Influence of exogenous melatonin on seasonality of reproduction in sheep

Two experiments were performed to examine the effects of exogenous melatonin on seasonality of reproduction in ewes. To simulate the secretory pattern observed on the shortest day of the year, 2.5 mg melatonin were administered i.m. at 1600 hr each day. This dosage has been shown to keep serum concentrations of melatonin elevated until the onset of darkness on the longest day of the year (i.e. ～ 6 hr). In experiment 1, ewes were administered melatonin (n=10) or vehicle (n=10) from June 20 until the onset of the breeding season. Jugular blood samples were collected three times weekly and the serum was stored for analysis of progesterone by radioimmunoassay. The interval from the first day of treatment until serum concentrations of progesterone remained above 1 ng/ml in two consecutive samples (indicative of the formation of a corpus luteum) was 39.3 ± 4.1 days in the melatonin-treated ewes and 61.4 ± 4.6 days in the ewes receiving vehicle (P < 0.05). In the second experiment, ewes were given injections of melatonin (n=6) or vehicle (n=6) from December 21 until September 1. Again, jugular blood samples were collected three times weekly for analysis of progesterone. The interval from initiation of treatment until serum concentrations of progesterone remained below 1 ng/ml for at least 5 consecutive samples was 80.2 ± 3.3 days in the melatonin-treated ewes and 37.8 ± 6.5 days in the control ewes (P < 0.05). However, when treatment was begun on January 1 the interval from the initiation of treatment until serum concentrations of progesterone were greater than 1 ng/ml in two consecutive samples after June 1 was not different between melatonin-treated and control ewes. These data indicate that adminis-tration of melatonin to ewes can hasten the onset of the breeding season in the fall or extend the length of the breeding season in the spring, but after prolonged administration the ewes may become refractory to the treatment.