Sexual Dimorphism in the Posterior Pituitary Response to Stress in the Rat*

Abstract

The posterior pituitary response to immobilization was studied in male and female rats. Plasma levels of arginine vasopressin (AVP) and oxytocin (OT) were measured both in control rats and in rats immobilized in an acrylic restrainer for 1 min. In male rats immobilization did not result in any change in AVP (control: 1.3 .+-. 0.2 pmol/l, mean .+-. SEM [standard error of the mean]; immobilized: 2.3 .+-. 0.6 pmol/l), although there was a small but significant increase in OT (control; 4.1 .+-. 0.5 pmol/l, immobilized: 10.2 .+-. 2.2 pmol/l; P < 0.005). In female rats a marked rise was observed in AVP (control: 1.4 .+-. 0.3 pmol/l; immobilized: 5.5 .+-. 1.3 pmol/l; P < 0.005), and the rise in OT was considerably greater (P < 0.01) than that found in males (control: 4.7 .+-. 0.8 pmol/l; immobilized: 26.0 .+-. 5.6 pmol/l, P < 0.001). Further groups of male and female rats were gonadectomized 2 wk before immobilization. Basal levels of AVP and OT were unchanged. Orchidectomized males had an increased OT response to immobilization compared with sham-operated males (P < 0.05); the AVP response was not significantly changed. Ovariectomy did not significantly affect either the AVP or OT responses. Although the neural pathways responsible for the neurohypophyseal response to immobilization are not known, the response is dependent on the sex of the rat.