Acquisition and maintenance of cocaine, nicotine or sucrose self-administration in WT and mGluR2-KO rats. (A) Total numbers of cocaine infusions within a daily 3 hr session over the initial 14 days of daily cocaine self-administration. (B) Cocaine infusion rates (calculated as the total number of infusions over time spent). (C) Representative records of cocaine self-administration, illustrating that mGluR2-KO rats displayed more rapid cocaine-taking behavior with shorter inter-infusion intervals compared with WT rats. (D) Total numbers of nicotine infusions during the initial 10 days of nicotine self-administration, illustrating enhanced nicotine self-administration in mGluR2-KO rats compared to WT rats. (E) Oral sucrose self-administration during the initial 10 self-administration days, illustrating no difference between two groups of rats. The data are represented as mean ± SEM. *p< 0.05; **p<0.01; compared with WT rats. FR1, fixed-ratio 1; FR2, fixed-ratio 2.