title: Serotonin, glutamate and glycerol are released after the injection of hypertonic saline into human masseter muscles - a microdialysis study.
abstract: BACKGROUND
  Chronic myalgia is associated with higher muscle levels of certain algesic biomarkers. The aim of this study was to investigate if hypertonic saline-induced jaw myalgia also leads to release of such biomarkers and if there were any sex differences in this respect.
  
  
  METHODS
  Healthy participants, 15 men and 15 aged-matched women (25.7 ± 4.3 years) participated. Intramuscular microdialysis into masseter muscles was performed to sample serotonin (5-HT), glutamate, lactate, pyruvate, glucose and glycerol. After 2 hours 0.2 mL hypertonic saline (58.5 mg/mL) was injected into the masseter on one side and 0.2 mL isotonic saline (9 mg/mL) into the contralateral masseter close to the microdialysis catheter. Microdialysis continued for 1 hour after the injections. Pressure pain thresholds (PPT) and pain were assessed before and after injections.
  
  
  RESULTS
  The median (IQR) peak pain intensity (0-100 visual analogue scale) after hypertonic saline was 52.5 (38.0) and after isotonic saline 7.5 (24.0) (p < 0.05). 5-HT, glutamate and glycerol increased after hypertonic saline injection (p < 0.05). Lactate, pyruvate and glucose showed no change. PPT after microdialysis was reduced on both sides (p < 0.05) but without side differences. Pain after hypertonic saline injection correlated positively to 5-HT (p < 0.05) and negatively to glycerol (p < 0.05).
  
  
  CONCLUSIONS
  5-HT, glutamate and glycerol increased after a painful hypertonic saline injection into the masseter muscle, but without sex differences. Since increased levels of 5-HT and glutamate have been reported in chronic myalgia, this strengthens the validity of the pain model. Glycerol warrants further investigations.
authors.affiliation: Orofacial Pain and Jaw Function, Department of Dental Medicine, Karolinska Institutet, Huddinge, Sweden. sofia.louca@ki.se.
authors.forename: Sofia
authors.initials: S
authors.lastname: Louca
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Christidis
authors.forename: Bijar
authors.initials: B
authors.lastname: Ghafouri
authors.forename: Björn
authors.initials: B
authors.lastname: Gerdle
authors.forename: Peter
authors.initials: P
authors.lastname: Svensson
authors.forename: Thomas
authors.initials: T
authors.lastname: List
authors.forename: Malin
authors.initials: M
authors.lastname: Ernberg
mesh_terms: D000328:Adult; D005260:Female; D018698:Glutamic Acid / Q000378:metabolism*; D005990:Glycerol / Q000378:metabolism*; D006801:Humans; D007273:Injections, Intramuscular; D008297:Male; D008406:Masseter Muscle / Q000187:drug effects / Q000378:metabolism*; D017551:Microdialysis; D063806:Myalgia / Q000139:chemically induced / Q000378:metabolism*; D010147:Pain Measurement / Q000379:methods; D017288:Pain Threshold / Q000502:physiology; D012462:Saline Solution, Hypertonic / Q000008:administration & dosage*; D012701:Serotonin / Q000378:metabolism*; D012727:Sex Characteristics; D055815:Young Adult
chemical_list: D012462:Saline Solution, Hypertonic; D012701:Serotonin; D018698:Glutamic Acid; D005990:Glycerol