A new study suggests that hamsters may suffer from symptoms of anxiety and depression during the dark days of winter, just as some humans do.
Using a variety of tests, researchers found more symptoms of depression and anxiety in adult hamsters that were housed for weeks in conditions with limited daylight, as they would find in winter, when compared to hamsters who had days with longer daylight.
The research also examined whether hamsters that developed prenatally and then were born during short days were more likely to suffer from depression and anxiety as adults. The results for these tests were mixed, but suggest that hamsters born in winter-like light conditions had increased depressive symptoms as adults.
Overall, the results suggest that the season the hamsters were born in, their sex, and the changing of the seasons all may play a role in levels of depression and anxiety.
"These results in hamsters may provide some insight into the development of seasonal affective disorders in humans," said Randy Nelson, co-author of the study and professor of psychology and neuroscience at Ohio State University.
"Our results do suggest a relationship between season and symptoms of depression and anxiety
Nelson conducted the study with Leah Pyter, a doctoral student in neuroscience at Ohio State. They presented their results Nov. 15 in Washington , D.C. at the annual meeting of the Society for Neuroscience.
The study involved 53 female and 48 male Siberian hamsters. They were offspring of breeding pairs housed in either long days (16 hours of light a day, such as found in summer) or short days (8 hours of light per day, such as found in winter). Following birth, the hamsters remained in the same light period until weaning (about 20 to 23 days of age). After weaning, about half were transferred into the opposite light period – in other words, some who were in short days went to long days, and vice versa.
At about 60 days of age, the hamsters began a series of tests that are used to measure anxiety and depression in the animals. Many of these tests are the same ones used by pharmaceutical companies to test anti-depressive and anti-anxiety drugs in animals before they are used in humans, Nelson said.
Tests for anxiety included one in which hamsters are placed in a large box and viewed for 60 minutes to see how much time they spend in the middle versus near the walls. Anxious hamsters will spend more time along the walls, where they feel more protected, while less anxious animals will explore the entire box, including the open middle area.
Among the tests for depression was one in which they measured how much of a sugar drink hamsters drank over the course of several days. Hamsters generally like this drink, but hamsters with depressive-like symptoms will not drink as much.
"We found that the amount of light hamsters were exposed to prenatally and up through weaning did have enduring effects on behavior in adulthood," Pyter said. "But these effects were tempered quite a bit by whether they spent their time as adults in long days or short days."
The researchers wanted to examine daylight exposure before and right after birth because of research in humans that suggest people who are born during the winter are more likely to suffer from depression as adults.
The results in hamsters were not always clear-cut. Specifically, hamsters exposed to short days before weaning showed more anxiety-like behaviors as adults in some tests, but not in others.
"Our results are generally consistent with the hypothesis that people born in the winter are more likely to suffer from depression, but a lot more work needs to be done," Nelson said.
Other results showed that short days post-weaning consistently increased anxiety-like behavior and depressive-like behaviors (with a few sex differences) compared to those exposed to long days.
In general, the results showed that female hamsters showed more evidence of depression than males, which corresponds to research in humans which shows more depression among women than men.
Previous studies suggested that the hormone melatonin plays a key role in the seasonal behavioral changes found in the hamsters in this study, Pyter said. Scientists know that levels of melatonin are associated with seasonal changes in daylight, Pyter said.
Melatonin is also found in humans, and that's one reason why this research may be applicable to human conditions like seasonal affective disorder, she said.
Pyter said future studies in the Ohio State lab will examine whether drugs used to treat anxiety and depression will reduce seasonal anxiety and depressive-like symptoms in hamsters.
Nelson said they will also look at the role of seasonal changes in stress hormones, as well as melatonin and testosterone, in depression and anxiety in hamsters.