A pretest procedure reliably predicts performance in two animal models of inescapable stress

Rats exposed to inescapable tailshock fail to learn a shuttle-escape task 24 hours later, an effect referred to as "learned helplessness." However, within most rat strains only 10-50% of the animals tested develop this syndrome. In the present study a significant correlation was found between rats that displayed learned helplessness on the first test and those that displayed learned helplessness on a second test performed either 2 weeks (r=.80, p<0.001) or 4 weeks (r=.74, p<0.001) later. An analysis of the mean session latency of the shuttlebox task in these two tests suggested a bimodal distribution of animals that failed and learned. A significant correlation was found between individual rats that learned this task on the first test and those which learned this task 2 or 4 weeks later. Similarly, in the "behavioral despair" test, a significant correlation was observed for floating time for individual rats on the first test and on the second test either 2 (r=.72, p<0.001) or 4 weeks (r=.63, p<0.001) later. However, for the forced-swim test, a unimodal and rather graded response was observed across individual subjects. Thus, performance on the first round predicted performance on the second round in both models. When rats experienced the learned helplessness paradigm on round 1 and the behavioral despair paradigm in round 2, there was no correlation between rats that displayed helplessness following inescapable tailshock and the rats that demonstrated "behavioral despair" on a later test. While both the "learned helplessness" and the "behavioral despair" models may assess the ability of individual animals to "cope" with stressors, the lack of cross-predictability strongly suggests that the two models may be mediated by different neurochenical mechanisms.