Nerve Growth Factor-Evoked Masseter Muscle Sensitization and Perturbation of Jaw Motor Function in Healthy Women

Aim: To replicate and extend previous findings of nerve growth factor (NGF)-induced mechanical sensitization in healthy young men to women and test for associations between mechanical sensitization and oral motor function. Combined these data would indicate if injection of NGF into the masseter muscle is a valid model of muscle pain related to temporomandibular disorders (TMD). Methods: A double-blind, placebo-controlled study was conducted on 14 healthy women. Each subject received an injection of NGF (5 µg in 0.2 mL) into 1 masseter muscle and buffered isotonic saline (control, 0.2 mL) into the other. Pressure pain thresholds (PPT) and pressure pain tolerance (PPTOL) as well as self-assessed pain intensity (numeric rating scale of 1 to 10) with the jaw at rest and in relation to various motor activities (chewing, yawning, talking, swallowing, drinking, and smiling) were recorded prior to and 3 hours, 1 day, 7 days, 14 days, and 21 days postinjection. ANOVAs were used to test data. Results: It was found that NGF significantly reduced PPT and PPTOL 3 hours, 1 and 7 days postinjection (P .001). Numerical rating scale (NRS) scores during chewing and yawning were significantly increased 3 hours and 1 day following NGF injection (P .001). After 3 hours, there were significant correlations between relative changes in PPTs and NRS scores during chewing (r = -0.556; P = .037), between relative changes in PPTOL and NRS scores during yawning (r = -0.607; P = .020), and between relative changes in PPTOL and maximum unassisted jaw-opening capacity (r = 0.868; P .001). Conclusion: This study shows that injection of NGF into the masseter muscle of women causes local signs of mechanical allodynia and hyperalgesia that persist for at least 7 days as well as pain during strenuous jaw movement. Taking the authors' previous results on NGF effects in men into consideration, these findings lend additional support to the suggestion that this model may serve as a proxy of some of the clinical features of TMD-related muscle pain. Keywords: allodynia, jaw motor function, nerve growth factor, pain measurement, TMD pain