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Dopamine

Dopamine

BIBLIOGRAPHY

Dopamine is a neurotransmitter that serves as a chemical messenger in the nervous system and permits individual nerve fibers (neurons) to communicate with each other. The dopamine neurotransmitter belongs to the class of compounds known as monoamines, and more specifically to a subclass of chemicals called catecholamines. Dopamine can act either as an inhibitory mechanism or an excitatory mechanism in the nervous system, depending on the location of dopamine neurons, and the receiving characteristics of the next neuron in the chain.

Dopamine activation has long been associated with increased motor output (i.e., increased physical activity) (Wise 2004). Hence, it is not surprising that dopamine depletion is associated with a variety of movement disorders, such as Parkinsons disease. Characterized by tremors, muscle rigidity, and lack of fine motor skills, Parkinsons is caused by a degeneration of dopamine projection fibers originating in a brain region called the substantia nigra. The fact that the administration of a substance (L-DOPA) that increases dopamine synthesis in this brain region is the primary approach to treating Parkinsonism underscores the importance of dopamine in the regulation of motor control and movement.

Changes in dopamine activity also are linked to the expression of certain psychological disorders, such as schizophrenia. Schizophrenia is characterized by shifting, illogical thought patterns, delusional thought processes, and hallucinations. The dopamine hypothesis of schizophrenia suggests that higher than normal levels of dopamine in the midbrain region of patients suffering from schizophrenia produce a biochemical imperative to engage in disordered behavior. Consistent with this position, the most commonly prescribed, and arguably the most effective, drug therapies for schizophrenia are dopamine receptor blockers. A compound labeled chlorpromazine (trade name Thorazine) is especially effective in reducing the symptoms of schizophrenia, and such dopamine antagonists when continued after treatment substantially lessen the chances for relapse compared to cases in which patients stop taking the drug.

There is evidence that blockade of dopamine transmission is associated with the devaluation of incentive systems, perhaps by affecting memory consolidation (Robbins and Everitt 2006). For instance, it is known that stamping-in of stimulus-response associations is blunted under conditions of reduced dopamine activity. Even once a behavior is learned, evidence shows that the ability to retrieve previously acquired information is reduced. Although the precise mechanisms responsible for these challenges to associative processes is not clear, it is clear that reward-seeking is diminished when dopamine systems are compromised.

There is a large literature that shows that a variety of rewarding events elevate the levels of dopamine in pleasure pathways of the brain. There are three major systems that are rich in dopamine fibers: the nigrostriatal system, the mesolimbic system, and the mesocortical system. Of the three, the pathway that has received the most attention from investigators of reward systems is the mesolimbic pathway. The dopamine projection neurons of the mesolimbic system originate in the ventral tegmental area of the midbrain and terminate in several forebrain regions, most importantly the nucleus accumbens. At one time it was believed that the nucleus accumbens constituted reward central and any events or substances that increased dopamine activity in this region served as rewards (Wise and Bozarth 1987). It is now known that other pathways and neurotransmitters are involved in defining reward properties, but the scientific community still maintains that elevated levels of dopamine in the nucleus accumbens contribute prominently to the rewarding effects associated with a variety of motivational processes, including the sex drive and hunger (Berridge and Robinson 1998).

Although dopamine plays a role in mediating a broad array of reinforcing (reward) activities, the topic that has been studied most is the modulatory role played by dopamine in determining the rewarding effects of psychoactive drugs. While dopamine is important for drugs such as heroin, marijuana, and alcohol, it does not appear to be crucial with respect to determining the reward value of these types of drugs. It is certain, however, that dopamine is the major neurotransmitter involved in defining the potency and addiction potential of psycho-stimulants such as cocaine and amphetamine. With respect to cocaine, the drug blocks the action of the dopamine transporter (DAT) in the nucleus accumbens. DAT is the reuptake chemical in the synaptic cleft (space between neurons where neurotransmitters are released) that moves dopamine back inside the releasing neuron and restores dopamine levels. When DAT is blocked by cocaine, dopamine remains in the cleft and continues to stimulate the postsynaptic neuron, thus producing euphoria. Amphetamine operates similarly to block dopamine reuptake, but also increases the frequency and amount of dopamine release.

SEE ALSO Happiness; Needs; Neuroscience; Psychology; Schizophrenia; Wants

BIBLIOGRAPHY

Berridge, K. C., and T. E. Robinson. 1998. What Is the Role of Dopamine in Reward: Hedonic Impact, Reward Learning, or Incentive Salience? Brain Research Review 28 (3): 309369.

