60-Hz magnetic field exposure effects on the melatonin rhythm and photoperiod control of reproduction.

Adult Djungarian hamsters in long (16 h of light) or short (10 h of light) days for 6 wk were acutely exposed to a 1-G 60-Hz magnetic field (MF) for 15 min 2 h before dark. The nighttime rise in melatonin was delayed and duration reduced in MF-exposed hamsters in both photoperiods compared with sham controls. In a second replicate experiment, MF effects on melatonin rhythm duration were repeated in hamsters in short but not long days, and amplitudes at some clock times differed between the same treatment groups in the two studies. To test the hypothesis that daily MF abbreviates melatonin rhythm duration and induces a long-day reproductive response, adults in shot days were exposed daily to MF. After 3 wk, pineal and serum melatonin rhythms were the same in MF and sham groups; reproduction remained suppressed. Irrespective of acute MF exposure effects on the melatonin rhythm, daily MF treatment does not alter photoperiodic time measurement or the clock mechanism controlling reproduction. Adaption to environmental MF exposures may be part of a normal physiological mechanism that maintains photoperiodic responsiveness in individuals and a seasonal pattern of reproduction.