ACUTE 60-HZ MAGNETIC-FIELD EXPOSURE EFFECTS ON THE MELATONIN RHYTHM IN THE PINEAL-GLAND AND CIRCULATION OF THE ADULT DJUNGARIAN HAMSTER

Adult male and female hamsters in long days (16 hr of light) were expo sed to a 1 gauss 60 Hz magnetic field for 15 min starting 2 hr before lights off. Sham-exposed controls were placed in an adjacent exposure system but current was not applied. Hamsters were decapitated at 0.5-2 hr intervals from 1 hr before lights off to 1 hr after lights on (n = 4-6/clocktime/group); sera were harvested and pineal glands obtained for melatonin radioimmunoassay. In controls, pineal melatonin signific antly increased from an average daytime baseline of less than 0.3 ng/g land to 3 ng/gland by 3 hr after lights off (P < 0.05, ANOVA). This in crease was sustained for the duration of the night and returned to bas eline within 1 hr after lights on. A similar melatonin rhythm was foun d in serum; concentrations ranged from 30 to 50 pg/ml at night and ret urned to a baseline of 12 pg/ml or less by 1 hr before lights on. The single magnetic field exposure reduced the duration and blunted the ri se in the nocturnal melatonin rhythm. The study was then repeated in i ts entirety 6 months later. The same magnetic field treatment signific antly suppressed pineal melatonin content at 5 hr after lights off and reduced serum melatonin concentrations at 3 and 5 hr after dark onset compared to sham-exposed controls. Thus, the acute magnetic field exp osure was again found to blunt the increase and suppress the duration of the nighttime melatonin rise. Point-by-point comparisons with the f irst study, however, did not replicate the magnetic field-associated r eduction in pineal melatonin content at 3 hr into the night, as well a s in pineal and serum melatonin at 7.5 hr after lights off. Concern ab out this divergence led us to repeat the experiment for a second time 6 months later. In both sham- and magnetic field-exposed groups, melat onin increased within 3 hr after lights off and this rise was sustaine d until 0.5 hr before lights on; nighttime melatonin content in the pi neal gland was approximately 2 ng while in circulation melatonin conce ntrations averaged 60 pg/ml or less. No statistical differences were e vident between the control and magnetic field exposed hamsters at any clocktime (P < 0.05, ANOVA). Thus the absence of an effect of magnetic field exposure on the melatonin rhythm in either the pineal gland or circulation in this second replicate study contrasts with the clear su ppression of the nocturnal melatonin rhythm in two previous experiment s. Further work is needed to define the parameters of magnetic field e xposure that consistently affect the pineal gland and its circadian me latonin rhythm. The time of the year for experimentation, animal age, or the endogenous response to exposure may be variables that might be understood before the physiological importance of magnetic fields for circadian time keeping mechanisms may be realized.