Dopamine is a hormone and neurotransmitter occurring in a wide variety of animals, both vertebrates and invertebrates. In the brain, this phenethylamine functions as a neurotransmitter, activating dopamine receptors and their variants. Dopamine is produced in several areas of the brain, including the substantia nigra and the ventral tegmental area.
It was discovered in 1952 by Arvid Carlsson in Sweden. Carlsson was awarded the 2000 Nobel Prize in Physiology or Medicine for showing that dopamine is not just a precursor of adrenaline but a neurotransmitter, as well.
Dopamine has many functions in the brain, including important roles in behavior and cognition, motor activity, motivation and reward, sleep, mood, attention, and learning. A common hypothesis is that dopamine has a function of transmitting reward prediction error. According to this hypothesis, the responses of dopamine neurons are observed when an unexpected reward is presented. These responses transfer to the onset of a conditioned stimulus after repeated pairings with the reward.
Further, dopamine neurons are depressed when the expected reward is omitted. Thus, dopamine neurons seem to encode the prediction error of rewarding outcomes. In nature, we learn to repeat behaviors that lead to maximize rewards. Dopamine is therefore believed to provide a teaching signal to parts of the brain responsible for acquiring new behavior. In insects, a similar reward system exists, using octopamine, a chemical relative of dopamine.
In the frontal lobes, dopamine controls the flow of information from other areas of the brain. Dopamine disorders in this region of the brain can cause a decline in neurocognitive functions, especially memory, attention, and problem solving. Reduced dopamine concentrations in the prefrontal cortex are thought to contribute to attention deficit disorder.
Dopamine is associated with the pleasure system of the brain, providing feelings of enjoyment and reinforcement to motivate a person proactively to perform certain activities. Dopamine is released by naturally rewarding experiences such as food, sex, drugs, and neutral stimuli that become associated with them. This theory is often discussed in terms of drugs such as cocaine, nicotine, and amphetamines, which seem to directly or indirectly lead to an increase of dopamine. Recent studies indicate that aggression may also stimulate the release of dopamine in this way.
Abnormally high dopamine action has also been strongly linked to psychosis and schizophrenia. Evidence comes partly from the discovery of a class of drugs called the phenothiazines that can reduce psychotic symptoms, and from the finding that drugs such as amphetamine and cocaine which are known to greatly increase dopamine levels can cause psychosis. Because of this, most modern antipsychotic medications are designed to block dopamine function to varying degrees.
Polyphenol oxidases are a family of enzymes responsible for the browning of fresh fruits and vegetables when they are cut or bruised. The natural substrate for the oxidation in bananas is dopamine. The product of their oxidation, dopamine quinone, oxidises to other quinones. The quinones then condense with amino acids to form brown pigments known as melanins. The quinones and melanins derived from dopamine help protect damaged fruit and vegetables against growth of bacteria and fungi.