Robbins, T. W., and B. J. Everitt. 2006. A Role for Mesencephalic Dopamine in Activation: Commentary on Berridge (2006). Psychopharmacology 191 (3): 433437.

Wise, Roy A. 2004. Dopamine, Learning, and Motivation. Nature Reviews in Neuroscience 5 (6): 483494.

Wise, Roy A., and Michael A. Bozarth. 1987. A Psychomotor Stimulant Theory of Addiction. Psychological Review 94 (4): 469492.

Jack Nation

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Dopamine

Dopamine


Dopamine belongs to a family of biological compounds called catecholamines (see Figure 1). Dopamine is synthesized from the compound L-dihydroxyphenylalanine (L-dopa) via the enzyme dopa decarboxylase. In noradrenergic neurons and in the adrenal glands, dopamine is the precursor for the neurotransmitter norepinephrine. In dopaminergic neurons, dopamine itself acts as a neurotransmitter. Although dopaminergic neurons are not as widely distributed in the brain as noradrenergic neurons, they act to coordinate movement, to control the secretion of some hormones, and to regulate mood and emotional stability.

Dopamine's role in the coordination of movement can be partially understood by examining Parkinson's disease. This illness is associated with low levels of dopamine in the brain and is characterized by spastic motion of the eyelids as well as rhythmic tremors of the hands and other parts of the body. One method of treating Parkinson's disease is to increase the concentration of dopamine in the brain. This is most effectively accomplished by administering the precursor of dopamine, L-dopa. In order to prevent concentrations of norepinephrine from increasing as well, L-dopa is given in conjunction with a drug that inhibits norepinephrine synthesis .

The role that dopamine plays in regulating mood and emotional stability can be at least partially grasped by examining dopamine's role in schizophrenia and drug addiction. Schizophrenia is a disorder characterized by delusions, hallucinations, withdrawal from external reality, and emotional unresponsiveness. The dopamine theory of schizophrenia, proposed in 1965, attributes the disorder to elevated brain concentrations of dopamine or to a hypersensitivity of dopaminergic receptors , especially the D2 and D4 receptor subtypes. Several drugs used to treat schizophrenic patients bind to D2 and D4 receptors and block the dopaminergic response.

Dopamine is also an important component of the brain's "reward system" and is believed to play a role in drug addiction. Increased levels of dopamine have been associated with cocaine, amphetamine , and marijuana use, as well as alcohol and nicotine addiction.

see also Neurotransmitters.

Jennifer L. Powers

Bibliography

Balter, Michael (1996). "New Clues to Brain Dopamine Control, Cocaine Addiction." Science 271:909.

Internet Resources

Indiana University School of Medicine, Terre Haute Center for Medical Education. The Medical Biochemistry Page. "Biochemistry of Nerve Transmission." Available from <http://web.indstate.edu/thcme/mwking/nerves.html>.

Northeastern University, Physical Therapy Department. Neuroanatomy Cyberlectures. "Pharmacology: The Chemistry of the Nervous System." Available from <http://www.ptd.neu.edu/neuroanatomy/cyberclass/Pharmacology>.

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dopamine

dopamine is a catecholamine, from which other important catecholamines (adrenaline and noradrenaline) are derived, but it is also an important neurotransmitter in its own right, especially in the brain. Of particular importance are central nervous pathways involved with the co-ordination of movement and with behaviour and emotion. As with the other catecholamines, dopamine is released from nerve endings where it acts upon receptors on other nerve cells to produce its effects. There are two sorts of receptors for dopamine, namely D1 and D2, of which the second are of greater importance. After release the dopamine is rapidly destroyed or taken up back into the nerve fibres for reuse. A number of medical conditions are associated with under-activity or over-activity of dopamine pathways in the brain. The rigidity and tremor, together with the hypokinesia (relative lack of voluntary movement), of Parkinson's disease are associated with lack of dopaminergic function in the nigrostriatal pathway in the brain. One form of treatment for this disease is to give large amounts of L-DOPA, the precurser of dopamine, so that some reaches the brain, is converted to dopamine, and restores some lost functions. Another approach is to transplant fetal dopamine-secreting cells into the relevant brain nuclei to take over the function of the diseased nerve cells. Over-activity of dopaminergic nerves, especially in the limbic system, is associated with schizophrenia. It has also been claimed that individuals with mutations affecting the dopamine D2 receptors show enhanced risk-taking behaviour. Some drugs, like the amphetamines, release dopamine from nerve endings in the brain, leading to hyperactivity and manic behaviour. Dopamine pathways are also associated with the vomiting centre in the brain, so drugs which block the dopamine receptors, such as the phenothiazines, have a calming effect on schizophrenics and are also useful anti-emetic drugs. There are a few dopamine pathways in other parts of the body, for example the kidney, where activation causes vasodilation. Dopamine increases the force of contraction of the heart and an infusion is sometimes used to treat shock resulting from blood loss.

