Prolactin release induced by stress and the influence of oestrogen and progesterone treatments, sex and daily rhythm.

The effect of sex, ovarian steroids and time of the day on the release of prolactin induced by stress was studied. Albino rats were bled by heart puncture and immediately anaesthetized with ether; 10 min later they were bled again. Ovariectomized, oestrogen-primed rats showed a daily rhythm in the stress (bleeding plus ether) induced prolactin release with maximal concentration of prolactin in the serum at night (01.00 h) and minimum in the morning (09.00 h). IN ovariectomized, oestrogen-treated rats the rise of prolactin in the serum after stress was higher than in the non-treated rats both in the morning and at night. However in the afternoon (17.00 h), when the pre-stress levels of prolactin were high, the response to stress was reversed and prolactin levels declined. A similar effect was seen in the ovariectomized, oestrogen-primed rats 4 h after the injection of progesterone. In these animals the concentration of prolactin in serum was high and declined after stress. However 28 h after injection of progesterone stress failed to induce any change in prolactin release. Progesterone injected into ovariectomized, non-primed rats did not influence the response to stress. In male rats, both intact and castrated, the increase of prolactin concentration in serum after stress was lower than in females and failed to exhibit a circadian rhythm. These results show the importance of the hormonal background and the time of the day in the magnitude of prolactin release induced by stress.