Benzodiazepine-induced motor impairment linked to point mutation in cerebellar GABAA receptor

Enhanced benzodiazepine and ethanol actions on cerebellar GABA(A) receptors mediating glutamate release in an alcohol-sensitive rat line

Granule cell axon terminals of rat cerebellum possess benzodiazepine-insensitive GABA(A) receptors mediating glutamate release. We have investigated the ability of benzodiazepines, ethanol and furosemide to modulate the function of these receptors in the cerebellum of alcohol-tolerant (AT) and alcohol-nontolerant (ANT) rats. AT and ANT synaptosomes, prelabeled with [3H]D-aspartate, were superfused with GABA and various drugs during the K+ -depolarization. GABA similarly enhanced [3H]D-aspartate overflow in AT (EC50 = 1.7 microM) and ANT (EC50 = 3.9 microM) rats in a bicuculline-sensitive manner. Diazepam or zolpidem, at 0.1 microM, potentiated GABA at the GABA(A) receptor of ANT rats, but were ineffective at the AT receptor. Zolpidem acted with great potency (EC50 = 13.6 nM). Ethanol, added at 50 mM, potentiated GABA in ANT rats, but it was inactive at the GABA(A) receptor of the AT cerebellum. Furosemide significantly inhibited the effect of GABA in ANT, but not in AT synaptosomes. Our results show that one GABA(A) receptor (the receptor sited on granule cell terminals which mediates glutamate release) exhibits functional responses to diazepam and ethanol that differ between AT and ANT rats. However, the data with zolpidem and furosemide differ from previous results obtained with membranes of the granule cell layer suggesting that distinct GABA(A) receptor subtypes may exist on axon terminals versus soma/dendrites of granule cells.

Cerebellar GABA(A) receptors and anxiolytic action of diazepam

Alcohol-sensitive ANT rats have a point mutation in the cerebellum-enriched GABA(A) receptor alpha6 subunit, which makes this subunit and the ANT rats in vivo highly sensitive to benzodiazepine agonists. In the elevated plus maze test of anxiety, diazepam produced a greater anxiolytic response in the ANT rats than in the control, alcohol-insensitive AT rats. The ANT rats were less sensitive to the sedative effect of diazepam in the staircase test of exploration. The results thus suggest that the mutant cerebellar granule cell layer receptors can participate in GABA(A) receptor-activation-induced anxiolysis.

Dopamine receptor D2 and D4 genes, GABA(A) alpha-1 subunit genes and response to lithium prophylaxis in mood disorders

Lithium is an effective prophylactic agent in mood disorders, and genetic factors are likely to modulate individual susceptibility to lithium treatment. The aim of this study is to investigate the influence of dopamine receptor D2 (DRD2), D4 exon 3 (DRD4), and gamma-aminobutyric acid type A (GABA(A)) receptor alpha-1 subunit (GABRA1) gene variants on the efficacy of lithium prophylaxis in mood disorders. Patients with mood disorders (N = 125: bipolar subtype, n = 100; major depressive disorder subtype, n = 25) were followed prospectively for an average of 53 months and were typed for DRD2 (Ser311/Cys311: n = 121, VNTR: n = 63), DRD4 (n = 125) and GABRA1 (n = 61) variants using polymerase chain reaction (PCR) techniques. DRD2, DRD4 and GABRA1 variants were not associated with response to lithium. A trend was observed toward a better outcome of DRD4* 2/4 subjects, but it was due to only two subjects. Consideration of possible stratification effects like gender, polarity, family history, age at onset and duration of lithium treatment did not reveal any association either. DRD2, DRD4 and GABRA1 variants therefore do not appear to be associated with the outcome of lithium prophylaxis in mood disorders.

Differential effects of zinc on native GABA(A) receptor function in rat hippocampus and cerebellum

Hippocampal noradrenergic and cerebellar glutamatergic granule cell axon terminals possess GABA(A) receptors mediating enhancement of noradrenaline and glutamate release, respectively. The hippocampal receptor is benzodiazepine-sensitive, whereas the cerebellar one is not affected by benzodiazepine agonists, indicating the presence of an alpha6 subunit. We tested here the effects of Zn2+ on these two native GABA(A) receptor subtypes using superfused rat hippocampal and cerebellar synaptosomes. In the cerebellum, zinc ions strongly inhibited (IC50 approximately 1 microM) the potentiation of the K(+)-evoked [3H]D-aspartate release induced by GABA. In contrast, the GABA-evoked release of [3H]noradrenaline from hippocampal synaptosomes was much less sensitive to Zn2+ (IC50 > 30 microM). The effects of Zn2+ were then studied in two rat lines selected for high (ANT) and low (AT) alcohol sensitivity because granule cell GABA(A) receptors in ANT, but not AT, rats respond to benzodiazepine agonists due to a critical mutation in the alpha6 subunit. GABA increased the K(+)-evoked release of [3H]DCNS REGIONS-aspartate from cerebellar synaptosomes of AT and ANT rats, an effect prevented by the GABAA selective antagonist bicuculline. In AT rat cerebellum, the effect of GABA was strongly inhibited by Zn2+ (IC50 < or = 1 microM), whereas in ANT rats, the divalent cation was about 100-fold less potent. Thus, native benzodiazepine-sensitive GABAA receptors appear largely insensitive to functional inhibition by Zn2+ and vice versa. Changes in sensitivity to Zn2+ inhibition consequent to mutations in cerebellar granule cell GABA(A) receptor subunits may lead to changes in glutamate release from parallel fibers onto Purkinje cells and may play important roles in cerebellar dysfunctions.

