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 drugs that act through the GABA(A) receptor (GABA(A)R) can evoke tolerance and dependence, suggesting the existence of an endogenous mechanism(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) recognition site and other agonist or modulator sites on the GABA(A)R, as measured by potentiation of [H-3]flunitrazepam ([H-3]FNZ) binding. To investigate the mechanism(s) of uncoupling, neuronal cultures were treated chronically with 3 alpha-hydroxy-5 beta-pregnan-20-one (pregnanolone), 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 inducing both heterologous and homologous uncoupling. Surprisingly, different 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 interactions, but different amounts of uncoupling of steroid-BZD site interactions. In addition, the EC(50) for pregnanolone-induced homologous uncoupling (1.7 mu M) is over an order of magnitude greater than that for heterologous uncoupling of GABA and BZD sites (82 nM). Moreover, heterologous uncoupling by pregnanolone is inhibited by the GABA site antagonist SR-95531, whereas homologous uncoupling by pregnanolone is resistant to SR-95531. Therefore, there are at least two distinct ways in which GABA(A)R modulatory site interactions can be regulated by chronic drug treatment.