The expression of mu (μ), delta (δ) and kappa (κ) opioid receptors was determined in vitro under the basal conditions as well as in the presence of cytokines (interleukin (IL)-1β, IL-6 and tumor necrosis factor α (TNFα)) in the endometrium of gilts on days 10–11, 12–13 and 15–16 of the oestrous cycle and pregnancy. These days were chosen due to their importance for the establishment of pregnancy in pigs. During the oestrous cycle, the basal expression of μ opioid receptor mRNA in endometrial explants was greater on days 10–11 than on days 12–13 and 15–16, during pregnancy its expression was greater on days 12–13 than on days 10–11 and 15–16. The expression of δ opioid receptor mRNA did not change during the cycle, but during pregnancy it was the greatest on days 10–11. The expression of κ opioid receptors did not significantly change during studied periods. The cytokines affected the expression of μ and δ, but not κ opioid receptors in endometrial explants. The expression of μ opioid receptor mRNA was significantly decreased by IL-1β on days 10–11, but IL-6 increased and decreased it on days 12–13 and 15–16 of the cycle, respectively. During pregnancy, the expression of μ opioid receptor mRNA in endometrial explants was increased by IL-1β, IL-6 and TNFα on days 10–11, decreased by IL-1β on days 12–13, and increased by TNFα on days 15–16. During the oestrous cycle, the expression of δ opioid receptor mRNA was elevated by IL-1β on all studied days, by IL-6 on days 10–11 and 12–13, and by TNFα on days 15–16. During pregnancy, IL-1β increased the expression of δ opioid receptor transcript on days 12–13. These data suggest that opioid receptors participate in a local regulation of uterine functions in gilts during the oestrous cycle and early pregnancy and their endometrial expression can be modulated by cytokines.
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