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 Ca(2+) 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.