Individual differences in response to novelty, amphetamine-induced activity and drug discrimination in rats

Rats were pre-tested in several individual difference screens--novelty-induced activity, novelty-induced place preference, novel-object interaction, and amphetamine-induced activity. Rats that were more sensitive to the locomotor effects of amphetamine were more active in an inescapable novel environment and displayed a greater preference for a novel environment. All animals were then trained to discriminate amphetamine (1 mg/kg) from saline in a two-bar discrimination procedure using food-maintained responding. After acquisition of the discrimination (mean = 37 trials), two amphetamine generalization tests (0.0625, 0.125, 0.25, 0.5, 1.0 and 2.0 mg/kg) were conducted. In the second generalization test, rats that were more sensitive to the activating effect of amphetamine were also more sensitive to the discriminative stimulus effects of amphetamine (i.e. lower median effective dose). Moreover, high responders in the novelty-induced activity and novelty-induced place preference screens were more sensitive than low responders to the bar-press suppressant effects of amphetamine in the first generalization test. The relationships are discussed in terms of identifying processes common to the screens (e.g. stress and reward).