14. An animal model of recurrent depression: Sensitized depression-like behavior when rats are re-exposed to chronic mild stress

Abstract

Major depression is a debilitating disorder, characterized by feelings of worthlessness, persistent sadness and lack of motivation. Depression is also recognized to be a chronic, recurrent disorder, and each depressive episode increases an individual’s susceptibility to future occurrences. The current study aimed to develop an animal model of recurrent depression in order to examine possible biological mechanisms responsible for this increased susceptibility. We hypothesized that animals with a prior depressive episode would be sensitive to future stressors, causing the animals to display depressive-like behaviors more rapidly or to a greater extent. Fisher rats ( n = 15) were exposed to chronic mild stress for 35 days or remained in their home cage as controls. During the initial stress, animals showed a slow decrease in sucrose consumption. Following the 35 days, animals went through a 20 day recovery phase where no stressors were present and animals showed a steady increase in sucrose consumption back to baseline levels. At this time animals were re-exposed to the chronic mild stress for 15 days. A rapid decline in sucrose consumption was observed. Linear regression lines were calculated for each animal’s sucrose consumption during the first and second stressor exposure and a paired t -test of the slope of each line revealed re-exposure to stress resulted in a significantly more rapid decline in sucrose consumption compared to that observed during the initial stressor exposure.