AN INHIBITORY EFFECT OF MELATONIN ON THE ESTROUS PHASE OF THE ESTROUS CYCLE OF THE RODENT.

Microgram doses of melatonin decrease the incidence of vaginal smears that demonstrate estrous phases in Sprague- Dawley rats and C3H/HeN mice. This property is not shared by the precursors of melatonin, serotonin and N-acetylserotonin, or its major metabolite, 6-hydroxymelatonin. The minimum effective dose of melatonin is decreased when daily melatonin injections are started prior to gonadal maturation. Pinealectomy is followed by an increase in the incidence of estrus; this increase is inhibited by melatonin treatment. (Endocrinology 75: 238, 1964)