Chronic diazepam exposure decreases transcription of the rat GABA(A) receptor gamma2-subunit gene

GABAA receptor subtypes and benzodiazepine use, misuse, and abuse

Benzodiazepines have been in use for over half a century. While they remain highly prescribed, their unfavorable side-effect profile and abuse liability motivated a search for alternatives. Most of these efforts focused on the development of benzodiazepine-like drugs that are selective for specific GABAA receptor subtypes. While there is ample evidence that subtype-selective GABAA receptor ligands have great potential for providing symptom relief without typical benzodiazepine side-effects, it is less clear whether subtype-selective targeting strategies can also reduce misuse and abuse potential. This review focuses on the three benzodiazepine properties that are relevant to the DSM-5-TR criteria for Sedative, Hypnotic, or Anxiolytic Use Disorder, namely, reinforcing properties of benzodiazepines, maladaptive behaviors related to benzodiazepine use, and benzodiazepine tolerance and dependence. We review existing evidence regarding the involvement of different GABAA receptor subtypes in each of these areas. The reviewed studies suggest that α1-containing GABAA receptors play an integral role in benzodiazepine-induced plasticity in reward-related brain areas and might be involved in the development of tolerance and dependence to benzodiazepines. However, a systematic comparison of the contributions of all benzodiazepine-sensitive GABAA receptors to these processes, a mechanistic understanding of how the positive modulation of each receptor subtype might contribute to the brain mechanisms underlying each of these processes, and a definitive answer to the question of whether specific chronic modulation of any given subtype would result in some or all of the benzodiazepine effects are currently lacking from the literature. Moreover, how non-selective benzodiazepines might lead to the maladaptive behaviors listed in DSM and how different GABAA receptor subtypes might be involved in the development of these behaviors remains unexplored. Considering the increasing burden of benzodiazepine abuse, the common practice of benzodiazepine misuse that leads to severe dependence, and the current efforts to generate side-effect free benzodiazepine alternatives, there is an urgent need for systematic, mechanistic research that provides a better understanding of the brain mechanisms of benzodiazepine misuse and abuse, including the involvement of specific GABAA receptor subtypes in these processes, to establish an informed foundation for preclinical and clinical efforts.

The Cys-loop pentameric ligand-gated ion channel receptors: 50 years on

This year, 2011, the Department of Pharmacology at the University of Alberta celebrated its 50th anniversary. This timeframe covers nearly the entire history of Cys-loop pentameric ligand-gated ion channel (pLGIC) research. In this review we consider how major technological advancements affected our current understanding of pLGICs, and highlight the contributions made by members of our department. The individual at the center of our story is Susan Dunn; her passing earlier this year has robbed the Department of Pharmacology and the research community of a most insightful colleague. Her dissection of ligand interactions with the nAChR, together with their interpretation, was the hallmark of her extensive collaborations with Michael Raftery. Here, we highlight some electrophysiological studies from her laboratory over the last few years, using the technique that she introduced to the department in Edmonton, the 2-electrode voltage-clamp of Xenopus oocytes. Finally, we discuss some single-channel studies of the anionic GlyR and GABA(A)R that prefaced the introduction of this technique to her laboratory.

Switching from long-term benzodiazepine therapy to pregabalin in patients with generalized anxiety disorder: a double-blind, placebo-controlled trial

To evaluate the efficacy of pregabalin in facilitating taper off chronic benzodiazepines, outpatients (N = 106) with a lifetime diagnosis of generalized anxiety disorder (current diagnosis could be subthreshold) who had been treated with a benzodiazepine for 8-52 weeks were stabilized for 2-4 weeks on alprazolam in the range of 1-4 mg/day. Patients were then randomized to 12 weeks of double-blind treatment with either pregabalin 300-600 mg/day or placebo while undergoing a gradual benzodiazepine taper at a rate of 25% per week, followed by a 6-week benzodiazepine-free phase during which they continued double-blind study treatment. Outcome measures included ability to remain benzodiazepine-free (primary) as well as changes in Hamilton Anxiety Rating Scale (HAM)-A and Physician Withdrawal Checklist (PWC). At endpoint, a non-significant higher proportion of patients remained benzodiazepine-free receiving pregabalin compared with placebo (51.4% vs 37.0%). Treatment with pregabalin was associated with significantly greater endpoint reduction in the HAM-A total score versus placebo (-2.5 vs +1.3; p < 0.001), and lower endpoint mean PWC scores (6.5 vs 10.3; p = 0.012). Thirty patients (53%) in the pregabalin group and 19 patients (37%) in the placebo group completed the study, reducing the power to detect a significant difference on the primary outcome. The results on the anxiety and withdrawal severity measures suggest that switching to pregabalin may be a safe and effective method for discontinuing long-term benzodiazepine therapy.

