Pineal sensitivity to pulsed static magnetic fields changes during the photoperiod.

The effect of pulsed static magnetic fields on the rat pineal melatonin synthesis was studied at different times of the photoperiod. Exposure to magnetic fields during mid- or late dark phase significantly suppressed pineal N-acetyltransferase activity, the rate-limiting enzyme in melatonin synthesis, as well as the melatonin content in the pineal gland. These parameters were not influenced by magnetic fields when the exposure occurred early in the dark phase or during the day. These results suggest that the responsiveness of the pineal gland to magnetic field perturbations changes throughout the photoperiod.