Stainless steel chambers

Contextual fear conditioning and testing was performed in stainless steel chambers (31 × 24 × 21 cm; Med Associates, St. Albans, VT). The floor comprised of metal bars (3.2 mm diameter) at 7.9 mm intervals. The front and back of the chamber were made of clear acrylic whereas the two sides were made of metal. An overhead camera monitored behavior and freezing was scored automatically with Freezeframe software (Actimetrics).
Animals were conditioned using 3 unsignaled foot-shocks of 0.5 mA for 2 s. The shocks were preceded by a 2-minute period and a 1-minute period after each shock, resulting in a 5-minute conditioning session. Mice were then returned to their home cage. Memory was tested 1 or 28 days after training by placing mice back in the chamber for 5 minutes and freezing during this period was assessed.

Operant testing took place in 4 stainless-steel chambers (29.2 × 24.1 × 20.3 cm) with acrylic doors, ceilings and rear walls (Med Associates, St. Albans, VT, USA). The left wall of each operant chamber was equipped with 2 retractable levers, 1 retractable sipper bottle module positioned between the levers, and 1 stimulus light above each lever. A white houselight was located on the right wall. The levers controlled the presentation of the sipper bottle into the chamber. Stainless-steel ball-bearing sipper spouts (5.7 cm) were inserted in rubber stoppers attached to 50-mL plastic centrifuge tubes. A plastic spacer (1.3 cm) was positioned on the spout, which ensured that each spout entered 0.4 cm into the chamber. Experimental events were recorded with a personal computer using Med-PC software (Med Associates) located in the same room. Chambers were enclosed in sound-attenuating plywood cabinets with individual exhaust fans to provide ventilation.