GAMMA-AMINOBUTYRIC ACID(A) RECEPTOR REGULATION - HETEROLOGOUS UNCOUPLING OF MODULATORY SITE INTERACTIONS INDUCED BY CHRONIC STEROID, BARBITURATE, BENZODIAZEPINE, OR GABA TREATMENT IN CULTURE

Prolonged administration of anxiolytic, sedative, and anticonvulsant d rugs that act through the GABA(A) receptor (GABA(A)R) can evoke tolera nce and dependence, suggesting the existence of an endogenous mechanis m(s) for altering the ability of such agents to interact with the GABA (A)R. Uncoupling appears to be one such mechanism. This is a decrease in the allosteric interactions between the benzodiazepine (BZD) recogn ition site and other agonist or modulator sites on the GABA(A)R, as me asured by potentiation of [H-3]flunitrazepam ([H-3]FNZ) binding. To in vestigate the mechanism(s) of uncoupling, neuronal cultures were treat ed chronically with 3 alpha-hydroxy-5 beta-pregnan-20-one (pregnanolon e), pentobarbital, flurazepam, or GABA, then tested for enhancement of [H-3]FNZ binding by these substances. The results indicate that BZDs, barbiturates, and steroids, as well as GABA itself, are capable of in ducing both heterologous and homologous uncoupling. Surprisingly, diff erent chronic drug treatments produce different patterns of homologous and heterologous uncoupling. Chronic exposure to pregnanolone, GABA, flurazepam or pentobarbitaI induces complete uncoupling of barbiturate -BZD site interactions, partial uncoupling of GABA-BZD site interactio ns, but different amounts of uncoupling of steroid-BZD site interactio ns. In addition, the EC(50) for pregnanolone-induced homologous uncoup ling (1.7 mu M) is over an order of magnitude greater than that for he terologous uncoupling of GABA and BZD sites (82 nM). Moreover, heterol ogous uncoupling by pregnanolone is inhibited by the GABA site antagon ist SR-95531, whereas homologous uncoupling by pregnanolone is resista nt to SR-95531. Therefore, there are at least two distinct ways in whi ch GABA(A)R modulatory site interactions can be regulated by chronic d rug treatment.