In mice, the circadian Clock gene is involved in regulating the brain's dopaminergic reward pathway. Colleen McClung and colleagues at the University of Texas Southwestern Medical Center at Dallas found that animals with no functional Clock protein were more susceptible to the behavioral effects of cocaine and had elevated dopamine transmission in the reward pathway.1

The researchers examined cocaine's effect in mice lacking Clock, a transcription factor with a central role in the body's suprachiasmatic nucleus circadian clock. The mutants showed greater cocaine sensitization than wild-type controls, as measured by their increase in movement with repeated cocaine doses. They also spent more time in parts of their cages where they could get cocaine.

The researchers also examined the Clock mutants' neural activity in the ventral tegmental area (VTA) dopamine system, which mediates the rewards felt from things such as drugs, food, and sex. They found that Clock was expressed...

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