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/liter, mean +/- SEM; immobilized: 2.3 +/- 0.6 pmol/liter), although there was a small but significant increase in OT (control; 4.1 +/- 0.5 pmol/liter; immobilized: 10.2 +/- 2.2 pmol/liter; P less than 0.005). In female rats a marked rise was observed in AVP (control: 1.4 +/- 0.3 pmol/liter; immobilized: 5.5 +/- 1.3 pmol/liter; P less than 0.005), and the rise in OT was considerably greater (P less than 0.01) than that found in males (control: 4.7 +/- 0.8 pmol/liter; immobilized: 26.0 +/- 5.6 pmol/liter; P less than 0.001). Further groups of male and female rats were gonadectomized 2 weeks 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 less than 0.05) whereas 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, this data demonstrate that the response is dependent on the sex of the rat.

The plasma OT increment following stress in rats is sexually dimorphic, females exhibiting greater responses than males. We have investigated the role of neonatal androgen secretion in determining the sextypical level of response. Castration of male pups either surgically or functionally (GnRH antagonist treatment) within either 2 h or 5 days of birth did not elevate the OT responses of adult males. In contrast, androgenization of female pups (testosterone, 1.25 mg/pup) within 5 days of birth markedly reduced the OT stress responses of adults to a level insignificantly different to males. The results show that neonatal androgens can exert organizational effects on OT regulatory mechanisms. Since neonatal castration was ineffective it would appear that a prenatal defeminization or masculinization event determines OT stress responsiveness in males.

The role of ascending noradrenergic pathways in the mediation of central opiate-induced cardiovascular effects has been investigated. The effects of selective kappa- and delta-opiate agonists microinjected into the nucleus tractus solitarius (NTS) of urethane anaesthetized rats were compared following 6-hydroxydopamine lesions of either the dorsal (DNAB) or ventral (VNAB) noradrenergic bundles. In sham lesioned animals both opiates elicited a significant pressor effect and a variable but consistent bradycardia. The delta-agonist responses were not modified in lesioned rats. In marked contrast the pressor effect of the kappa-agonist was abolished in both DNAB and VNAB lesioned rats. The bradycardic response was not significantly modified. These findings are consistent with previous observations that the cardiovascular effects of kappa- and delta-opiates in the NTS are mediated via different mechanisms, and provide evidence for selective functional actions of endogenous opioids within brain nuclei.

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