The circadian clock is a cell-autonomous endogenous system that generates circadian rhythms in the behavior and physiology of most organisms. We previously reported that the harmala alkaloid, harmine, lengthens the circadian period of Bmal1 transcription in NIH 3T3 fibroblasts. Clock protein dynamics were examined using real-time reporter assays of PER2::LUC to determine the effects of harmine on the central Key words carboline alkaloid; circadian clock; PER2::LUC protein; real-time reporter assay; suprachiasmatic nucleus The circadian clock is an endogenous cell-autonomous system that generates circadian rhythms in the behavior and physiology of most organisms. The master clock in mammals is located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus, 1,2) which controls most aspects of physiology such as sleep-wake cycles, body temperature, hormone secretion, blood pressure and metabolism.3,4) The circadian clock mechanism consists of a network of autoregulatory transcription-based feedback loops that drive the rhythmic expression of clock genes such as Per1, Per2 and Bmal1, 5) and these cell-autonomous and self-sustained oscillators are found not only in the SCN but also in peripheral tissues as well as dissociated cells. Acute induction of the mRNA expression of clock genes plays a critical role in phase shifting the circadian clock both in cells cultured in vitro and in vivo. In addition to transcriptional feedback regulations, some essential roles of post-translational modifications such as phosphorylation and ubiquitination have been reported. [6][7][8] The harmala alkaloid, harmine, is a β-carboline alkaloid that was originally isolated from seeds of Peganum harmala and Banisteropsis caapi, both of which have traditionally been used in ritual and medicinal preparations of the Middle East, Central Asia and South America. 9) Harmine is also found in common plant-derived foods and animal tissues, 10) and it has antiplasmodial, antioxidative, antimutagenic and antigenotoxic activities, as well as antidepressant properties.11-13) Several psychiatric conditions, including depression, are closely associated with the circadian clock.14) Although we previously reported that harmine dose-dependently lengthens the circadian period of Bmal1 expression in NIH 3T3 cells, 15) the effects of harmine on the master clock in the SCN remain unknown. We therefore evaluated clock protein dynamics in neuronal cells and in SCN slices by monitoring PER2::LUC bioluminescence in real-time to determine the effects of harmine on the master clock in the SCN.