The goal of this project is to understand the physiological and biochemical processes that contribute to learning. Habituation, dishabituation and sensitization are widespread forms of learning that are caused by changes in transmission in the neural pathways that mediate behavior. Using the relatively simple nervous system of the marine mollusk Aplysia, several types of short and long-term synaptic plasticity have bene found to contribute to these forms of learning. The aim of this proposal is to extend the understanding of these cellular processes at individual synapses and to examine the contribution of some of the elements of the neural pathways to the alterations in overall output. Individual synapses will be studied in intact ganglia and in a reconstituted culture system of pairs of synaptically connected neurons that display virtually all of the forms of synaptic plasticity seen in situ. The contributions of pathway elements will be studied in intact ganglia. The specific aims of the project include: 1) Roles of second messengers in plasticity. This part of the project will examine the role of second-messenger systems in heterosynaptic facilitation, the increase in transmitter release that accompanies sensitization and dishabituation. a) Intracellular recording, voltage clamp and optical measurements of intracellular calcium concentration will be used in order to delineate the contributions of protein kinases A and C and CaM kinase to facilitation. b) Other experiments will test the hypothesis that some second messengers modulate release by acting at steps in the release process that are dependent on calcium influx while others act further downstream. 2) Changes in the pool of active synapses in plasticity. The hypothesis will be tested that a portion of the changes in transmission with plasticity is caused by the progressive silencing of individual synapses during synaptic depression and the recruitment of silent synapses in synaptic facilitation. These experiments will involve statistical analysis of synaptic transmission and direct observations of silent synapses which are recruited by facilitating stimuli. 3) Changes in action potential firing at different sites in the pathway as expressions of plasticity. The question to be examined here is how the changes in synaptic transmission are translated into changes in firing behavior.