The long-term objective of this project is to elucidate the neuronal mechanisms of learning and memory in the marine mollusk Aplysia californica. The defensive withdrawal reflex of this animal constitutes an important model system for this purpose. The reflex exhibits several forms of learning, including classical conditioning. Moreover, the neuronal circuitry which underlies the withdrawal reflex, particularly its monosynaptic component--the synapse between sensory and motor neurons--is relatively simple and well-understood. Neuronal changes at the sensorimotor synapse have been shown to parallel, and may have mechanistic roles in, behavioral modification of the withdrawal reflex. Therefore, this synapse is a useful starting point for a neurobiological analysis of learning in Aplysia. An additional advantage of central sensorimotor synapses of Aplysia is that they can be reconstituted in cell culture. These in vitro synapses greatly facilitate experimental analysis of learning-related cellular modifications. The proposed experiments will focus on a form of synaptic plasticity known as Hebbian long-term potentiation (LTP), a form of neuronal change prominently implicated in vertebrate memory and cognition. Hebbian LTP has recently been described for Aplysia sensorimotor synapses in cell culture. The specific aims of this proposal are to analyze the cellular mechanisms of this novel form of invertebrate LTP, and to determine its potential role in learning in Aplysia. The proposed experiments will utilize synapses in cell culture, as well as preparations comprising the central nervous system of Aplysia. Some of the questions which will be experimentally addressed are: (i) Does transmission at Aplysia sensorimotor synapses involve N-methyl-D-aspartate (NMDA)-related receptors? Activation of NMDA receptors is known to mediate the induction of Hebbian LTP of vertebrate synapses. (ii) What are the long-term cellular changes that mediate the expression of LTP of Aplysia sensorimotor synapses? (iii) Does Hebbian potentiation of Aplysia sensorimotor synapses play a role in classical conditioning of the withdrawal reflex of Aplysia? These questions will be addressed utilizing both electrophysiological and imaging techniques, among which are: quantal analysis; video fluorescence microscopic detection of structural changes and changes in intracellular calcium, in living Aplysia neurons; and experiments involving a cellular analogue of classical conditioning of the withdrawal reflex. The proposed research will serve as the basis for improving our understanding of human learning and memory. It may thereby contribute to ameliorating human memory-associated diseases, such as Alzheimer's.