Abstract
Gonadal steroid feedback to oxytocin neurons during pregnancy is in part mediated via the neurosteroid allopregnanolone (3alpha-OH-DHP), acting as allosteric modulator of postsynaptic gamma-aminobutyric acid type A (GABA(A)) receptors. We describe here a form of nongenomic progesterone signaling by showing that 3alpha-OH-DHP not only potentiates GABA(A) receptor-channel activity but also prevents its modulation by protein kinase C (PKC). Application of oxytocin or stimulation of PKC suppressed the postsynaptic GABA responses of oxytocin neurons in the absence, but not in the presence of 3alpha-OH-DHP. This finding was true at the juvenile stage and during late pregnancy, when the GABA(A) receptor is sensitive to 3alpha-OH-DHP. In contrast, after parturition, when the GABA(A) receptors expressed by oxytocin neurons are less sensitive to 3alpha-OH-DHP, this neurosteroid no longer counteracts PKC. The change in GABA(A)-receptor responsiveness to 3alpha-OH-DHP helps to explain the onset of firing activity and thus the induction of oxytocin release at parturition.