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 f or food reinforcement was assessed in animals trained and tested with the s ame 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 t he same level of reinforcement magnitude regardless of the absolute level o f 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 o f reinforcement. In contrast, foot shock enhanced the increase in runway la tencies produced by a reduction in reinforcement magnitude. Because reducti ons in reinforcement magnitude are known to be aversive for animals, these data indicate that foot shock stress can alter the behavioral response to a n aversive stimulus without disrupting behavioral responding for an appetit ive reinforcer. They also suggest that stressor-induced alterations in appe titively motivated behaviors may be secondary to alterations in sensitivity to subtle aversive stimuli rather than by directly altering appetitive mot ivation. (C) 1999 Elsevier Science Inc.