Toll-like receptor 4 signaling in trigeminal ganglion neurons contributes tongue-referred pain associated with tooth pulp inflammation

Abstract

BackgroundThe purpose of the present study is to evaluate the mechanisms underlying tongue-referred pain associated with tooth pulp inflammation.MethodUsing mechanical and temperature stimulation following dental surgery, we have demonstrated that dental inflammation and hyperalgesia correlates with increased immunohistochemical staining of neurons for TLR4 and HSP70.ResultsMechanical or heat hyperalgesia significantly enhanced in the ipsilateral tongue at 1 to 9 days after complete Freund’s adjuvant (CFA) application to the left lower molar tooth pulp compared with that of sham-treated or vehicle-applied rats. The number of fluorogold (FG)-labeled TLR4-immunoreactive (IR) cells was significantly larger in CFA-applied rats compared with sham-treated or vehicle-applied rats to the molar tooth. The number of heat shock protein (Hsp) 70-IR neurons in trigeminal ganglion (TG) was significantly increased on day 3 after CFA application compared with sham-treated or vehicle-applied rats to the molar tooth. About 9.2% of TG neurons were labeled with DiI applied to the molar tooth and FG injected into the tongue, and 15.4% of TG neurons were labeled with FG injected into the tongue and Alexa-labeled Hsp70-IR applied to the tooth. Three days after Hsp70 or lipopolysaccharide (LPS) application to the tooth in naive rats, mechanical or heat hyperalgesia was significantly enhanced compared with that of saline-applied rats. Following successive LPS-RS, an antagonist of TLR4, administration to the TG for 3 days, the enhanced mechanical or heat hyperalgesia was significantly reversed compared with that of saline-injected rats. Noxious mechanical responses of TG neurons innervating the tongue were significantly higher in CFA-applied rats compare with sham rats to the tooth. Hsp70 mRNA levels of the tooth pulp and TG were not different between CFA-applied rats and sham rats.ConclusionsThe present findings indicate that Hsp70 transported from the tooth pulp to TG neurons or expressed in TG neurons is released from TG neurons innervating inflamed tooth pulp, and is taken by TG neurons innervating the tongue, suggesting that the Hsp70-TLR4 signaling in TG plays a pivotal role in tongue-referred pain associated with tooth pulp inflammation.

Novel methods of applying direct chemical and mechanical stimulation to the oral mucosa for traditional behavioral pain assays in conscious rats

“…Therefore, it is important to understand the mechanisms that underlie chemically or mechanically induced pain in the oral mucosa. Because of the technical difficulties associated with the application of these types of stimulations to the oral mucosa of conscious animals, most intraoral pain studies have used anesthetized rats and electromyograms to measure the head withdrawal reflex following mechanical and/or thermal stimulations (Katagiri et al, 2012;Ohara et al, 2013). Because general anesthetic agents have been reported to affect peripheral nociception and central pain perception (Cornett et al, 2008;Devor and Zalkind, 2001), the results obtained from anesthetized animals are complicated by the potential effects of anesthesia.…”

Involvement of TRPV1 and TRPA1 in Incisional Intraoral and Extraoral Pain

“…ACC-030; Alomone, Jerusalem, Israel) (Shinoda et al 2011), guinea pig anti-TRPV1 polyclonal antiserum (1:1000, cat. AB5566; Millipore, Billerica, MA, USA) (Ohara et al 2013), or rabbit anti-TRPA1 polyclonal antiserum (1:1000, cat. ab58844; Abcam, Cambridge, UK) (Honda et al 2014), and then TG sections were reacted with Alexa Fluor 488-conjugated goat anti-rabbit IgG (1:200 in 0.01 M PBS; Thermo Fisher Scientific), Alexa Fluor 568-conjugated goat anti-rabbit IgG (1:200 in 0.01 M PBS; Thermo Fisher Scientific), and/ or Alexa Fluor 568-conjugated goat anti-guinea pig IgG (1:200 in 0.01 M PBS; Thermo Fisher Scientific).…”

Preclinical Evaluation of a P2X7 Receptor–Selective Radiotracer: PET Studies in a Rat Model with Local Overexpression of the Human P2X7 Receptor and in Nonhuman Primates

“…Moreover, caspase-1 activation, needed for IL-1b release, occurs within the NLRP3 inflammasome in response to an increased concentration of extracellular adenosine triphosphate and thus P2X7R activation. In turn, IL-1b is one of the proinflammatory cytokines that initiate the neuroinflammatory cascade (7,8). In addition, several animal models for neurodegenerative disorders such as Alzheimer's disease and Huntington's disease (9)(10)(11) showed that proinflammatory cytokines correlated with upregulated P2X7R expression and increased adenosine triphosphate sensitivity, putting forward P2X7R as a valuable alternative marker for microglial activation and inflammation (8,12).…”