Every other neurotransmitter we have discussed flows in one direction: the presynaptic neuron releases, the postsynaptic neuron receives. The endocannabinoid system does the opposite. It is made by the receiving neuron, travels backward across the synapse, and binds receptors on the sending neuron — telling the sender to release less.
This pattern is called retrograde signaling, and the endocannabinoid system is the most clinically important example. The two main endogenous ligands — anandamide and 2-AG — are synthesized on demand from membrane lipids in the postsynaptic neuron, diffuse backward across the cleft, and bind CB1 receptors on the presynaptic terminal. When CB1 fires, presynaptic neurotransmitter release drops.
The function is a local feedback brake. When a postsynaptic neuron has been driven hard, it manufactures a brief retrograde signal that says that was enough, ease off. The sender adjusts. Synaptic activity self-regulates without needing input from the rest of the brain.
This system is everywhere. CB1 is one of the most densely expressed G-protein-coupled receptors in the central nervous system. The endocannabinoid feedback brake operates at synapses across cortex, hippocampus, basal ganglia, cerebellum, brainstem, and limbic system. It is not focal. It is the regulatory background of essentially every circuit the brain runs.
Which is why cannabis produces such a strange, diffuse clinical picture. THC is a partial CB1 agonist. It does not bind one circuit. It perturbs the regulator that touches nearly every circuit. The user experiences this as broad dis-coordination — appetite, mood, perception, memory, motor function, anxiety, time sense all shift at once. There is no single mechanism to point to because there is no single mechanism. The drug has been dropped into the master volume knob of the brain.
Therapeutic applications exist: appetite stimulation in cachexia, antiemesis in chemotherapy, partial relief of certain pain syndromes, dramatic reduction in seizure burden for specific epilepsies (cannabidiol). But the same diffuseness that gives cannabis its broad therapeutic claims also makes it a poor scalpel for any one indication. And in the adolescent brain — where the endocannabinoid system is still organizing synaptic pruning and circuit refinement — heavy use carries developmental costs that are now well documented.
Hold the image: the receiver makes the signal, sends it backward, dials the sender down. A feedback brake unique among neurotransmitters — and a system that touches almost every synapse the brain has.