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

The study investigates the impact of melatonin, a hormone that regulates sleep-wake cycle, on the first ovulation after ovarian inactivity in mares, under different light conditions. Researchers hypothesized melatonin applications at dusk or dawn could mimic night lengthening and thus repress ovarian stimulation induced by longer daylight periods.

Methodology and Experimentation

• The researchers created two experiments to test the studied hypothesis. Melatonin was administered 4 hours before nightfall (evening melatonin) or close to sunrise (morning melatonin) to attempt to simulate a longer nighttime.
• Experiment 1: Five groups were established, including an unstimulated control group, a group exposed to 14.5 hours of light, a group exposed to 14.5 hours of light with evening melatonin, a control group exposed to 17.5 hours of light, and a group exposed to 17.5 hours of light plus morning melatonin. The researchers measured the average intervals from the start of treatment to the first ovulation.
• Experiment 2: Three groups were exposed to 14.5 hours of light — a control group, and two groups receiving melatonin treatment in the morning and evening respectively. Again, the average intervals to first ovulation were determined for comparison.

Findings and Observations

• The research found that the evening melatonin had the effect of suppressing the stimulatory impact of the light on the mares, but the morning melatonin did not have this effect.
• In untreated mares, blood melatonin levels were higher during the night than during the day, a normal pattern for mammals with functional melatonin systems.
• In the melatonin-treated mares, significantly high plasma melatonin concentrations occurred shortly after treatment, and levels started to decline immediately after reaching this peak. This decline was more precipitous after morning treatments compared to evening treatments.
• The research observed that the natural night-time plasma melatonin concentrations before the morning treatment were lower than the night-time levels measured in the treated animals.

Proposed Hypotheses

• Two possible explanations were put forth. The researchers suggested that the melatonin treatment in the morning might be seen as dusk by the mares’ bodies, leading to only about 5 hours of elevated melatonin levels in the blood.
• Alternatively, the researchers have proposed that in the morning, the mare’s body might be sensitive to light regardless of the presence of high levels of melatonin in the bloodstream, and might thus respond naturally to the longer daylight hours by enhancing ovarian activity.