Physiological regulation of the picrotoxin receptor by γ-butyrolactones and γ-thiobutyrolactones in cultured hippocampal neurons

We examined the effects of alkyl-substituted γ-butyrolactones (GBLs), and γ-thiobutyrolactones (TBLs) on GABA currents in cultured, voltage-clamped rat hippocampal neurons. Convulsant GBLs and TBLs reversibly diminished GABA responses in a concentration-dependent manner. β-Ethyl-β-methyl GBL (β-EMGBL) completely abolished GABA responses at 3 mM (IC₅₀ 390 μM), while TBL and β-ethyl-β-methyl TBL (β-EMTBL)-induced inhibition of GABA currents was incomplete, saturating at about 50% of control at 300 μM and 10 mM for β-EMTBL and TBL, respectively. β-EMGBL and β-EMTBL both increased the rate of decay of inhibitory postsynaptic currents (IPSCs) and β-EMGBL also decreased IPSC peak amplitude. In contrast, the anticonvulsant α-ethyl-α-methyl TBL (α-EMTBL) potentiated GABA currents at all GABA concentrations tested; maximal potentiation was 190% of control at 1 mM α-EMTBL (EC₅₀ 102 μM). Another anticonvulsant, α-ethyl-α-methyl GBL (α-EMGBL), potentiated responses to low (0.5 μM) but not high (≥ 10 μM) GABA. It also blocked the inhibitory effects of picrotoxin and β-EMGBL and the facultative effect of α-EMTBL on responses to 30 μM GABA. α-EMGBL did not interfere with other agents which augment GABA currents. Both α-EMTBL and α-EMGBL decreased the rate of IPSC decay without altering IPSC peak amplitude. None of these compounds had any direct membrane effects. We propose that β-alkyl GBLs diminish GABA currents, and therefore, we hypothesize that these compounds are picrotoxin receptor agonists. β-Alkyl TBLs partially diminish GABA currents and may be partial agonists. α-Alkyl TBLs potentiate GABA currents and may be inverse agonists. Finally, α-EMGBL potentiates responses to low but not high GABA concentrations and may represent a mixed inverse agonist, antagonist at the picrotoxin receptor. The present results demonstrate that the picrotoxin receptor is capable of modulating GABA responses in opposing ways.