title: Expression of 5-HT3 receptors and TTX resistant sodium channels (Na(V)1.8) on muscle nerve fibers in pain-free humans and patients with chronic myofascial temporomandibular disorders.
abstract: BACKGROUND
  Previous studies have shown that 5-HT3-antagonists reduce muscle pain, but there are no studies that have investigated the expression of 5-HT3-receptors in human muscles. Also, tetrodotoxin resistant voltage gated sodium-channels (NaV) are involved in peripheral sensitization and found in trigeminal ganglion neurons innervating the rat masseter muscle. This study aimed to investigate the frequency of nerve fibers that express 5-HT3A-receptors alone and in combination with NaV1.8 sodium-channels in human muscles and to compare it between healthy pain-free men and women, the pain-free masseter and tibialis anterior muscles, and patients with myofascial temporomandibular disorders (TMD) and pain-free controls.
  
  
  METHODS
  Three microbiopsies were obtained from the most bulky part of the tibialis and masseter muscles of seven and six healthy men and seven and six age-matched healthy women, respectively, while traditional open biopsies were obtained from the most painful spot of the masseter of five female patients and from a similar region of the masseter muscle of five healthy, age-matched women. The biopsies were processed by routine immunohistochemical methods. The biopsy sections were incubated with monoclonal antibodies against the specific axonal marker PGP 9.5, and polyclonal antibodies against the 5-HT3A-receptors and NaV1.8 sodium-channels.
  
  
  RESULTS
  A similar percentage of nerve fibers in the healthy masseter (85.2%) and tibialis (88.7%) muscles expressed 5-HT3A-receptors. The expression of NaV1.8 by 5-HT3A positive nerve fibers associated with connective tissue was significantly higher than nerve fibers associated with myocytes (P < .001). In the patients, significantly more fibers per section were found with an average of 3.8 ± 3 fibers per section in the masseter muscle compared to 2.7 ± 0.2 in the healthy controls (P = .024). Further, the frequency of nerve fibers that co-expressed NaV1.8 and 5-HT3A receptors was significantly higher in patients (42.6%) compared to healthy controls (12.0%) (P < .001).
  
  
  CONCLUSIONS
  This study showed that the 5-HT3A-receptor is highly expressed in human masseter and tibialis muscles and that there are more nerve fibers that express 5-HT3A-receptors in the masseter of women with myofascial TMD compared to healthy women. These findings indicate that 5-HT3-receptors might be up-regulated in myofascial TMD and could serve as potential biomarkers of chronic muscle pain.
authors.affiliation: Orofacial Pain and Jaw Function, Department of Dental Medicine, Karolinska Institutet, Huddinge, Sweden. nikolaos.christidis@ki.se.
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Christidis
authors.forename: Isabell
authors.initials: I
authors.lastname: Kang
authors.forename: Brian E
authors.initials: BE
authors.lastname: Cairns
authors.forename: Ujendra
authors.initials: U
authors.lastname: Kumar
authors.forename: Xudong
authors.initials: X
authors.lastname: Dong
authors.forename: Annika
authors.initials: A
authors.lastname: Rosén
authors.forename: Sigvard
authors.initials: S
authors.lastname: Kopp
authors.forename: Malin
authors.initials: M
authors.lastname: Ernberg
mesh_terms: D000328:Adult; D002908:Chronic Disease; D005260:Female; D006801:Humans; D008297:Male; D008406:Masseter Muscle / Q000378:metabolism; D018482:Muscle, Skeletal / Q000378:metabolism*; D062559:NAV1.8 Voltage-Gated Sodium Channel / Q000378:metabolism*; D009412:Nerve Fibers / Q000378:metabolism*; D044406:Receptors, Serotonin, 5-HT3 / Q000378:metabolism*; D026941:Sodium Channel Blockers / Q000494:pharmacology; D013705:Temporomandibular Joint Disorders / Q000378:metabolism*; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D062559:NAV1.8 Voltage-Gated Sodium Channel; D044406:Receptors, Serotonin, 5-HT3; C568317:SCN10A protein, human; D026941:Sodium Channel Blockers; D013779:Tetrodotoxin