Natural mutation of GABAA receptor alpha 6 subunit alters benzodiazepine affinity but not allosteric GABA effects

The Interplay between Neurotransmitters and Calcium Dynamics in Retinal Synapses during Development, Health, and Disease

The intricate functionality of the vertebrate retina relies on the interplay between neurotransmitter activity and calcium (Ca2+) dynamics, offering important insights into developmental processes, physiological functioning, and disease progression. Neurotransmitters orchestrate cellular processes to shape the behavior of the retina under diverse circumstances. Despite research to elucidate the roles of individual neurotransmitters in the visual system, there remains a gap in our understanding of the holistic integration of their interplay with Ca2+ dynamics in the broader context of neuronal development, health, and disease. To address this gap, the present review explores the mechanisms used by the neurotransmitters glutamate, gamma-aminobutyric acid (GABA), glycine, dopamine, and acetylcholine (ACh) and their interplay with Ca2+ dynamics. This conceptual outline is intended to inform and guide future research, underpinning novel therapeutic avenues for retinal-associated disorders.

Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAA and GABAC receptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

The neurobiology of alcohol consumption and alcoholism: an integrative history

Studies of the neurobiological predisposition to consume alcohol (ethanol) and to transition to uncontrolled drinking behavior (alcoholism), as well as studies of the effects of alcohol on brain function, started a logarithmic growth phase after the repeal of the 18th Amendment to the United States Constitution. Although the early studies were primitive by current technological standards, they clearly demonstrated the effects of alcohol on brain structure and function, and by the end of the 20th century left little doubt that alcoholism is a "disease" of the brain. This review traces the history of developments in the understanding of ethanol's effects on the most prominent inhibitory and excitatory systems of brain (GABA and glutamate neurotransmission). This neurobiological information is integrated with knowledge of ethanol's actions on other neurotransmitter systems to produce an anatomical and functional map of ethanol's properties. Our intent is limited in scope, but is meant to provide context and integration of the actions of ethanol on the major neurobiologic systems which produce reinforcement for alcohol consumption and changes in brain chemistry that lead to addiction. The developmental history of neurobehavioral theories of the transition from alcohol drinking to alcohol addiction is presented and juxtaposed to the neurobiological findings. Depending on one's point of view, we may, at this point in history, know more, or less, than we think we know about the neurobiology of alcoholism.

The GABRA6 mutation, R46W, associated with childhood absence epilepsy, alters 6β22 and 6β2 GABA(A) receptor channel gating and expression

A GABA(A) receptor α6 subunit mutation, R46W, was identified as a susceptibility gene that may contribute to the pathogenesis of childhood absence epilepsy (CAE), but the molecular basis for alteration of GABA(A) receptor function is unclear. The R46W mutation is located in a region homologous to a GABA(A) receptor γ2 subunit missense mutation, R82Q, that is associated with CAE and febrile seizures in humans. To determine how this mutation reduces GABAergic inhibition, we expressed wild-type (α6β2γ2L and α6β2δ) and mutant (α6(R46W)β2γ2L and α6(R46W)β2δ) receptors in HEK 293T cells and characterize their whole-cell and single-channel currents, and surface and total levels. We demonstrated that gating and assembly of both α6(R46W)β2γ2L and α6(R46W)β2δ receptors were impaired. Compared to wild-type currents, α6(R46W)β2γ2L and α6(R46W)β2δ receptors had a reduced current density, α6(R46W)β2γ2L currents desensitized to a greater extent and deactivated at a slower rate, α6(R46W)β2δ receptors did not desensitize but deactivated faster and both α6(R46W)β2γ2L and α6(R46W)β2δ single-channel current mean open times and burst durations were reduced. Surface levels of coexpressed α6(R46W), β2 and δ, but not γ2L, subunits were decreased. 'Heterozygous' coexpression of α6(R46W) and α6 subunits with β2 and γ2L subunits produced intermediate macroscopic current amplitudes by increasing incorporation of wild-type and decreasing incorporation of mutant subunits into receptors trafficked to the surface. Finally, these findings suggest that similar to the γ2(R82Q) mutation, the CAE-associated α6(R46W) mutation could cause neuronal disinhibition and thus increase susceptibility to generalized seizures through a reduction of αβγ and αβδ receptor function and expression.

