Subjects were male 8- to 12-week-old 129S1/Sv1mJ (S1) mice obtained from The Jackson Laboratory. This inbred strain exhibits normal fear, but is impaired in fear extinction learning and retrieval, which is rescued by various pharmacological interventions (for example, fluoxetine, yohimbine, zinc depletion).23–25 Mice were housed 2 per cage in a temperature- and humidity-controlled vivarium under a 12 h light/dark cycle (lights on 0600 h). The number of mice used in each experiment is indicated in the figure legends.
