Drug addiction has long been associated with disruptions in circadian rhythms. Understanding how repeated administration of addictive psychostimulant drugs, like cocaine, affects the circadian clock and its output is fundamental to understanding drug addiction. Furthermore, understanding how circadian genes regulate the response to drug use and abuse is critical in designing new and more effective treatments for this devastating disease. Drugs of abuse alter circadian gene expression, resembling how a synchronizer entrains activity rhythms. Part of this proposal seeks to identify the role of cocaine as a synchronizer in terms of molecular rhythms in reward-related areas of the brain. In specific aim 1, we plan to characterize the effects of repeated cocaine exposure on the rhythmic expression patterns of circadian genes, as well as genes involved in dopaminergic neurotransmission in mesolimbic dopaminergic regions of the brain, like the Nucleus Accumbens (NAc) and the Caudate Putamen (CP). We will determine the effects of cocaine in both the diurnal and circadian rhythms of these genes. Increasing evidence suggests the role of circadian genes in drug-mediated responses and behaviors. Thus, in specific aim 2 we plan to examine the role of the circadian gene Npas2 in the NAc in the regulation of cocaine reward. Our preliminary data suggests that this gene regulates drug reward in a region-specific manner. We will use viral-mediated gene knockdown via RNAi to disrupt the expression of Npas2 in the desired regions. AAV viruses expressing a short hairpin RNA (shRNA) toward Npas2 or a scrambled control will be injected into the NAc of wild-type mice. Behavioral assays like cocaine sensitization and conditioned place preference for cocaine will be performed on injected mice to assess changes in drug sensitivity, craving and reward. In specific aim 3, we plan to establish a role for Npas2 in cocaine seeking behavior by using the more clinically relevant addiction model of cocaine self-administration. As in aim 2, animals will be injected with shRNA viruses to knock-down gene expression in the aforementioned area. Animals will be tested for amount of cocaine self-administration, motivation for drug-taking and -seeking, as well as extinction and reinstatement of drug-seeking behavior.