Daily repeated magnetic field shielding induces analgesia in CD-1 mice.

We have recently observed that a single exposure of mice to a magnetically shielded environment can attenuate opioid induced analgesia. Here, we report the effect of repeated exposures to the same magnetically shielded environment. Adult male Swiss CD-1 mice were placed in a Mu-metal lined box or an opaque Plexiglas box (sham condition) for 1 h per day for 10 consecutive days. Nociception was measured as the latency time to a foot lift/lick in response to an aversive thermal stimulus (hotplate analgesiometer, 50 +/- 1 degrees C) before and immediately after exposure. Multiple experiments were conducted in which thermal latency was tested on each of the 10 days or on days 1, 5, and 10, with some utilizing post-exposure testing only. It was shown that mice can detect and will respond to the repeated absence of the ambient magnetic field, with a maximum analgesic response occurring over days 4-6 of exposure and returning to baseline thereafter. The effect was robust, independent of pre-exposure and intermittent testing, and seems to be opioid related, since the results obtained on day 5 were similar to those from a 5 mg/kg dose of morphine and were abolished with the opioid antagonist, naloxone.