Opioid modulation of LH secretion by pig pituitary cells in vitro

Summary. The effects of naloxone and β-endorphin on LH secretion by pig pituitary cells were studied in primary cultures. On Day 4 of culture, cells (105 seeded/well) were challenged with 10−9, 10−8 or 10−7m gonadotrophin-releasing hormone (GnRH), 10−10, 10−9, 10−8 or 10−7m-β-endorphin or 10−6m-naloxone individually or in combinations. Secreted LH was measured at 4 h and 24 h after treatment and cellular content of LH was measured after 24 h. Basal LH secretion (control) was 23·5 ± 7·6 and 36·9 ± 10·3 ng/well at 4 h and 24 h, respectively. Relative to control at 4 h, 10−9, 10−8 or 10−7m-GnRH stimulated (P < 0·05) LH secretion 140%, 210% and 250%, respectively. At 24 h, LH secretion was increased (P < 0·05) by GnRH compared to control, but the does–response to GnRH was absent. Naloxone increased (P < 0·01) LH secretion 166 ± 13% at 4 h and 141 ± 13% (P < 0·06) at 24 h. Secretion of LH after simultaneous addition of 10−8m-GnRH plus naloxone was greater (P < 0·01) than after GnRH alone at 4 h but not at 24 h. β-Endorphin at 10−10, 10−9, 10−8 or 10−7m failed to alter basal LH secretion at 4 h but decreased secretion at 24 h, while cellular LH content was similar to control at 24 h. LH secretion after simultaneous addition of 10−7m-GnRH and 10−7m-β-endorphin was less (P < 0·01) than after GnRH at 4 h but not at 24 h, while 10−10m-β-endorphin plus 10−9m-GnRH failed to suppress LH secretion, compared to GnRH alone. These results indicate that endogenous opioids may directly modulate LH secretion at the level of the pituitary.