Abstract

Benz[e]indenes are tricyclic analogs of neuroactive steroids and can be modulators of GABA_A receptor activity. We have examined the mechanisms of action of the benz[e]indene compound [3S-(3α,3aα,5aβ,7β,9aα,9bβ)]-dodecahydro-7-(2-hydroxyethyl)-3a-methyl-1H-benz[e]indene-3-carbonitrile (BI-2) using single-channel patch-clamp and whole-cell recordings from human embryonic kidney cells transfected with rat GABA_A receptor α1, β2, and γ2L subunits. The data demonstrate that BI-2 is a positive modulator of GABA_A receptor activity with a peak effect at 2 μM. The mechanism of modulation is similar but not identical to that of neuroactive steroids. Similar to steroids, BI-2 acts by prolonging the mean open time duration through an effect on the duration and prevalence of the longest open time component. However, in contrast to many steroids, BI-2 does not selectively reduce the channel closing rate. The potentiating action of BI-2 seems to be mediated through interactions with the classic neuroactive steroid binding site. Mutation to the membrane-spanning region in the α1 subunit Q242W and the double mutation α1N408A/Y411F, previously shown to abolish potentiation by neurosteroids, also diminish potentiation by BI-2. At higher concentrations (>5 μM), BI-2 inhibits receptor function by enhancing the apparent rate of desensitization. From single-channel recordings, we estimate that the entry rate into the inhibited or blocked state, k_+B, is 0.50 μM^-1 s^-1. Based on the kinetic mechanism of action, and the finding that this effect is blocked by the α1V256S mutation, we propose that BI-2 acts through an inhibitory site first postulated for the inhibitory neurosteroid pregnenolone sulfate.