Analgesic Signaling in Drosophila

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Analgesia, the dampening of nociceptive responses to noxious stimuli capable of damaging tissues, is an adaptive behavioral response for organisms- allowing them to feel less pain in physiologically appropriate situations such as physical trauma or the fight/flight response. Analgesia has traditionally been studied in vertebrate models, where endogenous opioid peptides bind to their cognate receptors to dampen behavioral responses to painful stimuli. Genetically tractable model organisms such as Drosophila have recently been used to explore the molecular/genetic bases of nociception and nociceptive sensitization following tissue injury but have not yet been used to dissect conserved analgesia signaling. Drosophila offer both speed of genetic analysis and a variety of sophisticated genetic tools for analyzing gene expression and function that should prove a valuable complement to existing experimental paradigms for the study of analgesia. Our long-term goal in this basic research project is to identify and characterize analgesic signaling pathways in Drosophila larvae. That such pathways are likely to exist is evidenced by our preliminary findings that the opiate compound morphine is analgesic for Drosophila larvae and that a conserved G-protein coupled receptor (GPCR) is required in this organism for thermal analgesia. Morphine feeding to fly larvae causes transient analgesia that spans multiple sensory modalities (heat, cold, touch, chemical), is mimicked by other opiates (fentanyl) and is partially naloxone reversible. Our short-term goals over the initial project period will be to characterize the GPCR required for thermal analgesia. Using our knowledge of which tissues express the GPCR we will determine which tissues functionally require it for analgesia, and assess whether the putative receptor’s analgesic effects extend to other sensory modalities beyond heat. At the biochemical level we will test whether the GPCR (and it’s clearest human ortholog) directly binds morphine to activate signaling. At the genetic level we will identify endogenous peptide ligand(s) and probe genetic interactions with previously identified nociceptive genes and nociceptive sensitization signaling pathways. In our final aim we will probe cellular effects of morphine administration (calcium changes) and whether known deleterious biological side effects of morphine, development of tolerance and constipation are also observed in our new model. Successful completion of these aims will provide a comprehensive cellular and genetic analysis of analgesic signaling in this new model- basic information that is likely to generate testable hypotheses for ongoing work in vertebrate models. Moving forward, this new model of analgesic signaling will provide the possibility of unbiased gene discovery approaches that should allow for identification of novel conserved genes required for analgesia- targets that may in some cases be potentially relevant to human and health and worthy of further exploration in vertebrate models and clinical settings.