The effect of selective agonists of opioid receptors on in vitro secretion of steroid hormones by porcine endometrium during the estrous cycle and early pregnancy.

Steroid hormones play an important role in the regulation of cyclic changes in the uterus and preparation of intrauterine environment for the egg fertilization, embryo implantation and maintenance of pregnancy. Their secretion by porcine uterus has been demonstrated. The present study aimed to establish the effect of opioid receptors (μ, δ and κ) activation by selective agonists (DAMGO, DPLPE and U 50.488, respectively) on in vitro secretion of steroid hormones (during 6-h and 24-h incubations) by the endometrial explants of gilts on days 2 - 3, 10 - 11, 12 - 3, 15 - 16, 18 - 20 of the estrous cycle and 10 - 11, 12 - 13, 15 - 16 of pregnancy. The agonists at certain of tested concentrations (10, 10 and 10 M) affected secretion of steroid hormones. Progesterone secretion was increased by μ-opioid receptor agonist on days 18 - 20 (6 h) and by δ-agonist on days 2 - 3 and 18 - 20 (24 h) of the cycle. During pregnancy (days 15 - 16), κ-agonist increased it (6 h), but μ-opioid agonist decreased (24 h). Androstenedione secretion was decreased during shorter incubation; by μ- and δ-receptor agonists on days 2 - 3, by all agonists on days 12 - 13, and by κ-receptor agonist on days 18 - 20 of the cycle. However, it was increased during longer incubation with agonists of κ- and μ-opioid receptors on days 10 - 11 and 18 - 20 of the cycle, respectively. Estradiol secretion was elevated by κ- and μ-agonists (6 h) on days 2 - 3 and 15 - 16 of the cycle, respectively, as well as following 24-h incubation with μ-agonist on days 15 - 16, and μ- and κ-agonists on days 18 - 20 of the cycle. During pregnancy, its secretion was increased (24 h) ondays 15 - 16 by μ- and κ-opioid agonists. Cortisol secretion did not significantly change (versus control) in response to applied treatments. These results indicate a potential involvement of EOPs in the modulation of endometrial steroidogenesis in the pig during the estrous cycle and pregnancy.