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.
Authors: Coelho Lia Alencar, Silva Luciano Andrade, Reway Ana Paula, Buonfiglio Daniella Do Carmo, Andrade-Silva Jéssica, Gomes Patrícia Rodrigues Lourenço, Cipolla-Neto José
Journal: Animals : an open access journal from MDPI
Summary
# Editorial Summary Melatonin's role in reproductive seasonality is well-established systemically, but little has been known about its local influence within developing ovarian follicles themselves. Researchers in the Southern Hemisphere monitored five mares across a full year, collecting follicular fluid and tissue monthly via ultrasound-guided aspiration to measure melatonin concentrations and gene expression patterns across small (<20 mm), medium (20–35 mm) and large (>35 mm) follicles under natural photoperiods. Spring and summer months showed substantially greater melatonin levels in follicular fluid alongside significantly elevated expression of melatonin-synthesising enzymes (AANAT and ASMT) and melatonin receptor genes (MT1 and MT2), with FSH and LH receptor expression also increasing across follicle sizes—patterns that contrasted sharply with autumn/winter findings, when large follicle numbers dropped to 3.0 versus 8.2 during the breeding season. These findings suggest melatonin functions not merely as a systemic seasonal signal but as a direct, local regulator within the follicle microenvironment, actively upregulating its own receptors and synthetic machinery during reproductively favourable periods. For practitioners managing mare reproduction, particularly in Southern Hemisphere operations or those utilising natural breeding seasons, this work reinforces the importance of photoperiod management and highlights melatonin's multifaceted role in optimising follicular development beyond traditional hormonal interventions.
Read the full abstract on PubMed
Practical Takeaways
- •Melatonin plays a direct role in regulating mare ovarian function through receptor expression and enzyme activity, with seasonal photoperiod being a critical environmental driver
- •Understanding seasonal melatonin fluctuations may inform breeding management timing and expectations for mare fertility across different seasons in the Southern Hemisphere
- •Local melatonin signaling in follicular tissue represents a potential mechanism for photoperiod-dependent reproductive seasonality in horses
Key Findings
- •Large follicles were 2.7 times more prevalent during spring/summer (8.2 ± 1.9) compared to autumn/winter (3.0 ± 0.5) in mares under natural photoperiods
- •Fshr and Aanat mRNA expression increased in small, medium and large follicles during spring/summer versus autumn/winter
- •Mt1 and Mt2 melatonin receptor mRNA expression increased in small and large follicles during the reproductive season
- •Melatonin concentration in follicular fluid was significantly higher during spring/summer seasons