The Discovery of 2-AG — The Brain's Primary Endocannabinoid and the Molecule That Actually Runs the System
A second endogenous cannabinoid that modulates long-term potentiation
In 1995, two independent groups discovered 2-AG, a second endocannabinoid present in the brain at 170 times the concentration of anandamide. A 1997 Nature paper then showed it was a full agonist that modulates memory formation — establishing 2-AG, not anandamide, as the dominant signaling molecule in the endocannabinoid system.
By 1995, the endocannabinoid system had a receptor, a ligand, and a name. Anandamide — the "bliss molecule" — was the star. It bound the CB1 receptor, it had a poetic name, and it was the subject of a growing body of research. But something about it didn't add up.
Anandamide was a partial agonist — a weak activator. Its brain concentrations were vanishingly small. Its half-life was measured in seconds. Was this really the molecule that the most abundant receptor in the brain had evolved for? Was this faint, fleeting whisper the signal that ran one of the body's most important regulatory systems?
On opposite sides of the planet, two laboratories were about to find the molecule that actually did.
Discovered Twice
In a pattern that happens more often in science than the public realizes, two independent groups identified the same molecule in the same year without knowing about each other's work.
In Jerusalem, Raphael Mechoulam's laboratory — the same lab that had isolated THC three decades earlier and discovered anandamide three years before — was screening tissue extracts for additional cannabinoid receptor ligands. Shimon Ben-Shabat, Mechoulam's PhD student, isolated a monoglyceride from canine intestinal tissue that bound both CB1 and CB2 receptors. They published in Biochemical Pharmacology: 2-arachidonoylglycerol, a new endogenous cannabinoid.
Six thousand miles away at Teikyo University in Kanagawa, Japan, Takayuki Sugiura's group was independently running binding assays on rat brain lipid extracts. They found the same compound — a monoglyceride with cannabinoid receptor affinity — and published in Biochemical and Biophysical Research Communications.
Same molecule. Same year. Two continents. Neither group knew the other was looking.
The molecule was 2-arachidonoylglycerol — 2-AG for short. An arachidonic acid ester of glycerol. Chemically, it's a monoglyceride — a class of molecules better known as components of dietary fat. The idea that a fat molecule could be a neurotransmitter was already controversial from the anandamide discovery. Now there were two of them.
But the real surprise was still two years away.
170 Times More
In 1997, Nephi Stella, Paul Schweitzer, and Daniele Piomelli at the Neurosciences Institute in San Diego published a paper in Nature that rewrote the hierarchy of the endocannabinoid system.
170×
more abundant than anandamide in the rat brain — making 2-AG, not the 'bliss molecule,' the dominant endocannabinoid in the nervous system. Some measurements put the difference even higher, at roughly 1,000-fold.
If anandamide is a whisper, 2-AG is a conversation happening in every room in the building.
Stella et al. (1997), Nature 388:773-778; Zou & Kumar (2018), PMC5877694
The paper reported three critical findings:
First: 2-AG is present in the brain at concentrations 170 times greater than anandamide. This wasn't a subtle difference. The molecule the field had been treating as secondary was, by mass, overwhelmingly dominant.
Second: 2-AG is a full agonist at CB1 receptors. Anandamide is a partial agonist — it binds the receptor but only weakly activates it. 2-AG binds and fully activates. Stronger signal, more abundant messenger.
Third: 2-AG modulates long-term potentiation (LTP) at hippocampal CA3-CA1 synapses — the cellular process underlying memory formation. The endocannabinoid system wasn't just mood and appetite. It was directly involved in how the brain encodes information.
Piomelli, the senior author, had trained with two future Nobel laureates — Eric Kandel and Paul Greengard at Columbia — before working with a third, Gerald Edelman, at the Neurosciences Institute. He would go on to elucidate the core pathways of endocannabinoid synthesis and degradation, publish over 400 papers, and hold 34 patents. The 2-AG brain characterization was among his most important contributions.
How 2-AG Actually Works
This is the most important mechanism in endocannabinoid biology. Everything about how cannabis affects the brain — tolerance, withdrawal, the high itself — traces back to this pathway.
This retrograde signaling mechanism is how the brain fine-tunes itself in real time. If a neuron is receiving too much input, it releases 2-AG to tell the neurons upstream to reduce their output. It's a feedback loop — a thermostat for neural activity.
And it's happening at virtually every synapse in the brain, all the time.
Not the Bliss Molecule — the Volume Knob
Myth vs. Reality
Anandamide — the 'bliss molecule' — is the main endocannabinoid that runs the endocannabinoid system.
2-AG is the dominant endocannabinoid by every measure: 170-1,000× more abundant in the brain, a full agonist (vs anandamide's partial agonism), and the primary retrograde messenger at synapses. Knocking out the enzyme that makes 2-AG abolishes retrograde endocannabinoid signaling entirely. Knocking out the enzyme that makes anandamide does not.
The Evidence
DAGLα knockout mice (which can't make 2-AG) lose DSI and DSE — the core retrograde signaling mechanisms. NAPE-PLD knockout mice (which can't make anandamide via the main pathway) retain normal retrograde signaling. This genetic evidence definitively establishes 2-AG, not anandamide, as the retrograde messenger.
Pan et al. (2011), J Neurosci, PMC3371386; Stella et al. (1997), Nature
Anandamide got the better name and the better press. "Bliss molecule" makes for a compelling headline. "2-arachidonoylglycerol" does not. But in terms of physiological importance, 2-AG is the molecule that matters most. Anandamide appears to function more as a tonic modulator — setting baseline mood and stress tone. 2-AG is the phasic signal — the rapid, point-to-point messenger that adjusts synaptic strength moment by moment.
Neither is dispensable. But if you had to choose one to explain how the endocannabinoid system actually works, you'd choose 2-AG.
Two Endocannabinoids, Two Jobs
They're not competitors. They're complementary systems using different synthesis and degradation pathways, operating on different timescales, serving different physiological roles through the same receptor. The brain didn't evolve redundancy — it evolved precision.
The Molecule That Explains Tolerance
Understanding 2-AG's role in retrograde signaling explains something every cannabis user experiences: tolerance.
CB1 receptors evolved to respond to brief, precisely-timed 2-AG pulses — signals that last seconds before MAGL clears the 2-AG away. Each pulse is a calibrated instruction: "reduce output at this synapse, right now, by this much."
When someone uses cannabis, THC floods every CB1 receptor in the brain simultaneously and stays there for hours. Instead of targeted pulses, the system gets a constant, indiscriminate signal. The brain's response is predictable: it pulls CB1 receptors off the cell surface (internalization), reduces their sensitivity (desensitization), and decreases their total number (downregulation).
This is tolerance at the molecular level. The system that was designed for 2-AG whispers is being overwhelmed by a THC shout.
When cannabis use stops, the brain temporarily has fewer functional CB1 receptors — and its own 2-AG signaling is impaired. Withdrawal symptoms — anxiety, insomnia, irritability, appetite changes — reflect the temporary disruption of 2-AG-mediated retrograde signaling across the brain's circuits.
The good news: CB1 receptors recover. Imaging studies show receptor density returning to normal within approximately 2-4 weeks of abstinence. The 2-AG system comes back online. The tolerance break works because the molecular machinery was never broken — just overwhelmed.
Frequently Asked Questions
Cite this study
Stella, N; Schweitzer, P; Piomelli, D. (1997). A second endogenous cannabinoid that modulates long-term potentiation. Nature, 388(6644), 773-778. https://doi.org/10.1038/42015