The efficacy and safety of alprazolam versus other benzodiazepines in the treatment of panic disorder

We performed a meta-analysis of all single- or double-blind, randomized controlled trials comparing alprazolam to another benzodiazepine in the treatment of adult patients meeting the Diagnostic and Statistical Manual of Mental Disorders, Third or Fourth Edition, criteria for panic disorder or agoraphobia with panic attacks. Eight studies met inclusion criteria, describing a total of at least 631 randomized patients. In the pooled results, there were no significant differences in efficacy between alprazolam and the comparator benzodiazepines on any of the prespecified outcomes: improvement in mean panic attack frequency (between-arm weighted mean difference of 0.6 panic attacks per week; 95% confidence interval [CI], -0.3 to 1.6), improvement in Hamilton Anxiety Rating Scale score (weighted mean difference of 0.8 points; 95% CI, -0.5 to 2.1), and proportion of patients free of panic attacks at the final evaluation (pooled relative risk, 1.1; 95% CI, 0.9-1.4). Statistical heterogeneity on prespecified outcomes was not eliminated by stratification on baseline anxiety level. The available evidence fails to demonstrate alprazolam as superior to other benzodiazepines for the treatment of panic disorder.

Tolerance to allopregnanolone with focus on the GABA-A receptor

Many studies have suggested a relationship between stress, sex steroids, and negative mental and mood changes in humans. The progesterone metabolite allopregnanolone is a potent endogenous ligand of the γ-amino butyric acid -A (GABA-A) receptor, and the most discussed neuroactive steroid. Variations in the levels of neuroactive steroids that influence the activity of the GABA-A receptor cause a vulnerability to mental and emotional pathology. There are physiological conditions in which allopregnanolone production increases acutely (e.g. stress) or chronically (e.g. menstrual cycle, pregnancy), thus exposing the GABA-A receptor to high and continuous allopregnanolone concentrations. In such conditions, tolerance to allopregnanolone may develop. We have shown that both acute and chronic tolerances can develop to the effects of allopregnanolone. Following the development of acute allopregnanolone tolerance, there is a decrease in the abundance of the GABA-A receptor α4 subunit and the expression of the α4 subunit mRNA in the ventral-posteriomedial nucleus of the thalamus. Little is known about the mechanism behind allopregnanolone tolerance and its effects on assembly of the GABA-A receptor composition. The exact mechanism of the allopregnanolone tolerance phenomena remains unclear. The purpose of this review is to summarize certain aspects of current knowledge concerning allopregnanolone tolerance and changes in the GABA-A receptors.

The effects of zolpidem treatment and withdrawal on the in vitro expression of recombinant alpha1beta2gamma2s GABA(A) receptors expressed in HEK 293 cells

Zolpidem, a widely used hypnotic drug which acts through benzodiazepine binding sites, is a positive allosteric modulator of gamma-aminobutyric acid (GABA) action with preferential affinity for GABA(A) receptors containing alpha1 subunit. The pharmacological profile of zolpidem is different from that of classical benzodiazepines. The aim of this study was to find out whether zolpidem treatment triggers adaptive changes in the recombinant alpha1 subunit-containing GABA(A) receptors other than those observed following treatment with classical benzodiazepine-diazepam. Radioligand binding studies showed that 2-day exposure of human embryonic kidney (HEK) 293 cells stably expressing recombinant alpha1beta2gamma2s GABA(A) receptors to zolpidem (10 muM) up-regulated the maximum number (B (max)) of [(3)H]flunitrazepam, [(3)H]muscimol, and [(3)H]t-butylbicycloorthobenzoate ([(3)H]TBOB) binding sites without changing their affinity (K (d)), suggesting an increase in total GABA(A) receptor number. Semi-quantitative RT-PCR analysis demonstrated increased levels of alpha1 subunit mRNA, while Western blot demonstrated up-regulated gamma2 subunit proteins, suggesting that zolpidem induced de novo synthesis of receptors proteins, at both the transcriptional and translational levels. GABA-induced potentiation of [(3)H]flunitrazepam binding to membranes obtained from zolpidem-treated cells was markedly reduced, indicating allosteric uncoupling between GABA and benzodiazepine binding sites. The number of benzodiazepine and convulsant binding sites as well as the functional coupling between GABA and benzodiazepine binding sites normalized in 24 h following discontinuation of zolpidem treatment. The results of our in vitro studies suggest that a 2-day exposure of recombinant alpha1 subunit-containing GABA(A) receptors stably transfected in HEK 293 cells to zolpidem induces adaptive changes in this selective GABA(A) receptor subtype, which are not substantially different from those obtained after prolonged exposure of cells to high concentrations of diazepam.

