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

Interactions between neurosteroids and GABA receptors have attracted particular attention in the supraoptic nucleus (SON). Although GABA(A) receptors (GABA(A)R) mediate a sustained tonic inhibitory current (I(tonic)), as well as conventional phasic inhibitory postsynaptic currents (IPSCs, I(phasic)) in the SON, whether the steroid modulation on I(tonic) 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-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol (THIP) (1 μM), a relatively selective extrasynaptic GABA(A)R agonist, facilitated I(tonic) without affecting the main characteristics of IPSCs, while DS-2, a relatively selective modulator of GABA(A)R δ-subunits, caused minimal changes in I(tonic) of SON MNCs. l-655,708, a relatively selective GABA(A)R α(5)-subunit inverse agonist, blocked ∼35% of the total I(tonic) both under basal and elevated ambient GABA concentration (3 μM). Facilitation of I(tonic) by benzodiazepines further supported the role of GABA(A)R γ(2)-subunit in I(tonic) of SON MNCs. Quantitative RT-PCR analysis showed much lesser expression of GABA(A)R δ-subunit than the α(5) or γ(2)-subunit in the SON. Allopregnanolone and 3α,5α-tetrahydrodeoxycorticosterone increased both I(tonic) and I(phasic) in SON MNCs, respectively, although more than 90% of the current increase was mediated by I(tonic) during the neurosteroid facilitation. Finally, l-655,708 attenuated the neurosteroid facilitation of I(tonic) but not of I(phasic). Altogether, our results suggest that I(tonic), mediated mainly by benzodiazepine-sensitive GABA(A)Rs containing α(5)-, β-, and γ(2)-, and to a lesser extent, δ-subunits, is a potential target of neurosteroid modulation in SON neurons.