title: Intramuscular pH modulates glutamate-evoked masseter muscle pain magnitude in humans.
abstract: BACKGROUND
  This study was conducted to determine whether glutamate-evoked jaw muscle pain is modulated by the acidity and temperature of the solution injected.
  
  
  METHODS
  Thirty two participants participated and received injections of high-temperature acidic (HT-A) glutamate (pH 4.8, 48 °C), high-temperature neutral (HT-N) glutamate (pH 7.0, 48 °C) and neutral temperature neutral (NT-N) glutamate (pH 7.0, 38 °C) solutions (0.5 mL) into the masseter muscle. Pain intensity was assessed with an electronic visual analogue scale (eVAS). Numerical rating scale (NRS) scores of unpleasantness and temperature perception, pain-drawing areas, mechanical sensitivity and pressure pain thresholds (PPT) were also measured. Participants filled out the McGill Pain Questionnaire (MPQ). One or two way ANOVAs were used for data analyses.
  
  
  RESULTS
  Injection of HT-A glutamate solutions significantly increased the area under the VAS-time curve compared with injection of HT-N glutamate and NT-N glutamate solution (p < 0.040). The duration of glutamate-evoked pain was significantly longer when HT-A glutamate was injected than when NT-N glutamate was injected (p < 0.017). No significant effects of acidity were detected on pain drawings, NRS unpleasantness and heat perception, but there was a significant effect of acidity on MPQ scores and mechanical sensitivity.
  
  
  CONCLUSIONS
  Acidity and temperature modulate glutamate-evoked jaw muscle pain suggesting an interaction between acid sensing and glutamate receptors which could be of importance for understanding clinical muscle pain conditions.
authors: Department of Dentistry & Oral Surgery, School of Medicine, Keio University, Tokyo, Japan.|Department of Dentistry & Oral Surgery, Kawasaki Municipal Kawasaki Hospital, Japan.
authors: H
authors: H
authors: Sato
authors: Department of Clinical Oral Physiology, School of Dentistry, Aarhus University, Denmark.|Scandinavian Center for Orofacial Neuroscience (SCON), Huddinge, Sweden.
authors: E E
authors: EE
authors: Castrillon
authors: Faculty of Pharmaceutical Sciences, The University of British Columbia, Vancouver, Canada.|Center for Sensory-Motor Interaction, The Faculty of Medicine, Department of Health Science and Technology, Aalborg University, Denmark.
authors: B E
authors: BE
authors: Cairns
authors: Department of Clinical Oral Physiology, School of Dentistry, Aarhus University, Denmark.|Scandinavian Center for Orofacial Neuroscience (SCON), Huddinge, Sweden.
authors: K H
authors: KH
authors: Bendixen
authors: Center for Sensory-Motor Interaction, The Faculty of Medicine, Department of Health Science and Technology, Aalborg University, Denmark.
authors: K
authors: K
authors: Wang
authors: Department of Dentistry & Oral Surgery, School of Medicine, Keio University, Tokyo, Japan.
authors: T
authors: T
authors: Nakagawa
authors: Department of Dentistry & Oral Surgery, School of Medicine, Keio University, Tokyo, Japan.
authors: K
authors: K
authors: Wajima
authors: Department of Clinical Oral Physiology, School of Dentistry, Aarhus University, Denmark.|Scandinavian Center for Orofacial Neuroscience (SCON), Huddinge, Sweden.|Department of Dental Medicine, Karolinska Institute, Huddinge, Sweden.
authors: P
authors: P
authors: Svensson
mesh_terms: D000328:Adult; D005260:Female; D018698:Glutamic Acid / Q000494:pharmacology*; D006358:Hot Temperature; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007273:Injections, Intramuscular; D008297:Male; D008406:Masseter Muscle / Q000187:drug effects*; D063806:Myalgia / Q000139:chemically induced*; D058748:Pain Perception / Q000187:drug effects*
chemical_list: D018698:Glutamic Acid