Figure 2.

Dose–response curves for the self-administration of binary mixtures of MDPV+caffeine (top row), methylone+caffeine (middle row), or MDPV+methylone (bottom row; n=12 per group). Each mixture was tested at four fixed-dose ratios (10 : 1 (left column), 3 : 1 (left-center column), 1 : 1 (right-center column), 1 : 3 (right column)) relative to the ED₅₀ of the drug in each pair that maintained the most responding. Experimentally determined dose–response curves (white circles) represent the mean (± SEM) for 12 rats. Predicted, additive dose–response curves (gray, dashed lines) represent the mean (± SEM) for 12 rats. Abscissa: doses refer to total MDPV (top and bottom rows) or methylone (middle row) equivalents available for infusion expressed as mg/kg/inf on a log scale. Ordinate: percent of the maximal effect, normalized to the drug in each pair that maintained the most responding (100%), and saline (0%) as shown in Figure 1.