Diffuse noxious inhibitory controls (DNIC). I. Effects on dorsal horn convergent neurones in the rat

(1) Sixty-eight convergent dorsal horn neurones have been recorded at the lumbar level in anaesthetized intact rats. All cells received prominent A alpha and C fibre afferents and correspondingly could be activated by high and low threshold stimuli applied to the peripheral excitatory receptive field. (2) The activity of 67/68 of these neurones was powerfully inhibited by noxious stimuli applied to various parts of the body. Since non-noxious stimuli were ineffective in this respect, the term "diffuse noxious inhibitory controls" (DNIC) is proposed. (3) DNIC could be evoked by noxious pinch applied to the tail, the contralateral hind paw, the forepaws, the ears and the muzzle; the most effective areas were the tail and muzzle. Noxious heat applied to and transcutaneous electrical stimulation of the tail were extemely effective in eliciting DNIC as was the intraperitoneal injection of bradykinin. (4) DNIC strongly depressed by 60-100% both the C fibre response following suprathreshold transcutaneous electrical stimulation and the responses to noxious radiant heat. (5) The spontaneous activity and the responses to low threshold afferents induced either by A alpha threshold electrical or natural stimulation were also powerfully inhibited. (6) In the majority of cases, long lasting post-effects directly related to the duration of conditioning painful stimulus were observed.