How the Cannabinoid System Controls Pain: From THC to Natural Brain Mechanisms | Research

The takeaway

A review described the analgesic mechanisms of cannabinoids, highlighting CB2-selective agonists as lacking psychoactive side effects and revealing a novel endocannabinoid-based pain control circuit in the midbrain.

Read this if you want to understand the science behind how cannabis and the endocannabinoid system control pain.

CB2-selective agonists provide pain relief without psychoactive side effects

What the researchers found

The review covered multiple cannabinoid pain-relief strategies: exogenous cannabinoids (THC acting mainly through CB1), CB2-selective agonists (which avoid psychoactive effects), and endocannabinoid enzyme inhibitors (FAAH and MAGL inhibitors). It also described a newly discovered analgesic mechanism in the periaqueductal gray (PAG), a midbrain region that initiates descending pain inhibition.

In the PAG, receptor activation triggers a chain of enzymatic reactions producing 2-AG, which then inhibits GABAergic (inhibitory) neurons, effectively removing the brake on pain-suppressing pathways. Multiple neurotransmitter systems, including glutamate, acetylcholine, and orexin, can initiate this chain. Notably, acetaminophen's metabolite AM404 also works through this pathway.

Why it matters

Understanding the multiple pathways through which cannabinoids control pain reveals several distinct therapeutic opportunities. CB2-selective agonists could provide pain relief without cognitive or psychoactive effects. FAAH/MAGL inhibitors could boost natural pain-suppressing endocannabinoids. The PAG mechanism explains how the brain naturally uses endocannabinoids in stress-induced pain suppression.

The numbers in context

CB1 receptors mediate central analgesic effects. CB2-selective agonists lack CNS side effects. Multiple receptor systems (mGluR5, M1/M3, OX1) activate the PAG 2-AG pathway. Acetaminophen's metabolite AM404 also activates this pathway.

How the study worked

Narrative review covering exogenous cannabinoid pharmacology, endocannabinoid biosynthesis and degradation, CB1/CB2 receptor mechanisms, and a detailed analysis of the 2-AG-mediated disinhibition mechanism in the periaqueductal gray.

What this study cannot tell us

Much of the PAG mechanism research was from the authors' own lab and awaited independent replication. The clinical relevance of the midbrain circuit had not been demonstrated in humans. The review was selective rather than systematic.

How to read the evidence

Narrative review combining established and novel mechanistic findings; moderate evidence overall.

When this study was published

Published in 2013. Cannabinoid pain mechanisms continue to be an active research area with clinical applications.

The bigger picture

This review connected the dots between multiple pain research areas: the cannabinoid system, the opioid system, stress-induced analgesia, and even the mechanism of common painkillers like acetaminophen. The emerging picture suggests the endocannabinoid system is a central hub of the body's natural pain control.

Questions still open

Can the PAG endocannabinoid pathway be targeted for clinical pain treatment? Would combining endocannabinoid-boosting drugs with existing pain medications improve outcomes? Is stress-induced analgesia disrupted in chronic pain patients?

Common questions

Can cannabis relieve pain without causing a high?

Yes, potentially. This review highlighted CB2-selective agonists, which activate cannabinoid receptors found primarily outside the brain and do not produce psychoactive effects. Additionally, FAAH and MAGL inhibitors boost the body's own endocannabinoids in a more targeted way than THC, potentially providing pain relief with fewer cognitive side effects.

How does acetaminophen connect to the cannabinoid system?

Surprisingly, acetaminophen (Tylenol) is metabolized in the brain to a compound called AM404, which activates the same endocannabinoid pain-control pathway in the periaqueductal gray. This means part of how acetaminophen relieves pain is through the endocannabinoid system, a connection that was not widely appreciated before this research.

Read the original research

Targeting the cannabinoid system for pain relief?

Acta anaesthesiologica Taiwanica : official journal of the Taiwan Society of Anesthesiologists, 51(4), 161-70

View on PubMed ↗Publisher’s paper ↗

Citation

Chiou, Lih-Chu; Hu, Sherry Shu-Jung; Ho, Yu-Cheng. (2013). Targeting the cannabinoid system for pain relief?. Acta anaesthesiologica Taiwanica : official journal of the Taiwan Society of Anesthesiologists, 51(4), 161-70. https://doi.org/10.1016/j.aat.2013.10.004