Hooked on benzodiazepines: GABAA receptor subtypes and addiction

Benzodiazepines are widely used clinically to treat anxiety and insomnia. They also induce muscle relaxation, control epileptic seizures, and can produce amnesia. Moreover, benzodiazepines are often abused after chronic clinical treatment and also for recreational purposes. Within weeks, tolerance to the pharmacological effects can develop as a sign of dependence. In vulnerable individuals with compulsive drug use, addiction will be diagnosed. Here we review recent observations from animal models regarding the cellular and molecular basis that might underlie the addictive properties of benzodiazepines. These data reveal how benzodiazepines, acting through specific GABA(A) receptor subtypes, activate midbrain dopamine neurons, and how this could hijack the mesolimbic reward system. Such findings have important implications for the future design of benzodiazepines with reduced or even absent addiction liability.

A major QTL for acute ethanol sensitivity in the alcohol tolerant and non-tolerant selected rat lines

The Alcohol Tolerant and Alcohol Non-Tolerant rats (AT, ANT) were selectively bred for ethanol-induced ataxia as measured on the inclined plane. Here we report on a quantitative trait locus (QTL) study in an F(2) intercross population derived from inbred AT and ANT (IAT, IANT) and a follow-up study of congenics that were bred to examine one of the mapped QTLs. Over 1200 F(2) offspring were tested for inclined plane sensitivity, acute tolerance on the inclined plane, duration of the loss of righting reflex (LORR) and blood ethanol at regain of the righting reflex (BECRR). F(2) rats that were in the upper and lower 20% for inclined plane sensitivity were genotyped with 78 SSLP markers. Significant QTLs for inclined plane sensitivity were mapped on chromosomes 8 and 20; suggestive QTLs were mapped on chromosomes 1, 2 and 3. Highly significant QTLs for LORR duration (LOD = 12.4) and BECRR (LOD = 5.7) were mapped to the same locus on chromosome 1. Breeding and testing of reciprocal congenic lines confirmed the chromosome 1 LORR/BECRR QTL. A series of recombinant congenic sub-lines were bred to fine-map this QTL. Current results have narrowed the QTL to an interval of between 5 and 20 Mb. We expect to be able to narrow the interval to less than 5 Mb with additional genotyping and continued breeding of recombinant sub-congenic lines.

Bacopa monniera ameliorates amnesic effects of diazepam qualifying behavioral-molecular partitioning

Benzodiazepines are known to produce amnesia by involvement of GABAergic system and by interference of long term potentiation (LTP). In this study, we examined effect of Bacopa monniera on downstream molecules of LTP after diazepam-induced amnesia in mice. We used a Morris water maze scale for evaluating the effect of Bacopa monniera after screening for muscle coordination by rota rod. The index of acquisition and retrieval was recorded as escape latency time (ELT). Behavioral results showed that Bacopa monniera (120 mg kg(-1) oral) significantly reversed diazepam- (1.75 mg kg(-1) i.p.) induced amnesia in Morris water maze task. The molecular studies revealed that diazepam upregulated mitogen activated protein kinase (MAP kinase), phosphorylated CREB (pCREB) and inducible nitric oxide synthase (iNOS), while it downregulated nitrite, nitrate, total nitrite, cAMP response element binding protein (CREB) expression, phosphodiesterase, cyclic adenosine monophosphate (cAMP) without affecting calmodulin levels. Bacopa monniera suppressed the diazepam induced upregulation of MAP kinase, pCREB and iNOS and attenuated the downregulation of nitrite. It did not affect the cAMP, PDE, nitrate, total nitrite, total CREB level. These behavioral findings displayed the reversal of diazepam-induced amnesia by Bacopa monniera without qualifying the molecular details although some downstream molecules of LTP may be involved.

Modulation of GABAA receptors in cerebellar granule neurons by ethanol: a review of genetic and electrophysiological studies

Cerebellar granule neurons (CGNs) receive inhibitory input from Golgi cells in the form of phasic and tonic currents that are mediated by postsynaptic and extrasynaptic gamma-aminobutyric acid type A (GABAA) receptors, respectively. Extrasynaptic receptors are thought to contain alpha6betaxdelta subunits. Here, we review studies on ethanol (EtOH) modulation of these receptors, which have yielded contradictory results. Although studies with recombinant receptors expressed in Xenopus oocytes indicate that alpha6beta3delta receptors are potently enhanced by acute exposure to low (>or=3 mM) EtOH concentrations, this effect was not observed when these receptors were expressed in Chinese hamster ovary cells. Slice recordings of CGNs have consistently shown that EtOH increases the frequency of phasic spontaneous inhibitory postsynaptic currents (sIPSCs), as well as the tonic current amplitude and noise. However, there is a lack of consensus as to whether EtOH directly acts on extrasynaptic receptors or modulates them indirectly; that is, via an increase in spillover of synaptically released GABA. It was recently demonstrated that an R to Q mutation of amino acid 100 of the alpha6 subunit increases the effect of EtOH on both sIPSCs and tonic current. These electrophysiological findings have not been reproducible in our hands. Moreover, it was shown the alpha6-R100Q mutation enhances sensitivity to the motor-impairing effects of EtOH in outbred Sprague-Dawley rats, but this was not observed in a line of rats selectively bred for high sensitivity to EtOH-induced motor alterations (Alcohol Non-Tolerant rats). We conclude that currently there is insufficient evidence conclusively supporting a direct potentiation of extrasynaptic GABAA receptors following acute EtOH exposure in CGNs.

