Fig. 1.

Conditioned place preferences induced by conditioning doses of morphine at 1 and 10 mg/kg administered intraperitoneally in C57BL/6J and 129/SvJ mice, tested after saline or morphine injections: animals given morphine on test day were given a dose that corresponded to their conditioning dose. Thus, the first saline (sal) data (on the left for each strain) correspond to the same mice also given 1 mg/kg morphine on the test day and conditioned with 1 mg/kg morphine during training (mor1). The second saline data correspond to the same mice also given 10 mg/kg morphine on the test day and conditioned with 10 mg/kg morphine during training (mor10). Data represent the mean difference scores + SEM of the time spent in morphine-paired environments minus the time spent in saline-paired environments during testing. Inset, Untreated baseline mean times + SEM (in seconds) spent in the black and white environments during a 10 min period by separate groups of C57BL/6J (n = 8) and 129/SvJ (n = 8) mice. Black bars indicate time spent in the black environment and white bars indicate time spent in the white environment.