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.

Differential effects of diazepam treatment and withdrawal on recombinant GABAA receptor expression and functional coupling

Prolonged exposure to benzodiazepines, drugs known to produce tolerance and dependence and also to be abused, leads to adaptive changes in GABA(A) receptors. To further explore the mechanisms responsible for these phenomena, we studied the effects of prolonged diazepam treatment on the recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors, stably expressed in human embryonic kidney (HEK) 293 cells. The results demonstrating that long-term (48 and 72 h) exposure of cells to a high concentration of diazepam (50 microM) enhanced the maximum number (B(max)) of [(3)H]flunitrazepam, [(3)H]muscimol and [(3)H]t-butylbicycloorthobenzoate ([(3)H]TBOB) binding sites, without changing their affinity (K(d)), suggested the up-regulation of GABA(A) receptors. As demonstrated by cell counting and WST-1 proliferation assay, the observed increase in receptor expression was not a consequence of stimulated growth of cells exposed to diazepam. Semi-quantitative RT-PCR and Western blot analysis, showing elevated levels of alpha(1) subunit mRNA as well as beta(2) and gamma(2) subunit proteins, respectively, suggested that prolonged high dose diazepam treatment induced de novo receptor synthesis by acting at both transcriptional and translational levels. The finding that the number of GABA(A) receptor binding sites returned to control value 24 h following diazepam withdrawal, makes this process less likely to account for the development of benzodiazepine tolerance and dependence. On the other hand, the results demonstrating that observed functional uncoupling between GABA and benzodiazepine binding sites persisted after the termination of diazepam treatment supported the hypothesis of its possible role in these phenomena.

Abuse and dependence liability of benzodiazepine-type drugs: GABA(A) receptor modulation and beyond

Over the past several decades, benzodiazepines and the newer non-benzodiazepines have become the anxiolytic/hypnotics of choice over the more readily abused barbiturates. While all drugs from this class act at the GABA(A) receptor, benzodiazepine-type drugs offer the clear advantage of being safer and better tolerated. However, there is still potential for these drugs to be abused, and significant evidence exists to suggest that this is a growing problem. This review examines the behavioral determinants of the abuse and dependence liability of benzodiazepine-type drugs. Moreover, the pharmacological and putative biochemical basis of the abuse-related behavior is discussed.

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.

Seizures and Antiepileptic Drugs: Does Exposure Alter Normal Brain Development?

Seizures and antiepileptic drugs (AEDs) affect brain development and have long-term neurological consequences. The specific molecular and cellular changes, the precise timing of their influence during brain development, and the full extent of the long-term consequences of seizures and AEDs exposure have not been established. This review critically assesses both the basic and clinical science literature on the effects of seizures and AEDs on the developing brain and finds that evidence exists to support the hypothesis that both seizures and antiepileptic drugs influence a variety of biological process, at specific times during development, which alter long-term cognition and epilepsy susceptibility. More research, both clinical and experimental, is needed before changes in current clinical practice, based on the scientific data, can be recommended.

Long-term effects of diazepam and phenobarbital treatment during development on GABA receptors, transporters and glutamic acid decarboxylase

Diazepam (DZ) and phenobarbital (PH) are commonly used to treat early-life seizures and act on GABAA receptors (GABAR). The developing GABAergic system is highly plastic, and the long-term effects of postnatal treatment with these drugs on the GABAergic system has not been extensively examined. In the present study, we investigated the effects of prolonged DZ and PH treatment during postnatal development and then discontinuation on expression of a variety of genes involved in GABAergic neurotransmission during adulthood. Rat pups were treated with DZ, PH or vehicle from postnatal day (P) 10-P40 and then the dose was tapered for 2 weeks and terminated at P55. Expression of GABAR subunits, GABAB receptor subunits, GABA transporters (GAT) and GABA synthesizing enzymes (glutamic acid decarboxylase: GAD) mRNAs in hippocampal dentate granule neurons (DGNs) were analyzed using antisense RNA amplification at P90. Protein levels for the alpha1 subunit of GABAR, GAD67, GAT1 and 3 were also assessed using Western blotting. At P90, mRNA expression for GAT-1, 3, 4, GABAR subunits alpha4, alpha6, beta3, delta and theta and GABAB receptor subunit R1 was increased and mRNA expression for GAD65, GAD67 and GABAR subunits alpha1 and alpha3 were decreased in DGNs of rats treated with DZ and PH. The current data suggest that prolonged DZ and PH treatment during postnatal development causes permanent alterations in the expression of hippocampal GABA receptor subunits, GATs and GAD long after therapy has ended.