Alan W. Cuthbert


See also basal ganglia; drug abuse.

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Dopamine

DOPAMINE

Dopamine (DA) is a catecholamine according to its chemical structure and a neurotransmitter of special importance for drug addiction. DA is a decarboxylated form of dopa (an amino acid) found especially in the basal ganglia. Chemically known as 3, 4 dihydroxyphenylethylamine, DA arises from dihydroxphenylacetic acid (dopa) by the action of the enzyme dopa decarboxylase. Dopamine-containing Neurons (nerve cells) are widespread in the brain and the body. Small interneurons are found in the autonomic ganglia, retina, hypothalamus, and medulla. Long axon neurons are found in two extensive circuits: (1) the nigrostriatal pathway links the substantia nigra neurons to the basal ganglia neurons and regulates locomotor events; (2) the mesocortical and mesolimbic circuits arise in the ventral tegmental area and project to the neocortex, limbic cortices, nucleus accumbens, and amygdala, where they regulate emotional events, including several forms of drug addiction, reinforcement, or reward. DA is also found in minute amounts in other catecholamine neurons as a precursor to norepinephrine. The DA transporter, which transports DA from outside the nerve terminal to inside the nerve terminal, functions to retrieve released DA and help terminate its action at receptors. The transporter is the target of psychostimulant drugs that produce their effects, at least in part, by blocking the transporter and preventing its removal from receptors. A consistent observation, for example, is the efflux of DA from nerve terminal regions in the nucleus accumbens in response to giving animals a psycho-stimulant such as cocaine or amphetamine. DA is also thought to be involved in schizophrenia and psychosis since DA-receptor-blocking drugs are clinically useful antipsychotic agents. Another disease, in which DA is lost due to the degeneration of DA-containing neurons, is Parkinson's disease, which can be treated by replacing DA with its precursor, dopa.

Floyd Bloom

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dopamine

dopamine (doh-pă-meen) n. a catecholamine derived from dopa that functions as a neurotransmitter, acting on specific dopamine receptors and also on adrenergic receptors throughout the body; it also stimulates the release of noradrenaline from nerve endings. Dopamine is used as a drug to increase the strength of contraction of the heart in heart failure, shock, severe trauma, and septicaemia. It is administered by injection in carefully controlled dosage.

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dopamine

dopamine Chemical normally found in the corpus striatum region of the human brain. Insufficient levels are associated with Parkinson's disease. Dopamine is a neurotransmitter and a precursor in the production of adrenaline and noradrenaline.

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dopamine

dopamine A catecholamine that is a precursor in the synthesis of noradrenaline and adrenaline. It also functions as a neurotransmitter, especially in the brain.

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dopamine

dopamine (dōp´əmēn), one of the intermediate substances in the biosynthesis of epinephrine and norepinephrine. See catecholamine.

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dopamine

dopamine •gamine • bromine • thiamine •dopamine • amphetamine • histamine •quinine • strychnine • mezzanine •spalpeen • Philippine • lycopene •gangrene • terrene • silkscreen •windscreen • citrine • Dexedrine •putting green • Benzedrine •Irene, polystyrene •widescreen • sight screen •chlorine, chorine, Doreen, Maureen, Noreen, taurine •smokescreen • rood screen •sunscreen • fluorine • helleborine •Gadarene • Hippocrene •glycerine (US glycerin), nitroglycerine (US nitroglycerin) •nectarine • wintergreen • Methedrine •evergreen • wolverine • vaccine •glassine • Essene • Rexine • piscine •epicene • glycine • pyroxene •Palaeocene (US Paleocene) •Pliocene • Miocene • Holocene •damascene • kerosene • Plasticine •Pleistocene

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Dopamine

Dopamine ★★½ 2003 (R)

Rand (Livingston), a partner in a dot-com startup, develops an interactive, AI-type computer character. When they decide to test in a classroom, Rand falls for the teacher, Sarah (Lloyd). Rand tries to sort out his feelings, and intellectual insticts, as he ponders whether love is emotional or chemical. Perhaps he's over-thinking a bit. 79m/C DVD . US John Livingston, Sabrina Lloyd, Bruno Campos, Reuben Grundy, Kathleen Antonia, Nicole Wilder; D: Mark Decena; W: Mark Decena, Timothy Breitbach; C: Robert Humphreys; M: Eric Holland.

