<unk>  <unk> , a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families.<unk>  constitutes about 80% of the catecholamine content in the brain. It is an amine synthesized by removing a carboxyl group from a molecule of its precursor chemical, L-DOPA, which is synthesized in the brain and kidneys.<unk>  is also synthesized in plants and most animals. In the brain,<unk>  functions as a neurotransmitter—a chemical released by neurons (nerve cells) to send signals to other nerve cells. Neurotransmitters are synthesized in specific regions of the brain, but affect many regions systemically. The brain includes several distinct<unk>  pathways, one of which plays a major role in the motivational component of reward-motivated behavior. The anticipation of most types of rewards increases the level of<unk>  in the brain, and many addictive drugs increase<unk>  release or block its reuptake into neurons following release. Other brain<unk>  pathways are involved in motor control and in controlling the release of various hormones. These pathways and cell groups form a<unk>  system which is neuromodulatory. In popular culture and media,<unk>  is often portrayed as the main chemical of pleasure, but the current opinion in pharmacology is that<unk>  instead confers motivational salience; in other words,<unk>  signals the perceived motivational prominence (i.e., the desirability or aversiveness) of an outcome, which in turn propels the organism's behavior toward or away from achieving that outcome. Outside the central nervous system,<unk>  functions primarily as a local paracrine messenger. In blood vessels, it inhibits norepinephrine release and acts as a vasodilator (at normal concentrations); in the kidneys, it increases sodium excretion and urine output; in the pancreas, it reduces insulin production; in the digestive system, it reduces gastrointestinal motility and protects intestinal mucosa; and in the immune system, it reduces the activity of lymphocytes. With the exception of the blood vessels,<unk>  in each of these peripheral systems is synthesized locally and exerts its effects near the cells that release it. Several important diseases of the nervous system are associated with dysfunctions of the<unk>  system, and some of the key medications used to treat them work by altering the effects of<unk> . Parkinson's disease, a degenerative condition causing tremor and motor impairment, is caused by a loss of<unk> -secreting neurons in an area of the midbrain called the substantia nigra. Its metabolic precursor L-DOPA can be manufactured; Levodopa, a pure form of L-DOPA, is the most widely used treatment for Parkinson's. There is evidence that schizophrenia involves altered levels of<unk>  activity, and most antipsychotic drugs used to treat this are<unk>  antagonists which reduce<unk>  activity. Similar<unk>  antagonist drugs are also some of the most effective anti-nausea agents. Restless legs syndrome and attention deficit hyperactivity disorder (ADHD) are associated with decreased<unk>  activity.<unk> rgic stimulants can be addictive in high doses, but some are used at lower doses to treat ADHD.<unk>  itself is available as a manufactured medication for intravenous injection: although it cannot reach the brain from the bloodstream, it's peripheral effects make it useful in the treatment of heart failure or shock, especially in newborn babies.
Dopamine
Octopamine
Norepinephrine
Norepinephrine 
Catecholamine
Synephrine
Beta-Endorphin
Prolactin-releasing peptide