Simulated weightlessness aggravates hypergravity-induced impairment of learning and memory and neuronal apoptosis in rats.

It has been demonstrated that altered gravity may lead to impairments in cognitive functions. However, the effect of a combination of hypergravity and weightlessness on cognitive functions is not well understood. In the present study, we report the effects of high sustained hypergravity after 7 days' simulated weightlessness on learning and memory abilities and neuronal apoptosis in rats. In the Y-maze tests, hypergravity (HG) or simulated weightlessness (SW) significantly decreases accuracy, and increases reaction time of rats compared to that of controls. On the contrary, in the passive avoidance test, HG or SW treatment significantly shortens latency and prolongs total time compared to those of controls. In addition, TUNEL staining shows a few apoptotic cells in cortex and hippocampus in the HG, SW and HG+SW groups, and the number of TUNEL positive cells was found to be the most in the HG+SW group. Furthermore, rats with combined HG and SW treatment reveal a synergistic effect in both the Y-maze and the passive avoidance tests, as well as increased neuronal cell death. These findings suggest that simulated weightlessness may exacerbate hypergravity-induced impairment of learning and memory, likely caused by neuronal cell death in rats.