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

Abstract GABA A receptor function is modulated by various important drugs including neuroactive steroids that act on allosteric modulatory sites and can directly activate GABA A 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 GABA A 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 GABA A receptor sites containing the δ subunit steroid auto-modulation may have a critical role in enhancing potentiation of GABA-induced currents.