[Role of serotonin in the development of the defensive reflex to food in the snail].

9-16 days after injection of 5,7-dihydroxytryptamine which elicits degeneration of serotonergic nerve cell terminals, pairing of food and electrical shock had no effect on responses of injected animals to food, while definite food-aversive reactions were observed in control animals. In neurophysiological experiments applications of serotonin in the chamber containing the nervous system was used as a reinforcing stimulus. The amplitude of synaptic responses to nerve stimulation increased significantly in preparations in which stimulation was paired with serotonin application. In neurons involved in defensive reactions after 3-7 pairings of a drop of juice to the chemoreceptive part of the skin with serotonin application a new spike response to food has appeared. Unpaired presentations of the same stimuli were not effective. It is concluded that serotonin is an important factor in formation of conditioned adverse reactions in the snail.