title: Intramuscular temperature modulates glutamate-evoked masseter muscle pain intensity in humans.
abstract: OBJECTIVE
  To determine whether glutamate-evoked jaw muscle pain is altered by the temperature of the solution injected.
  
  
  METHODS
  Sixteen healthy volunteers participated and received injections of hot (48°C), neutral (36°C), or cold (3°C) solutions (0.5 mL) of glutamate or isotonic saline into the masseter muscle. Pain intensity was assessed with an electronic visual analog scale (eVAS). Numeric rating scale (NRS) scores of unpleasantness and temperature perception, pain-drawing areas, and pressure pain thresholds (PPTs) were also measured. Participants filled out the McGill Pain Questionnaire (MPQ). Two-way or three-way repeated measures ANOVA were used for data analyses.
  
  
  RESULTS
  Injection of hot glutamate and cold glutamate solutions significantly increased and decreased, respectively, the peak pain intensity compared with injection of neutral glutamate solution. The duration of glutamate-evoked pain was significantly longer when hot glutamate was injected than when cold glutamate was injected. No significant effect of temperature on pain intensity was observed when isotonic saline was injected. No effect of solution temperature was detected on unpleasantness, heat perception, cold perception, area of pain drawings, or PPTs. There was a significantly greater use of the "numb" term in the MPQ to describe the injection of cold solutions compared to the injection of both neutral and hot solutions.
  
  
  CONCLUSIONS
  Glutamate-evoked jaw muscle pain was significantly altered by the temperature of the injection solution. Although temperature perception in the jaw muscle is poor, pain intensity is increased when the muscle tissue temperature is elevated.
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Sato
authors.forename: Eduardo E
authors.initials: EE
authors.lastname: Castrillon
authors.forename: Brian E
authors.initials: BE
authors.lastname: Cairns
authors.forename: Karina H
authors.initials: KH
authors.lastname: Bendixen
authors.forename: Kelun
authors.initials: K
authors.lastname: Wang
authors.forename: Taneaki
authors.initials: T
authors.lastname: Nakagawa
authors.forename: Koichi
authors.initials: K
authors.lastname: Wajima
authors.forename: Peter
authors.initials: P
authors.lastname: Svensson
mesh_terms: D000328:Adult; D001831:Body Temperature / Q000502:physiology; D003080:Cold Temperature; D005260:Female; D018698:Glutamic Acid / Q000008:administration & dosage / Q000494:pharmacology*; D006358:Hot Temperature; D006801:Humans; D006987:Hypesthesia / Q000503:physiopathology; D007273:Injections, Intramuscular; D007552:Isotonic Solutions; D008297:Male; D008406:Masseter Muscle / Q000187:drug effects*; D063806:Myalgia / Q000139:chemically induced* / Q000503:physiopathology; D018377:Neurotransmitter Agents / Q000008:administration & dosage / Q000494:pharmacology*; D010147:Pain Measurement / Q000379:methods; D017288:Pain Threshold / Q000502:physiology; D011312:Pressure; D012965:Sodium Chloride; D013697:Thermosensing / Q000502:physiology; D055815:Young Adult
chemical_list: D007552:Isotonic Solutions; D018377:Neurotransmitter Agents; D018698:Glutamic Acid; D012965:Sodium Chloride