Physiological comparison of α-ethyl-α-methyl-γ-thiobutyrolactone with benzodiazepine and barbiturate modulators of GABA_A receptors

The GABA_A receptor/chloride ionophore (GABAR) is allosterically modulated by several classes of anticonvulsant agents, including benzodiazepines and barbiturates, and some alkyl-substituted butyrolactones. To test the hypothesis that the anticonvulsant butyrolactones act at a distinct positive-modulatory site on the GABAR, we examined the physiological effects of a butyrolactone, a benzodiazepine and a barbiturate on GABA-mediated currents in voltage-clamped neurons and cells transfected with various subunit combinations. The butyrolactone, α-ethyl-α-methyl-γ-thiobutyrolactone (αEMTBL), altered the EC₅₀ for GABA and changed the apparent cooperativity of GABA responses. In contrast, the benzodiazepine chlordiazepoxide altered the EC₅₀ for GABA with no effect on apparent cooperativity. The barbiturate phenobarbital altered both the EC₅₀ and the amplitude of the maximal GABA response without altering apparent cooperativity. The GABA-mediated effect of the barbiturate, but not the benzodiazepine, added to the maximal effect of the butyrolactone, supporting the hypothesis that butyrolactones do not exert their effects at the barbiturate effector site. Both αEMTBL and phenobarbital potentiated GABA currents in transfected cells containing the α1/β2 and α1γ2 subunit combinations, as well as α1 subunits alone. Chlordiazepoxide had the minimum requirement of an α subunit and a γ subunit. Specific GABARs lacking benzodiazepine or barbiturate modulation were tested for modulation by αEMTBL. The α6β2γ2 combination was modulated by the butyrolactone but not chlordiazepoxide. However, GABARs comprising ρ1 subunits were insensitive to both phenobarbital and αEMTBL. Although the molecular determinants for αEMTBL action appear similar to the barbiturates, our data support the conclusion that αEMTBL interacts with GABARs in a distinct manner from barbiturates and benzodiazepines.