Exposure of female rats to a 100-mu T 50 Hz magnetic field does not induceconsistent changes in nocturnal levels of melatonin

The hypothesis whereby alternating (50 or 60 Hz) magnetic fields such as th ose produced by electric power reduce the nocturnal production of melatonin in the pineal gland and thereby indirectly enhance development and growth of breast cancer has attracted a great deal of interest. In view of the pot ential importance of this hypothesis that there is a link between electric power and breast cancer, which is also known as the "melatonin hypothesis", we undertook various experiments in female Sprague-Dawley rats to evaluate whether 100-mu T 50 Hz magnetic-field exposure, i.e. a flux density shown recently to exert a tumor (co)promoting effect in the 7,12-dimethylbenz[a]a nthracene (DMBA) model of breast cancer in Sprague-Dawley rats, consistentl y reduces melatonin levels and, if not, which factors may be involved in th e inconsistent effects of magnetic-field exposure on production of melatoni n. Long-term exposure of female Sprague-Dawley rats to magnetic fields for 13 weeks did not alter the nocturnal levels of melatonin in the pineal glan d or serum (determined 5 h after the onset of darkness) significantly, irre spective of whether rats were treated with DMBA or not. In one experiment, when blood was sampled 3, 5 and 6 h after the onset of darkness after 2 wee ks of magnetic-field or sham exposure, a significant decrease in melatonin was seen in magnetic-field-exposed rats at 6 h. However, the results could not be reproduced in two subsequent experiments in other groups of rats. Sh orter (1 day, 1 week) or longer (4, 8, 13 weeks) exposure periods also did not result in any significant effects of the magnetic field on melatonin le vels when blood sampling was performed either 5 or 6 h after onset of the d ark phase. Various potential sources of variation in melatonin levels or in magnetic-field effects on melatonin levels were evaluated, but the reason( s) for the inconsistent effect of magnetic-field exposure remains unclear. Thus the present study failed to demonstrate a consistent effect of 100-mu T 50 Hz magnetic-field exposure on melatonin levels in Sprague-Dawley rats. (C) 1998 by Radiation Research Society.