Individual groups of rats were then shifted to either shorter or longer days, followed by a shift to constant dark (0L:24D) or constant light (24L:0D), respectively, and then shifted back to the original 12L:12D photoperiod. that this suppression of cocaine reinstatement was not due to repeated testing. In an additional experiment, we examined the photoperiodic regulation of Mirogabalin tyrosine hydroxylase (TH) and dopamine transporter (DAT) proteins in drug-naive rats. These results revealed photoperiodic modulation of proteins in the prefrontal cortex and dorsal striatum, but not in the nucleus accumbens or ventral tegmental area. Together, these findings add further support to the circadian genesis of cocaine-seeking behaviors and demonstrate that drug-induced reinstatement is usually modulated by photoperiod. Furthermore, the results suggest that photoperiod partly contributes to the seasonal expression of certain drug-related behaviors in humans living at different latitudes and thus our findings may have implications for novel targeting of circadian rhythms in the treatment of dependency. Keywords:circadian, cocaine, conditioned place preference, photoperiod, dopamine transporter, prefrontal cortex, tyrosine hydroxylase == INTRODUCTION == Biological clocks located in the brain are thought to provide a temporal business of behavior (Pittendrigh, 1960,Aschoff and Wever, 1976,Reppert and Weaver, 2002,Hastings et al., 2008). Specifically, neurons located within the suprachiasmatic nucleus of the hypothalamus (SCN) comprise a so-called grasp clock a hierarchy of molecular interactions responsible for generating a wide variety of daily rhythms (Davidson et al., 2003). Evidence supporting this role for the SCN includes: 1) loss of physiological rhythmicity following SCN destruction or clock gene disruption, 2) continued expression of behavioral rhythmicity in the absence of environmental light:dark cues, and 3) entrainment (i.e., resetting) of clock rhythms daily to the environmental light:dark cycleviainputs of the retinohypothalamic tract. It is also clear that Mirogabalin this circadian system is usually important for the translation of seasonal time (Sumova et al., 1995,Schwartz et al., 2001). If these characteristics of the natural time-keeping system impact the addiction process in humans, then methods could be developed to take advantage of this temporal gating of behavior. Indeed, daily variation in drug overdoses (Morris, 1987,Raymond et al., 1992), seasonal variation in drug arrests (Langworthy and McKelvie, 2005) and a wide variety of other human actions have been reported (Bronson, 2004,Foster and Roenneberg, 2008). Additionally, a recent longitudinal study of cocaine and cocaine metabolites in wastewater reveal clear seasonal differences indicative of human seasonal cocaine use patterns (Mari et al., 2009). In Mirogabalin addition, several groups have examined the seasonal expression of illicit behaviors in humans living at different latitudes (Sandyk and Kanofsky, 1992,Uitenbroek, 1996,Paschane, 1998,Sher, 2002,Rocchi et al., 2004,Veldhuizen et al., 2007) suggesting a link between season and behavior. Lastly, the frequent co-morbidity between drug abuse and depressive disorder (Quello et al., 2005), the positive effect of light treatment for seasonal affective disorder and its variation with latitude (Potkin et al., 1986), and the recent discovery of the role of clock genes in the etiology of bipolar disorder (McClung, 2007,Westrin and Lam, 2007), together suggest that a common mechanism may be involved. The goal of the current series of experiments was to determine if daily and seasonal (i.e., photoperiodic) cues contribute to drug-seeking behavior in a rat model of drug-relapse: cocaine-induced reinstatement of conditioned place preference (CPP). To this end, we trained and tested rats at different times of day and examined their cocaine-induced reinstatement Mirogabalin behavior before and after shifts to longer or shorter photoperiods. We have previously shown that cocaine sensitization, cocaine-induced reinstatement of CPP, and the dopamine transporter (DAT) and tyrosine hydroxylase (TH) protein expression vary with time of day (Sleipness et al., 2005,2007a,b). P2RY5 Much of Mirogabalin this variation is due to an important role of the suprachiasmatic.