The association between darkness and depression is well established. Now a March 25 study in the Proceedings of the National Academy of Sciences reveals for the first time the profound changes that light deprivation causes in the brain.

Neuroscientists at the University of Pennsylvania kept rats in the dark for six weeks. The animals not only exhibited depressive behavior but also suffered damage in brain regions known to be underactive in humans during depression. The researchers observed neurons that produce norepi­nephrine, dopamine and serotonin—common neurotransmitters involved in emotion, pleasure and cognition—in the process of dying. This neuronal death, which was accompanied in some areas by compromised synaptic connections, may be the mechanism underlying the darkness-related blues of seasonal affective disorder.

Principal investigator Gary Aston-Jones, now at the Medical University of South Carolina, speculates that the dark-induced effects stem from a disruption of the body’s clock. “When the circadian system is not receiving normal light, that in turn might lead to changes in brain systems that regulate mood,” he says.

Treating the rats with an antidepressant significantly ameliorated brain damage and depressive behaviors. “Our study provides a new animal system for antidepressant devel­opment. Many existing animal models depend on stress. Our model is a stress-free means of producing a depression. It might be parti­cularly relevant to seasonal affective disorder, but we think that it is relevant to depression overall,” Aston-Jones says.

Editor's Note: This story was originally printed with the title "Down in the Dark"