Neurosteroid modulation of benzodiazepine-sensitive GABA_A tonic inhibition in supraoptic magnocellular neurons

Interactions between neurosteroids and GABA receptors have attracted particular attention in the supraoptic nucleus (SON). Although GABAa receptors (GABAaR) mediate a sustained tonic inhibitory current (I_tonic), as well as conventional phasic inhibitory postsynaptic currents (IPSCs, Iphasic) in the SON, whether the steroid modulation on Itonic is present in SON magnocelluar neurosecretory cells (MNCs) is unknown. Here, we addressed this question and gained insights into the potential molecular configuration of GABA_A receptors mediating I_tonic and conferring its neurosteroids sensitivity in SON MNCs. 4,5,6,7-tetra-hydroisoxazolo[5,4-c]-pyridin-3-ol (THIP) (1 μM, a relatively selective extrasynaptic GABAaR agonist, facilitated Itonic without affecting the main characteristics of IPSCs, while DS-2, a relatively selective modulator of GABAaR δ-subunits, caused minimal changes in Itonic of SON MNCs. L-655,708, a relatively selective GABAaR ct₅-subunit inverse agonist, blocked ~35% of the total I_tonic both under basal and elevated ambient GABA concentration (3 (μM). Facilitation of Itonic by benzodiazepines further supported the role of GABAaR 72-subunit in Itonic of SON MNCs. Quantitative RT-PCR analysis showed much lesser expression of GABAaR 8-subunit than the α₅ or γ-subunit in the SON. Allopregnanolone and 3α,5α-tetrahydrodeoxycorticosterone increased both Itonic and Iphasic in SON MNCs, respectively, although more than 90% of the current increase was mediated by Itonic during the neurosteroid facilitation. Finally, L-655,708 attenuated the neurosteroid facilitation of Itonic but not of Iphasi_c. Altogether, our results suggest that Itonic, mediated mainly by benzodiazepine-sensitive GABAaRs containing α₅-β, γ₂ and to a lesser extent, δ-subunits, is a potential target of neurosteroid modulation in SON neurons.