The milk-ejection reflex in the peri-partum rat: effects of oestradiol and progesterone on basal milk-ejection frequency and the facilitatory response to central oxytocin.

Experiments were undertaken to examine the effects of ovarian steroids on the functional characteristics of the milk-ejection reflex during late pregnancy. Basal milk-ejection frequency and response to i.c.v. oxytocin (OT) were compared in different experimental groups, using intramammary pressure recordings obtained in suckling tests under urethane anaesthesia. Ovariectomy (OVX) on day 20 of pregnancy significantly (P < 0.05) increased milk-ejection frequency on day 22, compared with sham-OVXed animals. I.c.v. injection of 2.2 ng OT during suckling had no consistent effect on milk ejection in either of these groups. Pretreatment with oestradiol (5 micrograms per day, s.c.) or progesterone (5 mg per day, s.c.) both resulted in a fall in milk-ejection frequency compared to oil-treated OVXed controls. However, whereas oestradiol-treated OVXed rats showed a facilitatory response to i.c.v. OT, with a significant (P < 0.05) increase in milk-ejection frequency in the 20 min period after injection, progesterone-treated OVXed rats showed only a delayed decrease in milk-ejection frequency (significant at P < 0.05 between 20-40 min after injection). Oil-treated OVXed rats showed no significant response to i.c.v. OT at any stage. Electrophysiological recordings from supraoptic OT neurones confirmed that bursting activity was increased by i.c.v. injection of OT in oestradiol-treated, but not progesterone-treated rats. Further experiments with hysterectomized ovariectomized rats indicated that the difference in response to i.c.v. OT in oestradiol- vs progesterone-treated rats was not related to changes in the timing of birth induced steroid treatments. These findings demonstrate the ability of ovarian steroids to alter the characteristics of the milk-ejection reflex in the peri-partum rat. In particular, the rise in oestradiol and fall in progestesterone near term, may contribute to programming of the facilitary response to central OT in preparation for lactation.