Functional Expression of Thermo-transient Receptor Potential Channels in Dental Primary Afferent Neurons

Abstract

Temperature signaling can be initiated by members of transient receptor potential family (thermo-TRP) channels. Hot and cold substances applied to teeth usually elicit pain sensation. This study investigated the expression of thermo-TRP channels in dental primary afferent neurons of the rat identified by retrograde labeling with a fluorescent dye in maxillary molars. Single cell reverse transcription-PCR and immunohistochemistry revealed expression of TRPV1, TRPM8, and TRPA1 in subsets of such neurons. Capsaicin (a TRPV1 agonist), menthol (a TRPM8 agonist), and icilin (a TRPM8 and TRPA1 agonist) increased intracellular calcium and evoked cationic currents in subsets of neurons, as did the appropriate temperature changes (>43°, <25°, and <17°C, respectively). Some neurons expressed more than one TRP channel and responded to two or three corresponding stimuli (ligands or thermal stimuli). Immunohistochemistry and single cell reverse transcription-PCR following whole cell recordings provided direct evidence for the association between the responsiveness to thermo-TRP ligands and expression of thermo-TRP channels. The results suggest that activation of thermo-TRP channels expressed by dental afferent neurons contributes to tooth pain evoked by temperature stimuli. Accordingly, blockade of thermo-TRP channels will provide a novel therapeutic intervention for the treatment of tooth pain.Recent studies have demonstrated that subfamilies of TRP 2 channels play critical roles in the transduction of temperature and pain sensation (1). The temperature-activated TRP channels (thermo-TRP channels) include TRPV1, TRPM8, and TRPA1; they are activated by Ͼ43°, Ͻ25°, and Ͻ17°C, respectively (1). They can also be activated by the exogenous ligands, capsaicin (TRPV1), menthol (TRPM8), and icilin (TRPM8 and TRPA1) (2-5). Given that the thermo-TRPs are involved in converting thermal information into electrical signals within the sensory nervous system (1), it is possible that thermo-TRPs expressed by dental primary afferents play a crucial role for transduction process of tooth pain, especially caused by noxious thermal stimulation.The different forms of pain are produced by distinctive molecular and cellular mechanisms. It is now thought that elucidation of the main mechanism involved in a certain form of pain is key to the development of pain treatments, which specifically target underlying the cause rather than just symptoms (6). Tooth pain is commonly induced by the presence of hot or cold foods in the oral cavity (7-9). Tooth pain results from the exposure of dentin, which is caused by dissolution of the protective enamel covering the tooth crown by dental caries or gingival recession (7,8). Since the proposition of Brännström, arguing that the fluid movement in dentinal tubules induced by diverse stimuli, including thermal stimuli, elicits tooth nerve firing to produce tooth pain (referred to as the hydrodynamic hypothesis) (10, 11), much evidence has been reported to support the hypothesis (12). However, it is not fully under...

Diversity in the Neural Circuitry of Cold Sensing Revealed by Genetic Axonal Labeling of Transient Receptor Potential Melastatin 8 Neurons

“…Whereas localization of TRPM8 endings in the pulp is consistent with models of sensory signaling in the tooth (Matthews, 1977;Park et al, 2006), Trpm8 GFP expression in fibers entering the dentin is surprising. Strong experimental evidence suggests that sensory afferents innervating the dentin are activated via hydrodynamic receptors detecting fluid movement through dentinal tubules, a process termed the dentinal hydrodynamic theory (Matthews, 1977;Byers and Narhi, 2002;Park et al, 2006;Chidchuangchai et al, 2007).…”

“…Strong experimental evidence suggests that sensory afferents innervating the dentin are activated via hydrodynamic receptors detecting fluid movement through dentinal tubules, a process termed the dentinal hydrodynamic theory (Matthews, 1977;Byers and Narhi, 2002;Park et al, 2006;Chidchuangchai et al, 2007). Thus, intradental afferents are thought to be activated by a mechanism detecting changes in movement, not temperature (Chidchuangchai et al, 2007).…”

Intracellular Acidification Is Associated with Changes in Free Cytosolic Calcium and Inhibition of Action Potentials in Rat Trigeminal Ganglion

“…Removal of extracellular Ca 2ϩ completely blocked the acidification induced by capsaicin, showing that the influx of Ca 2ϩ is both necessary and sufficient to induce intracellular acidification by neurotransmitter receptor activation. We found that TRPV1, the receptor activated by capsaicin, was expressed only in small and intermediate sized TG neurons that are thought to be nociceptive (28). Glutamate has been shown to sensitize mechanosensitive afferents (29), and capsaicin has been shown to activate and sensitize trigeminal afferents; TRPV1 may also mediate Ca 2ϩ -dependent intracellular acidification in dorsal root ganglion neurons (7).…”

Activation of Src-Family Kinases in Spinal Microglia Contributes to Mechanical Hypersensitivity after Nerve Injury

“…TRPV1, one of the transducer proteins, can generate depolarizing currents in response to noxious thermal stimuli, with an activation temperature of ϳ43°C, whereas TRPA1 is activated at ϳ17°C, a temperature that is reported as painfully cold by humans (Jordt et al, 2003;Patapoutian et al, 2003;Park et al, 2006). Although the role of TRPA1 in cold transduction is still controversial (Jordt et al, 2004;Bautista et al, 2006), TRPV1 and TRPA1 upregulation in undamaged sensory neurons has been implicated in nerve injury-induced heat and cold hypersensitivity, respectively Obata et al, 2004Obata et al, , 2005Katsura et al, 2006).…”