Oxytocin responses to stress in lactating and hyperprolactinaemic rats

The plasma oxytocin (OT) response to acute stress was compared between virgin, lactating, and hyperprolactinaemic female rats. In virgin rats, brief immobilization was associated with a significant elevation of plasma OT to 24.7 +/- 3.7 pmol/l compared with 7.7 +/- 1.1 pmol/l in controls. In contrast, the stress response was absent in lactating (6 days post-partum) animals: control OT 9.4 +/- 2.2, immobilized OT 9.0 +/- 1.1 pmol/l. Hyperprolactinaemia produced by treatment with either dopamine antagonists (domperidone or haloperidol) or ovine prolactin was also associated with an impairment of the OT stress response in intact females, whereas domperidone treatment failed to modify the response in ovariectomized (OVX) rats. Following ovarian steroid replacement with oestradiol and progesterone, the inhibitory effect of domperidone was observed in OVX rats: control OT 11.1 +/- 2.5, immobilized OT 16.0 +/- 3.7 pmol/l. Treatment of OVX rats with oestradiol and progesterone, either separately or combined, did not modify the OT stress response. Plasma levels of vasopressin were not significantly modified in either control or immobilized rats of any experimental groups. The results indicate that hyperprolactinaemia may be a causative factor in the impairment of OT stress responses observed in lactating rats.