BINDING CHARACTERISTICS AND INTERACTIONS OF DEPRESSANT DRUGS WITH [S-35]TERT-BUTYLBICYCLOPHOSPHOROTHIONATE, A LIGAND THAT BINDS TO THE PICROTOXININ SITE

[35S]t-Butylbicyclophosphorothionate (TBPT), a cage convulsant with picrotoxinin-like activity, binds to rat brain membranes to a single site with an apparent Kd of 25.1 .+-. 5.6 nM and a Bmax of 1.40 .+-. 0.22 pmol/mg protein. TBPT binding to rat brain membranes was inhibited by a variety of convulsant, depressant, anxiolytic, and anticonvulsant drugs that had previously been shown to inhibit [3H].alpha.-dihydropicrotoxinin binding. Depressant drugs such as pentobarbital and the nonbarbiturate (+)-etomidate inhibited TBPT binding in an uncompetitive manner. Pentobarbital and (+)-etiomidate decreased both the affinity and the number of binding sites of TBPT to whole brain membranes. The IC50 [concentration which inhibits 50% of activity] values of (+)-etomidate (9 .mu.M) and pentobarbital (90 .mu.M) are similar to the EC50 values at which they enhance both [3H]GABA and [3H]diazepam binding in cerebral cortex membranes. RO5-4864 [7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepinone] which has recently been shown convulsant, also inhibited TBPT binding (IC50 = 10 .mu.M). TBPT binds to the picrotoxinin site. The picrotoxinin site is an important modulatory site at the benzodiazepine-GABA receptor-iontophore complex.