The GABAA receptor channel is a highly regulated receptor. The function of the receptor may be modified by drugs which alter the rates of binding of GABA, modify the gating of the channel or block the channel. It is also likely that phosphorylation of the receptor subunits modifies the biophysical properties, stability or assembly of the receptor. While GABAergic inhibition plays a major role in the regulation of neuronal excitability, a role for altered GABAergic inhibition in the pathogenesis of epilepsy remains to be proven. The demonstration that GABAA receptors are composed of multiple subunits and that the properties and pharmacology of GABAA receptors are different for different subunit combinations suggests that GABAA receptor heterogeneity may be of importance in determining the properties of GABAergic inhibition in different regions of the nervous system. While it is clear that GABAA receptor heterogeneity is present in the nervous system, a role for receptor heterogeneity in the pathogenesis of epilepsy remains uncertain. GABAA receptor heterogeneity may have implications for the treatment of epilepsy. It is quite possible that drugs which regulate GABAergic function may have variable efficacy in different regions of the nervous system due to expression of receptors with subunits that have different sensitivity to allosteric regulators. Furthermore, it is likely that there are developmental changes in the stoichiometry or subunit composition of GABAA receptors rendering the developing nervous system more or less sensitive to the effects of GABAergic anticonvulsant drugs. In addition to the heterogeneous expression of GABAA receptors, other issues concerning the regulation of GABAergic function are of potential importance. The regulatory events that control the expression of specific receptor subtypes and levels of GABA receptors are unknown. The post-translational events that regulate GABAA receptor function are uncertain. It is possible that post-translational regulation of GABAA receptors by phosphorylation may contribute to altered GABAA receptor function in epilepsy. To understand the role of GABAA receptor heterogeneity in the pathogenesis of epilepsy will require the combination of biophysical and molecular biological techniques. It will be important to determine not only whether the properties of GABAA receptors have been altered in a specific form of epilepsy, but also whether gene expression has been altered.
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"The BZD binding site is located on the extracellular surface of the GABA-A receptor, and is formed by residues located in at least six noncontiguous regions at the α/γ interface historically designated Loops A-F (reviewed in (Sigel, 2002)). The BZD recognition site binds a large selection of ligands, agonists that potentiate GABA induced current (positive modulators) (Macdonald and Barker, 1978), inverse agonists that inhibit GABA current (negative modulators) (Macdonald et al., 1992; Oakley and Jones, 1980) and antagonists that competitively bind at the BZD binding site but have no effect on GABA current (zero modulators) (Braestrup et al., 1982). Because the therapeutic value of BZDs depends upon their efficacy in modulating I GABA , mapping structural rearrangements involved in mediating the full range of BZD actions from positive to negative modulation of I GABA is essential. "
[Show abstract][Hide abstract] ABSTRACT: The mechanisms by which the GABA and benzodiazepine (BZD) binding sites of the GABA-A receptor are allosterically coupled remain elusive. In this study, we separately monitored ligand-induced structural changes in the BZD binding site (alpha/gamma interface) and at aligned positions in the alpha/beta interface. alpha(1)His101 and surrounding residues were individually mutated to cysteine and expressed with wild-type beta2 and gamma2 subunits in Xenopus laevis oocytes. The accessibilities of introduced cysteines to modification by methanethiosulfonate ethylammonium (MTSEA)-Biotin were measured in the presence and absence of GABA-site agonists, antagonists, BZDs, and pentobarbital. The presence of flurazepam or the BZD-site antagonist flumazenil (Ro15-1788) decreased the rate of modification of alpha(1)H101C at the BZD binding site. GABA and muscimol each increased MTSEA-Biotin modification of alpha(1)H101C located at the BZD-site, gabazine (SR-95531, a GABA binding site antagonist) decreased the rate, whereas pentobarbital had no effect. Modification of alpha(1)H101C at the alpha/beta interface was significantly slower than modification of alpha(1)H101C at the BZD site, and the presence of GABA or flurazepam had no effect on its accessibility, indicating the physicochemical environments of the alpha/gamma and alpha/beta interfaces are different. The data are consistent with the idea that GABA-binding site occupation by agonists causes a GABA binding cavity closure that is directly coupled to BZD binding cavity opening, and GABA-site antagonist binding causes a movement linked to BZD binding cavity closure. Pentobarbital binding/gating resulted in no observable movements in the BZD binding site near alpha(1)H101C, indicating that structural mechanisms underlying allosteric coupling between the GABA and BZD binding sites are distinct.
"Because diazepam exerts its anticonvulsant effect primarily by enhancing GABAergic inhibition by acting on GABARs (Macdonald et al., 1992), we hypothesized that seizures altered the functional properties of GABARs. The seizures could potentially alter the modulation of GABAR by various drugs, such as enhancement by benzodiazepines, barbiturates, and neurosteroids and antagonism by penicillin, picrotoxin, bicuculline, and Zn 2+ . "
[Show abstract][Hide abstract] ABSTRACT: Fast synaptic inhibition in the forebrain is mediated primarily by GABA acting on GABAA receptors (GABARs). GABARs are regulated by numerous positive (barbiturates, benzodiazepines, and neurosteroids) and negative (picrotoxin, bicuculline, and Zn2+) allosteric modulators. The sensitivity of GABARs to GABA and to allosteric modulators changes gradually during normal development, during development of chronic epilepsy, and after prolonged exposure to GABAR agonists. Here we report the development of rapid functional plasticity of GABARs occurring over 45 min of continuous seizures (status epilepticus) in rats. Seizures induced in rats by administration of lithium followed by pilocarpine were readily terminated by the benzodiazepine diazepam when administered early during the seizures (after 10 min of seizures). However, during status epilepticus, there was a substantial reduction of diazepam potency for termination of the seizures. To determine whether the loss of sensitivity of the animals to diazepam was caused by an alteration of GABAR functional properties, we obtained whole-cell GABAR currents from hippocampal dentate granule cells isolated acutely from control rats and from rats undergoing status epilepticus. GABAR properties were characterized by determining GABA sensitivity and the sensitivity of GABARs to regulation by benzodiazepines, barbiturates, and Zn2+. When compared with those from naive controls, GABAR currents from rats undergoing status epilepticus were less sensitive to diazepam and Zn2+ but retained their sensitivity to GABA and pentobarbital. We conclude that the prolonged seizures of status epilepticus rapidly altered the functional properties of hippocampal dentate granule cell GABARs.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 11/1997; 17(19):7532-40. · 6.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.