Melatonin enhances sexual behavior in the male rat.

Anecdotal reports suggest that melatonin enhances libido in men. However, controlled trials remain to be published for any species. Accordingly, adult male rats were chronically treated for 12 weeks with melatonin via the drinking water. On the 13th week, all males were tested in the presence of sexually receptive females on measures of sexual behavior. Moreover, because of the established inverse relationship between male sexual behavior and serotonergic type 2A (5-HT(2A)) receptor activity, "wet-dog shakes" (WDS), a 5-HT(2A) receptor mediated behavior, were measured concurrently. All aspects of sexual activity were significantly facilitated in males treated with melatonin. In addition, there was a consistent, progressive reduction in the frequency of WDS, suggestive of a temporal decrement in serotonergic receptor activity and supportive of previous indications that melatonin possesses 5-HT(2A) antagonistic properties. These results provide the first empirical evidence for a facilitatory role of melatonin in sexual behavior, and suggest that its mechanism of action may involve the 5-HT(2A) receptor.