Behavioral evidence showing the predominance of diffuse pain stimuli over discrete stimuli in influencing perception

Chronic back pain, acute postoperative pain and the activation of diffuse noxious inhibitory controls (DNIC)

The effect of the presence of either chronic or acute clinical pain on pain threshold and on the nociceptive flexion reflex (RIII) threshold was studied. The experimental pain sensation and the flexion reflex were evoked by trains of short electrical pulses. It was hypothesized that both kinds of clinical pain would be able to induce 'diffuse noxious inhibitory controls' (DNIC) and thereby raise the 2 experimental thresholds. Patients with chronic low back pain, patients with postoperative pain from oral surgery, and pain-free subjects were tested in 3 conditions: during baseline, after i.v. administration of a placebo, and after i.v. administration of naloxone. In comparison with 2 pain-free control groups, the 2 pain groups had a significantly higher pain threshold in all conditions. However, the RIII threshold was not significantly elevated in chronic or acute pain patients compared to controls. Naloxone had no effect on the RIII or pain threshold in any of the groups. It is concluded that the increased pain threshold which is frequently found in chronic pain patients, and which could be confirmed in the present study, does not result from a DNIC effect. The adaptation level theory offers an alternative explanation. Also, the acute postoperative pain in this study did not seem to induce DNIC. Because other forms of acute pain have been found to be effective in activating DNIC, future research should establish which pains are and which pains are not effective.

Effects of tractotomy on nociceptive reactions induced by tooth pulp stimulation in the rat

The effects of a trigeminal tractotomy on nociceptive reactions induced by electrical stimulation of an inferior incisor were studied in freely moving rats. Behavioral tests were based on the observation of three reactions: jaw opening reflex, face scratching with forepaws, and head rotation toward the stimulated side. These reactions appear successively when the stimulation intensity increases. The results were evaluated by comparing a test group with a sham-operated group. Both groups were prepared surgically in the same way but only the former had undergone tractotomy. The results were as follows: The threshold of the jaw opening reflex triggered by stimulation of the tooth pulp ipsilateral to the tractotomy was not modified; however, the threshold of the face-scratching and the head rotation reactions rose (P less than or equal to 0.05) after tractotomy. These data suggest that the rostral part of the trigeminal sensory complex is involved in oral nociception. However, the modification of the face-scratching and head rotation thresholds reveals that the subnucleus caudalis also participates in this function when a certain level of pain is reached. The thresholds of the three nociceptive reactions evoked by stimulation of the contralateral tooth pulp were significantly lower in the test group than those in the sham-operated group (P less than or equal to 0.001), which is interpreted as being indicative of an modulating effect of the subnucleus caudalis on the contralateral trigeminal sensory complex. This effect was probably revealed by the use of a sham-operated group as a control.

Diffuse noxious inhibitory controls (DNIC). II. Lack of effect on non-convergent neurones, supraspinal involvement and theoretical implications

(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.

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.