Psychopharmacology (2011) 214:769–778 suture to the subdermal tissue and the guide cannula protruded through the hole perpendicular to the surface of the animal’s back. During surgery, rats were treated with atropine (0.02 mg/kg, intramuscularly) to prevent respira- tory congestion and the non-opiate analgesic flunixin meglumine (Phoenix Pharmaceuticals, Belmont, CA, USA; 1.3 mg/kg, subcutaneously) to reduce post-surgical pain. Following surgery, rats received the antibiotic ticarcillin disodium/clavulanate potassium (Timetin; 50 mg/kg, i.v.) and 0.1 ml of heparin (6.0 IU/0.1 ml prepared in 0.9% physiological saline, i.v.) as a prophylac- tic measure against microbial infection and to promote catheter patency. One week was provided for subjects to recover from surgery during which the catheters were flushed daily with 0.1 ml of antibiotic (ticarcillin disodium/clavulanate potas- sium, 20 mg/kg, i.v.) and 0.1 ml of heparinized 0.9% physiological saline to maintain catheter patency. In addition, rats received 3.0 ml of 0.9% physiological saline subcutaneously once each day for three consecutive days post-surgery to prevent dehydration. Topical antibiotics bacitracin, neomycin, and polymyxin B (Neosporin, Pfizer, New York, NY, USA) were applied to all incisions to aid healing and prevent infection. Once each week, catheter patency was confirmed by observing the behavioral impact of an i.v. injection of the fast-acting barbiturate, methohex- ital sodium (Brevital; 2.0 mg/kg/0.1 ml). Animals that did not lose their righting reflex in response to this challenge were re-implanted with a new catheter using the left jugular vein and given additional days for recovery. Drugs Both the cocaine hydrochloride (cocaine, 1.0 mg/kg) and diacetylmorphine (heroin, 0.1 mg/kg) were provided by the National Institute of Drug Abuse. Each drug was dissolved in a vehicle of 0.9% physiological saline and delivered in a volume of 0.1 ml over a period of 4.3 s via a 10-ml syringe that was seated in a motorized syringe pump (Razel Scientific Instruments, St. Albans, Vermont, USA). The doses selected for study had been shown to be maximally effective (produced the fastest start times, run times, and fewest retreats) in prior runway studies (e.g., Ettenberg et al. 1996; Ettenberg and Geist 1993; Guzman and Ettenberg 2004; McFarland and Ettenberg 1997; Raven et al. 2000). Runway apparatus All runway testing was conducted in two straight-arm alleys (160 cm in length× 12 cm wide ×44 cm high). A start box and goal box (each 23× 20 × 44 cm) were attached to opposite ends of each runway. The floor of each apparatus consisted of 3 cm steel rods laid in parallel 1.2 cm apart 771 perpendicular to the runway walls. Embedded in the side walls of each runway were 13 evenly spaced pairs of infrared photoemitters (on one side wall) with corresponding infrared detectors on the opposite wall. The first pair was located within the start box and the 13th pair inside the goal box. The output from these sensors was fed to a desktop computer via a custom Any–Maze interface (AMi; Stoetling Co, Wood Dale, IL, USA) that recorded the precise location of the animal in the alley in real time throughout the course of each trial. Suspended above each runway were two long magnetic rails (spaced 3 cm apart) that ran in parallel along the entire length of the apparatus. Positioned between the rails was a liquid swivel (375-22PS, Instech Labs Inc.) that connected the guide cannula on the animal’s back to the 10-ml drug- filled syringe, via PE-20 tubing. A flat plastic disk was secured to the midsection of each swivel to prevent it from falling through the gap between the magnetic rails. A pot magnet was attached to the underside of the disk with the polarity arranged to repel the charge of the magnetic rails. The resulting magnetic repulsion between the swivel assembly and the rails permitted the swivel to float slightly above the tracks. Thus, this system served as a low-friction mechanism that allowed the rat to move freely throughout the alley, pulling the swivel assembly along behind and above it as it moved (for more details, see Geist and Ettenberg 1990). Runway self-administration Rats were habituated to the apparatus during a single 10- min trial during which they were permitted to explore the runway and start box (but not the goal box). On the following day, each animal was individually connected to the drug delivery system and placed into the start box where, after 5 s, the start door opened and the trial commenced. Upon the rat’s entry into the goal box, the goal door was closed (to prevent re-tracing) and the syringe pump activated to deliver the drug reinforcer. Rats were left in the goal box for an additional 5-min and then returned to their home cages. Prior to each runway trial, an olfactory cue was provided (McCormick’s pure orange or almond food extract; Sparks, MD, USA) by placing cotton swabs infused with 3 ml of the scented extract under the start box, halfway down the runway, and under the goal box. A fan was used to continuously aerate the runways with the odor. Animals experienced 15 single-daily runway training trials for either heroin or cocaine reinforcement in the presence of a single distinct olfactory cue (either almond or orange scents). Beginning on the trial/day 16, the drug reinforcer was switched to the alternative drug and 15 additional daily trials were conducted in the presence of the alternate scent.