Modulatory actions of light on the behavioural responses to magnetic fields by land snails probably occur at the magnetic field detection stage

Abstract

The attenuation of opioid peptide–mediated antinociception or analgaesia is a well–established effect of extremely low frequency (ELF) magnetic fields. Results of prior studies indicated a modulatory role for light such that when the ELF exposures were carried out in the absence of light, the inhibitory effect on analgaesia was reduced. Here, we investigated whether this modulatory effect of light occurs at either the magnetic field detection stage or is associated with a post–detection mechanism. We compared the effects of the presence and absence of light on the attenuation of opioid–induced analgaesia in the land snail,Cepaea nemoralis, by (i) an ELF magnetic field (15 min, 60 Hz, 141 –T peak), and (ii) the prototypic opiate antagonist, naloxone. Determinations were performed during the subjective ‘day’ and ‘night’ in the presence (1.9 W m⁻² and 1.0 mW m⁻², respectively) and absence of light (less than 10⁻⁶W m⁻²). The inhibitory effects of the ELF magnetic fields and naloxone on opioid–induced analgaesia were similar in the presence of light; whereas in the absence of light the inhibitory effects of the ELF magnetic fields as a percentage of sham were markedly reduced, while those of naloxone were unaffected. This indicates that the modulatory effects of light on the actions of the ELF magnetic fields probably affect the detection mechanism prior to its coupling to the opioid system.