SUMMARY Foraging animals have distinct exploration and exploitation behaviors that are organized into discrete behavioral states. Here we characterize a neuromodulatory circuit that generates long-lasting roaming and dwelling states in Caenorhabditis elegans. We find that two opposing neuromodulators, serotonin and the neuropeptide pigment dispersing factor (PDF), each initiate and extend one behavioral state. Serotonin promotes dwelling states through the MOD-1 serotonin-gated chloride channel. The spontaneous activity of serotonergic neurons correlates with dwelling behavior, and optogenetic modulation of the critical MOD-1-expressing targets induces prolonged dwelling states. PDF promotes roaming states through a Gαs-coupled PDF receptor; optogenetic activation of cAMP production in PDF receptor-expressing cells induces prolonged roaming states. The neurons that produce and respond to each neuromodulator form a distributed circuit orthogonal to the classical wiring diagram, with several essential neurons that express each molecule. The slow temporal dynamics of this neuromodulatory circuit supplement fast motor circuits to organize long-lasting behavioral states.

Neuronal responses to sensory inputs can vary based on genotype, development, experience, or stochastic factors. Existing neuronal recording techniques examine a single animal at a time, limiting understanding of the variability and range of potential responses. To scale up neuronal recordings, we here describe a system for simultaneous wide-field imaging of neuronal calcium activity from at least 20 Caenorhabditis elegans animals under precise microfluidic chemical stimulation. This increased experimental throughput was used to perform a systematic characterization of chemosensory neuron responses to multiple odors, odor concentrations, and temporal patterns, as well as responses to pharmacological manipulation. The system allowed recordings from sensory neurons and interneurons in freely moving animals, whose neuronal responses could be correlated with behavior. Wide-field imaging provides a tool for comprehensive circuit analysis with elevated throughput in C. elegans.M odern neuronal recording techniques are labor-and equipment-intensive, and generally designed to obtain maximal information from individual animals. However, individuals differ from one another. To understand the full range and variability of neuronal responses, it is desirable to apply highthroughput methods and systematic data collection to many animals under controlled stimulation conditions.The nematode Caenorhabditis elegans is particularly amenable to high-throughput studies of neural and behavioral activity, which are facilitated by its small size, compact nervous system, ease of genetic modification, well-defined behavioral repertoire, and transparent body with optical access to single defined neurons. Optical neural recordings in C. elegans have primarily used high-magnification imaging of neurons expressing a genetically encoded calcium indicator such as GCaMP or cameleon. Glued preparations (1) or partial-body traps (2-4) enable imaging of calcium dynamics in head neurons, but immobilization limits the animal's behavioral repertoire. More complex behaviors can be visualized in freely moving animals on agar surfaces by using a moving stage or objective and computer-controlled feedback to track a moving animal and keep a specific neuron in view (5-8). These methods monitor a single animal at a time, as do optical and electrophysiological recording methods in flies, fish, and rodents, which rely upon complex surgeries, implanted sensors, and dedicated equipment for each animal (9-13).Here we describe a strategy for recording neuronal activity evoked by precise chemical stimulation of freely moving or anesthetized C. elegans by using wide-field microscopy. We adapted previous microfluidic arenas optimized for normal C. elegans crawling behavior and repeatable spatiotemporal stimulation (14) to simultaneous optical recording of calcium transients in individual neurons. The automated microscope is capable of continuous recording from more than 20 animals at once for hours during repeated stimulation without user interaction. We demonstrat...

Animals have a remarkable ability to track dynamic sensory information. For example, the nematode Caenorhabditis elegans can locate a diacetyl odor source across a 100,000-fold concentration range. Here, we relate neuronal properties, circuit implementation, and behavioral strategies underlying this robust navigation. Diacetyl responses in AWA olfactory neurons are concentration- and history-dependent; AWA integrates over time at low odor concentrations, but as concentrations rise it desensitizes rapidly through a process requiring cilia transport. After desensitization, AWA retains sensitivity to small odor increases. The downstream AIA interneuron amplifies weak odor inputs and desensitizes further, resulting in a stereotyped response to odor increases over three orders of magnitude. The AWA-AIA circuit drives asymmetric behavioral responses to odor increases that facilitate gradient climbing. The adaptation-based circuit motif embodied by AWA and AIA shares computational properties with bacterial chemotaxis and the vertebrate retina, each providing a solution for maintaining sensitivity across a dynamic range.