Ethanol acts directly on extrasynaptic subtypes of GABAA receptors to increase tonic inhibition

Based on the similarity of ethanol intoxication to the behavioral effects of drugs known to target gamma-aminobutyric acid type A (GABAA) receptors (GABARs), it has been suspected for decades that ethanol facilitates the activity of GABA. Even so, it has been surprisingly difficult to identify molecular targets of ethanol. Research conducted over the past several years suggests that a subclass of GABARs (those containing delta subunits) responds in a relevant concentration range to ethanol. Although delta subunit-containing GABARs are not ubiquitously expressed at inhibitory synapses like their gamma subunit-containing, synaptic counterparts, they are found in many neurons in extrasynaptic locations. Here, they give rise to a tonic form of inhibition that can potently suppress neuronal excitability. Studies have shown that both recombinant and native delta subunit-containing GABARs (1) are modulated by behaviorally relevant (i.e., low millimolar) concentrations of ethanol, (2) directly bind ethanol over the same concentration range, (3) show altered function upon single amino substitutions linked to changes in behavioral responsiveness to ethanol, and (4) are a site of action of Ro15-4513, a competitive antagonist of ethanol binding and a drug which prevents many of the behavioral aspects of ethanol intoxication. Despite such comprehensive evidence, however, the field is not free from controversy. This review evaluates published data for and against a central role of delta subunit-containing GABARs in ethanol actions and suggests future directions that might help settle points of controversy.

Does ethanol act preferentially via selected brain GABAA receptor subtypes? the current evidence is ambiguous

In rodent models, gamma-aminobutyric acid A (GABAA) receptors with the alpha6 and delta subunits, expressed in the cerebellar and cochlear nucleus granule cells, have been linked to ethanol sensitivity and voluntary ethanol drinking. Here, we review the findings. When considering both in vivo contributions and data on cloned receptors, the evidence for direct participation of the alpha6-containing receptors to increased ethanol sensitivity is poor. The alpha6 subunit-knockout mouse lines do not have any changed sensitivity to ethanol, although these mice do display increased benzodiazepine sensitivity. However, in general the compensations occurring in knockout mice (regardless of which particular gene is knocked out) tend to fog interpretations of drug actions at the systems level. For example, the alpha6 knockout mice have increased TASK-1 channel expression in their cerebellar granule cells, which could influence sensitivity to ethanol in the opposite direction to that obtained with the alpha6 knockouts. Indeed, TASK-1 knockout mice are more impaired than wild types in motor skills when given ethanol; this might explain why GABAA receptor alpha6 knockout mice have unchanged ethanol sensitivities. As an alternative to studying knockout mice, we examined the claimed delta subunit-dependent/gamma2 subunit-independent ethanol/[3H]Ro 15-4513 binding sites on GABAA receptors. We looked at [3H]Ro 15-4513 binding in HEK 293 cell membrane homogenates containing rat recombinant alpha6/4beta3delta receptors and in mouse brain sections. Specific high-affinity [3H]Ro 15-4513 binding could not be detected under any conditions to the recombinant receptors or to the cerebellar sections of gamma2(F77I) knockin mice, nor was this binding to brain sections of wild-type C57BL/6 inhibited by 1-100 mM ethanol. Since ethanol may act on many receptor and channel protein targets in neuronal membranes, we consider the alpha6 (and alpha4) subunit-containing GABAA receptors unlikely to be directly responsible for any major part of ethanol's actions. Therefore, we finish the review by discussing more generally alcohol and GABAA receptors and by suggesting potential future directions for this research.

