The influence of season and age on circulating melatonin and leptin concentrations in Lipizzan fillies

Daily serum melatonin and leptin concentrations in 48 Lipizzan fillies, aged 1 to 3 years, have been measured during one year Mean melatonin concentrations ranged from 7.08 +/- 0.43 pg/mL in 3-year olds to 9.02 +/- 0.73 pg/mL in yearlings (P > 0.05). In all age groups, circulating melatonin concentrations peaked in spring and early autumn and decreased in summer and early winter (P < 0.001). The decrease of circulating melatonin levels could serve as a signal for resumption of ovarian activity in spring and for onset of anoestrus in autumn, but further investigations are needed to confirm this hypothesis. Mean annual leptin concentrations ranged from 2.55 +/- 0.15 ng/mL in yearlings to 3.94 +/- 0.32 ng/mL in 3-year olds (P < 0.01); age-dependent differences were also observed during monthly samplings. In all age groups, the circulating leptin values were lowest in early spring and peaked in autumn (P < 0.001). The rise of leptin levels in spring coincides with the period of resumption of ovarian activity and the decrease in winter with the anoestrous period of Lipizzan fillies, thus supporting the findings about the role of leptin in the regulation of the seasonal reproductive activity in mares.