|CAS number||, (hydrochloride)|
|Jmol-3D images||Image 1|
|Molar mass||153.18 g/mol|
128 °C, 401 K, 262 °F
|Solubility in water||60.0 g/100 ml|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Dopamine is a simple organic chemical in the catecholamine family that plays a number of important roles in the brains and bodies of animals. Its name derives from its chemical structure, which consists of an amine group (NH2) linked to a catechol structure called dihydroxyphenethylamine, the decarboxylated form of dihydroxyphenylalanine (known as L-DOPA).
In the brain, dopamine functions as a neurotransmitter—a chemical released by nerve cells to send signals to other nerve cells. It plays a major role in the brain system that is responsible for reward-motivated behavior. Every type of reward that has been studied increases the level of dopamine in the brain, and a variety of addictive drugs, including stimulants such as cocaine, amphetamine, and methamphetamine, act by amplifying the effects of dopamine. Personality traits such as extraversion and reward seeking have been linked to higher sensitivity to rewarding stimuli in the dopamine system.
Several important diseases of the nervous system are associated with dysfunctions of the dopamine system. Parkinson's disease, an age-related degenerative condition causing tremor and motor impairment, is caused by loss of dopamine-secreting neurons in a small brainstem area called the substantia nigra. There is evidence that schizophrenia involves altered levels of dopamine activity, and the antipsychotic drugs that are frequently used to treat it have a primary effect of attenuating dopamine activity. Attention deficit hyperactivity disorder (ADHD) and restless legs syndrome (RLS) are also believed to be associated with decreased dopamine activity.
Outside the central nervous system dopamine acts as a hormone, released by the adrenal gland into the bloodstream, where it circulates to every part of the body. This peripheral dopamine has significant effects on the kidneys, heart, immune system, and sympathetic nervous system.
Dopamine is available as an intravenous medication, producing effects such as increased heart rate and blood pressure. However, because dopamine cannot cross the blood–brain barrier, dopamine given as a drug does not directly affect the central nervous system. To increase the amount of dopamine in the brains of patients with diseases such as Parkinson's disease, L-DOPA (the precursor of dopamine) is often given because it crosses the blood–brain barrier relatively easily.
Dopamine was first synthesized in 1910 by George Barger and James Ewens at Wellcome Laboratories in London, England. It was named dopamine because it is a monoamine whose precursor in the Barger-Ewens synthesis is 3,4-dihydroxyphenylalanine (levodopamine or L-DOPA). Dopamine's function as a neurotransmitter was first recognized in 1958 by Arvid Carlsson and Nils-Åke Hillarp at the Laboratory for Chemical Pharmacology of the National Heart Institute of Sweden. Carlsson was awarded the 2000 Nobel Prize in Physiology or Medicine for showing that dopamine is not only a precursor of norepinephrine (noradrenaline) and epinephrine (adrenaline), but also a neurotransmitter.
Biochemical mechanisms 
Structurally, dopamine belongs to the catecholamine and phenethylamine classes. In biological systems, dopamine is synthesized in brain cells and adrenal cells from the precursor L-DOPA. In brain cells, it is transported to synaptic sites and packaged into vesicles for release, which occurs during synaptic transmission. After release, free dopamine is either reabsorbed into the presynaptic terminal for reuse, or broken down by the enzymes monoamine oxidase or COMT, producing a variety of degradation metabolites.
- L-Phenylalanine → L-Tyrosine → L-DOPA → Dopamine
Thus the direct precursor of dopamine is L-DOPA, but this itself can be synthesized from the essential amino acids phenylalanine or tyrosine. These amino acids are found in nearly every protein and as such are provided from ingestion of protein-containing food, with tyrosine being the most common. Although dopamine itself is also found in many types of food, it is incapable of crossing the blood–brain barrier that surrounds and protects the brain. It must therefore be synthesized inside the brain in order to perform its neural actions.