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Dopamine

Dopamine

Definition

Description

Definition

Dopamine, identified as a central nervous system agent in 1959, is a neurotransmitter (nerve-signaling molecule) the body makes from the amino acid tyrosine. Dopamine in turn serves as the molecule the body uses to make adrenaline and noradrenaline. In addition to operating in nervous system signaling, it also acts as a hormone in an area of the brain called the hypothalamus, regulating release of the hormone prolactin, which is involved in parenting behavior and milk production. The body regulates dopamine’ activity in the brain in part by using proteins called dopamine transporters, which can take up dopamine and dump it back into a cell, preventing the signaling molecule from exerting its activity. The body also has five types of proteins, called dopamine receptors, responsible for recognizing the dopamine molecule, binding to it, and transmitting its signal to the cell. Dopamine is at the center of the development of a number of psychiatric disorders, including addiction and schizophrenia , and it also plays a prominent role in the manifestations of Parkinson’s disease.

Description

The brain produces dopamine in three primary areas: the substantia nigra, the ventral tegmental area, and the arcuate nucleus. The first two are of particular interest in terms of psychiatric disorders; the arcuate nucleus is associated with dopamine’s role as a neurohormone in prolactin regulation.

Disorders associated with dopamine signaling have a biological basis in the brain that appears to be site-specific. The brain has four major dopamine-signaling pathways.

  • The mesocortical pathway connects the ventral tegmental area to the cortex, the part of the brain involved in cognition and that may play a role in motivation. This pathway features in hypotheses of dopamine’s association with schizophrenia.
  • The mesolimbic pathway also begins in the ventral tegmental area, which is linked to the nucleus accumbens, the largest component of the ventral striatum. Much research has associated the nucleus accumbens and the mesolimbic pathway with brain reward processes and addiction and also with different aspects of schizophrenia.
  • The nigrostriatal pathway connects the dopamine-producing nigrostriatal area with the striatum and plays a high-profile role in the development of Park-sinon’s symptoms.
  • The tuberoinfundibular pathway involves the hypothalamus and dopamine as a neurohormone.

In terms of neuropsychiatric disorders, dopamine is probably best known as the neurotransmitter underlying the development and persistence of addiction as part of the mesolimbic reward pathway. In brief, experiences we find rewarding, such as food or sex, can become associated with increased dopamine, as can some pathological behaviors, such as compulsive gambling. Some drugs also directly elicit an increase in dopamine, setting off the reward pathway and leading more use of the drug. Ultimately, some people become addicted to substances or behaviors because of the dopamine release they trigger and the feelings of euphoria or tension relief that can follow the release.

Anatomically, these distinct dopamine-signaling pathways, variously involved in specific pathologies, may overlap with one another. For example, there is some comorbidity among schizophrenia, depression , and drug dependence and some anatomical overlap in the dopamine-signaling areas of the brain underlies this.

Dopamine receptors

The dopamine receptors, the five proteins responsible for receiving the dopamine signal for a cell, are divided into two general classes: those that are D1-like, and those that are D2-like. Of the five, the D1A through D1D and D5 receptors are all D1-like, and the D2, D3, and D4 receptors fall into the D2-like category. The distribution of these receptors differs in different dopamine-related areas of the brain. For example, the ventral striatum and limbic cortex of the mesolimbic pathway have more D2-like receptors, and D2 and D4 receptors are more closely associated with people with substance abuse problems. The dorsal striatum, involved in dopamine-related disorders such as Parkinson’s, has more also D2- than D1-like receptors. But in the prefrontal cortex, where dopamine-signaling dysfunction is associated with schizophrenia, the ratio of D1-like receptors to D2-like receptors is higher. The two general classes of receptors have opposite effects at the molecular level, but they act together in complex ways.

