Impact of shock on pain reactivity: III. The magnitude of hypoalgesia observed depends on test location.

ABSTRACT

Pain reactivity is often assessed in rodents by measuring the latency of tail withdrawal from radiant heat (the tail-flick test). Using this test, the authors show that the magnitude of antinociception observed in spinal rats depends on test location; antinociception is observed at, and distal to, where shock is applied, but not at more proximal sites (Experiments 1 & 2). Experiment 3 evaluates the generality of this observation by testing 3 other shock schedules that are known to elicit distinct forms of antinociception. In all but 1 case, the magnitude of antinociception varied as a function of test location. Experiment 4 shows that morphine also has a greater impact at distal test locations. Experiment 5 assessed the impact of tailshock on reactivity to radiant heat applied to the foot. Of the 5 distinct forms of shock-induced antinociception studied, only 2 produce a robust antinociception at this test location.