L-Phenylalanine is converted into L-tyrosine by the enzyme phenylalanine hydroxylase (PAH), with molecular oxygen (O2) and tetrahydrobiopterin (THB) as cofactors. L-Tyrosine is converted into L-DOPA by the enzyme tyrosine hydroxylase (TH), with tetrahydrobiopterin (THB), O2, and ferrous iron (Fe2+) as cofactors. L-DOPA is converted into dopamine by the enzyme aromatic L-amino acid decarboxylase (AAAD; also known as DOPA decarboxylase (DDC)), with pyridoxal phosphate (PLP) as the cofactor.
Dopamine itself is also used as precursor in the synthesis of the neurotransmitters norepinephrine and epinephrine. Dopamine is converted into norepinephrine by the enzyme dopamine β-hydroxylase (DBH), with O2 and L-ascorbic acid as cofactors. Norepinephrine is converted into epinephrine by the enzyme phenylethanolamine N-methyltransferase (PNMT) with S-adenosyl-L-methionine (SAMe) as the cofactor.
It should be noted that some of the cofactors also require their own synthesis. Deficiency in any required amino acid or cofactor will result in subsequent dopamine, norepinephrine, and epinephrine biosynthesis impairment and deficiency.
Storage, release, and reuptake 
Upon synthesis, dopamine is transported from the cell cytosol into synaptic vesicles by the vesicular monoamine transporter 2 (VMAT2). Dopamine is stored in and remains in these vesicles until an action potential occurs and forces them to merge with the cell membrane via a process known as exocytosis, thereby dumping dopamine into synapses.
Once in the synapse, dopamine binds to and activates postsynaptic dopamine receptors, resulting in the signal of the presynaptic cell being propagated to the postsynaptic neuron. Dopamine also binds to presynaptic dopamine receptors, which can either excite the presynaptic cell or inhibit it depending on their electrical potential. Presynaptic receptors with an inhibitory potential are called autoreceptors and inhibit neurotransmitter synthesis and release. They serve to keep dopamine levels normalized in certain pathways when release is acutely disrupted and becomes too high or too low.
After dopamine has performed its synaptic duties, it is taken up via reuptake back into the presynaptic cell by either the high-affinity dopamine transporter (DAT) or the low-affinity plasma membrane monoamine transporter (PMAT). Once back in the cytosol, it is subsequently repackaged into vesicles by VMAT2.
Dopamine is directly broken down into inactive metabolites by two enzymes, monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT). It is equally metabolized by the two respective isoforms of MAO, MAO-A and MAO-B.
Dopamine is metabolized by MAO into 3,4-dihydroxyphenylacetaldehyde (DOPAL). DOPAL is further metabolized into 3,4-dihydroxyphenylacetic acid (DOPAC) by the enzyme aldehyde dehydrogenase (ALDH). DOPAL can also be reduced to 3,4-dihydroxyphenylethanol (DOPET; also known as hydroxytyrosol) by aldose reductase (AR) to a lesser extent. Finally, COMT reduces DOPAC and DOPET to homovanillic acid (HVA) and 3-methoxy-4-hydroxyphenylethanol (MOPET), respectively, which are then excreted in the urine. COMT can also directly metabolize dopamine into 3-methoxytyramine (3-MT), which is then subsequently metabolized to HVA by MAO and is excreted in the urine as well. The reactions are illustrated and summarized here:
- Dopamine → DOPAL → DOPAC → HVA
- Dopamine → DOPAL → DOPET → MOPET
- Dopamine → 3-MT → HVA
In most areas of the brain, including the striatum and basal ganglia, dopamine is inactivated by reuptake via the DAT, then enzymatic breakdown by MAO into DOPAC. In the prefrontal cortex, however, there are very few DAT proteins, and dopamine is inactivated instead by reuptake via the norepinephrine transporter (NET), presumably on neighboring norepinephrine neurons, then enzymatic breakdown by COMT into 3-MT. The DAT pathway is roughly an order of magnitude faster than the NET pathway: in mice, dopamine concentrations decay with a half-life of 200 milliseconds in the caudate nucleus (which uses the DAT pathway) versus 2,000 milliseconds in the prefrontal cortex. Dopamine that is not broken down by enzymes is repackaged into vesicles for reuse by VMAT2.
