Project Summary In 2020, an estimated 5.2 million Americans reported using cocaine, with 1.3 million meeting criteria for Cocaine Use Disorder (CUD). Despite the increasing need, there are no FDA-approved treatments for CUD. One variable that is often overlooked in CUD research is the resulting disruption of circadian (24-hour) and diurnal (night/day) rhythms. Disrupted rhythms have been shown to prolong the cycle of cocaine abuse and increase the risk of relapse. CUD can be modeled with cocaine self-administration (SA) using three different patterns of cocaine intake: short continuous access (ShA), long continuous access (LgA), and intermittent access (IntA). Though our lab has established diurnal variation in rhythms of dopamine (DA) dynamics, including cholinergic interneuron (CIN) modulation and DA reuptake in naïve animals, it is not yet understood how adapting the type of cocaine SA intake pattern differentially affects these rhythms and the mechanistic pathways involved following long-term cocaine intake. The mesolimbic DA system in the nucleus accumbens core (NAc) is an important mediator of motivated and reward-associated behaviors that are maladapted in CUD. CINs are critical modulators of mesolimbic DA release in the NAc via their activation of acetylcholine (ACh) receptors on DA terminals. While the effects of chronic cocaine on DA signals and CIN/ACh activity have been studied extensively at single time points, a major obstacle in the field is that cocaine-induced changes in DA signaling rhythms and potential mechanisms for rhythmic disruptions, such as alterations in CIN signaling, have not been investigated. Thus, my central hypothesis is twofold: 1) to discover the effect of long-term, voluntary cocaine intake on time- of-day rhythms in DA and ACh NAc signaling, and 2) outline a mechanistic link between rhythms of ACh and DA signaling. The proposed research plan will examine the role of rhythms in DA release and CIN activity under various patterns of cocaine access with the following aims: (1) Assess the cocaine history and pattern of intake on diurnal variation in NAc DA release, (2) Examine a mechanism for disrupted diurnal rhythms in DA release in cocaine-exposed animals, and (3) Define the temporal architecture of CIN modulation of DA release magnitude in cocaine-exposed animals. These studies will highlight the importance of endogenous diurnal rhythms in NAc neurochemistry following chronic cocaine intake patterns to provide greater mechanistic insight into the role of rhythms in CUD. Understanding the influence of rhythms underlying CUD neurochemistry will provide novel treatment targets and propose times throughout the day in which to target them most effectively.