Mesolimbic Dopamine System

The mesolimbic dopamine system is a pathway in the brain in which dopamine is carried from one area of the brain to another. Dopamine is responsible for controlling the brain’s pleasure and reward centers. Starting in the midbrain’s ventral tegmental area, it is linked by the amygdale, the nucleus accumbens, the medial prefrontal cortex, and the hippocampus parts of the brain to another set of brain structures known as the limbic system. In addition to pleasure and reward, the mesolimbic dopamine system is associated with the functions of movement, preservation, and compulsion.

Dopamine
[Dopamine](http://en.wikipedia.org/wiki/Dopamine) is a chemical messenger that is created in a number of areas of the brain. Essential in maintaining normal dopamine activity are [dopamine receptors](http://en.wikipedia.org/wiki/Dopamine_receptors). There are five known kinds of dopamine receptors in the human brain. These receptors are able to detect the neurotransmitter dopamine and activate a response. Dopamine receptors are found in the brain, the blood vessels, the kidneys, and the central nervous system. Function and number of dopamine receptors play a role in the brain processes of cognition, motivation, memory, and learning, among others. Any damage in the function or variation in the number of these receptors may lead to a mild to severe change in these processes.

Mesolimbic and Dopaminergic Pathways
These pathways carry dopamine from one part of the brain to another. Covering the length of the pathways are neurons featuring [axons](http://en.wikipedia.org/wiki/Axon). Among other functions, these signal-transmitting neurons play a role in the synthesizing of dopamine.

Among the eight dopaminergic pathways is the mesolimbic pathway. From the midbrain’s ventral tegmental area (VTA), this pathway transmits dopamine to the limbic system’s nucleus accumbens, a collection of neurons in the brain’s striatum.

Included in the mesolimbic pathway are the below structures:

* __Bed nucleus of the stria terminalis__ (BStc): this structure on the surface of the thalamus is associated with anxiety and drug-seeking behavior.
* __Ventral tegmental area__ (VTA): this area of the midbrain contains various types of neurons that are capable of traveling to a number of different areas of the brain. Through the unmyelinated medial forebrain bundle, the VTA communicates with the nucleus accumbens. It is considered to play an important role in the [reward system](http://en.wikipedia.org/wiki/Reward_system). It is also observed as being a factor in drug addiction, motivation, and some psychiatric disorders. The disruption of this pathway has been linked with Parkinson’s disease, schizophrenia, and attention deficit hyperactivity disorder (ADHD). This area of the brain is one of the key locations where commonly abused drugs such as alcohol and heroin act.
* __Amygdala__: located in the temporal lobe anterior to the hippocampus, this large nuclear mass is linked with fear and anxiety, among other emotions.
* __Nucleus accumbens__: this part of the ventral striatum is made up of medium spiny neurons. Two sub-regions known as the shell and core are the limbic and motor areas, respectively. Neurons in this part of the brain receive messages from neurons in the VTA, the hippocampus, the medial prefrontal cortex, and the amygdala
* __Hippocampus__: situated in the temporal lobe’s medial portion, this structure is recognized for its relationship with memory.

Mesolimbic Dopamine System Function
Dopamine is commonly linked with the reward system. Yet its role in this system remains controversial. There are three leading explanations of dopamine’s role in the reward system:

* __Hedonia__: this theory puts forward dopamine as a pleasure chemical in the brain’s nucleus accumbens. Supporting this idea is the effect select drugs of abuse, such as cocaine, have on dopamine activity. Yet pleasure can be experienced without activating the reward system, suggesting the mesolimbic dopamine system is not only sparked by pleasurable things
* __Learning__: this theory is based on the idea that learning is dependent on motivation for a reward. Damage to the ventral tegmental and nucleus accumbens areas of the brain has been shown to remove the motivation―not the capability―for learning
* __Incentive salience__: this explanation associates the release of dopamine with a reason worth working toward

Addiction and the Mesolimbic Dopamine System
Drugs of abuse interact with the mesolimbic pathway to produce addictive and rewarding effects. The dopaminergic pathways, in particular, use dopamine to send messages that are responsible for the pleasurable effects of these drugs. Repeated dopamine stimulation in this way can cause a permanent change in dopamine levels, which has been linked with compulsive drug taking. It is therefore thought that the mesolimbic dopamine system has a relationship with drug addiction.

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