Rat strain differences in open-field behavior and the locomotor stimulating and rewarding effects of amphetamine.

Fischer 344 (F344) and Lewis (LEW) rats show considerable neuroanatomical and neurophysiological differences within the mesolimbic dopamine system. The aim of our experiments was to study the functional correlates of such differences by examining open-field behavior and the sensitivity towards the psychostimulant and rewarding effects of amphetamine in male and female, F344 and LEW rats. In addition, the consequences of short versus extended habituation to open-field testing on amphetamine locomotion in these two rat strains was assessed. LEW but not F344 rats irrespective of gender showed between-session habituation of open-field activity. Amphetamine-induced locomotion was higher in F344 compared to LEW rats and in females compared to male rats. In addition, extended habituation increased the locomotor effects of amphetamine. The rewarding effects of amphetamine as measured by the conditioned place preference test were more pronounced in F344 than in LEW rats. Our results suggest that the two rat strains differed in their behavioral response to mild stress and to amphetamine and that these differences may depend upon differences within the mesolimbic dopamine system.

Differential effects of prenatal stress in two inbred strains of rats.

The long-term effects of prenatal stress (three times daily restraint stress during the last week of gestation) on the behavioral response to stress, as assessed by novelty-induced locomotion, performance in the forced swim test, and the acquisition of a two-way active avoidance, were investigated in two inbred strains of rats, Fischer 344 (F344/NHsd/Zur) and Lewis (LEW/SsNHsd/Zur). Additional measures included birth weights, pain threshold on the hot plate, and basal and stress-induced corticosterone secretion. In all of the behavioral paradigms strain differences were found: LEW rats showed poorer acquisition of avoidance conditioning, displayed higher levels of activity on the open plate, less immobility time in the forced swim test, and lower pain thresholds in the hot-plate test compared with F344 rats. LEW rats had higher birth weights after prenatal stress, whereas F344 rats were lighter. Following prenatal stress the pattern of behavioral effects obtained in LEW rats in stress-related tests could be interpreted as improved coping abilities with stress, i.e., improved acquisition of active avoidance, less immobility in the forced swim test, and reduced novelty-induced locomotion. Prenatal stress was much less effective in inducing long-term behavioral changes in F344 rats, yielding only one effect, namely, enhanced novelty-induced locomotion in female F344 rats. Pain thresholds were increased as a consequence of prenatal stress, irrespective of strain and gender. Basal and stress-induced corticosterone release differed in the two strains, with LEW rats showing less stress-induced corticosterone release. Prenatal stress did not, however, affect basal or stress-induced corticosterone release. The results suggest that prenatal stress exerts long-term effects on behavior, which depend on the genetic background.