Dopamine binds to and activates a group of receptors called the dopamine receptors to mediate its physiological effects in the body. The dopamine receptors are a series of five G protein-coupled receptors (GPCRs), which consist of the D1, D2, D3, D4, and D5 receptors. As GPCRs, they work by modulating the cyclic adenosine monophosphate (cAMP) second messenger system to produce a cellular response. The five receptors are individually categorized into two distinctive groups based on their varying properties and effects, the D1-like and D2-like subfamilies. The D1 and D5 receptors belong to the D1-like subfamily. They are coupled to Gs and increase the cellular concentrations of cAMP by the activation of the enzyme adenylate cyclase. The D2, D3, and D4 receptors belong to the D2-like subfamily. They are coupled to Gi/Go and decrease the cellular concentrations of cAMP by inhibition of adenylate cyclase. Ultimately, the cAMP second messenger system, through several downstream mechanisms, works by modulating the opening of plasmalemmal ion channels that allow positively charged ions such as Na+ and K+ to enter or exit the cytoplasm of the cell, thereby generating or inhibiting an action potential. The receptors also couple directly to ion channels via the G-proteins. The D1-like receptors have various effects on neuronal activity, while the D2-like receptors tend to decrease action potential generation and are therefore usually inhibitory.
|D1-like||D1||DRD1||Gs-coupled.||Increasing intracellular levels of cAMP by activating adenylate cyclase.|
|D2-like||D2||DRD2||Gi/Go-coupled.||Decreasing intracellular levels of cAMP by inhibiting adenylate cyclase.|
The D1 receptor is the most widespread dopamine receptor in the central nervous system. The D3, D4, and D5 receptors are present in significantly lower levels than are the D1 and D2 receptors. In fact, the D1 receptors are approximately 100x more common than the D5 receptors. However, dopamine binds to the D3, D4, and D5 receptors with nanomolar or submicromolar affinity constants, while its corresponding constants for D1 and D2 receptors are in the micromolar ranges. As an example, dopamine has 20-fold higher binding affinity for the D3 receptor in comparison to the D2 receptor, and 10-fold higher binding affinity for the D5 receptor over the D1 receptor. Hence, overall activation of the system seem to be more or less well-balanced.
Functions in the brain 
Dopamine has many functions in the brain, including important roles in behavior and cognition, voluntary movement, motivation, punishment and reward, inhibition of prolactin production (involved in lactation and sexual gratification), sleep, dreaming, mood, attention, working memory, and learning. Dopaminergic neurons (i.e., neurons whose primary neurotransmitter is dopamine) are present chiefly in the ventral tegmental area (VTA) of the midbrain, the substantia nigra pars compacta, and the arcuate nucleus of the hypothalamus.
It has been hypothesized that dopamine transmits reward prediction error, although this has been questioned. According to this hypothesis, the phasic responses of dopamine neurons are observed when an unexpected reward is presented. These responses transfer to the onset of a conditioned stimulus after repeated pairings with the reward. Further, dopamine neurons are depressed when the expected reward is omitted. Thus, dopamine neurons seem to encode the prediction error of rewarding outcomes. In nature, we learn to repeat behaviors that lead to maximizing rewards. Dopamine is therefore believed to provide a teaching signal to parts of the brain responsible for acquiring new behavior. Temporal difference learning provides a computational model describing how the prediction error of dopamine neurons is used as a teaching signal.
The reward system in insects uses octopamine, which is the presumed arthropod homolog of norepinephrine, rather than dopamine. In insects, dopamine acts instead as a punishment signal and is necessary to form aversive memories.
Dopaminergic neurons form a neurotransmitter system which originates in substantia nigra pars compacta, ventral tegmental area (VTA), and hypothalamus. These project axons to large areas of the brain which are typically divided into four major pathways:
- Mesocortical pathway connects the ventral tegmental area to the frontal lobe of the pre-frontal cortex. Neurons with somas in the ventral tegmental area project axons into the pre-frontal cortex.
