Previously we showed that rats in long-term constant darkness (DD) exhibit anatomical and behavioral features associated with depression, including decreased norepinephrine (NE) and serotonin (5-HT) innervation of frontal cortex, hypersomnia during the active period, and increased immobility in the forced swim test (FST). We proposed that such light deprivation is a potential environmental factor that could trigger depression by impairing, at least in part, the NE/5-HT systems (Gonzalez and Aston-Jones, SFN 2004). Here, we seek to test this hypothesis by attempting to reverse the depression-like behavior by chronic treatment with the antidepressant desipramine (DMI). Pairs of cages of individually housed adult Sprague-Dawley male rats (200-250 g) were placed together in an environmentally controlled compartment (23±1°C) with free access to food and water. During six weeks, the animals were kept in DD. Subsequently, DMI (~30 mg/kg, along with 0.02% saccharin to increase palatability) was administered in dri...