Simulated weightlessness procedure, head-down bed rest impairs adult neurogenesis in the hippocampus of rhesus macaque

The microgravity environment in space can impact astronauts' cognitive and behavioral activities. However, due to the limitations of research conditions, studies of biological changes in the primate brain, such as neurogenesis, have been comparatively few. We take advantage of - 6° head-down bed rest (HDBR), one of the most implemented space analogue on the ground, to investigate the effects of weightlessness on neurogenesis of non-human primate brain. Rhesus Macaque monkeys were subjected to HDBR for 42 days to simulate weightlessness. BrdU (5-bromodeoxyuridin) and IdU (iododeoxyuridine) were intraperitoneally injected separately before or after HDBR to label the survival and proliferation of newborn neurons. Immunohistochemistry was performed to study the effect of simulated weightlessness on neurogenesis. BrdU staining showed that survival of newborn neurons was reduced, while there were fewer BrdU-positive neurons in the HDBR group compared with the control. Furthermore, IdU-positive neurons also decreased in the HDBR group suggesting a reduced proliferation capacity for these newborn neurons. Our results demonstrate the definite neurogenesis in the adult rhesus macaque hippocampus, and simulated weightlessness HDBR procedure impairs the adult neurogenesis.