Project Summary The long-term goal of this project is to delineate at an unprecedented level of precision how all the neurotransmitter signaling events within a model neural circuit together produce its dynamic pattern of activity. Neural circuits are a basic unit of brain function, and altered signaling within neural circuits can disrupt circuit function to produce mood disorders and other human brain diseases. To date, our understanding of such diseases remains vague because we understand only a few individual features within each of the many different neural circuits that have been studied. Therefore, our approach is to analyze all the signaling events within one simple neural circuit, anticipating that this will yield new insights into how neural circuits in general work. This approach is inspired by past successes in other areas of biology in which deep analysis of a simple model in a genetically tractable organism (e.g. the lac operon in E. coli or pattern formation in the Drosophila embryo) led to conceptual breakthroughs that generalized to human biology. Thus, we are intensely studying the simple egg-laying circuit of C. elegans, an experimental system in which we have developed a powerful combination of genetic, optogenetic, chemogenetic, and Ca2+ imaging approaches that together can provide unprecedented mechanistic insights into circuit function. Our analysis has already discovered two instances in which a small-molecule neurotransmitter signals alongside co-released neuropeptides, and also that ongoing circuit activity responds to homeostatic feedback from the postsynaptic cells. These features are likely conserved in other neural circuits. In the next period of this project we aim to understand how the egg-laying circuit turns itself on. This circuit alternates between ~20 minute inactive phases and ~2.5 minute periods of rhythmic activity. Our recent work shows that the two HSN command neurons release a combination of serotonin and neuropeptides encoded by the nlp-3 gene to activate the circuit. These neuromodulators signal through a set of at least five receptors to alter the postsynaptic muscle response to acetylcholine released from presynaptic motor neurons. Aim 1. We will determine how the HSN neurons “decide” when to release serotonin and NLP-3 neuropeptides to activate the circuit. Aim 2. We will determine how a diverse set of receptors for serotonin and NLP-3 neuropeptides alter activity of various specific cells of the circuit to make the circuit active. Aim 3. We will identify the cells and signals that act with HSN-released serotonin and NLP-3 neuropeptides to help activate the circuit and ultimately trigger egg-laying muscle contraction.