Comparative biology and evolution::Dopamine


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Comparative biology and evolution


There are no reports of dopamine in archaea, but it has been detected in some types of bacteria and in the protozoan called Tetrahymena.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> Perhaps more importantly, there are types of bacteria that contain homologs of all the enzymes that animals use to synthesize dopamine.<ref name=Iyer/> It has been proposed that animals derived their dopamine-synthesizing machinery from bacteria, via horizontal gene transfer that may have occurred relatively late in evolutionary time, perhaps as a result of the symbiotic incorporation of bacteria into eukaryotic cells that gave rise to mitochondria.<ref name=Iyer>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>


Dopamine is used as an intercellular messenger in virtually all multicellular animals.<ref name=Barron/> In sponges only a single report exists of the presence of dopamine, with no indication of its function;<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> however, dopamine has been reported in the nervous systems of numerous radially symmetric species, including the jellyfish, hydra and some coral species of the cnidaria phylum.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> This dates the emergence of dopamine as a neurotransmitter back to the earliest appearance of the nervous system, over 500 million years ago in the Cambrian era. Dopamine functions as a neurotransmitter in vertebrates, echinoderms, arthropods, molluscs, and several types of worm.<ref name=Cottrell>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

In every type of animal that has been examined, dopamine has been seen to modify motor behavior.<ref name=Barron>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> In the model organism, nematode Caenorhabditis elegans, it reduces locomotion and increases food-exploratory movements; in planarian flatworms it produces "screw-like" movements; in leeches it inhibits swimming and promotes crawling. Across a wide range of vertebrates, dopamine has an "activating" effect on behavior-switching and response selection, comparable to its effect in mammals.<ref name=Barron/>

Dopamine also consistently plays a role in reward learning, in all animal groups that have been examined except arthropods, although recent evidence suggests that dopamine at least mediates reward learning in fruit flies.<ref name=Waddell/> In nematodes, planarians, molluscs, D. melanogaster and vertebrates, animals can be trained to repeat an action if it is consistently followed by an increase in dopamine levels.<ref name=Barron/> Arthropods have long been believed to be an exception. In these species—insects, crustaceans, etc.—dopamine was thought to have an aversive effect, with reward instead mediated by octopamine, a neurotransmitter that is not found in vertebrates but is closely related to norepinephrine.<ref name=Waddell/> More recent studies, however, have found that the rewarding effect of octopamine comes from its activation of a set of dopaminergic neurons that had not been accessed in previous efforts.<ref name=Waddell>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>


Photo of a bunch of bananas.
Dopamine can be found in the peel and fruit pulp of bananas.

Many plants, including a variety of food plants, synthesize dopamine to varying degrees.<ref name=Kulma/> The highest concentrations have been observed in bananas—the fruit pulp of red and yellow bananas contains dopamine at levels of 40 to 50 parts per million by weight.<ref name=Kulma/> Potatoes, avocados, broccoli, and Brussels sprouts may also contain dopamine at levels of 1 part per million or more; oranges, tomatoes, spinach, beans, and other plants contain measurable concentrations less than 1 part per million.<ref name=Kulma>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The dopamine in plants is synthesized from the amino acid tyrosine, by biochemical mechanisms similar to those that animals use.<ref name=Kulma/> It can be metabolized in a variety of ways, producing melanin and a variety of alkaloids as byproducts.<ref name=Kulma/> The functions of plant catecholamines have not been clearly established, but there is evidence that they play a role in the response to stressors such as bacterial infection, act as growth-promoting factors in some situations, and modify the way that sugars are metabolized. The receptors that mediate these actions have not yet been identified, nor have the intracellular mechanisms that they activate.<ref name=Kulma/>

Dopamine consumed in food cannot act on the brain, because it cannot cross the blood–brain barrier.<ref name="Nice-pharma"/> However, there are also a variety of plants that contain L-DOPA, the metabolic precursor of dopamine.<ref name=Ingle>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The highest concentrations are found in the leaves and bean pods of plants of the genus Mucuna, especially in Mucuna pruriens (velvet beans), which have been used as a source for L-DOPA as a drug.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Another plant containing substantial amounts of L-DOPA is Vicia faba, the plant that produces fava beans (also known as "broad beans"). The level of L-DOPA in the beans, however, is much lower than in the pod shells and other parts of the plant.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The seeds of Cassia and Bauhinia trees also contain substantial amounts of L-DOPA.<ref name=Ingle/>

In a species of marine green algae Ulvaria obscura, a major component of some algal blooms, dopamine is present in very high concentrations, estimated at 4.4% of dry weight. There is evidence that this dopamine functions as an anti-herbivore defense, reducing consumption by snails and isopods.<ref name="pmid16489461">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

As a precursor for melanin


Melanins are a family of dark-pigmented substances found in a wide range of organisms.<ref name=Simon/> Chemically they are closely related to dopamine, and there is a type of melanin, known as dopamine-melanin, that can be synthesized by oxidation of dopamine via the enzyme tyrosinase.<ref name=Simon>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The melanin that darkens human skin is not of this type: it is synthesized by a pathway that uses L-DOPA as a precursor but not dopamine.<ref name=Simon/> However, there is substantial evidence that the "neuromelanin" that gives a dark color to the brain's substantia nigra is at least in part dopamine-melanin.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Dopamine-derived melanin probably appears in at least some other biological systems as well. Some of the dopamine in plants is likely to be used as a precursor for dopamine-melanin.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The complex patterns that appear on butterfly wings, as well as black-and-white stripes on the bodies of insect larvae, are also thought to be caused by spatially structured accumulations of dopamine-melanin.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Dopamine sections
Intro  Structure  Biochemistry  Functions  Medical uses  Pharmacology  Diseases and disorders  Comparative biology and evolution  History and development  See also   References   

Comparative biology and evolution
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