Extremely low frequency magnetic field exposure modulates the diurnal rhythm of the pain threshold in mice

The aim of this study was to determine whether exposure to extremely low frequency magnetic field (ELF‐MF) affects the normal diurnal rhythm of the pain threshold in mice. Pain thresholds were evaluated in mice using the hot plate test. A significant increase of pain threshold during night was observed compared to that during day. This rhythm was attenuated by both constant exposure to light (LL) and constant exposure to darkness (DD) for 5 days. Under DD exposure, the diurnal rhythm in pain threshold was restored when mice were exposed to ELF‐MF (60 Hz, 1.5 mT for 12 h daily, from 08:00 to 20:00 h) for 5 days. The diurnal rhythm was not reversed under dark with reversed ELF‐MF cycle (exposure to 1.5 mT from 20:00 to 08:00 h, next day) for 5 days, although pain threshold in the ELF‐MF exposed period of night was slightly decreased. The diurnal rhythm of melatonin analgesic effect related to pain threshold was also observed under DD by the exposure of ELF‐MF for 5 days, but not for 5 nights. The present results suggest that ELF‐MF may participate in the diurnal rhythm of pain threshold by acting on the system that is associated with environmental light‐dark cycle. Bioelectromagnetics 24:206–210, 2003. © 2003 Wiley‐Liss, Inc.