Opposite responses of mesolimbic dopamine system to controllable and uncontrollable aversive experiences.
The immediate and proactive effects of controllable and uncontrollable stressors on plasma corticosterone were assessed in CD-1 mice. A progressive increase of plasma corticosterone concentrations was associated with graded increases in stressor severity. When a footshock stressor was employed, however, the magnitude of the glucocorticoid response, as well as the decay of plasma corticosterone concentrations, was independent of stressor controllability. This was the case regardless of the number of escapable vs. yoked inescapable shock trials mice received, the spacing of shock trials (i.e., applied within a single session or spaced over days), or the degree to which the escape response had been established. In contrast, in a swim task stressor controllability influenced plasma corticosterone concentrations provided that the escape response required of the animal was a highly prepared one (i.e., swim to an illuminated region). When mice were required to emit a contraprepared response (swim to dark) corticosterone concentrations did not differ between escapable and inescapable swim. It is suggested that glucocorticoid secretion is a fundamental response to stressors, and the differential effects of controllable and uncontrollable stressors will be most apparent when the response required of the animal is a highly prepared one.