NEUROSTEROID PROLONGS GABA(A) CHANNEL DEACTIVATION BY ALTERING KINETICS OF DESENSITIZED STATES

Fast applications of GABA (1 mM) to nucleated and outside-out patches excised from granule neurons in cerebellar slices from developing rats evoked currents with a double exponential time course reminiscent of that of IPSCs. A neurosteroid 3 alpha, 21 dihydroxy-5 alpha-pregnan-20 -one (THDOC) remarkably increased the slow deactivation time constant and slowed down recovery from desensitization, as estimated by paired- pulse GABA applications. THDOC also reduced the amplitude of GABA curr ents, whereas it failed to affect the fast deactivation component and its relative contribution to peak amplitude. The effects of THDOC on s low deactivation were greater in rats younger than postnatal day 13 (P 13) as compared with rats at P30-P35. THDOC failed to alter deactivati on of short responses induced by a less-potent agonist taurine at satu rating doses. These responses had deactivation kinetics described by a fast single exponential decay, little desensitization, and quick reco very. However, THDOC slowed deactivation if taurine responses were lon g enough to allow consistent desensitization, suggesting that desensit ized states are required for the neurosteroid to modulate GABA respons es. In outside-out patches, just as desensitized states prolonged GABA responses by producing reopening of channels activated by brief GABA pulses, THDOC increased the channel open probability by further increa sing the number of late channel openings, resulting in a prolongation of the slow deactivation. Our data suggest that neurosteroid potentiat es the inhibitory postsynaptic transmission via the prolongation of th e slow deactivation and that the alteration of kinetics of entry and e xit from desensitized states underlies the allosteric modification of GABA(A) receptors by neurosteroids.