Fig. 1.
HRs acquire cocaine SA (175 μg/kg per infusion), whereas LRs do not. a, HRs and LRs discriminated differently between the active and inactive holes [group × hole interaction, F(1,8) = 25.0;p < 0.001]. LRs showed no preference for the active hole versus the inactive one throughout the testing procedure [hole effect, F(1,4) = 1.4;p > 0.3; hole × days interaction,F(6,24) = 0.85; p> 0.5], indicating that they did not acquire cocaine SA. Instead, HRs developed active hole preference during the 7 d of testing [hole effect, F(1,4) = 48.1;p < 0.002; hole × days interaction,F(6,24) = 2.67; p< 0.05], denoting the acquisition of cocaine SA behavior. HRs also displayed greater responding in the active hole [group effect,F(1,8) = 37.1; p < 0.001] and slightly higher inactive hole responding [group effect,F(1,8) = 4.3; p = 0.08] compared with LR rats. b, HR rats showed greater cocaine intake (number of self-infusions) compared with LRs [group effect, F(1,8) = 23.4;p < 0.001]. This difference was present throughout the cocaine SA experiment [group × days interaction,F(6,48) = 1.08; p> 0.39]; however, an examination of the figure indicates that no differences were present during the first SA session. Eachpoint represents the mean ± SEM of each group.