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

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

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

Selective genotyping for the role of 5-HT2A, 5-HT2C, and GABA alpha 6 receptors and the serotonin transporter in the level of response to alcohol: a pilot study

BACKGROUND

The vulnerability to alcohol dependence appears to be genetically influenced through a variety of mechanisms. One potentially genetically mediated channel may be a low level of response (LR) to alcohol, which has been seen in children of alcoholics and noted to predict future alcohol abuse and dependence. This pilot study uses a case and control genetic association approach to evaluate the possible role of five genotypes in both LR and alcoholism in informative subgroups of men with high and low LR scores documented 15 years earlier.

METHODS

As part of a larger study, 41 men, about 39 years old, were selected from among the first 113, completed 15-year follow-ups in a prospective study. The 17 subjects whose LRs at age 20 were in the lower third were compared on five polymorphisms of four genes with 24 men whose reactions to alcohol had been above the median.

RESULTS

The 14 men with the LL genotype of the serotonin transporter (5-HTT) polymorphism and the seven with the Pro/Ser genotype of the GABAA alpha 6 polymorphism had demonstrated lower LR scores at about age 20, and had significantly higher proportions of alcoholics than the other genotypes for those loci. All four subjects with combined LL and Pro/Ser genotypes had developed alcoholism and demonstrated the lowest LR scores overall. There was no evidence that two polymorphisms of the 5-HT2A receptor gene and one of the 5-HT2C receptor gene were related to LR or alcoholism in this sample.

CONCLUSIONS

These results are consistent with animal and human studies suggesting a possible role for genetic variation in the GABAA alpha 6 and the serotonin transporter in the reaction to alcohol and the alcoholism risk.

Cerebellar granule-cell-specific GABAA receptors attenuate benzodiazepine-induced ataxia: evidence from alpha 6-subunit-deficient mice

Benzodiazepine- and alcohol-induced ataxias in rodents have been proposed to be affected by the gamma-aminobutyric acid type A (GABAA) receptor alpha 6 subunit, which contributes to receptors specifically expressed in cerebellar granule cells. We have studied an alpha 6 -/- mouse line for motor performance and drug sensitivity. These mice, as a result of a specific genetic lesion, carry a precise impairment at their Golgi-granule cell synapses. On motor performance tests (rotarod, horizontal wire, pole descending, staircase and swimming tests) there were no robust baseline differences in motor function or motor learning between alpha 6 -/- and alpha 6 +/+ mice. On the rotarod test, however, the mutant mice were significantly more impaired by diazepam (5-20 mg/kg, i.p.), when compared with alpha 6 +/+ control and background C57BL/6J and 129/SvJ mouse lines. Ethanol (2.0-2.5 g/kg, i.p.) produced similar impairment in the alpha 6 -/- and alpha +/+ mice. Diazepam-induced ataxia in alpha 6 -/- mice could be reversed by the benzodiazepine site antagonist flumazenil, indicating the involvement of the remaining alpha 1 beta 2/3 gamma 2 GABAA receptors of the granule cells. The level of activity in this synapse is crucial in regulating the execution of motor tasks. We conclude that GABAA receptor alpha 6 subunit-dependent actions in the cerebellar cortex can be compensated by other receptor subtypes; but if not for the alpha 6 subunit, patients on benzodiazepine medication would suffer considerably from ataxic side-effects.

Genetic dissection of the molecular target(s) of anesthetics with the gene knockout approach in mice

Techniques have recently been developed that enable the creation of mice that harbor specific, predetermined genetic changes. These 'gene knockout mice', which contain a single genetic modification that is determined by the investigator, can subsequently be analyzed with tests that span the molecular, cellular, and behavioral levels. Application of such a multi-level approach to mechanisms of drug action should ultimately allow general anesthetic responses to be properly attributed to a molecular site.

Decreased expression of GABAA receptor alpha6 and beta3 subunits in stargazer mutant mice: a possible role for brain-derived neurotrophic factor in the regulation of cerebellar GABAA receptor expression?

The cerebellar granule cells of the spontaneous recessive mutant mouse strain, stargazer (stg/stg), fail to express brain-derived neurotrophic factor mRNA. This deficit is exclusive to these neurons and is believed to underlie the motor irregularities displayed by stg/stg, though the molecular basis for their phenotype has still to be resolved. Brain-derived neurotrophic factor has been shown to play a role in the postnatal maturation of cerebellar granule cells. Differentiation of these neurons, postnatally, is characterised by a switch in their GABAA receptor subunit expression profile. Notably, the GABAA receptor alpha6 subunit, which is specific to these neurons, becomes detectable at postnatal days 10-14 (P10-14). To determine whether cerebellar GABAA receptor expression has been compromised in stg/stg mice, the expression levels of GABAA receptor alpha1, alpha6, beta2 and beta3 subunits were compared between stg/stg mice and the appropriate wild-type background strain, C57BL/6J (+/+). By quantitative immunoblotting, it was found that the expression of the alpha6 and beta3 subunits was 23+/-8% and 38+/-12% (mean+/-S.E.M., n=6) of control (+/+) levels, respectively. In contrast, the expression of the alpha1 and beta2 subunits was not significantly different from controls, being 116+/-11% and 87+/-24% (mean+/-S.E.M., n=6) of +/+ levels, respectively. Total specific [3H]Ro15-4513 binding activity detected in cerebellar membranes prepared from stg/stg was not significantly different from +/+ mice. However, the benzodiazepine agonist-insensitive subtype of [3H]Ro15-4513 binding activity, a pharmacological motif of alpha6 subunit-containing GABAA receptors, was lower in stg/stg mice relative to the +/+ strain which correlated with the lowered level of alpha6 subunit expression. Thus, we have identified an abnormality in the GABAA receptor profile of stg/stg mutant mice that might underpin its irregular phenotype.