Dopamine and schizophrenia

A much-discussed proposed explanation for the manifestations of schizophrenia is the “dopamine hypothesis of schizophrenia.” This hypothesis implicates dopamine-signaling dysfunction along different dopamine pathways in the symptoms associated with schizophrenia. The hypothesis finds its origins in the fact that antipsychotic medications (also called “neuroleptics”) exert their effects by blocking or inhibiting D2 receptors. The mesolimbic pathway may be involved, a conclusion based on studies showing a link between dysfunction of this system and the delusions and hallucinations of schizophrenia, with an

increase in striatal dopamine in association with these occurrences. On the other hand, the mesocortical pathway is also probably involved because of its role in working memory, memorization, and manipulation of spatial information, all of which are affected in schizophrenia. A decrease in dopamine in the prefrontal area, which is linked to the ventral tegmental area in the mesocortical pathway, appears to lead to the cognitive deficits of schizophrenia. In addition, the nigrostriatal pathway may be involved: there is an increase in dopamine transmission from the substantia nigra to the striatum in people with schizophrenia.

Dopamine, the brain reward system, and addiction

The nucleus accumbens (in the ventral striatum and part of the mesolimbic pathway) is the focal point of dopamine’s involvement in the brain’s reward pathway and addiction. There is an increase in dopamine release in the nucleus accumbens in addiction, and activity in this area is a target of models exploring the mechanisms of behavioral or substance addictions. Human imaging studies , which have become quite revelatory in terms of the biological underpinnings of psychiatric disorders, show that endogenous release of dopamine in the striatum is correlated with drug-induced feelings of pleasure. For example, a dose of amphetamine or of alcohol will promote dopamine release in the ventral striatum. Dopamine also is associated with the cravings of addiction and may play a role in the significance an addicted person may assign to cues that others perceive as neutral (known as salience). This system has also been implicated in process or behavioral addiction.

Dopamine and movement and repetitive disorders

Dopamine’s role in the extrapyramidal (movement and coordination) symptoms of Parkinson’s is seated in a shortage of the neurotransmitter in the nigrostriatal pathway, specifically involving the putamen and caudate nucleus. Tourette’s, a syndrome characterized by onset in childhood, involuntary tics, stereotypic behaviors, and repetitive thoughts and rituals, is also seated in the dorsal striatum. This syndrome can occur as a comorbidity with obsessive-compulsive disorder and/or attention deficit/hyperactivity disorder (ADHD), which some studies also have associated with dopamine-signaling dysfunction.

Dopamine and mood disorders

Changes in dopamine signaling may contribute to symptoms of depression, such as an inability to experience

KEY TERMS

D1, D2, etc. —Dopamine receptor proteins.

Dopamine —A neurotransmitter and neurohormone.

Mesolimbic pathway —The “reward pathway” of the brain.

Nucleus accumbens —A part of the brain involved in the mesolimbic reward pathway, which receives dopamine signaling from the ventral tegmental area.

Ventral tegmental area —Produces dopamine and signals to the nucleus accumbens the rest of the striatum.

pleasure or loss of motivation. Although low levels of dopamine binding to the D2 receptor are associated with social anxiety , an increase in dopamine can be associated with the hypersocial behavior of someone experiencing the manic aspects of bipolar disorder.

Drugs related to dopamine/dopamine receptor regulation

Drugs may act at any point along a dopamine-signaling pathway. L-dopa, used in treating Parkinson’s, is a dopamine precursor that is synthesized into dopamine in the brain and ameliorates the effects of low dopamine levels in the dorsal striatum. Monoamine oxidase inhibitors (MAOIs) block the activity of the enzyme that breaks down dopamine; these may be used as antidepressants and can affect dopamine-related pathways. Antipsychotics are divided into two classes, the typical and atypical antipsychotics, and can target different types of dopamine receptors. Atypical antipsychotics, including clozapine , may target the D4 receptor more strongly than the D2. Bromocriptine targets D2 and is a partial inhibitor of D1. The recently approved aripirprazole is a partial dopamine agonist (mimic), and amantadine is also a dopamine agonist.

Resources

BOOKS

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, 4th edition, Text revision. Washington, D.C.: American Psychiatric Association, 2000.

PERIODICALS

Franken, Ingmar H.A., Jan Booij, and Wim van den Brink. “The Role of Dopamine in Human Addiction: From Reward to Motivated Attention.” European Journal of Pharmacology 526 (2005): 199–206.

Grant, Jon E., J.D., M.D., M.P.H., Judson A. Brewer, M.D., Ph.D., and Marc N. Potenza, M.D., Ph.D. “The Neurobiology of Substance and Behavioral Addictions.” CNS Spectrum 11 (2006): 924–930.

