Acute and chronic stress effects on open field activity in the rat: implications for a model of depression.

The initial activity of a rat placed in novel surroundings (i.e., open field activity) has been taken as an indicator of its emotional state. We have investigated the effects of immediately antecedent stress upon open field activity in comparison with basal (i.e., unstressed) activity, and additionally, the effects of a history of chronic stress upon the above behavioral patterns. Acute exposure to a non-traumatic, non-debilitating stress (noise and light) consistently increased activity in comparison with basal activity. A history of chronic stress on the other hand reduced basal activity from control levels, and eliminated the activation response to acute stress. This lack of acute activation may bear some resemblance to depression on several grounds. Behaviorally it represents a "refractory loss of interest." Also, chronically stressed rats showed endocrine changes similar to those seen in human depressives. Finally, antidepressant treatment with the monoamine oxidase inhibitor pargyline restored the ability of chronically stressed rats to respond actively to stress.