The Third Endocannabinoid: A Disputed Discovery from the Lab That Found THC
2-Arachidonyl glyceryl ether, an endogenous agonist of the cannabinoid CB1 receptor
Mechoulam's lab reported a third type of endocannabinoid — an ether-type compound called noladin ether — but whether the body actually makes it remains one of the most unresolved questions in cannabinoid science.
By 2001, Raphael Mechoulam's laboratory at the Hebrew University of Jerusalem had already rewritten the textbook twice. In 1964, they isolated THC — the molecule that makes cannabis psychoactive. In 1992, they discovered anandamide — the brain's own version of THC. By 1995, a second endocannabinoid, 2-AG, had been identified independently by two groups. The endocannabinoid system had receptors, ligands, and enzymes. It was becoming a real system.
But Mechoulam's team wasn't done looking. If the body made two cannabinoid-like molecules, it might make more. And Lumír Hanuš — the Czech analytical chemist who had physically isolated anandamide from pig brain tissue nine years earlier — was back at the same bench, running the same kind of extraction, hunting for whatever else might be hiding in the brain's lipid soup.
What he found was real, active, and interesting. Whether the body actually makes it remains one of the most unresolved questions in endocannabinoid science.
The Third Molecule
The technique was the same one that had produced anandamide in 1992: extract lipids from animal brain tissue, fractionate them by chemical properties, and test each fraction against cannabinoid receptors. Whatever binds is your candidate. It's conceptually simple and practically grueling — thousands of fractions, each requiring purification and testing.
From porcine brain, Hanuš isolated a compound that bound CB1 receptors with high affinity. When Mechoulam's team determined its structure, they found something unexpected. This wasn't another amide like anandamide. It wasn't an ester like 2-AG. It was an ether — a fundamentally different chemical bond connecting the arachidonic acid chain to the glycerol backbone.
They named it 2-arachidonyl glyceryl ether. It would become known as noladin ether, catalogued as HU-310.
A third structural class of endocannabinoid, from the same laboratory that had found the first.
The paper appeared in Proceedings of the National Academy of Sciences in March 2001. Eight authors. NIH/NIDA funding. And a claim that, if confirmed, would significantly expand what "endocannabinoid" meant.
Why an Ether Matters
The difference between an amide, an ester, and an ether might sound like a chemistry footnote. It's not. The type of chemical bond determines how long the molecule survives in the body.
Ester and amide bonds are vulnerable to hydrolysis — water-assisted cleavage by enzymes. The body has dedicated enzymes (FAAH, MAGL) precisely designed to snap these bonds and terminate endocannabinoid signals within seconds. This is by design: the endocannabinoid system operates on brief, precisely timed pulses.
An ether bond is fundamentally harder to break. No carbonyl group for enzymes to attack. Noladin ether resists the same enzymatic degradation that rapidly destroys anandamide and 2-AG. If it functions as an endocannabinoid, it would be a slower, longer-lasting signal — a different kind of message entirely.
This distinction matters for drug development. If you're designing a synthetic cannabinoid for therapeutic use, ether-type compounds offer inherently longer duration of action without requiring enzyme inhibitors. The Hanus discovery, regardless of the controversy that followed, opened that chemical door.
The Data
21.2 nM
binding affinity (Ki) of noladin ether at the CB1 receptor — comparable to anandamide (~60-90 nM at CB1) and within the range of physiologically relevant endocannabinoid signaling. The compound also showed weak CB2 binding (Ki = 480 nM) and partial agonism at TRPV1 vanilloid receptors.
For reference, THC binds CB1 at approximately 10-40 nM depending on the assay system. Noladin ether's affinity is in the same ballpark.
Hanus et al. (2001), PNAS 98:3662-3665
In mice, noladin ether produced a classic tetrad of cannabinoid effects:
- Sedation — reduced locomotor activity
- Hypothermia — lowered body temperature
- Intestinal immobility — slowed gut motility
- Mild antinociception — reduced pain sensitivity
These are the same four effects used as a standard screen for cannabinoid activity since the 1980s. Any compound that produces all four is behaving like a cannabinoid agonist. Noladin ether passed every test.
The compound is biologically active. That has never been in dispute.
What happened next is.
The Replication Problem
This is what scientific uncertainty actually looks like. Not a clear-cut fraud. Not a consensus confirmation. Two camps with plausible evidence, and a question that remains open more than two decades later.
The biosynthesis problem is particularly damning. For every other endocannabinoid, the synthesis pathway is mapped: we know the precursor, the enzyme, and the mechanism. For noladin ether, no pathway exists. The known ether lipid biosynthesis route in mammals (the peroxisomal pathway) acts at the sn-1 position on glycerol. Noladin ether's ether bond is at sn-2. Either there's an unknown enzyme, or the compound was formed during the extraction process itself.
Neither possibility can be ruled out.
What This Tells Us About Endocannabinoids
Myth vs. Reality
There are five or more confirmed endocannabinoids: anandamide, 2-AG, noladin ether, virodhamine, and NADA.