Amino acid residue 200 on the alpha1 subunit of GABA(A) receptors affects the interaction with selected benzodiazepine binding site ligands

Mutant alph1 subunits of the GABA(A) receptor were coexpressed in combination with the wild-type beta2 and gamma2 subunits in human embryonic kidney (HEK) 293 cells. The binding properties of various benzodiazepine site ligands were determined by displacement of ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5a]-[1,4]benzodia zepine-3-carboxylate ([3H]Ro 15-1788). The mutation G200E led to a decrease in zolpidem and 3-methyl-6-[3-(trifluoromethyl)phenyl]-1,2,4-triazolo[4,3-b]pyridazine (CL 218872) affinity amounting to 16- and 8-fold. Receptors containing a conservative T206V substitution showed a 41- and 38-fold increase in methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) and CL 218872 affinity combined with a decrease in diazepam and zolpidem affinity, amounting to 7- and 10-fold. Two mutations, Q203A and Q203S showed almost no effects on the binding of benzodiazepine site ligands, indicating that this residue is not involved in the binding of benzodiazepines and related compounds.

Ethanol: Novel Actions on Nerve Cell Physiology Explain Impaired Functions

Molecular biological tools have revealed receptor proteins for excitatory and inhibitory neurotransmitters on cell membranes as targets of ethanol action. Behavioral and pharmacogenetic assays using rodent lines have supported this neurotransmitter theory of ethanol action and given a firm basis for future identification of the relevant genes and the central physiological processes vulnerable to ethanol.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

GABAA-benzodiazepine receptors in the striatum are involved in the sedation produced by a moderate, but not an intoxicating ethanol dose in outbred Wistar rats

The role of the dorsal striatum in mediating the sedation produced by a moderate (0.75 g/kg) and an intoxicating (1.25 g/kg) EtOH dose was investigated in the open field by determining the capacity of direct intrastriatal injections of RY 008, a partial inverse agonist of the benzodiazepine (BDZ) receptor, to antagonize EtOH's effects. SR 95531, the competitive high-affinity GABAA antagonist was used as a reference compound. Intrastriatal RY 008 (50, 500 ng) and SR 95531 (50 ng) antagonized the sedation produced by the 0.75 g/kg EtOH dose. However, RY 008 did not alter the sedation produced by the 1.25 g/kg dose. RY 008 alone was without effect. RY 008 also failed to negatively modulate GABAergic function at alpha1beta2gamma2 or alpha6beta2gamma2 receptor subtypes expressed in Xenopus oocytes. Intrastriatal modulation of the moderate EtOH dose was site specific: no antagonism by RY 008 after intraaccumbens infusions was observed. The results suggest that central GABAA-BDZ receptors in the dorsal striatum play an important role in mediating the sedation produced by a moderate EtOH dose in the open field.

Search for mutations near the alternatively spliced 8-amino-acid exon in the GABAA receptor gamma 2 subunit gene and lack of allelic association with alcoholism among four aboriginal groups and Han Chinese in Taiwan

The alternatively spliced 8-amino-acid exon for the GABAA receptor gamma2 subunit gene (GABRC2) has been postulated to mediate behavioral actions of alcohol. A rapid search for splice-junction mutations near the 8-amino-acid exon using restriction enzymes which normally recognize sequences near or in the exon gave negative results among 217 alcoholics in four aboriginal groups (Ami, Atayal, Bunun and Paiwan) and Han Chinese in Taiwan. The role of the GABRC2 gene in alcoholism was further assessed by a comparison of allelic frequencies revealed by a NciI RFLP between case and control groups. No significant association of alcohol dependence with GABRC2 alleles was observed. These results suggest that the GABRC2 gene probably does not play an essential role in predisposition to alcoholism in the sample population.

Anomalous rectifying properties of 'diazepam-insensitive' GABA(A) receptors

Studies using recombinant systems indicate that 'diazepam-insensitive' GABA(A) receptors in the central nervous system contain alpha4 and alpha6 subunits while 'diazepam-sensitive' GABA(A) receptors contain alpha1, alpha2, alpha3 and alpha5 subunits. Both native and recombinant diazepam-sensitive GABA(A) receptors typically exhibit large, outwardly rectifying currents. For example, in patch clamp studies, Human Embryonic Kidney (HEK) 293 cells transfected with cDNAs encoding alpha1beta2gamma2 subunits exhibit a rectification ratio (I +60 mV/I -60 mV) of 1.95 +/- 0.21. However, anomalous rectification was observed in recombinant diazepam-insensitive GABA(A) receptors composed of either alpha4beta2gamma2 (rectification ratio, 0.74 +/- 0.09) or alpha6beta2gamma2 (rectification ratio, 0.67 +/- 0.11) subunits. Based on sequence differences between diazepam-sensitive and -insensitive GABA(A) receptor alpha subunits in the vicinity of the putative channel lining, a point mutation was introduced at His273 on the alpha4 subunit. The rectification ratio in cells expressing a mutated alpha4(Asn273)beta2gamma2 receptor increased to 1.92 +/- 0.17. Moreover, mutation of the homologous residue in the alpha1 subunit to histidine reduced the rectification ratio of alpha1(His274)beta2gamma2 to 1.02 +/- 0.12. The affinities of benzodiazepine site ligands at diazepam-sensitive and -insensitive GABA(A) receptors were unaffected by these mutations. Thus, the electrophysiological properties of diazepam-sensitive and -insensitive GABA(A) receptors may be as divergent as their pharmacological characteristics.

