Masseter corticomotor excitability is decreased after intramuscular administration of nerve growth factor

Background Quantification of motor-evoked potentials (MEPs) can contribute to better elucidate the central modulation of motor pathways in response to nociceptive inputs. The primary aim of this study was to assess the modulatory effects of nerve growth factor (NGF) injection on masseter corticomotor excitability. Methods The healthy participants of this randomized, double blind placebo-controlled experiment were assigned to have injected into the right masseter muscle either NGF (n = 25) or isotonic saline (IS, n = 17). The following variables were assessed at baseline and 48 hr after the injection: right masseter MEP amplitude and corticomotor mapping and clinical assessment of jaw pain intensity and function. Repeated Measures ANOVA was applied to the data. Results NGF caused jaw pain and increased jaw functional disability after the injection (p < 0.050). Also, the participants in the NGF group decreased the MEP amplitude (p < 0.001) but the IS group did not present any significant modulation after the injection (p > 0.050). Likewise, the participants in the NGF group reduced corticomotor map area and volume (p < 0.001), but the IS group did not show any significant corticomotor mapping changes after the injection (p > 0.050). Finally, there was a significant correlation between the magnitude of decreased corticomotor excitability and jaw pain intensity on chewing 48 hr after the NGF injection (r = -0.51, p = 0.009). Conclusion NGF-induced masseter muscle soreness can significantly reduce jaw muscle corticomotor excitability, which in turn is associated with lower jaw pain intensity and substantiates the occurrence of central changes that most likely aim to protect the musculoskeletal orofacial structures. Significance Intramuscular administration of nerve growth factor into masseter muscle causes inhibitory corticomotor plasticity, which likely occurs to prevent further damage and seems associated with lower pain intensity on function.