772 Half of the animals experienced heroin trials first and then cocaine (and vice versa), and the two scents were also counterbalanced across groups. Following the completion of this 30-day runway self-administration phase, rats underwent either 6, 13, or 20 days of non-reinforced, non- cued extinction trials followed by a single test trial in which runway responding was assessed upon the presentation of either the cocaine- or heroin-paired cue. Three dependent measures were collected on every trial: start latency (time from opening start door to a break in photobeam #2 located just outside of the start box), run time (the time required to traverse the runway and enter the goal box, i.e., the time elapsed between breaking photocell #2 outside the start box and #12 just inside the goal box), and retreat frequency (the number of times the animal ceased its forward locomotion toward the goal, stopped, and reversed its direction back toward the start box; the criterion for a retreat was a reversal in direction for a distance spanning a minimum of two photocell breaks). Conditioned Place Preference apparatus A conditioned place preference (CPP) test was conducted to ensure that the doses of the drug reinforcers used in the runway were sufficiently salient to produce drug–place associations after daily 5-min pairings. Two identical CPP apparatus consisted of a wood-constructed rectangular enclosures (156 cm long ×34 cm wide× 30 cm high) that could be sub-divided into three separate compartments: Two larger chambers (61× 30 cm), one painted black and the other white, were separated by a smaller gray intermediate chamber (34× 30 cm). The black side of the apparatus contained acrylic (Plexiglas ®) flooring, the floor of the white compartment was covered with soft wood bedding, and the gray middle section had a painted wood surface floor. The black compartment was scented with a dilute 2% acetic acid solution (a single wipe along each wall 5 cm from the floor); the white compartment received no additional odor cues. Thus, each CPP apparatus provided three distinct environments that differed in color, texture, and scent. We have found these procedures to yield two conditioning environments (the white and dark com- partments) for which rats exhibit no reliable inherent preferences prior to conditioning. During preference tests, the dividing walls were removed to allow the animal full access to each of the three compartments of the apparatus. Situated above each apparatus was a digital camera that detected and recorded the precise location of the animal in real time via a desktop computer running Any-Maze software (Stoetling Co, Wood Dale, IL, USA). The CPP procedure consisted of a 10-day protocol that involved a one-trial pre-conditioning preference test, eight place conditioning trials, and a final post-conditioning Psychopharmacology (2011) 214:769–778 preference test. On day 1, the interior walls separating each compartment were removed, each rats was placed into the middle gray section of the apparatus, and the time spent in each of the three compartments was recorded over a 15-min “baseline”. This was done to ensure that no animal had strong initial side preferences prior to drug–place condi- tioning. Each rat then completed 8 days of conditioning: On the first such day, each animal received an i.v. injection of either vehicle or drug and was then placed into either the white or black compartment for 5 min (thereby modeling the temporal parameters of the drug–goal box associations in the runway). On the following day, each rat received the alternate treatment (either drug or vehicle) and was placed in the alternate colored environment. This continued in an alternating manner until each subject had experienced eight conditioning trials. One group of animals experienced the effects of i.v. cocaine (1.0 mg/kg) in one side and i.v. saline in the other, a second group experienced i.v heroin (0.1 mg/kg) in one side and saline in the other, and a final group experienced cocaine in one compartment and heroin in the other. The order in which subjects received the drug or vehicle, as well as the side of the apparatus paired with a given treatment, was counterbalanced across all animals and between groups (i.e., an unbiased CPP design was employed; see Carr et al. 1988). The day after the final conditioning trial, each subject underwent a 15-min CPP test precisely as described for the baseline session. CPPs were determined by comparing the time spent in the drug-paired environment with that spent in the vehicle-paired environment or, in the case of cocaine versus heroin, in one drug-paired environ- ment compared to the other. Results Runway self-administration Animals traversed the alley once a day over 15 days for either single cocaine (n=24) or heroin (n=26) i.v. infusions followed by 15 additional days/trials for the alternate drug reinforcer. Figure 1 depicts the runway behavior of these subjects expressed as 3-day average scores (to smooth the variability inherent of single daily runway measures). A mixed two- factor (Group × Trial) analysis of variance (ANOVA) computed on the start latency (Fig. 1a) yielded a significant Group × Trial interaction (F(9, 48)=4.291, p<0.01), but no main effects for Trial or Group (p>.05). Thus, changes in start latencies differed across groups as trials progressed. Indeed, while both groups improved their start times with repeated testing, cocaine-reinforced animals started much more slowly than heroin-reinforced animals during the early trials but initiated responding with increasing speed (shorter latencies) as testing continued. An independent t test conducted on very