From Scientific American:
According to a report published online this week by the Proceedings of the National Academy of Sciences, circadian rhythm genes help to regulate the brain’s reward system and could influence the addictive properties of drugs such as cocaine.
Colleen A. McClung of the University of Texas Southwestern Medical Center and her colleagues studied mice lacking a circadian rhythm gene known as Clock. Compared to control animals, mice without Clock were hyperactive and became even more so after being given cocaine. What is more, they also found the drug more rewarding than normal mice did. Finally, Clock-deficient animals exhibited increased activity in the dopamine neurotransmitter system in the brain, which is heavily stimulated by cocaine use. “We found that the Clock gene is not only involved in regulating sleep/wake cycles, but is also very involved in regulating the rewarding responses to drugs of abuse,” McClung says.