The effects of mu-, delta- and kappa-opioid receptor activation on luteinizing and follicle-stimulating hormone secretion from porcine pituitary cells.

Endogenous opioid peptides (EOP) are involved, among others, in the regulation of endocrine systems, including gonadotropin (LH and FSH) secretion in females during the estrous cycle. EOP preferentially act through three major types of opioid receptors: mu (MOP), delta (DOP) and kappa (KOP). Their influence on gonadotropin secretion at the hypothalamic level was extensively studied in different species, but information pertaining to their modulatory action on LH and FSH secretion at the pituitary level is scarce. Therefore, the aim of the present study was to examine the effects of opioid receptor agonists (mu--DAMGO, delta--DPDPE and kappa--U 50.488) at doses 10⁻⁹, 10⁻⁸, 10⁻⁷ mol/L on both basal and GnRH-stimulated gonadotropin (LH and FSH) secretion in vitro from the anterior pituitary cells of gilts on days 8-10 (luteal phase) and 19-20 (follicular phase) of the estrous cycle. The exposition of pituitary cells in vitro to kappa-receptor agonist (U 50.488; 10⁻⁷ mol/L) significantly reduced (p<0.05) basal secretion of LH during both phases of the estrous cycle. The GnRH-stimulated LH secretion was also decreased (p<0.05) by this agonist during the luteal phase (10⁻⁷ mol/L) and follicular phase (10⁻⁹, 10⁻⁸ and 10⁻⁷ mol/L). In turn, the FSH secretion was reduced (p<0.05) by kappa-agonist only in the presence of GnRH during the luteal phase (10⁻⁸ and 10⁻⁷ mol/L) and follicular phase (10⁻⁷ mol/L). The delta-opioid agonist (DPDPE) significantly reduced (p<0.05) the GnRH-affected secretion of LH during the follicular phase (10⁻⁸ mol/L) and FSH during the luteal phase (10⁻⁷ mol/L). The mu-opioid agonist (DAMGO) affected neither LH nor FSH secretion. These results indicate that opioid peptides, acting mainly through kappa- and delta-opioid receptors, may participate in the modulation of gonadotropin (LH and FSH) secretion at the pituitary level in cyclic gilts.