Individual differences in novelty- and cocaine-induced locomotor activity as predictors of food-reinforced operant behavior in two outbred rat strains

A goal of the current study was to determine if individual differences in cocaine-induced locomotion, which has been shown in outbred Sprague–Dawley (SD) rats to be correlated with differential function of dopamine transporters, were also evident in Long–Evans (LE) rats. Another objective was to determine if differences in locomotion following exposure to novelty or cocaine predicted food-reinforced behavior. Between-strain comparisons of open-field activity revealed similar effects of 10 mg/kg cocaine, although increases in rearing were prominent in LE rats. Both strains exhibited robust individual differences in cocaine-induced locomotion, with nearly identical ambulatory behavior observed in low and high cocaine responders (LCRs and HCRs, respectively) from the two strains. In a cued-discrimination operant task, LE rats learned the contingency in fewer sessions, whereas SD rats obtained more food pellets at fixed ratio 10 and maintained higher progressive ratio (PR) breakpoints. HCRs from both strains also tended to maintain higher PR breakpoints; low and high responders to novelty (LR and HR, respectively) had no consistent differences in food-reinforced behavior. Overall, these studies suggest that wide individual differences in cocaine-induced behavior are common to SD and LE strains and certain differences in food-reinforced behavior are associated with HCRs compared to LCRs.