43,44 Yellon45,46 and Wilson et al,47 documenting the effects of magnetic fields, were the first to report a reduction of both in pineal and plasma melatonin in Djungarian hamsters with a short exposure to a sinusoidal 100-μT magnetic

field. In addition, Wilson et al47 also reported an increase in the concentration of norepinephrine in the suprachiasmatic nuclei, the central rhythm-generating system. The majority of laboratory studies were then carried Inhibitors,research,lifescience,medical out on rats. Kato et al,48 in exposing male Wistar-King rats for 6 weeks to a 50-Hz circularly polarized sinusoidal magnetic field using increasing intensities, showed a decrease in pineal and plasma melatonin concentrations without any dose-response relationship. With the same protocol of exposure and species, but with a horizontal or vertical magnetic field,

the same authors failed to find any effect on melatonin levels:49 Suspecting a possible interference of pigmentation, Kato et al50,51 then documented in Long-Evans rats the same intensities Inhibitors,research,lifescience,medical of a circularly polarized magnetic field and did indeed show a Inhibitors,research,lifescience,medical reduction of pineal and plasma melatonin concentrations. Other studies on rats or mice,52-55 baboons,56 and hamsters57,58 also showed a reduction in the nighttime peak of melatonin. The same team reported a phase delay in the nocturnal peak time of melatonin in hamsters,46,57,58 Inhibitors,research,lifescience,medical though they acknowledged in one paper that they were unable to replicate these findings, which make them inconclusive.58 Some authors have reported an increase in nighttime melatonin levels.59-61 With the aim of comparing short-term and long-term exposure effects, Selmaoui and Touitou62 used male Wistar Inhibitors,research,lifescience,medical rats LY2157299 concentration housed in a 12:12 light:dark schedule and submitted to a 50-Hz sinusoidal magnetic field of 1, 10, or 100 μT intensity, either once for 12 h or repeatedly 18 h per day for 30 days. While a single 12-h exposure to a 1- or 10-μT magnetic field had no effect on plasma melatonin levels or NAT and hydroxyindole-O-methyltransferase (HIOMT)

pineal activities, a 100-μT exposure significantly decreased 30% plasma concentrations of melatonin and depressed 23% pineal NAT activity (HIOMT activity unchanged) when compared with sham-exposed rats. In turn, the 30 days’ repeated exposure showed that while the 1-μT intensity showed no effects on pineal function, both the L-NAME HCl 10- and 100-μT intensities resulted in an approximately 42% decrease of plasma melatonin levels. NAT activity was also decreased, and HIOMT activity remained unchanged. This study showed that a sinusoidal magnetic field alters plasma melatonin levels and pineal NAT activity, and that the sensitivity threshold varies with the duration of exposure, thus suggesting that magnetic fields may have a cumulative effect upon pineal function.