ASIC Channels and Pain

University Of California, San Francisco, San Francisco CA

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Abstract

DESCRIPTION (provided by applicant): Nociception is the process whereby primary afferent somatosensory neurons recognize and respond to noxious stimuli. In addition to initiating acute pain responses, nociceptor activation can produce local inflammation leading to pain hypersensitivity. Tissue acidosis (i.e. reduction in local pH) is an important hallmark of this response, and is associated with a range of physiological insults, such as infection, ischemia, tumor growth, and arthritis. Indeed, extracellular protons enhance excitability of primary afferent nociceptors, thereby producing acute pain and/or pain hypersensitivity. Members of the acid sensing ion channel (ASIC) family are believed to play important roles in nociception and pain by functioning as sensors for extracellular protons. For example, the ASIC3 subtype likely accounts for ischemic pain associated with large, rapidly inactivating proton-evoked currents in neurons that innervate skeletal or cardiac muscle. However, additional roles for ASIC channels in nociception have remained enigmatic for a variety of reasons. First, a dearth of pharmacological tools has made it difficult to manipulate these channels in vivo. Second, mice lacking specific ASIC channel subtypes have failed to revealed clear or robust phenotypes in regard to acid-evoked pain or other aspects of nociception. Third, a comprehensive analysis of ASIC channel expression and localization - which is critical to deciphering physiological roles for these channels in nociception and pain - remains incomplete. Finally, some ASIC subtypes (e.g. ASIC2a) respond only to extreme extracellular acidosis (pH < 5), suggesting the existence of other endogenous modulators for these channels that may be produced under pathophysiological conditions of tissue injury and/or chronic inflammation. The goal of this proposal is to address these and other questions by applying genetic, physiologic, and biochemical methods to develop a comprehensive view of ASIC subtype function, pharmacology, and expression in the somatosensory system. The specific aims are to (i) develop a comprehensive map of ASIC channel expression using gene targeted reporter mice to visualize ASIC-positive nerve fibers with exquisite sensitivity and fidelity; (ii) characterize functional properties of ASIC-expressing sensory neurons and nerve fibers from these genetically-labeled mice using a range of electrophysiological and live-cell imaging methods; (iii) screen for novel endogenous ASIC modulators in extracts of normal and injured tissues using a range of biochemical, functional, and behavioral assays. Together, these aims will address important unresolved questions concerning ASIC function as key steps toward the rational development of novel therapeutic agents that target a range of chronic inflammatory pain syndromes.