A neural analysis of avoidance conditioning with the feeding attractant glycine in Pleurobranchaea japonica

Glycine (Gly) is one of the amino acids that most strongly provoke feeding behavior in the carnivorous opisthobranch sea slug Pleurobranchaea japonica. Placing of an aliquot of a Gly solution in front of the anterior end of this animal induced feeding responses such as orientation to the origin of the stimulus and extrusion of the proboscis. In contrast, light stimulation of the body of the animal with a glass-fiber light guide evoked aversive responses involving the gill withdrawal response. Animals were trained by pairing a Gly solution stimulus (conditioned) and a light stimulus (unconditioned). After training with repetitive paired stimuli, we found that animals exhibited aversive responses to the Gly solution. We confirmed achievement of a conditioned Gly-aversive reflex in intact animals by recording of motor impulses with an electrode implanted on the branchial nerve responsible for the gill withdrawal response, the most reliable index of the reflex. Motor discharge of the branchial nerve associated with the conditioned Gly-aversive reflex persisted even in a preparation isolated from an animal which had previously acquired the reflex. This study provides an experimental model for neural analysis with in vivo long-term nerve recording using an electrode implanted in a nerve of an intact animal for neuroethological training.