Effects of extremely low frequency magnetic field on the development of tolerance to the analgesic effect of morphine in rats.

This study examined the effects of low frequency magnetic field (MF) on tolerance to analgesic effect of morphine in rats. Rats were made tolerant to morphine by injecting morphine (10 mg/kg, s) once daily for 8 consecutive days. Rats were simultaneously exposed to an MF (50 Hz at 1, 50, and 100 μT for 30 min) before, immediately, or 30 min after injection of morphine, and also exposed to a 0.5, 6, 12, and 30 Hz at 100 μT for 30 min before injection of morphine. The percentage of maximum possible effect of morphine (% MPE) was measured on the 1st, 4th, and 8th days by hot plate test. We observed that MF radiation (50 Hz at 1 µT and 30 Hz at 100 µT) immediately before and MF radiation (50 Hz at 100 µT) after morphine injection prevented the development of morphine tolerance compared to control. Also, we found that exposure to MF (50 Hz at 1, 50, and 100 µT) 30 min after injection of morphine failed to maintain the analgesic effect of morphine. Our results showed that exposure to low frequency electromagnetic field (30 and 50 Hz) immediately before or after the injection of morphine may be a potential method for treating the development of morphine tolerance in rats. Bioelectromagnetics. 38:618-625, 2017. © 2017 Wiley Periodicals, Inc.