Researchers found that hamsters that were exposed to 4 weeks of light during the dark cycle at night displayed symptoms of depression, which disappeared around two weeks, after switching back to a normal day and night cycle. The researchers noted that changes in the hamsters brain, which occurred during the chronic light period, also reversed when they returned to a normal light cycle.
Leading researcher, Tracy Bedrosian, a doctoral student in neuroscience at Ohio State University, says that these study results add to existing evidence that chronic exposure to artificial light at night may have an influence on the growing rates of depression in humans during the past 5 centuries, adding: "The results we found in hamsters are consistent with what we know about depression in humans."
The new study also offers some hope for those suffering from depression.
"The good news is that people who stay up late in front of the television and computer may be able to undo some of the harmful effects just by going back to a regular light-dark cycle and minimizing their exposure to artificial light at night. That's what the results we found in hamsters would suggest."
The study was a collaboration of Bedrosian, Zachary Weil, research assistant professor in neuroscience and Randy Nelson, professor of neuroscience and psychology, whose lab conducted previous studies in which he linked chronic exposure to light at night to depression and obesity in animal models.
The team discovered that one particular protein called tumor necrosis factor (TNF) that is present in the brain of hamsters, which is also present in humans, may play a key part in exposure to light at night can lead to depression. By blocking this protein, the team managed to prevent the hamsters from developing depressive-like symptoms - even when they were exposed to light at night.
The researchers conducted two experiments with female Siberian hamsters - with surgically removed ovaries - to ensure that the results were not distorted due to the production of hormones.
The first experiment exposed half of the hamsters to eight weeks in a standard light-dark cycle of 16 hours of light (150 lux) and 8 hours of total darkness each day, whilst the other hamsters were kept in 16 hours of daylight and 8 hours of dim light, i.e. 5 lux, for the first four weeks, which is similar to an on-switched television in a darkened room.
After eight weeks, the hamsters were returned to live under normal light cycle conditions for a period of either 1,2 or 4 weeks before testing began.
The animals were subsequently subjected to various behavior tests, which showed that hamsters exposed to chronic dim light at night were less active overall during their active period each day than those in standard lighting conditions.
The animals that experienced the dim light also displayed greater depressive symptoms compared with the other hamsters, which was observed by showing less interest in drinking sugar water they usually enjoy. However, the team observed that within two weeks of returning to a normal light cycle, the depressive-like symptoms in those in the dim light group were no different than that in hamsters that always had standard lighting and that the animals regained their normal levels of activity. The animals were sacrificed after the behavioral test and the researchers examined the animals' hippocampus, the region in the brain that plays a key role in depressive disorders. The team discovered that the hippocampus of those that were exposed to dim light displayed various changed linked to depression, one of which was an increased expression of the gene that produces TNF.
TNF is a chemical messenger, which is activated during an injury or infection. It belongs to a large family of proteins called cytokines, which cause inflammation in an effort to repair the body's injured or infected site. However, when the inflammation is constant it can be damaging, as seen in the hamsters that were exposed to dim light at night.
Nelson, who is a member of Ohio State's Institute for Behavioral Medicine Research stated: "Researchers have found a strong association in people between chronic inflammation and depression. That's why it is very significant that we found this relationship between dim light at night and increased expression of TNF."
The team also discovered that hamsters in the dim light group had a considerably reduced density of dendritic spines, i.e. hair like growths on brain cells that transmit chemical messages between cells. Bedrosian stated that changes such as this have been associated with depression.
The team did discover though that in hamsters that were returned to a regular light-dark cycle after four weeks of dim light during the night were able to restore their TNF levels and even their density of dendritic spines to essentially normal levels.
Bedrosian commented: "Changes in dendritic spines can happen very rapidly in response to environmental factors."
The second experiment involved testing TNFs importance in causing the negative effects those hamsters that were exposed to light at night. Some hamsters received a drug named XPro1595, a TNF inhibitor that negates the effects of some forms of TNF in the brain. The results demonstrated that those in the dim light group displayed no more depressive-like symptoms than standard-light hamsters if they were given XPro1595. The team points out that the drug was not able to prevent the reduction of dendritic spine density in those exposed to dim light.
Nelson concludes that these results add to existing evidence of TNFs potential role in causing depressive symptoms in hamsters exposed to dim light. He continues saying that the fact that XPro1595 did not affect dendritic spine density means that more research is necessary to gain further insight into the workings of TNF.
Written by Petra Rattue