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Many of us experience mood shifts during the colder, shorter, and gloomier winter days. However, for those diagnosed with seasonal affective disorder (SAD), winter blues take on a whole new meaning. While mental health professionals have attributed the symptoms of SAD, which include depression and a feeling of hopelessness, to the lack of sunlight, no one was sure how the brain made the connection. Now, some researchers have found the culprit – a brain circuit which connects special light-sensing cells in the retina with the areas of the brain that impact our moods.
The research that eventually led to this groundbreaking discovery began in the early 2000s when Samer Hattar, chief of the section on light and circadian rhythms at the US National Institute of Mental Health, and David Berson, a neuroscience professor at Brown University, were studying retinal cells. Back then, most scientists were under the impression that there were only two kinds of photoreceptor cells in the retina – rods and cones. However, Hattar and Berson were confident that there were more. "People used to laugh at us if we say there are other photoreceptors distinct from rods and cones in the retina," says Hatter.
But the skeptics turned into believers when the team found the retinal ganglion cells (RGCs) in 2002. However, the photoreceptor cells, which contained a light-sensitive substance called melanopsin not found in rods and cones, were not part of the ocular system.
Instead, they played a crucial role in helping keep the brain's internal clock in sync with changes in the amount of light received. The finding led many scientists to assume that this circadian function also explained seasonal depression.
But Hattar and Berson were not satisfied. They were confident there was a better explanation for the disorder. Sure enough, in 2018 while exploring the function of these new cells in a study involving mice, Hattar’s team found a pathway linking RGCs to the area of the brain responsible for regulating moods. When these cells were present, an artificially-shortened cycle of light and dark caused a version of depression in the mouse. But when the team removed the cells with gene-editing tools, the mouse did not become depressed. The researchers, who published their findings in the online journal Cell on August 30, 2018, concluded that the direct connection between melanopsin and moods might be a key factor for the seasonal disorder, which impacts over 10 million Americans.
Intrigued by Hattar and Berson's findings, Jerome Sanes, also a neuroscientist at Brown University, decided to investigate whether the results apply to human brains as well. His team put young adults in an MRI machine and measured their brain activity as they were exposed to different levels of light. This enabled them to identify brain areas that were receiving signals from the photoreceptors. Sure enough, they observed that the regions of the brain which responded to the variations in light intensity were the same ones that are responsible for depression and other affective disorders. "It's very likely that things like seasonal affective disorder involve this pathway," says Sanes.
While Sanes’ findings need to be confirmed, the researchers involved in the two studies are quite confident that this newly-found circuit is the link between light exposure and mood. The researchers are not sure why the lack of light impacts some people more severely than others. Hattar is also curious about another thing. "You will understand why you would need light to see," he says, "but why do you need light to make you happy?" Whether we are able to solve these mysteries remains to be seen. Meanwhile, the scientists recommend getting as much daylight as possible to keep those winter blues at bay!
Resources: NPR.org, cell.com.