The long-term goal of the proposed investigations is to elucidate the roles of peptides as transmitters, using as an example the system of bag cell neurons in the marine mollusk, Aplysia. Repetitive impulse activity in the bag cells produces four types of prolonged responses in neurons of the abdominal ganglion. Two types of prolonged excitatory responses are very likely mediated by egg laying hormone, which is thought to act as a nonsynaptic transmitter. Two other peptides are candidate transmitters for mediating inhibition and transient excitation, respectively. All three peptides are encoded onto a single bag-cell-specific gene. Using combined biochemical and electrophysiological techniques we plan to test the hypothesis that the bag cells are a multitransmitter neural system in which these peptides are enzymatically cleaved from a common precursor molecule and then secreted to act on various target neurons. We plan studies to demonstrate the release of two or more of the putative transmitters, to investigate their cellular and subcellular mechanisms of action, and to study the pharmacology of these and other bag cell peptides. We will also attempt to purify and characterize other neuroactive peptides from the CNS of Aplysia. These studies may result in a clearer understanding of fundamental mechanisms of chemical signalling between neurons.