Behavioural responses to magnetic fields by land snails are dependent on both magnetic field direction and light

A variety of sensing-transduction mechanisms have been proposed to explain the biological effects of magnetic fields. Previous studies have established the well-defined inhibitory effects which magnetic fields have on opioid-peptide-mediated antinociception or 'analgaesia' as an appropriate system for examining these mechanisms. Here, we show that the inhibitory effects of magnetic fields on opioid-mediated 'analgaesia' in the land snail, Cepaea nemoralis, are dependent on: (i) the relative direction of a weak static and 60 Hz magnetic field; and (ii) the presence of light. In the absence of light, the attenuation of opioid-induced analgaesia was markedly reduced in extent and no longer dependent on the relative direction of the magnetic fields. The directional sensitivity in the presence of light is consistent with the direct detection of magnetic fields through amplitude-frequency dependent resonance mechanisms such as described in the 'parametric resonance model'. This mechanism has properties similar to magnetoreception models involving resonant reactions of excited triplet state macromolecules originally proposed for avian orientation and recently evoked in light-dependent magnetic compass orientation. We suggest that the biophysical sensing-transduction mechanisms for the present light-dependent, non-orientational responses to magnetic fields and light-dependent magnetic compass orientation may be closely related. This raises the possibility of a common underlying mechanism(s) for various behavioural effects of magnetic fields.