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.
The researchers plan to study human drug users next, to see if the new role for Clock also pertains to people. "Most work on Clock has focused on the brain's master pacemaker, located in a brain area called the suprachiasmatic nucleus," explains study co-author Eric J. Nestler, also at the University of Texas Southwestern Medical Center. "The novelty of [the] findings is the role Clock plays in brain reward pathways. The next step is to examine Clock and related genes in human addicts."