Addiction can be defined as a state where an organism behaves compulsively, even if the consequences of the behavior do not benefit the organism. Drug addiction is a multi-pronged process. Lifestyle has a role, but the biology of an individual is of prominent importance. The pleasure afforded by various drugs and the craving for these sensations as the effects of a drug begin to dissipate has a physiological basis.
Drugs like morphine, cocaine and heroin work by entering a "reward system" in the brain. As the brain develops a tolerance for the pleasurable sensations evoked by a drug, more of the drug is necessary to elicit those sensations. The term reward system refers to the tendency of humans and other creatures to perform tasks that are rewarding. Pleasurable feelings provide positive reinforcement that encourages repeating of the behavior. Studies in rats have shown that the ventral tegmental area of the brain is implicated in reward behavior. The neurons found in this region contain a neurotransmitter (a substance that functions to pass signal from one neuron to the next) called dopamine. In drug addiction, dopamine is released, triggering the sought-after pleasurable sensations.
Transmission of impulses from neuron to neuron depends upon the rapid release and rapid reabsorption of neurotransmitters such as dopamine. Drugs such as cocaine and amphetamine change the flow of neurotransmitters by slowing down the reabsorption of dopamine, in the case of cocaine, or increasing the release of dopamine, in the case of amphetamine. Both drugs act to increases the concentration of dopamine in the synapse--the space between the neurons. Upon repeated stimulation, the dopamine receptors become less sensitive to dopamine, producing an effect known as tolerance.
In more recent years, another component of the brain called the glutamate receptor has been implicated as another focus of addiction. The glutamate receptor is a site that influences the activity of glutamate neurotransmitters. Glutamate is an important neurotransmitter, accounting for upwards of 40 percent of all nerve signals in the brain. Studies with rats have demonstrated that blocking the transmission of glutamate prevents the development of increased sensitivity to drugs such as cocaine. The stimulation of glutamate transmission elicits sensations that require the repeated presence of glutamate.
For both neurotransmitters, a hallmark of addiction is loss of control in limiting the intake of the addictive substance. Recent research indicates that the reward pathway involving dopamine and glutamate can become even more important in the craving associated with addiction than the pleasurable reward itself. Thus, drug addiction can lead to a state where a craving for the drug is overpowering, even though the pleasurable effects produced by the drug might no longer be produced.
A new research avenue being pursued views addictive drugs as foreign invaders. Thus, similar to the body's response to a bacterial invader, the production and administration of antibodies to the particular drug may be a means of preventing the drug from reaching its target in the brain.
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