Chuck Bednar for redOrbit.com – @BednarChuck
Up to six percent of all Americans suffer from seasonal affective disorder (SAD), a form of depression that tends to make people sad, tired, and irritable during the winter months. And now thanks to a team of researchers from Vanderbilt University, we know why this phenomenon occurs.
Scientists had long known that the amount of sunlight a person is exposed to played a role in the condition, as did his or her internal or circadian clock. They suspected that the neurotransmitters serotonin and melatonin were also involved, but the underlying neurobiological mechanisms that were directly responsible for SAD had remained a mystery.
Now, Noah Green, a graduate student in the VU Department of Biological Sciences and his fellow investigators have narrowed down the origin of this condition in experiments involving mice. They found that the seasonal light cycle effects that drive SAD come from a part of the mid-brain known as the dorsal raphe nucleus.
Probing the role of the dorsal raphe nucleus
According to their research, which appears in Thursday’s edition of the journal Current Biology, the dorsal raphe nucleus is home to many of the specialized neurons that control serotonin levels. Since low levels of this neurotransmitter have been linked to depression, and higher levels play a role in feelings of happiness, it plays a key role in regulating a person’s mood.
In a statement, Green explained that his team’s research was inspired by the work of a team of Viennese psychiatrists who had found “a season of birth correlation in SAD patients,” and that previous studies had shown that this part of the brain is linked to the brain’s master biological clock. Furthermore, it has also been found to respond to melatonin, a hormone that is involved in physiological functions such as sleep, blood pressure and seasonal reproduction.
Confirming the involvement of serotonin in SAD
The VU researchers divided mice into three groups: One that was born and raised in a summer-like environment and experienced 16 hours of light followed by eight hours of darkness; another that was raised in a cycle of 12 hours of dark and 12 hours of light (similar to spring and fall); and a third that was born in winter-like conditions (eight hours of day, 12 of darkness).
Except for the light cycle differences, the environments were identical, the researchers explained. The mice were then run through a series of tests to see whether or not they exhibited depression-like behaviors, through which the authors found that the summer-light-cycle mice showed fewer signs of depression than those born and raised in spring/fall- or winter-light-cycles.
After examining the brains of members of that group, they found that the neurons responsible for serotonin fired faster in the summer-light-cycle mice, meaning that they had higher levels of both that neurotransmitter and another (norepinephrine) that is known to excite serotonergic neurons. The findings confirm that serotonin is involved in SAD, the authors concluded.
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