(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.

(1) Diffuse noxious inhibitory controls (DNIC) were tested for their effect on noxious only, non-noxious and proprioceptive cells in the dorsal horn of the intact anaesthetized rat. Unlike convergent neurones, as described in the previous paper, there was no effect of DNIC on these neurones. It is concluded that convergent neurones are specifically inhibited by DNIC. (2) The effect of DNIC could not be demonstrated for convergent neurones in the spinal animal. Thus the neuronal substrate for DNIC must involve supraspinal structures. (3) Because of the level of firing in convergent neurones induced by hair and touch receptors, presumably constantly and randomly activated in the freely moving animal, a noxious message arriving at higher centres may be partly masked by this background noise. On the basis of the known role of convergent neurones in nociception, we propose the following mechanism which may interpret this paradoxical convergence: two pools of convergent neurones are influenced by a painful peripheral stimulation, one segmental pool being activated whilst the remaining population of cells is inhibited; the "contrast" between the messages from these two pools may well produce a significant pain signalling output from the convergent dorsal horn cells. (4) These results and their theoretical implications are discussed with regard to the concept of the "analgesic system", certain clinical observations and the paradoxical pain relieving effects of counterirritation and some forms of acupuncture.

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The nociceptive flexion reflex (RIII reflex) and the concurrent subjective pain score elicited by right sural nerve stimulation at random intensities were studied in 10 healthy volunteers. A close relationship was found between the recruitment curves of the reflex and the pain score as a function of stimulus intensity. As a consequence, the threshold of the RIII reflex (Tr) and of pain sensation (Tp) were found to be almost identical (mean: 9.8 and 11.3 mA, respectively). Similarly, the threshold for obtaining a maximal reflex response (Tmr) was found to be very close to that for intolerable pain (Tip): 33.5 and 35.1 mA, respectively. These four parameters were studied before and during the immersion of the left hand into a heated thermoregulated waterbath at various temperatures (from 40 to 47.5 degrees C). While nonnociceptive temperatures (40 to 44 degrees C) were without effect, higher conditioning temperatures induced an increase in the four thresholds. In addition, a highly significant linear relationship was observed between the increase in these thresholds and the intensity of the conditioning stimulus in the 44 to 47.5 degrees C range. These four parameters were also studied before and during three other nociceptive conditioning stimuli: immersion of the left hand into a 6 degrees C waterbath, 10 watts muscular exercise of the left hand performed under ischaemia and a painful (5.5 kg/cm2) pinch applied on the nasal septum. These three conditioning situations induced a very significant increase of the four thresholds considered in this study with the greatest being observed during nociceptive cold applied to the left hand. During all the conditioning situations, variations in Tr and Tp as well as in Tmr and Tip were found to be linearly related. This indicates a close relationship between the effects of the conditioning nociceptive stimuli on the reflex and the related pain sensation. These results suggest that the modulation of pain by heterotopic nociceptive stimuli can be explained at least in part by a depression in the transmission of nociceptive messages at the spinal level. They are discussed with reference to the counterirritation phenomena and common features with 'diffuse noxious inhibitory controls' (DNIC) are underlined.

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