Seasonal Variation of Melatonin Concentration and mRNA Expression of Melatonin-Related Genes in Developing Ovarian Follicles of Mares Kept under Natural Photoperiods in the Southern Hemisphere

This study investigated the seasonal variations in mRNA expression of FSH (Fshr), LH (Lhr) receptors, melatonin (Mt1 and Mt2) receptors, melatonin-synthetizing enzymes (Asmt and Aanat) and melatonin concentration in developing follicles from mares raised in natural photoperiods. For one year, ultrasonographic follicular aspiration procedures were performed monthly, and small (<20 mm), medium (20 to 35 mm) and large (>35 mm) follicles were recovered from five mares. One day before monthly sample collections, an exploratory ultrasonography conducted to record the number and the size of all follicles larger than 15 mm. The total number of large follicles were higher during the spring/summer (8.2 ± 1.9) than during autumn/winter (3.0 ± 0.5). Compared to autumn/winter seasons, there was an increase of Fshr and Aanat mRNA expressions in small, medium and large follicles, an increase of Lhr and Asmt mRNA expressions in medium and large follicles and an increase of Mt1 and Mt2 mRNA expressions in small and large follicles during spring/summer. The melatonin levels in follicular fluid were also higher during the spring/summer seasons. The present data show that melatonin locally upregulates the mRNA expression of Mt1 and Mt2 receptors and melatonin-forming enzymes in mare developing follicles during reproductive seasons.

Maternal pineal melatonin in gestation and lactation physiology, and in fetal development and programming

Pregnancy and lactation are reproductive processes that rely on physiological adaptations that should be timely and adequately triggered to guarantee both maternal and fetal health. Pineal melatonin is a hormone that presents daily and seasonal variations that synchronizes the organism's physiology to the different demands across time through its specific mechanisms and ways of action. The reproductive system is a notable target for melatonin as it actively participates on reproductive physiology and regulates the hypothalamus-pituitary-gonads axis, influencing gonadotropins and sexual hormones synthesis and release. For its antioxidant properties, melatonin is also vital for the oocytes and spermatozoa quality and viability, and for blastocyst development. Maternal pineal melatonin blood levels increase during pregnancy and triggers the maternal physiological alterations in energy metabolism both during pregnancy and lactation to cope with the energy demands of both periods and to promote adequate mammary gland development. Moreover, maternal melatonin freely crosses the placenta and is the only source of this hormone to the fetus. It importantly times the conceptus physiology and influences its development and programing of several functions that depend on neural and brain development, ultimately priming adult behavior and energy and glucose metabolism. The present review aims to explain the above listed melatonin functions, including the potential alterations observed in the progeny gestated under maternal chronodisruption and/or hypomelatoninemia.