Involvement of Trigeminal Transition Zone and Laminated Subnucleus Caudalis in Masseter Muscle Hypersensitivity Associated with Tooth Inflammation

Abstract

A rat model of pulpitis/periapical periodontitis was used to study mechanisms underlying extraterritorial enhancement of masseter response associated with tooth inflammation. Periapical bone loss gradually increased and peaked at 6 weeks after complete Freund’s adjuvant (CFA) application to the upper molar tooth pulp (M1). On day 3, the number of Fos-immunoreactive (IR) cells was significantly larger in M1 CFA rats compared with M1 vehicle (veh) rats in the trigeminal subnucleus interpolaris/caudalis transition zone (Vi/Vc). The number of Fos-IR cells was significantly larger in M1 CFA and masseter (Mass) capsaicin applied (M1 CFA/Mass cap) rats compared with M1 veh/Mass veh rats in the contralateral Vc and Vi/Vc. The number of phosphorylated extracellular signal-regulated kinase (pERK)-IR cells was significantly larger in M1 CFA/Mass cap and M1 veh/Mass cap rats compared to Mass-vehicle applied rats with M1 vehicle or CFA in the Vi/Vc. Pulpal CFA application caused significant increase in the number of Fos-IR cells in the Vi/Vc but not Vc on week 6. The number of pERK-IR cells was significantly lager in the rats with capsaicin application to the Mass compared to Mass-vehicle treated rats after pulpal CFA- or vehicle-application. However, capsaicin application to the Mass did not further affect the number of Fos-IR cells in the Vi/Vc in pulpal CFA-applied rats. The digastric electromyographic (d-EMG) activity after Mass-capsaicin application was significantly increased on day 3 and lasted longer at 6 weeks after pulpal CFA application, and these increase and duration were significantly attenuated by i.t. PD98059, a MEK1 inhibitor. These findings suggest that Vi/Vc and Vc neuronal excitation is involved in the facilitation of extraterritorial hyperalgesia for Mass primed with periapical periodontitis or acute pulpal-inflammation. Furthermore, phosphorylation of ERK in the Vi/Vc and Vc play pivotal roles in masseter hyperalgesia after pulpitis or periapical periodontitis.

Activation of trigeminal ganglion satellite glial cells in CFA-induced tooth pulp pain in rats

“…Although this data was not statistically different, there are important variations when comparing the experimental groups to the naïve controls. Our results are somewhat contradictory to that showed by Shimizu et al (2014), describing no alterations of body weight, general behavior, and feeding in rats submitted to CFA tooth pulp inflammation, according to the evaluation at 3 days and 6 weeks [ 14 ]. However, the results presented by the authors are descriptive and no graphical data comparing the different groups was presented.…”

“…The surgical dental procedures, the behavioral assessments and the sample collection were carried between 8:00 AM and 5:00 PM. Sample size (N = 4–8 per group, depending on the evaluated parameter) was established based on previous studies [ 14 , 16 ].…”

“…The induction of tooth pulp inflammatory pain was adapted from the method described by Shimizu et al [ 14 ]. The animals were anesthetized by an i.p.…”

“…A few studies have also analyzed the effects of CFA application into the rat tooth pulp. In two of these studies, the authors investigated the mechanisms involved in the ectopic persistent pulp pain or masseter hypersensitivity, by applying CFA into the pulp of the first upper molar, and capsaicin to the pulp of the second molar or into the masseter muscle, respectively [ 13 , 14 ]. Additionally, the application of CFA into the pulp of the right first and second maxillary molars caused an up-regulation of the vesicular glutamate transporter-2 (VGLUT2) in the pulp tissue or trigeminal ganglion (TG) [ 15 ].…”

Neuron-glia interaction is a key mechanism underlying persistent orofacial pain

“…Many previous studies used complete Freund's adjuvant (CFA) to induce tooth pulp inflammation (4,8,9). Small dental paper points are soaked in a CFA emulsion (50% CFA and saline) and inserted into the tooth pulp to produce pulpal inflammation.…”