Benzodiazepine receptor antagonists modulate the actions of ethanol in alcohol-preferring and -nonpreferring rats

The pyrazoloquinoline CGS 8216 (2-phenylpyrazolo-[4,3-c]-quinolin-3 (5H)-one, 0.05-2 mg/kg) and the beta-carboline ZK 93426 (ethyl-5-isopropyl-4-methyl-beta-carboline-3-carboxylate, 1-10 mg/kg) benzodiazepine receptor antagonists were evaluated for their capacity to modulate the behavioral actions of ethanol in alcohol preferring and -nonpreferring rats. When alcohol-preferring rats were presented with a two-bottle choice test between ethanol (10% v/v) and a saccharin (0.0125% g/v) solution, both antagonists dose-dependently reduced intake of ethanol by 35-92% of control levels on day 1 at the initial 15 min interval of the 4 h limited access. Saccharin drinking was suppressed only with the highest doses. CGS 8216 (0.25 mg/kg) and ZK 93426 (4 mg/kg) unmasked the anxiolytic effects of a hypnotic ethanol dose (1.5 g/kg ethanol) on the plus maze test in alcohol-preferring rats, but potentiated the ethanol-induced suppression in alcohol-nonpreferring rats. CGS 8216 (0.25 mg/kg) and ZK 93426 (4 mg/kg) attenuated the ethanol (0.5 and 1.5 g/kg)-induced suppression in the open field in alcohol-nonpreferring rats; however, CGS 8216 potentiated the depressant effects of the lower ethanol dose (0.5 g/kg) in alcohol-preferring rats. These findings provide evidence that benzodiazepine receptor antagonists may differentially modulate the behavioral actions of ethanol in alcohol-preferring and-nonpreferring rats. It is possible that the qualitative pharmacodynamic differences seen in the present study may be related to selective breeding for alcohol preference. The findings indicate the potential for development of receptor specific ligands devoid of toxic effects which may be useful in the treatment of alcohol abuse and alcoholism.

Adenosine A1 receptor antisense infused in striatum of rats: actions on alcohol-induced locomotor impairment, blood alcohol, and body temperature

Previous pharmacological studies show that adenosine receptors in the corpus striatum may be involved in locomotor coordination. The purpose of this investigation was to determine whether the adenosine A1 receptor subtype would alter the locomotor response due to incapacitating doses of alcohol. In these experiments, an antisense oligodeoxynucleotide (ODN) targeted to the adenosine A1 receptor was used to elucidate its possible role in locomotor function. After bilateral cannulation of the caudate nuclei of two strains of adult male rats, the animals were trained to remain on a rotorod for an entire 3-min interval. Then, a standard dose of 2.0 micrograms per 2.0 microliters of the A1 adenosine antisense (A1AS), dissolved in a pyrogen-free artificial cerebrospinal fluid (aCSF), was microinjected four times bilaterally into the caudate nuclei of the rats at successive 12-h intervals over 2 days. Three sets of controls were utilized: intragastric gavage with tap water alone: intragastric gavage of 3.5-4.0 g/kg 20% alcohol alone; and the aCSF vehicle alone microinjected identically in the caudate nuclei. The results showed that the intragastric administration of 20% alcohol in a dose of 3.5-4.0 g/kg caused a complete incapacitation of locomotor performance. Moreover, the A1AS injected in the striatum failed to alter significantly the action of alcohol in its impairment of the rats' ability to negotiate the rotorod. Concurrent measures of blood alcohol and body temperature taken to validate the efficacy of alcohol administration correlated precisely with the blockade of locomotor behavior of the animals. These findings thus suggest that because of the specificity of the A1AS probe, the A1 receptor in the striatum is not involved in the alcohol-induced incapacitation of locomotor activity.

A gene in human chromosome band Xq28 (GABRE) defines a putative new subunit class of the GABAA neurotransmitter receptor

We have isolated and sequenced a novel human gene (GABRE) of the GABAA neurotransmitter receptor family. A cDNA sequence of the gene coding for a 506 amino acid protein was identified, representing a member of a putative new class (epsilon) of the GABAA receptor. The gene is transcribed at least at low level in several different tissues, with the highest levels being detected in adult heart and placenta. Alternative splicing of GABRE transcripts isolated from different tissues was observed at multiple positions of the gene, yielding an unusually complex variety of cDNA variants. The structure of the 5' region of most cDNAs is compatible with expression of protein sequence epsilon only in adult brain, whereas in other tissues, the majority of transcripts code for truncated protein sequences. The GABRE gene extends over 14 kb and is clustered together with the alpha 3 and the putative beta 4 GABAA receptor subunit genes in an approximately 0.8-Mb interval in chromosome band Xq28, located in the candidate regions of two different neurologic diseases. Based on features of conservation of protein sequences, gene structure, and genomic organization of GABAA receptor gene clusters, we propose that the epsilon and gamma subunit genes have a common ancestor and that GABAA receptor gene clusters in the human genome have diverged by multiple duplication events of an ancestral gene cluster containing one each alpha, beta, and gamma/epsilon precursor gene.

Polymorphisms and intron sequences flanking the alternatively spliced 8-amino-acid exon of gamma2 subunit gene for GABAA receptors

Gamma-amminobutyric acid (GABA) is a major inhibitory neurotransmitter. Two alternatively spliced forms of the gamma2 subunit of GABAA receptor (gamma2L and gamma2S), which differ by an exon of eight amino acids, show different sensitivities to modulatory effects of ethanol on receptor activities. A 2.7 kb DNA fragment and an 1.7 kb DNA fragment covering respectively the introns upstream and downstream from the 8-amino-acid exon were obtained through PCR-amplification of human genomic DNA using primers derived from cDNA sequences. Total sequencing of these fragments showed a composite 4.2 kb segment containing the 8-amino-acid exon and consensus sequences for RNA splice junctions. Restriction fragment length polymorphisms (RFLP) based on NciI restriction digestion were found among Chinese in Taiwan. This RFLP provides a useful DNA marker for allelic association or linkage analyses of the role of GABAA receptors in predisposition to alcoholism or other neuropsychiatric disorders.

