Prediction and control of food rewards modulate endogenous pain inhibitory systems.

Tail-flick latencies were measured in food-deprived rats submitted to various schedules of food reinforcement with experimental and yoked animals differing by the possibility of either predicting the occurrence of food by means of a pre-food signal, controlling its delivery by lever-pressing or developing adjunctive activities (schedule-induced drinking). In the first two cases, yoked animals that could not predict or control food deliveries displayed higher tail-flick latencies at the end of sessions than did experimental animals. In the polydipsia experiment, rats that did not develop schedule-induced drinking had higher tail-flick latencies from the start of the experiment than rats that did develop drinking and for these latter animals, drinking was accompanied by a significant reduction in tail-flick latencies. These results demonstrate that prediction and control over external events modulate the activation of endogenous pain-suppressing systems.