Phosphorylation of p38 in Trigeminal Ganglion Neurons Contributes to Tongue Heat Hypersensitivity in Mice.

Aims: To develop a tongue pain model with no mucosal pathologic changes and to examine whether phosphorylation of p38 in trigeminal ganglion (TG) neurons innervating the tongue is associated with tongue heat hypersensitivity in mice. Methods: Tongue heat sensitivity in mice was assessed following application of the irritant 2,4,6-trinitrobenzene sulfonic acid (TNBS) to the tongue. After TNBS application, the expressions of p38, phosphorylated p38 (pp38), and transient receptor potential vanilloid 1 (TRPV1) were examined in TG neurons innervating the tongue. To further assess changes in tongue heat sensitivity and TRPV1 expression, a specific inhibitor of p38 phosphorylation (SB203580) was also administered into the TG. Student t test or two-way repeated-measures analysis of variance followed by Sidak multiple comparison test were used for statistical analysis, and P <.05 was considered statistically significant. Results: TNBS application to the tongue induced noninflammatory heat hypersensitivity accompanied by the enhancement of p38 phosphorylation in TG neurons innervating the tongue and by an increase in the number of TRPV1 and pp38-immunoreactive (IR) TG neurons innervating the tongue. Intra-TG administration of SB203580 suppressed the increase in the TRPV1 and pp38-IR TG neurons and alleviated the noninflammatory tongue heat hypersensitivity induced by TNBS. Conclusion: p38 signaling cascades are involved in tongue heat hyperalgesia in association with TRPV1 upregulation in TG neurons innervating the TNBS-treated tongue.