Procedures

Procedure 1: osmotic minipump treatment

Reagents and solutions

• saline
• nicotine hydrogen tartrate salt, Mice received 0.9% sterile saline (SAL) or nicotine hydrogen tartrate salt (NIC, 6.3 mg/kg/day, free base weight-Fisher Scientific, Waltham, MA) dissolved in 0.9% sterile saline, delivered subcutaneously via osmotic mini-pumps (Alzet, Model 1004, Durect, Cupertino, CA) for 12 days. (Fisher Scientific)

Procedure 2: contextual fear conditioning

Equipment, software, and supplies

• Mouse fear conditioning chamber, Briefly, mice were trained and tested in noise-attenuating chambers (18.8 × 20 × 18.3 cm3, 65 dB background noise; MED Associates, St. Albans, VT). Mice were fear conditioned with two conditioned stimulus (CS, 30 s, 85 dB white noise)–unconditioned stimulus (US, 2 s, 0.57 mA foot shocks) pairings separated by 120 s. Twenty-four hours after training, mice were returned to the training context for 5 min to assess contextual freezing. After contextual testing, mice were placed in distinct chambers (Med Associates)