35S]tert.-butylbicyclophosphorothionate and avermectin bind to different sites associated with the gamma- aminobutyric acid-benzodiazepine receptor complex

Low concentrations of avermectin B₁a (AVM) stimulated the specific high affinity binding of ³⁵S]tert.-butylbicyclophosphorothionate (³⁵S]TBPT) to membranes from rat cerebral cortex in the absence or presence of chloride or bromide ions. In contrast, TBPT either weakly stimulates or does not significantly influence the specific high affinity binding of ³H]AVM to the same membranes in the absence or presence of chloride ions, respectively. These results indicate that ³H]AVM and ³⁵S]TBPT bind to different but closely associated binding sites.