Comparison of extinction and reinstatement behavioral responses. The same 3 groups of animals: (1) H-RI, the heroin SA rats with “high” reinstatement induced by acute foot shock stress, n = 10; (2) L-RI, the heroin SA rats with “low” reinstatement induced by acute foot shock stress, n = 10; and (3) NR, the rats below the criterion for acquisition of heroin SA, n = 6. The upper panels show the mean (sem) responses on the active lever (A) and inactive lever (B) during the four 3-h extinction sessions. * p<0.001 vs. Session 4 within the group; + p<0.05 vs. L-RI group; # p<0.05 vs. NR group. The middle panels (C, D) show individual responses and the lower panels (E, F) show the mean (sem) responses on both the active and inactive levers following exposure to 15-min intermittent foot shock stress (C, E) and to heroin (0.25 mg/kg, s.c.) (D, F) during the 3-h session of two reinstatement tests. * p<0.05 vs. inactive lever within the group; ++ p<0.001 vs. L-RI group; # p<0.05, ## p<0.01 vs. NR group.