Stress enhances the response to reward reduction but not food-motivated responding.

The effect of a single exposure to foot shock stress on runway responding for food reinforcement was assessed in animals trained and tested with the same or changed reinforcement magnitude. Foot shock (30 1-s shocks, 1.0 mA) exerted no impact on runway responding in animals trained and tested with the same level of reinforcement magnitude regardless of the absolute level of reinforcement magnitude (i.e., either 15 pellets or 1 pellet). Similarly, foot shock exerted no impact on runway responding in animals trained with a small magnitude of reinforcement but tested with an increased magnitude of reinforcement. In contrast, foot shock enhanced the increase in runway latencies produced by a reduction in reinforcement magnitude. Because reductions in reinforcement magnitude are known to be aversive for animals, these data indicate that foot shock stress can alter the behavioral response to an aversive stimulus without disrupting behavioral responding for an appetitive reinforcer. They also suggest that stressor-induced alterations in appetitively motivated behaviors may be secondary to alterations in sensitivity to subtle aversive stimuli rather than by directly altering appetitive motivation.