Diffuse noxious inhibitory controls (DNIC): evidence for post-synaptic inhibition of trigeminal nucleus caudalis convergent neurones

Activity produced by direct microelectrophoretic application of glutamate onto 19 convergent neurones in trigeminal nucleus caudalis, was strongly depressed during and after the application of heterotopic noxious conditioning stimuli: noxious heat (52 °C) applied to the tail, noxious pinches applied to the tail or hindpaws and intraperitoneal injections of bradykinin produced mean reductions in activity of 80–90%. The same noxious conditioning stimuli had no effect on the activities of any of 5 noxious-only or 5-non-noxious-only neurones. These effects were similar to those previously reported to influence peripherally evoked activities of nucleus caudalis convergent neurones and which have been termed diffuse noxious inhibitory controls (DNIC). It is therefore proposed that DNIC act on nucleus caudalis convergent neurones by a final post-synaptic inhibitory mechanism involving hyperpolarisation of the neuronal membrane. Consistent with this hypothesis, it was also found that the noxious conditioning stimuli could restore firing of convergent neurones which had been excessively depolarized by large doses of glutamate.     

The effect of systemic morphine upon diffuse noxious inhibitory controls (DNIC) in the rat: evidence for a lifting of certain descending inhibitory controls of dorsal horn convergent neurones

The effect of exogenous opiates upon diffuse noxious inhibitory controls (DNIC) was investigated in intact anaesthetized rats. 58 convergent neurones, responding to both noxious and innocuous stimuli applied to their cutaneous receptive fields, were recorded at the lumbar level. These cells received A- and C-peripheral fibre inputs as shown by electrical stimulation of their receptive fields and were mainly located in the medial part of the dorsal horn.The immersion of the distal two-thirds of the tail in hot water (52 °C) induced strong inhibition of the responses to both A-(23%) and C-(69%) fibres. Post-effects of long duration were commonly observed after cessation of the conditioning stimulus.While systematic injection of morphine at a low dose-range (0.1–1 mg/kg) did not significantly affect the unconditioned responses, the DNIC-mediated inhibitions were profoundly altered.(a) DNIC of responses to C fibres were dose-dependently (P < 0.01) lifted by morphine: (b) the post-effects observed after cessation of conditioning stimuli were dose-dependently (P < 0.01) diminished; (c) DNIC of responses to A-fibre were similarly altered but this effect was less significant (P < 0.05); (d) DNIC of responses to sustained moderate pressure were greatly diminished by morphine (P < 0.01); and (e) these effects were specific since they were antagonized by the opiate antagonist, naloxone. In addition, they were shown to be stereospecific since while the dextrogyre stereoisomer, dextrorphan, was ineffective the levogyre derivative, levorphanol, induced a significant lifting of DNIC.It is concluded that morphine decreases the supraspinal inhibitory controls of dorsal horn convergent neurones, at least when these controls are triggered by noxious stimuli. Assuming that a basic somatosensory background activity (noise) is transmitted to higher centres by dorsal horn convergent neurones, and that the pain-signalling message is the contrast between the activity of the segmental pool of neurones induced by the noxious stimulus and the DNIC-mediated silence of the remaining neuronal population, it is proposed that, by a reduction in DNIC, low-dose morphine could restore the initial level of background activity, the final result being analgesia.     

Depression of activities of dorsal horn convergent neurones by propriospinal mechanisms triggered by noxious inputs; comparison with diffuse noxious inhibitory controls (DNIC)

