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.

No abstract

Orientation and nociception (pain sensitivity) are affected by exposure to geomagnetic or low frequency (<1,000 Hz) magnetic fields of approximately the earth's field strength, i.e., 50 microT. However, these effects are often dependent on the simultaneous presence of visible light. Recently, it was shown that nociception was affected in mice acutely exposed to an electromagnetic-shielded environment in the dark (<0.05 W/m(2)) during the mid-light phase of the diurnal cycle. Here, we report for the first time that if mice are exposed to magnetic shielding in the presence of visible light (0.6 W/m(2), 400-750 nm) that most of the effects of shielding are eliminated. This simple experimental protocol may be useful in investigating the role that light plays in the detection of ambient electromagnetic fields.