Masticatory myalgia to headache-like secondary hypersensitivity induces gene plasticity at dura mater

BACKGROUND: Patients with myofascial pain in the head and neck region often report widespread, referred pain or secondary hypersensitivity, including headache-like. Secondary hypersensitivity originating from masticatory myalgia may result from myalgia-induced plasticity within the central nervous system, affecting referred site sensitization. The main aims of this study were to develop animal models that mimic secondary hypersensitivity and to investigate whether stimulated myalgia induces gene expression plasticity at referred sites, which may contribute to the secondary hypersensitivity phenomenon. METHODS: A majority of experiments were conducted in male and female mice. Masticatory myalgia was assessed as mechanical hypersensitivity in the region over the masseter muscle (MM). Secondary hypersensitivity was evaluated by measuring mechanical hypersensitivity at sites anatomically distinct from the stimulated muscle: periorbital and MM areas. Stimulated myalgia was achieved by either a single high-dose collagenase type-II (10U; Col) or repeated low-dose Col (0.2–0.5U) injections into the MM or the temporal muscle (TM), repetitive gentle vibration applied over the MM, a single forceful mouth opening (FMO), or repeated FMO. Statistical analyses were one-way or two-way ANOVA followed by Bonferroni post-hoc tests. RESULTS: Stimulation of the MM, whether by single, repeated Col injections or FMO, produced inconsistent and short-lasting (1–2 days) referred pain at a periorbital area in both males and females. In contrast, stimulation of the TM using multiple paradigms reliably induced mechanical secondary hypersensitivity in two referred sites: MM and periorbital areas. MM stimulation did not exhibit sex-dependent mechanical hypersensitivity in the MM area. In contrast, TM-induced secondary hypersensitivity at both the MM and periorbital areas was sex-dependent. Secondary hypersensitivity in the MM and periorbital regions following TM stimulation was accompanied by significant gene expression plasticity in both tissues. Notably, transcriptional changes in the MM after Col injection into the TM closely resembled those observed following direct Col injection into the MM. CONCLUSIONS: The presented data suggest that secondary hypersensitivity from masticatory myalgia can be effectively modeled in mice through stimulation of the TM. Importantly, TM stimulation-induced transcriptomic changes at MM and dura mater may generate nociceptive signaling from these sites, thereby contributing to an input network underlying secondary hypersensitivity.