Acquisition of methamphetamine self-administration (SA). (A) Mean total number of lever responses of Wistar rats and SHR/NCrlCrlj during the 2-hour methamphetamine SA. Open symbols indicate lever responses on the active (reinforced) lever and filled symbols show responses for the inactive (non-reinforced) lever. (B) Mean infusions obtained by Wistar and SHR/NCrlCrlj during the 2-hour methamphetamine self-administration. Empty squares are responses of Wistar rats and circles are for SHR/NCrlCrlj. Data are presented as means ± SEM (n = 6).