Effects of single doses and daily melatonin treatments on serotonin metabolism in rat brain regions

J Pineal Res. 1994 Nov;17(4):170-6. doi: 10.1111/j.1600-079x.1994.tb00129.x.


The acute effects of two doses (0.5 and 1 mg/kg) of melatonin on the levels of tryptophan, serotonin, and 5-hydroxyindoleacetic acid in several rat brain regions were studied. Tryptophan content in the brain regions was unchanged by the treatments. Melatonin at a dosage of 0.5 mg/kg increased medial hypothalamic serotonin levels at 60 and 90 min after the injection. However, the dose of 1 mg/kg increased the levels of this amine or its metabolite in the preoptic area-anterior hypothalamus, medial and posterior hypothalamus, amygdala, and midbrain. These results suggest a specific regional sensitivity to melatonin as well as a dose-dependent response. The stimulatory melatonin effect on the serotoninergic system was also observed after a daily treatment with this hormone (0.5 mg/kg, twice daily during 10 days) in both intact or pinealectomized 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 melatonin increased the serotonin content in the medial hypothalamic region. The data support the idea that melatonin has a selective action on serotonin metabolism in regions that contain serotoninergic terminals, especially at medial hypothalamic level.

MeSH terms

  • Animals
  • Brain / drug effects
  • Brain / metabolism*
  • Dose-Response Relationship, Drug
  • Drug Administration Schedule
  • Hydroxyindoleacetic Acid / metabolism*
  • Injections, Subcutaneous
  • Male
  • Melatonin / administration & dosage
  • Melatonin / pharmacology*
  • Pineal Gland / physiology
  • Pineal Gland / surgery
  • Rats
  • Rats, Sprague-Dawley
  • Serotonin / metabolism*
  • Tryptophan / metabolism


  • Serotonin
  • Hydroxyindoleacetic Acid
  • Tryptophan
  • Melatonin