Neural mechanisms of age-dependent changes in avoidance behaviour of the snail Helix lucorum.

In order to study the emergence of different components of learning and memory, we investigated developmental changes in behavioural, neurophysiological and histochemical experiments in newborn and adult terrestrial snails (Helix lucorum L.). The absence of sensitization and inability to develop avoidance-conditioned reactions were revealed in behavioral experiments in juvenile snails under 1 month old. Investigation of neural mechanisms of this behavioral deficit showed the absence of sensitization in spike reactions to repeated nerve stimulation in command neurons for the avoidance behaviour in juvenile snails. The same dynamics of response amplitude, as in juvenile snails, was seen in adults only after selective impairment of serotonergic neurons by treatment with 5,7-dihydroxytryptamine. During 1 month after hatching, the serotonin content investigated with fluorescence techniques was very low in the nervous system. This result suggests that the absence of sensitization, as well as inability to be aversively conditioned is related to 5-hydroxytryptamine level, which changes during postnatal development in the snail.