A point mutation in the gamma2 subunit of gamma-aminobutyric acid type A receptors results in altered benzodiazepine binding site specificity

Benzodiazepines allosterically modulate gamma-aminobutyric acid (GABA) evoked chloride currents of gamma-aminobutyric acid type A (GABAA) receptors. Coexpression of either rat gamma2 or gamma3, in combination with alpha1 and beta2 subunits, results both in receptors displaying high [3H]Ro 15-1788 affinity. However, receptors containing a gamma3 subunit display a 178-fold reduced affinity to zolpidem as compared with gamma2-containing receptors. Eight chimeras between gamma2 and gamma3 were constructed followed by nine different point mutations in gamma2, each to the homologous amino acid residue found in gamma3. Chimeric or mutant gamma subunits were coexpressed with alpha1 and beta2 in human embryonic kidney 293 cells to localize amino acid residues responsible for the reduced zolpidem affinity. Substitution of a methionine-to-leucine at position 130 of gamma2 (gamma2M130L) resulted in a 51-fold reduction in zolpidem affinity whereas the affinity to [3H]Ro 15-1788 remained unchanged. The affinity for diazepam was only decreased by about 2-fold. The same mutation resulted in a 9-fold increase in Cl 218872 affinity. A second mutation (gamma2M57I) was found to reduce zolpidem affinity by about 4-fold. Wild-type and gamma2M130L-containing receptors were functionally expressed in Xenopus oocytes. Upon mutation allosteric coupling between agonist and modulatory sites is preserved. Dose-response curves for zolpidem and for diazepam showed that the zolpidem but not the diazepam apparent affinity is drastically reduced. The apparent GABA affinity is not significantly affected by the gamma2M130L mutation. The identified amino acid residues may define part of the benzodiazepine binding pocket of GABAA receptors. As the modulatory site in the GABAA receptor is homologous to the GABA site, and to all agonist sites of related receptors, gamma2M130 may either point to a homologous region important for agonist binding in all receptors or define a new region not underlying this principle.

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.

Subtle changes in residue 77 of the gamma subunit of alpha1beta2gamma2 GABAA receptors drastically alter the affinity for ligands of the benzodiazepine binding site

Recombinant alpha1beta2gamma2 gamma-aminobutyric acid type A (GABAA) receptors were functionally expressed in Xenopus oocytes. Upon the mutation F77L, diazepam and Ro 15-1788 retained the ability to interact with the benzodiazepine binding site, but zolpidem lost this ability. To quantify these data, radioligand binding experiments were performed using membrane preparations of transiently transfected human embryonic kidney 293 cells. The amino acid gamma77, phenylalanine, was also mutated to tyrosine, tryptophan, and isoleucine. Although there was little effect on Ro 15-1788 binding upon mutation to tyrosine, the loss in affinity for diazepam was from 12 to 2,720 nM. The change to leucine, in contrast, resulted in little change in the diazepam affinity, whereas there was a strongly reduced affinity for zolpidem from 17 to 4,870 nM and for methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) from 1.9 to 1,780 nM, respectively. The change to tryptophan resulted in two-phasic displacement curves, and only about 50% of the [3H]flunitrazepam binding could be displaced by zolpidem, DMCM, and Ro 15-1788, respectively, whereas midazolam and diazepam still resulted in 100% displacement, indicating the presence of two sites upon expression of this mutant receptor. Functional expression in Xenopus oocytes showed that all mutant channels displayed a comparatively small change (<4.3-fold) in their apparent agonist affinity and that these channels could still be functionally modulated by ligands of the benzodiazepine binding site. We conclude that subtle changes in gammaF77 drastically affect benzodiazepine pharmacology and that this residue probably interacts directly with most ligands of the benzodiazepine binding site and therefore defines part of the benzodiazepine binding pocket.

Mice devoid of gamma-aminobutyrate type A receptor beta3 subunit have epilepsy, cleft palate, and hypersensitive behavior

gamma-Aminobutyric acid type A receptors (GABA(A)-Rs) mediate the bulk of rapid inhibitory synaptic transmission in the central nervous system. The beta3 subunit is an essential component of the GABA(A)-R in many brain regions, especially during development, and is implicated in several pathophysiologic processes. We examined mice harboring a beta3 gene inactivated by gene targeting. GABA(A)-R density is approximately halved in brain of beta3-deficient mice, and GABA(A)-R function is severely impaired. Most beta3-deficient mice die as neonates; some neonatal mortality, but not all, is accompanied by cleft palate. beta3-deficient mice that survive are runted until weaning but achieve normal body size by adulthood, although with reduced life span. These mice are fertile but mothers fail to nurture offspring. Brain morphology is grossly normal, but a number of behaviors are abnormal, consistent with the widespread location of the beta3 subunit. The mice are very hyperactive and hyperresponsive to human contact and other sensory stimuli, and often run continuously in tight circles. When held by the tail, they hold all paws in like a ball, which is frequently a sign of neurological impairment. They have difficulty swimming, walking on grids, and fall off platforms and rotarods, although they do not have a jerky gait. beta3-deficient mice display frequent myoclonus and occasional epileptic seizures, documented by electroencephalographic recording. Hyperactivity, lack of coordination, and seizures are consistent with reduced presynaptic inhibition in spinal cord and impaired inhibition in higher cortical centers and/or pleiotropic developmental defects.

