Role of opioid peptides in pulsatile release of gonadotropins and prolactin in the rat

Abstract

To determine the role of endogenous opioid peptides in the pulsatile release of gonadotropins and prolactin in the ovariectomized rat, the opiate receptor blocker, naloxone, was administered intravenously, and its effect on plasma FSH, LH and prolactin was determined by multiple sampling prior to and after injection. Naloxone produced a dose-related increase in plasma LH and to a lesser extent FSH and decreased prolactin levels in the experiment in which they were examined. Higher doses of naloxone produced a significant increase in plasma LH pulse amplitude and lengthened the interpulse interval with a consequent decrease in pulse frequency. Minimum values between pulses were also increased. There was no clear effect on FSH pulsations but pulses of prolactin were blocked. Intraventricular (third ventricle) injection of a specific anti β endorphin antiserum (3 μl) produced an initial decline followed by an elevation of LH but had no effect on plasma FSH. The normal rabbit serum control injections were without effect. It is hypothesized that initiation of LH pulses in the castrated rat may be related to a periodic removal of tonic beta endorphinergic tone.