Dopamine

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Dopamine Summary

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Dictionary of Biological Psychology

dopamine

Dopamine (DA) is a neurotransmitter; neurons that contain dopamine are said to be DOPAMINERGIC. It is one of the CATECHOLAMINE neurotransmitters and therefore a member of the even larger family of MONO AMINE neurotransmitters. The full chemical name for dopamine is 3,4-DIHYDROXYPHENYLETHYLAMINE. Dopamine is synthesized from the amino acid L-TYROSINE. The enzyme tyrosine hydroxylase catalyses the conversion of tyrosine to L-DOPA (L-3,4-DIHYDROXYPHENYLALANINE). L-DOPA is then converted by AROMATIC L-AMINO ACID DECARBOXYLASE to dopamine. In dopaminergic neurons, this is as far as the conversion goes, dopamine being transported and packaged for use as a neurotransmitter. However, further conversion of dopamine can produce another catecholamine neurotransmitter, NORADRENALINE. The presence of the enzyme dopamine beta hydroxylase, involved in the conversion of dopamine to noradrenaline, is the principal way to discriminate noradrenaline- from dopamine-containing neurons, dopamine of course being present in all of them. There are multiple receptors for dopamine present in the brain (see D1-D5 DOPAMINE RECEPTORS). The synaptic action of dopamine is terminated in two ways: destruction by enzymes or by reuptake, for which there are specific dopamine transporters. Enzymatic destruction can be achieved by either of two enzymes: MONOAMINE OXIDASE (MAO) or CATECHOL-O-METHYLTRANSFERASE (COMT).

(Both MONOAMINE OXIDASE-A and MONOAMINE OXIDASE-B effectively degrade dopamine.) Monoamine oxidase destruction of dopamine leads to formation of the metabolite 3,4-DIHYDROXYPHENYLACETALDEHYDE (DHPA) which ALDEHYDE DEHYDROGENASE further converts to 3,4-DIHYRODXYPHENYLACETICACID(DOPAC).COMT destruction of dopamine leads to production of the metabolite 3-METHOXYTYRAMINE (3-MT) which monoamine oxidase can convert to 3-METHOXY-4-HYDROXYPHENYLACETALDEHYDE (MHPA). DOPAC can be further converted by COMT and MHPA by aldehyde dehydrogenase to produce a final metabolite, HOMOVANILLIC ACID (HVA). Measurement of the metabolites of dopamine (or indeed any other neurochemical) can give important information regarding its synthesis in brain.

Dopamine neurons are found in a variety of places within the brain: the NIGROSTRIATAL DOPAMINE SYSTEM, MESOLIMBIC DOPAMINE SYSTEM and the MESOLIMBICOCORTICAL SYSTEM are the most widely studied, being involved in a variety of psychological process to do with MOTOR CONTROL, cognition and REWARD. Loss of dopamine from the nigrostriatal system gives rise to PARKINSON’S DISEASE, while changes in the dopamine content of the NUCLEUS ACCUMBENS and PREFRONTAL CORTEX have been associated with schizophrenia (see DOPAMINE HYPOTHESIS OF SCHIZOPHRENIA). However, these are just the best known and most often studied dopamine systems. In fact dopamine has a wide variety of functions, indicated by the variety of places in which it is found. Dopamine neurons are also found in all of these places: the DIENCEPHALOSPINAL DOPAMINE SYSTEM projects from the HYPOTHALAMUS to the SPINAL CORD; the PERIVENTRICULAR DOPAMINE SYSTEM projects from the PERIAQUEDUCTAL GREY to the hypothalamus and THALAMUS; the INCERTOHYPOTHALAMIC DOPAMINE SYSTEM is contained within the hypothalamus; the TUBEROHYPOPHYSEAL DOPAMINE SYSTEM projects from the hypothalamus to the PITUITARY GLAND (see also TUBEROINFUNDIBULAR DOPAMINE SYSTEM); the PERIGLOMERULAR DOPAMINE NEURONS are found in the OLFACTORY BULB; and dopamine neurons are also found in the RETINA. (Note that many of these systems also have an ‘A’ number: the SUBSTANTIA NIGRA pars compacta dopamine neurons are for example the A9 neurons, while those of the VENTRAL TEGMENTAL AREA are the A10 neurons. These relate to the CLASSIFICATION OF DAHLSTRÖM AND FUXE.)

One final note: dopamine has received extensive study since its first description as a neurotransmitter in the 1950s. It is worth considering though the very many tools there are to do this with: there are histological techniques (HISTOFLUORESCENCE and IMMUNOHISTOCHEMISTRY) and biochemical analysis using HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC). There are specific lesioning agents, 6-HYDROXYDOPAMINE and MPTP, and a variety of drugs specific for all aspects of dopamine synthesis and degradation, as well as drugs to activate or inactivate receptors, or promote or inhibit dopamine release. Other neurotransmitters have not been nearly so well studied: to what extent is our interest in a given system simply the product of having the tools available with which to study it?