- Mesolimbic pathway carries dopamine from the ventral tegmental area to the nucleus accumbens via the amygdala and hippocampus. The somas of the projecting neurons are in the ventral tegmental area.
- Nigrostriatal pathway runs from the substantia nigra to the neostriatum. Somas in the substantia nigra project axons into the caudate nucleus and putamen. The pathway is involved in the basal ganglia motor loop.
- Tuberoinfundibular pathway runs from the hypothalamus to the pituitary gland.
This innervation explains many of the effects of activating this dopamine system. For instance, the mesolimbic pathway connects the VTA and nucleus accumbens; both are central to the brain reward system.
Whilst the distinction between pathways is widely used, and is regarded as a "convenient heuristic when considering the dopamine system", it is not absolute, and there is some overlap in the projection targets of each group of neurons.
Cellular effects 
Tonic and phasic activity 
The level of extracellular dopamine is modulated by two mechanisms: tonic and phasic dopamine transmission. Tonic dopamine transmission occurs when small amounts of dopamine are released independently of neuronal activity, and is regulated by the activity of other neurons and neurotransmitter reuptake. Phasic dopamine release results from the activity of the dopamine-containing cells themselves. This activity is characterized by irregular pacemaking activity of single spikes, and rapid bursts of typically 2-6 spikes in quick succession. Concentrated bursts of activity result in a greater increase of extracellular dopamine levels than would be expected from the same number of spikes distributed over a longer period of time.
Reuptake inhibition and synaptic release 
Cocaine and amphetamines inhibit the re-uptake of dopamine; however, they influence separate mechanisms of action. Cocaine is a dopamine transporter and norepinephrine transporter blocker that competitively inhibits dopamine uptake to increase the lifetime of dopamine and augments an overabundance of dopamine (an increase of up to 150 percent) within the parameters of the dopamine neurotransmitters. Like cocaine, amphetamines increase the concentration of dopamine in the synaptic gap, but by a different mechanism. Amphetamines and methamphetamine are similar in structure to dopamine, and so can enter the terminal bouton of the presynaptic neuron via its dopamine transporters as well as by diffusing through the neural membrane directly. By entering the presynaptic neuron, amphetamines force dopamine molecules out of their storage vesicles and expel them into the synaptic gap by making the dopamine transporters work in reverse.
Motor control 
Dopamine reduces the influence of the indirect pathway while increasing the actions of the direct pathway within the basal ganglia. Insufficient dopamine biosynthesis in the dopaminergic neurons can cause Parkinson's disease, a condition in which one loses the ability to execute smooth, controlled movements.
Regulating prolactin secretion 
Dopamine is the primary neuroendocrine inhibitor of the secretion of prolactin from the anterior pituitary gland. Dopamine produced by neurons in the arcuate nucleus of the hypothalamus is secreted into the hypothalamo-hypophysial blood vessels of the median eminence, which supply the pituitary gland. The lactotrope cells that produce prolactin, in the absence of dopamine, secrete prolactin continuously; dopamine inhibits this secretion. Thus, in the context of regulating prolactin secretion, dopamine is occasionally called prolactin-inhibiting factor (PIF), prolactin-inhibiting hormone (PIH), or prolactostatin.
Cognition and frontal cortex 
In the frontal lobes, dopamine controls the flow of information from other areas of the brain. Dopamine disorders in this region of the brain can cause a decline in neurocognitive functions, especially memory, attention, and problem-solving. Reduced dopamine concentrations in the prefrontal cortex are thought to contribute to attention deficit disorder. It has been found that D1 receptors as well as D4 receptors are responsible for the cognitive-enhancing effects of dopamine, whereas D2 receptors are more specific for motor actions.
Chemoreceptor trigger zone 
Dopamine is one of the neurotransmitters implicated in the control of nausea and vomiting via interactions in the chemoreceptor trigger zone. Metoclopramide is a D2-receptor antagonist that functions as a prokinetic/antiemetic.
