Age-related degradation of behavioral and network features of Aplysia escape locomotion

Aplysia californica has been a useful model system for studies of the neural basis of behavior, learning, and aging. While the latter topic has been explored with respect to several of its simple reflex behaviors, this study represents the first examination of how one of Aplysia's more complex behaviors, escape locomotion, is affected in animals nearing the end of their natural lifespan. Old animals (12-13mo) showed a greatly reduced gallop response compared with middle-aged adults (5-7mo), together with a loss of locomotion onset latency sensitization. Large-scale VSD imaging was used to record motor programs in isolated brain preparations from middle-aged vs. elderly animals. Old brains displayed the same loss of onset latency sensitization seen in the intact old animal behavior, and also a reduced number of cycles per locomotion episode. Brains from middle-aged animals showed an unchanged number of motor program cycles from that observed in intact animals, but a much more transient motor program onset latency sensitization. A further age-related finding was that while in middle-aged brains repeatedly eliciting the motor program led to progressively increasing cumulative activity across trials, in old brains this same procedure led to progressively decreasing activity. Some of our results are consistent with peripheral processes working in concert with the CNS as animals age to support healthy locomotion behavior and its modification by learning, or with early changes in the brain that are not yet expressed in behavior.