The properties of neurons can be altered by modulatory transmitters, a process which contributes to the plasticity of neural circuits. Our long-te m goal is to delineate how the intracellular signal transduction pathway utilizing cAMP is involved in neuronal modulation. The information acquired will contribute especially to our understanding of learning and memory, and also to the action of heart muscle, the origin of biological rhythms, and t e mechanism of peptide hormone secretion. Our findings will aid in the design of pharmacological agents to intervene in these processes. Specifically, the techniques of molecular biology will be used to investiga e the roles played by cAMP-dependent protein kinases (cAPKs) when modulatory neurotransmitters alter the properties of mechanosensory neurons involved i simple reflex behaviors. The mollusk Aplysia californica will be used in these studies because it will be possible to relate our findings to ongoing behavioral and electrophysiological investigations, which are already well advanced. First, the extent of diversity of Aplysia cAPK will be investigat d by examining cDNAs and genomic DNA fragments encoding the regulatory and catalytic subunits of the enzyme. Next, the forms and properties of the enzyme involved in sensory cell modulation, in particular facilitation of sensorimotor synapses, will be determined by microinjecting each polypeptid (obtained by expression) into sensory neurons and monitoring the electrophysiological properties of these cells and associated motor neurons An examination of the cellular and subcellular locations and concentrations of the corresponding mRNAs and polypeptides will also be carried out, followed by experiments to determine whether the forms, levels or locations are altered during synaptic modulation, and if so by what mechanisms? Finally, the molecular properties of the physiologically important forms of Aplysia cAPK will be examined, including the identification of substrates a d the effects phosphorylation has on them. Using this information, existing models for short- and long-term modulation will be critically evaluated and, if necessary, new models will be devised and tested.