Serotonin leads to whitening of brown adipocytes, shifting their metabolism to fat accumulation rather than oxidation. In addition, serotonin interferes with the differentiation process into brown adipocytes.

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Objective Serotonin was shown to interfere with the differentiation of brown adipocytes. In addition, clock components inhibit brown adipogenesis through direct transcriptional control of key components of the transforming growth factor β pathway. The aim of this study was to investigate whether serotonin abrogates brown adipogenesis by affecting clock functionality. Methods Nondifferentiated and differentiated HIB1B brown adipocytes were treated with serotonin, and their clock expression and functionality and differentiation state were examined. Results Nondifferentiated HIB1B brown adipocytes treated with serotonin showed increased brown adipocyte markers alongside increased brain‐muscle Arnt‐like protein 1 (Bmal1) and RAR related orphan receptor A (Rora) but decreased nuclear receptor Rev‐erbα mRNA levels. BMAL1 overexpression together with serotonin led to significantly lower brown adipocyte markers. Serotonin in the differentiation cocktail led to reduced brown adipocyte markers as well as clock gene expression. After differentiation, serotonin treatment significantly decreased brown adipocyte markers and reduced BMAL1 and RORα but increased REV‐ERBα protein levels. Addition of serotonin to the differentiation medium or addition after differentiation reduced activity of calcium/calmodulin‐dependent protein kinase type II subunit gamma, which interferes with circadian locomoter output cycles protein kaput (CLOCK):BMAL1 dimerization and transactivation. Conclusions Clock expression is required at the early stages of differentiation to brown adipocytes, and serotonin interferes with this process by modulating clock functionality. Serotonin interferes with clock functionality by reducing the levels of the active form of calcium/calmodulin‐dependent protein kinase type II subunit gamma.