EFFECTS OF NALOXONE ON PULSATILE LUTEINIZING-HORMONE IN EXPERIMENTAL HYPERPROLACTINEMIA

It is well known that opioids regulate luteinizing hormone (LH) secret ion through not fully understood mechanisms. This study describes the effects of naloxone on the episodic release of LH in adult sham-operat ed and pituitary-grafted female rats. Animals were rendered hyperprola ctinemic by transplanting 2 pituitary glands beneath the kidney capsul e. Naloxone (2 mg/kg/h) or saline (0.5 ml/h) were administered Iv thro ugh jugular cannulae and subjects were bled at 7 min intervals for a p eriod of 3 h. As expected, pituitary-grafting was followed by an incre ase In mean values of prolactin during the bleeding period. Naloxone a dministration decreased mean serum prolactin levels in sham-operated r ats and did not further change them in pituitary-grafted animals. Hype rprolactinemia was associated with increases In mean serum-LH levels d uring the bleeding period and in the absolute amplitude of LH peaks. N aloxone administration increased mean values of LH and the absolute am plitude of LH peaks, and decreased their frequency in sham-operated ra ts. Neither pituitary grafting nor naloxone administration modified th e frequency, duration or relative amplitude of LH peaks. However, nalo xone administration reduced the mean half-life of LH in sham-operated rats to a similar extent than did pituitary grafting. Naloxone failed to further change the mean half-life of LH in pituitary-grafted rats. These results suggest that opioids modulate the pulsatile pattern of L H and that these effects are blunted in pituitary-grafted animals.