Acute low doses of melatonin restore full sexual activity in impotent malerats

Evidence exists that repeated injections of melatonin in rather large doses inhibit sexual performance in male rats. In contrast, systemic injection o f small doses of this hormone stimulates sexual activity of normal male rat s. In these experiments, systemic acute administration of melatonin in smal l doses (10-100 mu g/kg) induced the appearance of ejaculations in impotent Wister male rats that were selected as showing null sexual approach or sho wing mounts, intromissions but no ejaculations. This effect was partially a bolished by the simultaneous peripheral injection of the non-selective mela tonin receptor antagonist, luzindole, or by the acute administration of ser otonin or of the 5HT(2A) receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2 -aminopropane (DOI), into the amygdala or the ventral hypothalamus. These r esults suggest that melatonin may stimulate, in a dose-dependent manner, se veral copulatory parameters of male sexual behavior and may restore sexual activity in impotent animals by interacting with brain receptors, i.e. mela tonin and serotonin receptors. (C) 2000 Elsevier Science B.V. All rights re served.