LONG-TERM SENSITIZATION AND ENVIRONMENTAL CONDITIONING IN TERRESTRIALSNAILS

The hypothesis that a long-term increase of behavioural responses in s nails (over a period of days) might be due to environmental conditioni ng was examined. Training consisted of delivering electric shocks nonc ontingently with test stimuli twice per day for 5 days to freely movin g snails on a ball floating in water. After training, a significant di fference in amplitude of a withdrawal reaction to tactile test stimula tion appeared between shocked and control snails. Responses were signi ficantly facilitated in shocked animals for up to 12 days after traini ng, but only if the animals were tested in the environment used for tr aining. Testing of the same groups of animals crawling freely on the g lass lid of a tank in which they lived between experimental sessions r evealed no difference in responses to the same stimuli between shocked and control snails. Injection of the neurotoxin 5,7-dihydroxytryptami ne, which selectively impairs serotonergic cells, eliminated the diffe rences between shocked and control animals. Changing the pH of the wat er in which the ball floated, by addition of citric acid, led to a sig nificant selective increase of responsiveness in snails sensitized in this environment relative to the responsiveness of the same snails wit h normal water in the tank. The results suggest that the long-term sen sitization of withdrawal reactions observed is at least in part a mani festation of an associative process, namely environmental conditioning .