Daily Rhythms in Clock Gene mRNA Expression in Serotonergic Brain Regions of Adult Male Rats

Dysregulation of the serotonergic system is consistently noted in cases of psychiatric pathology. Circadian rhythm dysregulation is also a common comorbidity in psychiatric populations, and the circadian and serotonergic systems have a long history of coregulation. Despite this, it is not yet known whether serotonergic neurons house circadian molecular clocks, the transcription and translation feedback loops that drive circadian rhythms at the cellular level. To investigate this, brain tissue was extracted from adult male rats every 4 h throughout the light-dark cycle. Radiolabeled in situ hybridization was used to quantify clock gene expression in the dorsal and median raphe, the two nuclei responsible for providing the majority of serotonin to the brain. We discovered oscillatory rhythms in the expression of clock genes Bmal1, Per1, and Per2 with a period of approximately 24 h, and confirmed via fluorescent in situ hybridization that serotonergic (positive for Tph2, the rate-limiting enzyme in serotonin synthesis) neurons do express clock genes. The roughly antiphasic relationship between Bmal1 and the Per genes supports the existence of a circadian molecular clock in these cells. We next measured clock gene expression in neighboring brainstem regions that were not serotonergic, and found that although they all had similar daily clock gene expression profiles, the dorsal and median raphe had higher amplitude Bmal1 expression, and trending higher amplitude Per1 expression. This study adds to the growing list of extra-SCN (suprachiasmatic nucleus) molecular clocks reported in the brain. The prevalence of this circadian machinery, especially in regions of the brain so relevant to psychiatric health, underscores the importance of circadian rhythms to well-being. A greater understanding of the unique nature of circadian rhythms in discrete brain regions is a fruitful frontier for improving psychiatric treatment outcomes and overall health.