IMPACT OF SHOCK ON PAIN REACTIVITY .3. THE MAGNITUDE OF HYPOALGESIA OBSERVED DEPENDS ON TEST LOCATION

Pain reactivity is often assessed in rodents by measuring the latency of tail withdrawal from radiant heat (the tail-hick test). Using this test, the authors show that the magnitude of antinociception observed in spinal rats depends on test location; antinociception is observed a t, and distal to, where shock is applied, but not at more proximal sit es (Experiments 1 & 2). Experiment 3 evaluates the generality of this observation by testing 3 other shock schedules that are known to elici t 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 location s. Experiment 5 assessed the impact of tailshock on reactivity to radi ant heat applied to the foot. Of the 5 distinct forms of shock-induced antinociception studied, only 2 produce a robust antinociception at t his test location.