Extremely low frequency magnetic fields can either increase or decrease analgaesia in the land snail depending on field and light conditions.

Results of prior investigations with opioid peptide mediated antinociception or analgaesia have suggested that these extremely low frequency (ELF) magnetic field effects are described by a resonance mechanism rather than mechanisms based on either induced currents or magnetite. Here we show that ELF magnetic fields (141-414 microT peak) can, in a manner consistent with the predictions of Lednev's parametric resonance model (PRM) for the calcium ion, either (i) reduce, (ii) have no effect on, or (iii) increase endogenous opioid mediated analgaesia in the land snail, Cepaea nemoralis. When the magnetic fields were set to parameters for the predictions of the PRM for the potassium ion, opioid-peptide mediated analgaesia increased and there was evidence of antagonism by the K(+) channel blocker, glibenclamide. Furthermore, these effects were dependent on the presence of light; the effects were absent in the absence of light. These observed increases and decreases in opioid analgaesia are largely consistent with the predictions of Lednev's PRM. Copyright 2000 Wiley-Liss, Inc.