Multiple effects of allopregnanolone on GABAergic responses in single hippocampal CA3 pyramidal neurons

3α-Hydroxy, 5α-reduced pregnane steroids, such as allopregnanolone, are potent modulators of GABA_A receptors and have many biological responses including sedative, anxiolytic, anticonvulsant and anesthetic actions. In the present study, we have investigated the effects of allopregnanolone on GABA_A receptors in acutely isolated single hippocampal CA3 pyramidal neurons using the whole cell patch-clamp technique. Allopregnanolone induced membrane Cl^- currents in a concentration-dependent manner, and the allopregnanolone-induced currents (I_AlloP) were blocked by noncompetitive GABA_A receptor antagonists. The I_AlloP was not affected by the intracellular loading of γ-cyclodextrin (γ-CD), which efficiently sequesters several kinds of endogenous neurosteroids including allopregnanolone, suggesting that allopregnanolone accesses extracellular but not intracellular sites to activate GABA_A receptors. Allopregnanolone prolonged the decay time constant of GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs), suggesting that allopregnanolone modulates the desensitization kinetics of postsynaptic GABA_A receptors. The picrotoxin-sensitive tonic currents (I _tonic), which were mediated by extrasynaptic GABA_A receptors, were recorded from CA3 pyramidal neurons. The intracellular loading of γ-CD or allopregnanolone significantly decreased or increased the amplitude of picrotoxin-sensitive I_tonic, respectively, suggesting that endogenous neurosteroids might, at least in part, be involved in the generation of picrotoxin-sensitive I_tonic. Allopregnanolone also increased the frequency of GABAergic sIPSCs, in a manner dependent on the integrity of voltage-dependent Na⁺ and Ca²⁺ channels, suggesting that allopregnanolone activates presynaptic GABA_A receptors to depolarize GABAergic nerve terminals. The present results suggest that allopregnanolone exerts its pharmacological and pathophysiological actions via the modulation of multiple types of GABA_A receptor-mediated responses.