Genetic and physiological analysis of inflammatory pain mechanisms
Although pain relief is one of the most common goals of clinical interventions, it remains a great challenge due to the variety of pain types and the significant side effects of available pharmacological agents. Pain perception is mediated by specialized sensory neurons known as nociceptors that signal the detection of tissue damage to the CNS often eliciting a behavioral response to limit further tissue damage. Work in a variety of species has identified potential diffusible substances released from epidermal wounds that are capable of attracting phagocytes to the wound but also capable of nociceptor activation. Recent work has described the endogenous reactive oxygen species(ROS), H2O2, as a potential signal released from wounded epithelium to attract phagocytes. Although ROS have been implicated as potential causative agents in aging, cancer and heart disease, they are also utilized at lower concentrations as endogenous signaling molecules during development and for regulation of various physiological processes. Model genetic organisms have been useful in these studies allowing the application of a variety of genetic and genomic approaches to the investigation of the molecular links between tissue damage and pain. Drosophila larvae display characteristic taxis behavior in response to a variety of sensory stimuli including a strong aversion to locomotion on dry surfaces. We have utilized genetic, molecular and physiological approaches to show that this behavioral aversion to dry surfaces is dependent upon the ROS-mediated activation of a subset of peripheral nociceptor neurons. The class IV multiple dendritic(mdIV) neurons extend a complex dendritic arbor tiling the larval body wall and specifically express the DEG/ENaC ion channel subunit, Pickpocket1(PPK1) and the Drosophila TRP channel Painless. Transgenic manipulation of PPK1 and mdIV neuronal activity demonstrated that mdIV neuron inactivation caused a loss of aversion to dry surfaces and the resulting accumulation of epithelial melanotic plugs consistent with increased tissue damage. Direct electrophysiological recordings from PPK1 neurons revealed an acute sensitivity to nanomolar levels of H2O2 capable of neuronal activation. Transgenic overexpression of catalase to degrade endogenous H2O2 signals caused the same loss of aversion to dry surfaces seen in ppk1 and mdIV loss-of-function mutants. These preliminary findings support a model in which PPK1-expressing neurons function as ROS-activated nociceptors. We are currently working to establish these neurons as a highly useful genetic and behavioral model system to study the molecular mechanisms linking wound inflammation and healing signals with nociceptor activation and pain aversive behavior.
1. Ainsley, J.A., Kim, M.-J., Wegman, L.J., Pettus, J.M. and W.A. Johnson(2008) Sensory mechanisms controlling the timing of larval developmental and behavioral transitions require the Drosophila DEG/ENaC subunit, Pickpocket1. Developmental Biology 322: 46-55.
2. Vermehren-Schmaedick, A., Ainsley, J.A., Johnson W.A., Davies, S.A. and D.B. Morton(2010) Behavioral Responses to Hypoxia in Drosophila Larvae Are Mediated by Atypical Soluble Guanyl Cyclases. Genetics 186: 183-196.
3. Wegman, L.J., Ainsley, J.A. and W.A. Johnson(2010) Developmental timing of a sensory-mediated larval surfacing behavior correlates with cessation of feeding and determination of final adult size. Developmental Biology 345: 170-179.