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In addition to its role in reproduction, oxytocin has central actions modulating behavioural and hypothalamic-pituitary-adrenal (HPA) axis responses during late pregnancy and lactation. The hypothesis that ovarian hormones modulate the effects of oxytocin on HPA axis activity was studied in 7-day ovariectomised rats receiving oestradiol with or without progesterone replacement and intracerebroventricular (i.c.v) minipump infusion of oxytocin (100 ng/h). In an initial experiment, i.c.v. oxytocin had no effect on basal or restraint-stimulated plasma adrenocorticotrophic hormone (ACTH) and corticosterone concentrations or hypothalamic corticotrophin-releasing factor (CRF) mRNA expression with low oestradiol replacement alone but it had a stimulatory effect in the presence of low oestradiol and progesterone. To investigate further whether oestradiol modulates central actions of oxytocin, rats received low dioestrous (low), pro-oestrous (medium) or pregnancy (high) oestradiol replacement levels, yielding plasma concentrations of < 5, 17.3 +/- 4.5 and 258 +/- 32 pg/ml, respectively, with or without i.c.v. oxytocin. Oestradiol caused dose-dependent increases in basal plasma ACTH and corticosterone concentrations but decreased the ACTH response to restraint stress. In parallel to the changes in basal plasma ACTH, high oestrogen increased basal CRF hnRNA, CRF mRNA in the paraventricular nucleus and pro-opiomelanocortin (POMC) mRNA in the pituitary gland, while decreasing restraint stress-stimulated levels. Intracerebroventricular administration of oxytocin reduced basal and stress-stimulated plasma ACTH, hypothalamic CRF hnRNA (30 min), CRF mRNA and pituitary POMC mRNA (4 h) levels parallel to the increases induced by elevating plasma oestradiol. The present study demonstrates the converse effects of oestradiol on basal and restraint stress-stimulated basal HPA axis activity, and that the ability of central oxytocin to inhibit HPA axis activity depends on the levels of circulating oestradiol.

The stimulation of ACTH release from anterior pituitary cells by corticotropin-releasing factor (CRF) is mediated by specific, high affinity receptors with a Ka of 10(9) M-1 for ovine CRF. The relationship between ACTH secretion and CRF receptor activation was analyzed in normal and adrenalectomized rats by comparison of ACTH release with changes in CRF receptors and adenylate cyclase activity. The marked increase in plasma ACTH levels that occurred after adrenalectomy (from 71 to 478 pg/ml after 4 days) was accompanied by a progressive decrease in pituitary CRF receptor concentration [by 29 +/- 1%, 75 +/- 2%, 77 +/- 6%, and 80 +/- 4% (+/- SE) after 1, 2, 3, and 4 days, respectively]. Most of this decrease was due to receptor down-regulation rather than occupancy by endogenous CRF, since high dose infusions of CRF (300-500 ng/min) for 30 min before pituitary membrane preparation reduced CRF-binding sites by only 40%. The marked reduction in CRF receptors after adrenalectomy was accompanied by comparable decreases in maximal CRF-stimulated adenylate cyclase activity and sensitivity to CRF (ED50, 3.8 +/- 2.8 vs. 58 +/- 3.7 X 10-9 M CRF in control and 2-day-adrenalectomized rats, respectively). Fluoride-stimulated adenylate cyclase activity was unchanged at 24 h, but was decreased by 28 +/- 7% at later times. Such decreases in CRF receptors and adenylate cyclase activity in adrenalectomized rats were prevented by dexamethasone treatment. In cultured anterior pituitary cells from 4-day-adrenalectomized rats, CRF-stimulated cAMP production was decreased by 40%. However, in contrast to the decreases in CRF receptors and cAMP production, there was a 3-fold increase in CRF-stimulated ACTH release, with no change in sensitivity to CRF. The ability of corticotrophs to maintain increased ACTH release, in conjunction with reduced CRF receptors and CRF-stimulated adenylate cyclase, indicates that elevated ACTH secretion can be maintained by occupancy and activation of only a small number of CRF receptors. This finding also suggests that synergistic interactions between CRF and other regulators of ACTH release may contribute to the sustained increase in ACTH secretion that follows adrenalectomy.