Effects of acute pentobarbital administration on GABAA receptor-regulated chloride uptake in rat brain synaptoneurosomes

Autoradiography of [3H]glutamate binding during pentobarbital tolerance and withdrawal in the rat

The influence of centrally administered pentobarbital on [3H]glutamate receptor binding in the rat brain was examined. Animals were rendered tolerant by intracerebroventricular (i.c.v.) infusion through osmotic minipumps with pentobarbital (300 microg/10 microl/h, for 6 days), and dependent, by 24 h after withdrawal from pentobarbital. Pentobarbital tolerant rats have significant increases in [3H]glutamate binding in the cortex and hippocampus area. Pentobarbital withdrawal produced increases in glutamate binding in many regions, e.g., the cortex, hippocampus area, thalamus, and cerebellum. These results show that chronic i.c.v. infusion with pentobarbital increases N-methyl-D-aspartate (NMDA) displaceable [3H]glutamate binding, suggesting that an increase in NMDA binding sites may play an important role in the development of tolerance to and withdrawal from pentobarbital.

GABA(A) Receptor-Related Chloride Flux of Rats Receiving Continuous Intracerebroventricular Infusion of Pentobarbital

The in vitro receptor-stimulated synaptoneurosomal chloride uptake induced by pentobarbital or muscimol was studied in the brains of rats which had been rendered tolerant to or dependent on pentobarbital. In the chronic study, rats received continuous administration of sodium pentobarbital by intracerebroventricular (i.c.v.) infusion. In rats continuously infused with pentobarbital for 6 days or in rats which had the infusion terminated for 24 h, the basal synaptoneurosomal chloride uptake was not altered in either the cerebral cortex or the cerebellum. On the other hand, pentobarbital (500 &mgr;M)-stimulated chloride uptake was significantly decreased in both cerebral cortices and cerebella of rats which received pentobarbital for 6 days as compared with the saline control groups. Twenty-four hours after termination of pentobarbital infusion, this high concentration of pentobarbital-stimulated chloride uptake remained low in both cerebral cortices and cerebella of rats which had been infused with pentobarbital for 6 days. In addition, similar results were also obtained in muscimol (2.5 &mgr;M)-stimulated chloride uptake in the cerebral cortices of pentobarbital-infused animals. However, pentobarbital infusion failed to alter muscimol-stimulated chloride uptake in the cerebellum. These results suggest that the GABA(A) receptor regulated chloride uptake is downregulated after chronic pentobarbital administration. The results also suggest that down-regulation of the GABA(A) receptor chloride channel complex takes place at different recognition sites on the complex. It further substantiates the allosteric effects of GABA and barbiturate recognition sites. Copyright 1996 S. Karger AG, Basel

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.

Studies on picrotoxin binding sites of GABAA receptors in rat cortical synaptoneurosomes

Experiments were performed to characterize [35S]TBPS binding in rat cortical synaptoneurosomes, which have vesicular structures containing both pre- and postsynaptic elements. Scatchard analysis revealed a single component of [35S]TBPS binding sites with KD and Bmax values of 76.1 nM and 1.97 pmoles/mg protein, respectively, under physiological conditions. GABA and muscimol inhibited [35S]TBPS binding in a concentration-dependent manner. IC50 values of these GABAA agonists in displacing synaptoneurosomal [35S]TBPS binding are comparable to previously reported EC50 values of the agonist-stimulated 36Cl- uptake in synaptoneurosomes by these agents. Furthermore, the IC50 values of these GABAA agonists were better correspondence to those determined by [3H]muscimol binding in synaptoneurosomal preparations as reported by Delorey and Brown (3) than those determined in membrane preparations. Although bicuculline increased [35S]TBPS binding in a concentration dependent manner in cortical membranes, it did not affect synaptoneurosomal [35S]TBPS binding. Benzodiazepine agonists and inverse agonists (0.1 to 10 microM) did not show any effects on the binding in the absence of muscimol. However, benzodiazepine agonists potentiated and inverse agonists antagonized muscimol-induced inhibition of synaptoneurosomal [35S]TBPS binding. In addition, an anesthetic steroid, THDOC, and pentobarbital inhibited synaptoneurosomal [35S]TBPS binding in a concentration dependent manner. These results suggest that allosteric modulation of [35S]TBPS binding by various ligands which interact with GABAA supramolecular complexes remain intact in synaptoneurosomes. It appears that this preparation is useful for investigating correlation between functional 36Cl- uptake and individual binding studies of each of the GABAA receptor complex.