Melatonin as well as serotonin, when injected into the third ventricle of the brain of male rats, appeared to stimulate the release of prolactin and inhibit the release of FSH as judged by the changes in the concentrations of these hormones in the plasma of the recipient animals. (FSH and prolactin were determined by radioimmunoassay.) For example, 30 min after the injection into the third ventricle of doses of 1, 5 and 50 jug melatonin, the plasma prolactin concentrations were 199 ± 11.3 % (mean and SE), 235 ± 16.1% and 304 ± 12.7%, respectively, of the pre-injection levels. The corresponding FSH values were 88 ± 3.7%, 85 ± 3.7% and 74 ± 2.6%. After 2 hr, the plasma prolactin concentration had returned to the pre-injection level; however, the plasma FSH concentration after 2 hr was only 50% of the pre-injection level. The effects of intraventricularly administered serotonin were similar to those seen with melatonin. Isotonic saline solution (2.5 μl) had no effect on the release of prolactin and FSH. When melatonin or serotonin was infused directly into the anterior pituitary by way of a cannulated portal vessel, no significant alterations in the plasma concentrations of prolactin and FSH were observed. These results suggest that melatonin or serotonin may have suppressed the discharge of PIF and FRF and thereby indirectly affected the release of prolactin and FSH.
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