Interaction between oestrogen and oxytocin on hypothalamic-pituitary-adrenal axis activity

Area of Science:

• Neuroendocrinology
• Reproductive Biology
• Stress Physiology

Background:

• Oxytocin modulates behavior and the hypothalamic-pituitary-adrenal (HPA) axis during late pregnancy and lactation.
• The influence of ovarian hormones on oxytocin's central actions on the HPA axis remains incompletely understood.

Purpose of the Study:

• To investigate how ovarian hormones, particularly oestradiol, modulate the effects of central oxytocin on HPA axis activity in rats.
• To determine the dose-dependent effects of oestradiol on basal and stress-induced HPA axis responses and the interaction with oxytocin.

Main Methods:

• Ovariectomized rats received varying levels of oestradiol replacement (low, medium, high) with or without progesterone.
• Intracerebroventricular (i.c.v.) minipump infusion of oxytocin was administered.
• Plasma adrenocorticotrophic hormone (ACTH) and corticosterone levels, and hypothalamic corticotrophin-releasing factor (CRF) and pituitary pro-opiomelanocortin (POMC) mRNA expression were measured under basal and restraint stress conditions.

Main Results:

• Oestradiol alone increased basal ACTH and corticosterone but blunted the response to restraint stress.
• High oestradiol levels increased basal CRF and POMC mRNA expression while decreasing stress-induced levels.
• Central oxytocin administration reduced basal and stress-stimulated ACTH and mRNA levels, an effect that was dependent on oestradiol levels.

Conclusions:

• Oestradiol exhibits converse effects on basal versus stress-stimulated HPA axis activity.
• The inhibitory capacity of central oxytocin on the HPA axis is contingent upon the prevailing levels of circulating oestradiol.