Previously we showed that animals maintained in long-term complete darkness (DD) exhibited hypersomnia during the active period and decreased concentration of norepinephrine (NE) as well as fewer fibers/boutons compared to animals maintained under 12:12 light:dark conditions (LD) (Gonzalez and Aston-Jones, SFN Abstr. 2002, 2003). Impairment of the NE-locus coeruleus (LC) system is associated with depression, raising the possibility that DD could induce depression as result of NE-LC dysfunction. Here we tested this hypothesis using a forced swim test (FST) to characterize depressive-like behavioral effects of DD. Adult Sprague Dawley male rats were placed in individual cages with free access to food and water. For six weeks, half of the animals were kept on a 12:12 LD schedule while the rest were kept in DD. Subsequently, at the beginning of the rest period each rat was submitted to a FST under their current lighting conditions. A flotation aid was fitted on the midscapular area and the rat was placed in a ...