Gene knockout of the alpha6 subunit of the gamma-aminobutyric acid type A receptor: lack of effect on responses to ethanol, pentobarbital, and general anesthetics

The alpha6 subunit of the gamma-aminobutyric acid type A receptor (GABA(A)-R) has been implicated in mediating the intoxicating effects of ethanol and the motor ataxic effects of general anesthetics. To test this hypothesis, we used gene targeting in embryonic stem cells to create mice lacking a functional alpha6 gene. Homozygous mice are viable and fertile and have grossly normal cerebellar cytoarchitecture. Northern blot and reverse transcriptase-polymerase chain reaction analyses demonstrated that the targeting event disrupted production of functional alpha6 mRNA. Autoradiography of histological sections of adult brains demonstrated that diazepam-insensitive binding of [3H]Ro15-4513 to the cerebellar granule cell layer of wild-type mice was completely absent in homozygous mice. Cerebellar GABA(A)-R density was unchanged in the mutant mice; however, the apparent affinity for muscimol was markedly reduced. Sleep time response to injection of ethanol after pretreatment with vehicle or Ro15-4513 did not differ between genotypes. Sleep time response to injection of pentobarbital and loss of righting reflex and response to tail clamp stimulus in mice anesthetized with volatile anesthetics also did not differ between genotypes. Thus, the alpha6 subunit of the GABA(A)-R is not required for normal development, viability, and fertility and does not seem to be a critical or unique component of the neuronal pathway mediating the hypnotic effect of ethanol and its antagonism by Ro15-4513 in mice. Similarly, the alpha6 subunit does not seem to be involved in the behavioral responses to general anesthetics or pentobarbital.

The GABAA receptor alpha 6 subunit gene (Gabra6) is tightly linked to the alpha 1-gamma 2 subunit cluster on mouse chromosome 11

We have established that the GABAA receptor alpha 6 (Gabra6) and alpha 1 (Gabra 1) subunit genes are tightly linked on mouse chromosome 11 by analysing the strain distribution patterns of RFLPs for the two genes and microsatellite markers flanking these genes in 26 BXD recombinant inbred strains. These results further demonstrate clustering of the GABAA receptor subunit genes on mouse chromosomes and the synteny for these clusters between the mouse and human genomes.

Localization and pharmacological characterization of pigeon diazepam-insensitive GABAA receptors

Transduction mechanisms associated with ligand binding at diazepam-insensitive subtypes of GABAA receptors remain largely unknown, but unique behavioral effects of ligands binding at these sites have been reported in pigeons. The present study further evaluated the pharmacological characteristics of diazepam-insensitive GABAA receptors in pigeon brain, using [3H]Ro 15-4513. Autoradiography detected diazepam-insensitive benzodiazepine sites on GABAA receptors in a number of brain regions, with the highest densities present in the olfactory bulb, hippocampus, thalamic nuclei and cerebellar granule cell layers, with densities of approximately 10-20% of total benzodiazepine receptor binding. Saturation analysis revealed significant densities (approximately 10% of total benzodiazepine receptor binding) of extracerebellar diazepam-insensitive benzodiazepine receptors in optic lobe, hippocampus, and brainstem compared to 27% in cerebellum. As reported for mammalian diazepam-sensitive benzodiazepine receptors, GABA (50 microM) generally increased the affinities of agonists and partial agonists, had little effect on the affinities of antagonists, and decreased the affinity of an inverse agonist for pigeon cerebellar diazepam-sensitive benzodiazepine receptors. GABA modulation of ligand binding to diazepam-insensitive benzodiazepine receptors was less than that observed for diazepam-sensitive sites, and no positive modulation was observed. These results demonstrate the presence of cerebellar and extracerebellar diazepam-insensitive benzodiazepine receptors in pigeon brain, with distribution patterns and pharmacology similar to those reported in mammals. The comparable central localization and pharmacological properties of drugs at diazepam-sensitive and -insensitive benzodiazepine receptors in pigeons and rats attests to the evolutionary conservation of GABAA systems.

Chloride channels: an emerging molecular picture

Chloride channels are probably found in every cell, from bacteria to mammals. Their physiological tasks range from cell volume regulation to stabilization of the membrane potential, signal transduction, transepithelial transport and acidification of intracellular organelles. These different functions require the presence of many distinct chloride channels, which are differentially expressed and regulated by various stimuli. These include various intracellular messengers (like calcium and cyclic AMP), pH, extracellular ligands and transmembrane voltage. Three major structural classes of chloride channels are known to date, but there may be others not yet identified. After an overview of the general functions of chloride channels, this review will focus on these cloned chloride channels: the CLC chloride channel family, which includes voltage-gated chloride channels, and the cystic fibrosis transmembrane regulator (CFTR), which performs other functions in addition to being a chloride channel. Finally, a short section deals with GABA and glycine receptors. Diseases resulting from chloride channel defects will be specially emphasized, together with the somewhat limited information about how these proteins work at the molecular level.

Subunit-dependent interaction of the general anaesthetic etomidate with the gamma-aminobutyric acid type A receptor

