Context b

On the day following completion of the last VI-60 schedule of pressing, rats were trained in a standard tone-shock fear conditioning paradigm consisted of a single 49-min session. In this task, subjects were tested in a novel chamber (Context B: 43cm × 43cm × 53cm, stainless steel grid floor; MED Associates) that was located in a different location in the research facility from the original Instrumental Context A. The first 5 min of the session consisted of a habituation phase where no cues were presented followed by a randomized presentation of a total of 14 trials with a 180s ITI (7 CS+ trials: 30s tone (5000Hz, 80dB) co-terminating with a 0.8mA footshock delivered through the stainless floor grid bars, and 7 CS− trials: 30s tone (3000 Hz, 80dB) presented without any programmed consequences). Average freezing during the cue presentations and inter-trial intervals (ITI) were automatically scored using MED-Associates software.

Four Coulbourn Instruments (Whitehall, PA) mouse-conditioning chambers (H10-11M-TC) were used with a Plexiglass cylinder (21.5 cm in diameter and 23 cm in height) placed on the chamber floor. Scrambled shock (2 sec, 0.35 mA) was delivered to the grid floor by a computer controlled shock generator (Coulbourn H13-15). For cued conditioning, an 85 dB white noise CS was administered through a sound generator (Coulbourn A12-33). Above the Plexiglas cylinder, an automated infrared activity monitor (Coulbourn H24-61) recorded activity in Graphic State 3.01 software. Contextual fear conditioning studies were conducted in these chambers, and fear conditioning and context testing occurred in these chambers for cued fear studies (Context A). Testing for cued conditioning was conducted in a separate room in rectangular conditioning chambers (Context B; Med-Associates, St. Albans, VT). For cued fear experiments, all data (in both Context A and Context B) were hand-scored due to the lack of an automated measurement system in Context B. For contextual fear experiments, an automated activity monitor was used to assess freezing behavior in Context A. Contexts were cleaned with 95% ethanol following each session. Fear renewal was tested in Context A.