Effects of drugs that reduce dopamine activity 
In humans, drugs that reduce dopamine activity (neuroleptics, e.g. antipsychotics) have been shown to impair concentration, reduce motivation, cause anhedonia (inability to experience pleasure), and long-term use has been associated with tardive dyskinesia, an irreversible movement disorder. Antipsychotics have significant effects on gonadal hormones including significantly lower levels of estradiol and progesterone in women, whereas men display significantly lower levels of testosterone and DHEA when undergoing antipsychotic drug treatment compared to controls. Antipsychotics are known to cause hyperprolactinaemia leading to amenorrhea, cessation of normal cyclic ovarian function, loss of libido, occasional hirsutism, false positive pregnancy tests, and long-term risk of osteoporosis in women. The effects of hyperprolactinemia in men are gynaecomastia, lactation, impotence, loss of libido, and hypospermatogenesis. Furthermore, antipsychotic drugs are associated with weight gain, diabetes, drooling, dysphoria (abnormal depression and discontent), fatigue, sexual dysfunction, heart rhythm problems, stroke and heart attack.
Opioid and cannabinoid transmission 
Opioid and cannabinoid transmission instead of dopamine may modulate consummatory pleasure and food palatability (liking). This could explain why animals' liking of food is independent of brain dopamine concentration. Other consummatory pleasures, however, may be more associated with dopamine. One study found that both anticipatory and consummatory measures of sexual behavior (male rats) were disrupted by DA receptor antagonists. Libido can be increased by drugs that affect dopamine, but not by drugs that affect opioid peptides or other neurotransmitters.
Learning, reinforcement, and reward-seeking behavior 
Dopamine is commonly associated with the reward system of the brain, providing feelings of enjoyment and reinforcement to motivate a person to perform certain activities. Dopamine is released (particularly in areas such as the nucleus accumbens and prefrontal cortex) as a result of rewarding experiences such as food, sex, drugs, and neutral stimuli that become associated with them. Recent studies indicate that aggression may also stimulate the release of dopamine in this way.
This theory can be discussed in terms of drugs such as cocaine, nicotine, and amphetamines, which directly or indirectly lead to an increase of dopamine in the mesolimbic reward pathway of the brain, and in relation to neurobiological theories of chemical addiction (not to be confused with psychological dependence), arguing that this dopamine pathway is pathologically altered in addicted persons. In recent studies, cholinergic inactivation of the nucleus accumbens was able to disrupt the acquisition of drug reinforced behaviors, suggesting that dopamine has a more limited involvement in the acquisition of both drug self-administration and drug-conditioned place-preference behaviors than previously thought.
Dopaminergic neurons of the midbrain are the main source of dopamine in the brain. Dopamine has been shown to be involved in the control of movements, the signaling of error in prediction of reward, motivation, and cognition. Cerebral dopamine depletion is the hallmark of Parkinson's disease. Other pathological states have also been associated with dopamine dysfunction, such as schizophrenia, autism, and attention deficit hyperactivity disorder, as well as drug abuse.
Dopamine is closely associated with reward-seeking behaviors, such as approach, consumption, and addiction. Recent research suggests that the firing of dopaminergic neurons is motivational as a consequence of reward-anticipation. This hypothesis is based on the evidence that, when a reward is greater than expected, the firing of certain dopaminergic neurons increases, which consequently increases desire or motivation towards the reward. However, recent research finds that while some dopaminergic neurons react in the way expected of reward neurons, others do not and seem to respond in regard to unpredictability. This research finds the reward neurons predominate in the ventromedial region in the substantia nigra pars compacta as well as the ventral tegmental area. Neurons in these areas project mainly to the ventral striatum and thus might transmit value-related information in regard to reward values. The nonreward neurons are predominate in the dorsolateral area of the substantia nigra pars compacta which projects to the dorsal striatum and may relate to orienting behaviour. It has been suggested that the difference between these two types of dopaminergic neurons arises from their input: reward-linked ones have input from the basal forebrain, while the nonreward-related ones from the lateral habenula.
