MODULATORY ACTIONS OF LIGHT ON THE BEHAVIORAL-RESPONSES TO MAGNETIC-FIELDS BY LAND SNAILS PROBABLY OCCUR AT THE MAGNETIC-FIELD DETECTION STAGE

The attenuation of opioid peptide-mediated antinociception or analgaes ia is a well-established effect of extremely low frequency (ELF) magne tic fields. Results of prior studies indicated a modulatory role for l ight such that when the ELF exposures were carried out in the absence of light, the inhibitory effect on analgaesia was reduced. Here, we in vestigated whether this modulatory effect of light occurs at either th e magnetic field detection stage or is associated with a post-detectio n mechanism. We compared the effects of the presence and absence of li ght on the attenuation of opioid-induced analgaesia in the land snail, Cepaea nemoralis, by (i) an ELF magnetic field (15 min, 60 Hz, 141 mu T peak), and (ii) the prototypic opiate antagonist, naloxone. Determi nations 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 l ight (less than 10(-6) W m(-2)). The inhibitory effects of the ELF mag netic 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 marke dly reduced, while those of naloxone were unaffected. This indicates t hat 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.