Cannabinoid actions on sensitized dural nociceptors in a non-surgical mouse model of migraine-like pain

Background Migraine is a debilitating neurological disorder affecting ~ 15% of the global population with greater prevalence in females. Cannabinoid 1 and 2 receptor (CB1R and CB2R) agonists alleviate migraine symptoms. However, central nervous system (CNS) side effects mediated by CB1Rs limit their widespread use. We developed peripherally restricted cannabinoids (PRCBs) which lack CNS side effects. Here we examine actions of a PRCB on behavioral and physiological parameters in a mouse migraine model. Methods Female 4-week-old C57BL/6J mice were acclimated to measurements of head withdrawal threshold (HWT) responses to periorbital von Frey filaments. After several days of baseline testing, mice (under brief isoflurane anesthesia) were administered pH-6.0 or pH-7.4 saline (4.5µL) supradurally via a 33-gauge cannula through the skin and connective tissue by taking advantage of unfused cranial sutures at bregma, followed 3-days later, by responses to a mild (pH-7.0 saline) supradural stimulation. The CB1R/CB2R agonist PRCB, PrNMI (5µM), was co-applied with pH-6.0 saline with or without the peripherally-restricted selective CB1R antagonist, 18A (20µM) or the selective CB2R antagonist, SR144528 (20µM). The potent selective CB2R agonist RNB61 (5µM) was also co-applied. Patch clamp recordings were obtained in retrogradely-labeled dural trigeminal ganglion (dTG) neurons acutely isolated from mice treated 3-days earlier with supradural pH-7.4 or pH-6.0 saline and 4% fluorogold. Results Supradural pH-6.0 (but not pH-7.4) treatment produced allodynia symptoms which recovered by 48-hrs but were renewed by pH-7.0 supradural treatment. Supradural co-administration of pH-6.0/PrNMI prevented both the initial and latent allodynia symptoms. Co-administration of pH-6.0/PrNMI with 18A, or SR144528, abolished the preventative effects of PrNMI. Co-administration of pH-6.0/RNB61 did not prevent the initial allodynia or latent sensitization. Co-administration of pH-7.0/PrNMI did not prevent latent hypersensitivity. All dTGs had Aδ- or C-type nociceptor characteristics. Acid-induced depolarizations were enhanced in dTGs from pH-6.0-treated mice. PrNMI (1µM) decreased acid-induced depolarizations in dTGs from pH-7.4- (but not pH-6.0)-treated mice. RNB61 (0.5µM) had no effect on acid-induced depolarizations in dTGs from either group. Conclusions These findings indicate that PrNMI prevents acid-induced sensitization and allodynia by activating peripheral neuronal CB1Rs and non-neuronal CB2Rs. CB2R activation alone is insufficient, and once sensitized, combined CB1R/CB2R activation loses efficacy.