemistry in consecutive brain sections. Pentobarbital was administered to rats either 60 mg/kg, i.p., once, for acute treatment, or 300 micrograms/10 microliters/h i.c.v. continuously for 6 days via osmotic minipumps to render rats tolerant to pentobarbital. Rats assigned to the dependent group were sacrificed 24 h after discontinuance of pentobarbital infusion, while those assigned to the tolerant group were sacrificed at the end of infusion. The alpha 6 subunit mRNA was increased in the tolerant group only. Diazepam-insensitive [3H]Ro15-4513 binding was increased in the cerebellar granule layer of pentobarbital-tolerant and -dependent rats. No alterations in these parameters were observed in acutely treated animals. These data suggest that chronic pentobarbital treatment induced expression of alpha 6-subunit mRNA. This was in contrast to alpha 1- and gamma 2-subunit mRNA, which in tolerant animals are unchanged, but for which withdrawal triggers a surge in levels. Because the alpha 6-subunit is a major component of the diazepam-insensitive [3H]Ro15-4513 binding site, the increased diazepam-insensitive [3H]Ro15-4513 binding implied de novo synthesis of the receptor subunit protein.

gamma-aminobutyric acidA receptor regulation: heterologous uncoupling of modulatory site interactions induced by chronic steroid, barbiturate, benzodiazepine, or GABA treatment in culture

Prolonged administration of anxiolytic, sedative, and anticonvulsant drugs that act through the GABAA receptor (GABAAR) can evoke tolerance and dependence, suggesting the existence of an endogenous mechanism(s) for altering the ability of such agents to interact with the GABAAR. Uncoupling appears to be one such mechanism. This is a decrease in the allosteric interactions between the benzodiazepine (BZD) recognition site and other agonist or modulator sites on the GABAAR, as measured by potentiation of [3H]flunitrazepam ([3H]FNZ) binding. To investigate the mechanism(s) of uncoupling, neuronal cultures were treated chronically with 3 alpha-hydroxy-5 beta-pregnan-20-one (pregnanolone), pentobarbital, flurazepam, or GABA, then tested for enhancement of [3H]FNZ binding by these substances. The results indicate that BZDs, barbiturates, and steroids, as well as GABA itself, are capable of inducing both heterologous and homologous uncoupling. Surprisingly, different chronic drug treatments produce different patterns of homologous and heterologous uncoupling. Chronic exposure to pregnanolone, GABA, flurazepam or pentobarbital induces complete uncoupling of barbiturate-BZD site interactions, partial uncoupling of GABA-BZD site interactions, but different amounts of uncoupling of steroid-BZD site interactions. In addition, the EC50 for pregnanolone-induced homologous uncoupling (1.7 microM) is over an order of magnitude greater than that for heterologous uncoupling of GABA and BZD sites (82 nM). Moreover, heterologous uncoupling by pregnanolone is inhibited by the GABA site antagonist SR-95531, whereas homologous uncoupling by pregnanolone is resistant to SR-95531. Therefore, there are at least two distinct ways in which GABAAR modulatory site interactions can be regulated by chronic drug treatment.

Regulation of GABAA receptor structure and function by chronic drug treatments in vivo and with stably transfected cells

In this article, we review the use of stably transfected cells to study the regulation of receptor structure and function by chronic drug treatments and compare results from these cells to results obtained from other systems, including neuronal cultures and intact animals. We focus on the gamma-aminobutyric acid type A (GABAA) receptor complex. Sedative/hypnotic drugs such as benzodiazepines, barbiturates and alcohol that potentiate GABAA receptor function produce tolerance and dependence. Chronic treatment of GABAA receptor preparations from brain and neuronal cultures with GABAA agonists, as well as these other three classes of drugs, results in regulation of several properties of the receptor. Drug treatments may regulate levels of binding sites, allosteric binding interactions, receptor function, levels of receptor subunit mRNA and levels of receptor subunit protein. Some or all of these effects may comprise the molecular mechanisms of tolerance to these GABAA-modulatory drugs. The use of cells stably transfected with neurotransmitter receptors provides a homogeneous population that can be cultured under controlled conditions. As most preparations contain mixed populations of GABAA receptor subunits, stably transfected cells offer the advantage of the expression of receptors with a defined subunit composition. We conclude that chronic drug treatments regulate allosteric coupling and function of GABAA receptors in stably transfected cells. This regulation does not appear to be due to decreases in the expression of alpha 1- or beta 1-receptor subunits or to expression of subunits other than alpha 1, beta 1, gamma 2L. Therefore, it is unlikely to be due to changes in receptor subunit composition and probably represents post-translational changes. The rapid regulation of allosteric coupling and function by drug treatment of the stably transfected cells should provide insights to the mechanisms of coupling between GABAA and benzodiazepine receptors as well as tolerance and dependence of benzodiazepines and ethanol.

