Expression of 5-HT(3) 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

BACKGROUND: Previous studies have shown that 5-HT(3)-antagonists reduce muscle pain, but there are no studies that have investigated the expression of 5-HT(3)-receptors in human muscles. Also, tetrodotoxin resistant voltage gated sodium-channels (Na(V)) 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-HT(3A)-receptors alone and in combination with Na(V)1.8 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-HT(3A)-receptors and Na(V)1.8 sodium-channels. RESULTS: A similar percentage of nerve fibers in the healthy masseter (85.2%) and tibialis (88.7%) muscles expressed 5-HT(3A)-receptors. The expression of Na(V)1.8 by 5-HT(3A) 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 Na(V)1.8 and 5-HT(3A) receptors was significantly higher in patients (42.6%) compared to healthy controls (12.0%) (P < .001). CONCLUSIONS: This study showed that the 5-HT(3A)-receptor is highly expressed in human masseter and tibialis muscles and that there are more nerve fibers that express 5-HT(3A)-receptors in the masseter of women with myofascial TMD compared to healthy women. These findings indicate that 5-HT(3)-receptors might be up-regulated in myofascial TMD and could serve as potential biomarkers of chronic muscle pain.