Effect of oral melatonin on the date of the first ovulation after ovarian inactivity in mares under artificial photoperiod.

Two experiments tested the hypothesis that exogenous melatonin (12 mg) given 4 h before dusk (evening melatonin), or near dawn (morning melatonin), would mimic a prolongation of the night and suppress stimulation of the ovaries induced by long days. Experiment 1 consisted of a non-stimulated control group, a control group stimulated by 14.5 h of light, a treated group stimulated with 14.5 h of light plus evening melatonin, a control group given 17.5 h of light and a treatment group given 17.5 h of light plus morning melatonin. The mean (+/- s.e.m.) intervals from the start of treatment to the first ovulation were 135 +/- 26, 68 +/- 4, 147 +/- 11, 94 +/- 6 and 107 +/- 13 days, respectively. Experiment 2 consisted of 3 groups exposed to 14.5 h of light, a control group and a morning and an evening melatonin group. The mean (+/- s.e.m.) intervals from treatment to first ovulation were 53 +/- 12, 62 +/- 8 and 101 +/- 5 days, respectively. Thus, evening melatonin suppressed the stimulatory effect of the applied light, but morning melatonin did not (P greater than 0.05). In non-treated mares, peripheral blood melatonin levels were higher at night than during the day. In the treated mares, very high plasma melatonin concentrations occurred 10-20 min after treatment and levels began to fall again immediately after this peak was reached. This decline was steeper after morning than the evening treatments. Conversely, the endogenous night-time plasma melatonin concentrations before the morning treatment were lower than the night-time levels measured in the treated animals. Two hypotheses are proposed: (a) exogenous treatment with melatonin in the morning is perceived as dusk and is followed by only approximately 5 h of high melatonin concentrations in the blood and (b) in the morning the mare is sensitive to light regardless of the presence of high levels of melatonin in her blood.