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Light therapy is a proven therapy for seasonal depression, but the neurobiology has been unclear. To further understand how light affects serotonin pathways, investigators examined a subset of retinal ganglion cells that project to serotonin neurons in the dorsal raphe.
Gerbils were housed in total darkness for 14 days, while matched controls had usual light–dark cycles. All animals underwent laboratory protocols similar to those used to test drugs for antidepressant efficacy by measuring helplessness, despair, and anhedonia.
Darkness produced worse outcomes than normal light exposure in gerbils on depression tests, and these effects were alleviated with fluoxetine or imipramine. Destroying retinal ganglion cells that project to the dorsal ra…