Effects of indoleamines and their newly identified metabolites on prolactin release in rats.

Plasma immunoreactive PRL responses to indoleamines and their metabolites were studied in urethane-anesthetized rats. All drugs were injected into the lateral ventricle and blood samples were serially collected from a jugular vein. Serotonin and melatonin caused a significant increase in plasma PRL with peak values at 10-20 min after the injection. Significant increase in plasma PRL were also observed after the administration of 5-hydroxykynurenamine (5-HK), a newly identified serotonin metabolite. The potency of 5-HK was less than that of serotonin but much greater than that of melatonin. In contrast, plasma PRL did not change significantly in response to N-acetyl-5-methoxykynurenamine, another newly identified metabolite of melatonin, or a vehicle solution. Simultaneous administration of melatonin significantly blunted the plasma PRL response to serotonin, whereas the rise in plasma PRL induced by 5-HK was not blunted by melatonin. These results suggest that indoleamines as well as their metabolites play a role in regulating PRL secretion in rats.