The analgesic effect of peripheral nerve stimulation in various tests of nociception in rats.

The present study examines analgesic properties of 2 Hz peripheral nerve stimulation in four different tests of nociception: the tail-flick, hot-plate, jump-response and formalin tests. Radial, sciatic and trigeminal nerve stimulation produced decreased response amplitudes to weak electrical foot shocks in the jump-response test. The fact that all three nerves produced the same effect suggests the involvement of general, non-segmental mechanisms. The nerve stimulation produced a characteristic after-effect, lasting several minutes, suggesting the involvement of supraspinal mechanisms. The radial nerve stimulation resulted in the most pronounced reduction in jump responses. Sciatic nerve stimulation also produced analgesia in the formalin test but had no effect on the responsiveness to a brief stimulus at threshold level in the hot-plate or the tail-flick tests. It is concluded that analgesia by peripheral nerve stimulation in animals is most readily detected by tests allowing a quantitative evaluation of responses to above-threshold noxious stimuli and not only measuring reactions occurring at the threshold for nociception.