Saturable binding of 35S-t-butylbicyclophosphorothionate to the sites linked to the GABA receptor and the interaction with gabaergic agents

Abstract

35S-t-Butylbicyclophosphorothionate ( 35S-TBPS) binds in a concentration-saturable manner to specific sites on membranes from rat cerebral cortex. Using a filtration assay at 25°C, in 250 mM NaCl, specific binding of 35S-TBPS constitutes about 84 to 94 percent of total binding, depending on radioligand concentrations. 35S-TBPS binding is optimal in the presence of NaCl or NaBr and substantially less in the presence of NaI or NaF. It is sensitive to the treatment with 0.05 percent Triton X-100 but not to repeated freezing and thawing, procedures which increase 3H-GABA binding. Pharmacological studies show that 35S-TBPS binding is strongly inhibited by GABA-A receptor agonists (e.g., GABA and muscimol) and by the noncompetitive antagonist, picrotoxin, but not the competitive antagonist, bicuculline. Compounds which enhance binding of radioactive GABA and benzodiazepines, such as the pyrazolopyridines, cartazolate and tracazolate, and a diaryl-triazine, LY81067, are also potent inhibitors of 35S-TBPS binding, with LY81067 being the most effective. The effects of GABA, picrotoxin and LY81067 on the saturable binding of 35S-TBPS in cortical membranes are compared. The present findings are consistent with the interpretation that 35S-TBPS binds at or near the picrotoxin-sensitive anion recognition sites of the GABA/benzodiazepine/ picrotoxin receptor complex.