Inhibition of nociceptive responses of lumbar dorsal horn neurones by remote noxious afferent stimulation in the cat.

In cats anaesthetized with nitrous oxide and sodium pentobarbital, multireceptive lumbar dorsal horn neurones excited by controlled noxious radiant heating of glabrous hind paw skin were recorded by extracellular microelectrodes. These noxious heat responses were inhibited by concomitant noxious stimulation of the ipsilateral forepaw or pinna, or repetitive electrical stimulation of the ipsilateral forelimb deep radial nerve. Similar extents of inhibition were produced by noxious peripheral stimulation and by deep radial nerve stimulation in repetitive trains at intensities sufficient to excite small myelinated fibres or unmyelinated fibres. A greater inhibitory effect was produced by continuous repetitive high-intensity stimulation of the deep radial nerve. With a constant frequency (5 Hz) of continuous deep radial nerve stimulation, graded increases in stimulation intensity revealed the threshold for inhibition in the small myelinated fibre range, and an additional increment of the inhibitory effect when unmyelinated fibres were also activated. When suprathreshold for unmyelinated fibres, the efficacy of continuous deep radial nerve stimulation increased with graded increases in stimulation frequency, with a threshold frequency for inhibition between 0.5 and 1 Hz and maximal effect at 5 Hz. Two nociceptive-specific neurones studied were also inhibited by deep radial nerve stimulation. The results indicate that 'diffuse noxious inhibitory controls' (DNIC) occur in the cat and can be activated by remote electrical or natural noxious stimulation.