Animal studies 
Clues to dopamine's role in motivation, desire, and pleasure have come from studies performed on animals. In one such study, rats were depleted of dopamine by up to 99 percent in the nucleus accumbens and neostriatum using 6-hydroxydopamine. With this large reduction in dopamine, the rats would no longer eat of their own volition. The researchers then force-fed the rats food and noted whether they had the proper facial expressions indicating whether they liked or disliked it. The researchers of this study concluded that the reduction in dopamine did not reduce the rat's consummatory pleasure, only the desire to eat. In another study, mutant hyperdopaminergic (increased dopamine) mice show higher "wanting" but not "liking" of sweet rewards. Mice who cannot synthesize dopamine are unable to feed sufficiently to survive more than a few weeks after birth, but will feed normally and survive if administered L-DOPA.
Dopamine modulates foraging behavior in animals, by activating brain systems registering reward when food sources are found. When monkeys are given a highly palatable food, dopamine levels rise, but levels then decline when the palatable food is available for prolonged periods of time and is no longer novel.
Dopamine may also have a role in the salience of potentially important stimuli, such as sources of reward or of danger. This hypothesis argues that dopamine assists decision-making by influencing the priority, or level of desire, of such stimuli to the person concerned.
Dopamine's role in experiencing pleasure has been questioned by several researchers. It has been argued that dopamine is more associated with anticipatory desire and motivation (commonly referred to as "wanting") as opposed to actual consummatory pleasure (commonly referred to as "liking").
Latent inhibition and creative drive 
Dopamine in the mesolimbic pathway increases general arousal and goal directed behaviors and decreases latent inhibition; all three effects increase the creative drive of idea generation. This has led to a three-factor model of creativity involving the frontal lobes, the temporal lobes, and mesolimbic dopamine.
Since dopamine drives reward-seeking behavior and successive sensations of contentment from social interactions, sociability is also closely tied to dopamine neurotransmission. Low D2 receptor-binding is found in people with social anxiety. Traits common to negative schizophrenia (social withdrawal, apathy, anhedonia) are thought to be related to a hypodopaminergic state in certain areas of the brain. In instances of bipolar disorder, manic subjects can become hypersocial, as well as hypersexual. This is credited to an increase in dopamine, because mania can be reduced by dopamine-blocking antipsychotics.
Processing of pain 
Dopamine has been demonstrated to play a role in pain processing in multiple levels of the central nervous system including the spinal cord, periaqueductal gray (PAG), thalamus, basal ganglia, insular cortex, and cingulate cortex. Accordingly, decreased levels of dopamine have been associated with painful symptoms that frequently occur in Parkinson's disease. Abnormalities in dopaminergic neurotransmission have also been demonstrated in painful clinical conditions, including burning mouth syndrome, fibromyalgia, and restless legs syndrome. In general, the analgesic capacity of dopamine occurs as a result of dopamine D2 receptor activation; however, exceptions to this exist in the PAG, in which dopamine D1 receptor activation attenuates pain presumably via activation of neurons involved in descending inhibition. In addition, D1 receptor activation in the insular cortex appears to attenuate subsequent pain-related behavior.
Behavior disorders 
Deficient dopamine neurotransmission is implicated in attention-deficit hyperactivity disorder, and stimulant medications that are used to treat its symptoms increase dopamine neurotransmission. Consistent with this hypothesis, dopaminergic pathways have a role in inhibitory action control and the inhibition of the tendency to make unwanted actions.
