Effects of naloxone on pulsatile luteinizing hormone in experimental hyperprolactinemia

It is well known that opioids regulate luteinizing hormone (LH) secretion through not fully understood mechanisms. This study describes the effects of naloxone on the episodic release of LH in adult sham-operated and pituitary-grafted female rats. Animals were rendered hyperprolactinemic by transplanting 2 pituitary glands beneath the kidney capsule. Naloxone (2 mg/kg/h) or saline (0.5 ml/h) were administered iv through jugular cannulae and subjects were bled at 7 min intervals for a period of 3 h. As expected, pituitary-grafting was followed by an increase in mean values of prolactin during the bleeding period. Naloxone administration decreased mean serum prolactin levels in sham-operated rats and did not further change them in pituitary-grafted animals. Hyperprolactinemia was associated with increases in mean serum LH levels during the bleeding period and in the absolute amplitude of LH peaks. Naloxone administration increased mean values of LH and the absolute amplitude of LH peaks, and decreased their frequency in sham-operated rats. Neither pituitary grafting nor naloxone administration modified the frequency, duration or relative amplitude of LH peaks. However, naloxone 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 LH and that these effects are blunted in pituitary-grafted animals.