Binding Characteristics and Interactions of Depressant Drugs with [³⁵S]t‐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. Thus, pentobarbital and (+)etomidate decreased both the affinity and the number of binding sites of TBPT to whole brain membranes. The IC50 values of (+)etomidate (9 microM) and pentobarbital (90 microM) are similar to the EC50 values at which they enhance both [3H]gamma-aminobutyric acid and [3H]diazepam binding in cerebral cortex membranes. RO5-4864, which has recently been shown to be a convulsant, also inhibited TBPT binding (IC50 = 10 microM). These results suggest that TBPT binds to the picrotoxinin site and further supports the notion that the picrotoxinin site is an important modulatory site at the benzodiazepine-GABA receptor-ionophore complex.