The ability of heterotopic noxious stimuli to inhibit the activity of dorsal horn convergent neurones was investigated in both intact anesthetized, and spinal unanesthetized rats. Forty-four convergent neurones in lumbar dorsal horn were recognized by their ability to respond to both noxious and non-noxious natural stimuli and by their characteristic responses corresponding to A- and C-fibre activity following electrical stimulation of their cutaneous excitatory receptive fields on the ipsilateral hindpaw. The application of a sustained pinch to the excitatory receptive field resulted in an initial phasic activation of the neurone, which adapted to a stable tonic level of activity (mean 31.8 +/- 2.2 spikes/s). The levels of activity produced in this fashion were not appreciably different between the two types of preparation. In the intact anesthetized rat, the tonic activity produced by the sustained pinch could be strongly depressed by noxious conditioning stimuli applied to various parts of the body for all 10 neurones studied: heating the tail or pinching the contralateral hindpaw, the tail or a forepaw during 30 s each resulted in comparable inhibitions which had mean values in the order of 80% and which were always marked by post-effects lasting for upwards of 30 s. These inhibitory effects have been called Diffuse Noxious Inhibitory Controls (DNIC). In the spinal unanesthetized rat, the tonic activity was depressed to some extent by the same conditioning stimuli, for only 16/34 neurones studied. By comparison with the intact animals these inhibitions were weak, adapted to base-line levels within 30 s and were more marked for conditioning stimuli applied to structures proximal (tail, contralateral hindpaw) to the excitatory receptive field than for stimuli applied more distally (forepaws). The differences between the inhibitions found in the intact and spinal preparations were subsequently confirmed in a series of experiments in which single convergent neurones were studied before and after the pharmacological blocking of the cervical spinal cord in anaesthetized rats. The results in the spinal preparations provide evidence for the existence of some propriospinal modulatory processes, triggered by the onset of noxious stimulation and acting on convergent neurones. These processes appear to be different from those mediating DNIC, which have been shown to involve supraspinal structures, to concern all convergent neurones, to be very potent and associated with long-lasting post-effects whether the conditioning noxious stimuli are applied to parts of the body proximal or distal to the excitatory receptive field.     

Pharmacological evidence for the involvement of serotonergic mechanisms in diffuse noxious inhibitory controls (DNIC)

The involvement of serotonergic mechanisms in diffuse noxious inhibitory controls (DNIC) acting on dorsal horn convergent neurones has been studied in the anaesthetized rat. 35 neurones activated by transcutaneous electrical stimulation of their hindpaw receptive fields giving clear large A-fibre and C-fibre responses were recorded. These activities were conditioned by DNIC, evoked by either noxious heat applied to the tail or noxious pinch of the nose. Cinanserin (4 mg/kg i.v.) and metergoline (5 mg/kg i.v.), serotonin (5-HT) receptor blockers, strongly reduced the inhibitory effects of DNIC whilst having no significant effect on the non-conditioned responses. 5-Hydroxytryptophan, a precursor for 5-HT synthesis, significantly potentiated the effect of DNIC. These results indicate an important role for descending serotonergic pathways in DNIC. The functional role of this system is discussed.     

The ability of inhibitory controls to ‘switch-off’ activity in dorsal horn convergent neurones in the rat

Unitary extracellular recordings were made from 51 convergent neurones in the dorsal horn of the lumbar spinal cords of urethane anaesthetized rats. All the cells tested responded to sustained noxious mechanical stimulation of their receptive fields on the ipsilateral hindpaw, but only 26/49 gave tonic responses lasting for more than 5 min. In all 26 cells, these tonic responses were depressed by diffuse noxious inhibitory controls (DNIC) triggered by applying noxious conditioning stimuli elsewhere on the body. In seven cells, the inhibitory effects could involve a complete abolition of activity and in five cells, when this occured, activity did not return during 2.5–6 min periods of observation following removal of the conditioning stimuli. However, in those cases, activity could be restored to pre-conditioning levels by further manipulations of the receptive field - either removal and re-application of the original stimulus or brief application of an additional stimulus. These results show that inhibitory controls can ‘switch-off’ activity in at least a small proportion of dorsal horn convergent neurones. One possible explanation would be that in these neurones, responses to sustained noxious stimuli may depend on activity in a positive feedback circuit within the central nervous system, which when interrupted, may be restored only by additional afferent inputs. The existence of such a loop could also explain the finding of convergent neurones which initially were not spontaneously active but which after stimulation of their receptive fields, developed on-going discharges which could be switched-off by DNIC.     

