Fig. 2—

The Stress-Alternatives Model (SAM) social interaction regimen, begins with A) placement of a young adult male C57BL/6 N mouse in the center of the oval arena, but hidden by an opaque cylinder, while a novel and much larger CD1 mouse is also placed in the arena, but outside of the cylinder. After a 30s acclimation period (during which freezing is measured for comparison), a 5s tone is played as a conditioned stimulus (CS), which is followed by a 10s trace period (freezing is also measured during the tone and trace). The opaque cylinder is then removed and social interaction, including vigorous aggression, ensues for 5 min, repeated with a new CD1 aggressor each day for 4 days. During the first 2 days, mice choose a relatively stable behavioral phenotype, either B) Escape, which consists of making use of one of the available tunnels at the ends of the oval interaction arena and shortening the duration of their aggressive interaction, or C) Stay, in which mice remain in the SAM arena for the full 5 minutes and continues to receive attacks from the CD1. Importantly, both phenotypes receive significant aggression, and the phenotypes, while relatively stable can be reversed by anxiolytic or anxiogenic drugs. Drug treatments occur on day 3, after stable phenotypes have been formed.