Toll-Like Receptor 4 in the Rat Caudal Medulla Mediates Tooth Pulp Inflammatory Pain.

Helena F Filippini,Barry J Sessle,Maryam Zanjir,Siew-Ging Gong,Yamini Arudchelvan,Maria M Campos,Graziella R Molska,Limor Avivi-Arber

doi:10.3389/fnins.2020.00643

Abstract

The aims of this study were to investigate if Toll-like receptor 4 (TLR4) is expressed in the medullary dorsal horn (MDH) and if medullary application of a TLR4 antagonist (lipopolysaccharides from Rhodobacter sphaeroides, LPS-RS) can attenuate changes in nociceptive sensorimotor responses or TLR4 expression that might be evoked by mustard oil (MO) application to the right maxillary first molar tooth pulp. Of 41 adult male Sprague-Dawley rats used in the study, 23 received intrathecal application of the TLR4 antagonist LPS-RS (25 μg/10 μl; LPS-RS group) or isotonic saline (10 μl; vehicle control group) 10 min before pulpal application of MO (95%; 0.2 μl). Bilateral electromyographic (EMG) activities of the anterior digastric and masseter muscles were recorded continuously before and until 15 min after the MO application to the pulp. In 6 of these 23 rats and an additional 18 rats, the caudal medulla containing the ipsilateral and contralateral MDH was removed after euthanasia for subsequent Western Blot analysis of TLR4 expression in LPS-RS (n = 8) and vehicle (n = 8) groups and a naïve group (n = 8). The % change from baseline in the MO-evoked EMG activities within the anterior digastric muscles were significantly smaller in the LPS-RS group than the control group (two-way ANOVA, post hoc Bonferroni, P < 0.0001). Western Blot analysis revealed similar levels of TLR4 expression in the caudal medulla of the naïve, vehicle and LPS-RS groups. These novel findings suggest that TLR4 signaling in the caudal medulla may mediate MO-induced acute dental inflammatory pain in rats.

Highlights

Tooth inflammatory pain is a common and debilitating condition with negative social and economic consequences that may result in an impaired quality of life if not appropriately treated
EMG recordings in the control group revealed that mustard oil (MO) application to the right maxillary molar tooth pulp resulted, within 2 s, in an increased EMG activity that was especially apparent in the left and right anterior digastric muscles (LAD) and RAD
The present study showed that LPS from Rhodobacter sphaeroides (LPS-RS) when applied to the caudal medullary surface overlying the medullary dorsal horn (MDH) can attenuate for several minutes the jaw muscle EMG activities evoked by MO application to the tooth pulp

Summary

Introduction

Tooth inflammatory pain is a common and debilitating condition with negative social and economic consequences that may result in an impaired quality of life if not appropriately treated. Several studies have shown that the medullary dorsal horn (MDH), which is often referred to as the trigeminal subnucleus caudalis, is an important caudal medullary site related to the processing and relaying of nociceptive information from the teeth and other orofacial structures to higher brain centers (for review, see Sessle, 1986, 2000, 2014; Byers and Närhi, 1999; Iwata et al, 1999, 2011; Chiang et al, 2011; Chichorro et al, 2017). Tooth pulp application of mustard oil (MO), an inflammatory irritant and transient receptor potential ankyrin 1 (TRPA1) agonist, can markedly enhance activity of MDH nociceptive neurons accompanied by nociceptive sensorimotor responses in jaw muscles reflected in MO-induced increases in electromyographic (EMG) activities (e.g., Chiang et al, 1998, 2007, 2011; Sunakawa et al, 1999; Narita et al, 2012). The first one induces nuclear factor-kB (NF-kB) translocation and expression of inflammatory cytokines as well as type I interferon genes, whereas the TRIF pathway activates type 1 interferon genes and delayed NF-kB translocation and expression of inflammatory cytokines via interferon regulatory factor 3 (IRF-3) (Vaure and Liu, 2014; Bruno et al, 2018)

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