TRP Channels at the Periphery of the Taste and Trigeminal Systems

The mammalian taste system consists of taste buds, which are groups of 50–100 taste cells that are found throughout the oral cavity. On the tongue, which is the focus of this chapter, taste buds are located on circumvallate, foliate, and fungiform papillae (Figure 7.1a). Taste cells synapse with afferent fibers from branches of the facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) cranial nerves (Figure 7.1b) that, in turn, transmit information to the central nervous system (CNS) attributes of tastant quality, intensity, and hedonic nature (Gutierrez and Simon, 2011; Carleton et al., 2010; Vincis and Fontanini, 2016). The list includes several classes of chemical stimuli such as sugars, salts, acids, proteins, and organic compounds that are perceived as bitter tasting (Simon et al., 2006). Taste buds are embedded in a stratified squamous epithelium, which contains somatosensory branches of the trigeminal (CN V), glossopharyngeal (CN IX), and vagus (CN X) cranial nerves. The terminals of these somatosensory fibers often surround taste buds (Figures 7.1b and 7.4), indicating the close association of the taste and somatosensory systems. Information from these general sensory nerves provides information to the CNS about mechanical, thermal, and painful stimuli (Julius, 2013; Kaneko and Szallasi, 2014). The painful stimuli can arise from strong or sharp mechanical stimuli, abnormally high or low temperatures, or chemical stimuli such as capsaicin, which is found in chili peppers and causes a burning taste sensation. As both the peripheral taste and somatosensory systems contain transient receptor potential (TRPs) (Ramsey et al., 2006; Julius, 2013), here we will initially review general properties of TRPs, and then describe their roles in the peripheral taste and somatosensory systems. We do not, however, discuss their presence in keratinocytes, but refer the reader to Chapter 5. Finally, for additional details regarding their roles in taste and as condiments used for cooking, we refer the reader to several excellent reviews on this topic (Vriens et al., 2008; Roper, 2013; Talavera, 2015; Roper, 2014). Since their discovery in 1989, as seen in Figure 7.2, numerous TRP channels have been identified. What they have in common is that they are all cation selective (most for Ca), and they are composed of four subunits with six transmembrane spans (S1–S6), with a pore region between S5 and S6 (Morales-Lázaro et al., 2013). Moreover, they often have ankyrin repeats in their N-terminus, exhibit inwardly rectifying current-voltage curves, and are modulated by calmodulin and phospholipases and kinases (Ramsey et al., 2006; Julius, 2013). As seen in Figure 7.2a, TRPs fall into six subfamilies: TRPC for “canonical” (TRPC1–7), TRPM for “melastatin” (TRPM1–8), TRPA for “ankyrin” (TRPA1), TRPV for “vanilloid” (TRPV1–6), TRPML for “mucolipin” (TRPML1–3), and TRPP for “polycystin” (TRPP2, TRPP3, TRPP5). Other interesting properties of many TRP channels, illustrated in Figure 7.2b and elaborated below, are that they are important in gustatory processing, are very sensitive to changes in temperature, and are activated by many compounds found in plants that are often used as spices (Figure 7.2b).