Cannabinoid actions in a nonsurgical mouse model of sensitized dural nociceptors

Abstract Cannabinoid 1 and 2 receptor (CB1R and CB2R) agonists alleviate inflammatory and neuropathic pain symptoms. However, central nervous system (CNS) side effects mediated by CB1Rs limit their widespread use. We developed peripherally restricted cannabinoids (PRCBs), which exhibit analgesic efficacy with minimal CNS side effects. Here, we examined PRCB actions in a mouse model of dural acid-induced hypersensitivity. Supradural injections of pH-6.0 (but not pH-7.4) saline (4.5 µL) in five-week-old female C57BL/6J mice induced periorbital allodynia (assessed with von Frey filaments) with recovery by 48 hours. By 72 hours, allodynia was elicited with a supradural pH-7.0 saline injection in pH-6.0 (but not pH-7.4) mice. The CB1R/CB2R agonist PRCB, 4-{2-[(1E)-1-[(4-propylnaphthalen-1-yl) methylidene]-1H-inden-3-yl]ethyl}morpholine (PrNMI) (5 µM) or the selective CB1R agonist arachidonyl-2′-chloroethylamide (ACEA, 5 µM), coapplied with pH-6.0 saline, prevented both the initial and latent allodynia, but coadministering pH 6.0 with the CB2R-selective agonist RNB61 (5 µM) did not. Coadministering pH-6.0/PrNMI with CB1R- or CB2R-selective inverse agonists abolished the preventative effects of PrNMI. Supradural pH-6.0 saline-evoked allodynia followed 72 hours later by coadministration of pH 7.0/PrNMI did not prevent latent allodynia. Patch recordings obtained from dural trigeminal ganglion neurons (dTGNs) acutely isolated from FluoroGold/pH-6.0 (but not pH-7.4)–injected mice had larger acid-induced depolarizations. All dTGNs had electrophysiological properties of Aδ- or C-type nociceptors. PrNMI (1 µM) suppressed acid-induced depolarizations in dTGNs from pH-7.4 (but not pH-6.0)–treated mice, whereas RNB61 (0.5 µM) had no effect. These findings indicate that PrNMI prevents acid-induced sensitization by coactivating neuronal CB1Rs and non-neuronal CB2Rs. Cannabinoid 2 receptor activation alone is insufficient, and once sensitized, combined CB1R/CB2R activation loses efficacy.