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.

The expression of genes coding for opioid precursors, opioid receptors, beta-LH subunit and GnRH receptor in the anterior pituitary of cyclic gilts.

In previous studies the effect of endogenous opioid peptides (EOP) on LH secretion was mainly considered at the hypothalamic level, while opioid involvement in the modulation of LH secretion at the pituitary level remains insufficiently elucidated. Therefore, the present study was undertaken to determine the expression of genes encoding opioid precursors--proopiomelanocortin (POMC), proenkephalin (PENK), prodynorphin (PDYN) and opioid receptors--mu, delta, kappa in the porcine anterior pituitary throughout the estrous cycle. Additionally, the mRNA content of beta-LH subunit and GnRH receptor (GnRH-R) was estimated. Pituitaries (5xN = 7) were collected from sows on days 3-5, 8-10, 13-15, 16-17 and 19-20 of the cycle and gene expression was determined using a semi-quantitative RT-PCR assay. The expression of POMC, PDYN, delta and kappa receptor genes was variable across the cycle, whereas the expression of PENK and mu receptor genes remained relatively stable. The POMC mRNA content was the lowest on days 19-20 of the cycle and the PDYN content was reduced on days 8-10. The delta receptor mRNA content was elevated on days 3-5, while the kappa receptor mRNA content was decreasing over the luteal phase. Changes in the expression of genes encoding beta-LH and GnRH-R were also demonstrated. These results indicate variable activity of pituitary opioid systems in cyclic pigs and suggest implication of EOP in the modulation of LH secretion at the pituitary level.

Expression of proopiomelanocortin, proenkephalin and prodynorphin genes in porcine luteal cells.

The objective of the study was to examine the expression of the genes coding for proopiomelanocortin (POMC), proenkephalin (PENK) and prodynorphin (PDYN) in porcine luteal cells isolated from corpora lutea (CL) collected on days 3-6, 8-10 and 13-16 of the oestrous cycle. Total RNA was purified from non-incubated cells and from cells incubated for 48 h in the absence or presence of luteinising hormone (LH). The semi-quantitative RT-PCR technique, involving coamplification of the target and control cDNA (beta-actin or 18S rRNA), was used to examine gene expression. It was found that the genes coding for opioid precursors are expressed in both non-incubated and incubated porcine luteal cells representing the early, mid- and late luteal phase. In non-incubated cells, only POMC mRNA content changed during CL development, whereas the expression of PENK and PDYN genes remained relatively constant. Additionally, the treatment of cells with LH markedly affected the expression of POMC and PENK, but no influence on PDYN expression was observed. The present study indicates that porcine luteal cells may produce opioid peptides and that gene expression of their precursors (except for PDYN) may be modulated in these cells by LH. Moreover, the present results support the involvement of opioid peptides in local regulation within the CL of the pig.