1. The GABA modulating and GABA-mimetic actions of the general anaesthetic etomidate were examined in voltage-clamp recordings performed on Xenopus laevis oocytes induced, by cRNA injection, to express human recombinant gamma-aminobutyric acidA (GABAA) receptor subunits. 2. Currents mediated by recombinant receptors with the ternary subunit composition alpha x beta y gamma 2L (where x = 1,2,3 or 6 and y = 1 or 2), in response to GABA applied at the appropriate EC10, were enhanced by etomidate in a manner that was dependent upon the identity of both the alpha and beta subunit isoforms. 3. For the beta 2-subunit containing receptors tested, the EC50 for the potentiation of GABA-evoked currents by etomidate (range 0.6 to 1.2 microM) was little affected by the nature of the alpha subunit present within the hetero-oligomeric complex. However, replacement of the beta 2 by the beta 1 subunit produced a 9-12 fold increase in the etomidate EC50 (6 to 11 microM) for all alpha-isoforms tested. 4. For alpha 1, alpha 2 and alpha 6, but not alpha 3-subunit containing receptors, the maximal potentiation of GABA-evoked currents by etomidate was greater for beta 2- than for beta 1-subunit containing receptors. This was most clearly exemplified by receptors composed of alpha 6 beta 1 gamma 2L compared to alpha 6 beta 2 gamma 2L subunits, where a maximally effective concentration of etomidate potentiated currents evoked by GABA at EC10 to 28 +/- 2% and 169 +/- 4% of the maximal GABA response, respectively. 5. For alpha 1 subunit-containing receptors, the potency and maximal potentiating effect of either pentobarbitone or propofol was essentially unaffected by the beta subunit isoform contained within the receptor complex. The potency of the anaesthetic neurosteroid 5 alpha-pregnan-3 alpha-ol-20-one was marginally higher for beta 1 rather than the beta 2 subunit-containing receptor, although its maximal effect was similar at the two receptor isoforms. 6. The GABA-mimetic action of etomidate was supported by beta 2- but not beta 1-subunit containing receptors, whereas that of pentobarbitone or propofol was evident with either beta isoform. For beta 2-subunit containing receptors, both the agonist EC50 and the maximal current produced by etomidate were additionally influenced by the alpha isoform. 7. It is concluded that the subtype of beta-subunit influences the potency with which etomidate potentiates GABA-evoked currents and that the beta isoform is a crucial determinant of the GABA-mimetic activity of this compound. The nature of the alpha-subunit also impacts upon the maximal potentiation and activation that the compound may elicit. Such pronounced influences may aid the identification of the site that recognises etomidate. More generally, these results provide a clear example of structural specificity in anaesthetic action.

Blockade of behavioral effects of bretazenil by flumazenil and ZK 93,426 in pigeons

Benzodiazepine receptor partial agonists manifest full efficacy in preclinical tests of anxiolytic drug action but do not fully reproduce the discriminative stimulus effects of benzodiazepine receptor full agonists in pigeons. The partial agonist, bretazenil, binds to both diazepam-sensitive and diazepam-insensitive GABAA receptors. Previous studies have suggested a role for each of these receptor populations in some behavioral effects of bretazenil in pigeons. A possible role for these receptor subtypes in the behavioral effects of bretazenil was further investigated through drug interaction studies with the benzodiazepine receptor antagonists, flumazenil and ZK 93,426. Whereas flumazenil binds with high affinity to both receptor isoforms, ZK 93,426 binds preferentially to diazepam-sensitive binding sites. Bretazenil markedly increased punished responding of pigeons without significantly affecting nonpunished responding. In pigeons discriminating the full benzodiazepine receptor agonist, midazolam, from saline, bretazenil produced only 60-75% maximal effect. Flumazenil and ZK 93,426 neither increased punished responding nor substituted for midazolam, but dose-dependently blocked the effects of bretazenil on punished responding. Flumazenil also dose-dependently blocked the effects of bretazenil in midazolam-discriminating pigeons, whereas ZK 93,426 only attenuated this effect. These results indicate that bretazenil's actions as a partial agonist at diazepam-sensitive benzodiazepine receptors mediate increases in punished responding and substitution for the discriminative stimulus effects of midazolam in pigeons. The differences in the effects of flumazenil and ZK 93,426 on the discriminative stimulus effects of bretazenil suggest a potential contribution of diazepam-insensitive sites to this behavioral effect.

Genetic approaches to CNS disorders with particular reference to GABAA-receptor mutations

The tools of molecular biology will bring the field of human genetics into a new era by permitting the analysis of the genetic contribution to disease. Most single gene disorders, inherited in a Mendelian fashion, will be molecularly diagnosed. In addition, the genetic susceptibility of common, complex diseases such a schizophrenia can be clarified, even though the conditions are not inherited as Mendelian characteristics. The mapping of the human genome will increase the rate at which new disease genes are identified and isolated. Finally, the development of genetically engineered animal models will help to dissect the steps involved in physiological and pathophysiological processes and thereby enhance our understanding of complex biological systems.

Propofol-induced ataxia and hypnosis in rat lines selected for differential alcohol sensitivity

An alcohol-sensitive rat line, selectively bred for high sensitivity to ethanol-induced motor impairment, also exhibits greater sensitivity to gamma-aminobutyric acid type A (GABAA) receptor agonists, such as benzodiazepines and barbiturates, than an alcohol-insensitive rat line. We have investigated whether this difference was also maintained for the most recent intravenous anaesthetic, propofol. Propofol (100 mg/kg, intraperitoneally) induced similar sleep times and produced identical plasma propofol concentrations in alcohol-sensitive and alcohol-insensitive rat lines. At lower doses (50 and 75, but not 25 mg/kg), propofol produced a greater motor impairment in a tilting plane test in alcohol-sensitive than alcohol-insensitive rats shortly after the injection. Binding of a convulsant, [35S]t-butylbicyclophosphorothionate, to cerebellar and cerebrocortical GABAA receptors in the presence of 2 microM GABA was similarly affected by low micromolar propofol concentrations in both rat lines, while in the absence of GABA, propofol was slightly less potent in the alcohol-sensitive than alcohol-insensitive line. These data indicate that alcohol-sensitive rats show transiently enhanced sensitivity to an ataxic, but not to a hypnotic dose of propofol, which cannot be explained by sensitivity differences to propofol in GABAA receptors determined in a binding assay using brain membrane homogenates.

