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CB1: The Brain’s Most Abundant GPCR

The CB1 receptor (gene: CNR1), cloned by Lisa Matsuda at NIMH in 1990, is the most abundant G protein-coupled receptor in the mammalian central nervous system. It is a seven-transmembrane domain receptor coupled primarily to G_i/o proteins, and its activation produces three principal downstream effects:

• Inhibition of adenylyl cyclase — reducing intracellular cAMP levels, which suppresses protein kinase A (PKA) activity
• Activation of inwardly rectifying potassium channels (GIRKs) — hyperpolarizing the presynaptic terminal
• Inhibition of voltage-gated calcium channels (N-type and P/Q-type) — reducing calcium influx required for vesicle fusion and neurotransmitter release

The net effect is suppression of neurotransmitter release from the presynaptic terminal. This applies to both excitatory (glutamate) and inhibitory (GABA) transmission, which is why CB1 activation can produce seemingly contradictory effects depending on circuit context.

Miles Herkenham’s autoradiographic mapping studies revealed CB1’s characteristic distribution: highest density in the basal ganglia (substantia nigra pars reticulata, globus pallidus, caudate-putamen), cerebellum (molecular layer), hippocampus (CA1, CA3, dentate gyrus), and cerebral cortex (layers I and VI). Moderate density in the amygdala, hypothalamus, and periaqueductal gray. Very low density in the brainstem, thalamus, and lower medulla — the critical absence in cardiorespiratory centers that explains why cannabis cannot produce fatal respiratory depression.

CB1 is also expressed peripherally — in adipose tissue, liver, gastrointestinal tract, skeletal muscle, and reproductive organs — where it influences energy metabolism, gut motility, and reproductive function. The failed obesity drug rimonabant (Acomplia, Sanofi) was a CB1 inverse agonist that demonstrated the receptor’s metabolic role: it produced significant weight loss but was withdrawn from the European market in 2008 after reports of depression, anxiety, and suicidal ideation, illustrating the danger of globally blocking a receptor with such widespread CNS expression.

CB2: The Immune Receptor

The CB2 receptor (gene: CNR2), cloned by Sean Munro at Cambridge MRC in 1993 from a human HL-60 promyelocytic cell line, was initially described as a “peripheral cannabinoid receptor” confined to immune tissues. CB2 shares only 44% amino acid identity with CB1 in the transmembrane domains, and its pharmacological profile is distinct.

CB2 is expressed at high levels in immune cells: B lymphocytes (highest expression), natural killer (NK) cells, monocytes, macrophages, and T cells (CD4+ and CD8+). The spleen — the body’s largest lymphoid organ — shows particularly high CB2 expression. CB2 activation generally produces immunosuppressive and anti-inflammatory effects: reduced pro-inflammatory cytokine release (TNF-α, IL-1β, IL-6), decreased immune cell migration, and modulation of antibody production.

The initial characterization of CB2 as “peripheral only” has been revised. CB2 is now known to be expressed in the CNS, particularly in microglia — the brain’s resident immune cells — where its expression is dramatically upregulated during neuroinflammation. This has made CB2 a therapeutic target of intense interest for neuroinflammatory and neurodegenerative conditions (Alzheimer’s disease, multiple sclerosis, Parkinson’s disease, traumatic brain injury), because CB2-selective agonists could potentially reduce neuroinflammation without producing the psychoactive effects mediated by CB1.

Orphan Receptors and Ion Channels

The two-receptor model is now recognized as insufficient. Several additional molecular targets respond to endocannabinoids and plant cannabinoids:

GPR55 is sometimes called the “third cannabinoid receptor,” though its pharmacology differs substantially from CB1/CB2. Activated by lysophosphatidylinositol (LPI), THC, and some synthetic cannabinoids, GPR55 couples to G_q and G_12/13 proteins rather than G_i/o, activating phospholipase C (PLC), RhoA, and intracellular calcium mobilization. It is expressed in brain (caudate, putamen, hippocampus), adrenal glands, GI tract, and various immune cells. Functionally, GPR55 has been implicated in bone metabolism (osteoclast function), cancer cell proliferation, vascular tone, and inflammatory pain. Notably, CBD acts as a GPR55 antagonist, which may contribute to CBD’s antiproliferative effects observed in preclinical cancer models.

GPR18 responds to N-arachidonoyl glycine (NAGly, a metabolite of anandamide) and resolvin D2. It is expressed in immune cells, spleen, and testis. GPR18 activation promotes microglial migration and has been implicated in inflammation resolution and intraocular pressure regulation.

