Acute low doses of melatonin restore full sexual activity in impotent male rats

Evidence exists that repeated injections of melatonin in rather large doses inhibit sexual performance in male rats. In contrast, systemic injection of small doses of this hormone stimulates sexual activity of normal male rats. In these experiments, systemic acute administration of melatonin in small doses (10-100 microg/kg) induced the appearance of ejaculations in impotent Wistar male rats that were selected as showing null sexual approach or showing mounts, intromissions but no ejaculations. This effect was partially abolished by the simultaneous peripheral injection of the non-selective melatonin receptor antagonist, luzindole, or by the acute administration of serotonin or of the 5HT(2A) receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), into the amygdala or the ventral hypothalamus. These results suggest that melatonin may stimulate, in a dose-dependent manner, several copulatory parameters of male sexual behavior and may restore sexual activity in impotent animals by interacting with brain receptors, i. e. melatonin and serotonin receptors.