Thursday, July 13, 2006

Monkeys on Cocaine

The presence of a particular brain receptor is likely to influence the possibility that an individual will become, and remain, addicted to cocaine, opening a promising new front in treatment options.

Researchers working under the leadership of Michael A. Nader, Ph.D., a professor of physiology and pharmacology at Wake Forest University’s School of Medicine, examined the level of a specific type of dopamine receptor in rhesus monkeys that had never before been exposed to cocaine in order to determine how this level influenced the monkeys’ reactions to subsequently administered cocaine.

Dopamine is a neurotransmitter that, like many other neurotransmitters, moves between nerve cells in the brain. It is sent by the end of one nerve cell and received by the receptor of a neighboring nerve cell in order to deliver a particular message. When there is an excess level of dopamine in the brain, “transporter” cells, which return unused dopamine to the sender nerve, intervene to restore balance.

Researchers believe that cocaine interrupts the work of “transporter” cells, thereby leaving an excess amount of dopamine in the space between nerve cells, causing what is known as a cocaine high. Nerve cells react to the excess of dopamine by reducing their number of receptors, specifically, D2 receptors.

Past tests comparing D2 receptor levels in rhesus monkeys (known as effective models for human drug users) which had been exposed to cocaine versus control monkeys, revealed that cocaine use did indeed reduce D2 levels. However, no test had ever been conducted comparing the levels of D2 receptors in the same monkeys before and after they were exposed to cocaine.

Wake Forest researchers used positron emission topography (PET) to measure the initial amount of D2 dopamine receptors in several rhesus monkeys, administered cocaine to the monkeys and then examined how the monkeys reacted to the drug. Monkeys with lower initial levels of D2 receptors tended to consume more cocaine, suggesting that their addiction to the drug was stronger than that of monkeys with a greater initial number of D2 receptors.

According to the researchers “these findings provide unequivocal evidence for a role of [dopamine] D2 receptors in cocaine abuse and suggest that treatments aimed at increasing levels of D2 receptors may have promise for alleviating drug addiction.”

At present, there no method has been established to increase dopamine D2 receptor levels. However, the study did suggest that stress reduction could prove effective, as stress has also been shown to reduce D2 receptor levels. Furthermore, previous Wake Forest investigations have linked stress with an inclination to consume cocaine.

Story here.

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