Investigating the cellular bases of cold nociception in Drosophila larvae. Luis Sullivan, Srividya C Iyer, Eswar P R Iyer, Kevin Armengol, Daniel N Cox. School of Systems Biol., Krasnow Inst. Adv. Study, George Mason University, Fairfax, VA.

   Thermosensory nociception, particularly in poikilothermic organisms, is essential for survival and provides a mechanism for sensory perception of noxious thermal stimuli that alert the organism to potential environmental dangers coupled with pain sensation and complex behavioral responses to protect the organism from incipient damage. Moreover, acute and chronic pain may manifest as altered thermosensory nociception in neuropathic pain states. Drosophila has emerged as a powerful model organism for dissecting the cellular and molecular mechanisms regulating both nociception and thermosensation, however, nothing is known regarding the bases of cold nociception. To address this knowledge gap, we have developed and implemented a novel behavioral assay to characterize responses of larvae to a declining thermal gradient. These analyses revealed a distinct and novel behavior we refer to as cringing which is characterized by a full body contraction along the anterior-posterior axis. Moreover, cringing behavior was uniquely observed only in the noxious cold range (10C) and is the predominant behavioral response below 8C. Using this behavioral platform, we demonstrate that synaptic transmission in a specific subset of dendritic arborization (da) sensory neurons is required for noxious cold thermosensation. Moreover, optogenetic-based activation of this da neuron subset is capable of phenocopying the noxious cold-induced cringing behavior. Conversely, optogenetic inhibition via halorhodopsin expression confirms our observations that this subset of da neurons is required for mediating the response to noxious cold in larvae. Finally, we demonstrate via GCaMP3 live imaging that these neurons actively respond to noxious cold stimulation. In summary, these studies provide insight into the cellular bases of noxious cold sensation in Drosophila larvae and the cellular mechanisms responsible for this sensory modality.