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 s pecific extremely low frequency pulsed magnetic field (Cnp) can induce significant partly opioid-mediated analgesia in the land snail, Cepae a nemoralis. Here, this Cnp-induced analgesia is examined for the deve lopment 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 reactio n to a warmed surface (40 degrees C), and the latency time to the aver sive 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 analg esia. During the daily (9 day) repeated acute exposures, the induction of analgesic response was significantly reduced, but not ablated. Alt ering the environmental conditions of the Cnp exposure restored a sign ificant proportion of the partly developed tolerance, consistent with previous reports of environmental specificity in the development of op ioid 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 cond itions. (C) 1998 Elsevier Science Inc.