Low dose acute alcohol effects on GABA A receptor subtypes

GABA(A) receptors (GABA(A)Rs) are the main inhibitory neurotransmitter receptors and have long been implicated in mediating at least part of the acute actions of ethanol. For example, ethanol and GABAergic drugs including barbiturates and benzodiazepines share many pharmacological properties. Besides the prototypical synaptic GABA(A)R subtypes, nonsynaptic GABA(A)Rs have recently emerged as important regulators of neuronal excitability. While high doses (> or =100 mM) of ethanol have been reported to enhance activity of most GABA(A)R subtypes, most abundant synaptic GABA(A)Rs are essentially insensitive to ethanol concentrations that occur during social ethanol consumption (< 30 mM). However, extrasynaptic delta and beta3 subunit-containing GABA(A)Rs, associated in the brain with alpha4 or alpha6 subunits, are sensitive to low millimolar ethanol concentrations, as produced by drinking half a glass of wine. Additionally, we found that a mutation in the cerebellar alpha6 subunit (alpha6R100Q), initially reported in rats selectively bred for increased alcohol sensitivity, is sufficient to produce increased alcohol-induced motor impairment and further increases of alcohol sensitivity in recombinant alpha6beta3delta receptors. Furthermore, the behavioral alcohol antagonist Ro15-4513 blocks the low dose alcohol enhancement on alpha4/6/beta3delta receptors, without reducing GABA-induced currents. In binding assays alpha4beta3delta GABA(A)Rs bind [(3)H]Ro15-4513 with high affinity, and this binding is inhibited, in an apparently competitive fashion, by low ethanol concentrations, as well as analogs of Ro15-4513 that are active to antagonize ethanol or Ro15-4513's block of ethanol. We conclude that most low to moderate dose alcohol effects are mediated by alcohol actions on alcohol/Ro15-4513 binding sites on GABA(A)R subtypes.

Gender-specific contribution of the GABA(A) subunit genes on 5q33 in methamphetamine use disorder

Family and twins studies have suggested that genetic factors are involved in the development of substance use disorders. Several unrelated case/control association studies have reported associations of the GABA(A) subunit genes on 5q33 with the development of alcohol dependence. We hypothesized that these particular GABA(A) subunit genes also contribute to the development of methamphetamine use disorder. To test our hypothesis, we recruited cases using a series of questionnaires. Among the polymorphic SNPs, significant differences between cases and controls were identified in the female sample in the rs2279020 of the GABA(A)alpha1 subunit gene, and the novel SNP rs4480617 in the GABA(A)gamma2 subunit gene. No associations were found in the male sample. Further haplotype analysis identified several marker blocks significantly associated with methamphetamine use disorder in females; each block consists of the rs4480617. Our study provides preliminary evidence that the GABA(A) subunit genes on 5q33 may preferentially contribute to methamphetamine use disorder in females.

Kainate down-regulates a subset of GABAA receptor subunits expressed in cultured mouse cerebellar granule cells

The effect of kainate, an agonist selective for ionotropic AMPA/kainate type of glutamate receptors, on GABAA receptor subunit expression in cultured mouse cerebellar granule cells was studied using quantitative RT-PCR, ligand binding and electrophysiology. Chronic kainate treatment, without producing excitotoxicity, resulted in preferential, dose- and time-dependent down-regulation of alpha1, alpha6 and beta2 subunit mRNA expression, the expression of beta3, gamma2 and delta subunit mRNAs being less affected. The down-regulation was reversed by DNQX, an AMPA/kainate-selective glutamate receptor antagonist. A 14-day kainate treatment resulted in 46% decrease of total [3H]Ro 15-4513 binding to the benzodiazepine sites. Diazepam-insensitive [3H]Ro 15-4513 binding was decreased by 89% in accordance with very low amount of alpha6 subunit mRNA present. Diazepam-sensitive [3H]Ro 154513 binding was decreased only by 40%, contrasting >90% decrease in alpha1 subunit mRNA expression. However, this was consistent with lower potentiation of GABA-evoked currents in kainate-treated than control cells by the alpha1-selective benzodiazepine site ligand zolpidem, suggesting compensatory expression of alpha5 (and/or alpha2 or alpha3) subunits producing diazepam-sensitive but zolpidem-insensitive receptor subtypes. In conclusion, chronic kainate treatment of cerebellar granule cells selectively down-regulates oil, alpha6 and beta2 subunits resulting in altered GABAA receptor pharmacology.

The cerebellar GABAA α6 subunit is differentially modulated by chronic ethanol exposure in normal (R100R) and mutated (Q100Q) sNP rats

