Binding Characteristics and Interactions of Depressant Drugs with [35S]t‐Butylbicyclophosphorothionate, a Ligand that Binds to the Picrotoxinin Site

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Abstract
[35S]r-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]a-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 μM) and pentobarbital (90 μM) are similar to the EC50 values at which they enhance both [3H]-γ-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 μM). 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.