Dopamine

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Schematic of the dopamine molecule

Dopamine is a chemical produced by the human body that acts as a neurotransmitter in the brain. Its chemical formula is (C6H3(OH)2-CH2-CH2-NH2) and its chemical name is 4-(2-aminoethyl)benzene-1,2-diol. Dopamine is abbreviated as DA. The brain contains five types of receptors to which dopamine binds (D1, D2, D3, D4 and D5).

Contents

Reward, Prediction and Motivation

Dopamine plays an important role in brain reward systems in the nucleus accumbens, reward prediction and error in the ventral tegmental area, and appears to be involved in various levels of behavioral motivation.

Actions and substances that increase the amount of DA that is released in the nucleus accumbens increase the hedonistic responses of the animal. While dopamine antagonist (blockers) in the nucleus accumbens decrease these hedonistic responses. Manipulations of dopamine in the ventral tegmental area and the substantia nigra do not alter the hedonistic response but decrease or increase the motivation to perform behaviors. It is believed that dopamine acting in these regions actually signals predictions of future reward and errors in predicted reward and actual reward. Because of its role in both direct hedonistic assessment and its predictive ability dopamine is theorized to be the central neurotransmitter mediating reinforcement learning. Researchers have attempted to model its role through use of Temporal difference learning.

Dopamine is released by natural behaviors such as eating and sexual activity, and also by drugs such as cocaine and amphetamine.

Muscle communication

Dopamine is also a central neurotransmitter in mediating communication with muscles. The D1 receptors in the cerebellum controls many elements of motion and stabalization. The deterioration of dopaminergic neurons within the substantia nigra of the brain is primarily responsible for the development of Parkinson's disease

Link to Mental Illness

Dopamine has also been linked to mental illness, particularly schizophrenia, which is characterized by psychotic episodes. Studies have shown that drugs which block D2 receptors (preventing dopamine from binding to them) reduce psychotic symptoms, and drugs which increase dopamine relase (e.g., cocaine) can lead to psychotic episodes. Most medications used in psychiatry to treat schizophrenia and other psychotic disorders work by inhibiting dopaminergic activity. It is thought that it is hyperactivation of the ventral tegmental area and dorsal striatum regions that cause the positive symptoms of schizophrenia.

There has also been animal models that have linked dopamine agonism to obsessive compulsive disorder. Most researchers have labeled serotonin as the primary mediator of OCD but combined treatments of a dopamine antagonist and serotonin reuptake site inhibitor have shown to be better than just the SRSI alone.

Therapeutic Uses

Dopamine is used to increase cardiac output and blood pressure. Levodopa (or L-DOPA), which is a dopamine precursor, is often used to treat Parkinsons' disease.

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