Animals as diverse as snails and humans sense noxious events (the process termed nociception); pain receptors, called nociceptors, respond to mechanical change, heat, and certain toxic chemicals. Pain serves to alert us to events such as tissue damage that are capable of producing injury and evokes behaviors that promote tissue healing. Persistent pain in response to tissue injury is common, and many individuals suffer from chronic pain. Thus understanding both acute and chronic pain is a major research goal, as is the development of new types of drugs to treat pain.

One of the first mammalian pain receptors to be cloned and identified was TRPV1, a Na⁺/Ca²⁺ channel that is found in many sensory pain neurons of the peripheral nervous system and is activated by a wide variety of exogenous and endogenous physical and chemical stimuli. The best-known activators of TRPV1 are heat greater than 43 °C, acidic pH, and capsaicin, the molecule that makes chili peppers seem hot. Activation of TRPV1 receptors leads to painful, burning sensations. Numerous TRPV1 antagonists have been developed by pharmaceutical companies as possible pain medications. However, a major side effect that has limited the utility of these drugs is that they result in an elevation in body temperature; this suggests that one “normal” function of TRPV1 is to sense and regulate body temperature, and that the drugs inhibit this function.

In a recent landmark study, scientists used single-particle cryoelectron microscopy (cryoEM, see Chapter 3) to obtain a high-resolution (0.34 nm) model of the rat TRPV1 channel in the closed configuration and in two open configurations, one bound to capsaicin and the other bound to two potent TRPV1 activators, one from plant and the other from spider venom. As shown in Figure 22-33, these studies revealed that the TRPV1 channel structure is similar to that of voltage-gated ion channels (see Figure 22-13), composed of four symmetrical subunits with six transmembrane helices (S1–S6) each. However, the charged amino acids in S1–S4 that function as voltage sensors in voltage-gated ion channels are replaced by aromatic residues in TRPV1. This stabilizes the channel core so that instead of moving like voltage sensors upon depolarization, the TRPV S1–S4 helices provide an anchor for movements within the pore that are triggered by ligand binding. Two constrictions, or gates, were identified in the pore region. The spider toxin bound to the extracellular surface of the channel, near the pore helix, and locked open the extracellular end of the channel. Capsaicin and the plant toxin bound to a site deep within the membrane toward the cytoplasmic end of the pore, with binding increasing the diameter of the pore. These findings indicate that the TRPV channel undergoes dual gating.

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Two channels were discovered in 2010 that directly convert mechanical stimuli into cation conductance in vertebrate cells, called Piezo1 and Piezo2 (from the Greek word piesi, which means “pressure”). Both form large cation-selective channels composed of four identical subunits, with each subunit containing over 30 membrane-spanning domains, creating a channel whose molecular weight is about 1.2 million daltons, and that has between 120 and 160 transmembrane segments! Expression of Piezo1 or Piezo2 induces mechanosensitive-cation currents in these cells. This can be assayed by expressing the channels in cell culture, and using calcium imaging to monitor the response of the cells to stretch induced by poking the cells with a small glass pipette (Figure 22-34). Reduction of Piezo2 expression in dorsal root ganglion sensory neurons in mice reduced their mechanosensitivity, and knockout of the single Piezo homolog in Drosophila melanogaster resulted in flies with severely reduced behavioral responses to noxious mechanical stimuli. Together, these experiments show that Piezo channels mediate mechanical signal transduction.

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