A subfraction of cortical tissue from rat brain, containing membrane vesicles was prepared freshly with added protease inhibitors and antioxidant. The preparation was used to measure stimulation of transmembrane 36Cl- flux and inhibition of bicuculline-sensitive [3H]muscimol binding by ((+)-(S) and (-)-(R) enatiomers of dihydromuscimol at 30° in physiological salt solution. Displacement of bound [3H]muscimol and stimulation of 36Cl- flux appeared in the 0.1-10 μM concentration range of the enantiomers, channel gating, however, required rather high concentrations. Degrees of enantioselectivity for channel gating, desensitization of and binding to GABAA receptors were estimated by the concentration ratios of dihydromuscimol enantiomers, [(-)-(R)]/[(+)-(S)], at the same level of response or displacement. Different enantioselectivities were observed for channel gating (6±3), receptor binding (3±2) and desensitization (no selectivity). The low and concentration-dependent enantioselectivities found for channel gating and receptor binding can be explained by desensitization and heterogeneity of GABAA receptors.
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