Effects of neurosteroid and benz[e]indene enantiomers on GABA(A) receptors in cultured hippocampal neurons and transfected HEK-293 cells

The effects of the enantiomers of the neurosteroid, 3α-hydroxy-5α-pregnan-20-one (DHP), and the benz[e]indene, BI-1, on γ-aminobutyric acid (GABA) responses were studied using whole-cell recording techniques in cultured rat hippocampal neurons and human embryonic kidney cells (HEK-293) transfected with either a1β2γ2 or α6β2γ2 GABA(A) receptor subunits. At 10 μM, the (+)-enantiomers enhanced currents gated by 2 μM GABA in all cells, whereas the (-)-enantiomers were significantly less effective. The enhancement of 2 μM GABA responses in HEK-293 cells transfected with α6β2γ2 subunits was about half that of hippocampal neurons or HEK-293 cells transfected with α1β2γ2. The lower sensitivity of α6β2γ2 receptors for (+)-DHP and (+)-BI-1 is accounted for by their greater apparent affinity for GABA. When the GABA concentration was decreased to 0.5 μM to take into account the four-fold higher apparent affinity of α6β2γ2 receptors, these receptors exhibited enhancement similar to a1β2γ2 receptors. These results indicate that both native and recombinant GABA(A) receptors have enantioselective sites at which neurosteroids and benz[e]indenes modulate GABA responses, and that differences in agonist affinity contribute to apparent differences in steroid sensitivity among GABA(A) receptors.