Partial Agonist
A drug that binds to a receptor and activates it, but produces a submaximal response even at full receptor occupancy. THC is a partial agonist at CB1 and CB2 receptors.
In Depth
THC's partial agonism at CB1 receptors (intrinsic efficacy ~50% of full agonists like 2-AG) is clinically important: it limits the maximum psychoactive effect achievable and contributes to a ceiling effect on some adverse effects. Full synthetic CB1 agonists (like those in illicit "Spice" products) produce far more intense and dangerous effects. CBD acts as a negative allosteric modulator at CB1 receptors, reducing the efficacy of THC and other agonists — a potential mechanism for CBD's ability to moderate THC's psychoactive effects.
Related Terms
More in Pharmacology
Endocannabinoid System (ECS)
A lipid-based retrograde neurotransmitter system comprising endogenous cannabinoids (endocannabinoids), their receptors (CB1, CB2), and metabolic enzymes.
CB1 Receptor
Cannabinoid receptor type 1. A G protein-coupled receptor (GPCR) primarily expressed in the central nervous system. The primary target of THC's psychoactive effects.
CB2 Receptor
Cannabinoid receptor type 2. A GPCR primarily expressed in immune tissues and peripheral organs. Less abundant in the CNS than CB1.
Anandamide (AEA)
N-arachidonoylethanolamine. The first endocannabinoid identified. A partial agonist at CB1 and CB2 receptors, named from the Sanskrit word "ananda" meaning bliss.
2-Arachidonoylglycerol (2-AG)
The most abundant endocannabinoid in the brain. A full agonist at both CB1 and CB2 receptors.