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Serotonin, like dopamine, is a neurotransmitter produced in the central nervous system from amino acids. Serotonin, the chemical 5-Hydroxytryptamine, is derived from the amino acid tryptophan and plays a big role in a wide range of physiological states, such as sexual behavior, intestinal functions, and affective states like depression.

As a chemical, serotonin has been launched to celebrity status in the past two decades because of its known involvement in depression, anxiety, and obsessive?compulsive disorders. A whole class of antidepressants known as the selective serotonin reuptake inhibitors (SSRI)?including Prozac and Zoloft?work by raising serotonin levels.

During the last two decades, scientists like Parsons have also come to recognize that serotonin levels are affected by alcohol and illegal drugs and that this may account for affective disorders similar to depression and anxiety often seen during withdrawal.

"When ethanol, cannabinoids, opioids, or psychostimulants are taken into the body, serotonin levels in the brain are elevated," says Parsons. Significantly, he adds, this elevation in serotonin plays a role in the motivation to continue taking drugs.

Early studies in the field showed that when serotonin receptor cells were removed or when the receptors themselves were blocked, drug intake increases in laboratory models. Early studies showed that the opposite was also true. If you increase the amount of serotonin in the brain, by giving an SSRI for instance, drug intake decreases.

"The conclusion [from these early studies] was that serotonin produced an inhibitory effect on drug intake," says Parsons. "If you increase the serotonergic component, you make the drugs less attractive. However, now that we have better pharmacologic tools for studying serotonin neurotransmission we're finding a much more complicated picture."

The main thrust of Parsons' laboratory is to investigate the function of individual serotonin receptors?proteins that sit on the surface of neurons and bind serotonin. He is interested in the mechanisms whereby these receptors influence drug intake: how they modulate the behavioral effects of alcohol, cocaine, amphetamine and opioids; the neurochemical processes through which these effects are produced; how these receptor mechanisms change with long-term drug use, and how these alterations in function may contribute to addiction.

These are not easy questions to answer.

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