Dopamine (C6H3(OH)2-CH2-CH2-NH2) is an important neurotransmitter, similar to adrenaline as a chemical messenger. Dopamine affects brain processes that control movement, emotional response, the experience of pleasure and pain, and the ability to focus attention. Dopamine is synthesized in the midbrain by cells clustered in the substantia nigra and the ventral tegmental area. It is transmitted to and released at synapses (nerve terminals) in the nucleus accumbens and striatum and the frontal and prefrontal cortex, where it binds to a receptor and generates a nerve signal. Normally after dopamine has been released and had its effect on neighboring neurons, excess amounts are pumped back into the transmitter cell axons by an active re-uptake mechanism. Cocaine is particularly destructive in binding to the re-uptake site and preventing termination of dopamine signaling. As a result, more dopamine remains to stimulate neurons, which prolongs feelings of pleasure and excitement. Many other drugs have a similar effect. For example, experiments on animals have shown that amphetamine produces a 1100% increase in dopamine levels compared with a 150% rise in a normal pleasure stimulus like food. Animals will self administer these drugs to the exclusion of all else and most eventually die.
Once returned to the sending neuron by the re-uptake system, dopamine is subject to an enzyme called monoamine oxidase (MAO), which breaks it down. Because of the vital role that MAOs play in the inactivation of neurotransmitters, MAO dysfunction is thought to be responsible for a number of neurological disorders such as depression, substance abuse, criminality, attention deficit disorder, and social phobias. Monoamine oxidase inhibitors are one of the major classes of drug prescribed for the treatment of depression. Substance abusers using cocaine, amphetamine, heroin, alcohol, and even obese individuals have lower numbers of D2 receptors (one of the 5 kinds of dopamine receptors), which may mean that they have an understimulated reward system. Susceptibility to addiction is affected by genetic make up and environment as well as chemistry. Low receptor levels may make people more susceptible. Experiments in animals have shown an increase of social interaction is protective in increasing D2 detector levels.
Wednesday, September 14, 2005