An ovarian steroid hormone regimen that increases hypothalamic oxytocin expression alters [H-3] muscimol binding in the hypothalamic supraoptic nucleus of the female rat

Administration of sequential estradiol (E-2) and progesterone (P) for 2 wee ks followed by withdrawal of P 48 h prior to sacrifice will increase oxytoc in (OT) messenger ribonucleic acid (mRNA) levels in the paraventricular and supraoptic nuclei (PVN and SON) of the ovariectomized rat. Progesterone is known to mediate certain of its effects via binding to the gamma aminobuty ric acid A (GABA(A)) receptor. E-2 and P are known to modulate the specific binding of the GABA(A) receptor agonist, muscimol, in certain brain region s, In the present study ovariectomized rats received empty or steroid-fille d Silastic capsules for 2 weeks according to one of the following schedules : E-2 only (E-2 group) vs. sequential E-2 and P in which P was either remov ed 48 h prior to killing (E-2/P - group) or sustained until sacrifice (E-2/ P + group). [H-3]muscimol binding was measured in several brain regions of the animals. The steroid sequence that is known to increase SON OT mRNA (E- 2/P -) selectively decreased [H-3]muscimol binding in the SON of ovariectom ized rats. The results suggest that changes in GABA(A) receptor binding may , in part, play a role in the regulation of steroid-induced increases in hy pothalamic OT expression. (C) 2000 Elsevier Science B.V. All rights reserve d.