Biological Factors of Addiction

Evidence suggests that drug-seeking behavior may be caused by genetically determined abnormalities in central nervous system neurotransmitters. Depressants such as heroin or stimulants such as cocaine mimic the structure of neurotransmitters at synaptic junctions of the brain. Stimulants such as amphetamines and cocaine mimic neurotransmitters that cause a stimulant reaction; heroin, other natural and synthetic opiates, and alcohol mimic neu-rotransmitters with a depressant effect. Dopamine has been suggested as a positive reinforcer, and drugs such as amphetamine, cocaine, and nicotine act by increasing the amount of dopamine in the synapse. Tetrahydrocannabinol (THC, marijuana) acts like dopamine in the brain and reinforces the stimulant effects of the neurotransmitter. Alcohol also stimulates the release of dopamine (12).

Illicit drugs may also act at the level of the axon. Phencyclidine hydrochloride (PCP), alcohol, and inhalants interfere with cell membrane function to influence cell transmission along the axon (12).

Despite superficial differences, all drugs activate the limbic system composed of the temporal lobes, amygdala, and hippocampus. The nucleus accumbens within the limbic system is involved in the perception of pleasure and may be the common site of action of all drugs. The limbic system has connections throughout the brain, especially those areas involving voluntary control and cognition (12).

Alcohol addiction may be caused by abnormalities in several central neurotransmitters, including serotonin and dopamine. Changes in serotonergic and dopaminergic systems are associated with states of alcohol intoxication and withdrawal (12).

Cocaine is thought to cause a surge of dopamine and serotonin in the brain that triggers the cocaine "high." Dopamine activation had been described as essential in drug reinforcement and is associated with pleasure and elation. Some authorities suggested it is the master molecule of addiction and the nucleus accumbens is the master organ. Reinforcement theory in drug use proposes that compulsive substance abusers use drugs because these same drugs have been positive reinforcers on previous occasions. Dopamine exercises power over learning and memory, creating a neurochemical support for addiction so powerful that the people, places, and thoughts associated with drug taking are also imprinted on the brain (12).

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