A lifelong persistent neurogenesis occurs in the dentate gyrus of the mammalian hippocampus. Research in peripheral cell tissue has shown that the timing of cellular division of these cells coincide with the light/dark cycle, however it remains unclear as to whether there is an association between the time of day and cellular proliferation in the brain. The timing of cellular division can be studied through the use of a cellular proliferation marker, such as 5-bromo-2-deoxyuridine (BrdU), which is taken up by the DNA of dividing cells during replication. The goal of this study was to determine whether the time of day affects the number of BrdU labeled cells in the subgranular zone of the dentate gyrus of adult male Syrian hamsters. Adult males received a single systemic injection of BrdU (300. mg/kg) at either the end of the light (ZT-13) or dark phase (ZT-23) of a 14:10 LD cycle and were sacrificed 24. h or 3 days later. Sections through the hippocampus were immunolabeled for BrdU. Cellular proliferation fluctuated across the light/dark cycle during the expansion phase rather than during initial cellular proliferation. A twofold increase in number was expected between 24 and 72. h following a single BrdU injection, but this increase was only seen in the population of cells injected at the end of the light phase. © 2010 Elsevier Ireland Ltd.
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