Auto-modulation of neuroactive steroids on GABAA receptors: A novel pharmacological effect

GABAA receptor function is modulated by various important drugs including neuroactive steroids that act on allosteric modulatory sites and can directly activate GABAA receptor channels at high concentrations. We used whole cell patch-clamp recordings and rapid applications of the neuroactive steroid alphaxalone to investigate repetitive steroid effects. Alphaxalone potentiation of submaximal GABA-evoked currents was enhanced significantly by repetitive coapplications at all investigated recombinant isoforms (α1β3δ, α1β3γ2L, α6β3δ, α6β3γ2L) and at GABAA receptors of differentiated human NT2 neurons. A similar increase of current amplitudes was induced by repetitive applications of a high steroid concentration without GABA. We refer to these reversible effects as auto-modulation because repeated interactions of steroids enhanced their own pharmacological impact at the receptor sites in a time and concentration dependent manner without affecting GABA controls. Pronounced auto-modulatory actions were also measured using the neurosteroid 5α-THDOC in contrast to indiplon, THIP, and pentobarbital indicating a steroid specificity. Protein kinase A inhibition significantly reduced alphaxalone auto-modulation at α1β3γ2L, α6β3γ2L, and α6β3δ subtypes while it enhanced potentiation at α1β3δ isoforms suggesting a crucial influence of receptor subunit composition and phosphorylation for steroid actions. Especially at extrasynaptic GABAA receptor sites containing the δ subunit steroid auto-modulation may have a critical role in enhancing potentiation of GABA-induced currents.