Comparison of the effects of MK-801, ketamine and memantine on responses of spinal dorsal horn neurones in a rat model of mononeuropathy

Selective ligation of the L5/L6 spinal nerves produces a partial denervation of the hindpaw and has proved to be a useful model for studying the mechanisms underlying neuropathic pain. Two weeks after surgery, in vivo electrophysiological studies were performed in sham operated and nerve injured rats and the responses of spinal dorsal horn neurones to controlled electrical and natural (mechanical and heat) stimuli were recorded. The systemic effects of three N-methyl-D-aspartate receptor (NMDA) antagonists, ketamine (1-10 mg/kg), memantine (1-20 mg/kg) and MK-801 (0.1-5 mg/kg) were compared. Ketamine a clinically available NMDA receptor antagonist, produced greater reductions of the postdischarge, thermal (10 mg/kg, P=0.02), and mechanical evoked responses in spinal nerve ligated (SNL) rats (von Frey 9 g, 1 mg/kg, P=0.04; 5 mg/kg, P=0.01; 10 mg/kg, P=0.05; von Frey 50 g, 5 mg/kg, P=0.02; 10 mg/kg, P=0.003). The inhibition of wind-up was comparable in both animal groups. Memantine produced powerful inhibitions of wind-up after nerve injury with little effect in sham controls (5 mg/kg, P=0.02). The postdischarge, mechanical and thermal evoked responses were reduced to similar extents by memantine in both experimental groups. The effects of MK-801 were comparable between SNL and sham operated rats for all neuronal measures (wind-up, postdischarge, thermal and noxious mechanical evoked responses). The differential blocking abilities of these antagonists on the various neuronal responses may relate to the characteristics of their voltage-dependent blockage of the channel associated with the receptor. The favourable side effect profile of memantine supports its potential use for the treatment of neuropathic pain.     

Effects of midazolam in the spinal nerve ligation model of neuropathic pain in rats

Potential changes in the spinal GABAergic activity after nerve injury were studied by comparing the effects of systemic administration of the benzodiazepine midazolam on the noxious evoked responses of dorsal horn in rats with spinal nerve ligation of neuropathy and control animals. The tight ligation of the L(5) and L6 spinal nerves was performed in adult male Sprague-Dawley rats and resulting mechanical and cold allodynia were assessed with von Frey hairs and the acetone drop test. Single unit extracellular recordings of dorsal horn neurones were performed 15-18 days after the surgery under halothane anaesthesia using transcutaneous electrical stimulation of the receptive field at three times the C-fibre threshold. The rats in the spinal nerve ligation group, but not in the sham-operated control group developed mechanical and cold allodynia. Subcutaneous administration of midazolam 0.1-3.0 mg/kg reduced the Adelta-fibre evoked activity in a dose-related manner in all study groups, but the C-fibre evoked activity was significantly reduced only in the spinal nerve ligation group. The inhibitory effects of s.c. midazolam were significantly reversed by i.t. administration of flumazenil, suggesting a spinal site of action. Midazolam reduced C-fibre evoked firing significantly more in the spinal nerve ligation model than in the non-operated or sham controls. These results indicate changes in the spinal GABAergic system in the neuropathic animals and could be of importance in the development of new treatments for neuropathic pain.     

Nerve injury induces plasticity that results in spinal inhibitory effects of galanin

Galanin is a 29-amino-acid neuropeptide that has been implicated in the processes of nociception. This study examines the effect of exogenous galanin on dorsal horn neurone activity in vivo in the spinal nerve ligation (SNL) model of neuropathic pain. SNL rats but not naive or sham-operated rats exhibited behaviour indicative of allodynia. In anaesthetized rats, extracellular recordings were made from individual convergent dorsal horn neurones following stimulation of peripheral receptive fields electrically or with natural (innocuous mechanical, noxious mechanical and noxious thermal) stimuli. Spinal administration of galanin (0.5-50 microg) caused a slight facilitation of the neuronal responses to natural and electrical stimuli in naive rats and up to a 65% inhibition of neuronal responses in sham-operated rats following 50 microg galanin. In contrast, there was a marked inhibition of up to 80% of responses to both natural and electrical stimuli in SNL rats following spinal galanin administration. These results suggest that following peripheral nerve injury, there is plasticity in the levels of galanin and/or its receptors at spinal cord level so that the effect of exogenous galanin favours inhibitory function.     

