Impact of shock on pain reactivity: II. Evidence for enhanced pain.

Shocked rats (Rattus norvegicus) often exhibit longer tail withdrawal latencies to radiant heat, which suggests that exposure to shock reduces pain. But at the same time, rats appear hyperreactive to shock, suggesting than pain is enhanced. Experiment 1 replicated these findings and showed that when tail movement was monitored, shocked rats were less responsive to heat and hyperreactive to shock even when the same behavioral criteria were used. When latency to vocalize was measured, shocked rats appeared hyperreactive to both test stimuli (Experiments 2 and 3). Prior exposure to shock also enhanced the acquisition of conditioned fear in a different context (Experiment 4) and the speed with which rats learned a response to avoid a thermal stimulus (Experiment 5). The results suggest that exposure to shock enhances pain.