Tachykininergic tone in the spinal cord of the rabbit: dependence on nociceptive input arising from invasive surgery

Selective inhibition from the anterior hypothalamus of C- versus A-fibre mediated spinal nociception

Modulation of spinal nociception from the anterior hypothalamus/preoptic area (AH/POA), and consequent alterations in the pain experience may contribute to integrated responses brought into play during fear or stress and as part of the sickness response. This study was designed to compare the effects of descending control from AH/POA on A- versus C-fibre-evoked spinal nociception, since any differential control is of behavioural and clinical importance given that A-fibre and C-fibre nociceptors convey different qualities of the pain signal (first and second pain, respectively), and play different roles in the development and maintenance of chronic pain states. In anaesthetised rats, electromyographic responses were recorded to monitor thresholds of withdrawal to slow (2.5 degrees Cs(-1)) or fast (7.5 degrees Cs(-1)) rates of skin heating of the hindpaw, to preferentially activate C- or A-nociceptors, respectively. Neuronal activation by microinjection of dl-homocysteic acid at sites within a specific region of AH/POA, lateral area of the anterior hypothalamus (LAAH), significantly increased response thresholds to slow heating rates (p<0.02, n=11), but not those to fast rates of heating (p=0.48, n=10). Injection of DLH adjacent to LAAH (n=9) had no significant effect on responses to slow (n=8) or fast (n=9) rates of skin heating. The functional significance of differential descending control of spinal processing of C- and A-nociceptive inputs is discussed with respect to roles both of the LAAH in pain processing, and of C- and A-nociceptive inputs in acute and chronic pain.

Midbrain control of spinal nociception discriminates between responses evoked by myelinated and unmyelinated heat nociceptors in the rat

Descending control of spinal nociception is a major determinant of normal and chronic pain. Myelinated (A-fibre) and unmyelinated (C-fibre) nociceptors convey different qualities of the pain signal (first and second pain, respectively), and they play different roles in the development and maintenance of chronic pain states. It is of considerable importance, therefore, to determine whether descending control has differential effects on the central processing of A- vs. C-nociceptive input. In anaesthetised rats, biceps femoris EMG was recorded to monitor the thresholds and encoding properties of responses evoked by fast (7.5 degrees Cs(-1)) or slow (2.5 degrees Cs(-1)) rates of skin heating of the dorsal surface of a hindpaw to preferentially activate myelinated or unmyelinated heat nociceptors, respectively. Activation of neurones in the periaqueductal grey (PAG) by microinjection of dl-homocysteic acid (DLH) or bicuculline (BIC) significantly increased response thresholds to slow rates of heating (P<0.001), but not those to fast rates of heating (P>0.05). The ability of the EMG to encode the stimulus intensity of fast rates of skin heating remained intact and unaltered (r2=0.99, P<0.001) following BIC but not DLH injection. In contrast, encoding of the stimulus intensity of slow rates of skin heating was abolished following BIC and DLH injection. The functional significance of differential descending control of the central processing of C- and A-nociceptive inputs is discussed with respect to role of the PAG in mediating antinociception as part of active coping strategies in emergency situations and the role of C- and A-nociceptive inputs in animal models of chronic pain.

A reliable method for the preferential activation of C- or A-fibre heat nociceptors

There is strong evidence that A- and C-fibre nociceptors evoke significantly different sensory experiences, are differentially sensitive to pharmacological intervention, and play different roles in pain pathology. It is therefore of considerable interest to be able to selectively activate one fibre type or the other in studies of nociceptive processing. Here, we report significant modifications to a non-invasive technique, first described by Yeomans et al. [Pain 59 (1994) 85; Pain 68 (1996) 141; Pain 68 (1996) 133], which uses different rates of skin heating to preferentially activate A- or C-nociceptors. A copper disk (diameter: 4mm) was used to transfer heat evenly across the dorsal surface of the rat hindpaw. Initial experiments established the relationship between the temperature at the skin surface and the sub-epidermal temperature. Subsequently, the vanilloid capsaicin, which sensitises unmyelinated C-mechanoheat nociceptors, was shown to decrease the thresholds of reflex responses evoked by slow rates of heating. In contrast thresholds of responses to fast rates of skin heating were unchanged, indicating that nociceptors activated by this stimulus were capsaicin-insensitive A-fibre heat nociceptors.