Analgesic effect of vitamin E is mediated by reducing central sensitization in neuropathic pain

Recent studies suggest that reactive oxygen species (ROS) are critically involved in neuropathic pain. Although vitamin E is a well-known antioxidant, its efficacy on chronic pain is not known. This study investigated the efficacy and mechanisms of vitamin E analgesia in a rat model of neuropathic pain produced by spinal nerve ligation. The effects of vitamin E were investigated using behavioral testing, electrophysiological recording of dorsal horn neurons, and determinations of phosphorylated NMDA receptor subunit 1 (pNR1) levels in the spinal dorsal horn. Results showed that a systemic single injection of a high dose or repetitive daily injections of low doses of vitamin E significantly reduced neuropathic pain behaviors. Vitamin E was also effective in producing analgesia by intrathecal injection, suggesting the importance of spinal mechanisms. In spinal dorsal horn neurons, vitamin E reduced evoked responses to mechanical stimuli as well as the sizes of their receptive fields. In addition, levels of pNR1 in neuropathic rats were also reduced by vitamin E injection. These data suggest that vitamin E produces analgesia in neuropathic rats that is, at least in part, mediated by reducing central sensitization which, in turn, is induced by peripheral nerve injury.     

Mu-opioid and noradrenergic α(2)-adrenoceptor contributions to the effects of tapentadol on spinal electrophysiological measures of nociception in nerve-injured rats

Multiple pathological mechanisms at multiple sensory sites may underlie the pain that follows nerve injury. This provides a basis for recommending more than one agent, either sequentially or in combination, for its treatment. According to this premise, new drugs that combine different mechanisms of analgesic action in a single molecule are gaining momentum, such as tapentadol which stimulates mu-opioid receptors (MOR) and acts as a noradrenaline reuptake inhibitor (NRI) in the CNS. Tapentadol is currently indicated for treating moderate to severe acute and severe chronic pain, and here we demonstrate its efficacy in an animal model of ongoing neuropathic pain. In particular, we performed a series of in vivo electrophysiological tests in spinal nerve ligated and sham-operated rats to show that systemic tapentadol (1 and 5 mg/kg) dose-dependently reduced evoked responses of spinal dorsal horn neurones to a range of peripheral stimuli, including brush, punctate mechanical and thermal stimuli. Furthermore, we showed that spinal application of the selective α₂-adrenoceptor antagonist atipamezole, or alternatively the mu-opioid receptor antagonist naloxone, produced near complete reversal of tapentadol’s inhibitory effects, which suggests not only that the spinal cord is the key site of tapentadol’s actions, but also that no pharmacology other than MOR-NRI is involved in its analgesia. Moreover, according to the extent that the antagonists reversed tapentadol’s inhibitions in sham and SNL rats, we suggest that there may be a shift from predominant opioid inhibitory mechanisms in control animals, to predominant noradrenergic inhibition in neuropathic animals.     

Spinal local anaesthetic actions on afferent evoked responses and wind-up of nociceptive neurones in the rat spinal cord: combination with morphine produces marked potentiation of antinociception.

Lignocaine was tested either alone or in combination with a low dose of morphine by intrathecal administration on the C- and A-beta evoked responses of nociceptive neurones in the dorsal horn of the halothane-anaesthetized rat. In addition the effect of prilocaine was compared to lignocaine. The effects of lignocaine on wind-up, a frequency-dependent increase in the responses of the cells produced by repeated C-fibre stimulation was also tested. Lignocaine produced dose-dependent inhibitions of the C-, A-delta and A-beta evoked responses of the cells which became more selective for the noxious evoked responses as the dose increased. The effective doses corresponded well to those used clinically. Wind-up was also decreased by lignocaine. In combination with a low dose of morphine, threshold doses of lignocaine produced a highly marked potentiation of the inhibitions of the C-fibre evoked responses compared to either agent alone. No potentiation of the inhibitions of the A-beta responses was observed. The potentiated inhibitory effects on the C-fibre responses were rapidly reversed by intrathecal naloxone. The finding that spinal local anaesthetic and morphine potentiate markedly to reduce spinal nociception is discussed both in terms of mechanisms of action of the agents and their clinical application.     

