Molecular and cellular mechanisms of cold allodynia

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Project Summary The detection of external stimuli such as temperature is critical for survival, yet inappropriate responses to temperature do have a significant negative impact on overall health. The sensations and the physiological effects of cold are distinct among somatosensory modalities in that cold provides a pleasant, soothing sensation at mild temperatures, but is also agonizing as temperatures decrease. Remarkably, how this one somatosensory modality mediates this diverse range of physiological effects is not known. The menthol receptor TRPM8 is considered the principal cold sensor in mammalian sensory neurons, but the irritant receptor TRPA1 have also been associated with cold pain. Mice lacking TRPM8 channels retain some limited cold sensitivity, but we find that ablation of TRPM8-expressing neurons in mice abolishes essentially all acute cold and cold pain behaviors, results implying TRPM8-independent cold transduction mechanisms in TRPM8+ neurons. TRPA1 channels appear to serve no role in acute cold, but likely contribute to injury-induced cold pain. TRPM8 and TRPA1 are not co-expressed, yet the interplay between TRPA1 and TRPM8 after injury has not been examined, nor have the molecular and cellular mechanisms leading to injury-induced sensitization. Recently, we found that the glial cell-line derived neurotrophic factor-like (GDNF) ligand artemin is a mediator of TRPM8-dependent cold pain, and that the artemin receptor GFRα3 is required for pathological cold allodynia. However, the molecular and cellular mechanisms whereby this pathway leads to cold allodynia is not known. Here we propose to use a combination of molecular, cellular, behavioral, and pharmacological approaches to fill in these gaps in our knowledge of the mechanisms underlying cold sensation. First, we will determine the role of TRPM8 and TRPA1 channels in cold allodynia. Second, we define the cellular basis for artemin-induced cold hypersensitivity. Third, we will test the necessity of the classical GFRα co-receptor Ret in cold allodynia and determine if other candidate co-receptors mediate this form of pathology. Lastly, we will generate transgenic mice in which genetically defined subpopulations of TRPM8 neurons are conditionally ablated in vivo to determine if innocuous cool, noxious cold, and analgesia are mediated cell autonomously. At the conclusion of these studies, we will have defined a signal transduction pathway leading to cold pain, and if these stimuli are transmitted via distinct neural circuits to mediate the range of behavioral and physiological responses to cold temperatures.