Diazepam biphasically modulates [³H]TBOB binding to the convulsant site of the GABA_A receptor complex

Abstract

Interactions of GABA, bicuculline methochloride and diazepam with [³H]TBOB binding to rat brain membranes were evaluated in vitro.

GABA displaced [³H]TBOB binding with and IC₅₀ of 4μM and a slope factor near unity. The competitive GABA antagonist bicuculline methochloride shifted the displacement curve of GABA parallelly to the right, indicating that the interaction of GABA with [³H]TBOB binding is of an allosteric nature.

In the presence of GABA, diazepam displaced the binding of [³H]TBOB according to a two-site model: a high affinity site with an IC₅₀ of about 50 nM and a lower affinity site with an IC₅₀ of about 30 μM. Bicuculline methochloride abolished the nanomolar displacement by diazepam and increased the micromolar IC₅₀ value. These results indicate that the interaction of the high affinity diazepam site with the [³H]TBOB binding site is totally GABA dependent and that the low affinity effect of diazepam on [³H]TBOB binding is at least partially GABA dependent.

It is likely that the low affinity potency of diazepam to displace [³H]TBOB binding has physiological relevance.