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 GABAA 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 GABAA 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 (IAₗₗₒP) were blocked by noncompetitive GABAA receptor antagonists. The IAₗₗₒP 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 GABAA receptors. Allopregnanolone prolonged the decay time constant of GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs), suggesting that allopregnanolone modulates the desensitization kinetics of postsynaptic GABAA receptors. The picrotoxin-sensitive tonic currents (Iₜₒₙᵢc), which were mediated by extrasynaptic GABAA receptors, were recorded from CA3 pyramidal neurons. The intracellular loading of γ-CD or allopregnanolone significantly decreased or increased the amplitude of picrotoxin-sensitive Iₜₒₙᵢc, respectively, suggesting that endogenous neurosteroids might, at least in part, be involved in the generation of picrotoxin-sensitive Iₜₒₙᵢc. 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 GABAA 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 GABAA receptor-mediated responses.