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

SIMPLE SUMMARY: Reproductive activity in mares shows a seasonal pattern that is associated with an increasing photoperiod with a pronounced incidence of ovulations during spring and summer. The photoperiodic control of reproductive activity is mediated by melatonin, secreted by the pineal gland, which plays an inhibitory role in ovulatory activity via the hypothalamic-pituitary axis. Considering that there is little information on the direct effect of melatonin on the development of equine ovarian follicles, we studied the seasonal variations in mRNA expression of gonadotropin receptors (Fshr and Lhr), melatonin receptors (Mt1 and Mt2), melatonin-synthetizing enzymes (Asmt and Aanat) and melatonin concentration in developing follicles (small—<20 mm, medium—20 to 35 mm and large—>35 mm) from five mares raised in natural photoperiods. There was an increased mRNA expression of gonadotropin receptors and melatonin-related genes and an increase of melatonin levels in developing follicles during the spring/summer seasons. The total number of large follicles (potential ovulatory follicles) was significantly higher during the spring/summer seasons. Our results demonstrate that melatonin upregulates the mRNA expression of melatonin receptors and melatonin-forming enzymes in mare developing follicles during reproductive seasons. ABSTRACT: 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.