Cannabinoid receptor structure and function
Answer first: Cannabinoid receptors are G protein-coupled receptors (GPCRs) that bind endogenous cannabinoids, plant cannabinoids like THC, and synthetic ligands. The two main types, CB1 and CB2, share a seven-transmembrane structure but differ in distribution, signaling, and physiological roles. CB1 is abundant in the brain and mediates psychoactive effects; CB2 is found on immune cells and regulates inflammation.
How Many Cannabinoid Receptors Are in the Brain? CB1 and CB2 Explained
What are cannabinoid receptors?
Cannabinoid receptors are part of the endocannabinoid system, which helps maintain homeostasis. They respond to endocannabinoids such as anandamide and 2-arachidonoylglycerol (2-AG). The receptors are class A GPCRs, the largest family of membrane receptors.
Endocannabinoid System Receptors Explained
Basic structure of CB1 and CB2 receptors
Both receptors share a common architecture:
- Seven transmembrane alpha helices (TM1 to TM7) that span the cell membrane and form a compact bundle.
- An extracellular N-terminus that varies in length and contributes to ligand recognition.
- Three extracellular loops and three intracellular loops that connect the helices.
- An intracellular C-terminus that interacts with signaling proteins and regulates receptor trafficking.
- A ligand-binding pocket located within the transmembrane domain, formed by residues from several helices.
How cannabinoid receptors function
Cannabinoid receptor activation follows a stepwise process:
- A ligand binds to the orthosteric site in the transmembrane pocket, causing a conformational change.
- The receptor acts as a guanine nucleotide exchange factor for the G protein alpha subunit, most often Gi/o.
- Activated Gi/o inhibits adenylyl cyclase, reducing cyclic AMP (cAMP) levels.
- The released G beta-gamma subunits modulate ion channels, such as inhibiting voltage-gated calcium channels and activating G protein-gated inwardly rectifying potassium channels.
- Downstream effects include altered neurotransmitter release, gene expression, and cell migration.
CB1 receptors also recruit beta-arrestins, which can lead to receptor desensitization and internalization. Signaling bias allows different ligands to favor specific pathways.
CB1 vs CB2 receptor differences
- Distribution: CB1 is expressed at high levels in the central nervous system, in regions such as the hippocampus, cerebellum, and basal ganglia. CB2 is found in immune tissues like the spleen, thymus, and tonsils, and on microglia.
- Ligand selectivity: THC binds both receptors with similar affinity, while CBD has low affinity for both but can modulate them through other pathways. Synthetic ligands like rimonabant (CB1 inverse agonist) and SR144528 (CB2 antagonist) show selectivity.
- Physiological roles: CB1 regulates mood, memory, appetite, and pain perception. CB2 modulates inflammation, immune cell migration, and cytokine release.
- Structure variations: CB1 has a longer N-terminus and a distinct binding pocket that accommodates lipophilic ligands. CB2 has a smaller binding pocket and different residues at key positions, affecting ligand access and signaling.
Why structure matters for cannabis effects
The precise shape of each receptor determines how cannabinoids produce their effects. For example, the CB1 binding pocket is deep and hydrophobic, which explains why THC, a lipophilic molecule, fits well. Structural studies using cryo-electron microscopy have revealed how ligands stabilize specific receptor conformations, leading to different signaling outcomes. This knowledge helps researchers design drugs that target CB1 or CB2 with selectivity, which may avoid side effects like psychoactivity or immune suppression.
Key takeaways
- Cannabinoid receptors are class A GPCRs with seven transmembrane helices.
- CB1 is neural, CB2 is immune, and their structures differ in the binding pocket and termini.
- Activation inhibits cAMP and modulates ion channels through Gi/o proteins.
- Ligand-receptor structure determines signaling bias and physiological response.