Dopamine and norepinephrine are catecholamines that are widely spread in the brain and peripheral nervous system. Dopamine is present in three circuits in the brain that control movements, cause psychiatric symptoms such as psychosis and regulate hormonal responses.

Dopamine: The Three Circuits

The dopamine circuit that influences movement has been directly linked to several diseases. People with Parkinson’s disease have brains which posses as good as no dopamine. So, medical scientists have discovered that the chemical called levodopa, a substance from which dopamine is synthesized, is an effective treatment for Parkinson’s, allowing patients to walk and perform finely tuned movements accurately.
The second circuit is quite important for memory and cognition. Abnormalities in this circuit have been implicated in schizophrenia. Drugs that block dopamine receptors in the brain are found to be very helpful in diminishing numerous psychotic symptoms. Learning more about this dopamine circuit could prove to be essential in understanding several mental illnesses.
In the third circuit, the endocrine system is regulated by dopamine. It directs the hypothalamus to produce hormones and store them in the pituitary gland. It orders the hormones to be released into the bloodstream or to trigger the release of hormones held within cells of the pituitary gland.

Norepinephrine

Norepinephrine containing nerve fibers are present throughout the brain. Deficiencies in this neurotransmitter are found to occur in patients with Alzheimer’s disease, Parkinson’s disease and Korsakoff’s syndrome, a cognitive disorder associated with chronic alcoholism. As a result of these findings, norepinephrine is thought to play a role in both learning and memory.

This neurotransmitter is also secreted by the sympathetic nervous system in the periphery to regulate heart rate and blood pressure. Acute stress increases the release of norepinephrine.

Serotonin

This neurotransmitter can be found in many tissues, particularly blood platelets, the lining of the digestive tract and the brain. It was first thought to be involved in high blood pressure since it is present in the blood and induces quite powerful contractions of the smooth muscles.

In the brain, it has been implicated in sleep, mood, depression and anxiety. Because serotonin controls the different switches controlling emotional states, it is believed that these switches might be manipulated by analogs, which are chemicals with a structure similar to serotonin.

Therapeutic drugs that alter serotonin’s actions, such as fluoxetine (A substance present in Prozac and several other anti-depressants), have relieved symptoms of both depression and obsessive compulsive disorder.