Dopaminergic mind hypothesis 
The dopaminergic mind hypothesis seeks to explain the differences between modern humans and their hominid relatives by focusing on changes in dopamine. It theorizes that increased levels of dopamine were part of a general physiological adaptation due to an increased consumption of meat around two million years ago in Homo habilis, and later enhanced by changes in diet and other environmental and social factors beginning approximately 80,000 years ago. Under this theory, the "high-dopamine" personality is characterized by high intelligence, a sense of personal destiny, a religious/cosmic preoccupation, an obsession with achieving goals and conquests, an emotional detachment that in many cases leads to ruthlessness, and a risk-taking mentality. High levels of dopamine are proposed to underlie increased psychological disorders in industrialized societies. According to this hypothesis, a "dopaminergic society" is an extremely goal-oriented, fast-paced, and even manic society, "given that dopamine is known to increase activity levels, speed up our internal clocks and create a preference for novel over unchanging environments." In the same way that high-dopamine individuals lack empathy and exhibit a more masculine behavioral style, dopaminergic societies are "typified by more conquest, competition, and aggression than nurturance and communality." Although behavioral evidence and some indirect anatomical evidence (e.g., enlargement of the dopamine-rich striatum in humans) support a dopaminergic expansion in humans, there is still no direct evidence that dopamine levels are markedly higher in humans relative to other apes. However, recent discoveries about the sea-side settlements of early man may provide evidence of dietary changes consistent with this hypothesis.
Links to psychosis 
Abnormally high dopaminergic transmission has been linked to psychosis and schizophrenia. However, clinical studies relating schizophrenia to brain dopamine metabolism have ranged from controversial to negative, with HVA levels in the CSF the same for schizophrenics and controls. Increased dopaminergic functional activity, specifically in the mesolimbic pathway, is found in schizophrenic individuals. However, decreased activity in another dopaminergic pathway, the mesocortical pathway, may also be involved. The two pathways are thought to be responsible for differing sets of symptoms seen in schizophrenia.
Antipsychotic medications act largely as dopamine antagonists, inhibiting dopamine at the receptor level, and thereby blocking the effects of the neurochemical in a dose-dependent manner. The older, so-called typical antipsychotics most commonly act on D2 receptors, while the atypical drugs also act on D1, D3 and D4 receptors, though they have a lower affinity for dopamine receptors in general. The finding that drugs such as amphetamines, methamphetamine and cocaine, which can increase dopamine levels by more than tenfold, can temporarily cause psychosis, provides further evidence for this link. However, many non-dopaminergic drugs can induce acute and chronic psychosis. The NMDA antagonists Ketamine and PCP both are used in research to reproduce the positive and negative symptoms commonly associated with schizophrenia.
Dopaminergic dysregulation has also been linked to depressive disorders. Early research in humans used various methods of analyzing dopamine levels and function in depressed patients. Studies have reported that there is decreased concentration of tyrosine, a precursor to dopamine, in the blood plasma, ventricular spinal fluid, and lumbar spinal fluid of depressed patients compared to control subjects.  One study measured the amount of homovanillic acid, the major metabolite of dopamine in the CSF, as a marker for the dopamine pathway turnover rate, and found decreased concentrations of homovanillic acid in the CSF of depressed patients. Postmordem real time reverse transcriptase-polymerase chain reaction (RT-PCR) has also been used to find that gene expression of a specific subtype of dopamine receptor was elevated in the amygdale of people suffering from depression as compared to control subjects.
The action of commonly used antidepressant drugs also has yielded information about possible alterations of the dopaminergic pathway in treating depression. It has been reported that many antidepressant drugs increase extracellular dopamine concentrations in the rat prefrontal cortex, but vary greatly in their affects on the striatum and nucleus accumbens.  This can be compared to electro convulsive shock treatment (ECT), which has been shown to have a multiple fold increase in striatal dopamine levels in rats.
More recent research studies with rodents have found that depression-related behaviors are associated with dopaminergic system dysregulation. In rodents exposed to chronic mild stress, decreased escape behavior and decreased forced swimming is reversed with activation of the dopaminergic mesolimbic pathway. Also, rodents that are susceptible to depression-related behavior after social defeat can have their behavior reversed with dopamine pathway activation. Depletion of dopamine in the caudate nucleus and nucleus accumbens has also been reported in cases of learned helplessness in animals. These symptoms can be reversed with dopamine agonists and antidepressant administration prior to the learned helplessness protocol.
