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Figure 2: Morphine dose–response curve in the conditioned place preference pattern. The preference of score was calculated as the difference between the time spent in the drugpaired compartment on the 5^{th} and 1^{st} day (a). The changes of locomotor activity parameters on the 5^{th} day were compared between groups. Time spent and locomotor activity parameters (line crossings and the distance traveled on the testing day [b and c, respectively]) were recorded. Data are expressed as mean ± standard error of the mean. *P < 0.05 different from the saline control group (n = 6–9) 