Effects of visceral distension on the activities of neurones receiving cutaneous inputs in the rat lumbar dorsal horn; comparison with effects of remote noxious somatic stimuli

(1) Unitary extracellular recordings were made from 92 lumbar dorsal horn neurones in urethane-anaesthetised rats. These neurones were classed as 'noxious-only' (4), 'non-noxious-only' (33) or 'convergent' (55) by their responses to stimulation of their cutaneous receptive fields on the ipsilateral hindpaw. (2) Distension of abdominal viscera (colon, urinary bladder) depressed the activities of the vast majority (93%) of the convergent neurones but of only one other cell (a non-noxious-only neurone). Similarly, noxious stimulation of widespread somatic structures depressed activity in all but one of the convergent neurones but in only 3 other cells (one non-noxious- and two noxious-only neurones). One or other of these procedures also excited 3 cells--one convergent neurone responding to distension of the colon, another to stimulation of widespread somatic structures and one non-noxious-only neurone being excited by stimulation on the contralateral hindpaw. (3) The inhibitory effects of the noxious somatic stimuli were very like those described previously and termed 'diffuse noxious inhibitory controls' (DNIC) and it seems likely that the effects of the visceral stimuli were also manifestations of DNIC, particularly in view of their similar, nearly total, specificity to convergent neurones. There were however, some small differences in the extent and temporal evolution of the inhibitory effects of the visceral and of the somatic stimuli--the visceral stimuli generally producing weaker inhibitions with slower rates of onset and recovery. It is proposed that these differences may have reflected different amounts and patterns of activity in the relevant primary afferent fibres rather than being due to different central neural mechanisms. (4) These results and the likely explanation that the effects of the visceral stimuli were mediated by a diffuse mechanism should be taken into account when interpreting the results of other studies in which inhibitory effects are produced by visceral stimulation.     

The encoding of thermal stimuli applied to the tail of the rat by lowering the excitability of trigeminal convergent neurones

Recordings were made from convergent neurones in the nucleus caudalis of the trigeminal complex. They could be excited by both innocuous and noxious stimuli applied to their excitatory receptive field located on the ipsilateral part of the muzzle. The responses to A- and C-fibre activation induced by supramaximal transcutaneous electrical stimulation of their receptive fields were conditioned by thermal stimuli applied to the tail, and the relationship between the conditioning temperatures and their ability to induce inhibitions of the evoked discharges of convergent neurones were investigated. During sequences of repetitive stimulation, the posterior two-thirds of the tail were immersed in a waterbath at various temperatures (36–52°C) and the resultant inhibitions of responses were calculated. The threshold for obtaining an inhibitory effect ranged between 40 and 44°C. Above this level, there was a highly significant correlation between the conditioning temperature and the degree of inhibition, which was very strong for the highest temperature (i.e. for 52°C: 54.4 ± 7.3and92.3 ± 3.7% inhibitions being obtained for A- and C-fibre-evoked responses respectively); such a correlation concerned the inhibitions observed both during immersion of the tail and after the removal of the conditioning stimuli. In further experiments, identical conditioning stimuli were tested upon responses of trigeminal convergent neurones to microelectrophoretic applications of an excitatory amino acid, dl-homocyteate, and very similar results were obtained: a threshold for inhibition in the 40–44°C range and a significant correlation between the conditioning temperature and the degree of inhibition in the 44–52°C range (52°C giving inhibitions of 98.8 ± 0.4%. These results demonstrate that the excitability of nucleus caudalis convergent neurones is reduced in direct relationship to the intensity of a thermal nociceptive stimulus applied to the tail. The encoding of noxious stimuli by hyperpolarization of convergent neurones remote from the segmentally depolarized neuronal population, is suggested.     

