Individual differences in vulnerability to inescapable shock in rats.

The present study determined whether individual differences in neophobia during an open-field pretest predict vulnerability to inescapable electric shock, as measured by 2 tests of learned helplessness in rats. Shuttle-escape latencies and saccharin finickiness increased across groups that had received increasing numbers of inescapable shocks 24 hr earlier. Dispersion in the test measure as well as the percentage of variance explained by pretest neophobia were greater when no or few shocks were delivered in the interpolated stress phase. Pretest neophobia was positively related to stress vulnerability in both tests under these conditions. Further increments in stressor severity overwhelmed even the most stress-resistant rats, thereby decreasing dispersion in the test measure and eliminating the predictive value of pretest neophobia. This pattern of outcomes was more robust for the shuttle-escape measure of helplessness.