SOURCE: Vanderbilt University, news release, May 7, 2015
FRIDAY, May 8, 2015 (HealthDay News) -- Researchers believe they've pinpointed the part of the brain responsible for seasonal affective disorder (SAD).
SAD -- which affects 4 to 6 percent of Americans -- is a type of depression that occurs during winter. It's thought to be caused by the lack of sunlight during that season.
In experiments with mice, Vanderbilt University biologists say they traced SAD to a small region of the mid-brain called the dorsal raphe nucleus. Mice are often used to study depression in humans.
In mice and humans, the dorsal raphe nucleus contains many of the neurons that control brain levels of serotonin, a mood-related chemical. High levels of serotonin are associated with feeling happy while low levels are associated with depression.
The researchers also found evidence that the season in which people are born may affect activity levels of the neurons in the dorsal raphe nucleus.
They divided mice into three groups: One group was born and raised in a summer-like cycle of 16 hours of light and eight hours of dark and another group was born and raised in a spring-like cycle of 12 hours of light and 12 hours of dark. The third group was born and raised in a winter-like cycle of eight hours of light and 16 hours of dark.
The mice born in the summer light cycle had lower levels of depression-like behavior than those born in the spring/fall or winter light cycles, the researchers found.
Also, the serotonin-producing neurons in the mice delivered in the summer light cycle fired faster than those born in the spring/fall or winter light cycles. The mice born in the summer light cycle also had higher levels of serotonin.
When scientists switched the summer-born mice to winter light cycles, their serotonin-producing neurons still fired at an increased rate for months, well into adulthood.
"This showed that early life seasonal photoperiods can have enduring effects on the serotonin neurons. If such an effect occurs in humans, and is long-lasting, it could contribute to the season of birth modulation of SAD risk," said researcher Douglas McMahon in a university news release. He is chair of biological sciences at Vanderbilt.
It's not clear, however, if the study results would apply to humans, since research with animals often fails to product similar results in people.
The study was published May 7 in the journal Current Biology.
The American Academy of Family Physicians has more about seasonal affective disorder.