Sardinian alcohol non-preferring (sNP) rats carry a point mutation (R100Q) in the cerebellar expressed GABAA receptor alpha6 subunit gene, leading to a higher sensitivity to ethanol and diazepam. The role of the alpha6 subunit gene cluster in the ethanol non-preferring phenotype was here investigated by measuring the levels of alpha1, alpha6 and gamma2 peptide in the cerebellum of normal (RR) and mutated (QQ) sNP rats after 2 weeks of chronic ethanol administration. Western blot analysis revealed that the alpha6 subunit is increased in RR sNP rats after chronic ethanol exposure (25.44%+/-8.69 versus control), while it remained unchanged in mutated QQ sNP rats. Interestingly, chronic ethanol administration decreased alpha1 peptide levels in the cerebellum of both rat lines to a similar extent (30.99%+/-6.74 and 27.12%+/-9.83 in RR and QQ rats, respectively), while gamma2 peptide levels remained unchanged. To further correlate the genetic and biochemical difference of the normal and mutated sNP rats with their aversive phenotype, we exposed sNP rats to a protocol of acquisition and maintenance of ethanol drinking. QQ sNP rats drank less ethanol than RR rats during the acquisition phase, but such difference was lost during the maintenance phase. These data may contribute to elucidating the mechanisms of alcohol avoidance in rat lines selected for this behavior when exposed to ethanol solution.

Characterization of wild-type (R100R) and mutated (Q100Q) GABAA alpha 6 subunit in Sardinian alcohol non-preferring rats (sNP)

Sardinian alcohol non-preferring (sNP) rats, selected for their low ethanol preference and consumption, carry a point mutation (R100Q) in the gene coding for GABA(A) receptor alpha(6) subunit, which becomes more sensitive to diazepam-evoked GABA currents. We performed binding studies in the cerebellum of normal (RR) and mutated (QQ) sNP rats using [3H]Ro 15-4513, an inverse agonist for the benzodiazepine site which binds both diazepam insensitive and diazepam sensitive sites. Saturation curves performed on cerebellar membrane from genotyped rats indicated an higher affinity of [3H]Ro 15-4513 for GABA(A) receptors in QQ with respect to RR rats (K(d) values 4.0+/-0.67 and 6.24+/-0.95 nM, respectively), with similar B(max) values (3.5+/-0.25 and 3.9+/-0.39 pmol/mg protein, respectively). Diazepam displacement curves showed a two component model for both genotypes, with similar K(i1) values for QQ and RR (3.6+/-0.62 and 4.9+/-0.33 nM, respectively). In QQ rats diazepam is able to completely displace [3H]Ro 15-4513 (K(i2)=1.48+/-0.27 microM), while in RR rats the diazepam sensitive sites are still present (K(i2)>10 microM). The basal mRNA and protein expression level of the alpha(6) subunit were similar in RR and QQ rats. The electrophysiological profile of oocytes of Xenopus laevis injected with cerebellar synaptosomes showed that ethanol positively modulated GABA-evoked currents significantly more in QQ than in RR rats. These data contribute to the characterization of the function of GABA(A) alpha(6) subunit and its involvement in determining alcohol related behavior.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Increased behavioral neurosteroid sensitivity in a rat line selectively bred for high alcohol sensitivity

Acute administration of a neurosteroid 5beta-pregnan-3alpha-ol-20-one induced a greater impairment in motor performance of the selectively bred alcohol-sensitive (ANT) than alcohol-insensitive (AT) rats. This difference was not associated with the sensitivity of gamma-aminobutyrate type A (GABA(A)) receptors, as 5alpha-pregnan-3alpha-ol-20-one (allopregnanolone) decreased the autoradiographic signals of t-butylbicyclophosphoro[35S]thionate binding to GABA(A) receptor-associated ionophores more in the brain sections of AT than ANT rats. Nor was the difference associated with baseline levels of neuroactive progesterone metabolites, as 5alpha-pregnan-3,20-dione (5alpha-DHP) and 5alpha-pregnan-3alpha-ol-20-one were lower in the ANT rats. After ethanol (2 g/kg, i.p.) administration and the subsequent motor performance test, the increased brain concentrations of these metabolites were still lower in the ANT than AT rats, although especially in the cerebellum the relative increases were greater in the ANT than AT rats. The present data suggest that the mechanisms mediating neurosteroid-induced motor impairment are susceptible to genetic variation in rat lines selected for differences in ethanol intoxication.

Role of the GABA(A)beta2, GABA(A)alpha6, GABA(A)alpha1 and GABA(A)gamma2 receptor subunit genes cluster in drug responses and the development of alcohol dependence

gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter of the central nervous system and it acts at the GABA(A) and GABA(B) receptors. A possible role for the GABA(A) receptors in alcohol action has been derived from in vitro cell models, animal studies and human research. GABA(A) subunit mRNA expression in cell models has suggested that the long form of the gamma2 subunit is essential for ethanol enhanced potentiation of GABA(A) receptors, by phosphorylation of a serine contained within the extra eight amino acids. Several animal studies have demonstrated that alterations in drug and alcohol responses may be caused by amino-acid differences at the GABA(A)alpha6 and GABA(A)gamma2 subunits. An Arg(100)/Glu(100) change at the GABA(A)alpha6 subunit conferring altered binding efficacy of the benzodiazepine inverse agonist Ro 15-4513, was found between the AT (alcohol tolerance) and ANT (alcohol non-tolerance) rats. Several loci related to alcohol withdrawal on mouse chromosome 11 which corresponds to the region containing four GABA(A) subunit (beta2, alpha6, alpha1 and gamma2) genes on human chromosome 5q33-34, were also identified. Gene knockout studies of the role of GABA(A)alpha6 and GABA(A)gamma2 subunit genes in mice have demonstrated an essential role in the modulation of other GABA(A) subunit expression and the efficacy of benzodiazepine binding. Absence of the GABA(A)gamma2 subunit gene has more severe effects with many of the mice dying shortly after birth. Disappointingly few studies have examined the effects of response to alcohol in these gene knockout mice. Human genetic association studies have suggested that the GABA(A)beta2, alpha6, alpha1 and gamma2 subunit genes have a role in the development of alcohol dependence, although their contributions may vary between ethnic group and phenotype. In summary, in vitro cell, animal and human genetic association studies have suggested that the GABA(A)beta2, alpha6, alpha1 and gamma2 subunit genes have an important role in alcohol related phenotypes (300 words).

Ethanol and amino acids in the central nervous system: assessment of the pharmacological actions of acamprosate

Ethanol induces alterations in the central nervous system by differentially interfering with a number of neurotransmitter systems, although the mechanisms by which such effects are executed are not well understood. The present review therefore, is designed to ascertain the effect of ethanol on both excitatory and inhibitory amino acid neurotransmitters, as well as the sulphonated amino acid taurine, assayed by the microdialysis technique within specific brain regions of rat during different types of alcohol intoxication, acute and chronic, as well as during the withdrawal period. Such an understanding of these pharmacological actions of ethanol on neurotransmitters is essential in order to provide the impetus for the development of appropriate therapeutic intervention to ameliorate the multitude of neurochemical disorders induced by ethanol. In addition the possible mode of action of a new therapeutic drug for the treatment of alcoholism, acamprosate will be discussed. The first part of this review will be limited to studies of the effect of ethanol on both amino acid neurotransmitters and the sulphonated amino acid taurine, a possible neuromodulator. While, the second part will seek to establish the possible mechanism of action of a new therapeutic drug, acamprosate, which is used to combat the effects of ethanol, particularly during the craving period, as well as maintaining abstinence in weaned alcoholics.

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.

Inhibition of GABAA ligand-gated Cl- channels by zinc in adult rat brain: a regional study

Zinc (Zn2+) was shown to invariably inhibit muscimol-stimulated 36Cl- uptake by synaptoneurosomes in the cerebral cortex, hippocampus and cerebellum. The Zn2+ sensitivity of the GABAA receptor-gated 36Cl- uptake in the cerebral cortex was comparable to that in the hippocampus, whereas the uptake in the cerebellum was less sensitive to Zn2+. Although diazepam-potentiation of muscimol-stimulated 36Cl- uptake was unaltered by 100 microM Zn2+ in the cerebral cortex and hippocampus, diazepam caused no enhancement in the presence of Zn2+ in the cerebellum. Zn2+ inhibited [3H]diazepam binding significantly at 1 mM in the cerebral cortex and cerebellum, whereas Ni2+ increased the binding in a concentration-dependent manner in both regions. Although lower concentrations of Zn2+ did not affect [3H]Ro 15-4513 binding to diazepam-sensitive sites, higher concentrations of ZN2+ increased the binding in both regions. Unlike the diazepam-sensitive sites, the diazepam-insensitive [3H]Ro 15-4513 binding was not affected by Zn2+ or Ni2+ at any of the tested concentrations. These results suggest that the GABAA ligand-gated Cl- flux and its diazepam-potentiation are heterogeneously modulated in various brain regions. It is also suggested that cerebellar diazepam-insensitive [3H]Ro 15-4513 binding sites are insensitive to Zn2+ and Ni2+.