Greene, James G. “Gene Expression Profiles of Brain Dopamine Neurons and Relevance to Neuropsychiatric Disease.” Journal of Physiology 575 (2006): 411–416.

Kienast, T., and A. Heinz. “Dopamine and the Diseased Brain.” CNS and Neurological Disorders-Drug Targets 5 (2006): 109–131.

Totterdell, Susan. “The Anatomy of Comorbid Neuropsychiatric Disorders Based on Cortico-limbic Synaptic Interactions.” Neurotoxicity Research 10 (2006): 65–86.

OTHER

“Schizophrenia.” National Institutes of Mental Health. (Janaury 15 2007) <http://www.nimh.nih.gov/healthinformation/schizophreniamenu.cfm>.

“Schizophrenia.” National Library of Medicine. (Janaury 15 2007) <http://www.nlm.nih.gov/medlineplus/schizophrenia.html>.

Emily Jane Willingham, Ph.D.

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Dopamine

Dopamine

Basic definitions and chemical information

Dopamine and Parkinson disease

Dopamine and schizophrenia

Dopamine as heart medicine

Dopamine and attention deficit hyperactivity disorder

Dopamine and drug addiction

Dopamine and aging

Resources

Dopamine is a neurotransmitter (a chemical used to send signals between nerve cells) in the same family as epinephrine (adrenaline). Dopamine is one of the primary neurotransmitters and it affects motor functions (movement), emotions, learning, and behavior. It was originally identified as the brain chemical associated with pleasure. A decrease in the amount of dopamine in specific sections of the brain has been implicated as a possible cause of Parkinson disease, while an excess of dopamine in some regions of the brain has been suggested as a possible cause of schizophrenia. Dopamine is also thought to play a role in depression, attention deficit hyperactivity disorder, high blood pressure, and drug addiction. Recently, dopamine has been used as a treatment for victims of heart attacks.

Basic definitions and chemical information

Dopamine is one of a group of chemicals known as catecholamine neurotransmitters. Catecholamines are a group of chemicals that include epinephrine (adrenalin); histamine, which is responsible for many of the symptoms of allergies; and serotonin, a molecule that has been suggested as aiding in sleep. This group of compounds is sometimes collectively known as the biogenic amines. Neurotransmitters are chemicals used by the body to signal or send information between nerve cells or nerve and muscle cells. The chemical structure of dopamine is shown in Figure 1. The NH2 group on the molecule is the amine group in the term biogenic amines. This entire group of chemicals has been implicated in depression and general moods.

Dopamine and Parkinson disease

Parkinson disease is a disorder of the nervous system that is characterized by slow movements and difficulty initiating movements, a shuffle when walking, and increased muscular rigidity. It is estimated to affect as many as one million Americans and is far more prevalent in the elderly. The main cause of Parkinson disease is thought to be a lack of dopamine in a region of the brain known as the substantia nigra. Whether the cells in that area do not produce enough dopamine or whether there are too few of the dopa-mine-producing cells is a matter of debate and active research. A chemical known as Levodopa or L-dopa, which our bodies rapidly metabolize to dopamine, is the main treatment. Levodopa reduces the symptoms of the disease, but does not stop the progression of the disease. A lack of dopamine in some areas of the brain also has been implicated in depression.

Dopamine and schizophrenia

Schizophrenia is a form of psychosis or loss of contact with reality. It is estimated to affect about 1% of the population, or over 2.5 million Americans. A great deal of research is being done on the origins of schizophrenia. One widely accepted theory is that it is caused by an excess of dopamine or dopamine receptors. Receptors are proteins on the surfaces of cells that act as signal acceptors for the cells. They allow cells to send information, usually through neurotransmitter molecules. This hypersensitivity to dopamine (the prefix hyper means over or excessive) is treated by using chemicals that block (or inactivate) the receptors for the dopamine signals. However, there are a number of different types of dopamine receptors and there are many differences among individuals in the structures of these receptors. Drugs usually block all of the receptors, not just the ones related to schizophrenic symptoms, resulting in many side effects. Other approaches to the treatment of schizophrenia have focused on decreasing the amounts of dopamine in the brain. In doing so, however, symptoms of Parkinson disease often result, since less dopamine (or the ability to respond to dopamine) is present. The origins of schizophrenia are unclear; dopamine excess is probably not the sole cause of the disease, as strong evidence for genetic and environmental factors exists as well. To date, treatments that focus on excess dopamine sensitivity have been the most successful.

