No stress response of the hypothalamo-pituitary-adrenal axis in parturient rats: Lack of involvement of brain oxytocin

During parturition, the basal activity of the hypothalamo-pituitary-adrenal (HPA) axis of Wistar rats is strongly attenuated, whereas the oxytocin system is activated. We investigated the secretory responses of the HPA axis and oxytocin to exposure to a mild emotional stressor (airpuff) comparing virgin female, d 22 pregnant, and parturient rats. Furthermore, as the brain oxytocin system is activated in parturition and oxytocin has been shown to inhibit HPA axis responses in virgin rats, the role of brain oxytocin in the regulation of stress responses during parturition was investigated by intracerebroventricular administration of an oxytocin receptor antagonist before stressor exposure (0.75 mug/5 mul). In virgin female rats, exposure to airpuff increased ACTH (2.5+/-0.34-fold) and corticosterone (5.1+/-2.3-fold) secretion, but in late pregnancy and parturition, the stress-induced increase in ACTH (pregnancy: 1.9+/-0.41-fold; parturition: 1.3+/-0.13-fold) and corticosterone secretion (parturition: 1.8+/-0.40-fold) were strongly attenuated. Oxytocin secretion remained unchanged in response to airpuff in both virgin and parturient rats despite higher overall plasma concentrations in the latter. Oxytocin receptor blockade in the brain elevated basal and stress-induced ACTH secretion in virgin but not pregnant or parturient rats and had no effect on oxytocin secretion either in virgin or parturient rats. We conclude that the reactivity of the HPA axis to external stressors is strongly attenuated during parturition, and this cannot be disinhibited by blocking the receptor-mediated action of brain oxytocin.