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.

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.

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.

Hippocampal hyperexcitability induced by GABA withdrawal is due to down-regulation of GABA(A) receptors

The sudden interruption of an intracortical instillation of exogenous gamma-aminobutyric acid (GABA) generates an epileptic focus in mammals. Seizures elicited by GABA withdrawal (GW) last for weeks. A similar withdrawal-induced hyperexcitability is also produced by several GABA(A) receptor agonists. This work reports a quantitative analysis of GW-induced hyperexcitability produced in the hippocampus in vitro. GW produced a left-ward displacement of the input/output (I/O) function, suggesting that the postsynaptic component is predominant to explain the hyperexcitability. A decrease in the inhibitory efficacy of the GABA(A) receptor agonist, muscimol, confirmed that inhibition was impaired. Binding saturation experiments demonstrated a decrease in [(3)H]-muscimol binding after GABA withdrawal showing a close correlation with the development of hyperexcitability. All these modifications coursed without changes in receptor affinity (K(D)) for muscimol or bicuculline as demonstrated by both binding studies and Schild analysis. It is concluded that, in the CA1 region of the hippocampus, it is the number of functional GABA(A) receptors, and not the affinity of the receptor, what is decreased during GW-induced hyperexcitability.

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.

Increase in expression of the GABA(A) receptor alpha(4) subunit gene induced by withdrawal of, but not by long-term treatment with, benzodiazepine full or partial agonists

The effects of long-term exposure to, and subsequent withdrawal of, diazepam or imidazenil (full and partial agonists of the benzodiazepine receptor, respectively) on the abundance of GABA(A) receptor subunit mRNAs and peptides were investigated in rat cerebellar granule cells in culture. Exposure of cells to 10 microM diazepam for 5 days significantly reduced the amounts of alpha(1) and gamma(2) subunit mRNAs, and had no effect on the amount of alpha(4) mRNA. These effects were accompanied by a decrease in the levels of alpha(1) and gamma(2) protein and by a reduction in the efficacy of diazepam with regard to potentiation of GABA-evoked Cl- current. Similar long-term treatment with 10 microM imidazenil significantly reduced the abundance of only the gamma(2)S subunit mRNA and had no effect on GABA(A) receptor function. Withdrawal of diazepam or imidazenil induced a marked increase in the amount of alpha(4) mRNA; withdrawal of imidazenil also reduced the amounts of alpha(1) and gamma(2) mRNAs. In addition, withdrawal of diazepam or imidazenil was associated with a reduced ability of diazepam to potentiate GABA action. These data give new insights into the different molecular events related to GABA(A) receptor gene expression and function produced by chronic treatment and withdrawal of benzodiazepines with full or partial agonist properties.

Effects of cocaine administration on receptor binding and subunits mRNA of GABA(A)-benzodiazepine receptor complexes

The effects of intermittent intraperitoneal (i.p.) administration of cocaine (20 mg/kg) on GABA(A)-benzodiazepine (BZD) receptors labeled by t-[(35)S]butylbicyclophosphorothionate (TBPS), and on several types of mRNA subunits were investigated in rat brain by in vitro quantitative receptor autoradiography and in situ hybridization. Phosphor screen imaging with high sensitivity and a wide linear range of response was utilized for imaging analysis. There was a significant decrease in the level of alpha 1, alpha 6, beta 2, beta 3, and gamma 2 subunits mRNA, with no alteration of [(35)S]TBPS binding in any regions in the brain of rats at 1 h following a single injection of cocaine. In chronically treated animals, the mean scores of stereotyped behavior were increased with the number of injections. The level of beta 3 subunit mRNA was decreased in the cortices and caudate putamen, at 24 h after a final injection of chronic administrations for 14 days. In the withdrawal from cocaine, the frontal cortex and hippocampal complexes showed a significant increase in [(35)S]TBPS binding and alpha1 and beta 3 subunit mRNA in the rats 1 week after a cessation of chronic administration of cocaine. These findings suggest that the disruption of GABA(A)-BZD receptor formation is closely involved in the development of cocaine-related behavioral disturbances. Further studies on the physiological functions on GABA(A)-BZD receptor complex will be necessary for an explanation of the precise mechanisms underlying the acute effects, development of hypersensitization, and withdrawal state of cocaine.

