title: Toll-like receptor 4 signaling in trigeminal ganglion neurons contributes tongue-referred pain associated with tooth pulp inflammation.
abstract: BACKGROUND
  The purpose of the present study is to evaluate the mechanisms underlying tongue-referred pain associated with tooth pulp inflammation.
  
  
  METHODS
  Using 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.
  
  
  RESULTS
  Mechanical 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.
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: Department of Endodontics, Nihon University School of Dentistry, 1-8-13 Kandasurugadai, Chiyoda-ku, Tokyo 101-8310, Japan. shimizu.kouhei01@nihon-u.ac.jp.
authors.forename: Kinuyo
authors.initials: K
authors.lastname: Ohara
authors.forename: Kohei
authors.initials: K
authors.lastname: Shimizu
authors.forename: Shingo
authors.initials: S
authors.lastname: Matsuura
authors.forename: Bunnai
authors.initials: B
authors.lastname: Ogiso
authors.forename: Daisuke
authors.initials: D
authors.lastname: Omagari
authors.forename: Masatake
authors.initials: M
authors.lastname: Asano
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Tsuboi
authors.forename: Masamichi
authors.initials: M
authors.lastname: Shinoda
authors.forename: Koichi
authors.initials: K
authors.lastname: Iwata
mesh_terms: D000818:Animals; D003782:Dental Pulp / Q000473:pathology*; D050884:HSP72 Heat-Shock Proteins / Q000378:metabolism; D006930:Hyperalgesia / Q000378:metabolism; D007150:Immunohistochemistry; D007249:Inflammation / Q000150:complications / Q000378:metabolism / Q000473:pathology; D008297:Male; D009474:Neurons / Q000378:metabolism*; D053591:Referred Pain / Q000209:etiology / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D060888:Real-Time Polymerase Chain Reaction; D015398:Signal Transduction / Q000502:physiology; D051197:Toll-Like Receptor 4 / Q000378:metabolism*; D014059:Tongue / Q000502:physiology; D012668:Trigeminal Ganglion / Q000378:metabolism*
chemical_list: D050884:HSP72 Heat-Shock Proteins; C495347:Tlr4 protein, rat; D051197:Toll-Like Receptor 4