Eun Jeong Lee ¹ Soo Hyun Kim ² Seongsik Yun ³ Yunmi Kim ³ Jihyun Kim ³ Hyo Kyeong Cha ³ Joo Yeon Oh ³ Hyun Kim ⁴ Kyungjin Kim ⁵ Sooyoung Chung ^2* Gi Hoon Son ^3*

• ¹ Department of Brain Science, Ajou University School of Medicine, Suwon, Gyeonggi, Republic of Korea
• ² Department of Brain and Cognitive Sciences, Scranton College, Ewha Womans University, Seoul, Republic of Korea
• ³ Department of Biomedical Sciences, Korea University College of Medicine, Seoul, Republic of Korea
• ⁴ Department of Anatomy, College of Medicine, Korea University, Seoul, Republic of Korea
• ⁵ Department of Brain and Cognitive Sciences, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu, North Gyeongsang, Republic of Korea

The circadian regulation of gene expression underlies orchestrated daily rhythms in physiology, metabolism, and behavior. The central dopamine (DA) system arising from DA neurons in the substantia nigra (SN) and ventral tegmental area (VTA) of the ventral midbrain has been implicated in a wide range of brain functions. Growing evidence suggests that DA transmission follows daily oscillations at multiple regulatory steps, and its dysfunction is closely related to the onset of various neuropsychiatric diseases. The present study aimed to determine diurnal gene expression profiles across key brain areas constituting the central DA pathways in male mice, such as the SN/VTA, prefrontal cortex, striatum, and habenula. For this purpose, we identified rhythmically expressed genes (REGs) in these brain tissues by transcriptome analysis, according to the time of day. Notably, only 18 REGs, including canonical clock genes, showed significant rhythmicity in all tested tissues, implying the involvement of cell type-specific transcriptional regulators in circadian transcription in discrete brain regions. Gene enrichment analyses revealed that a subset of genes responsible for protein processing in the endoplasmic reticulum also exhibited diurnal expression in multiple brain regions. In contrast, a group of genes constituting a certain biological pathway often showed coordinated rhythmic expression in restricted brain areas; for example, multiple genes mediating dopaminergic synapses were diurnally expressed in the SN/VTA area. Furthermore, gene-disease association analyses strongly suggest that the REGs in the DA pathways are significantly associated with various affective and substance use disorders, as well as sleep-related disturbances, in which both circadian and dopaminergic dysfunctions have been implicated. In conclusion, we have provided valuable information on cyclic gene expression and its potential implications in the circadian regulation of the central DA system.