Procedure: Catheter implantation, maintenance, and patency

1. Mice are anesthetized under an oxygen/isoflurane (1.5-2.0%) ventilation system and supported on a circulating-water heating pad.
2. A catheter is inserted through a ventral supra-jugular incision (placed 1.2 cm dorsal to the heart, anchored with suture knots above and below a silicon bead) and is tunneled subcutaneously over the shoulder and externalized through a midscapular dorsal incision.
3. Incisions are sutured and treated with triple antibiotic ointment.
4. Mice are allowed to recover 6 days prior to testing.
5. A 20 µL solution containing the antibiotic ticarcillin clavulanate is dissolved in heparinized (30 units/mL) 0.9% saline and flushed through the catheter immediately after implantation, daily throughout recovery, and before and after each self-administration session.
6. Catheter patency is evaluated with a 20 µL infusion of 1% propofol (10 mg/mL) before starting self-administration and between each experimental phase (if prominent sedation is not apparent within 3 s of infusion, the mouse is removed from the study).

Procedure: Cocaine self-administration

Operant conditioning chambers are enclosed in sound-attenuating cabinets and computer controlled. Chambers are fitted with two levers, assigned as active and inactive (in a counterbalanced fashion). A single-channel fluid swivel connects a pump-driven syringe to the indwelling catheter (infusion speed = 10 µL/s). Completion of a response schedule on the active lever triggers a 20 µL infusion containing a unit dose of 0.5 mg/kg cocaine and initiates flashing of the house light for 20 s (post-infusion period). Daily self-administration sessions last 2h and are conducted 7 days a week. A maximum of 65 cocaine infusions can be obtained in each session.

1. Mice train for acquisition of cocaine self-administration under a fixed ratio 1 (FR1) schedule of reinforcement (5 days).
2. If mice acquire and show stable behavior for FR1 (see criteria below), they are evaluated under a FR2 schedule (4 days).
3. If mice acquire and show stable behavior for FR2 (see criteria below), they are evaluated under a FR5 schedule (5 days)

Criteria for stable acquisition and cocaine-maintained responding for FR1 and FR2 (FR5 tests are administered without completion criteria):

• a) 20% or less variation in number of infusions earned and greater than 70% discrimination index (active responses divided by total responses) across three consecutive days
• b) 10 infusions minimum earned per session
• c) 5 and 4 days minimum per schedule, respectively

Mice with patent catheters are considered to have failed to acquire self-administration if they earned no infusions over 7 consecutive days or if they earned fewer than 10 infusions per session over 3 consecutive days.

Procedure: Locomotor activity

Activity chambers consist of large acrylic cages placed within an infrared grid; clean cages are used for each session and each subject.

1. Mice are habituated to the experimental setup for 30 min across three consecutive days.
2. Mice are injected with cocaine (1 mL/100g body weight) at four doses: 0, 5, 10, 20 mg/kg using a repeated measures, cyclic Latin square design, with 2-3 days between tests.
3. Number of photobeam breaks is recorded, and activity measured as the average activity counts, excluding the first 15 min to allow for drug onset.