Abstract

The binding of the cage convulsant, [35S]t-butylbicyclophosphorothionate ([35S]TBPS), to a picrotoxin-sensitive site in rat cerebral cortical homogenates was used to identify and characterize the site of action of the phenylquinolines PK 8165 and PK 9084, the isoquinoline PK 11195 and the atypical benzodiazepine (BZ) Ro5 4864. These agents were found to allosterically modulate the binding of 2 nM [35S]TBPS in a pharmacologically relevant fashion. Evidence is presented to suggest that these compounds share a common site of action as modulators of [35S]TBPS binding. The relative potencies of these compounds in vitro are in the submicromolar to micromolar concentration range and correlate well with the concentrations reported to elicit specific responses in behavioral and electrophysiologic studies. Modulation of [35S]TBPS binding in vitro is affected by micromolar quantities of gamma-aminobutyric acid in a (+)-bicuculline-sensitive fashion and is unaffected by the central BZ receptor "antagonist" Ro15 1788. Collectively, the evidence presented suggests the existence of a novel drug binding site that is functionally coupled to a gamma-aminobutyric acid-A receptor and a [35S]TBPS-labeled chloride ionophore. Moreover, this site is distinct from the central BZ receptor recognized by clonazepam and the high-affinity peripheral BZ binding site labeled by [3H] Ro5 4864. The hypothesis is proposed that the novel "Ro5 4864 site" identified in the present study is a functionally relevant binding site that mediates some of the pharmacologic effects of Ro5 4864, PK 8165, PK 9084 and PK 11195 in the mammalian central nervous system.