Barbiturate recognition site on the GABA/benzodiazepine receptor complex is distinct from the picrotoxinin/TBPS recognition site

The cage convulsant [35S]tert-butylbicyclophosphorothionate [( 35S]TBPS) labels a presumed sedative-convulsant receptor complex. The relative potencies of barbiturates in competing for [35S]TBPS binding parallels their potencies in enhancing benzodiazepine receptor binding. Barbiturates inhibit [35S]TBPS binding in a complex, mixed competitive fashion, leading to a decrease in both the apparent affinity of TBPS for its sites and the apparent number of TBPS sites. All of the barbiturates examined markedly accelerate the dissociation of [35S]TBPS from its recognition sites, while picrotoxinin does not affect the dissociation. These results suggest that the barbiturate and picrotoxinin/TBPS recognition sites are distinct but allosterically linked.