Fig. 1.

Spontaneous acquisition of 0.032 mg/kg/infusion MDPV (n = 32, left) and 0.32 mg/kg/infusion cocaine (n = 16, middle) over the course of the 10-day acquisition phase in male Sprague-Dawley rats. Solid squares represent active lever responses and the X symbols represent inactive lever responses. Abscissa: numbers refer to consecutive days during the acquisition period. Ordinate: total responses emitted on each lever during the active portion of the 90-minute session. Error bars represent ±S.E.M. (Right panel) Percent of rats to acquire self-administration of 0.032 mg/kg/infusion MDPV (solid squares) and 0.32 mg/kg/infusion cocaine (solid circles) over the course of 10 days. Abscissa: numbers refer to consecutive days during the acquisition period. Ordinate: cumulative percent of rats within each training group to acquire self-administration of the training drug/dose.