Acetaminophen attenuates pathological pain through a mechanism that requires CB1 cannabinoid receptors and the enzyme diacylglycerol lipase in mice.

Acetaminophen (APAP) is commonly used as a pain and fever reliever, but its mechanisms remain unclear. Conflicting evidence implicates the endocannabinoid system in the effects of APAP. We tested the hypothesis that the analgesic effects of APAP were dependent upon both CB1 cannabinoid receptors and diacylglycerol lipase (DAGL), an enzyme which catalyzes formation of the endocannabinoid 2-arachidonoylglycerol. We examined the impact of APAP, administered in the presence and absence of DAGL inhibitors, on mechanical hypersensitivity in mice using models of inflammatory (induced by intraplantar injection of complete Freunds adjuvant (CFA)) and post-surgical (induced by incisional injury) pain. Pharmacological specificity was assessed using global (Rimonabant, AM251) and peripherally restricted (AM6545) CB1 antagonists. APAP produced a dose-dependent attenuation of inflammation-induced mechanical hypersensitivity, but did not alter peripheral edema in the CFA-injected paw. APAP also attenuated mechanical hypersensitivity in mice with incisional injury. The DAGL inhibitors, RHC-80267 or DO34, attenuated the anti-allodynic effects of APAP in both models of pain. CB1 receptor antagonists (Rimonabant and/or AM251) suppressed the antinociceptive effect of APAP in both pain models. The peripherally-restricted CB1 antagonist AM6545 did not alter the anti-allodynic effects of APAP. We also assessed the impact of APAP on tail-flick antinociception, locomotor behavior, and body temperature. APAP produced hypothermia and hypolocomotion at the highest dose, but these effects were not blocked by RHC-80267 or AM251. APAP did not produce tail flick antinociception. Our studies demonstrate that the analgesic effects of APAP observed in mouse models of pathological pain require both DAGL and CB1 activation. Our findings support a potential mechanism of APAP-induced analgesic action involving the enzyme DAGL and CB1 receptors.