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TRPV1 Receptors and Signal Transduction.


In: Liedtke WB, Heller S, editors.


TRP Ion Channel Function in Sensory Transduction and Cellular Signaling Cascades. Boca Raton (FL): CRC Press; 2007. Chapter 5.
Frontiers in Neuroscience.


The perception of pain throughout the body arises when neural signals originating from the terminals of nociceptors are propagated to second-order neurons in the spinal cord or brainstem, whereupon they are transmitted to specific higher order brain areas (Price, 2000). Recent studies have begun to elucidate some of the molecular mechanisms underlying the transduction of noxious stimuli. Many stimuli have been found to activate ion channels present on nociceptor terminals that act as molecular transducers to depolarize these neurons, thereby setting off nociceptive impulses along the pain pathways (Price, 2000; Costigan and Woolf, 2000). Among these ion channels are the members of the transient receptor potential (TRP) family. To date, the most studied member of the TRP family is the TRPV1 receptor. This is because it is the only one activated by capsaicin, the compound in chili pepper responsible for its “hot” taste; also, inhibiting TRPV1 has been shown to have therapeutic value (DiMarzo et al., 2002; Cortright and Szallasi, 2004). Although we will focus on the presence of these channels in nociceptors, we note that they have been identified in many other cell types and in various cortical and subcortical areas (Toth et al., 2005). The transient receptor potential vanilloid 1 (TRPV1) channel is predicted to have six transmembrane domains and a short, pore-forming hydrophobic stretch between the fifth and sixth transmembrane domains (see Figure 5.1A). It is activated not only by the vanilloid capsaicin (Caterina et al., 1997), but also by noxious heat (>43°C) and low pH (Caterina et al., 1997; Tominaga et al., 1998), voltage (Gunthorpe et al., 2000; Piper et al., 1999), and various lipids (Julius and Basbaum, 2001; Caterina and Julius, 2001; Clapham, 2003; Cortright and Szallasi, 2004, Szallasi and Blumberg, 1999; Prescott and Julius, 2003; Jung et al., 2004; Bhave et al., 2003). In cells, TRPV1 is inactivated by its binding to PIP2 and is released from this block by PLC-mediated PIP2 hydrolysis (Prescott and Julius, 2003). Since its cloning in 1997, many amino acid regions within the TPRV1 protein have been shown to be involved in specific functions, such as capsaicin, proton, and heat activation; voltage dependence; permeability and ion selectivity; antagonist regions; desensitization; phosphorylation; modulation by lipids; and multimerization. In regard to its subunit composition, functional TRPV1 channels likely exist as homomeric or heteromeric complexes composed of four subunits that assemble to form functional cation-(including calcium) permeable pores (Clapham, 2003; Kedei et al., 2001; Kuzhikanathil et al., 2001). Moreover, like other ion channels, these channels have been shown to be associated with regulatory proteins (see Figure 5.1B and Kim et al., 2006). There are many signaling pathways that become activated (or inhibited) by the activation of TRPV1 (Farkas-Szallasi et al., 1995; Wood et al., 1988). Similar to many other channels, TRPV1 contains multiple phosphorylation sites in its amino acid sequence for protein kinase C (PKC) (Bhave et al., 2003; Dai et al., 2004; Premkumar et al., 2004), protein kinase A (PKA) (Bhave et al., 2002; De Petrocellis et al., 2001; Rathee et al., 2002) and Ca2+/calmodulin-dependent protein kinase II (CaMKII). The presence of multiple phosphorylation sites in TRPV1 implies possible regulatory actions by these kinases (Wood et al., 1988). Discussed later in this chapter are several lines of evidence that show that two types of lipids—endocannabinoids and eicosanoids that are products of lipoxygenase (LOX)—activate TRPV1 channels (Zygmunt et al., 1999; Hwang et al., 2000). Because TRPV1 functions as a molecular integrator for multiple types of sensory input, in this chapter we will explore the molecular mechanisms underlying the activation and modulation of this channel.

Copyright © 2007, Taylor & Francis Group, LLC

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