Differential effects of Pavlovian conditioned stimuli for cocaine or for food on baseline response rates and as conditioned reinforcers

Separate groups of rats were trained to nose-poke for either food or cocaine on a variable-interval schedule of reinforcement. Superimposing a cocaine-paired stimulus while a rat nose-poked for cocaine did not affect ongoing nose-poke response rates. However, when the cocaine-paired stimulus was presented contingent on the performance of a novel operant response (chain pulling), it increased the frequency of that response. In contrast, superimposing a food-paired stimulus while a rat was nose-poking for food suppressed ongoing nose-poke response rates, and presenting this stimulus contingent on chain pulling did not significantly increase the frequency of this response. These results suggest that (1) the conditioned reinforcing properties of cocaine-paired conditioned stimuli may not be revealed from their effects on ongoing cocaine-maintained behavior, whereas (2) food-paired conditioned stimuli that decrease ongoing food-maintained behavior may not produce effects expected of conditioned reinforcers. The processes operating that may explain this pattern of results are discussed.