Long-Term Exposure to Dim Light Shown to Alter Hippocampus of Rats and Impair Learning and Memory

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Researchers at Michigan State University found that in rats and possibly in humans long-term exposure to dim light conditions may be detrimental to learning and memory and in rats, they showed that it actually changes the structure of the brain. The researchers examined the brains of Nile grass rats after exposing them to dim or bright light for four weeks. The rats they chose, like humans, sleep at and and are awake during daylight hours.

According to the study, the rats lost about 30 percent of the capacity in the hippocampus, a brain region critical for learning and memory. Also, these rats performed badly on a spatial task that they had trained on previously. However, the rats exposed to bright light over the same four week period demonstrated considerable improvement on the spatial task.

Study Showed Changes in Hippocampus

Moreover, when the rodents that were exposed to dim light were then exposed to bright light for four weeks after a month-long break, their brain capacity and task performance fully recovered. The study claims to be the first to demonstrate that changes in light in an environment lead to alteration of brain structures. The researchers expect analogous findings in studies of humans.

Antonio “Tony” Nunez, psychology professor and co-investigator on the study pointed out that when they exposed the rats to dim light, which mimicked typical indoor lighting, the animals spatial learning was impaired. Nunez worked with Lily Yan, associate professor of psychology and primary researcher on the project, and Joel Soler, a doctoral graduate student in psychology and lead author of a paper detailing the findings published in the journal Hippocampus.

Soler noted that prolonged exposure to dim light resulted in significant reductions in a substance called brain-derived neurotrophic factor – a peptide that supports the maintenance of the hippocampus’s healthy connections and neurons. Also, Soler indicated that such sustained exposure to dim light considerably reduced dendritic spines or the connections that allow neurons communicate.

The fewer connections translate to diminished learning and memory performance on tasks that are dependent on the hippocampus.  “In other words, dim lights are producing dimwits,” Soler said.

The researchers know that the light does not directly affect the hippocampus, but first affects other regions of the brain after passing through the eyes. The group hopes to figure out how light exposure influences learning and memory. Yan said the team is now looking into a group of neurons in the rodent hypothalamus that generate a peptide called orexin that’s known to influence brain functions

The group hopes that the project could lead to insights about the hippocampus, as well as learning and memory in the elderly and people with glaucoma, retinal degeneration, or cognitive impairments.

 

Reference

Soler JERobison AJNúñez AAYan LLight modulates hippocampal function and spatial learning in a diurnal rodent species: A study using male nile grass rat (Arvicanthis niloticus)Hippocampus2017;00:112https://doi.org/10.1002/hipo.22822

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