Effects of Neonatal Melatonin Administration on the Extra-Hypothalamic Regions in Rat Brains: Effects on the Serotonergic System

The effects of 100g melatonin injection at postnatal day 5 (PD 5) on the development of the central serotonergic systems in male and female rats were investigated. The contents of serotonin (5-HT) and 5-hydroxy-3-indolacetic acid (5-HIAA) were measured in several extra-hypothalamic regions at 3, 10 and 42 weeks of age. The neonatal melatonin administration increased both 5-HT and 5-HIAA levels in the striatum throughout the examined period. In the hippocampus, an increase in 5-HIAA contents by neonatal melatonin administration was found at 3 weeks but not 10 or 42 weeks of age. There were no significant differences in the effects of melatonin between male and female rats. These results indicated that exogenous melatonin administration during the early neonatal period influenced the development of the serotonergic systems in extrahypothalamic regions including the hippocampus and the striatum.