Stress vocalization. The footshock stimulus to which animals audibly vocalized were assessed following the generation of a mild footshock by Med Associates, Inc. Shock Titration Package for Mice (model ENV-307 W, St Albans, VT, USA). Specifically, on each test day, mice were moved from the mouse colony room to a holding room adjacent to the footshock chambers. Following at least a 25-min wait period (to allow for acclimation to the move), audible vocalization thresholds was assessed. Mice were placed individually in a shock chamber and allowed to adapt to the chamber for 5 min. Each mouse then received a mild footshock via the floor grid every 30 seconds for 500 milliseconds. The intensity of the first footshock was 0.05 mA and each subsequent footshock increased in increments of 0.05 mA until the mouse vocalized as determined by a single technician who performed all assays, positioned within 1 m of the shock chamber. Once the mouse vocalized, the experiment was terminated and the mouse was subsequently removed from the chamber. Each chamber was cleaned between test subjects. To control for experimenter-related
