Analgesic effects of a specific pulsed magnetic field in the land snail, Cepaea nemoralis:: Consequences of repeated exposures, relations to tolerance and cross-tolerance with DPDPE

It has been demonstrated previously that a short acute exposure to a specific extremely low frequency pulsed magnetic field (Cnp) can induce significant partly opioid-mediated analgesia in the land snail, Cepaea nemoralis. Here, this Cnp-induced analgesia is examined for the development of tolerance to daily repeated acute exposures of 15 or 30 min duration. Acute cross-tolerance to the delta opioid receptor directed agonist DPDPE, [D-Pen(2), D-Pen(5)]enkephalin, was also found. Before (pre-exposure) and after (0, 15, 30 and 60 min) exposure to either a sham or Cnp magnetic field, snails were tested for an aversive reaction to a warmed surface (40 degrees C), and the latency time to the aversive reaction was recorded. Snails that were exposed to the Cnp showed a significant increase in the latency time (F-1,F-55 = 2856.4; p < 0.001; Eta(2) = 0.95), which may be interpreted as an induction of analgesia. During the daily (9 day) repeated acute exposures, the induction of analgesic response was significantly reduced, but not ablated. Altering the environmental conditions of the Cnp exposure restored a significant proportion of the partly developed tolerance, consistent with previous reports of environmental specificity in the development of opioid tolerance. These findings suggest that the partial development of tolerance to the opioid-mediated portion of Cnp-induced analgesia may be countered by altering the specific environmental Cnp exposure conditions. (C) 1998 Elsevier Science Inc.