REGULATION OF THE PHOTOPERIOD-INDUCED CYCLE IN THE PERIPHERAL-BLOOD CONCENTRATIONS OF BETA-ENDORPHIN AND PROLACTIN IN THE RAM - ROLE OF DOPAMINE AND ENDOGENOUS OPIOIDS

In a group of adult Soay rams housed indoors under an artificial light cycle of alternating 16-week periods of long and short days, there was a conspicuous long-term cycle in the peripheral plasma concentrations of beta-endorphin and prolactin. The levels of beta-endorphin were highest under short days and lowest under long days (15-fold change), and inversely related to the changes in the plasma levels of prolactin (120-fold change). The role of dopamine in the control of beta-endorphin and prolactin was investigated in a series of experiments, conducted under both long and short days, in which rams were treated with dopamine receptor agonists (dopamine and bromocriptine) and antagonists (pimozide and sulpiride). Naloxone (opioid antagonist) was also administered to assess the additional involvement of endogenous opioids. Dopamine injected i.v. (6.6 mg/kg every 10 min) did not significantly affect the mean plasma concentrations of beta-endorphin and prolactin under either long or short days. Pimozide (0.08 mg/kg i.m. every 2 h) caused a large increase in the mean plasma concentrations of beta-endorphin and prolactin under long days but not short days. Naloxone (1.6 mg/kg, i.v.), administered alone or in combination with dopamine or pimozide, had no effect on the mean plasma concentrations of beta-endorphin and prolactin, except under short days when, combined with pimozide, it induced an increase in the plasma concentrations of the two polypeptides. Bromocriptine (0.06 mg/kg, s.c.) caused a significant decrease in the plasma concentrations of both beta-endorphin and prolactin; this effect was most marked at the times of increased secretion (under short days for beta-endorphin and under long days for prolactin). Sulpiride (0.59 mg/kg, s.c.) produced the converse effect and caused an increase in the plasma concentrations of beta-endorphin and prolactin with the amplitude and duration of the effect varying with the stage of the photoperiod-induced cycle. From these results in the Soay ram, we conclude that dopamine inhibits beta-endorphin and prolactin secretion by way of D2 receptors under both long and short days. Endogenous opioids interact with dopamine, augmenting this inhibition under short days. Differences in the acute responses in the secretion of beta-endorphin and prolactin, and the inverse relationship between beta-endorphin and prolactin during the cycle, indicate that different regulatory systems involving dopamine influence the two pituitary polypeptides.