INCREASED AGE AFFECTS PROPERTIES CHARACTERIZING BEHAVIORAL PLASTICITYIN FREELY BEHAVING APLYSIA

In the marine molluse Aplysia, in vitro studies showed that the gill w ithdrawal reflex (GWR) and its neuronal substrates were altered by age . In contrast, age minimally affected the gill respiratory pumping mov ements (GPM) and its neuronal substrates. Based on the respective prop erties of the GWR- and GPM-pathways in vitro, we proposed that the mor e pronounced the effect of age, the greater the expression of plastici ty in a pathway. This conclusion may hold for in vitro preparations, b ut it remained to be demonstrated in intact animals. Based on this con clusion, the GWR should exhibit greater plasticity than the GPM in int act animals. Using freely behaving Aplysia, we tested for plasticity o f the GWR and the GPM in three age groups (young, mature, and old). Th e tests for behavioral plasticity were: Graded responses to varying st imulus strength, response decrement (or habituation) to repetitive sti mulation, enhanced response to dishabituating stimuli, and the effect of the GWR stimulus on the GPM and the GPM stimulus on the GWR. The GW R in mature animals exhibited all four properties, but in old animals, graded responses and habituation were significantly altered and in yo ung animals habituation and dishabituation were absent. The GPM exhibi ted fewer of the properties than the GWR, only graded responses and re sponse decrement, both of which were generally the same in the three g roups. We found that behavioral plasticity and age-induced plasticity are related in freely behaving animals and are consistent with in vitr o findings. The effect of age on properties characterizing plasticity at both the behavioral and pathway levels is discussed.