Abstract:
Nociception, the process of encoding and transmitting noxious stimuli within a nervous system, is an essential mechanism through which organisms are alerted to potentially life-threatening insults or conditions that originate externally or internally and may lead to incipient tissue damage. These insults can be classified into three broad types of stimuli: chemical, mechanical and thermal. Despite significant progress in this field of study, our understanding of the neural bases of noxious cold detection remains poorly understood. The Drosophila melanogaster larval peripheral nervous system (PNS) has emerged as an excellent model system for studying both the cellular and molecular mechanisms of nociception, particularly with regards to noxious heat and mechanical stimuli. Using this model system, we have implemented a systems neuroscience approach to dissect the underlying cellular, genetic, molecular, and behavioral bases of noxious cold detection. We have developed the first behavioral assay for investigating cold nociception in Drosophila and have identified unique noxious cold-evoked behavioral responses. These cold-specific behaviors require synaptic transmission predominantly from class III dendritic arborization (da) sensory neurons. Optogenetic activation of class III neurons, independent of thermal stimulation, is sufficient to behaviorally phenocopy noxious cold-evoked behavior. Moreover, GCaMP6 functional analyses reveal specific activation of class III da neurons in response to noxious cold stimulation. At a molecular level, microarray analyses revealed these neurons are enriched with a broad spectrum of ion channels, including numerous Transient Receptor Potential (TRP) channels. Mutant and cell-type specific RNAi analyses reveal that select TRP channels Trpm, nompC and Pkd2 are required for normal cold-evoked behaviors and function in the transduction step of noxious cold stimulation. Collectively, our results establish a novel systems-level framework for exploring cellular and molecular mechanisms underlying thermosensory nociception.