Only two endocannabinoids are firmly established: anandamide and 2-AG. Both have known synthesis and degradation pathways, documented physiological roles, and consistent detection across laboratories. Noladin ether (2001), virodhamine (2002), and NADA (2000) have all been reported but their endogenous status and physiological relevance remain debated. Listing them alongside anandamide and 2-AG without qualification overstates what the science actually supports.
The Evidence
Multiple independent groups have failed to reliably detect noladin ether in mammalian brain tissue (Oka et al., 2003; Richardson et al., 2007). No biosynthetic enzyme has been identified. Virodhamine's endogenous role is similarly unclear. NADA has stronger evidence but functions primarily through TRPV1, not cannabinoid receptors.
Oka et al. (2003), J Neurochem; Bisogno et al. (2008), J Neuroendocrinol 20(s1):1-9
The honest inventory of the endocannabinoid family looks like this:
Confirmed:
- Anandamide (1992) — partial CB1 agonist, also binds TRPV1, synthesized by NAPE-PLD, degraded by FAAH
- 2-AG (1995) — full CB1 agonist, dominant retrograde messenger, synthesized by DAGLα, degraded by MAGL
Putative (biologically active but endogenous status debated):
- Noladin ether (2001) — ether-type, CB1 agonist, detection disputed, no known biosynthetic pathway
- Virodhamine (2002) — CB1 antagonist/CB2 agonist, detection inconsistent
- NADA (2000) — N-arachidonoyldopamine, dual CB1/TRPV1 agonist, better evidence but low brain concentrations
This doesn't diminish the research. It reflects it honestly. Science doesn't resolve every question quickly, and the endocannabinoid system is almost certainly more complex than the two-molecule model suggests. The lipidome of the brain contains hundreds of signaling molecules we haven't fully characterized. Noladin ether may eventually be confirmed with better analytical methods — or it may remain a pharmacological tool compound that taught us about ether-type cannabinoid chemistry without being a true endogenous signal.
A Lab That Changed Everything
Raphael Mechoulam, who passed away in 2023 at age 92, built one of the most productive pharmacology programs in history. His lab's contributions to cannabinoid science span from the structural chemistry of a plant molecule to the discovery of an entire signaling system in the human body. Noladin ether was part of that arc — an ambitious attempt to show that the endocannabinoid family was larger and more diverse than anyone had imagined.
Lumír Hanuš, who had come to Jerusalem from the Czech Republic as a visiting scientist and never left, was the hands-on chemist behind both anandamide and noladin ether. His technical skill in isolating vanishingly small quantities of lipid from biological tissue made both discoveries possible.
“It was me who then isolated the compound in the brain, but he was running various tests to assess its activity, and I wouldn't have been able to do that without him, so it was our teamwork.”
— Lumir Hanus
Hebrew University of Jerusalem
On his collaboration with William Devane during the anandamide discovery — the same collaborative approach defined the noladin ether work
The Drug Design Legacy
Even if noladin ether turns out not to be endogenous, the discovery was not wasted. It demonstrated that ether-type cannabinoids are pharmacologically active, metabolically stable, and capable of selectively engaging CB1 over CB2. This opened a chemical design space.
The logic: if you're designing a therapeutic cannabinoid, you want it to last long enough to be useful but not so long that it causes problems. Anandamide and 2-AG are destroyed in seconds — too fast for most drug applications. THC persists for hours — too long for precise dosing. An ether-type compound occupies the middle ground: resistant to enzymatic cleavage, but not as persistent as THC.
Researchers have since explored noladin ether's effects on:
- Intraocular pressure — potential glaucoma application
- Appetite stimulation — enhanced motivation to eat in animal models
- Neuroprotection — PPARα receptor activation
- GABA uptake in the globus pallidus — basal ganglia function
The compound was also included in the cannabinoid ligand panel used by Ryberg et al. (2007) when they identified GPR55 as a putative third cannabinoid receptor. Noladin ether activated GPR55 at nanomolar concentrations — adding another receptor target to its profile.
What We Don't Know
This study sits at the boundary between established science and open questions. The honest answer to "Is noladin ether an endocannabinoid?" is: we don't know yet.
What we do know:
- It binds CB1 with high affinity and produces cannabinoid effects in animals
- Some labs can detect it in brain tissue; others cannot
- No biosynthetic pathway has been identified
- It is pharmacologically interesting regardless of its endogenous status
The endocannabinoid system almost certainly involves more signaling molecules than just anandamide and 2-AG. The brain's lipidome is vast and poorly characterized. Better analytical methods may eventually resolve the noladin ether question — or reveal entirely different molecules that fill the roles noladin ether was hypothesized to play.
In science, "we don't know" is not a failure. It's a frontier.
Frequently Asked Questions
Cite this study
Hanus et al.. (2001). 2-Arachidonyl glyceryl ether, an endogenous agonist of the cannabinoid CB1 receptor. Proc Natl Acad Sci U S A. https://doi.org/10.1073/pnas.061029898