Role of calcium channels in the spinal transmission of nociceptive information from the mesentery

Opioids, alpha(2)-adrenoceptor agonists and blockers of voltage-gated calcium channels (VGCCs) have been attributed antinociceptive activity in various experimental set-ups. The present study tested the ability of morphine, clonidine and drugs acting at various VGCCs to inhibit the transmission of noxious stimuli from the mesentery at the level of the spinal cord. In rats under barbiturate anaesthesia traction of 20 g was applied to a bundle of mesenteric blood vessels. This caused immediate transient changes of mean arterial pressure that were taken as indication of nociception. Similar reflexes were elicited by applying 0.6% acetic acid to the same bundle of vessels. The reflexes were dose-dependently reduced by intrathecal administration of morphine or clonidine, but were left unaltered by intrathecal administration of verapamil, Bay-K 8644 or omega-conotoxin MVIIA. Neither verapamil nor Bay-K 8644 influenced clonidine-induced analgesia. Conotoxin markedly enhanced the effectiveness of all doses of clonidine against both types of mesenteric stimuli. Verapamil, Bay-K 8644, as well as conotoxin reduced the ability of morphine to inhibit mechanically evoked reflexes, while there was no statistically significant effect in chemonociception. These data suggest that, at the spinal level, both morphine and clonidine are effective drugs to decrease the cardiovascular changes caused by acute mesenteric pain. In the dorsal spinal cord neither L-type nor N-type VGCCs are responsible on their own for the transmission of noxious stimuli from the mesentery. Inhibition of N-type channels markedly augments the action of clonidine, whereas blocking either VGCC seems to inhibit antinociceptive mechanisms induced by morphine. It is suggested that in patients the combined administration of clonidine with omega-conotoxin MVIIA might lead to effective pain control with reduced side effects.     

Functional MRI of the brain detects neuropathic pain in experimental spinal cord injury

Functional magnetic resonance imaging (fMRI) has been used to map cerebral activations related to nociceptive stimuli in rodents. Here, we used fMRI to investigate abnormally increased responses to noxious or innocuous stimuli, in a well-established rat model of chronic neuropathic pain induced by photochemical lumbar spinal cord injury. In this model, a subpopulation of rats exhibits allodynia-like hypersensitivity to mechanical and cold stimulation of the trunk area. In those rats that do not develop overt hypersensitivity after identical spinal cord injury (i.e. non-hypersensitive rats), touch evoked pain can be triggered by the opioid receptor antagonist, naloxone. We show that cerebral activations in contralateral primary somatosensory cortex (SI) are markedly correlated with different behavioural characteristics of these animals. Identical electrical stimulation, applied on trunks of spinally injured hypersensitive and non-hypersensitive rats, evoked significantly higher responses in SI of the former than the latter. Although levels of fMRI signals in SI of the trunk territory were not significantly different between normal and spinally injured non-hypersensitive rats, the administration of naloxone significantly increased fMRI signals in the non-hypersensitive rats, but not in the normal rats. We conclude that increased activation of contralateral SI is a key feature of behavioural neuropathic pain in spinally injured rats and that fMRI is an effective method to monitor experimental neuropathic pain in small animals.     

Effect of morphine on deep dorsal horn projection neurons depends on spinal GABAergic and glycinergic tone: implications for reduced opioid effect in neuropathic pain

The mu opioid agonist morphine has distinct effects on spinal dorsal horn neurons in the superficial and deep laminae. However, it is not clear if the inhibitory effect of morphine on dorsal horn projection neurons is secondary to its potentiating effect on inhibitory interneurons. In this study, we tested the hypothesis that removal of GABAergic and glycinergic inhibitory inputs attenuates the effect of morphine on dorsal horn projection neurons and the reduced spinal GABAergic tone contributes to attenuated morphine effect in neuropathic pain. Single-unit activity of deep dorsal horn projection neurons was recorded in anesthetized normal/sham controls and L(5) and L(6) spinal nerve-ligated rats. Spinal application of 10 microM morphine significantly inhibited the evoked responses of dorsal horn neurons in both normal/sham controls, and this effect was abolished by the specific mu opioid antagonist. However, the effect of morphine on dorsal horn projection neurons was significantly reduced in nerve-injured rats. Furthermore, topical application of the GABA(A) receptor antagonist bicuculline (20 microM) almost abolished the effect of morphine in normal/sham control rats but did not significantly attenuate the morphine effect in nerve-injured rats. On the other hand, the glycine receptor antagonist strychnine (4 microM) significantly decreased the effect of morphine in both nerve-injured and control animals. These data suggest that the inhibitory effect of opioids on dorsal horn projection neurons depends on GABAergic and glycinergic inputs. Furthermore, reduced GABAergic tone probably contributes to diminished analgesic effect of opioids in neuropathic pain.     

