Cortisol: a potent biphasic modulator at GABAA-receptor complexes in the guinea pig isolated ileum.

Cortisol had a biphasic action on GABAA-receptor-mediated contractile responses, enhancing at picomolar concentrations (1-10 pM) and inhibiting at higher concentrations (10-1000 nM). There was a sinistral shift of the GABA dose-response curve in the presence of 10 pM cortisol, with a significant potentiation of the GABA-induced contractions over the lower dose range of GABA (3-30 microM), whereas 100 nM cortisol caused a non-parallel dextral shift of the GABA dose-response curve, with a depression of the maximum GABA response indicative of non-competitive antagonism. Cortisol at various concentrations did not affect GABAB-receptor-mediated ileal relaxations, or the baclofen-induced depression of twitch contractions to transmural stimulation. Such concentrations of cortisol also did not affect ileal responses to exogenously applied acetylcholine or cholinergic twitch contractions themselves. These results suggest that cortisol is a specific, and very potent modulator at GABAA-receptor complexes in the guinea pig ileum.