APLYSIA

[Since the mid-1960s, the marine mollusk Aplysia has proved to be an extremely useful model system for gaining insights into the neural and molecular mechanisms of simple forms of memory. Indeed, the pioneering discoveries of Eric Kandel using this animal were recognized by his receipt of the Nobel Prize in physiology or medicine in 2000. A number of characteristics make Aplysia well suited to the examination of the molecular, cellular, morphological, and network mechanisms underlying neuronal modifications (plasticity) and learning and memory. The animal has a relatively simple nervous system with large, individually identifiable neurons that are accessible for detailed anatomical, biophysical, biochemical, and molecular studies. Neurons and neural circuits that mediate many behaviors in Aplysia have been identified. In several cases, these behaviors have been shown to be modified by learning. Moreover, specific loci within neural circuits where modifications occur during learning have been identified, and aspects of the cellular mechanisms underlying these modifications have been analyzed. The three entries that follow review several aspects of research on Aplysia.C lassicalC onditioning andO perantC onditioningdescribes several of the behaviors of Aplysia that exhibit these associative forms of learning and the progress that has been made in examining the underlying mechanisms.M olecularB asis ofL ong-T ermS ensitizationdescribes the elucidation of second-messenger cascades and the roles of specific genes and proteins in the induction and maintenance of this example of nonassociative learning.D evelopment ofP rocessesU nderlyingL earningdescribes the ways in which it has been possible to identify and dissociate multiple components of nonassociative learning on both behavioral and cellular levels. For entries on other invertebrates that have proved useful for examining mechanisms of learning and memory, see INSECT LEARNING, INVERTEBRATE LEARNING, and MORPHOLOGICAL BASIS OF LEARNING AND MEMORY: INVERTEBRATES.]