Focal Laser Stimulation of Fly Nociceptors Activates Distinct Axonal and Dendritic Ca2+ Signals

Abstract

Drosophila Class IV neurons are polymodal nociceptors that detect noxious mechanical, thermal, optical and chemical stimuli. Escape behaviors in response to attacks by parasitoid wasps are dependent on Class IV cells, whose highly branched dendritic arbors form a fine meshwork that is thought to enable detection of the wasps needle-like ovipositor barb. To understand how mechanical stimuli trigger cellular responses, we used a focused 405-nm laser to create highly local lesions to probe the precise position needed in evoke responses. By imaging calcium signals in dendrites, axons, and soma in response to stimuli of varying positions, intensities and spatial profiles, we discovered that there are two distinct nociceptive pathways. Direct stimulation to dendrites (the contact pathway) produces calcium responses in axons, dendrites and the cell body whereas stimulation adjacent to the dendrite (the non-contact pathway) produces calcium responses in the axons only. We interpret the non-contact pathway as damage to adjacent cells releasing diffusible molecules that act on the dendrites. Axonal responses have higher sensitivities and shorter latencies. In contrast, dendritic responses have lower sensitivities and longer latencies. Stimulation of finer, distal dendrites leads to smaller responses than stimulation of coarser, proximal dendrites, as expected if the contact response depends on the geometric overlap of the laser profile and the dendrite diameter. Because the axon signals to the CNS to trigger escape behaviors, we propose that the density of the dendritic meshwork is high not only to enable direct contact with the ovipositor, but also to enable neuronal activation via diffusing signals from damaged surrounding cells. Dendritic contact evokes responses throughout the dendritic arbor, even to regions distant and distal from the stimulus. These dendrite-wide calcium signals may facilitate hyperalgesia or cellular morphological changes following dendritic damage. Statement of SignificanceAnimals encounter a wide range of noxious stimuli in the natural world. Nociceptive neurons are specialized cells that sense harmful stimuli and trigger avoidance responses. Class IV cells, located under the cuticle in Drosophila larvae, are polymodal nociceptors that respond to noxious mechanical, thermal, optical, and chemical stimuli. To investigate the spatial requirements of mechanoreception in Class IV neurons, we measured calcium signals evoked by a focused laser beam that creates highly localized tissue damage. We discovered that different cellular compartments - axons and dendrites - responded differentially depending on whether the stimulus makes direct contact with the neuron or not. This provides evidence that mechanical nociception in Class IV cells occurs via two distinct pathways.