The Pineal Gland and the Menstrual Cycle

Abstract

The menstrual cycle reflects the expression of a cyclical process involving the interaction between the hypothalamic-pituitary axis and the ovaries. This complex process requires an integrated neural and humoral control mechanism. It is now well established that a hypothalamic "transducer" located in the medial basal hypothalamus integrates neural and humoral information and translates it into an oscillatory signal which eventually results in the release of the gonadotropin releasing hormone (GnRH), triggering the secretion of gonadotropins from the pituitary gland. Recent animal studies indicate that melatonin influences the functions of the hypothalamic-pituitary-gonadal axis by modifying the firing frequency of the hypothalamic GnRH pulse generator. Consequently, the pineal gland, through the action of melatonin, may exert an important modulatory effect on the mechanisms controlling menstrual cyclicity. Furthermore, abnormal melatonin functions may be involved in the pathogenesis of several disorders of the menstrual cycle including some forms of hypothalamic amenorrhea such as exercise and malnutrition-induced amenorrhea. Consideration of pineal melatonin functions provides a new dimension into the understanding of the neuroendocrine mechanisms governing the cyclical phenomena of the female reproductive system.

Melatonin as a Biomarker of Circadian Dysregulation

“…Animal studies have also shown that melatonin can inhibit luteinizing hormone-induced testosterone production in rats (16)(17)(18), inhibits prolactin cell activity in male and female hamsters (19), and suppresses several aspects of reproductive physiology in male hamsters (20). Human studies indicate that decreased concentrations of circulating melatonin (such as those brought about by circadian disruption) can result in increased release of the gonadotropins luteinizing hormone and follicle-stimulating hormone from the pituitary and estrogen release by the ovaries (21)(22)(23)(24)(25). There is also evidence from clinical studies of a relationship between melatonin and male reproductive hormones (26)(27)(28)(29).…”

Melatonin Patterns and Levels During the Human Menstrual Cycle and After Menopause

“…It is possible that the level of intrafollicular melatonin (not captured by the present study) is what supports the subsequent production of progesterone and stabilizes the CL. Another distinct function of melatonin may be to govern the menstrual cycle through its reciprocal relation with gonadotropin-releasing hormone pulse frequency [ 27 ]. The gonadotropin-releasing hormone pulse generator is slowest (approximately every 4-6 hours) in the late luteal phase, when melatonin is high, and is fastest (approximately circhoral) at midcycle, when melatonin is low [ 28 ].…”