Scientists Find a Brain Circuit That Could Explain Seasonal Depression

An anonymous reader quotes a report from NPR: Just in time for the winter solstice, scientists may have figured out how short days can lead to dark moods. Two recent studies suggest the culprit is a brain circuit that connects special light-sensing cells in the retina with brain areas that affect whether you are happy or sad. When these cells detect shorter days, they appear to use this pathway to send signals to the brain that can make a person feel glum or even depressed. The research effort began in the early 2000s, when [Samer Hattar, an author of the mouse study and chief of the section on light and circadian rhythms at the National Institute of Mental Health] and David Berson, a professor of neuroscience at Brown University, were studying cells in the retina. At the time, most scientists thought that when light struck the retina, only two kinds of cells responded: rods and cones. But Hattar and Berson thought there were other light-sensitive cells that hadn't been identified. The skeptics stopped laughing when the team discovered a third kind of photoreceptor that contained a light-sensitive substance called melanopsin not found in rods and cones. These receptors responded to light but weren't part of the visual system.

[Jerome Sanes, a professor at neuroscience at Brown University, and his team] team put young adults in an MRI machine and measured their brain activity as they were exposed to different levels of light. This allowed the team to identify brain areas that seemed to be receiving signals from the photoreceptors Hattar and Berson had discovered. Two of these areas were in the front of the brain. 'It's interesting because these areas seem to be the areas that have been shown in many studies to be involved in depression and other affective disorders,' Sanes says. The areas also appeared to be part of the same circuit found in mice. The finding needs to be confirmed. But Hattar is pretty confident that this circuit explains the link between light exposure and mood.

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