Background & Aims

Acetaminophen is commonly used as an over-the-counter pain and fever reliever. The mechanism of action of acetaminophen remains unclear [1]. The endocannabinoid system, which consists of cannabinoid receptors, endocannabinoids, and enzymes implicated in endocannabinoid synthesis and degradation, consists of multiple targets for analgesic drug development. Conflicting evidence implicates a role for the endocannabinoid system in antinociceptive effects of acetaminophen [2–4]. However, published studies have largely relied on in vitro approaches or in vivo models of acute nociception (e.g. hotplate test), that lack translational relevance of chronic pain models. We tested the hypothesis that the analgesic effects of acetaminophen in preclinical pain models were dependent upon both CB1 cannabinoid receptors and diacylglycerol lipase-alpha, an enzyme that controls the biosynthesis of the endocannabinoid 2-AG.

Methods

We examined the impact of acetaminophen on behavioral hypersensitivity to mechanical stimulation in mice in both a complete Freund’s adjuvant (CFA) model of inflammatory pain and a paw incision model of post-surgical pain. Mechanical paw withdrawal thresholds were evaluated before and after injury and pharmacological manipulations. Paw diameter was also measured to assess peripheral edema in the CFA model. We assessed the impact of acetaminophen and the diacylglycerol lipase (DAGL) inhibitor RHC-80267, administered either alone or in combination, in these preclinical pain models. Pharmacological specificity was assessed using both global (rimonabant, AM251) and peripherally restricted (AM6545) cannabinoid type 1 receptor (CB1) antagonists. Lastly, we assessed the impact of acetaminophen and a DAGL inhibitor on cardinal signs of CB1 activation using the tail-flick test, activity meter test and rectal temperature test.

Results

Acetaminophen, administered intraperitoneally, produced a dose-dependent attenuation of mechanical hypersensitivity in the CFA-injected (ipsilateral) paw without altering responsiveness in the non-inflamed (contralateral) paw. Acetaminophen did not alter paw edema in CFA-injected mice. Acetaminophen, given orally, attenuated mechanical allodynia in a mouse model of incisional injury. The DAGL inhibitor, RHC-80267, attenuated the anti-allodynic effects of acetaminophen in both the CFA model and incisional injury model. Furthermore, CB1 receptor antagonists (rimonabant and/or AM251) suppressed the antinociceptive effect of acetaminophen in both pain models whereas AM6545 failed to do so. Acetaminophen produced hypothermia at the highest dose, but this effect was not blocked by RHC-80267. High but not low doses of acetaminophen produced hypolocomotion in mice, but this effect was not blocked by treatment with RHC-80267. Acetaminophen did not produce tail-flick antinociception.

Conclusions

Antinociceptive effects of acetaminophen observed in mouse models of persistent pain require both DAGL alpha and CB1 activation. Our findings support a potential mechanism of acetaminophen-induced analgesic action involving the enzyme DAGL.
This work was supported by DA009158 and DA047858 (to AGH).

References

1. Ohashi N, Kohno T. Analgesic Effect of Acetaminophen: A Review of Known and Novel Mechanisms of Action. Front Pharmacol. 2020;11:580289. doi:10.3389/FPHAR.2020.580289/BIBTEX
2. Klinger-Gratz PP, Ralvenius WT, Neumann E, et al. Acetaminophen Relieves Inflammatory Pain through CB1 Cannabinoid Receptors in the Rostral Ventromedial Medulla. J Neurosci. 2018;38(2):322-334. doi:10.1523/JNEUROSCI.1945-17.2017
3. Ottani A, Leone S, Sandrini M, Ferrari A, Bertolini A. The analgesic activity of paracetamol is prevented by the blockade of cannabinoid CB1 receptors. Eur J Pharmacol. 2006;531(1-3):280-281. doi:10.1016/J.EJPHAR.2005.12.015
4. Topuz RD, Gunduz O, Karadag CH, Dokmeci D, Ulugol A. Endocannabinoid and N-acylethanolamide levels in rat brain and spinal cord following systemic dipyrone and paracetamol administration. Can J Physiol Pharmacol. 2019;97(11):1035-1041. doi:10.1139/CJPP-2019-0015

Presenting Author

Carlos Henrique Alves Jesus

Poster Authors

Carlos Alves Jesus

PhD

Indiana University

Lead Author

E-mail

Jonah Wirt

Indiana University Bloomington

Lead Author

E-mail

John Hainline

Indiana University

Lead Author

E-mail

Alex Makriyannis

Northeastern University

Lead Author

E-mail

Andrea G. Hohmann

PhD

Indiana University, Dept. of Psychological and Brain Sciences

Lead Author

E-mail

Topics

  • Models: Acute Pain