Dopamine and the reward system

Dopamine is a chemical that nerve cells use to pass messages to each other, which is why it's called a neurotransmitter. It belongs to a family of chemicals called catecholamines. In the brain, dopamine affects movement, emotions, and the reward system, the circuit that responds to things we find rewarding. One of its routes is the mesolimbic pathway: nerve cells that start in the ventral tegmental area, a region at the base of the brain, and extend their long branches all the way to the nucleus accumbens, another cluster of nerve cells.

Here's how it works on the inside. Even when nothing special is happening, the midbrain, the top part of the brainstem, keeps releasing small amounts of dopamine into the striatum, a group of structures deep inside the brain, and into the prefrontal cortex, the part right behind your forehead. That steady background trickle is called tonic release. Dopamine delivers its message by fitting into receptors, the parts of a nerve cell that pick it up. D2 receptors are high-affinity, meaning very sensitive. Think of them as a doorbell that rings at the lightest touch, and the background trickle sets off almost nothing but them. D1 receptors are more like a stiff doorbell you have to press hard. When a rewarding substance comes along, dopamine transmission goes up, the stiff doorbell finally rings, and the euphoria of those first times follows, that intense rush of feeling good.

Over time, this system can change. In addiction, dopamine neurons, the cells that release dopamine, shift from responding to the new reward itself to anticipating the cues that announce it. It's as if the system started reacting to the ice cream truck's jingle before the ice cream is even in sight. This change is called incentive salience: the system's attention moves away from the reward and toward the cue that signals it's coming.