THE GPCR ACTIVATION REPORTER PEPSEE UNCOVERS PLASTICITY IN NEUROPEPTIDE SIGNALING IN THE NEMATODE <EM>C. ELEGANS</EM>

Neuropeptides are important modulators of neural function and organismal physiology that mainly exert their effect by binding to G protein-coupled receptors (GPCRs). Yet, when and where neuropeptides signal and how neuropeptide transmission is shaped by context remain poorly understood. To understand the functional organization of peptidergic circuits in C. elegans, we developed a light-gated transcriptional reporter, “PepSee”, that couples ligand-induced GPCR activation to the production of a stable, fluorescent signal by release of a GPCR-tethered transcription factor. We use PepSee to visualize the activation of the NMUR-1 receptor of the neuropeptide Neuromedin U (NMU), which is an evolutionarily conserved pleiotropic regulator of food-related behavior and physiology. We uncovered that basal fed or starved conditions elicit reporter activity in two cell types that are intimately associated with the animal’s excretory system. In contrast, salt aversive learning, in which animals are exposed to a chemoattractive salt under starvation conditions, elicits NMUR-1-PepSee activity in multiple neurons linked to navigation in addition to the excretory system. We show that this context-specific signaling pattern stems, at least in part, from increased axonal neuropeptide release from the ASG sensory neurons that express the NMU homolog CAPA-1 using a sensor for peptidergic release and by optogenetically-inducing neuropeptide release from ASG. Since parallel work in other brains is revealing neuropeptide signaling to be similarly organized to that in the C. elegans brain, the concepts and tools presented here can act as a scaffold to investigate how flexible behaviors and physiological responses emerge from neuromodulatory networks.