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.
Anyone interested in how the endocannabinoid system actually works at the synapse level, why cannabis produces tolerance, or the science behind how neurons communicate and regulate each other.
170×more abundant than anandamide in the brain — making 2-AG, not the 'bliss molecule,' the dominant endocannabinoid in the nervous system
The Backstory
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.
Biological Mechanism
Retrograde Signaling: How 2-AG Runs the Synapse
Postsynaptic neuron fires
When a neuron receives enough excitatory input, it depolarizes and calcium floods in through voltage-gated channels.
Calcium activates DAGLα
The calcium influx triggers diacylglycerol lipase alpha (DAGLα), an enzyme sitting in the postsynaptic membrane. DAGLα cleaves 2-AG from membrane lipids.
2-AG crosses the synapse backwards
Unlike conventional neurotransmitters (which travel from presynaptic to postsynaptic), 2-AG travels in reverse — from the postsynaptic neuron back to the presynaptic terminal. This is retrograde signaling.
2-AG binds presynaptic CB1
2-AG activates CB1 receptors on the presynaptic terminal as a full agonist. CB1 activation inhibits voltage-gated calcium channels and suppresses the cAMP/PKA signaling pathway.
Neurotransmitter release is suppressed
The presynaptic neuron releases less neurotransmitter — whether it's GABA (inhibitory) or glutamate (excitatory). This is the 'volume knob' function: the postsynaptic neuron is telling the presynaptic neuron to quiet down.
MAGL degrades 2-AG
Monoacylglycerol lipase (MAGL), located on the presynaptic side, rapidly breaks 2-AG into arachidonic acid and glycerol. The signal ends. The synapse returns to baseline.
Stella et al. (1997), Nature; Ohno-Shosaku & Kano (2014), Curr Opin Neurobiol; Zou & Kumar (2018)
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
2-AG (This Discovery)
- 170–1,000× more abundant in the brain
- Full agonist at CB1 — strong receptor activation
- Primary retrograde messenger (DSI/DSE)
- Made by DAGLα from membrane lipids
- Degraded by MAGL
- Rapid, phasic, point-to-point signaling
- Discovered 1995 (two groups independently)
The workhorse — runs the synapse
Anandamide (Discovered 1992)
- 170–1,000× less abundant in the brain
- Partial agonist at CB1 — weaker activation
- Also binds TRPV1 vanilloid receptors (unique)
- Made by NAPE-PLD from membrane phospholipids
- Degraded by FAAH
- Slower, more tonic, modulatory signaling
- Named 'anandamide' from Sanskrit for bliss
The modulator — sets the tone
Stella et al. (1997), Nature; Zou & Kumar (2018), PMC5877694
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.
Research Timeline
2-AG: From Discovery to System Understanding
Anandamide discovered (Devane, Hanuš, Mechoulam)
First endocannabinoid — but a partial agonist at low concentrations
2-AG independently identified by Mechoulam (gut) and Sugiura (brain)
Second endocannabinoid found — by two groups, on two continents, in the same year
Stella, Schweitzer & Piomelli characterize 2-AG in brain
170× more abundant than anandamide, full agonist, modulates LTP — published in Nature
DSI and DSE shown to be endocannabinoid-mediated
Retrograde synaptic signaling finally linked to endocannabinoids
DAGLα identified as the enzyme that synthesizes 2-AG
The 'on switch' for 2-AG production is mapped
MAGL characterized as primary 2-AG degradation enzyme
The 'off switch' — MAGL terminates the retrograde signal
DAGLα knockout mice lose all retrograde endocannabinoid signaling
Genetic proof: 2-AG, not anandamide, is the retrograde messenger
MAGL inhibitors in development for neuroinflammation and pain
Boosting 2-AG by blocking its degradation — a precision therapeutic strategy
Mechoulam et al. (1995); Sugiura et al. (1995); Stella et al. (1997); Pan et al. (2011)
Related Research
The Endocannabinoid Family
2-AG is the second endocannabinoid discovered and the most physiologically dominant. These are the studies that built the system around it.
Isolation and structure of a brain constituent that binds to the cannabinoid receptor
Devane, Hanuš, Mechoulam et al. (1992)
The first endocannabinoid — but not the dominant one. Anandamide opened the door; 2-AG filled the room.
Structure of a cannabinoid receptor and functional expression of the cloned cDNA
Matsuda et al. (1990)
The receptor 2-AG was made for — CB1, the most abundant GPCR in the brain
FAAH discovery (enzyme that breaks down anandamide)
Cravatt et al. (1996)
FAAH degrades anandamide; MAGL degrades 2-AG — parallel but separate enzymatic pathways
Isolation, Structure, and Partial Synthesis of an Active Constituent of Hashish
Gaoni & Mechoulam (1964)
The plant molecule that mimics what 2-AG does naturally at every synapse in the brain
What is 2-AG and how is it different from anandamide?
