Toll-like receptor 4 signaling in neurons of trigeminal ganglion contributes to nociception induced by acute pulpitis in rats.

Jia-Ji Lin,Wen-Ke Cai,Fang Kuang,Zhu-Yi Li,Rong Kuang,Zhuo Zhang,Chao Sun,Yi Du,Ting Chang,Yong-Xiang Yang

doi:10.1038/srep12549

Abstract

Pain caused by acute pulpitis (AP) is a common symptom in clinical settings. However, its underlying mechanisms have largely remained unknown. Using AP model, we demonstrated that dental injury caused severe pulp inflammation with up-regulated serum IL-1β. Assessment from head-withdrawal reflex thresholds (HWTs) and open-field test demonstrated nociceptive response at 1 day post injury. A consistent up-regulation of Toll-like receptor 4 (TLR4) in the trigeminal ganglion (TG) ipsilateral to the injured pulp was found; and downstream signaling components of TLR4, including MyD88, TRIF and NF-κB, and cytokines such as TNF-α and IL-1β, were also increased. Retrograde labeling indicated that most TLR4 positve neuron in the TG innnervated the pulp and TLR4 immunoreactivity was mainly in the medium and small neurons. Double labeling showed that the TLR4 expressing neurons in the ipsilateral TG were TRPV1 and CGRP positive, but IB4 negative. Furthermore, blocking TLR4 by eritoran (TLR4 antagonist) in TGs of the AP model significantly down-regulated MyD88, TRIF, NF-κB, TNF-α and IL-1β production and behavior of nociceptive response. Our findings suggest that TLR4 signaling in TG cells, particularly the peptidergic TRPV1 neurons, plays a key role in AP-induced nociception, and indicate that TLR4 signaling could be a potential therapeutic target for orofacial pain.

Highlights

Deregulation of cytokines and/or chemokines in neuronal ganglia (such as tumor necrosis factor α (TNF-α ) and interleukin (IL) 1β ) triggers a cascade of events that includes the release of prostanoids and neuropeptides, and induces a change in the properties of neurons and ion channels, leading to production of other cytokines/chemokines and recruitment of macrophages[7,8,9,10]
There is no significant difference between the right and left Trigeminal ganglion (TG) of the SHAM and acute pulpitis (AP)-3 groups
There is no significant difference between the right and left TG of the SHAM group; the pixels of the blots are normalized by the left TG of the SHAM group for comparison

Summary

Introduction

Deregulation of cytokines and/or chemokines in neuronal ganglia (such as tumor necrosis factor α (TNF-α ) and interleukin (IL) 1β ) triggers a cascade of events that includes the release of prostanoids and neuropeptides, and induces a change in the properties of neurons and ion channels, leading to production of other cytokines/chemokines and recruitment of macrophages[7,8,9,10]. TLR4 is widely expressed in the glial cells and primary sensory neurons to sense exogenous pathogen-associated molecular patterns (PAMPs) and endogenous danger-associated molecular patterns (DAMPs) released by tissues after injury or cellular stress[11,12]. This is well documented in inflammatory hypernociception (stimulated by complete Freund’s adjuvant, CFA), neuropathic pain (caused by spared nerve injury) and other pain related models[13,14]. The specific objectives were to: (1) establish an AP model in the rat and verify the progression of AP by pulp histology and serum cytokine detection; (2) evaluate the behavior of nociceptive response by head-withdrawal reflex thresholds (HWTs) measurement and open-field test; (3) explore the expression and signaling of TLR4 in the pulp and TGs in the AP models; and (4) observe the rescue effect of eritoran, an antagonist of TLR4, on the nociceptive response

Methods

Results

Conclusion