People who use cocaine regularly may be less likely to experience feelings of pleasure -- and not just the "high" received from the drug, Reuters reported Jan. 1.When studying the effects of cocaine on humans, researchers at the University of Michigan examined the brain's dopamine neurons, which are believed to be involved in the experience of pleasure.
The researchers examined the brain tissue of 35 deceased individuals who were highly dependent on cocaine, and 35 deceased nonusers. The research focused mainly on the level of dopamine and the quantity and activity of the vesicular monoamine transporter protein (VMAT2), which helps return dopamine to storage in the neuron.
The researchers found that people who had used cocaine had much lower dopamine and VMAT2 levels in their brains than nonusers. As a result, while cocaine use increases dopamine level to create a "high," it eventually caused a long-term reduction of dopamine, making it harder for cocaine users to experience a "high" from the drug or other positive feelings.
"The neurons in the brain are trying to decrease the effect of cocaine, but in the process they make the person less responsive to all pleasures," said Dr. Karley Little, who led the research.
Other researchers cautioned that additional research is necessary to confirm the study results. Similar research in rats and human subjects has not shown comparable decreases in dopamine and VMAT2 levels, said Dr. Glen Hanson, acting director of the National Institute of Drug Abuse.
The researchers have begun another study to determine whether cocaine causes permanent cell death or temporary cell inactivity.
"Studying how dopamine neurons work, change, adapt, and are damaged after being exposed to cocaine can explain why people get addicted or go into withdrawal," Little said. "The more you understand the brain's biochemistry and how these molecular machines are working or not working, the more ideas you have about how to rationally change them."
The study is published in the January 2003 issue of the American Journal of Psychiatry.
Little, K., Krolewski, D., Zhang, L., & Cassin, B. (2003). Loss of Striatal Vesicular Monoamine Transporter Protein (VMAT2) in Human Cocaine Users. American Journal of Psychiatry, 160: 47-55.
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