Ionotropic Receptors (IRs) are a large subfamily of variant ionotropic glutamate receptors present across Protostomia. While these receptors are most extensively studied for their roles in chemosensory detection, recent work has implicated two family members, IR21a and IR25a, in thermosensation in Drosophila. Here we characterize one of the most evolutionarily deeply conserved receptors, IR93a, and show that it is co-expressed and functions with IR21a and IR25a to mediate physiological and behavioral responses to cool temperatures. IR93a is also co-expressed with IR25a and a distinct receptor, IR40a, in a discrete population of sensory neurons in the sacculus, a multi-chambered pocket within the antenna. We demonstrate that this combination of receptors is required for neuronal responses to dry air and behavioral discrimination of humidity differences. Our results identify IR93a as a common component of molecularly and cellularly distinct IR pathways important for thermosensation and hygrosensation in insects.DOI: http://dx.doi.org/10.7554/eLife.17879.001

Complex animal behaviors are built from dynamical relationships between sensory inputs, neuronal activity, and motor outputs in patterns with strategic value. Connecting these patterns illuminates how nervous systems compute behavior. Here, we study Drosophila larva navigation up temperature gradients toward preferred temperatures (positive thermotaxis). By tracking the movements of animals responding to fixed spatial temperature gradients or random temperature fluctuations, we calculate the sensitivity and dynamics of the conversion of thermosensory inputs into motor responses. We discover three thermosensory neurons in each dorsal organ ganglion (DOG) that are required for positive thermotaxis. Random optogenetic stimulation of the DOG thermosensory neurons evokes behavioral patterns that mimic the response to temperature variations. In vivo calcium and voltage imaging reveals that the DOG thermosensory neurons exhibit activity patterns with sensitivity and dynamics matched to the behavioral response. Temporal processing of temperature variations carried out by the DOG thermosensory neurons emerges in distinct motor responses during thermotaxis. N avigation toward environmental conditions that improve survival and fitness is of near-universal importance in motile biological organisms. Quantitative analysis of such animal behaviors to defined sensory inputs is a powerful approach to elucidate how behavior is encoded in underlying neurons and circuits. The advantage of studying navigation in small, optically transparent, genetically modifiable animals like Caenorhabditis elegans (1) or Drosophila larvae (2) is the opportunity to dissect sensory, neuronal, and behavioral dynamics in live animals by using optical neurophysiology and optogenetics throughout the nervous system.The Drosophila melanogaster larva navigates gradients of many sensory cues, including light, temperature, odors, and tastes, but with fewer neurons in its sensory periphery and brain than the adult. Moreover, the simpler body plan and crawling movements of the larva facilitate the precise quantification of behavioral dynamics. Poikilotherms like C. elegans or Drosophila use sensitive thermosensory mechanisms to navigate moderate temperature ranges, thereby enabling them to use their environments to regulate their own body temperatures (3, 4). Here, we study sensory and behavioral dynamics during positive thermotaxis (i.e., cool avoidance) by the Drosophila larva. Tracking the movements of Drosophila exploring temperature, olfactory, or gaseous gradients has shown that their navigation is generated by a sequence of two alternating motor programs: runs involving peristaltic forward movement that are interrupted by turns involving probing side-to-side head sweeps until the initiation of a new run (5-8). Larvae negotiating temperature gradients stochastically transition between runs and turns by strategies that cause runs pointed in favorable directions to be more frequent and longer than runs pointed in unfavorable directions. These transitions b...

The avoidance of light by fly larvae is a classic paradigm for sensorimotor behavior. Here, we use behavioral assays and video microscopy to quantify the sensorimotor structure of phototaxis using the Drosophila larva. Larval locomotion is composed of sequences of runs (periods of forward movement) that are interrupted by abrupt turns, during which the larva pauses and sweeps its head back and forth, probing local light information to determine the direction of the successive run. All phototactic responses are mediated by the same set of sensorimotor transformations that require temporal processing of sensory inputs. Through functional imaging and genetic inactivation of specific neurons downstream of the sensory periphery, we have begun to map these sensorimotor circuits into the larval central brain. We find that specific sensorimotor pathways that govern distinct light-evoked responses begin to segregate at the first relay after the photosensory neurons.N avigating organisms must extract spatial information about their surroundings to orient and move toward preferred environments. Phototaxis of fly larvae has long been a paradigm for understanding the mechanisms of animal orientation behavior (1). The study of phototaxis in the Drosophila larva provides an opportunity to investigate the circuits for orientation behavior from sensory input to motor output in a small nervous system. First, however, the sensorimotor structure of responses to illumination must be defined by studying larval behavior in controlled environments.The tropism theory of Jacques Loeb states that bilateral body plans allow animals to extract spatial information through the sensation of external forces acting asymmetrically on symmetric body halves. The navigation of fly larvae away from incident light rays was interpreted as a direct demonstration of tropism. However, temporal comparisons performed by moving animals, also known as klinotaxis, also can encode spatial information (2). Like most fly larvae, Drosophila larvae are negatively phototactic during most of their development (3-9). To navigate away from light, the Drosophila larva uses two sets of photosensors, the Rhodopsin-expressing Bolwig's organs (BO) that mediate phototaxis at low light levels and the non-Rhodopsin-expressing class IV multidendritic (md) neurons that respond to intense light levels comparable to direct sunlight (10). Here, we sought to resolve the sensorimotor structure of larval phototaxis to understand how these photosensitive structures extract and use information about ambient light conditions to control motor behavior.We developed a tracking assay and illumination system that allowed us to quantify the movements of individual animals in defined spatiotemporal illumination patterns at both low and high light intensities. We uncovered a set of sensorimotor relationships that allow the larva to navigate away from light based on temporal processing of sensory inputs. Even the capacity to navigate away from directed illumination is mediated by temporal process...

