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.     

The ß-adrenergic receptor system in human glioma-derived cell lines: The mode of phosphodiesterase induction and the macromolecules phosphorylated by cyclic AMP-dependent protein kinase

Nineteen convergent neurones, 19 'non-noxious only' neurones and 13 'proprioceptive' neurones all with peripheral excitatory receptive fields on the ipsilateral hindpaw, were recorded in the lumbar dorsal horn of non-spinalized, anaesthetized rats. These neurones were all excited by the electrophoretic application of glutamate; using 20 s applications of appropriate electrophoretic currents, almost identical levels of activity (around 30 spikes/s) were produced for each of the 3 types of neurones. The application of heterotopic noxious stimuli resulted in strong inhibitions of the glutamate-evoked activity of the convergent neurones. During the application of noxious heat (52 degrees C) to the tail and noxious pinches to the tail, contralateral hindpaw and muzzle, the glutamate-evoked activity was depressed by 78%, 84%, 63% and 59%, respectively. It was also found that these inhibitions outlasted the period of conditioning stimulation by several minutes (post-effects). Twenty seconds after i.p. injection of bradykinin (20 micrograms) the glutamate-evoked activity was depressed by 56% and this effect also lasted for several minutes. The application of identical heterotopic noxious stimuli did not affect the glutamate-evoked activity of the 'non-noxious only' neurones or the 'proprioceptive' neurones. The application to convergent neurones, of doses of glutamate, which were very much larger than the threshold for firing, produced an intense discharge followed by a progressive decrease in spike amplitude and finally a blocking of the spike discharge. During such sequences, which are typical of excessive depolarization, the application of noxious conditioning stimuli (e.g pinch of the contralateral hindpaw or muzzle) resulted in recovery of the spike discharge. In several cases, this recovery long outlasted the period of conditioning noxious stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

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.     

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.     

Responses of rat lateral hypothalamic neurons to periaqueductal gray stimulation and nociceptive stimuli

The effects of dorsal periaqueductal gray (D-PAG) stimulation and noxious stimuli on neural activity in the lateral hypothalamic area (LHA) were investigated in 56 adult male anesthetized rats. Strong tail pinch was used as noxious stimulation. We examined 234 extracellular and 75 intracellular recordings of LHA responses to electrical stimulation of D-PAG. To determine neurotransmitter candidates, the effects of the opioid agonist, morphine, and its antagonist, naloxone were investigated by systemic administration and microelectrophoresis. Of 234 spontaneously firing LHA neurons, 70 (30%) were inhibited by D-PAG stimulation. Of these 70 neurons, 26/40 tested (65%) were glucose-sensitive, 16/19 (84%) were inhibited by morphine and 12/18 (67%) were inhibited by tail pinch. Glucose-sensitive neurons were selectively inhibited by morphine and tail pinch. Naloxone attenuated inhibitory responses to D-PAG stimulation, tail pinch and electrophoretic morphine. From intracellular recordings these polysynaptic inhibitory responses to D-PAG stimulation were considered to be inhibitory postsynaptic potentials (IPSPs) with 6.1 +/- 3.2 ms (mean +/- S.D.) latency and reversal membrane potential of about -78 mV. Since LHA glucose-sensitive neurons receive, selectively, both inhibitory opioid inputs from the D-PAG and inhibitory inputs through noxious stimulation, we suggest that D-PAG might be an intermediate site for transmission of noxious stimuli to the LHA.     

Effects of cardiac administration of bradykinin on thoracic spinal neurons in the cat

Extracellular action potentials were recorded from single cells located in gray matter of the T2 to T4 spinal segments in chloralose-anesthetized cats. Bradykinin either was injected indirectly into the coronary circulation and the heart via a cannula placed in the left atrium or was applied directly to the epicardium. Bradykinin excited 26 of 56 cells after a latency of 11 ± 1 s and inhibited two cells. Ten of eighteen cells demonstrated tachyphylaxis to repeated injections of bradykinin; the other eight cells were excited similarly to successive injections. All cells in this study received visceral and somatic inputs and were found in laminae I through VIII with a majority located in laminae V and VII. A noxious pinch applied to the somatic receptive fields was required to excite 94% of the cells (high-threshold cells), and hair movement and touch as well as pinch excited the other 6% of the cells (wide dynamic range cells). The results demonstrated that bradykinin excited cardiac receptors whose afferent fibers influenced the activity of spinal neurons. Because convergence of visceral and somatic inputs onto the same neurons provides a basis for referred pain and bradykinin is an algesic substance, these cells could potentially be involved in pain sensation and cardiovascular adjustments associated with angina pectoris.     

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.