2-AG (2-arachidonoylglycerol) is the most abundant endocannabinoid in the brain — present at 170 to 1,000 times the concentration of anandamide. While anandamide is a partial agonist (weak activator) at CB1 receptors, 2-AG is a full agonist (strong activator) and serves as the primary retrograde messenger at synapses. Think of 2-AG as the workhorse that handles moment-to-moment signaling between neurons, while anandamide is more of a background tone-setter for mood and stress regulation.
If 2-AG is more important, why is anandamide more famous?
Anandamide was discovered first (1992 vs 1995), has a memorable name ("bliss molecule" from Sanskrit), and benefited from a more dramatic discovery narrative — Mechoulam's Hebrew word joke and the pig brain story are widely retold. "2-arachidonoylglycerol" is a mouthful, and its discovery was split across two independent groups. But in terms of physiological importance, 2-AG dominates: it is far more abundant, a stronger receptor activator, and the molecule actually responsible for retrograde signaling at virtually every synapse in the brain.
What the researchers found
sn-2 arachidonylglycerol (2-AG) was identified in brain tissue at concentrations 170 times greater than anandamide. Unlike anandamide (a partial agonist), 2-AG activated neuronal cannabinoid receptors as a full agonist. 2-AG was produced in hippocampal slices by stimulation of the Schaffer collaterals through a calcium-dependent mechanism involving phospholipase C and diacylglycerol lipase. Functionally, 2-AG prevented the induction of long-term potentiation (LTP) at CA3-CA1 hippocampal synapses, demonstrating that it can modulate synaptic plasticity — the cellular basis of learning and memory.
Why it matters
This paper established 2-AG — not anandamide — as the brain's dominant endocannabinoid. At 170 times the concentration and with full agonist activity (compared to anandamide's partial agonism), 2-AG turned out to be the primary retrograde messenger at synapses throughout the brain. The endocannabinoid system is not run by the 'bliss molecule' — it's run by 2-AG. This fundamentally reshaped understanding of how the ECS works and explained why chronic THC exposure (which overwhelms the 2-AG signaling system) produces tolerance, dependence, and withdrawal.
How the study worked
Brain lipid extracts from rat tissue were analyzed to quantify 2-AG and anandamide concentrations. Hippocampal brain slices were electrically stimulated at the Schaffer collateral pathway and the resulting lipid release was measured. Calcium dependence was tested by manipulating extracellular calcium. Enzyme involvement was determined using inhibitors of phospholipase C and diacylglycerol lipase. 2-AG's agonist activity was confirmed through receptor binding and functional assays. The effect on long-term potentiation was measured by electrophysiological recording at CA3-CA1 synapses before and after 2-AG application.
What this study cannot tell us
The Stella 1997 paper used rat hippocampal slices — an ex vivo preparation, not intact behaving animals. The 170× concentration difference was measured in whole brain extract and may vary by region. The study demonstrated that exogenously applied 2-AG modulates LTP but did not prove that endogenously released 2-AG does the same under physiological conditions (this was established by subsequent work). The 1995 discovery papers had similar limitations: Mechoulam's group isolated 2-AG from canine gut (not brain), and Sugiura's group measured binding affinity but did not characterize physiological function.
How to read the evidence
Rated strong because the identification of 2-AG as an endocannabinoid was independently confirmed by two groups in 1995, the brain characterization was published in Nature in 1997, and subsequent knockout mouse studies have definitively established 2-AG as the primary retrograde messenger at synapses.
When this study was published
The 1995 discovery papers and 1997 Nature characterization are nearly 30 years old. The core finding — that 2-AG is the dominant endocannabinoid — has been massively validated. Subsequent research has mapped its synthesis (DAGLα), degradation (MAGL), and role in retrograde synaptic signaling in extraordinary detail.
The bigger picture
2-AG completed the core cast of the endocannabinoid system: two receptors (CB1 and CB2), two endogenous ligands (anandamide and 2-AG), and the enzymes that make and break them down (DAGLα/MAGL for 2-AG, NAPE-PLD/FAAH for anandamide). Subsequent research proved 2-AG is the primary retrograde messenger at synapses — responsible for depolarization-induced suppression of inhibition (DSI) and excitation (DSE), two fundamental forms of short-term synaptic plasticity. Knocking out the enzyme that makes 2-AG (DAGLα) abolishes retrograde endocannabinoid signaling entirely; knocking out the enzyme that makes anandamide does not. 2-AG is the workhorse. MAGL, the enzyme that degrades 2-AG, is now a therapeutic target for neuroinflammation, pain, and neurodegeneration.
Questions still open
- If 2-AG is 170 times more abundant than anandamide, why does the brain need both endocannabinoids?
- Can drugs that boost 2-AG levels (by inhibiting MAGL) treat neurological conditions?
- Does chronic cannabis use specifically disrupt 2-AG retrograde signaling, and is this reversible?
Common questions
What is 2-AG and how is it different from anandamide?
If 2-AG is more important, why is anandamide more famous?
Read the original research
A second endogenous cannabinoid that modulates long-term potentiation
Nature, 388(6644), 773-778
Citation
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