Daily rhythm of blood melatonin concentrations in sheep of different ages

The aim of the study was to investigate the daily pattern of melatonin during short photoperiods in sheep of different age. Thirty Sarda sheep kept under natural light conditions and divided into three equal groups were used: Group 1 (12-18 months); Group 2 (3-4 years); Group 3 (8-9 years). Blood samples were collected every 2 h for 22 h, starting at 21 December (12:00 h) until 22 December (10:00 h) for plasma melatonin radioimmunoassay determination. Statistical significant effect of time of day and age on melatonin concentration was registered. Mesor and amplitude values were statistically lower in Group 3 than in Groups 1 and 2. This study provides the first characterization of the relationship between age and daily pattern of melatonin in sheep, highlighting a decrease in melatonin production in the older animals.