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.