Pharmacological properties of recombinant "diazepam-insensitive" GABAA receptors

Both native and recombinant "diazepam-insensitive" GABAA receptors (DI) are characterized by the very low affinities of prototypic 1,4-benzodiazepines such as diazepam and the high affinity of an imidazobenzodiazepine, Ro 15-4513. The presence of either an alpha 4 or alpha 6 subunit imparts this unusual pharmacological profile to DI. Based on the affinities of these compounds at recombinant DI, the pharmacological properties of alpha 4- and alpha 6-bearing receptor isoforms appear to be very similar if not identical. Using a larger sample of structurally diverse compounds, we now demonstrate distinct but related ligand binding profiles of recombinant alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 DI. Comparison of 18 ligands drawn from three principal structural groups (beta-carbolines, imidazobenzodiazepines and pyrazoloquinolinones) revealed that the affinity of at least one representative from each group differed by > 5-fold between alpha 4- and alpha 6 beta 2 gamma 2 receptors. While the high correlation (r2 = 0.926; p < 0.001) obtained between the affinities of these ligands at alpha 4- and alpha 6-containing receptors underscores the similarity between these receptor isoforms, a significant deviation of the slope of this correlation (0.792; 95% C.I. 0.673-0.911) from unity is substantive evidence that these DI possess distinct pharmacological profiles. These findings indicate that it is feasible to develop selective ligands for these DI isoforms.

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.

The cerebellum: a model system for studying GABAA receptor diversity

The basic unsolved questions concerning GABAA receptors are: "How many receptor subtypes exist?", "What subtypes are used by which types of neuron and where are they located on the cell?", and "What are the functions of the different subtypes?" As described in this Review, the cerebellum is an ideal vertebrate brain region for investigating these issues.

Genetic analysis of anxiety-related behaviors and responses to benzodiazepine-related drugs in AXB and BXA recombinant inbred mouse strains

Recombinant inbred (RI) strains derived from the C57BL/6J and A/J mouse strains were used for behavioral studies designed to estimate the number and location of chromosomal loci responsible for anxiety-related behaviors and differential sensitivity to agonists and inverse agonists of the gamma-aminobutyric acidA (GABAA)/benzodiazepine receptor complex. The phenotypes of the parental inbred strains and of 28 RI strains were characterized for the number of transitions in the light<-->dark exploratory model, anxiolytic response to diazepam, vertical and ambulatory activities in an open field, and sensitivity to the convulsant properties of methyl-beta-carboline-3-carboxylate (beta-CCM). The strain distribution patterns and estimates of the minimal number of loci obtained for each trait suggest that multiple chromosomal loci contribute to differences in anxiety-related behavioral phenotypes and the behavioral responses to diazepam and beta-CCM between C57BL/6J and A/J mice. The best probabilities of linkage were found between the variables characterizing response to diazepam and loci on chromosomes 1 (Xmv-41) and 10 (D10Mit2) and between the sensitivity to the convulsant actions of beta-CCM and locus D15Mit5 on chromosome 15.

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.

Effects of ethanol on recombinant rat GABAA receptors: [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding study

To determine the roles of the alternatively spliced short and long forms of the gamma 2 subunit in the effect of ethanol on the GABAA receptor function, picrotoxin-sensitive [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding was studied in recombinant rat alpha 1 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 receptors expressed in human embryonic kidney 293 cells. Ethanol (10-500 mM) in the absence of added GABA had only minor effects on [35S]TBPS binding irrespective of the gamma 2 splice variant, its effects being greater in alpha 6 beta 2 gamma 2 than in alpha 1 beta 2 gamma 2 receptors. Ethanol (100 mM) decreased the binding in all four subunit combinations at various concentrations of GABA, again an effect independent of the gamma 2 variant. The two gamma 2 variants had different effects on GABA modulation of the binding, with the long gamma 2 variant decreasing the efficiency of GABA inhibition in alpha 6 beta 2 gamma 2 receptors and enhancing the biphasic GABA stimulation and inhibition in alpha 1 beta 2 gamma 2 receptors. The findings confirm the importance of the alpha subunits in the allosteric interactions between the convulsant binding site and other effector sites, which can be modified only to a minor extent by the type of the gamma 2 splice variant.

Stereospecific transduction of behavioral effects via diazepam-insensitive GABAA receptors

Previous studies reported a positive correlation between ligand affinities at diazepam-insensitive GABAA receptors and substitution for the discriminative stimulus effects of the benzodiazepine receptor antagonist, flumazenil, in pigeons. In the present experiments, bretazenil and Ro 14-5974 (ethyl-(S)-11,12,13,13 a-tetrahydro-9-oxo-9H-imidazo[1,5-a]-pyrrolo-[2,1-c] [1,4]benzodiazepine-1-carboxylate) partially substituted for, and blocked the discriminative stimulus effects of midazolam, congruent with their actions at diazepam-sensitive GABAA receptors in vitro. In addition, bretazenil and Ro 14-5974, but not their R-enantiomers, had high affinity for diazepam-insensitive receptors and fully substituted for the discriminative stimulus effects of flumazenil. The R-enantiomers of these compounds had low affinity (Ki > 1 microM) for diazepam-sensitive and diazepam-insensitive receptors, and did not share discriminative stimulus effects with flumazenil or midazolam. Ro 19-0528 (7-chloro-3-(3-cyclopropyl-1,2,4-oxadiazol-5-yl)-4,5-dihydro-5-met hyl-6H- imidazo[1,5-a][1,4]benzodiazepin-6-one), a structurally related compound with full agonist actions at diazepam-sensitive GABAA receptors, had high diazepam-insensitive receptor affinity (Ki = 96 nM) and partially substituted for the discriminative stimulus effects of flumazenil. These results are consistent with stereospecific mediation of the discriminative stimulus effects of flumazenil through high affinity binding to diazepam-insensitive receptors in pigeons.

