Controllability of stressors and rewarding brain stimulation: effect on the rate-intensity function

Previous research in mice has shown that an uncontrollable stressor, but not a controllable stressor, depresses responding for rewarding electrical stimulation of the brain (ESB) (33). We reexamined this effect in rats by determining rate-intensity functions for rewarding brain stimulation 24 h and 48 h following stressor exposure. In each rat, two electrodes, one on each side of the brain, were aimed at the posterior, lateral hypothalamus. Self-stimulation rate-intensity functions were obtained for each electrode; stimulation of one electrode always caused stronger activity than equivalent stimulation of the other. If the rats had been stressed with uncontrollable shock the preceding day, their responding was significantly depressed to stimulation at the weaker electrode site but not the stronger electrode site. However, if the rat had control over the stressor, then rate-intensity functions were unchanged. Repeating the test for rate-intensity function 48 h after exposure to the stressor showed no effect of prior stress on self-stimulation. This work suggests that relatively brief exposure to an uncontrollable stressor can leave a residual, 24-h depressive influence on reward system substances. Some effects of exposure to uncontrollable stressors on acquisition and maintenance of responding for rewards may be due, therefore, to attenuation of reward strength.