The serotonin receptor type 3 in chronic and experimental human muscle pain

Abstract: The main aim of this thesis was to gain a better understanding of the influence of serotonin on chronic craniofacial muscle pain. The thesis comprised a basic science part and a human experimental part. The aim for the basic science part was to investigate if the expression of 5-HT3 receptors in muscle tissue differ between myalgic and healthy muscles and if sex or muscle site influence this expression. The aim for the human experimental part was to investigate if the specific 5-HT3 antagonist granisetron influences pain variables in patients with chronic craniofacial myalgia and experimental pain induced by hypertonic saline, as well as the mechanical pain threshold of healthy muscles. A second aim was to investigate if any effect by granisetron was influenced by sex, muscle site or route of administration, i.e. local or systemic. In the basic science part, patients with chronic local myalgia of the masseter muscle and age- and sexmatched healthy controls were examined clinically according to the Research Diagnostic Criteria for Temporomandibular Disorders. Traditional biopsies were taken intra-orally with a 4-mm Bergström needle from the most painful part of the masseter muscle in patients, and from a standardized point in the healthy controls. In order to compare if there were any differences in receptor expression due to sex or between muscles a microbiopsy technique was used to obtain masseter muscle and tibialis anterior muscle biopsies in healthy age-matched men and women. The patients were found to have a significantly higher amount of co-expressed 5-HT3A receptors and NaV1.8 sodium channels in the connective tissue compared to their healthy controls. This novel finding makes it presumable that in myalgic muscles proliferation of nociceptive nerve fibers occurs in the connective tissue surrounding the myocytes. Further, in healthy subjects, the co-expression of 5-HT3A receptors and NaV1.8 sodium channels in the masseter muscle was higher in women than in men. The tibialis anterior muscle expressed more 5-HT3A receptors than the masseter muscle. However, since these fibers did not co-express NaV1.8 sodium channels extensively, they are presumed being motor neurons. The first experimental part, investigating the effect of granisetron on the mechanical pain threshold, comprised both healthy men and age-matched women. Granisetron was administrated in a randomized, placebo-controlled and double-blind manner both systemically and locally by intramuscular injection. Sugar pills and isotonic saline served as placebo. The pressure pain threshold (PPT) was examined over the masseter, temporalis anterior, trapezius and tibialis anterior muscles before and after drug administration. The PPT of the masseter muscle increased significantly after local administration of granisetron and for all muscles combined after systemic administration. However, the increase of PPT was only significant over the trapezius and tibialis anterior muscles. Further, the PPT at baseline was significantly higher in the men and the increase of PPT after administration of granisetron was also only significant in the men. This blockage of the 5-HT3 receptors with granisetron seems to inhibit serotonin to excite and sensitize nociceptors, leading to a reduced sensitivity to mechanical stimuli. The second experimental part used a randomized, placebo-controlled and double-blind methodology to investigate how granisetron influences masseter muscle pain variables both in patients with chronic local myalgia, and in an experimental pain situation including sex- and age-matched healthy individuals. In the experimental pain situation injections with hypertonic saline were used to induce pain, followed by injections of granisetron or placebo as pre-treatment before a second pain induction with hypertonic saline. Granisetron significantly reduced both clinical and experimentally induced pain variables. The painful area was greater in the healthy women compared to the men before pre-treatment with granisetron. However, after pre-treatment with granisetron there were no significant sex differences in any pain variable. These findings indicate that 5-HT3 receptors play a significant role in pain transmission and pain modulation in human muscles, since granisetron diminishes experimentally hypertonic saline-induced pain and chronic myalgia of the masseter muscle. Conclusively, our novel findings that the 5-HT3 receptor is up-regulated in painful craniofacial muscles and that blocking of this receptor decreases clinical and experimental human muscle pain indicate that the 5-HT3 receptor has an important role in peripheral pain transmission in localized chronic muscle pain.