Cell proliferation in the dentate gyrus of the adult rat fluctuates with the light–dark cycle

This study measured cell proliferation in the hippocampal dentate gyrus in the adult rat at different times within a 12:12 h light–dark cycle. The experiments were conducted in animals living in either a complex environment or in standard lab cages. A single dose of the thymidine analog 5-Bromo-2′-deoxyuridine (BrdU) was injected 2 h before animals were sacrificed either 4, 11, 16, or 23 h after the beginning of the light phase of the light–dark cycle (designated ZT0). In both studies, we found a significant increase in the number of BrdU-positive cells in the subgranular cell layer (SGZ) following BrdU administration at ZT9 and sacrifice at ZT11, compared to other circadian times examined. BrdU administration at ZT9 was timed to primarily identify proliferating cells that were in the S phase of the cell cycle during the light phase. Our results suggest that cell proliferation is enhanced either by sleep or by other variables coupled to the light phase of the circadian cycle.