Novelty-induced opioid analgesia in the terrestrial snail, Cepaea nemoralis

Exposure for 1-15 min to the surface of either a non-functional (22 degrees C) hot-plate or a polypropylene tube increased the thermal (38.5 degrees C) nociceptive thresholds of the terrestrial snail, Cepaea nemoralis. This "analgesic response," which was evident 10-15 sec after exposure to the new stimuli and lasted for 15-30 min, was blocked by the opiate antagonist naloxone. The analgesic response was not observed in snails previously made familiar with the test situation. Snails exposed to a functional (36.5 degrees C) stressful hot-plate surface to which they were aversive also displayed a naloxone-reversible analgesia. However, the level and duration of analgesia was markedly greater than that observed following exposure to the novel non-physically stressful stimuli and was not inhibited by prior familiarization with the stimuli. These observations demonstrate that novelty per se can activate endogenous opioid system(s) and induce an analgesic response in a snail. They also suggest a phylogenetic continuity in responses to novel stimuli and environmental conditions.