PHYSIOLOGICAL COMPARISON OF ALPHA-ETHYL-ALPHA-METHYL-GAMMA-THIOBUTYROLACTONE WITH BENZODIAZEPINE AND BARBITURATE MODULATORS OF GABA(A) RECEPTORS

The GABA(A) receptor/chloride ionophore (GABAR) is allosterically modu lated by several classes of anticonvulsant agents, including benzodiaz epines and barbiturates, and some alkyl-substituted butyrolactones. To test the hypothesis that the anticonvulsant butyrolactones act at a d istinct positive-modulatory site on the GABAR, we examined the physiol ogical effects of a butyrolactone, a benzodiazepine and a barbiturate on GABA-mediated currents in voltage-clamped neurons and cells transfe cted with various subunit combinations. The butyrolactone, alpha-ethyl -alpha-methyl-gamma-thiobutyrolactone (alpha EMTBL), altered the EC(50 ) for GABA and changed the apparent cooperativity of GABA responses. I n contrast, the benzodiazepine chlordiazepoxide altered the EC(50) for GABA with no effect on apparent cooperativity. The barbiturate phenob arbital altered both the EC(50) and the amplitude of the maximal GABA response without altering apparent cooperativity. The GABA-mediated ef fect of the barbiturate, but not the benzodiazepine, added to the maxi mal effect of the butyrolactone, supporting the hypothesis that butyro lactones do not exert their effects at the barbiturate effector site. Both alpha EMTBL and phenobarbital potentiated GABA currents in transf ected cells containing the alpha 1 beta 2 and alpha 1 gamma 2 subunit combinations, as well as alpha 1 subunits alone. Chlordiazepoxide had the minimum requirement of an ct subunit and a gamma subunit. Specific GABARs lacking benzodiazepine or barbiturate modulation were tested f or modulation by alpha EMTBL. The alpha 6 beta 2 gamma 2 combination w as modulated by the butyrolactone but not chlordiazepoxide. However, G ABARs comprising rho 1 subunits were insensitive to both phenobarbital and alpha EMTBL. Although the molecular determinants for alpha EMTBL action appear similar to the barbiturates, our data support the conclu sion that alpha EMTBL interacts with GABARs in a distinct manner from barbiturates and benzodiazepines.