dopamine

Chemically, dopamine is a benzene ring (aka phenyl group), with two hydroxyl (OH) groups at adjacent sites, and a two-carbon (ethyl) chain hanging off the other side capped by an amine (NH2) group:

It is very similar to norepinephrine, which just has an extra hydroxyl group hanging off of the carbon chain, and epinephrine, which also has an extra methyl (CH3) group as a sibling of the amine group. Both are catecholamines: the benzene ring with two adjacent hydroxyl groups is the 'catechol', attached to an amine side chain. They are also substituted phenethylamines.

Dopamine is derived in the brain from L-DOPA, which has an extra COOH hanging off of the carbon chain as a sibling of the amine group. This is in turn derived from (L-)tyrosine, which is like L-DOPA but with only one hydroxyl group attached to the benzene ring. Here the amine group and the COOH act as the standard amino acid links, and the benzene ring + (single) hydroxyl is the side chain.

• Dopamine is not in itself a source of pleasure. We think it signals 'motivational prominence', i.e., that a recent experience should guide our future motivations, either positively (towards good things) or negatively (avoiding something bad).
• Dopamine is necessary for reinforcement learning. If you try to condition an animal, but inhibit their dopamine system during the training, then the positive association won't be 'stamped-in', and the conditioning doesn't work. Interestingly, if you only sometimes inhibit the dopamine system, it acts as a form of intermittent reinforcement, and the learned stimulus-response is actually stronger.
  • This is an after-the-fact effect. A trained animal with dopamine disabled will continue to perform the behaviors they've been conditioned to perform. It's only after a long period of this that the conditioning fades as there is no new reinforcement of the behavior.
  • One might hypothesize that this is due to anhedonia; that the rewards are not actually pleasurable once you take away the dopamine. There doesn't seem to be conclusive evidence for this. The alternative is that reinforcement can itself be a subconscious event, independent of subjective pleasure.
• Dopamine is strongly tied to the motor system.
• 'Dopaminergic neurons' release dopamine in the brain. Parkinson's disease is associated with the death of dopaminergic neurons. L-DOPA is a treatment.

There are five known types of dopamine receptors, but D1 and D2 are by far the most common (D3 and D4 are 'D2-like', while D5 is 'D1-like'). All are G protein-coupled receptors, which themselves are proteins that pass through the cell membrane seven times, and act inside the cell via the connected G protein. Such receptors are slower to act than 'direct' receptors in which the ligand binding directly gates a channel. The D1 receptor is linked to a 'stimulatory' G protein, while D2 is linked to a' inhibitory' protein.

• How do these actually affect neuron signaling?

Dopamine also does stuff elsewhere in the body, but it does not cross the blood-brain barrier, so this is mostly unrelated to its role as a neurotransmitter.

Sources:

• Sci-Hub | Dopamine, learning and motivation. Nature Reviews Neuroscience, 5(6), 483–494 | 10.1038/nrn1406
• Sci-Hub | The dopamine motive system: implications for drug and food addiction. Nature Reviews Neuroscience, 18(12), 741–752 | 10.1038/nrn.2017.130