EFFECTS OF SINGLE DOSES AND DAILY MELATONIN TREATMENTS ON SEROTONIN METABOLISM IN RAT-BRAIN REGIONS

The acute effects of two doses (0.5 and 1 mg/kg) of melatonin on the l evels of tryptophan, serotonin, and 5-hydroxyindoleacetic acid in seve ral rat brain regions were studied. Tryptophan content in the brain re gions was unchanged by the treatments. Melatonin at a dosage of 0.5 mg /kg increased medial hypothalamic serotonin levels at 60 and 90 min af ter the injection. However, the dose of 1 mg/kg increased the levels o f this amine or its metabolite in the preoptic area-anterior hypothala mus, medial and posterior hypothalamus, amygdala, and midbrain. These results suggest a specific regional sensitivity to melatonin as well a s a dose-dependent response. The stimulatory melatonin effect on the s erotoninergic system was also observed after a daily treatment with th is hormone (0.5 mg/kg, twice daily during 10 days) in both intact or p inealectomized rats. In intact rats, melatonin treatment increased the levels of 5-hydroxyindole-3-acetic acid in the preoptic area-anterior hypothalamus and medial hypothalamus, while in pinealectomized rats m elatonin increased the serotonin content in the medial hypothalamic re gion. The data support the idea that melatonin has a selective action on serotonin metabolism in regions that contain serotoninergic termina ls, especially at medial hypothalamic level.