Therapeutic use 
Under the trade names Intropan, Inovan, Revivan, Rivimine, Dopastat, and Dynatra, dopamine, as well as norepinephrine and epinephrine, are also used as pharmaceutical drugs in injectable forms in the emergency clinical treatment of severe hypotension and/or bradycardia, circulatory shock, and cardiac arrest, the latter of which for the purpose of cardiopulmonary resuscitation.
Levodopa is a dopamine precursor used in various forms to treat Parkinson's disease and dopa-responsive dystonia. It is typically co-administered with an inhibitor of peripheral decarboxylation (DDC, dopa decarboxylase), such as carbidopa or benserazide. Inhibitors of alternative metabolic route for dopamine by catechol-O-methyl transferase are also used. These include entacapone and tolcapone.
Nonneural functions 
Renal and cardiovascular 
Dopamine (brand name Intropin or Giludop) also has effects when administered through an IV line outside the central nervous system. The effects in this form are dose dependent.
- Dopamine induces natriuresis (sodium loss) in the kidneys, and has a diuretic effect, potentially increasing urine output from 5 ml/kg/hr to 10 ml/kg/hr. Dosages from 2 to 5 μg/kg/min are considered the "renal dose". It was once thought that at this low dosage provided increased renal perfusion in critically ill patients. The mechanism was thought to involve dopamine binding D1 receptors, dilating blood vessels, increasing blood flow to renal, mesenteric, and coronary arteries, which would thus increase overall renal perfusion. However, recent multi-center, randomized trials have shown that this is not clinically effective. Thus, "renal dose" dopamine is largely considered a myth that has been propagated in medicine for the past 30 years.
- Intermediate dosages from 5 to 10 μg/kg/min, known as the "cardiac dose", additionally have a positive inotropic and chronotropic effect through increased β1 receptor activation. Dopamine is used in patients with shock or heart failure to increase cardiac output and blood pressure. Dopamine begins to affect the heart at lower doses, from about 3 μg/kg/min IV.
- High doses from 10 to 20 μg/kg/min are the "pressor dose". This dose causes vasoconstriction, increases systemic vascular resistance, and increases blood pressure through α1 receptor activation, but can cause the vessels in the kidneys to constrict to the point that urine output is reduced.
Dopamine acts upon receptors present on immune cells, with all subtypes of dopamine receptors found on leukocytes. There is low expression of receptors on T lymphocytes and monocytes, moderate expression on neutrophils and eosinophils, and high expression on B cells and natural killer cells. The sympathetic innervation of lymphoid tissues is dopaminergic, and increases during stress. Dopamine can also affect immune cells in the spleen, bone marrow, and blood circulation. In addition, dopamine can be synthesized and released by the immune cells themselves.
The effects of dopamine on immune cells depend upon their physiological state. While dopamine activates resting T cells, it inhibits them when they are activated. Disorders such as schizophrenia and Parkinson's disease, in which there are changes in brain dopamine receptors and dopamine signaling pathways, are also associated with altered immune functioning.
The LD50, or dose which is expected to be lethal in 50% of the population, has been found to be: 59 mg/kg (mouse; administered i.v.); 950 mg/kg (mouse; administered i.p.); 163 mg/kg (rat; administered i.p.); 79 mg/kg (dog; administered i.v.)[clarification needed]
In plants 
Fruit browning 
Polyphenol oxidases (PPOs) are a family of enzymes responsible for the browning of fresh fruits and vegetables when they are cut or bruised. These enzymes use molecular oxygen (O2) to oxidise various 1,2-diphenols to their corresponding quinones. The natural substrate for PPOs in bananas is dopamine. The product of their oxidation, dopamine quinone, spontaneously oxidises to other quinones. The quinones then polymerise and condense with amino acids and proteins to form brown pigments known as melanins. The quinones and melanins derived from dopamine may help protect damaged fruit and vegetables against growth of bacteria and fungi.
See also 
- Catechol-O-methyl transferase
- Classical conditioning
- Dopamine hypothesis of schizophrenia
- Dopamine reuptake inhibitor
- Epinine (N-methyldopamine)
- Limbic system
- Operant conditioning
- Parkinson's disease
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