Use-dependent regulation of GABAA receptors

Prolonged occupancy of GABAA receptors by ligands, including GABA and benzodiazepine agonists, sets in motion a series of mechanisms that can be termed use-dependent regulation. These mechanisms can be subdivided into two distinct pathways, one for GABAA receptor downregulation and another for upregulation. Treatment of cortical neurons with GABA or benzodiazepines in cultures opens the pathway for GABAA receptor downregulation, which includes (in putative temporal order): (1) desensitization (tachyphylaxis), (2) sequestration (endocytosis) of subunit polypeptides and uncoupling of allosteric interactions between GABA and benzodiazepine binding sites, (3) subunit polypeptide degradation, and (4) repression of subunit gene expression. The end-point of GABAA receptor downregulation, a reduction in receptor number, is postulated to be established initially by degradation of the receptor protein and then maintained by a diminished level of de novo synthesis. Benzodiazepine treatment of many preparations, including cells expressing recombinant GABAA receptors, may elicit only desensitization, sequestration, or uncoupling, without a decline in receptor number. Components of the GABAA receptor downregulation pathway are also evoked by chronic administration of GABAmimetics, benzodiazepines, barbiturates, and neurosteroids in animals. This downregulation correlates with the establishment of tolerance to and physical dependence on the pharmacological effects of these drugs, suggesting a cellular model for this behavior. The upregulation of GABAA receptors is observed as one of the neurotrophic actions of GABA, primarily in cultured cerebellar granule cells. Upregulation in culture is caused by enhanced expression of genes for GABAA receptor subunits and correlates with the establishment of GABAergic circuitry in the developing cerebellum. Thus, both the upregulation and downregulation of GABAA receptors appear to represent use-dependent pathways for guiding synaptic plasticity in the vertebrate central nervous system.

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.

GABAA receptor function and binding in stably transfected cells: chronic ethanol treatment

Effects of chronic ethanol exposure on GABAA receptors may contribute to tolerance and dependence to alcohol. Ethanol treatment of mice and rats can produce alterations of GABAA receptor binding, function, and subunit mRNA and protein levels. We treated a cell line (PA3 cells) that stably expresses GABAA receptors chronically with ethanol. Expression of bovine alpha 1, beta 1, and gamma 2L GABAA receptor subunits genes in these cells is controlled by a dexamethasone-sensitive promoter, and this provides an excellent system to study the regulation by chronic ethanol treatment of receptors with a defined subunit composition. The actions of the GABA agonist muscimol on receptor function (36Cl- uptake) were not affected by 100 mM ethanol treatment for 4 days, but the actions of flunitrazepam (1 microM) were decreased in cells treated with ethanol. The functional coupling between benzodiazepine and GABA sites on the receptors was affected by chronic ethanol treatment in a manner consistent with results from mice. Ethanol treatments (50 or 100 mM) for 4 days did not affect the affinity (Kd) or receptor density (Bmax) of [3H]flunitrazepam binding, or the levels of alpha 1 subunit mRNA, or alpha 1 or beta 1 subunit proteins. These results demonstrate that the regulation of the stably expressed GABAA receptors by chronic ethanol, in the absence of neuronal receptor gene promoters, is posttranscriptional and likely posttranslational.

Long-term and regional specific changes in [3H]flunitrazepam binding in kindled rat hippocampus

The binding of the GABAA/benzodiazepine receptor agonist [3H]flunitrazepam was studied in the hippocampus of rats kindled by daily stimulation of the Schaffer collaterals, using semi-quantitative autoradiography. Two kindled stages were investigated: (i) 24 h after the last generalized tonic-clonic seizure (fully kindled) and (ii) 28 days after the last generalized seizure (long-term). The binding of [3H]flunitrazepam was determined at two concentrations, 3 and 16 nM. In the CA1 area, we found a small but significant decrease (ca. 10%), both in the 3 and 16 nM [3H]flunitrazepam binding at the fully kindled stage. In contrast, there was a significant increase in the 3 nM binding (c. 15%) at the long-term stage. The 16 nM binding was not significantly different from control binding at this stage. In the granular and molecular layers of the fascia dentata, we found at both kindled stages a significantly increased 3 nM (ca. 9 and 19%, respectively) and 16 nM (ca. 19 and 14%, respectively) binding. Furthermore, we found that muscimol was still able to enhance the [3H]flunitrazepam binding in kindled animals, indicating that the GABAA receptor agonist binding site and benzodiazepine agonist binding site are still functionally coupled. The changes in [3H]flunitrazepam binding at the fully kindled stage are in agreement with the recently observed kindling-induced changes in [3H]muscimol binding in the hippocampal formation of the same animals [Titulaer M. N. G. et al. (1994) Neuroscience 59, 817-826] and extend these observations to the benzodiazepine modulatory site of the GABAA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of GABAA receptor subunit mRNAs in hippocampal pyramidal and granular neurons in the kindling model of epileptogenesis: an in situ hybridization study

