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 reuptake inhibitor
AM 404. However, the contribution of the cannabinoid system to antihyperalgesia 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 antihyperalgesic action, which was mimicked by intrathecally injected
AM 404. Both compounds lost their antihyperalgesic activity in CB1(-/-) mice, confirming
the involvement of the cannabinoid system. Consistent with a mechanism downstream
of proinflammatory prostaglandin formation, acetaminophen also reversed hyperalgesia
induced by intrathecal prostaglandin E2 To distinguish between a peripheral/spinal
and a supraspinal action, we administered acetaminophen and AM 404 to hoxB8-CB1(-/-)
mice, which lack CB1 receptors from the peripheral nervous system and the spinal cord.
These mice exhibited unchanged antihyperalgesia indicating a supraspinal site of action.
Accordingly, local injection of the CB1 receptor antagonist rimonabant into the rostral
ventromedial medulla blocked acetaminophen-induced antihyperalgesia, while local rostral
ventromedial medulla injection of AM 404 reduced hyperalgesia in wild-type mice but
not in CB1(-/-) mice. 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 rostral ventromedial
medulla.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.