Figure 1.

Yohimbine facilitates long-term cue fear extinction with massed CS exposures. Separate experiments examined the effects of vehicle, yohimbine, and propranolol on extinction with 0 (A), 5 (B), 10 (C), 20 (D), or 30 (E) nonreinforced CS presentations (seven to eight mice/group). The data shown represent freezing during a single 2-min test CS given 1 d after the extinction sessions (drug free). The final summary panel (F) shows the results of the five experiments with fear normalized to freezing in groups of mice that were injected with vehicle and not presented with any extinction CSs on Day 2 (retention control mice, RC). (^*) p < 0.05 versus RC mice; (+) p < 0.05 versus vehicle-extinction mice.