“Acetaminophen (paracetamol) is a widely used analgesic and antipyretic drug with only incompletely understood mechanisms of action.
Previous work, using models of acute nociceptive pain, indicated that analgesia by acetaminophen involves an indirect activation of CB1 receptors by the acetaminophen metabolite and endocannabinoid re-uptake inhibitor AM 404. However, the contribution of the cannabinoid system to anti-hyperalgesia against inflammatory pain, the main indication of acetaminophen, and the precise site of the relevant CB1 receptors have remained elusive.
Here, we analyzed acetaminophen analgesia in mice of either sex with inflammatory pain and found that acetaminophen exerted a dose-dependent anti-hyperalgesic action, which was mimicked by intrathecally injected AM 404. Both compounds lost their anti-hyperalgesic activity in CB1-/- mice confirming the involvement of the cannabinoid system.
Our results indicate that the cannabinoid system contributes not only to acetaminophen analgesia against acute pain but also against inflammatory pain, and suggest that the relevant CB1 receptors reside in the RVM.
SIGNIFICANCE STATEMENT: Acetaminophen is a widely used analgesic drug with multiple but only incompletely understood mechanisms of action including a facilitation of endogenous cannabinoid signaling via one of its metabolites. Our present data indicate that enhanced cannabinoid signaling is also responsible for the analgesic effects of acetaminophen against inflammatory pain. Local injections of the acetaminophen metabolite AM 404 and of cannabinoid receptor antagonists as well as data from tissue specific CB1 receptor deficient mice suggest the rostral ventromedial medulla as an important site of the cannabinoid-mediated analgesia by acetaminophen.”