Differential modulation of withdrawal reflexes by a cannabinoid in the rabbit

Inhibition of spinal and trigeminal withdrawal reflexes by morphine and by the cannabinoid agonist HU 210 has been studied in anaesthetized and in decerebrated rabbits. In intact, pentobarbitone-anaesthetized animals, the jaw-depressor reflex (JDR) evoked by stimulation of the tongue, and the reflex elicited in the ankle flexor tibialis anterior (TA) by stimulation of the toes were inhibited to the same extent by morphine (1-30 mg kg(-1) i.v. cumulative). In spinalized, anaesthetized rabbits morphine depressed the JDR to the same level as in non-spinal preparations, but the effect of the opioid on the TA reflex was significantly reduced. All effects of morphine were reversed by naloxone (0.25 mg kg(-1), i.v.). In anaesthetised intact animals, HU 210 depressed the JDR at a dose of 100 nmol kg(-1) i.v. cumulative, reduced reflexes evoked in the knee flexor muscle semitendinosus (ST) by stimulation at the toes at a dose of 30 nmol kg(-1) i.v. cumulative, but had no consistent or significant effects on the TA reflex to toe stimulation. The same results were obtained in spinalized, anaesthetised animals. In decerebrated, spinalized rabbits with no anaesthesia, HU 210 (30 nmol kg(-1)) depressed both ST and TA reflexes evoked by toe stimulation. These data reveal that trigeminal and spinal withdrawal reflexes are equally sensitive to morphine provided the spinal cord is intact, suggesting that at least part of the action of systemic morphine is due to activation of descending inhibition. The present results also show for the first time that cannabinoid agonists can inhibit trigeminal withdrawal reflexes. HU 210 had differential effects on the three reflexes studied depending on the presence or absence of anaesthesia. This is the first occasion on which we have found pharmacological distinctions between withdrawal reflexes, and indicates that spinal sensorimotor processing is more heterogeneous than has been suspected previously.

Organisation of sensitisation of hind limb withdrawal reflexes from acute noxious stimuli in the rabbit

Spatial aspects of central sensitisation were investigated by studying the effects on three hind limb withdrawal reflexes of an acute noxious stimulus (20 % mustard oil) applied to a number of locations around the body in decerebrate and in anaesthetised rabbits. Reflex responses to electrical stimulation of the toes were recorded from the ankle flexor tibialis anterior (TA) and the knee flexor semitendinosus (ST), whereas responses to stimulation of the heel were recorded from the ankle extensor medial gastrocnemius (MG). In non-spinalised, decerebrated, pentobarbitone-sedated preparations, flexor reflexes were facilitated significantly from sites on the plantar surface of the ipsilateral foot but were either inhibited or unaffected by stimulation of sites away from this location. The heel-MG reflex was facilitated from the ipsilateral heel and was inhibited from a number of ipsilateral, contralateral and off-limb sites. In decerebrated, spinalised, pentobarbitone-sedated animals, mustard oil applied to any site on the ipsilateral hind limb enhanced both flexor reflexes, whereas the MG reflex was enhanced only after stimulation at the ipsilateral heel and was inhibited after stimulation of the toe tips or TA muscle. Mustard oil on the contralateral limb had no effect on any reflex. In rabbits anaesthetised with pentobarbitone and prepared with minimal surgical interference, the sensitisation fields for the heel-MG and toes-TA reflexes were very similar to those in non-spinal decerebrates whereas that for toes-ST was more like the pattern observed in spinalised animals. In no preparation was sensitisation or inhibition of reflexes related to the degree of motoneurone activity generated in direct response to the sensitising stimulus. This study provides for the first time a complete description of the sensitisation fields for reflexes to individual muscles. Descending controls had a marked effect on the area from which sensitisation of flexor reflexes could be obtained, as the sensitisation fields for the flexor reflexes evoked from the toes were larger in spinalised compared to decerebrated, non-spinalised animals. The intermediate sizes of sensitisation fields in anaesthetised animals suggests that the area of these fields can be dynamically controlled from the brain. On the other hand, the sensitisation field for the heel-MG reflex varied little between preparations and appears to be a function of spinal neurones.

