The brain is awash in chemicals, called neurotransmitters, that carry messages from one cell to the next. Recently, scientists have learned that each neurotransmitter has a number of receptor subtypes that carry different messages into cells. The neurotransmitter dopamine has five subtype receptors. The neurotransmitter serotonin has 14 subtype receptors. Different subtypes tend to cluster in different parts of the brain.
Researchers study the mouse's behavior to learn the function of the missing gene.
Two years ago, Caron developed a knockout mouse that was missing the dopamine transporter. This molecule works by mopping up or reabsorbing excess dopamine in the brain. The mutant mice were hyperactive and did not increase their activity when given cocaine. This seemed to confirm the dopamine hypothesis, he said. But in the new study, the researchers let the knockout mice give themselves cocaine.