Impact of shock on pain reactivity: I. Whether hypo- or hyperalgesia is observed depends on how pain reactivity is tested.

Prior research has shown that exposure to shock can induce a decrease in pain reactivity (hypoalgesia). The present experiments show that, at the same time points that subjects are less responsive to radiant heat applied to the tail (the tail-flick test), tailshock elicits enhanced motor reactivity and vocalization. This enhanced responsiveness, or hyperalgesia, is observed with both magnitude (Experiment 1) and threshold (Experiment 2) measures and decays within 32 min (Experiment 2). Experiment 3 shows that the hyperalgesia decays irrespective of whether or not subjects remain in the shock context, which suggests that the loss of hyperalgesia does not reflect extinction of the context-shock association. Neither removing subjects from the shock context (Experiment 4) nor the presentation of a postshock distractor (Experiment 5) affected the hyperalgesia.