To investigate the molecular changes underlying kindling epileptogenesis in the rat hippocampus, the expression levels of the genes encoding for 13 different gamma-aminobutyric acid type-A receptor (GABAAR) subunits were measured in hippocampal principal neurons using in situ hybridization techniques and semi-quantitative analysis of the autoradiograms. Schaffer collateral-commissural pathway kindled rats were investigated at three different stages of kindling acquisition, at 24 h after the last seizure and at long-term (28 days) after termination of kindling stimulations. Changes were distinct for the different subunits in the three analyzed regions (CA1, CA3, fascia dentata) and also different for the various kindling stages. In all hippocampal areas at the early phases of kindling epileptogenesis, before the appearance of generalized seizures, an increase was found of those transcripts that constituted the majority of the expressed variants in control animals (alpha 1, alpha 2, alpha 4, beta 1, beta 2, beta 3, gamma 2/gamma 2L mRNA). In these stages, the increased levels of different variants in the granular neurons of the fascia dentata were more pronounced when compared to the pattern of changes in pyramidal cells of CA1 and CA3. In fully kindled animals, the expression levels of several subunits returned to control levels, whereas beta 3 and gamma 2/gamma 2L mRNA expression was still significantly enhanced in all areas. At long-term, few changes were encountered. The long-splice variant of gamma 2 was decreased within pyramidal and granular neurons while the total level of gamma 2 mRNA was not different from controls. The increased GABAAR subunit expression in the fascia dentata may underly the reported increased GABAAR ligand binding and the increased GABA mediated inhibition. However, the decreased GABAAR binding and the attenuation of GABAergic inhibition in CA1, could not be explained by a decrement of receptor subunit expression.

Autoradiographic analysis of [35S]t-butylbicyclophosphorothionate binding in kindled rat hippocampus shows different changes in CA1 area and fascia dentata

We studied the binding of [35S]t-butylbicyclophosphorothionate to the GABAA receptor-mediated chloride channel in the CA1 area and fascia dentata of control and Schaffer collateral kindled rats, by means of semi-quantitative autoradiography. The [35S]t-butylbicyclophosphorothionate binding was determined at three stages during kindling acquisition: (i) after six afterdischarges, (ii) after 14 afterdischarges and (iii) after the induction of fully kindled seizures. Furthermore, the binding was studied at the long-term stage, 28 days after the last generalized tonic-clonic seizure [Racine R. J. (1972) Electroenceph. clin. Neurophysiol. 32, 281-294]. The binding was investigated at three [35S]t-butylbicyclophosphorothionate concentrations, 4, 47.5 (KD value) and 180 nM (Bmax value). A significant decrease in [35S]t-butylbicyclophosphorothionate binding in the CA1 area (-6 to -20%) and hilar formation (-17 to -37%), in one or more of the three [35S]t-butylbicyclophosphorothionate concentrations tested at the six and 14 afterdischarges and fully kindled stages was observed, but no significant changes at the long-term kindling stage were found. In contrast, the granular and molecular layers of the fascia dentata presented a significant increase in [35S]t-butylbicyclophosphorothionate binding (+15 to +38%) at the 14 afterdischarges, fully kindled and long-term kindled stages.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and sequence analysis of the chicken GABAA receptor alpha 1-subunit gene promoter

A genomic clone containing the 5'-flanking sequence of the chicken GABAA receptor alpha 1-subunit-encoding gene (GabR alpha 1) was isolated and characterized. An intron was found to interrupt the 5'-untranslated region. The transcription start point (tsp) was determined by primer extension, RNase protection and the amplification of chick brain first-strand cDNA. DNA sequence analysis revealed a number of putative transcriptional regulatory motifs, including a TATA box 30 nucleotides upstream from the tsp, and that this region is a CpG island. While there is conservation between the chicken and human GabR alpha 1 sequences, the chicken GabR alpha 1 promoter has a different structure to those reported for the GABAA receptor beta 3- and delta-subunit-encoding genes.