Reduced oxytocin response to osmotic stimulus and immobilization stress in lactating rats.

The release of oxytocin in response to an osmotic stimulus and immobilization stress was compared in lactating rats 8-12 days after delivery and in non-lactating rats. Intravenous injection of hypertonic saline or immobilization stress induced an increase in blood oxytocin levels in both lactating and non-lactating rats, but the increment in the former was significantly lower than that in the latter. The lower responsiveness of oxytocin release to stress in lactating rats was not altered by ovariectomy 2 days after parturition. Oxytocin release induced by electrical stimulation of the anteroventral third ventricle (an osmoreceptive area), paraventricular nucleus and neurohypophysis was significantly lower, to a similar extent, in lactating rats compared with non-lactating rats. These findings indicate that the structural reorganization reported in the hypothalamo-neurohypophysial system may not function to facilitate release of oxytocin in response to stress and osmotic stimulus in lactating rats. The reduced responsiveness of the release of oxytocin is independent of the influence of ovarian hormones, and may be due to the low ability of the oxytocin neurone itself to release oxytocin, and/or due to the activated inhibitory influence on the oxytocin neurone in the lactating rat.