Pineal actions and mechanisms in reproductive physiology

The pineal gland secretes hormones which have obvious effects on the reproductive system. The bulk, if not all, of these effects are of an inhibitory nature. The pineal is regulated primarily by photoperiodic information which, in mammals, is perceived by the lateral eyes. Secondary regulators of pineal biosynthetic and secretory activity may be gonadotrophins and gonadal steroids. Darkness promotes the antigonadotrophic activity of the pineal while light suppresses it. Melatonin may be only one of the hormones responsible for the gonad-inhibiting activity of the pineal gland. The sites of action of the pineal substances on the reproductive axis still have not been definitely identified although there is a general belief among individuals working in the field that the most likely locus of action is in the hypothalamo-pituitary axis.