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.

New insights into the mechanism of action of hypnotics

Between 1987 and 1989, the different protein subunits that make up the receptor for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) were identified. These make up the alpha, beta, gamma and delta families, for each of which exist several subtypes. This receptor is the molecular target of modern hypnotic drugs (i.e. benzodiazepines, zopiclone, zolpidem and zaleplon). In the 10 years that have followed this milestone, significant progress has been made in exploring the molecular mechanisms of hypnotic drug action. Receptor subtype specificity of hypnotics has been explained in terms of differential affinity for receptors containing different alpha subunits, which are expressed in different brain regions. Zolpidem and zaleplon bind preferentially to alpha1-containing receptors, whereas benzodiazepines and zopiclone are aspecific. Different sets of subunits are encoded in contiguous 'cassettes' on the genome, and the transcription of each set appears to be regulated coherently. The predominant GABA(A) receptor composition found in the brain is alpha1beta2gamma2, which are all encoded on human chromosome 5. Targeted gene disruption has provided clues to the physiological functions served by GABA(A) receptors containing different subunits. Receptors containing gamma2 appear to have a vital role in maintaining appropriate central inhibition, beta3-containing receptors may also be important determinants of excitability in certain brain regions, whereas a clear role for alpha5-, alpha6- and gamma3-containing receptors has not yet been established by these techniques. Site-directed mutagenesis has indicated that benzodiazepines bind to a cleft on the GABA(A) receptor surface at the interface between the alpha and gamma subunits. Other drugs (flumazenil, zopiclone, zolpidem) also bind to the a subunit, but interact with amino acids in different binding domains to the benzodiazepines. The molecular mechanism of hypnotic dependence has been explored, and seems to involve downregulation of transcription of the normally prevalent alpha1, beta2 and gamma2 subunits, and the reciprocal upregulation of the expression of rarer subunits. Chronic treatment with hypnotic drugs that may have less dependence potential, such as zopiclone and zolpidem, appears to produce more limited change in GABA(A) receptor subunit expression. These ideas will be important both for designing new hypnotic drugs with a better safety/efficacy profile, and for evaluating more appropriate ways of using the drugs available today.

Specificity of chronic effects of diazepam on responding of rats under fixed-ratio schedules

Behavioral effects of diazepam were studied in rats responding in different daily sessions using different operant chambers and manipulanda. In one experiment, key pressing was maintained in a first session under a 40-response fixed-ratio schedule; lever pressing was maintained in a second session under a 40-response fixed-ratio schedule; and a third session consisted of a multiple schedule comprising both key and lever pressing maintained under a 40-response fixed-ratio schedule. In a second experiment, the first session consisted of a multiple schedule with both key and lever pressing maintained under a 40-response fixed-ratio schedule and the second session consisted of lever pressing maintained under a 40-response fixed-ratio schedule. After studying effects of widely spaced injections of diazepam (0.3-3.0 mg/kg) on responding for each separate schedule, animals received 1.7 mg/kg/day diazepam in order to study chronic effects of the diazepam on behavior among the different schedule-conditions. In both experiments, tolerance to rate-decreasing effects of diazepam in a particular schedule component did not extend to any other schedule component when the manipulandum or chamber was different, but did extend to other schedule components when the manipulandum or chamber was the same. These results suggest that behavioral effects of chronically administered diazepam were influenced not only by pharmacologic processes, but also by learned relationships among its interoceptive effects, reinforcement contingencies, and particular behavioral environments.

Decreased GABA enhancement of benzodiazepine binding after a single dose of diazepam

The GABA and benzodiazepine binding sites on GABA(A) receptors are allosterically coupled. The in vitro binding of 2 nM [3H]flunitrazepam to cortical and cerebellar membranes prepared from drug-naive rats was potentiated approximately 1.6-fold by 100 microM GABA. Potentiation in both regions was significantly reduced 4 or 12 but not 24 h after a single dose of 15 mg/kg diazepam. At 24 h after the last of 14 daily doses of diazepam, no differences in GABA potentiation were observed. Diazepam-induced changes in GABA(A) receptor gamma2-subunit gene transcription and alpha1-, beta2-, and gamma2-subunit steady-state mRNA levels did not appear to be temporally related to allosteric uncoupling.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.