Muscle and Brain Changes of Calcitonin Gene-Related Peptide in Experimentally Induced Unilateral Rat Masseter Myositis

Aims: To investigate changes in calcitonin gene-related peptide (CGRP)–like immunoreactivity (CGRP-LI) in the rat masseter muscle and brain after the unilateral experimental induction of masseter myositis. Methods: Ipsilateral and contralateral changes of the CGRP were examined in rat masseter muscle after the induction of unilateral myositis on the right side with an intramus-cular injection of 0.01 mL Freund’s adjuvant. The left masseter, and left and right masseters of control rats, were injected with 0.01 mL saline (0.9%). After 21 days, tissue samples from the masseter muscles and the hypothalamic-pituitary-adrenal (HPA) axis were analyzed for the presence of CGRP by immunohisto-chemistry, radioactive immunoassay, and high performance liquid chromatography. Hematoxylin-eosin staining was used to confirm inflammation in the masseter muscles. Results: Elevated CGRP-LI was detected bilaterally in the masseter muscles (P ＜ .001) in the myositis group. CGRP-immunoreactive nerve fibers were mainly detected in close proximity to muscle cells and in the walls of the blood vessels. Compared to the control rats, a significant differ-ence in scratching behavior was seen in the myositis group from day 9 until day 21. In the myositis group, CGRP-LI was increased in the pituitary gland concomitant with the increase in CGRP-LI in the masseter muscles but was decreased in the hypothalamus. A possible explanation for these changes could be that rats with chronic myositis develop an abnormal function of the HPA axis triggered by masseter muscle inflammation. Conclusion: The results of this study demonstrate that CGRP may play an impor-tant role both peripherally and centrally in masseter muscle myosi-tis in association with presumed nociceptive behavior.

calcitonin gene-related peptide; Freund’s adjuvant; inflammation; masseter muscles; myositis; neurogenic inflammation; neuropeptides; scratching

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