Magnetic fields and stress: day-night differences.

An exposure for 30 min to a 0.5 Hz rotating magnetic field (1.5-90 G) significantly reduced warm water swim stress-induced opioid analgesia in CF-1 male mice. Pre-treatment with naloxone (1.0 mg/kg) had comparable inhibitory effects on warm water swim induced analgesia. The magnetic stimuli also eliminated the day-night rhythm in stress-induced analgesia, with maximum inhibitory effects occurring in the dark period when peak analgesia was present. These results indicate that magnetic stimuli can significantly alter day-night rhythms of stress-induced activation of endogenous opioid systems and their behavioral and physiological consequences. These elevated night time effects may involve actions on the pineal gland, while the day time actions may involve alterations in the distribution and transport of Ca++ and or other divalent ions.