IX. DOPAMINE
The primary monoamine neurotransmitters are dopamine, norepinephrine and serotonin. Dopamine and norepinephrine are catecholeamines, whereas serotonin is an indolamine.
The amino acid tyrosine is not an essential amino acid because it can be synthesized in the liver from phenylalanine by the enzyme phenylalanine hydroxylase. But it cannot be synthesized in the brain, and therefore must enter the brain by the large neutral amino acid transporter, which also transports phenylalanine, tryptophan, methionine and the branch-chained amino acids. These amino acids all compete for the transporter, so a large quantity of one of the other amino acids in the blood stream could greatly limit the amount of tyrosine entering the brain. One case in which this occurs is when there is a liver deficiency of phenylalanine hydroxylase. In that case, Phenylalanine reaches high concentrations in the blood and monopolizes the large neutral amino acid transporter, producing the mental retardation of phenylketonuria.
Dopamine synthesis [Dopamine synthesis]
Once in the brain, tyrosine can be converted to DihydrOxyPhenylAlanine (DOPA) by the tyrosine hydroxylase enzyme using oxygen, iron and TetraHydroBiopterin (THB) as co-factors. High concentrations of dopamine inhibit tyrosine hydroxylase activity through an influence on the THB co-factor. DOPA is converted to dopamine by Aromatic Amino Acid Decarboxylase (which is fairly nonspecific insofar as it will decarboxylate any aromatic amino acid) using PyridoxaL Phosphate (PLP) as a co-factor. This reaction is virtually instantaneous unless there is a Vitamin B6 deficiency.
Dopamine & epinephrine are primarily inhibitory neurotransmitters that produce arousal. This may sound paradoxical, but the most likely explanation for this effect is that the postsynaptic cells for catecholamines themselves are inhibitory. There are 3-4 times more dopaminergic cells in the CNS than adrenergic cells. Dopamine in the caudate nucleus facilitates posture, whereas dopamine in the nucleus accumbens is associated with an animal's speed (and pleasure).
[dopamine receptors]
There are two primary dopamine receptor-types: D1 (stimulatory) and D2 (inhibitory), both of which act through G-proteins. D2 receptors often occur on the dopaminergic neurons, partially for the purpose of providing negative feedback. These so-called autoreceptors can inhibit both dopamine synthesis and release.