Dopamine as heart medicine

Since dopamine can increase blood pressure, it is used as a treatment for shock (low blood pressure throughout the body) which carries the risk of damage to major organs in patients who have suffered serious heart attacks. Dopamine causes small blood vessels to constrict, thus raising the blood pressure throughout the body. Chemically related molecules such as adrenaline act similarly and both are often used to help patients.

Dopamine and attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD), a syndrome that affects as many as 3.5 million American children, and many adults as well, is characterized by an inability to pay attention, over-activity, and impulsive behaviors. ADHD has been associated with certain forms of the dopamine D4 receptor, and with individual differences in the gene that encodes the dopamine transporter, a molecule that binds and carries dopamine. ADHD often is treated with stimulatory drugs such as Ritalin, which increase the availability of dopamine in the brain.

KEY TERMS

Neurotransmitter A chemical used to send information between nerve cells or nerve and muscle cells.

Psychosis A loss of contact with reality. It may be caused by drugs, chemical imbalances, or even severe stress or depression.

Receptors Protein molecules on a cells surface that acts as a signal receiver and allows communication between cells.

Dopamine and drug addiction

Alcohol, nicotine, and a variety of other drugs including marijuana, cocaine, amphetamines, and heroin all appear to raise the level or the availability of dopamine in different parts of the brain. Pathways of nerve cells that produce dopamine and contain dopamine receptors are affected by all of these drugs. There is evidence that certain forms of the dopamine D4 receptor may predispose a person to drug addiction. Based on this information, researchers are attempting to develop drugs to treat addictions.

Dopamine and aging

Although individuals vary greatly in the amount of dopamine activity in their brains, in general dopa-mine appears to decline with age in those parts of the brain responsible for thinking. In particular, as people age, the number of dopamine D2 receptors decreases significantly. Thus, dopamine may be involved with the age-related loss of intellectual skills.

Resources

BOOKS

Ackerman, S. Discovering the Brain. National Academy Press, 1992.

Restak, Richard M. Receptors. Bantam Press, 1994.

PERIODICALS

Bower, Bruce. The Birth of Schizophrenia: A Debilitating Mental Illness May Take Root in the Fetal Brain. Science News (29 May 1993): 346.

Concur, Bruce. A Dangerous Pathway. New Scientist (5 July 1997).

Miller, Susan. Picking up Parkinsons Pieces. Discover, (May 1991): 22.

Zamula, Evelyn. Drugs Help People with Parkinsons Disease. FDA Consumer (Jan-Feb 1992): 28.

OTHER

Hedweb: Good Drug Guide. Dopamine: Pharmacologic and Therapeutic Aspects <http://www.biopsychiatry.com/dopapharm.htm> (accessed November 16, 2006).

University of Groningen, Faculty of Behavioural and Social Sciences. The Dopamine Theory of Parkinsons Disease <http://tcw2.ppsw.rug.nl/vdbosch/pd.html> (accessed November 19, 2006).

Louis J. Gotlib

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Dopamine

Dopamine

Dopamine is a neurotransmitter (a chemical used to send signals between nerve cells) in the same family as epinephrine (adrenaline). Dopamine is one of the primary neurotransmitters and it affects motor functions (movement), emotions, learning , and behavior . It was originally identified as the brain chemical associated with pleasure. A decrease in the amount of dopamine in specific sections of the brain has been implicated as a possible cause of Parkinson's disease , while an excess of dopamine in some regions of the brain has been suggested as a possible cause of schizophrenia . Dopamine is also thought to play a role in depression , attention deficit hyperactivity disorder, high blood pressure , and drug addiction . Recently, dopamine has been used as a treatment for victims of heart attacks.


Basic definitions and chemical information

Dopamine is one of a group of chemicals known as catecholamine neurotransmitters. Catecholamines are a group of chemicals that include epinephrine (adrenalin); histamine , which is responsible for many of the symptoms of allergies; and serotonin, a molecule that has been suggested as aiding in sleep . This group of compounds is sometimes collectively known as the biogenic amines. Neurotransmitters are chemicals used by the body to signal or send information between nerve cells or nerve and muscle cells. The chemical structure of dopamine is shown below. The NH2 group on the molecule is the amine group in the term biogenic amines. This entire group of chemicals has been implicated in depression and general moods.


