The structure-function relationships underlying Drosophila larval chemotaxis

The Drosophila larva is an excellent model organism to study the neural correlates of behavior. It possesses a tractable yet complex nervous system that is capable of integrating and transforming multimodal sensory stimuli into complex navigational decisions. In the larva, activity of individual neurons –the building blocks of the nervous system– could be reliably monitored and manipulated thanks to the unmatched genetic tools available in Drosophila. Recent efforts to reconstruct the connectome of the whole larval nervous system enable circuit-level analysis of the neural mechanisms underlying the larval behavior. The larva exhibits robust navigation in the presence of volatile chemical cues (chemotaxis). Larval chemotaxis comprises alternations between different behavioral modes: runs, pauses and turns. Here, we performed two independent forward screens to identify neurons that are involved in action selection during Drosophila larval chemotaxis. In our first screen, we identified neurons that are involved in run-to-turn transitions. High-resolution behavioral analysis upon manipulation of activity in a subset of neurons in the subesophageal zone revealed that these neurons are necessary and sufficient to trigger reorientation maneuvers. Our findings suggest that the SEZ is a premotor center that mediates action selection based on integrated sensory stimuli. In the second screen, we combined functional analysis with electron microscopy reconstruction to identify a descending neuron (PDM) that is necessary and sufficient to trigger run-to-turn transitions. EM reconstruction revealed that PDM receives olfactory inputs in the lateral horn region and connects to premotor neurons involved in peristaltic wave propagation through a set of SEZ descending neurons. By combining optogenetic activation with high-resolution analysis of behavior, we showed that PDM is responsible for terminating runs by inhibiting peristaltic wave-generating circuits in the ventral nerve cord of the larva.La larva de Drosophila es un excelente organismo modelo para estudiar las correlaciones neuronales del comportamiento. Posee un dócil pero complejo sistema nervioso capaz de integrar y transformar estímulos sensoriales multimodales en decisiones de navegación complejas. En la larva, la actividad de neuronas individuales –las piezas fundamentales del sistema nervioso-puede ser controlada y manipulada de manera fiable gracias a las inigualables herramientas genéticas disponibles en Drosophila. Esfuerzos recientes para reconstruir el conectoma completo del sistema nervioso de la larva nos permite analizar el mecanismo neuronal a nivel de circuito subyacente al comportamiento de la larva. La larva presenta una navegación robusta en presencia de señales químicas volátiles (quimiotaxis). La quimiotaxis de la larva alterna entre distintos modos de comportamiento: carreras, pausas y giros. Aquí, hemos llevado a cabo dos cribados independientes que nos permiten identificar neuronas de la larva de Drosophila involucradas en la selección de acciones durante la quiomitaxis. En nuestro primer cribado, hemos identificado neuronas involucradas en transiciones correr-para-girar. El análisis del comportamiento a alta resolución, habiendo manipulado la actividad de un grupo de neuronas de la zona subesofageal (SEZ), reveló que dichas neuronas son necesarias y suficientes para activar maniobras de reorientación. Nuestros descubrimientos sugieren que la SEZ es un centro premotor que media la selección de acciones basándose en estímulos sensoriales integrados. En el segundo cribado, hemos combinado el análisis funcional con la reconstrucción mediante microscopia electrónica para identificar la neurona descendiente (PDM) que es necesaria y suficiente para activar las transiciones correr-para-girar. La reconstrucción mediante EM reveló que la PDM recibe señales olfativas en la región del asta lateral y se conecta con neuronas premotoras involucradas en la propagación de ondas peristálticas a través de un conjunto de neuronas SEZ descendientes. Combinando la activación optogenética con el análisis del comportamiento de alta resolución, hemos demostrado que la PDM es la responsable de terminar carreras inhibiendo los circuitos de generación de ondas peristálticas en el cordón nervioso ventral de la larva

Role of the subesophageal zone in sensorimotor control of orientation in Drosophila larva

Chemotaxis is a powerful paradigm to investigate how nervous systems represent and integrate changes in sensory signals to direct navigational decisions. In the Drosophila melanogaster larva, chemotaxis mainly consists of an alternation of distinct behavioral modes: runs and directed turns. During locomotion, turns are triggered by the integration of temporal changes in the intensity of the stimulus. Upon completion of a turning maneuver, the direction of motion is typically realigned toward the odor gradient. While the anatomy of the peripheral olfactory circuits and the locomotor system of the larva are reasonably well documented, the neural circuits connecting the sensory neurons to the motor neurons remain unknown. We combined a loss-of-function behavioral screen with optogenetics-based clonal gain-of-function manipulations to identify neurons that are necessary and sufficient for the initiation of reorientation maneuvers in odor gradients. Our results indicate that a small subset of neurons residing in the subesophageal zone controls the rate of transition from runs to turns-a premotor function compatible with previous observations made in other invertebrates. After having shown that this function pertains to the processing of inputs from different sensory modalities (olfaction, vision, thermosensation), we conclude that the subesophageal zone operates as a general premotor center that regulates the selection of different behavioral programs based on the integration of sensory stimuli. The present analysis paves the way for a systematic investigation of the neural computations underlying action selection in a miniature brain amenable to genetic manipulations.M.L. acknowledges funding from the Spanish Ministry of Science and Innovation (MICINN, BFU2008-00362, BFU2009-07757-E/BMC, and BFU2011-26208), the EMBL/CRG Systems Biology Program, and a Marie Curie Reintegration Grant (PIRG02-GA-2007-224791). J.W.T. was supported by the Howard Hughes Medical Institute. I.T. acknowledges funding from the Marie Curie FP7 Programme through FLiACT (ITN

Sensorimotor pathway controlling stopping behavior during chemotaxis in the Drosophila melanogaster larva

Sensory navigation results from coordinated transitions between distinct behavioral programs. During chemotaxis in the Drosophila melanogaster larva, the detection of positive odor gradients extends runs while negative gradients promote stops and turns. This algorithm represents a foundation for the control of sensory navigation across phyla. In the present work, we identified an olfactory descending neuron, PDM-DN, which plays a pivotal role in the organization of stops and turns in response to the detection of graded changes in odor concentrations. Artificial activation of this descending neuron induces deterministic stops followed by the initiation of turning maneuvers through head casts. Using electron microscopy, we reconstructed the main pathway that connects the PDM-DN neuron to the peripheral olfactory system and to the pre-motor circuit responsible for the actuation of forward peristalsis. Our results set the stage for a detailed mechanistic analysis of the sensorimotor conversion of graded olfactory inputs into action selection to perform goal-oriented navigation.This work was initiated as part of the multi-lab Larval Olympiad conducted at the Janelia Research Campus. We are in debt to the work of S Reid (Louis lab) during the initial phase of the screen. We thank I Andrade, A Fushiki, J Jonaitis, I Larderet, P Schegel, C Schneider-Mizell and M Zwart for contributing to the EM reconstruction. We thank H Aberle for glutamate antibodies, as well as V Jayaraman, A Nern, S Pulver, G Rubin and J. Simpson for sharing fly lines. We thank V Jayaraman and R Francoville for training and access to the functional imaging setup. ML and DT acknowledges support of the Spanish Ministry of Economy and Competitiveness (MICINN and BFU2011-26208), ‘Centro de Excelencia Severo Ochoa 2013–2017’, the CERCA Programme/Generalitat de Catalunya, the EMBL/CRG Systems Biology Program and the University of California, Santa Barbara. IT was supported by the Marie Curie FP7 Programme through FLiACT (ITN). AK was supported by the ‘La Caixa’ International PhD Programme. JT, MZ and AC acknowledge funding from the Howard Hughes Medical Institute