Procedures

General Information for All Procedures: IVSA # Intravenous Drug Self-Administration. The IVSA testing procedure and chambers are described in detail by Dickson et al. (2011)

Procedure 1: Catheter implantation

Equipment, software, and supplies

• Catheter
• Individual housed cage

Reagents and solutions

• Oxygen/isoflurane anesthesia

Steps

1. At 12 weeks of age, an indwelling catheter is implanted into the right external jugular vein under oxygen/isoflurane anesthesia. Mice are individually housed following catheter implantation and allowed 3 days of post-surgical recovery.

Procedure 2: Behavioral testing

Equipment, software, and supplies

• Med Associates operant conditioning chamber
• Syringe filter, 0.22 µm

Reagents and solutions

• Cocaine hydrochloride, cocaine doses are calculated as the salt, filtered through 0.22 µm syringe filter (Sigma-Adrich Chemical Co., St. Louis, MO, USA)
• Heparin lock solution, 100 U/ml heparin/saline, filtered through 0.22 µm syringe filter (Hospira, Lake Forest, IL, USA)
• Enrofloxacin/saline solution, 10 mg/kg, filtered through 0.22 µm syringe filter (Henry Schein, Baytril, Melville, NY, USA)
• Methohexital/saline solution, 5 mg/kg, filtered through 0.22 µm syringe filter (Henry Schein, Brevital, Melville, NY, USA)

Steps

1. Mice begin cocaine IVSA testing on a fixed-ratio 1 schedule at a dose of 0.56 mg/kg/infusion, in operant conditioning chambers. Manipulanda are two retractable response levers (active and inactive).
2. Mice are tested in two-hour sessions at the same time daily, seven days per week, throughout the experiment. Acquisition of cocaine IVSA are defined as two consecutive sessions during which greater than (or equal to) 10 infusions occurred, infusions do not vary by more than 20%; and at least 80% of lever presses are on the active lever.
3. Following acquisition of cocaine IVSA at a dose of 0.56 mg/kg/infusion, the self-administration response is allowed to stabilize at 7 additional doses to establish a dose-response curve. Doses are presented in the order: 0.56, 1.8, 1.0, 0.32, 0.18, 0.1, 0.056, 0.032 mg/kg/infusion. Stabilization is defined as two consecutive sessions during which infusions do not vary by more than 20%, and at least 70% of lever presses are on the active lever.
4. To maintain patency, catheters are flushed before and after each daily testing session with 25 µL of a heparin lock solution. To avert bacterial infection, mice are infused (2 µL/g) with enrofloxacin solution immediately prior to the heparin flush at the end of each session. Catheters are tested for patency with an infusion (2 µL/g) of methohexital/saline solution between each cocaine dose. Rapid loss of muscle tone is interpreted as indicator of patency.