Dopamine is a neurotransmitter occurring in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this phenethylamine functions as a neurotransmitter, activating the five types of dopamine receptors — D1, D2, D3, D4, and D5, and their variants. Dopamine is produced in several areas of the brain, including the substantia nigra and the ventral tegmental area.[1] Dopamine is also a neurohormone released by the hypothalamus

Monoamine oxidases (singular abbreviation MAO) (EC 1.4.3.4) are enzymes that catalyze the oxidation of monoamines. They are found bound to the outer membrane of mitochondria in most cell types in the body. The enzyme was discovered by Mary Hare in the liver, and received the name of tyramine oxidase.

Monoamine neurotransmitters are neurotransmitters and neuromodulators that contain one amino group that is connected to an aromatic ring by a two-carbon chain (-CH2-CH2-). All monoamines are derived from aromatic amino acids like phenylalanine, tyrosine, tryptophan, and the thyroid hormones by the action of aromatic amino acid decarboxylase enzymes.

Serotonin is a monoamine neurotransmitter that is primarily found in the gastrointestinal (gi) tract and central nervous system (cns) of animals. Approximately 80 percent of the human body's total serotonin is located in the enterochromaffin cells in the gut, where it is used to regulate intestinal movements.