TRPV1 (transient receptor potential vanilloid 1) is the capsaicin receptor — the same ion channel activated by hot peppers. Anandamide is an endogenous TRPV1 agonist at higher concentrations, and CBD activates then desensitizes this channel. TRPV1 is a nonselective cation channel permeable to Ca²⁺, expressed in sensory neurons (dorsal root ganglia, trigeminal ganglia), brain, and various peripheral tissues. Its activation produces burning pain and neurogenic inflammation, but sustained activation leads to desensitization — a mechanism that may underlie some of CBD’s analgesic effects. The overlap between endocannabinoid and vanilloid signaling at TRPV1 represents a critical integration point that Vincenzo Di Marzo has termed part of the “endocannabinoidome.”

Anandamide: The “Bliss Molecule”

Anandamide (N-arachidonoylethanolamine, AEA) was isolated by Raphael Mechoulam, William Devane, and Lumír Hanuš in 1992 from porcine brain. It is an arachidonoyl derivative of ethanolamine, synthesized from the membrane phospholipid precursor N-arachidonoyl phosphatidylethanolamine (NAPE) by the enzyme NAPE-specific phospholipase D (NAPE-PLD).

Key pharmacological characteristics of anandamide:

• Partial agonist at CB1 — lower intrinsic efficacy than 2-AG or THC at stimulating G protein coupling
• Weak partial agonist at CB2 — very low efficacy at immunological concentrations
• Endogenous TRPV1 agonist — at higher concentrations, anandamide activates vanilloid receptors, producing a dual cannabinoid/vanilloid signaling profile
• Rapidly degraded by fatty acid amide hydrolase (FAAH), which is located primarily in postsynaptic membranes and intracellular compartments, yielding arachidonic acid and ethanolamine
• Tonic signaling — anandamide appears to maintain a basal tone at CB1, modulating ongoing neurotransmission rather than producing acute phasic signals

FAAH inhibition has been pursued as a therapeutic strategy: by blocking anandamide degradation, endocannabinoid tone could be enhanced without flooding the system with exogenous cannabinoids. However, a Phase I trial of the FAAH inhibitor BIA 10-2474 (Bial Pharmaceuticals, 2016) resulted in one death and four hospitalizations in France — likely due to off-target enzyme inhibition rather than FAAH-specific effects, but the disaster significantly set back the field.

2-AG: The Workhorse Endocannabinoid

2-Arachidonoylglycerol (2-AG), identified by Shimon Ben-Shabat in Mechoulam’s laboratory in 1995, is the predominant endocannabinoid in both brain and peripheral tissues. It is present at concentrations 170 to 1,000 times higher than anandamide, depending on the tissue and measurement technique.

Key pharmacological characteristics of 2-AG:

• Full agonist at both CB1 and CB2 — substantially higher intrinsic efficacy than anandamide at both receptors
• Synthesized by diacylglycerol lipase alpha (DAGLα) in postsynaptic neurons from diacylglycerol (DAG), a membrane lipid intermediate
• Degraded primarily by monoacylglycerol lipase (MAGL), which is located on presynaptic terminals — in contrast to FAAH’s postsynaptic localization. MAGL accounts for approximately 85% of 2-AG hydrolysis in the brain, yielding arachidonic acid and glycerol
• Phasic signaling — 2-AG appears to mediate the on-demand, activity-dependent retrograde signaling that produces DSI and DSE

The spatial separation of degradative enzymes is functionally elegant: FAAH sits postsynaptically to terminate anandamide near its site of synthesis, while MAGL sits presynaptically to terminate 2-AG at its site of action. This arrangement enables precise spatiotemporal control of endocannabinoid signaling.

An important metabolic consideration: both anandamide and 2-AG are hydrolyzed to arachidonic acid, a precursor to prostaglandins and other eicosanoids. This means endocannabinoid degradation directly feeds into inflammatory/anti-inflammatory lipid signaling pathways, creating another level of cross-talk between systems.

Pharmacological Implications

Understanding the distinction between anandamide and 2-AG is essential for interpreting cannabinoid pharmacology. When THC enters the system, it acts as a partial agonist at CB1 — similar to anandamide but with a much longer duration of action (hours vs. minutes) because THC is not a substrate for FAAH or MAGL. It effectively “hijacks” the retrograde signaling system, producing sustained suppression of neurotransmitter release across circuits that normally experience only brief, precisely timed endocannabinoid modulation.

CBD, by contrast, has negligible direct affinity for CB1 (Ki ≈ 2,010 nM) but influences endocannabinoid signaling indirectly: by inhibiting FAAH (raising anandamide levels), by acting as a negative allosteric modulator at CB1 (reducing THC’s efficacy at the receptor without competing for the binding site), and through multiple non-cannabinoid receptor targets. This indirect mechanism explains why CBD does not produce intoxication but can modulate THC’s effects.

The endocannabinoid system acts as a retrograde signaling system that fine-tunes synaptic transmission. It doesn’t generate signals — it modulates them.

Roger Pertwee, British Journal of Pharmacology, 2008