Interactions between cutaneous afferent inputs to a withdrawal reflex in the decerebrated rabbit and their control by descending and segmental systems

Previous studies have suggested that activation of nociceptive afferents from the heel recruits a supraspinal mechanism, which is modulated by adrenergic descending inhibition, that augments withdrawal reflexes in medial gastrocnemius (MG) motoneurones. To test this idea, we have studied the temporal evolution of reflexes evoked in MG by electrical stimulation of sural nerve A(beta)-, A(delta)- and C-fibre axons at 1 Hz, in decerebrated rabbits. Reflexes were analysed in three time bands, estimated to accord to afferent drive from A(beta)- (phase 1), A(delta)- (phase 2) and C-fibre (phase 3) inputs. Stimulation of A(delta)- and C-fibres gave significant temporal summation of all reflexes. The alpha(2)-adrenoceptor antagonist RX 821002 ((2-(2,3-dihydro-2-methoxy-1,4-benzodioxin-2-yl)-4,5-dihydro-1-H-imidazole)-HCl) (100 microg intrathecal (i.t.)) potentiated, and the alpha(2)-agonist dexmedetomidine (1-30 microg i.t.) depressed all reflexes per se, but the effects of these drugs on temporal summation were secondary to changes in baseline excitability. When C-fibres were stimulated, the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (1 mg i.t.) reduced temporal summation of phase 2 and 3 but not phase 1 reflexes. Spinalisation at L1 in the absence of drugs increased phase 2 and 3 reflexes but had no effect on phase 1, whereas spinalisation after RX 821002 resulted in decreased phase 1 responses with no significant change in later phases. Spinalisation in the presence of dizocilpine resulted in small reductions in phase 3 reflexes only. In all cases spinalisation virtually abolished temporal summation. In spinalised animals, dizocilpine selectively reduced late reflexes, and the opioid antagonist naloxone (100 microg i.t.) augmented all reflexes but gave rise to temporal subtraction of reflexes when C-fibres were stimulated.The present experiments have revealed a number of novel and important features of the sural-MG reflex pathway: (i) activity in fine afferent axons augments the reflexogenic potential of all subsequent afferent input, thereby allowing all afferent drive from the sural field to contribute to withdrawal of the heel; (ii) endogenous adrenergic control of this reflex pathway is completely non-selective; (iii) there is a non-adrenergic element of descending inhibition that is selective for the late components of MG reflex responses, and this element is directed particularly against transmission through NMDA receptors; (iv) temporal summation in this reflex is dependent on NMDA receptor-dependent and -independent mechanisms; and (v) this temporal summation is in some way dependent on the integrity of descending pathways.

Tachykinins as modulators of the micturition reflex in the central and peripheral nervous system

In the normal urinary bladder, tachykinins (TKs) are expressed in a population of bladder nociceptors that is sensitive to the excitatory and desensitizing effects of capsaicin (i.e., capsaicin-sensitive primary afferent neurons (CSPANs)). Several endobiotics or xenobiotics excite CSPANs and release TKs and other mediators at both the peripheral and spinal cord level. The peripheral release of TKs determines a set of responses (known as neurogenic inflammation) that includes vasodilatation, plasma protein extravasation, smooth muscle contraction and stimulation of afferent nerves. Following chronic inflammation, both immune cells and capsaicin-resistant sensory neurons can de novo express TKs: whether these pools of TKs are releasable and contribute to inflammatory processes is presently unsettled. At the spinal cord level, the release of TKs contributes in determining an altered pattern of vesicourethral reflexes in response to nociceptive stimulation of the bladder by conveying: (a) the afferent transmission to supraspinal sites, and (b) descending or sensory inputs to the sacral parasympathetic nucleus (SPN). Recent evidence also attribute a synergetic role of TKs in the supraspinal modulation of the sensory arm of the micturition reflex. The overall available information suggests that TK receptor antagonists may affect bladder motility/reflexes which occur during different pathological states, while having little influence on the normal motor bladder function.

Tonic adrenergic and serotonergic inhibition of a withdrawal reflex in rabbits subjected to different levels of surgical preparation

