Figure 2.

Effect of D2 dopaminergic agents on long-term extinction memory. (A) Protocol for experiments. Mice were fear conditioned on Day 1 with three CS–US pairings in Context A. On Day 2, they were injected with vehicle or sulpiride (20 mg/kg) 45 min before or with quinpirole (1 mg/kg) 30 min before extinction with the indicated number of CS presentations in Context B. Retention control (RC) groups spent equivalent times in Context B, but received no CS presentations. On Day 3, all mice were tested for freezing during three CS presentations in Context B, and the results are averaged in the graphs. Separate experiments examined the effects of vehicle, sulpiride, and quinpirole on extinction with 0 (B), 5 (B,C), 10 (D), 20 (E), or 40 (F) nonreinforced CS presentations (n's = 7 or 8/group). (G) Presentation of these data normalized to the RC group in each experiment. (^*) P < 0.05 vs. RC; (+) P < 0.05 vs. sulpiride extinction; (#) P < 0.05 vs. quinpirole extinction.