Clip-induced analgesia: noxious neck pinch suppresses spinal and mesencephalic neural responses to noxious peripheral stimulation.

Pinch of the nape of the neck, of mice, with a serrated clip, produces immobility and lack of responsiveness to noxious stimulation. In this study we attempted to determine whether clip application produces true blockade of nociception, independent of its immobilizing effect, and examined the level of the neuroaxis at which such an effect takes place. To this end nociception was measured using indices not requiring a motor response. Neck pinch eliminated the elevation of heart rate induced by noxious pinch of the tail without affecting heart rate by itself providing evidence for its analgesic effect. Direct evidence that neck pinch suppresses the transmission of noxious information is also provided. Neck pinch inhibits neural activity evoked by noxious peripheral stimulation while exerting minimal effects on the effects of nonnoxious stimuli. Thus, sensory evoked activity in the periaqueductal gray area, elicited by noxious electrical stimulation, but not innocuous stimuli, is inhibited by neck pinch. Similarly, neck pinch inhibits the response of spinal cord neurons to noxious but not nonnoxious stimulation. It, therefore, appears that neck pinch produces true analgesia by activating supraspinal systems which in turn acts to inhibit the transmission of nociception both at spinal and supraspinal levels.