The excitability of the heel-gastrocnemius withdrawal reflex pathway has been monitored in rabbits undergoing surgical preparation for electrophysiological experimentation under Saffan anaesthesia. Reflexes were evoked by percutaneous electrodes inserted at the heel and recorded as electromyograph signals from the ipsilateral medial gastrocnemius muscle. Two levels of surgery were carried out. The "full surgical" preparation was performed under deep Saffan anaesthesia. The trachea, carotid artery, jugular vein and intrathecal space (via a small laminectomy at L1) were cannulated, the animals were decerebrated by suction, and the left hindlimb was immobilized by screw clamps applied to the tibia and the femur. The sciatic nerve and its branches were exposed by bisection of the posterior biceps muscle and the anaesthetic was withdrawn. In the "reduced surgery" preparation, procedures were carried out with a lighter level of Saffan anaesthesia and operated tissues were infiltrated with local anaesthetic. Only the cannulations were performed in these animals. The excitability of the heel-gastrocnemius reflex declined throughout the full surgical preparation, with the median threshold increasing from 0.8 to 4.2 mA (n=19) and responses to suprathreshold stimuli reducing in size. Most of this effect was reversed after surgery was complete and anaesthesia withdrawn subsequent to decerebration. There were no significant changes in reflex excitability during the reduced surgery preparation (n = 15). Animals prepared by each of these protocols were given increasing intrathecal doses of either the selective alpha2-adrenoceptor antagonist RX 821002 (0.3 to 300 microg) or the serotonin/5-hydroxytryptamine (5-HT)1A-receptor antagonist WAY-100635 (0.01 to 30 microg). Both drugs caused significant, dose-dependent increases in reflex responses, to four to six times pre-drug control in both groups of animals. There were no differences in the effects on reflexes of either drug between the preparations. Thus, surgical preparation of decerebrated rabbits for electrophysiological recording results in depression of hindlimb withdrawal reflexes, although much of this effect did not persist beyond the completion of surgery. Tonic monoaminergic inhibition of reflexes was present to the same extent in both preparations investigated and is not therefore an epiphenomenon of the way in which the animals were prepared.

Differences in opioidergic inhibition of spinal reflexes and Fos expression evoked by mechanical and chemical noxious stimuli in the decerebrated rabbit

Noxious mechanical and chemical stimuli were applied to the toes of the left hind limb of decerebrated, spinalized rabbits and their effects on a hind limb spinal withdrawal reflex and expression of Fos-like immunoreactivity in the spinal cord were measured. The animals were prepared so as to minimize nociceptive inputs arising from surgery. A single crush stimulus applied with a pair of haemostatic forceps caused long-lasting (c. 20 min) inhibition of reflexes evoked in medial gastrocnemius motoneurons by electrical stimulation of the skin at the heel. Naloxone (0.25 mg/kg i.v.) increased reflexes to more than 1000% of pre-drug controls and reversed crush-evoked inhibition. Mustard oil applied to the toes had no consistent effects on the heel-gastrocnemius reflex before or after naloxone. Both crush and mustard oil stimuli gave rise to unilateral increases in the number of Fos-immunopositive profiles in the superficial dorsal horn of spinal segments L7 and S1. There were significantly more Fos-immunoreactive elements in the central and lateral parts of lamina I of both segments in animals receiving the crush stimulus than there were in animals receiving the mustard oil stimulus. Immunochemical localization of enkephalins in rabbit spinal cord showed a dense network of fibres and terminals in laminae I and II, accompanied by infrequent but distinctly stained neuronal cell bodies. The same pattern, with increased numbers of visible cell bodies, was seen after treatment with colchicine. The present data show that tonic and stimulus-evoked opioidergic inhibition of the heel-gastrocnemius reflex of the rabbit are not epiphenomena of surgical preparation of the hindlimb. Opioid-mediated inhibition of the heel-gastrocnemius withdrawal reflex of the rabbit was evoked by noxious mechanical but not by chemical stimulation of the toes. Of these stimuli, the former gave rise to greater activation of neurons in central and lateral lamina I of segments L7 and S1, the region of termination of afferent fibres from the heel and the location of some enkephalin-positive neuronal cell bodies. Thus, noxious mechanical stimulation of the toes elicits inhibition of the heel-gastrocnemius withdrawal reflex, probably via activation of enkephalinergic neurons in the lateral half of lamina I in the L7 and S1 segments.