We present an imaging system for pan-neuronal recording in crawling Caenorhabditis elegans. A spinning disk confocal microscope, modified for automated tracking of the C. elegans head ganglia, simultaneously records the activity and position of ∼80 neurons that coexpress cytoplasmic calcium indicator GCaMP6s and nuclear localized red fluorescent protein at 10 volumes per second. We developed a behavioral analysis algorithm that maps the movements of the head ganglia to the animal's posture and locomotion. Image registration and analysis software automatically assigns an index to each nucleus and calculates the corresponding calcium signal. Neurons with highly stereotyped positions can be associated with unique indexes and subsequently identified using an atlas of the worm nervous system. To test our system, we analyzed the brainwide activity patterns of moving worms subjected to thermosensory inputs. We demonstrate that our setup is able to uncover representations of sensory input and motor output of individual neurons from brainwide dynamics. Our imaging setup and analysis pipeline should facilitate mapping circuits for sensory to motor transformation in transparent behaving animals such as C. elegans and Drosophila larva.C. elegans | Drosophila | calcium imaging | thermotaxis U nderstanding how brain dynamics creates behaviors requires quantifying the flow and transformation of sensory information to motor output in behaving animals. Optical imaging using genetically encoded calcium or voltage fluorescent probes offers a minimally invasive method to record neural activity in intact animals. The nematode Caenorhabditis elegans is particularly ideal for optical neurophysiology owing to its small size, optical transparency, compact nervous system, and ease of genetic manipulation. Imaging systems for tracking the activity of small numbers of neurons have been effective in determining their role during nematode locomotion and navigational behaviors like chemotaxis, thermotaxis, and the escape response (1-6).Recordings from large numbers of interconnected neurons are required to understand how neuronal ensembles carry out the systematic transformations of sensory input into motor patterns that build behavioral decisions.Several methods for fast 3D imaging of neural activity in a fixed imaging volume have been developed for different model organisms (7)(8)(9)(10)(11)(12)(13)(14). High-speed light sheet microscopy, light field microscopy, multifocus microscopy, and two-photon structured illumination microscopy have proved effective for rapidly recording large numbers of neurons in immobilized, intact, transparent animals like larval zebrafish and nematodes (15)(16)(17)(18)(19). However, these methods are problematic when attempting to track many neurons within the bending and moving body of a behaving animal. Panneuronal recording in moving animals poses higher demands on spatial and temporal resolution. Furthermore, extracting neuronal signals from recordings in a behaving animal requires an effective analysis pipel...

Animals rely on highly sensitive thermoreceptors to seek out optimal temperatures, but the molecular mechanisms of thermosensing are not well understood. The Dorsal Organ Cool Cells (DOCCs) of the Drosophila larva are a set of exceptionally thermosensitive neurons critical for larval cool avoidance. Here, we show that DOCC cool-sensing is mediated by Ionotropic Receptors (IRs), a family of sensory receptors widely studied in invertebrate chemical sensing. We find that two IRs, IR21a and IR25a, are required to mediate DOCC responses to cooling and are required for cool avoidance behavior. Furthermore, we find that ectopic expression of IR21a can confer cool-responsiveness in an Ir25a-dependent manner, suggesting an instructive role for IR21a in thermosensing. Together, these data show that IR family receptors can function together to mediate thermosensation of exquisite sensitivity.DOI: http://dx.doi.org/10.7554/eLife.13254.001

Brain circuits endow behavioral flexibility. Here, we study circuits encoding flexible chemotaxis in C. elegans, where the animal navigates up or down NaCl gradients (positive or negative chemotaxis) to reach the salt concentration of previous growth (the setpoint). The ASER sensory neuron mediates positive and negative chemotaxis by regulating the frequency and direction of reorientation movements in response to salt gradients. Both salt gradients and setpoint memory are encoded in ASER temporal activity patterns. Distinct temporal activity patterns in interneurons immediately downstream of ASER encode chemotactic movement decisions. Different interneuron combinations regulate positive vs. negative chemotaxis. We conclude that sensorimotor pathways are segregated immediately after the primary sensory neuron in the chemotaxis circuit, and sensory representation is rapidly transformed to motor representation at the first interneuron layer. Our study reveals compact encoding of perception, memory, and locomotion in an experience-dependent navigational behavior in C. elegans.

Highlights d Key Drosophila thermosensors detect heating and cooling, rather than hot and cold d Ionotropic Receptors (IRs) mediate cooling detection d IRs specify both the morphogenesis and thermosensitivity of sensory endings d Thermoregulation requires context-dependent interpretation of phasic sensory inputs

When placed on a temperature gradient, a Drosophila larva navigates away from excessive cold or heat by regulating the size, frequency, and direction of reorientation maneuvers between successive periods of forward movement. Forward movement is driven by peristalsis waves that travel from tail to head. During each reorientation maneuver, the larva pauses and sweeps its head from side to side until it picks a new direction for forward movement. Here, we characterized the motor programs that underlie the initiation, execution, and completion of reorientation maneuvers by measuring body segment dynamics of freely moving larvae with fluorescent muscle fibers as they were exposed to temporal changes in temperature. We find that reorientation maneuvers are characterized by highly stereotyped spatiotemporal patterns of segment dynamics. Reorientation maneuvers are initiated with head sweeping movement driven by asymmetric contraction of a portion of anterior body segments. The larva attains a new direction for forward movement after head sweeping movement by using peristalsis waves that gradually push posterior body segments out of alignment with the tail (i.e., the previous direction of forward movement) into alignment with the head. Thus, reorientation maneuvers during thermotaxis are carried out by two alternating motor programs: (1) peristalsis for driving forward movement and (2) asymmetric contraction of anterior body segments for driving head sweeping movement.