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 degrees , <25 degrees , and <17 degrees 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.
Highlights
Temperature signaling can be initiated by members of transient receptor potential family channels
We found evidence for expression of TRPV1, TRPM8, and TRPA1 in these neurons by measuring mRNA level through single cell reverse transcription (RT)-PCR, by immunohistochemistry, and by measuring inward currents or changes in intracellular calcium in response to the appropriate ligands or temperature changes
Single Cell RT-PCR Analysis of Thermo-TRP Channel Expression by Dental Primary Afferent Neurons—We investigated using single cell RT-PCR whether dental primary afferent neurons would express TRPV1, TRPM8, and TRPA1, transmitting heat, cool, and cold sensa
Summary
Temperature signaling can be initiated by members of transient receptor potential family (thermo-TRP) channels. Since the proposition of Brannstrom, 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] It is not fully understood yet how such temperature changes are perceived as painful by the teeth, given that tooth afferents are generally considered to be purely nociceptive. We found evidence for expression of TRPV1, TRPM8, and TRPA1 in these neurons by measuring mRNA level through single cell reverse transcription (RT)-PCR, by immunohistochemistry, and by measuring inward currents or changes in intracellular calcium in response to the appropriate ligands or temperature changes. The results indicate that TRP channels could be responsible for the transduction of heat and cold stimuli into the sensation of pain
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