Project Summary/Abstract Substance use disorders (SUDs) pose a significant global public health concern, yet effective treatment options remain limited. One often overlooked aspect is the role of circadian rhythms in SUDs. Clinical studies have shown rhythmic patterns in drug-taking and drug-craving behaviors. People with SUDs also often experience disrupted rhythms, including sleep disturbances. Moreover, irregularities in circadian rhythms can contribute to the susceptibility of developing SUDs. Despite this, the mechanisms underlying the connection between circadian rhythms and SUDs are not yet fully understood. Circadian rhythms have been found are present in the mesolimbic dopamine system, which plays a critical role in behaviors associated with SUDs, such as drug-taking. For instance, diurnal rhythms have been observed in dopamine release and extracellular tone in the nucleus accumbens (NAc), a vital brain region in the mesolimbic system. Our lab has also found that disturbing the molecular clock affects the firing patterns of medium spiny neurons, which encompass both DR1 and non-DR1 cells in the NAc. Interestingly, the NAc is also made up of cholinergic interneurons (CINs), potent regulators of dopamine dynamics and motivated behaviors. For example, CIN firing patterns mediate dopamine release via the release of acetylcholine acting on nicotinic receptors on dopamine terminals. Additionally, activation of CINs enhances extinction behaviors in a rodent model of addiction-like behaviors. However, research on CIN activity rhythms and their potential connection to motivated behaviors like drug-taking remains limited. This proposal seeks to bridge the existing gap by investigating the correlation between circadian rhythms and the intrinsic properties and activity of CINs in the NAc, aiming to elucidate their potential relevance to addiction-like behaviors. My prior research suggests that there are diurnal rhythms in CIN modulation of dopamine release and that CIN activity may be higher midway through the light cycle in the NAc. Based on these data, I will directly determine the rhythms in intrinsic properties and activity of CINs in the NAc with electrophysiology. Additionally, I will explore how chronic drug exposure, specifically cocaine self-administration, influences diurnal variations in CINs. My hypothesis is that there are specific times within the light cycle when CIN activity is elevated, aligning with drug- taking behaviors, and that chronic drug exposure alters these rhythms. Furthermore, I will investigate the effects of disrupting the molecular clock in CINs within the NAc and determine its impact on drug-taking behaviors. I hypothesize that disrupting the molecular clock in CINs will alter the diurnal rhythms in addiction-like behaviors. These novel studies will contribute to a better understanding of the relationship between rhythms and the underlying mechanisms, particularly the involvement of CINs, in SUDs. By gaining insights...