Altered effects of an A1 adenosine receptor agonist on the evoked responses of spinal dorsal horn neurones in a rat model of mononeuropathy

There is clinical evidence that adenosine might be a useful treatment for neuropathic pain states, although little is known regarding its mechanisms. In this study, we use the selective (L5/L6) spinal nerve ligation model to investigate the effects of an adenosine A₁ receptor agonist, N⁶-cyclopentyladenosine (CPA), on the evoked responses of dorsal horn neurones after nerve injury in vivo. Two weeks after surgery, the responses of dorsal horn neurones to controlled electrical and natural (mechanical and thermal) stimuli were recorded and the effects of intrathecal CPA were compared between nerve-ligated and sham-operated rats. CPA produced significant inhibitions of the C-fiber—evoked response, postdischarge, wind-up, mechanical, and thermal-evoked responses in both groups, but only minor inhibitions of the Aβ-fiber response. Overall, the drug effects in spinal nerve-ligated rats were greater than those of sham-operated rats. Spinal theophylline reversed these inhibitions. In contrast, CPA produced marked facilitations of the Aδ-fiber—evoked neuronal responses in sham-operated animals, yet this effect was completely absent after nerve injury. These results suggest that nerve injury induces plasticity in the spinal A₁ receptor system, which might form the basis for the therapeutic use of adenosine.     

Pregabalin modulation of spinal and brainstem visceral nociceptive processing

Brainstem and spinal mechanisms mediating visceral nociception are investigated here using electrophysiology and immunohistochemistry techniques in a model of acute visceral pain. Colorectal distension (CRD) produced graded visceromotor responses (VMR) in normal rats, and these were facilitated by intracolonic mustard oil (MO) that generated acute visceral hyperalgesia. The neuropathic pain drug pregabalin (PGB) is thought to have state-dependent effects in attenuating neuropathic, but not acute somatic pain, likely by impairing calcium-channel trafficking. We found that systemic PGB produced antinociceptive effects on CRD-evoked VMRs in naïve rats lacking pathophysiology and in MO-pretreated rats. Systemic PGB also significantly reduced Fos labelling in lumbosacral spinal cords of rats given noxious repetitive CRD; however, PGB did not alter this measure of neural activity in the brainstem. Differential brainstem processing of noxious somatic and visceral stimuli may underlie the unique lack of state-dependent actions of PGB in this visceral pain model. Single-unit recordings in the rostral ventromedial medulla (RVM) verify that brainstem processing of somatic and visceral stimuli differs. The effects of CRD on RVM cells classed as ON, OFF, or NEUTRAL were independent of their somatic responses, with surprising changes in RVM cell activity to innocuous visceral stimulation. PGB also markedly reduced the visceral responses of RVM ON-cells to noxious CRD. These results illustrate clear differences in the central processing of visceral and somatic stimuli, yet a common role for descending modulation by brainstem activity in mediating evoked pain measures.     

Intrathecal but not intravenous opioids release adenosine from the spinal cord.

Opioids increase spinal release of adenosine in rats, and analgesia from systemic and intrathecal morphine is reduced in animals by adenosine receptor antagonists. We performed 3 studies to determine whether opioid administration also induces adenosine release in humans. To determine the effect of intrathecal opioid exposure, 15 women received intrathecal fentanyl, 50 microg, or saline, and cerebrospinal fluid was sampled at 2-minute intervals for 6 minutes before surgery. In a second study, 8 healthy volunteers received intrathecal morphine, 50 microg, plus fentanyl, 50 microg, with cerebrospinal fluid sampled 20 and 60 minutes later. To determine the effect of intravenous opioid exposure, 9 healthy volunteers received intravenous remifentanil for 60 minutes, and cerebrospinal fluid was sampled before and at the end of the infusion. Adenosine concentrations were similar in the 3 studies before opioid administration. Intrathecal fentanyl or saline did not affect adenosine concentrations during the 6 minutes in the first study. Adenosine concentrations increased significantly 20 and 60 minutes after intrathecal morphine plus fentanyl was administered. In contrast, adenosine concentrations were unaffected by intravenous remifentanil. These results suggest that intrathecal but not systemic opioid analgesia in humans is associated with spinal release of adenosine. PERSPECTIVE: Although the role of adenosine release in the spinal cord for opioid receptor activation in subsequent analgesia from opioids is controversial in laboratory studies, these clinical data suggest that local opioid receptor stimulation in the spinal cord of humans does release adenosine. Whether adenosine participates in analgesia from spinal opioids in humans is not known, but spinal adenosine itself is analgesic in humans, consistent with an opioid-adenosine role in analgesia.     