Diazepam-insensitive GABAA receptors in rat cerebellum and thalamus

Three major populations of GABAA receptor binding sites are present in cerebellar membranes: diazepam-sensitive [3H]Ro15-4513 binding sites, diazepam-insensitive [3H]Ro15-4513 binding sites and high-affinity [3H]muscimol binding sites. All three populations contain a beta subunit as shown by immunoprecipitation with antibodies that recognize all beta subunits. The beta 3 subtype of beta subunit is contained in all three populations, but only a similar low fraction (< 20%) in each. Thus, the majority contain beta subunits other than beta 3 (beta 2 and beta 1) and beta 3 subunits are not selectively associated with nor lacking in any of the three binding populations. Antibodies to the gamma 2 subunit precipitated similar fractions of [3H]Ro15-4513, [3H]flunitrazepam and [3H]muscimol binding sites, showing that gamma 2 subunits are present in high-affinity muscimol binding isoforms, as well as a significant fraction of the diazepam-insensitive [3H]Ro15-4513 binding sites. Under conditions that identify the 56 kDa alpha 6 subunit on SDS-PAGE as the diazepam insensitive site of [3H]Ro15-4513 binding in cerebellum, no polypeptide showing diazepam-insensitive binding of [3H]Ro15-4513 could be photoaffinity-labeled in rat thalamus. These results suggest that alpha 4 subunits in the thalamus participate primarily in subunit combinations which bind muscimol but not any benzodiazepine site ligands.

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+.

Functional evidence for two native GABAA receptor subtypes in adult rat hippocampus and cerebellum

Studies of molecular cloning predict great heterogeneity for the GABAA receptor; however, evidence for functionally and pharmacologically distinct native GABAA receptors is relatively scarce. In this work we have compared some of the functional and pharmacological properties of two GABAA receptors previously shown to be present in the adult rat central nervous system. In superfused hippocampal synaptosomes activation of GABAA receptors increased the basal release of [3H]noradrenaline (EC50 for GABA = 3.2 microM). In contrast, the overflow evoked by depolarization with high-K+ (12 or 35 mM) was not affected. Conversely, GABAA receptor activation led to potentiation of the K(+)-evoked overflow of [3H]D-aspartate from cerebellar synaptosomes (EC50 for GABA = 1.3 microM) whereas the basal release remained unchanged. GABA and muscimol also potentiated the K(+)-evoked overflow of endogenous glutamate in cerebellum. Diazepam enhanced the GABA (3 microM)-evoked [3H]noradrenaline release (EC50 = 65 nM). The diazepam potentiation of the GABA- or muscimol-evoked release of [3H]noradrenaline was inversely related to the agonist concentration. The effect of diazepam was reversed by the benzodiazepine antagonist flumazenil. Zolpidem mimicked diazepam (EC50 = 14 nM). The increase of the K(+)-evoked overflow of [3H]D-aspartate (or of endogenous glutamate) elicited by GABA or muscimol in cerebellar synaptosomes was not affected by benzodiazepines (diazepam or clonazepam) or by zolpidem. On the other hand, Ro 15-4513, an inverse agonist at the benzodiazepine site, strongly inhibited (EC50 = 7 nM) the enhancement by GABA (3 microM) of the K(+)-evoked [3H]D-aspartate overflow in cerebellar synaptosomes; the effect of Ro 15-4513 was reversed by flumazenil. These results suggest the existence in the central nervous system of the adult rat of two native pharmacological-subtypes of the GABAA receptor having different function, regional distribution and neuronal location; the receptors require different membrane potential to be activated and display different sensitivity to benzodiazepines and to drugs acting at benzodiazepine sites.

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

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

Benzodiazepines on trial: a research strategy for their rehabilitation

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

Pharmacologic actions of subtype-selective and novel GABAergic ligands in rat lines with differential sensitivity to ethanol

Alcohol-nontolerant (ANT) rats, produced by selective breeding for high sensitivity to motor-impairing effects of ethanol, have a point mutation in the cerebellar gamma-aminobutyric acid type A (GABAA) receptor alpha 6 subunit, which has been proposed to underlie enhanced sensitivity to benzodiazepine agonists as well. We compared ANT and alcohol-tolerant (AT) rats using behavioral and neurochemical methods to assess the significance of alpha 6- and non alpha 6-containing GABAA receptor subtypes. Motor performance in a tilting plane test was largely unaffected by a type I benzodiazepine receptor-preferring agonist, zolpidem [1-10 mg/kg, intraperitoneally (IP)], partial benzodiazepine agonists bretazenil and ZG-63 (both at 40 mg/kg, IP), and a novel broad-spectrum anticonvulsant loreclezole (40 mg/kg, IP) in both ANT and AT rats. In contrast, diazepam (10 mg/kg, IP) impaired performance of the ANT but not AT animals. These data, supported by results from brain regional autoradiography of [3H]Ro15-4513 and membrane binding of [3H]ZG-63 and [35S]TBPS as influenced by these ligands, strongly suggest that only ligands with full agonist actions on mutant (ANT) but not wild-type (AT) alpha 6-containing GABAA receptors are able to produce motor impairment in the ANT rats.

GabaA Receptors: Pharmacology, Behavioral Roles, and Motor Disorders

τ-Aminobutyric acid (GABA), the most prevalent inhibitory neurotransmitter in the mammalian brain, exerts its main action through GABAA receptors. They belong to the superfamily of ligand-gated ion channels and respond to GABA by the opening of an intrinsic anion channel. Multiple GABAA receptor subtypes in the brain show differential regional and developmental expression patterns. The receptors have a pentameric structure and are formed from members of at least three different subunit families (α1–6, β1–3, and τ1–3). The regulation of functional properties by GABA and its analogs and by benzodiazepine (BZ) receptor ligands differs dramatically with the type of α variant in the receptor complex. Additional variations of GABAA receptors result from substitution of γ subunits. The role of the β subunits, which are essential for receptor assembly, is less well defined on a functional basis. Besides their involvement in anxiolysis and sedation, GABAA receptors clearly have an impact on motor coordination. However, with the possible exception of the alcohol-and BZ-sensitive alcohol non-tolerant (ANT) rat line, it is not well documented whether a genetic alteration in this receptor system is directly involved in the impairment of animal or human motor activity.

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.