Enhancement of γ‐Aminobutyric Acid Binding by the Anxiolytic β‐Carbolines ZK 93423 and ZK 91296

: The effects of two anxiolytic β‐carboline derivatives, ZK 93423 and ZK 91296, on the binding of γ‐[3H]aminobutyric acid ([3H]GABA) to brain membrane preparations from rat cerebral cortex were examined. ZK 93423 concentration‐dependently enhanced the specific binding of [3H]GABA, with a maximal increase of 45% above control at a 50 μM concentration. A less pronounced increase was induced by diazepam and by the partial agonist ZK 91296. Scatchard plot analysis revealed that the effect of ZK 93423 was due to an increase in the total number of high‐ and low‐affinity GABA binding sites. The action of ZK 93423 was mediated by benzodiazepine recognition sites since it was blocked by the benzodiazepine antagonists Ro 15–1788 and ZK 93426 at concentrations that failed to modify [3H]GABA binding on their own. Moreover the stimulatory effect of ZK 93423 on [3H]GABA binding was also blocked by the β‐carboline inverse agonist ethyl β‐carboline‐3‐carboxylate. These results are consistent with the view that ZK 93423 and ZK 91296, similarly to benzodiaze‐pines, exert their pharmacological effects by enhancing the GABAergic transmission at the level of the GABA/benzodiazepine receptor complex.