Fig. 2.

Number of infusions of 0.032 mg/kg/infusion MDPV self-administered per session by rats (n = 16/drug) with a history of responding for MDPV (circles), cocaine (squares), fentanyl (diamonds), nicotine (downward triangles), or ketamine (upward triangles). Open symbols indicate rats classified as low-responders (<20% of total active lever responses made during 5-second postinfusion timeout), where filled symbols indicate rats classified as high-responders (≥20% of total active lever responses made during 5-second postinfusion timeout). Symbols represent mean (±1 S.E.M.) number of infusions. Abscissa: session number for the final 10 sessions of responding for MDPV. Ordinates: mean number of infusions earned. Bottom right panel: Percent of rats classified as high- or low-responders when responding for MDPV, separated by prior self-administration history. Open bars represent percent of low-responders, where filled bars represent percent of high-responders. Abscissa: self-administration history. Ordinates: percent of rats classified as high- or low-responders. A two-way repeated measures ANOVA compared number of infusions earned by high- and low-responders for each drug history (except cocaine, due to n = 1 high-responder) and found significantly different levels of drug intake by phenotype (see text for details).