A time contraction effect of acute tail-pinch stress on the associative learning of rats

The effect of tail-pinch stress interpolated between the saccharin conditioned stimulus (CS) and the illness-inducing unconditioned stimulus (US) during long-trace taste-aversion conditioning was examined in young- and old adult rats with a two-cylinder (saccharin versus water) test. A 2 × 2 × 4 factorial ANOVA was performed on percent-preference-for-saccharin data, with age (young, old), stress condition (stressed, non-stressed), and CS–US interval (22.5-, 45-, 90-, and 180-min) being the factors under consideration. The ANOVA yielded only significant main effects of stress condition and CS–US interval. These findings indicate that stress weakens the CS–US association as evidenced by a higher percent preference for saccharin in the stressed rats than in non-stressed rats at all CS–US intervals. A comparison of the stressed and non-stressed conditioned rats with pseudo-conditioned controls showed that the non-stressed rats formed strong aversions up to the 45-min CS–US interval whereas the stressed rats showed no conditioning beyond the 22.5 min CS–US interval, indicating that stress decreases the effective CS–US interval. Results were interpreted in terms of time-contraction and an internal biological countdown timer hypothesized to govern processes involved in associative learning over long delays.