DIAZEPAM BIPHASICALLY MODULATES [H-3] TBOB BINDING TO THE CONVULSANT SITE OF THE GABA(A), RECEPTOR COMPLEX

Interactions of GABA, bicuculline methochloride and diazepam with [H-3 ]TBOB binding to rat brain membranes were evaluated in vitro. GABA dis placed [H-3]TBOB binding with and IC50 of 4 mu M and a slope factor ne ar unity. The competitive GABA antagonist bicuculline methochloride sh ifted the displacement curve of GABA parallelly to the right, indicati ng that the interaction of GABA with [H-3]TBOB binding is of an allost eric nature. In the presence of GABA, diazepam displaced the binding o f [H-3]TBOB according to a two-site model: a high affinity site with a n IC50 of about 50 nM and a lower affinity site with an IC50 of about 30 mu M. Bicuculline methochloride abolished the nanomolar displacemen t by diazepam and increased the micromolar IC50 value. These results i ndicate that the interaction of the high affinity diazepam site with t he [H-3]TBOB binding site is totally GABA dependent and that the low a ffinity effect of diazepam on [H-3]TBOB binding is at least partially GABA dependent. It is likely that the low affinity potency of diazepam to displace [H-3]TBOB binding has physiological relevance.