Functional gastrointestinal disorders such as irritable bowel syndrome (IBS) are characterized by pain and discomfort and enhanced sensitivity to gastrointestinal stimuli in the absence of a demonstrable organic cause (i.e., there are no mechanical, biochemical or inflammatory conditions to explain the symptoms). Visceral hypersensitivity thus differs from somatic hyperalgesia, which is commonly associated with tissue injury and inflammation. Because the anatomical organization of visceral afferent innervation and adequate noxious stimuli for viscera significantly differ from the innervation and adequate stimuli in the somatic realm, peripheral mechanisms of visceral hypersensitivity differ from those of somatic hyperalgesia and are not well understood. The long term objective of this research program is to understand peripheral mechanisms of visceral hypersensitivity. The current application proposes to continue to establish behavioral relevance of stimuli and treatments before subsequent examination of peripheral contributions to the development of visceral hypersensitivity. The peripheral receptors to be examined for contributions to colon hypersensitivity include ASIC3, TRPV1, P2X2-3, and PAR2, all of which have been implicated in visceral hypersensitivity. For each of these receptors: 1) Their contribution to baseline and enhanced responses (hypersensitivity) to colon distension 1-14 days after colon insult (intracolonic zymosan) will be examined in awake, behaving mice, 2) Mechano-and chemo-sensitivity of pelvic nerve and lumbar splanchnic nerve fiber terminals in the colon will be studied using an in vitro colon-nerve preparation, and 3) Whole cell currents and excitability of colon sensory neurons, identified by content of retrograde tracer will be studied. The overall hypothesis is that these 4 ligand-gated ion channels contribute to mechano-transduction in the colon. We also hypothesize that protons and/or endogenous mediators (e.g., ATP, mast cell tryptase, serotonin) contribute to visceral hypersensitivity and can do so in the absence of frank tissue damage, which is relevant to IBS and other functional gastrointestinal disorders.