Characterization of novel ligands for wild-type and natural mutant diazepam-insensitive benzodiazepine receptors

A series of benzodiazepine receptor ligands with different chemical structures were evaluated for their affinities at diazepam-sensitive and diazepam-insensitive binding sites for [3H]Ro 15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo-[1,5a][1,4] benzodiazepine-3-carboxylate) in cerebellar GABAA receptors. Rats of Wistar strain and of alcohol-sensitive (ANT) and alcohol-insensitive (AT) lines were used. The ANT rats possess a single point mutation in their GABAA receptor alpha 6 subunit, which makes their diazepam-insensitive sites sensitive to benzodiazepine agonists, unlike those of AT and Wistar rats. All compounds evaluated displayed high-affinity binding to diazepam-sensitive sites (Ki < 50 nM). In contrast, a wider range of affinities were observed at diazepam-insensitive sites which depended upon the basic structure and substitutions. The 7- and 8-halogen substituted imidazobenzodiazepines and 12-halogen substituted diimidazoquinazolines displayed the highest affinities (Ki < 15 nM), while intermediate to low affinities (100 < Ki < 4000 nM) were displayed by imidazoquinazolines, thienopyrimidines, one oxoimidazoquinoxaline, and some cyclopyrrolones. The imidazoquinoxalines evaluated displayed the lowest affinity (Ki > 10000 nM). The oxoimidazoquinoxaline, 6-chloro-3-(5-cyclopropyl-1,2,4-oxadiazol-3-yl)-4,5-dihydro-5-isop ropyl-4-oxo-imidazo[1,5-a]quinoxaline (NNC 14-0578) and suriclone represent the first benzodiazepine receptor full agonists to bind with relatively high affinity (Ki approximately 100 nM) to diazepam-insensitive sites. The 5 position substituted methoxybenzyl, dimethylallyl, and 4-fluorobenzyl oxoimidazoquinoxaline analogs demonstrated a 58-336-fold higher affinity for ANT than AT diazepam-insensitive sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological function of GABAA/benzodiazepine receptor heterogeneity

gamma-Aminobutyric acid (GABA) is the most prominent of the inhibiting neurotransmitters in the brain. It exerts its main action through GABAA receptors. The receptors respond to the presence of GABA by the opening of an intrinsic anion channel. Hence, they belong to the molecular superfamily of ligand-gated ion channels. There exist in the brain multiple GABAA receptors that show differential distribution and developmental patterns. The receptors presumably form by the assembly of five proteins from at least three different subunits (alpha 1-6, beta 1-3 and gamma 1-3). The regulation of functional properties by benzodiazepine (BZ) receptor ligands, neurosteroids, GABA and its analogs differs dramatically with the alpha variant present in the complex. Additional variation of the GABAA receptors comes with the exchange of the gamma subunits. No clear picture exists for the role of the beta subunits, though they may play an important part in the sensitivity of the channel-receptor complex. The effects of BZ receptor ligands on animal behavior range from agonist effects, e.g. anxiolysis, sedation, and hypnosis, to inverse agonist effects, e.g. anxiety, alertness, and convulsions. The diversity of effects reflects the ubiquity of the GABAA/BZ receptors in the brain. Recent data provide some insight into the mechanism of action of BZ ligands, but no clear delineation can be drawn from a single ligand to a single behavioral effect. This may be due to the fact that intrinsic efficacies of the ligands differ between receptor subtypes, so that the diversity of native receptors is further complicated by the diversity of the mode the ligands act on GABAA receptor subtypes. The behavioral actions of alcohol (ethanol) are similar to those produced by GABAA receptor agonists. In agreement, alcohol-induced potentiation of GABAergic responses has often been observed at behavioral, electrophysiological and biochemical levels. Thus, there is clearly a GABAA-dependent component in the actions of alcohol. However, the site and mode of action of ethanol on GABAA/BZ receptors remain controversial.