Reward system · Lobeworks/17

The reward system is the set of circuits that makes some outcomes worth seeking, centred on dopamine neurons of the ventral tegmental area and their targets in the nucleus accumbens and prefrontal cortex, and it turns out to hold at least three separable jobs: wanting a reward, liking it, and learning what predicts it.

Reward system. The reward system is the set of circuits that makes some outcomes worth seeking, centred on dopamine neurons of the ventral tegmental area and their targets in the nucleus accumbens and prefrontal cortex, and it turns out to hold at least three separable jobs: wanting a reward, liking it, and learning what predicts it.

The core is the mesolimbic pathway, from the ventral tegmental area in the midbrain to the nucleus accumbens in the ventral striatum (basal ganglia), with a branch to the prefrontal cortex. Dopamine along it carries the prediction error that teaches which cues come before good outcomes. The pleasure itself sits elsewhere: small hedonic hotspots, about a cubic millimetre in the rat's accumbens shell and another in the ventral pallidum, where opioid stimulation doubles or triples the liking reactions to a sweet taste.

Wanting and liking come apart. Rats whose dopamine is almost entirely depleted stop working for food yet still show normal liking reactions when sugar is put in their mouth; dopamine drives the pursuit (incentive salience), not the enjoyment.

That split explains craving without pleasure. Addictive drugs act on the dopamine pathway directly, so cues for the drug can grow ever more wanted while the drug is liked less and less.

Salience is the old note's filters what matters through noise. Dopamine marks which signals deserve action, a gain on the important ones; as a literal filter of noise it is a loose picture.

A blunted response to reward (anhedonia) is a core symptom of major depressive disorder, and one reason the circuit is studied there.

Wanting is dopamine, liking is a few opioid hotspots, and they can drift apart.

The system that makes a reward attractive is not the one that makes it pleasant.

Questions: Does dopamine make a reward pleasant, or make it wanted? Wanted. Rats with almost all their dopamine destroyed stop seeking food, yet when sugar is placed in their mouth they make the same liking reactions as healthy rats; raising dopamine makes animals work harder for a reward without making them like it more. Pleasure depends on small opioid hotspots in the nucleus accumbens shell and ventral pallidum. Berridge and Robinson called dopamine's part incentive salience: it makes a reward and its cues attractive and worth pursuing. The split explains craving that outlasts pleasure in addiction, where the drug's cues keep growing more wanted as the drug itself is liked less.