Influence of physical training on the Syrian hamster's melatonin rhythm.

Melatonin secretion can be influenced by acute exercise. Using female Syrian hamsters we assessed the influence of physical exercise in the serum melatonin rhythm. Melatonin concentrations were determined by RIA. After a gradually increased training program in which experiments were performed at the end of the light phase of the photoperiod (14-h light:10-h dark; lights on at 21:00) animals were killed immediately after exercise (14 h light) and at different time points including 11h after light and 3, 6, 8 and 9h after dark. No differences in melatonin concentrations immediately after exercise practice between control and trained animals were observed. A slower melatonin nighttime increase in exercised hamsters, with significantly lower levels 6h after dark in comparison with controls was found. The peak value appeared 8h after dark but high melatonin levels were prolonged in exercised hamsters, showing significantly higher melatonin levels 9h after. In conclusion, our results indicate for the first time that Syrian hamsters submitted to a training program of increased activity at the end of the light phase, the rest period, showed an altered melatonin rhythm, resembling the response observed in humans.