Dopamine is one of two neurotransmitters that are important in ADD and ADHD. The other neurotransmitter is norepinephrine. Dopamine is both a hormone and neurotransmitter, which occurs in a wide variety of vertebrates and invertebrates. In the brain dopamine activates five types of dopamine receptors: D1,D2, D3, D4, and D5. Arid Carlsson and Niels Ake-Hillarp, Swedish scientists, discovered dopamine in 1952. Carlsson won the 2000 Nobel Prize in Physiology or Medicine for showing that dopamine is a neurotransmitter, in addition to being a precursor or forerunner of norepinephrine (noradrenaline) and epinephrine (adrenaline).

Chemically dopamine is a member of the catecholamine family, a class of molecules that serve both as neurotransmitters and hormones. The chemical formula is C6H3(OH)2-CH2-CH2-NH2. It is a monoamine compound containing nitrogen formed from ammonia by replacement of one or more of the hydrogen atoms. As a neurotransmitter, dopamine is ''packaged'' after manufacture in the brain into chemicals that help transmit electrical impulses in the brain from one neuron to another. The chemical is stored in tiny bubbles called vesicles, located near the back end of each neuron. When a message comes through a neuron, the vesicles release small amounts of the neurotransmitter to help carry the electrical impulse across the synapse or gap between neurons. The action is very quick. Each molecule of a transmitter stays on the receptor for only about 50 milliseconds (one thousandth of a second). Twelve messages can be carried across the synapse in 1 millisecond. Any transmitter not used in the process is pumped back into the vesicle by specialized cells called transporters. This movement allows the system to reload for more action in a fraction of a second. At any one given time, vast numbers of messages are surging through millions of circuits in the brain all supported by neurotransmitters.

Dopamine has many functions in the brain, including important roles in behavior and cognition, motor activity, motivation and reward, inhibition, sleep, mood, inattention, and learning. The neurons whose primary neurotransmitter is dopamine are located in the ventral tegmental area of the midbrain, substantia nigra, and the arcuate nucleus of the hypothalamus. Dopamine is believed to provide a teaching signal to parts of the brain responsible for acquiring new behaviors and is also related to the reward system and acquiring of habits. In the frontal lobes, dopamine controls the flow of information from other areas of the brain. Disorders of dopamine in this area can cause a decline in neurocognitive functions, including memory, attention, and problem solving. Reduced dopamine concentrations in the prefrontal cortex are thought to contribute to ADHD. D1 receptors are responsible for the cognitive-enhancing effects of dopamine.

Both cocaine and amphetamines inhibit the reuptake of dopamine, but in different ways. Cocaine blocks the dopamine transporter and then inhibits dopamine uptake, creating an overabundance of dopamine within the neurotransmitters. Amphetamines are similar in structure to dopamine and can enter the terminal of the presynaptic neuron by way of its dopamine transporter and also by diffusing through the neural membrane. When the stimulants enter, they force dopamine molecules out of their storage vesicles and expel them into the synaptic gap.

Depletion of dopamine in the brain is associated with several pathological conditions. It is the hallmark of Parkinson's disease and has been studied for connections to schizophrenia, autism, ADHD, and drug abuse.

Bibliography:

Heijtz, R. D. et al. 2007. Motor inhibitory role of dopamine D1 receptors: Implications for ADHD. Physiological Behavior 92(1-2):155-60

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