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Dopamine

Explanations > Brains > Brain Chemistry > Neurotransmitters > Dopamine

Composition | Function | Dysfunction | See also

Dopamine is a monoamines that has many functions in the body and brain. It's adjective is dopaminergic.

Composition

Chemically, Dopamine is created by hydroxylation of L-tyrosine to L-DOPA via the enzyme tyrosine hydroxylase, and then by the decarboxylation of L-DOPA by dopamine beta-hydroxylase.

It is created in several areas of the brain, including the medulla, substantia nigra and Ventral tegmental area.

Dopamine is de-activated by reuptake by the presynaptic neuron and is broken down by the enzymes catechol-O-methyl transferase (COMT) and monoamine oxidase (MAO).

There are five dopamine receptors: D1, D2, D3, D4 and D5 all of which are metabotropic. D1 and D2 are most common. D1 receptors are postsynaptic, whilst D2 are both pre- and postsypnaptic.

Stimulating D1 increases production of the second messenger cAMP whilst stimulating D2, D3 and D4 decreases it.

Function

Dopamine has many functions, including effects in behavior and cognition, movement, attention, motivation and reward, mood, sleep, and learning. Its impact on motivation and learning is of particular note for changing minds. It has also been linked with sociability and creativity.

It is believed dopaminergic neuron firing increases when a reward is expected and depressed when the reward is not forthcoming. This makes it highly significant in learning and behaving.

Dopamine creates pleasurable feelings when it is released in areas such as the nucleus accumbens and pre-frontal cortex. This happens when positive actions are undertaken, such as eating food and having sex.

The general arousal and goal focus, plus decreases in inhibition can make dopamine important in creative thinking and problem solving.

In practice there are three conflicting hypotheses about dopamine and reward:

• Hedonia: dopamine in the nucleus accumbens  causing sensations of pleasure, sometimes intense. However, not all rewards or pleasurable things cause this system to be activated.
• Learning: focuses on future rewards and associative learning and the role of dopamine in motivation.
• Incentive salience (wanting): dopamine is released when there are stimuli worth working for, thus making us work harder to get it.

Dopamine has also more recently been associate more with motivation rather than feelings of pleasure (although it can lead to anticipated pleasure). In contrast, opioids have been linked directly with pleasure sensations of satiation and consummation, including rest, bliss and sedation.

Dopamine controls the flow of information in the frontal lobes from other areas of the brain. It also inhibits the secretion of prolactin in the anterior pituitary gland.

It has also been linked with the production of meaning and been used to explain deep narcotic experiences.

Neurons which mainly use dopamine as a neurotransmitter are in the ventral tegmental area, substantia nigra, and arcuate nucleus of the hypothalamus. These have long axons which reach through four major dopamine pathways.

Dysfunction

Dopamine disorders in the frontal lobes can cause a decline in cognitive function, including memory, attention, and problem-solving.  

It can also act as an analgesic and decreased production has been associated with the pain in Parkinson's Disease, which itself is caused by insufficient dopamine production.

Higher levels of dopamine are thought to increase salience, where more experiences and items are taken to be significant and important. This is a characteristic of conditions such as autism, schizophrenia and paranoia. Reduced dopamine concentrations, particularly in the pre-frontal cortex are thought to contribute to attentional problems such as Attention Deficit Disorder (ADD).

Drugs - narcotics

Narcotics such as cocaine, nicotine, and amphetamines act to increase the rewarding effects of dopamine, giving pleasurable feelings. Dopamine can also increase libido and post-coital pleasure.

• Cocaine works by blocking reuptake of dopamine, thus increasing its effects. It also blocks sodium channels and can be used as an anesthetic.
• Amphetamines act to stimulate neurotransmitter release, forcing dopamine vesicules to release their cargo. They are similar in structure to dopamine and get into the terminal button via dopamine transporters as well as diffusing through the membrane.

Perhaps surprisingly, blocking of dopamine receptors increases rather than decreases motivation to take narcotics, which calls into question their chemical role in motivation and suggests more psychological causes.

Other drugs

Dopamine antagonists and are used to treat schizophrenia.

Dopamine does not cross the blood-brain barrier, so it cannot be injected. L-DOPA does cross the barrier so it can be used.

As a medication, dopamine increases the heart rate and blood pressure, so is used with caution.

Apomorphine has a greater affinity for presynaptic than postsynaptic D2 receptors, inhibiting production and release of dopamine. Higher doses, however, also stimulate postsynaptic D2 receptors and hence the drug then becomes an agonist.

See also

Monoamines, Attraction vs. Avoidance Preference