Role of transient receptor potential vanilloid type 1 in the trigeminal ganglion and brain stem following dental pulp inflammation.

Aim: To verify whether experimentally induced pulpitis activates the expression of transient receptor potential vanilloid type 1 (TRPV1) and c-Fos, both peripherally and centrally.

Methodology: Acute pulpitis was induced in Sprague-Dawley rats via pulp exposure and application of complete Freund's adjuvant (CFA; n = 13). Saline-treated (n = 13) rats and rats that did not undergo tooth preparation (n = 13) served as control groups. Three days post-CFA or post-saline application, face grooming activity was recorded, and the rats were then euthanized to allow for immunohistochemical analysis of the trigeminal ganglion (TG) and spinal trigeminal nucleus. anova with Student's t-test for post-hoc analysis was used to quantify the differences in behavioural tests and immunohistochemical labelling (c-Fos and TRPV1) in TG amongst groups. Kruskal-Wallis test with Dunnett's test for post-hoc analysis was used to compare immunohistochemical labelling (c-Fos and TRPV1) in the brainstem amongst groups.

Results: Histological evidence of severe pulp inflammation was found, and there was a significant increase in pain-like behaviour (P < 0.05) in CFA-treated animals. C-Fos labelling and TRPV1 immunoreactivity in the TG were significantly higher (both P < 0.05) in the CFA group than in the control groups. In the spinal trigeminal nucleus, the immunoreactivity for c-Fos was absent in the intermediate region (trigeminal subnucleus interpolaris) in all animals, with comparable expression of TRPV1 amongst all groups. In contrast, neurons in the trigeminal subnucleus caudalis (TSC) exhibited significant c-Fos immunoreactivity in the CFA group (P = 0.0063). The expression of TRPV1 did not differ amongst the three groups, but the superficial laminae of the TSC exhibited significantly greater expression of TRPV1 than did the deep layers (P = 0.0014).

Conclusions: Following acute pulp inflammation, the TRPV1 channel was significantly involved in nociceptive signal processing in the peripheral nervous system, but not in the CNS. Because pulpitis induced some neuronal activation at the brainstem levels, further studies are needed to identify additional transducers that mediate signal transmission from pulpal afferents to their central targets.