Differences in Neuropathology Between Nitroglycerin-Induced Mouse Models of Episodic and Chronic Migraine

Abstract

Abstract Background Multiple animal models of migraine have been used in the development of new therapies. The transformation from episodic migraine (EM) to chronic migraine (CM) is not fully understood but warrants differentiation. We established mouse models mimicking EM and CM pain, then assessed neuropathological differences between the two types of migraine. Methods EM and CM models were generated with either a single injection of 10 mg/kg nitroglycerin (NTG) or five injections over 9 days, respectively. Saline with the same amount of propylene glycol was used as the vehicle control (VEH). Mechanical hypersensitivity was assessed using the von Frey filament test. Immunofluorescence analyses were performed using c-Fos, NeuN, and Iba1 as markers. Proinflammatory (TNF-α, IL-1β, and IL-6) and anti-inflammatory (IL-4 and IL-10) markers were analyzed. Neuropeptides (CGRP, VIP, PACAP, and substance P) were assessed. Results Mechanical thresholds were similar in NTG-injected EM and CM models. Notable neuropathological distinctions were observed in the spinal trigeminal nucleus caudalis (Sp5C) and anterior cingulate cortex (ACC). The ACC showed increased levels of c-Fos and NeuN expression in CM (p < 0.001), but these levels were unchanged in EM. Conversely, the Sp5C showed higher c-Fos and NeuN expression in EM (p < 0.001) than in CM. Iba1 expression was increased in the Sp5C of EM mice and ACC of CM mice (p < 0.001). Proinflammatory markers, such as TNF-α, IL-1β, and IL-6, were strongly expressed in the Sp5C of EM mice and ACC of CM mice. Neuropeptide analysis revealed that CGRP expression was elevated in both the Sp5C and ACC of both models compared with the VEH group; expression was higher in the CM model. VIP exhibited higher levels in the Sp5C of EM mice and ACC of CM mice, whereas PACAP and substance P were predominantly expressed in the Sp5C in both EM and CM models. Conclusions Although mechanical thresholds were similar, distinctive neuropathological differences were observed in the Sp5C and ACC regions between EM and CM models. These findings suggest that these differences contribute to the transformation from EM to CM.