Taste sensing in the colon

The colonic lumen is continually exposed to many compounds, including beneficial and harmful compounds that are produced by colonic microflora. The intestinal epithelia form a barrier between the internal and luminal (external) environments. Chemical receptors that sense the luminal environment are thought to play important roles as sensors and as modulators of epithelial cell functions. The recent molecular identification of various membrane receptor proteins has revealed the sensory role of intestinal epithelial cells. Nutrient sensing by these receptors in the small intestine is implicated in nutrient absorption and metabolism. However, little is known about the physiological roles of chemosensors in the large intestine. Since 1980s, researchers have examined the effects of short-chain fatty acids (SCFA), the primary products of commensal bacteria, on gut motility, secretion, and incretin release, for example. In this decade, the SCFA receptor genes and their expression were identified in the mammalian colon. Furthermore, many other chemical receptors, including taste and olfactory receptors have been found in colonic epithelial cells. These findings indicate that the large intestinal epithelia express chemosensors that detect the luminal contents, particularly bacterial metabolites, and induce the host defense systems and the modulation of systemic metabolism via incretin release. In this review, we describe the local effects of chemical stimuli on the lumen associated with the expression pattern of sensory receptors. We propose that sensory receptors expressed in the colonic mucosa play important roles in luminal chemosensing to maintain homeostasis.