ALLOSTERIC EFFECTS OF A GABA RECEPTOR-ACTIVE STEROID ARE ALTERED BY STRESS

Recently, ring A reduced metabolites of naturally occurring steroids have been shown to act as allosteric modulators of GABA-gated chloride ion conductance. Specifically, 5 alpha-pregnane-3 alpha,21-diol-20-one (allotetrahydrodeoxycorticosterone; 5 alpha THDOC) was shown to be a positive allosteric effector. For example, 5 alpha-THDOC enhances the specific binding of [H-3]flunitrazepam, a benzodiazepine receptor agonist, among other pharmacological actions. Swim stress has been shown to reduce the ability of flurazepam, a prototypic benzodiazepine agonist, to antagonize the electrical precipitation of tonic hindlimb extension in mice. This stress-induced reduction in flurazepam's antiseizure efficacy persists for up to 72 h and is associated with alterations in the specific binding of ligands to the GABA(A) receptor complex. In the current study, the potentiation of [H-3]flunitrazepam binding by 5 alpha-THDOC was greater in cerebral cortical membranes prepared from stressed mice compared with unstressed controls. Moreover, nanomolar concentrations of 5 alpha-THDOC that were ineffective in potentiating the specific binding of [S-35]TBPS in cerebral cortical membranes prepared from unstressed control mice were capable of potentiating this binding in membranes prepared from stressed animals. Specifically, 50 nM 5 alpha-THDOC caused a 23% increase in the specific binding of [S-35]TBPS in membranes from stressed mice, whereas it was without any significant effect in unstressed controls. This apparent ability of 5 alpha-THDOC to distinguish between the binding of [S-35]TBPS to crude membranes prepared from stressed and unstressed control mice was eliminated in the presence of a 5 mu M concentration of GABA. The data are consistent with an effect of stress on the ability of a GABA-positive steroid to allosterically modulate the binding of both a benzodiazepine agonist and GABA(A)-associated channel ligand. These data provide additional support for a stress-induced alteration of the GABA(A) receptor complex.