<P> Several important diseases of the nervous system are associated with dysfunctions of the dopamine system, and some of the key medications used to treat them work by altering the effects of dopamine . Parkinson's disease, a degenerative condition causing tremor and motor impairment, is caused by a loss of dopamine - secreting neurons in an area of the midbrain called the substantia nigra . Its metabolic precursor L - DOPA can be manufactured, and in its pure form marketed as Levodopa is the most widely used treatment for the condition . There is evidence that schizophrenia involves altered levels of dopamine activity, and most antipsychotic drugs used to treat this are dopamine antagonists which reduce dopamine activity . Similar dopamine 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 dopamine activity . Dopaminergic stimulants can be addictive in high doses, but some are used at lower doses to treat ADHD . Dopamine itself is available as a manufactured medication for intravenous injection: although it cannot reach the brain from the bloodstream, its peripheral effects make it useful in the treatment of heart failure or shock, especially in newborn babies . </P> <P> A dopamine molecule consists of a catechol structure (a benzene ring with two hydroxyl side groups) with one amine group attached via an ethyl chain . As such, dopamine is the simplest possible catecholamine, a family that also includes the neurotransmitters norepinephrine and epinephrine . The presence of a benzene ring with this amine attachment makes it a substituted phenethylamine, a family that includes numerous psychoactive drugs . </P> <P> Like most amines, dopamine is an organic base . As a base, it is generally protonated in acidic environments (in an acid - base reaction). The protonated form is highly water - soluble and relatively stable, but can become oxidized if exposed to oxygen or other oxidants . In basic environments, dopamine is not protonated . In this free base form, it is less water - soluble and also more highly reactive . Because of the increased stability and water - solubility of the protonated form, dopamine is supplied for chemical or pharmaceutical use as dopamine hydrochloride--that is, the hydrochloride salt that is created when dopamine is combined with hydrochloric acid . In dry form, dopamine hydrochloride is a fine colorless powder . </P> <Table> <Tr> <Td> Biosynthetic pathways for catecholamines and trace amines in the human brain L - Phenylalanine L - Tyrosine L - DOPA Epinephrine Phenethylamine p - Tyramine Dopamine Norepinephrine N - Methylphenethylamine N - Methyltyramine p - Octopamine Synephrine 3 - Methoxytyramine AADC AADC AADC primary pathway PNMT PNMT PNMT PNMT AAAH AAAH brain CYP2D6 minor pathway COMT DBH DBH In humans, catecholamines and phenethylaminergic trace amines are derived from the amino acid phenylalanine . It is well established that dopamine is produced from L - tyrosine via L - DOPA; however, recent evidence has shown that CYP2D6 is expressed in the human brain and catalyzes the biosynthesis of dopamine from L - tyrosine via p - tyramine . </Td> </Tr> </Table>

What role does dopamine play in executive function