Enantioselective modulation of GABAergic synaptic transmission by steroids and benz[e]indenes in hippocampal microcultures

The effects of enantiomers of the neurosteroid analogues, 3α-hydroxy- 5α-pregnan-20-one (DHP) and 3α-hydroxy-5α-androstane-17β-carbonitrile (ACN), and the benz[e]indene, BI-1, on synaptic currents were examined in microcultures of rat hippocampal neurons. Over the range of 0.1-10 μM, the (+)-enantiomers were more potent and effective than their (-)-enantiomeric counterparts in enhancing γ-aminobutyric acid (GABA)A receptor-mediated evoked synaptic currents. The (+)-enantiomers had small effects on peak currents, but slowed the decay of inhibitory synaptic currents, resulting in 2-3-fold increases in charge transfer during inhibitory synaptic events at 10 μM. Similar prolongations of spontaneous miniature inhibitory postsynaptic currents (IPSCs) and responses to brief GABA pulses to outside-out patches suggest that the prolongations of evoked synaptic currents result primarily from postsynaptic effects. In contrast, the (-)-enantiomers had little effect on evoked IPSCs at concentrations ≤1 μM, but enhanced inhibitory transmission at 10 μM. At concentrations ≤1 μM, neither the (+)- nor (-)- enantiomers altered glutamate-mediated excitatory synaptic currents. At 10 μM, (+)-DHP and (+)-ACN depressed excitatory responses in a bicuculline- sensitive fashion, suggesting that direct chloride channel gating by the steroids contributed to the depression. These data indicate that certain steroids and benz[e]indenes augment inhibitory synaptic transmission enantioselectively and provide strong support for the hypothesis that steroids act at specific sites on synaptic GABA(A) receptors rather than via alteration of membrane lipids.