The threshold for the depressive effect of intrathecal morphine on the spinal nociceptive flexor reflex is increased during autotomy after sciatic nerve section in rats.

The effect of intrathecal (i.t.) morphine on the spinal nociceptive flexor reflex in doses ranging between 10 ng and 10 micrograms was studied in decerebrate, spinalized, unanesthetized rats with intact sciatic nerves or in rats in which the sciatic nerve had been unilaterally sectioned. In rats with intact nerves the initial effect of i.t. morphine on the flexor reflex was a brief facilitation followed by depression. The threshold dose of morphine for reflex depression was 100 ng. In animals which did not develop autotomy after nerve section or in which autotomy had ceased for several days prior to the acute experiments, i.t. morphine had a similar depressive effect on the flexor reflex as in animals with intact nerves. However, in rats which were autotomizing at the time of the acute experiment, the threshold dose of the depressive effect of morphine was increased 3-5 fold. With higher doses of morphine (1-3 micrograms), similar depression of the reflex was found in all groups. The present results revealed a decreased sensitivity of spinal reflex mechanisms to low, but not high, doses of morphine after sciatic nerve section accompanied by autotomy. Nerve section per se did not alter opioid sensitivity. Thus, decreased effectiveness of morphine in this model for neuropathic pain may be partially due to a desensitization to the analgesic action of opioids in the spinal cord. Since after sciatic nerve section there is a differential sensitivity to the antinociceptive effect of i.t. morphine between autotomizing and non-autotomizing rats, it is further suggested that autotomy after peripheral nerve section in rats is a useful model for the study of neuropathic pain.     

Alterations in neuronal excitability and the potency of spinal mu, delta and kappa opioids after carrageenan-induced inflammation

These electrophysiological results show that the development of inflammation following peripheral injection of carrageenan into the paw is accompanied by alterations in the magnitude of the C-fibre evoked response of multireceptive dorsal horn neurones. The evoked response of the dorsal horn cells was found to either increase or decrease in the 3 h following the carrageenan injection, and the direction of this change was related to the degree of wind-up exhibited by the cell.

Regardless of whether a cell was facilitated or inhibited by carrageenan, mu, delta and kappa opioids applied topically onto the spinal cord (equivalent to an intrathecal injection) exhibited increased antinociceptive potency. This increased effectiveness was especially marked for the mu opioid, morphine, which showed a 30-fold increase in potency. Interestingly the facilitations seen with the lowest doses of the mu and kappa opioids in this model in normal animals were absent after carrageenan. In addition, a very low dose of spinal naloxone caused a small but significant reduction in the C-fibre evoked responses.

These results demonstrate that following peripheral inflammation, functional changes develop in both spinal transmission and modulatory systems. Alterations in the antinociceptive potency of opioid agonists occurs, with the mu agonist, morphine, showing the greatest change.     

A spinal mechanism of action is involved in the antinociception produced by the capsaicin analogue NE 19550 (olvanil)

We have studied the effect of NE 19550 (olvanil, N-(4-hydroxy-3-methoxyphenyl) methyl-9Z-octadecenamide), a capsaicin analogue with approximately equipotent antinociceptive activity in vivo compared with capsaicin, on nociceptive responses recorded from spinal dorsal horn neurones in vivo and from a spinal ventral root in vitro. In adult rats anaesthetized with halothane, antinociceptive doses of olvanil (20-40 mumol/kg, s.c.) reduced C-fibre responses evoked in wide dynamic range, lumbar dorsal horn neurones, by peripheral transcutaneous electrical stimulation. Intradermal injection of olvanil, localized to a discrete region of the peripheral receptive field, did not activate C-fibres nor change C-fibre evoked activation of dorsal horn neurones. Spinal intrathecal administration of olvanil attenuated C-fibre evoked responses and, at the highest concentration, significantly reduced A beta-fibre evoked activity. In the neonatal rat spinal cord/tail preparation maintained in vitro, superfusion of the cord with olvanil (500 nM-5 microM) did not evoke a depolarization but responses to peripheral noxious stimulation were attenuated. In a similar in vitro preparation of the neonatal rat spinal cord, the release of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) was measured in spinal cord superfusates. Capsaicin (2-10 microM) evoked a large release of CGRP-LI but olvanil (2-10 microM) produced only a small or undetectable release. Following the administration of each substance, however, the release of CGRP-LI evoked by a depolarizing potassium stimulus was significantly attenuated. These data indicate that C-fibre input to the dorsal horn was attenuated by acute systemic doses of olvanil that were antinociceptive in behavioural tests. This effect was unlikely to be due to impairment of C-fibre function by a peripheral locus of action but was more consistent with an action in the spinal cord in which the reduced release of a neurotransmitter substance from afferent nerve terminals may play a prominent role.     