Spinal section and opioid receptor blockade induce the appearance of Fos-like immunoreactivity in the spinal cord of the decerebrated rabbit

The expression of Fos-like immunoreactivity has been studied in spinal segments L5-S1 of decerebrated, unanaesthetized, but otherwise unstimulated rabbits. The aim of the study was to establish baseline levels of Fos in such preparations, and to examine how these might change after spinalization and opioid receptor blockade. In animals with an intact spinal cord, approximately 30 Fos-positive profiles per section were found in the superficial dorsal horns (i.e. laminae I and II) of each 40-microm section, while about 20 profiles per section were found immediately adjacent to the central canal (lamina X). Fos-like immunoreactive profiles were rare elsewhere in the gray matter. When the spinal cord was sectioned at L1 (after blockade with local anaesthetic), significantly more Fos-like immunoreactivity was found in superficial and central regions of the gray matter (approximately 90 profiles per section) in animals perfused 4 h after decerebration, but not when perfusion was performed 2 or 8 h after decerebration. The opioid antagonist naloxone (0.25 mg/kg/h) had little effect on expression of Fos-like immunoreactivity in spinalized preparations, but significantly increased the numbers of Fos-positive profiles in all but the ventral areas of the spinal gray matter in non-spinalized preparations. The present data show that spinal section induces a transient increase in expression of Fos in the superficial and central parts of the spinal gray matter. It appears that spinalization induces spontaneous activity in some neurons in these regions of the cord, presumably as a result of relief of descending inhibition. The effects of naloxone indicate that endogenous opioids exert tonic inhibition over Fos-expressing spinal neurons in non-spinalized rabbits.

Bladder distension and activation of the efferent function of sensory fibres: similarities with the effect of capsaicin

1. The effects of the tachykinin NK2 receptor antagonist MEN 11420 (100 nmol kg(-1), i.v.) and isoprenaline (400 nmol kg(-1), i.v.) were compared in a model of distension-induced bladder activity in isovolumetric conditions. MEN 11420 induced a relaxation of the basal tone of the urinary bladder that was dependent on the volume of the viscus: the effect was absent at low volumes (0.2 and 0.5 ml) and it was maximal at high volumes of distension (1 and 2 ml), approaching about 60% of the isoprenaline-induced relaxation. The relaxant effect of isoprenaline was always evident at all volumes of distension. 2. Tetrodotoxin (1-100 microM, intravesically applied) abolished distension-evoked micturition contractions, but did not prevent the relaxant effect of MEN 11420- or isoprenaline on the bladder tone. 3. The cyclo-oxygenase inhibitor S-ketoprofen (0.5 micromol kg(-1), i.v.) produced a marked decrease of the bladder tone and a concomitant reduction of bladder motility at 1 ml volume of distension. At 2 ml of distension, S-ketoprofen still decreased the minimal pressure but had no significant effect on other parameters of vesical motility. In S-ketoprofen-pretreated rats, the relaxant effect of MEN 11420 was significant at 2 but not at 1 ml of distension, and that of isoprenaline was reduced by 50% at both 1 and 2 ml. 4. Ruthenium red (10 micromol kg(-1), i.v.) had no effect at a low volume of distension (0.2 ml) or at highest volume (2 ml) but decreased the basal tone and the frequency of bladder contractions at 1 ml of distension. In ruthenium red-pretreated rats, MEN 11420 failed to decrease bladder tone at 1 ml, whereas at 2 ml the effect of MEN 11420 was not different from that observed in controls (43 vs 60% of isoprenaline-induced relaxation, respectively). 5. At both 1 and 2 ml of distension, capsaicin pretreatment (164 micromol kg(-1), s.c. 5 days before) reduced the frequency of micturition contractions but had no effect on the bladder tone. Capsaicin pretreatment prevented the relaxant effect of MEN 11420 on the bladder tone both at 1 and at 2 ml of distension. 6. It is concluded that the release of tachykinins from capsaicin-sensitive afferent nerves induced by bladder distension is resistant to tetrodotoxin and to prostaglandin synthesis inhibition. Tachykinins modulate the vesical tone by acting through NK2 receptors.

