Although there are clear interactions between circadian rhythms and drug addiction, mechanisms for such interactions remain unknown. Here we establish a role for the Clock gene in regulating the brain's reward circuit. Mice lacking a functional Clock gene display an increase in cocaine reward and in the excitability of dopamine neurons in the midbrain ventral tegmental area, a key brain reward region. These phenotypes are associated with increased expression and phosphorylation of tyrosine hydroxylase (the rate-limiting enzyme in dopamine synthesis), as well as changes in several genes known to regulate dopamine activity in the ventral tegmental area. These findings demonstrate the involvement of a circadian-associated gene, Clock, in regulating dopamine function and cocaine reward.circadian rhythms ͉ dopamine ͉ drug addiction ͉ tyrosine hydroxylase D rug addiction is associated with disruptions in sleep and circadian rhythmicity (1-3). Moreover, in animal models of addiction, several reward-related behaviors exhibit clear circadian regulation. For example, levels of drug self administration and drug-induced locomotor sensitization vary according to the day͞night cycle (4-6). These observations suggest interactions between the brain's circadian and reward systems.Although many of the genes involved in circadian rhythms are expressed outside the suprachiasmatic nucleus (SCN), the brain's master circadian pacemaker, and are found in limbic regions of the brain, little is known about their function in these other brain regions. The first indication that circadian-associated genes may be involved in drug-related behaviors came from studies in Drosophila, which showed that behavioral sensitization to cocaine depended on expression of Period, Clock, Cycle, and Doubletime (7). More recently, it was reported that locomotor sensitization and conditioned preference for cocaine are abnormal in mice lacking the Period-1 (mPer1) or Period-2 (mPer2) gene (6). These genes are induced as well by cocaine in the dorsal striatum and nucleus accumbens, brain regions important for cocaine's behavioral effects (8, 9). Although these findings support a role for circadian-associated genes in behavioral responses to drugs of abuse, little is known about the mechanisms by which these genes function, or are regulated, within the brain's reward and motor circuits.Cocaine and other drugs of abuse produce their behavioral effects in part by modulating dopamine neurotransmission in the midbrain ventral tegmental area (VTA), a key component of the brain's reward circuit (10). Several interactions between dopamine and circadian function have been reported. For example, dopamine neurons in the retina regulate adaptations to light (11). Moreover, dopamine D1 receptors in the prenatal SCN are necessary for synchronizing the master circadian clock during development (12). However, a direct link between circadian genes and the VTA dopamine reward system has not been described. CLOCK is a member of the basic helix-loop-helix-PAS (PER-ARNT-SIM) transc...