A CENTRAL ACTION OF ALPHA-MELANOCYTE-STIMULATING HORMONE ON SERUM LEVELS OF LH AND PROLACTIN IN RATS

We have investigated the effect of administration of α-MSH into the median eminence (ME) of rats on the release of LH and prolactin. Continuous infusion of α-MSH (0.5 μg/h) into the ME from the afternoon of the second day of dioestrus and over the 24 h of pro-oestrus inhibited the preovulatory LH and prolactin surge and the occurrence of ovulation. This inhibitory effect on LH and prolactin release was also observed in chronically ovariectomized rats given a single injection of α-MSH (1 μg/μl per rat) into the ME (blood samples were collected 0, 20, 60, 90, 105 and 120 min after injection). The intraperitoneal injection of the dopamine receptor blocker, haloperidol (2 mg/kg), 30 min before the injection of α-MSH into the ME prevented the inhibitory effect of α-MSH on the release of LH and prolactin. These results suggest that hypothalamic α-MSH might be involved in the regulation of LH and prolactin release via the tuberoinfundibular dopaminergic system and that this sytem also modifies the serum concentrations of α-MSH.