Bicuculline-produced regional differences in the modulation of 35S-TBPS binding by GABA, pentobarbital and diazepam...

The modulation of 35S-t-butylbicyclophosphorothionate (35S-TBPS) binding by in vitro addition of gamma-aminobutyric acid (GABA), diazepam, pentobarbital, etomidate and etazolate was studied in the presence of KCl (100 mM) in the cerebellum and in the cortex of C57Bl mice. In the cortex, all of the depressant drugs caused a biphasic effect (stimulation followed by inhibition) on 35S-TBPS binding, whereas in the cerebellum only inhibition was observed. Saturation analysis revealed that the enhancement of 35S-TBPS binding was due to an increase in the affinity of 35S-TBPS for its binding sites. The introduction of a GABAA receptor antagonist, bicuculline methiodide (10 microM), into assay media altered the effects of the depressants, except for those of diazepam, on 35S-TBPS binding in a complex manner. Our results indicate that bicuculline, in addition to its GABAA receptor blocking properties, also influenced the binding of 35S-TBPS through some other, as yet unknown, mechanism(s). Thus, bicuculline not only produced a rightward shift of the dose-response curves of the central depressant drugs in the cortex, but also increased the maximal stimulation of 35S-TBPS binding. Furthermore, in the cerebellum, the previously observed drug-induced inhibition of 35S-TBPS binding was replaced by stimulation followed by inhibition in the presence of bicuculline. Finally, it was found that the in vitro addition of bicuculline had no effect on the diazepam-induced stimulation of 35S-TBPS binding. It is suggested that the regional differences in the modulation of 35S-TBPS binding in various brain structures may be due to endogenous differences in the molecular composition of GABAA receptors in various brain areas.