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updated 11/17/2010 6:24:21 PM ET 2010-11-17T23:24:21

Exposure to a dim light at night, such as the glow of a TV screen, may prompt changes in the brain that lead to mood disorders, including depression, according to a new study in hamsters.

While more work is needed to see if the results are true of humans, the findings might explain why night-shift workers and others constantly exposed to light at night are at increased risk for mood disorders, the researchers said.

The findings are being presented today (Nov. 17) at the annual meeting of the Society for Neuroscience in San Diego.

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Over the last century, artificial nighttime lights have become ubiquitous in industrialized countries, but it's not clear whether exposure to illuminated darkness affects the brain.

To find out, Tracy Bedrosian, a doctoral student in neuroscience at Ohio State University, and colleagues placed hamsters in two environments. In one, hamsters were exposed to 16 hours of daylight and eight hours of complete darkness each day. In the other, the animals experienced 16 hours of daylight, but at nighttime, a dim light was kept on, about the intensity of a TV screen lighting up a dark room, the researchers said.

After eight weeks, the researchers tested the hamsters for behaviors that would suggest they were depressed. For example, they looked to see whether the hamsters still engaged in activities they normally enjoy, such as drinking sugar water.

In people, loss of enjoyment is known as anhedonia and is a major symptom of depression.

Hamsters in both groups were given a choice between drinking tap water or sugar water. The hamsters exposed to light at night drank similar amounts of tap and sugar water — they'd lost their preference for the sweet treat.

"That suggests to us that they are not getting the same pleasurable and rewarding feeling from drinking their sugar water, and that it may be interpreted as a depression-like response," Bedrosian said.

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These changes in behavior were associated with changes in the brain region known as the hippocampus. The hamsters exposed to night light had a reduced number of so-called dendritic spines on the surface of cells in this region. Dendritic spines are hair-like protrusions that brain cells use to communicate with one another.

The findings agree with studies on humans that have found the hippocampus to be involved in depression. A patient with major depression has a smaller hippocampus, Bedrosian said.

The brain changes in the hamsters might arise from fluctuations in the production of the hormone melatonin, Bedrosian said. Melatonin signals to the body that it's nighttime, but a light at night dampens its production. The hormone has been shown to have some antidepressant effects, and so a decrease in melatonin might spur depression symptoms, Bedrosian said.

If the same mechanism is at work in people, then "people might want to try to avoid falling asleep with their TVs on all night, or they might want to try to minimize light exposure during the night," Bedrosian said.

 

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