Dopamine and Parkinsons disease

Parkinson disease is a disorder of the nervous system that is characterized by slow movements and difficulty initiating movements, a shuffle when walking, and increased muscular rigidity. It is estimated to affect as many as one million Americans and is far more prevalent in the elderly. The main cause of Parkinson disease is thought to be a lack of dopamine in a region of the brain known as the substantia nigra. Whether the cells in that area do not produce enough dopamine or whether there are too few of the dopamine-producing cells is a matter of debate and active research. A chemical known as Levodopa or L-dopa, which our bodies rapidly metabolize to dopamine, is the main treatment. Levodopa reduces the symptoms of the disease, but does not stop the progression of the disease. A lack of dopamine in some areas of the brain also has been implicated in depression.


Dopamine and schizophrenia

Schizophrenia is a form of psychosis or loss of contact with reality. It is estimated to affect about 1% of the population, or over 2.5 million Americans. A great deal of research is being done on the origins of schizophrenia. One widely accepted theory is that it is caused by an excess of dopamine or dopamine receptors. Receptors are proteins on the surfaces of cells that act as signal acceptors for the cells. They allow cells to send information, usually through neurotransmitter molecules. This hypersensitivity to dopamine (the prefix "hyper" means over or excessive) is treated by using chemicals that block (or inactivate) the receptors for the dopamine signals. However, there are a number of different types of dopamine receptors and there are many differences among individuals in the structures of these receptors. Drugs usually block all of the receptors, not just the ones related to schizophrenic symptoms, resulting in many side effects. Other approaches to the treatment of schizophrenia have focused on decreasing the amounts of dopamine in the brain. In doing so, however, symptoms of Parkinson disease often result, since less dopamine (or the ability to respond to dopamine) is present. The origins of schizophrenia are unclear; dopamine excess is probably not the sole cause of the disease as strong evidence for genetic and environmental factors exists as well. To date, treatments that focus on excess dopamine sensitivity have been the most successful.


Dopamine as heart medicine

Since dopamine can increase blood pressure, it is used as a treatment for shock (low blood pressure throughout the body) which carries the risk of damage to major organs in patients who have suffered serious heart attacks. Dopamine raises the blood pressure and causes small blood vessels to constrict, thus raising the blood pressure throughout the body. Chemically related molecules such as adrenaline act similarly and both are often used to help patients.

Dopamine and attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD), a syndrome that affects as many as 3.5 million American children, and many adults as well, is characterized by an inability to pay attention, over-activity, and impulsive behaviors. ADHD has been associated with certain forms of the dopamine D4 receptor, and with individual differences in the gene that encodes the dopamine transporter, a molecule that binds and carries dopamine. ADHD often is treated with stimulatory drugs such as Ritalin, which increase the availability of dopamine in the brain.


Dopamine and drug addiction

Alcohol , nicotine , and a variety of other drugs including marijuana , cocaine , amphetamines , and heroin all appear to raise the level or the availability of dopamine in different parts of the brain. Pathways of nerve cells that produce dopamine and contain dopamine receptors are affected by all of these drugs. There is evidence that certain forms of the dopamine D4 receptor may predispose a person to drug addiction. Based on this information, researchers are attempting to develop drugs to treat addictions.


Dopamine and aging

Although individuals vary greatly in the amount of dopamine activity in their brains, in general dopamine appears to decline with age in those parts of the brain responsible for thinking. In particular, as people age, the number of dopamine D2 receptors decreases significantly. Thus, dopamine may be involved with the age-related loss of intellectual skills.


Resources

books

ackerman, s. discovering the brain. washington, dc: national academy press, 1992.

restak, richard m. receptors. new york: bantam press, 1994.

periodicals

bower, bruce. "the birth of schizophrenia: a debilitating mental illness may take root in the fetal brain." science news (may 29, 1993): 346.

Concur, Bruce. "A Dangerous Pathway." New Scientist (July 5, 1997).

Miller, Susan. "Picking up Parkinson's Pieces." Discover (May 1991): 22.

Zamula, Evelyn. "Drugs Help People with Parkinson's Disease." FDA Consumer (January-February 1992): 28.


Louis J. Gotlib

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Neurotransmitter

—A chemical used to send information between nerve cells or nerve and muscle cells.

Psychosis

—A loss of contact with reality. It may be caused by drugs, chemical imbalances, or even severe stress or depression.

Receptors

—Protein molecules on a cells surface that acts as a "signal receiver" and allows communication between cells.

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"Dopamine." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 10, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/dopamine-0

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