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Biology 202
2000 First Web Report
On Serendip

Addiction and the Reward Circuit

Sabah Quraishi

Most people are affected directly or indirectly by drug addiction. Many stereotypes including race and socio-economic class are associated with drug addiction. Despite longstanding stereotypes there is more and more evidence being discovered pointing to an explanation from within the brain of the addict. Free will is generally associated with drug addicts as is their choice to use drugs, but free will may not be a factor in addiction. The key to addiction lies solely in the brain, without influence form the "I-function", or free will.

The affect of all drugs of abuse, which are characterized by heroine, cocaine, LSD, opiates, and others, is on what is known as the reward circuit of the brain (1). The circuit includes the ventral tegmental area (VTA), which is connected to the nucleus accumbens and the prefrontal cortex in the pathway where they communicate through neurons (4). Other areas of the brain may be involved depending on the drug (2). The VTA and nucleus accumbens are involved in the reward circuit of all drugs (3). The natural function of the reward circuit is to provide a reward and associated pleasurable feelings in response to life sustaining functions, such as eating, to encourage repetition of that function.

The reward circuit functions through the use of neurotransmitters, mainly dopamine. When activated, the circuit triggers the release of dopamine, a neurotransmitter that induces a pleasurable feeling, from the neurons in the VTA. Drugs of abuse stimulate this same response, in many cases to a greater degree than the natural response causing a spike in dopamine levels. Certain drugs, especially cocaine, can block dopamine receptors in the reward circuit. After being released, if the receptors are blocked, dopamine has nowhere to go and it remains in the synapse where it was originally released causing a build up of dopamine. Naturally dopamine would be reabsorbed into the cells that released it to complete the cycle of the circuit. The use of drugs to influence the reward circuit can lead a user to bypass survival activities and repeat drug use, because it is being rewarded over other activities such as eating (4).

Addiction comes about from changes resulting in the brain due to a high dose, or frequent long-term use (1). A "negative state" of dopamine level can be produced, since the natural level has been elevated, causing a need for drugs by the user to restore normalcy (1). Changes that take place in the brain also affect behavior. Circuits are altered in the brain change the way the neurons in the system work. Dopamine's production, how its message is received, and how it is inactivated are all affected (2). Affects on the brain can be short term or long term depending on the usage. Damage to the dopamine neurons may be reversed using nerve growth factors, which are natural protective molecules that protect against drug-induced damage (2).

The direct evidence of drugs of abuse on the brain, and in turn behavior is supportive of the hypothesis that brain equals behavior. When the brain is altered, in turn behavior is altered. The behavior of the person as well as the behavior of the brain itself in terms of its activities is influenced by a physical change. Addiction, the behavioral affect, is not caused by the addict, but by the drugs on the brain. The only influence of the addict is the first decision to take the drug of choice, which is argued by some saying there is a genetic predisposition towards addiction to drugs of abuse. The brain may be solely responsible for the addiction but if this is true, how do people overcome their own brains to recover from addiction. Could it be that the brain changes back to its normal state in turn allowing the addict to recover or could it be the addict forcing his brain to change back to its normal state, causing recovery.

WWW Sources

1) Introduction: Addiction as a Disease

2) Addiction's Path

3) The Brain's Drug Reward System

4) The Neurobiology of Drug Addiction

Other Sources

5) Routtenberg, Aryeh. "The Reward System of the Brain". Scientific American. November 1978

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