Systemic morphine inhibits dorsal horn projection neurons through spinal cholinergic system independent of descending pathways

Cholinergic circuitry and muscarinic receptors within the spinal cord have been proposed to contribute to the analgesic effects of systemic morphine. In this study, we determined whether the descending pathways are involved in the inhibitory effect of systemic morphine on dorsal horn projection neurons mediated by activation of the spinal cholinergic system. Single-unit activity of dorsal horn projection neurons was recorded in anesthetized rats. The neuronal responses to mechanical stimuli applied to the receptive field were determined before and after intravenous injection of morphine. The inhibitory effect of intravenous morphine on dorsal horn neurons was also tested before and after topical spinal application of the muscarinic antagonist atropine in both intact and spinally transected rats. Intravenous injection of 2.5 mg/kg morphine significantly inhibited the evoked response of dorsal horn neurons in both intact and spinally transected rats. Spinal topical application of the mu opioid antagonist H-d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) completely blocked the effect of morphine on dorsal horn neurons. In addition, spinal application of 10 microM atropine significantly attenuated the effect of systemic morphine. In rats subjected to cervical spinal transection, atropine produced a similar attenuation of the inhibitory effect of systemic morphine on dorsal horn neurons. Data from this electrophysiological study suggest that systemic morphine inhibits ascending dorsal horn neurons through stimulation of spinal mu opioid receptors. Furthermore, activation of the local spinal cholinergic circuitry and muscarinic receptors is involved in the inhibitory effect of systemic morphine on dorsal horn projection neurons independent of descending pathways.     

Systemic morphine selectively depresses a thalamic link of widespread nociceptive inputs in the rat.

The lateral part of the ventromedial thalamus (VM l) relays nociceptive inputs from the whole body surface to the dorsolateral frontal cortex. The aim of the present study was to investigate the effects of systemic morphine on nociceptive activity evoked in VM l neurones either by thermal (48 degrees C) or by supramaximal percutaneous electrical stimuli. The noxious thermal evoked responses were depressed by 10.8 +/- 10.1%, 48.3 +/- 23.0% and 67.3 +/- 10.1%, 5 min after i.v. injections of 1.0, 1.73 and 3.0 mg/kg of morphine, respectively. Moreover, strong depressive effects on the Adelta- and C-fibre responses were already present 5 min after the injection. The responses were significantly reduced by 7.2 +/- 5.9%, 32.5 +/ 11.1% and 37.2 +/- 11.8% for Adelta fibres after i.v. injections of 1.0, 1.73 and 3.0 mg/kg of morphine, respectively. The corresponding values for C-fibre evoked responses were 16.3 +/- 16.2%, 57.0 +/- 12.0% and 69.0 +/- 8.2%. The dose of morphine that reduced VM l neuronal nociceptive responses by 50% (1.73 mg/kg) was around 3.5 times lower than that necessary to inhibit the responses of its spinal or medullary relays under similar experimental conditions. These results, added to the data of the literature, suggest that supraspinal effects of morphine are primarily mediated at the thalamic level. It is tempting to speculate that morphine-induced reductions of attentional or psychomotor responses related to pain may be mediated by its action on VM l.     

