Light modulates hippocampal function and spatial learning in a diurnal rodent species. A study using male nile grass rat Arvicanthis niloticus.
The effects of light on cognitive function have been well‐documented in human studies, with brighter illumination improving cognitive performance in school children, healthy adults, and patients in early stages of dementia. However, the underlying neural mechanisms are not well understood. The present study examined how ambient light affects hippocampal function using the diurnal Nile grass rats Arvicanthis niloticus as the animal model.
Grass rats were housed in either a 12:12 hours bright light-dark 1000 lux or dim light‐dark 50 lux cycle. After 4 weeks, the dim light‐dark group showed impaired spatial memory in the Morris Water Maze task. The impairment in their Morris Water Maze performance were reversed when the dim light‐dark group were transferred to the bright light-dark condition for another 4 weeks. The results suggest that lighting conditions influence cognitive function of grass rats in a way similar to that observed in humans, such that bright light is beneficial over dim light for cognitive performance. In addition to the behavioral changes, grass rats in the dim light‐dark condition exhibited reduced expression of brain‐derived neurotrophic factor in the hippocampus, most notably in the CA1 subregion. There was also a reduction in dendritic spine density in CA1 apical dendrites in dim light‐dark as compared to the bright light-dark group, and the reduction was mostly in the number of mushroom and stubby spines. When dim light‐dark animals were transferred to the bright light-dark condition for 4 weeks, the hippocampal brain‐derived neurotrophic factor and dendritic spine density significantly increased.
The results illustrate that not only does light intensity affect cognitive performance, but that it also impacts hippocampal structural plasticity. These studies serve as a starting point to further understand how ambient light modulates neuronal and cognitive functions in diurnal species. A mechanistic understanding of the effects of light on cognition can help to identify risk factors for cognitive decline and contribute to the development of more effective prevention and treatment of cognitive impairment in clinical populations.