Abstract

In mammals, a light-entrainable circadian clock located in the suprachiasmatic nucleus (SCN) regulates circadian behavioural and physiological rhythms by synchronizing oscillators throughout the brain and body. Synchrony between these multiple oscillators is believed to be essential for normal daily functions, and disruptions of the phase relationships between them are associated with several disorders and disease. Importantly, the nature of the relationship between the master SCN clock and subordinate oscillators in the rest of the brain is not well defined. We performed an unparalleled high temporal resolution analysis of potential extra-SCN brain oscillators by analyzing the expression of the clock protein PERIOD2 (PER2) throughout the forebrain of the inbred Lewis (LEW/Crl) rat. In addition, we analyzed the transcript levels of two core clock genes, Per2 and Bmal1, and a clock-controlled gene, Dbp, in the SCN and two limbic forebrain regions. Eighty-four LEW/Crl male rats were individually housed in cages equipped with running wheels and were entrained to a regular 12-hr:12-hr light/dark cycle. After 3-4 weeks on this schedule, rats were perfused every 30 min across the 24-hr day, giving a total of 48 time-points (n=1-4/time-point). In this thesis, I report the presence of circadian rhythms in clock gene expression in brain areas important for locomotion, stress, emotion, motivation, and learning and memory, and further explore the relationship between these rhythms. Twenty-two brain areas showed PER2 expression including the SCN, bed nucleus, and several regions of the amygdala, hippocampus, striatum, and cortex. Of these, 20 structures displayed circadian rhythms in PER2 expression. Remarkably, none of the PER2 rhythms in any of the regions analyzed were in phase with the SCN. Instead, an intricate network of brain oscillators with clock gene expression peaking at different times throughout the day was revealed. Furthermore, Per2, Bmal1, and Dbp were rhythmic in the SCN, CEAl, and DG, albeit to different degrees, consistent with the presence of a functional circadian clock in these regions. The data demonstrate the presence of complex and previously unappreciated associations of clock phases throughout the mammalian brain. This comprehensive atlas of clock gene rhythms in the normal rat brain will provide a sound baseline for studies of circadian clock function in animal models of disease.