Pharmacological analysis of inhomogeneous static magnetic field‐induced antinociceptive action in the mouse

The effect of inhomogeneous, 2–754 mT static magnetic field (SMF) on visceral pain elicited by intraperitoneal injection of 0.6% acetic acid (writhing test) was studied in the mouse. Exposure of mice to static magnetic field (permanent NdFeB N50 grade 10 mm × 10 mm cylindrical magnets with alternating poles) during the nociceptive stimulus (0–30 min) resulted in inhibition of pain reaction: the number of writhings decreased from 9 ± 2, 32 ± 4 and 30 ± 3 to 2 ± 0.03, 15 ± 1.6, and 14 ± 1.6, respectively, measured in 0–5th, 6–20th, and 21–30th min following the acetic acid challenge. The pain reaction during the total observation period was reduced by 57% ( P  < 0.005). The analgesic action induced by SMF was inhibited by subcutaneous administration of naloxone (1 and 0.2 mg kg −1 ), irreversible µ‐opioid receptor antagonist β‐funaltrexamine (20 mg kg −1 ) and δ‐opioid receptor antagonist naltrindole (0.5 mg kg −1 ), but the κ‐opioid receptor antagonist norbinaltorphimine (20 mg kg −1 ) failed to affect the SMF‐induced antinociception. In contrast to the subcutaneous administration, the intracerebroventricularly injected naloxone (10 µg mouse −1 ) did not antagonize the antinociceptive effect of SMF. The results suggest that acute exposure of mice to static magnetic field results in an opioid‐mediated analgesic action in the writhing test in the mouse. The antinociceptive effect is likely to be mediated by µ and (to a lesser extent) δ‐opioid receptors. Bioelectromagnetics 29:456–462, 2008. © 2008 Wiley‐Liss, Inc.