A group of amines (derivatives of amino acids), three of which are known to exert important influences on the nervous system: epinephrine, norepinephrine and serotonin.Epinephrine (adrenalin) and norepinephrine (noradrenalin) are both secreted by the adrenal medulla, and are frequently described as neurohormones since they activate the sympathetic nervous system during emotional and physical stress. Their specific effects on heart rate, blood pressure, blood sugar level, etc., are described under ADRENAL GLANDS.Serotonin is chemically related to these hormones, but is an indole amine rather than a catechol amine. It is produced in certain gastrointestinal cells and carried to the internal organs through the blood stream. One of its effects is to reduce hemorrhaging when the organism is damaged, since it lowers the blood pressure. It is also found in the brain and is believed to increase its metabolism and exert a stimulating effect on the activity of the nerve cells.Recent investigations have shown that certain psychic energizers, the monoamine oxidase inhibitors (MAOI), cause the serotonin level to rise in the brain and liver, and this may be one reason for their antidepressant effect. It has also been found that the tranquilizing drug, Reserpine, causes a marked and persistent fall in the level of brain serotonin (Shore et al., 1957). Findings of this kind have led some investigators to believe that disturbances in the brain serotonin level might be a major causal factor in serious mental disturbance (Wooley and Shaw, 1954). So far, research has failed to confirm this hypothesis. See ADRENAL GLANDS, ADRENERGIC REACTION, ENERGIZERS, SCHIZOPHRENIC REACTION: (ETIOLOGY), TRANQUILIZERS.