Levorphanol but not dextrorphan suppresses the foot-lifting response to an aversive thermal stimulus in the terrestrial snail, Cepaea nemoralis.

The terrestrial snail, Cepaea nemoralis, when placed on a surface heated at 40 degrees C lifts the anterior portion of its foot from the source of heat. This stereotyped response, which suggests aversion, can be inhibited by injections of small doses of morphine and levorphanol, but is not affected by similar or larger doses of dextrorphan. It is suggested that the "analgesic" effect of the opiates in these animals involves interaction with stereochemically specific opiate receptors that may be fundamentally similar to those occurring in mammals.