The tachykinin NK1 receptor. Part II: Distribution and pathophysiological roles

The tachykinin NK1 receptor is widely distributed in both the central and peripheral nervous system. In the CNS, NK1 receptors have been implicated in various behavioural responses and in regulating neuronal survival and degeneration. Moreover, central NK1 receptors regulate cardiovascular and respiratory function and are involved in activating the emetic reflex. At the spinal cord level, NK1 receptors are activated during the synaptic transmission, especially in response to noxious stimuli applied at the receptive field of primary afferent neurons. Both neurophysiological and behavioural evidences support a role of spinal NK1 receptors in pain transmission. Spinal NK1 receptors also modulate autonomic reflexes, including the micturition reflex. In the peripheral nervous system, tachykinin NK1 receptors are widely expressed in the respiratory, genitourinary and gastrointestinal tracts and are also expressed by several types of inflammatory and immune cells. In the cardiovascular system, NK1 receptors mediate endothelium-dependent vasodilation and plasma protein extravasation. At respiratory level, NK1 receptors mediate neurogenic inflammation which is especially evident upon exposure of the airways to irritants. In the carotid body, NK1 receptors mediate the ventilatory response to hypoxia. In the gastrointestinal system, NK1 receptors mediate smooth muscle contraction, regulate water and ion secretion and mediate neuro-neuronal communication. In the genitourinary tract, NK1 receptors are widely distributed in the renal pelvis, ureter, urinary bladder and urethra and mediate smooth muscle contraction and inflammation in response to noxious stimuli. Based on the knowledge of distribution and pathophysiological roles of NK1 receptors, it has been anticipated that NK1 receptor antagonists may have several therapeutic applications at central and peripheral level. At central level, it is speculated that NK1 receptor antagonists could be used to produce analgesia, as antiemetics and for treatment of certain forms of urinary incontinence due to detrusor hyperreflexia. In the peripheral nervous system, tachykinin NK1 receptor antagonists could be used in several inflammatory diseases including arthritis, inflammatory bowel diseases and cystitis. Several potent tachykinin NK1 receptor antagonists are now under evaluation in the clinical setting, and more information on their usefulness in treatment of human diseases will be available in the next few years.

Stimulus intensity, cell excitation and the N-methyl-D-aspartate receptor component of sensory responses in the rat spinal cord in vivo

The importance of receptors for N-methyl-D-aspartate in synaptic plasticity and in triggering long-term pronociceptive changes is explained by their voltage-dependence. This suggests that their contribution to acute nociceptive responses would be determined both by the magnitude of synaptic input and by the level of background excitation. We have now examined the role of N-methyl-D-aspartate receptors in acute nociceptive transmission in the spinal cord. Drugs selectively affecting activity mediated by these receptors were tested on responses of dorsal horn neurons to noxious stimuli of different intensities and at different levels of ongoing spike discharge. The drugs used were the N-methyl-D-aspartate receptor channel blocker ketamine; the competitive antagonists, 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (D-CPP) and D-2-amino-5-phosphonopentanoic acid (D-AP5), and the positive modulator thyrotropin-releasing hormone. The activity of dorsal horn wide dynamic range neurons was recorded extracellularly in alpha-chloralose-anaesthetized spinalized rats. Their responses to noxious stimuli (pinch, heat and electrical) were monitored in parallel with responses to iontophoretic N-methyl-D-aspartate and (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA). Drugs were given i.v. or (D-AP5) iontophoretically. At doses that selectively inhibited responses to exogenous N-methyl-D-aspartate, ketamine (4 or 8, mean 5 mg/kg i.v.) reduced the nociceptive responses of the majority of the cells in deep dorsal horn. Ketamine also reduced wind-up of the responses to repetitive electrical stimulation. Ketamine (4 or 8 mg/kg). D-CPP (2 mg/kg), D-AP5 (iontophoretically) and thyrotrophin-releasing hormone (1 mg/kg) were tested on different magnitude nociceptive responses evoked by alternating intensities of noxious heat or pinch. In percentage terms, the less vigorous responses were affected by all four drugs as much as or more than the more vigorous responses. When background activity of neurones was enhanced by continuous activation of C-fibres with cutaneous application of mustard oil, ketamine was less effective against superimposed noxious pinch responses. Ongoing background activity was affected in parallel with evoked responses. When background discharge of the cells was maintained at a stable level with continuous ejection of kainate, neither the N-methyl-D-aspartate antagonists nor thyrotrophin-relasing hormone affected the responses to noxious pinch or heat, although responses to exogenous N-methyl-D-aspartate were still blocked. The wind-up of the electrical responses was, however, reduced by ketamine irrespective of the level of background activity. The results indicate that under these conditions in vivo, N-methyl-D-aspartate receptors mediate ongoing low-frequency background activity rather than phasic high-frequency nociceptive responses. The effects of N-methyl-D-aspartate antagonists and positive modulators on nociceptive responses are evidently indirect, being secondary to changes in background synaptic excitation. These results cannot be explained simply in relation to the voltage-dependence of N-methyl-D-aspartate receptor-mediated activity; other factors, such as modulation by neuropeptides, must be involved.