Qualitative and quantitative differences in the operant runway behavior of rats working for cocaine and heroin...

Animals were trained to traverse a straight alley for drug reinforcement consisting of five IV injections of either 0.75 mg/kg/injection cocaine (n = 6) or 0.06 mg/kg/injection heroin (n = 6). Testing involved single daily trials during which the latency to leave the start box and the time required to traverse the alley were recorded for each animal. In addition, input from 12 pairs of infrared photocell detector/emittors placed along the length of the alley provided information on the precise location of the animal at 0.1-s intervals throughout the course of each trial. This information was recorded by computer and provided the basis for construction of graphic representations of each trial in the form of spatiotemporal records that revealed the precise route the subject took in getting to the goal box. The experiment revealed substantial differences in the runway behavior of heroin and cocaine animals. While the heroin group exhibited typical patterns of operant performance in that both start latency and goal times decreased gradually over the course of the experiment, cocaine animals were reliably slower than heroin subjects to leave the start box and exhibited a progressive increase in goal times over trials. The latter effect appeared to be a consequence of a "stop and retreat" behavior that was observed in all six cocaine subjects and increased in frequency as the experiment progressed. Because the runway behaviors exhibited here were emitted prior to delivery of the drug reinforcer, they suggest that the motivational state underlying drug-seeking behavior is qualitatively different for heroin- and cocaine-reinforced animals.