2.4. Intravenous catheterization surgery

Rats before surgery were pre-trained by once daily 40-min sessions with sucrose pellet, so that animals acquired lever-press behavior to shorten cocaine-training time after surgery. Animals weighing around 300 grams were used at the beginning of the experiments. Each rat was surgically implanted with an intravenous (i.v.) catheter following a previously used procedure (Thrivikraman et al., 2002). Under anesthesia with inhalation of isoflurane, intravenous catheters (#C30PU-RFV1418; Instech Laboratories, Inc.) made of a 3Fr polyurethane (22 ga) were aseptically inserted in the left femoral vein. The catheter, with one silicone collar at 5.0 cm from one end, was secured in place by anchored with nonabsorbable surgical sutures (#51–7623 Size 3–0; Harvard Apparatus). The distal end of catheter exited through a small incision on the back was connected to a support harness (#VAH95AB; Instech Laboratories, Inc.) wearing around scapula, and the base was sealed with a small plastic cover cap to keep the catheter base sterile and protected. Carprofen (analgesic; 5 mg/kg, s.c.) from Norbrook and Cefozoline (antibiotic; 20 mg/kg, i.v. through catheters,) from WG Critical Care were injected for three and seven days, respectively, after the surgery. The catheters were flushed daily with heparinized saline (10 U/ml, Heparin Sodium Injection, Sagent Pharmaceuticals). Rats were allowed to recover seven days before SA training. The catheter patency was checked by examining whether there was little or no resistance during the daily catheter flushing with the heparinized saline (100 μl) before and after each cocaine SA session. It should be noted that this method is not ideal, because it cannot be used to determine whether a catheter is leaking. In principle, catheter patency may be determined more reliably by infusion of a small volume of short-acting anesthetic into the vein to induce temporary loss of muscle tone; rats with functioning catheters should show immediate loss of muscle tone. This more reliable method was not used in our study because we wanted to avoid any possible effects of an anesthetic on cocaine intake during the daily SA tests.