A dissociative change in the efficacy of supraspinal versus spinal morphine in the neuropathic rat

The efficacy of spinally versus supraspinally administered morphine was studied in rats with a spinal nerve ligation-induced neuropathy. Behavioural assessment indicated that the effect of intrathecally administered morphine on pain-related responses was attenuated when compared with unoperated controls. The decreased efficacy of spinal morphine was associated with neuropathic symptoms, since sham ligation or nerve ligation without accompanying tactile allodynia did not lead to spinal inefficacy of morphine. In contrast, the pain attenuating effect of morphine in the periaqueductal gray (PAG) was enhanced in neuropathic animals. The effect of systemically administered morphine on pain-related behavior of neuropathic rats was in the same range as in controls or decreased, depending on the test. Coadministration of lidocaine or MK-801, a N-methyl-D-aspartate (NMDA) receptor antagonist, into the rostroventromedial medulla enhanced the tactile antiallodynic but not the thermal antinociceptive effect of intrathecally administered morphine in neuropathic animals. Supraspinal administration of MK-801 or lidocaine did not influence efficacy of spinal morphine in sham-operated animals. Electrophysiological recordings of nociceptive wide-dynamic range (WDR) neurons in the deep spinal dorsal horn of pentobarbitone-anesthetized animals corresponded to a large extent with behavioral results. The inhibitory effect of spinally and systemically administered morphine on WDR neuron responses was attenuated whereas that induced by morphine in the PAG was enhanced in neuropathic animals. The results indicate that in spinal nerve ligation-induced neuropathy the efficacy of spinal morphine is decreased whereas that of supraspinal morphine is increased. Descending influence from brainstem-spinal pathways, involving NMDA receptors in the rostroventromedial medulla, may contribute to the selective reduction in tactile antiallodynic efficacy of spinal morphine.     

Evidence for an exclusive antinociceptive effect of nociceptin/orphanin FQ, an endogenous ligand for the ORL1 receptor, in two animal models of neuropathic pain

Nociceptin/orphanin FQ (noci/OFQ), the endogenous ligand for the orphan ORL1 (opioid receptor-like1), has been shown to be anti- or pronociceptive and modify morphine analgesia in rats after central administration. We comparatively examined the effect of noci/OFQ on hyperalgesia and morphine analgesia in two experimental models of neuropathic pain: diabetic (D) and mononeuropathic (MN) rats. Noci/OFQ, when intrathecally (i.t.) injected (0.1, 0.3, or 1, to 10 microg/rat) was ineffective in normal rats, but reduced and suppressed mechanical hyperalgesia (paw-pressure test) in D and MN rats, respectively. This spinal inhibitory effect was suppressed by naloxone (10 microg/rat, i.t.) in both models. Combinations of systemic morphine with spinal noci/OFQ resulted in a strong potentiation of analgesia in D rats. In MN rats, an isobolographic analysis showed that the morphine+noci/OFQ association (i.t.) suppressed mechanical hyperalgesia in a superadditive manner. In summary, the present findings reveal that spinal noci/OFQ produces a differential antinociception in diabetic and traumatic neuropathic pain according to the etiology of neuropathy, an effect possibly mediated by opioid receptors. Moreover, noci/OFQ combined with morphine produces antinociceptive synergy in experimental neuropathy, opening new opportunities in the treatment of neuropathic pain.     

坐骨神经切断大鼠脑脊液中CCK-8含量的动态变化及其与吗啡镇痛的关系

目的：研究神经源性疼痛时吗啡镇痛效应的降低与中枢八肽胆囊收缩素(CCK-8)的释放量之间的关系。方法：以切断大鼠单侧坐骨神经作为引起神经痛的动物模型，用放射免疫分析法，观察术后第3，7，10和14天脑脊液中CCK-8-ir含量的变化，并在相应的时间点分别皮下注射吗啡(4mg／kg)和CCKB受体拮抗剂L-365，260(0．5mg,／kg)，观察痛阈(辐射热甩尾潜伏期)的变化。结果：(1)大鼠单侧坐骨神经切断后一周，脑脊液中CCK-8样免疫活性物质(CCK-8-ir)的浓度(代表中枢CCK-8的释放量)增加了125％，此时吗啡的镇痛效果降低，而CCK拮抗剂使吗啡镇痛效果提高。(2)坐骨神经切断后1．5～2周，中枢CCK-8释放减少或保持正常水平，此时吗啡镇痛效果正常，CCK拮抗剂也不能使其效应进一步提高。(3)假手术组大鼠于第14天(第四次注射吗啡)时，吗啡作用减弱(发生耐受)，此时CCK拮抗剂显示出对吗啡镇痛的加强作用。结论：单侧坐骨神经切断后一周吗啡镇痛效果减弱，可能与当时中枢CCK-8释放过多有关。切断坐骨神经后中枢释放CCK-8水平的变化，是影响阿片镇痛的重要因素。