Diffuse noxious inhibitory controls (DNIC) in the rat with or without pCPA pretreatment

Diffuse Noxious Inhibitory Controls (DNIC) were investigated in anaesthetized intact rats, with or without p-chlorophenylalanin (pCPA) pretreatment. Dorsal horn convergent neurones responding to both noxious and non-noxious stimuli applied to their excitatory receptive field located on the distal part of the hindlimb, were recorded in the lumbar spinal cord. These cells received Aα and C fibre inputs as shown by electrical stimulation of their receptive field.In control animals, the evoked responses to C fibre inputs coulb be strongly inhibited by various noxious stimuli applied to widespread areas of the body: the inhibitory effects induced by intraperitoneal administration of bradykinin, pinch applied to the tail or muzzle and noxious heat applied to the tail were of 77%, 87%, 83% and 61% respectively. Long-lasting post-effects were seen in most cases after cessation of the application of the conditioning stimulus.Pretreatment with pCPA (300 mg/kg, i.p., 3 days) resulted in a strong reduction of DNIC. The inhibitory effects induced by intraperitoneal administration of bradykinin, pinch applied to the tail or muzzle and noxious heat applied to the tail were reduced by 47%, 63%, 87% and 63%, respectively. The post-effects were also reduced both in terms of magnitude and duration.These results strongly suggest that serotonergic pathways are partially involved in     

Response patterns to noxious and non-noxious stimuli in rostral trigeminal relay nuclei.

Postimulus time histogram analysis of second-order neuron responses in rostral trigeminal relay nuclei of cat demonstrated characteristic firing patterns after noxious (tooth pulp) and non-noxious (tooth tap) stimuli. The response to noxious stimulation was prolonged and frequently bimodal while the response to non-noxious stimulation was brief. The same neurons were fired by electrical stimuli applied directly to nucleus caudalis but with longer latencies suggesting a contributory role of nucleus caudalis to the characteristic prolonged bimodal response pattern to noxious stimuli. Interacting noxious and non-noxious stimuli using condition-test sequences demonstrated further stimulus mode-related changes in firing patterns. Electrical conditioning stimuli in nucleus caudalis reduced some responses while strychnine sulfate applied into nucleus caudalis augmented the responses evoked in rostral nuclei by both noxious and non-noxious peripheral stimuli. Nucleus caudalis appeared to contain elements which may modulate activity in rostral trigeminal nuclei by either augmenting or reducing specific firing patterns of second-order neurons in rostral relay nuclei.     

Distribution of fos-like immunoreactivity in the caudal brainstem of the rat following noxious chemical stimulation of the temporomandibular joint

Central expression of the protooncogene c-fos was used to examine areas receiving noxious sensory input from the rat temporomandibular joint (TMJ). Fos-like immunoreactivity (Fos-LI) in the caudal brainstem was visualized 2 hours after unilateral injection of the small-fiber-specific excitant /inflammatory irritant mustard oil into the TMJ region. Control animals received injection of either mustard oil into the subcutaneous fascia overlying the masseter muscle or mineral oil vehicle into the TMJ region. In all groups, Fos-LI was consistently observed ipsilaterally in the spinal trigeminal nucleus and cervical dorsal horn and, bilaterally, in the nucleus of the solitary tract and. the ventrolateral medulla. The expression of Fos-LI ipsilaterally in the paratrigeminal nucleus was variable. Within the trigeminal sensory complex, Fos-LI was restricted to subnucleus caudalis and the caudal portions of subnucleus interpolaris near the level of the obex. Approximately 12% of Fos-LI cells in subnucleus caudalis and in the cervical dorsal horn were found in laminae III-VI. Compared to TMJ mustard oil injection, mineral oil injection produced less Fos-LI at all rostrocaudal levels, whereas subcutaneous mustard oil injection produced less Fos-LI in caudal subnucleus caudalis but similar amounts in the cervical dorsal horn. Neither of these injections yielded significant ipsilateral responses in subnucleus caudalis, indicating that Fos-LI in this region following TMJ mustard oil injection could be ascribed solely to small-fiber stimulation in the deep TMJ region. The wide rostrocaudal distribution of Fos-LI within the caudal brainstem reflects the distribution of TMJ-responsive nociceptive neurons that may underlie the spread and referral of pain from the TMJ region. © 1995 Wiley-Liss, Inc.