Selective antagonist of neurosteroid potentiation and direct gating of GABA A receptors. Selectively reduces the effects of 5 α -reduced steroids compared to 5 β -reduced steroids and displays no effect on potentiation evoked by barbiturates and benzodiazepines. Attenuates 3 α ,5 α -THP-induced loss of righting reflex and total sleep time following i.c.v administration in rats.