Fig. 2.  (A ) The observed concentration–time profiles of unbound oxycodone in brain (▪) and blood (□) (n = 10) from the microdialysis experiment, and of oxycodone in total brain tissue (▴) (n = 3 per time point) from the total brain experiment, in rats receiving a 60-min constant rate infusion of 0.3 mg/kg oxycodone. The data are presented as mean and SEM.  (B ) The observed concentration–time profiles of unbound morphine in brain (♦) and blood (⋄) (n = 9) from the microdialysis experiment, and of morphine in total brain tissue (▴) (n = 3 per time point) from the total brain experiment, in rats receiving a 60-min constant rate infusion of 0.9 mg/kg morphine. The data are presented as mean and SEM.