Characterization of U-97775 as a GABAA receptor ligand of dual functionality in cloned rat GABAA receptor subtypes.

1. U-97775 (tert-butyl 7-chloro-4,5-dihydro-5-(1-(3,4,5-trimethyl)piperazino)carbonyl]- imidazo1,5-a])quinoxaline-3-carboxylate) is a novel GABAA receptor ligand of dual functionality and was characterized for its interactions with cloned rat GABAA receptors expressed in human embryonic kidney cells. 2. The drug produced a bell-shaped dose-response profile in the alpha 1 beta 2 gamma 2 receptor subtype as monitored with GABA-induced Cl- currents in the whole cell patch-clamp technique. At low concentrations (< 0.5 microM), U-97775 enhanced the currents with a maximal increase of 120% as normalized to 5 microM GABA response (control). An agonist interaction of U-97775 with the benzodiazepine site is suggested, because Ro 15-1788 (an antagonist at the benzodiazepine site) abolished the current increase and 3H]-flunitrazepam binding was inhibited by U-97775 with a Ki of 1.2 nM. 3. The enhancement of GABA currents progressively disappeared as the U-97775 concentration was raised above 1 microM, and the current amplitude was reduced to 40% below the control at 10 microM U-97775. The current inhibition by U-97775 (10 microM) was not affected by Ro 15-1788. It appears that U-97775 interacts with a second site on GABA receptors, distinct from the benzodiazepine site, to reverse its agonistic activity on the benzodiazepine site and also to inhibit GABA currents. 4. U-97775 at low concentrations reduced and at high concentrations enhanced 35S]-TBPS binding. Ro 15-1788 selectively blocked the effect of U-97775 at low concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)