Binding interactions of convulsant and anticonvulsant γ-butyrolactones and γ-thiobutyrolactones with the picrotoxin receptor

Alkyl-substituted γ-butyrolactones (GBLs) and γ-thiobutyrolactones (TBLs) are neuroactive chemicals. β-Substituted compounds are convulsant, whereas α-alkyl substituted GBLs and TBLs are anticonvulsant. The structural similarities between β-alkyl GBLs and the convulsant pictrotoxinin suggested that alkyl substituted GBLs and TBLs act at the picrotoxin receptor. To test this hypothesis we examined the interactions of convulsant and anticonvulsant GBLs and TBLs with the picrotoxin, benzodiazepine and γ-aminobutyric acid (GABA) binding sites of the GABA receptor complex. All of these convulsants and anticonvulsants studied competitively displaced ³⁵S-t-butylbicyclophosphorotionate (³⁵S-TBPS), a ligand that binds to the picrotoxin receptor. This inhibition of 35S-TBPS binding was not blocked by the GABA antagonist bicuculline methobromide. The convulsant GBLs and TBLs also partially inhibited [³H]muscimol binding to the GABA site and [³H]flunitrazepam binding to the benzodiazepine site, but they did so at concentrations substantially greater than those that inhibited ³⁵S-TBPS binding. The anticonvulsant GBLs and TBLs had no effect on either [³H]muscimol or [³H]flunitrazepam binding. In contrast to the GBLs and TBLs, pentobarbital inhibited TBPS binding in a manner that was blocked by bicuculline methobromide, and it enhanced both [³H]flunitrazepam and [³H]muscimol binding. Both ethosuximide and tetramethylsuccinimide, neuroactive compounds structurally similar to GBLs, competitively displaced ³⁵S-TBPS from the picrotoxin receptor and both compounds were weak inhibitors of [³H] muscimol binding. In addition, ethosuximide also partially diminished [³H]flunitrazepam binding. These data demonstrate that the site of action of alkyl-substituted GBLs and TBLs is different from that of GABA, barbiturates and benzodiazepines. We suggest that the GBLs and TBLs act at the picrotoxin receptor.