Non-noxious Mechanical Stimuli Research Articles

Touch or pain? Spatio-temporal patterns of cortical fMRI activity following brief mechanical stimuli

Most imaging studies on the human pain system have concentrated so far on the spatial distribution of pain-related activity. In the present study, we investigated similarities and differences between the spatial and temporal patterns of brain activity related to touch vs. pain perception. To this end, we adopted an event-related functional magnetic resonance imaging (fMRI) paradigm allowing us to separately assess the activity related to stimulus anticipation, perception, and coding. The fMRI signal increases following brief mechanical noxious or non-noxious stimulation of the hand dorsum were largely overlapping in the contralateral and ipsilateral hemispheres, including portions of the parietal, insular, frontal and cingulate cortices. Higher activity following noxious stimulation was found in the contralateral mid-anterior insular cortex, in the anterior mid-cingulate cortex (aMCC) and in the adjacent dorso-medial frontal cortex. Significant decreases in fMRI signals following both tactile and painful stimuli were found in perigenual cingulate (pACC)/medial prefrontal cortex (MPF) and in the posterior cingulate/precuneus/paracentral lobule; more intense decreases were found in the pACC/MPF following painful stimuli. fMRI signal increases in the contralateral insula and in aMCC, but not in the parietal cortex, were more prolonged following painful than tactile stimuli. Moreover, a second peak of signal increases (albeit of lower intensity) was found in anterior insula and aMCC during pain intensity rating. These results show specific spatio-temporal patterns of cortical activity related to processing noxious vs. non-noxious mechanical stimuli.

Involvement of Dorsal Column Nucleus Neurons in Nociceptive Transmission in Aged Rats

To clarify the functional role of the dorsal column nucleus (DCN) in nociception in rats with advancing age, single neuronal activity and substance P-like immunoreactivity (SP-LI) of the gracile nucleus (GN) were studied in aged rats (29 to 34 mo old) and adult rats (9 to 12 mo old). A total of 122 neurons [aged: 34 wide-dynamic-range (WDR), two nociceptive-specific (NS), and 32 low-threshold mechanical (LTM) neurons; adult: 22 WDR and 32 LTM neurons] were recorded from GN. For WDR neurons, the latency to antidromic activation of the ventral posterior lateral nucleus of the thalamus showed no difference between the aged and adult rats. Sciatic nerve stimulation with C-fiber intensity induced responses of GN with significantly longer latency in aged rats than in adults, whereas there was no difference in the response latency to A-fiber intensity stimulation. Background activity and afterdischarges were significantly higher in the aged rats than those in the adult rats. Responses to noxious mechanical and thermal stimuli were significantly greater in the aged rats during application of graded stimuli. There were no significant differences in responses to nonnoxious mechanical stimulus, mechanical response threshold, and the size of the receptive fields between neurons in the aged and adult rats. The area occupied by SP-LI fibers in the GN and the size of SP-LI dorsal root ganglia neurons were significantly larger in aged rats than in adults. The present findings suggest that the hyperexcitability of GN neurons could be involved in abnormal noxious pain sensations with advancing age.

Pronociceptive effects of spinal dynorphin promote cannabinoid-induced pain and antinociceptive tolerance

Recent studies indicate that sustained opioid administration produces increased expression of spinal dynorphin, which promotes enhanced sensitivity to non-noxious and noxious stimuli. Such increased ‘pain’ may manifest behaviorally as a decrease in spinal antinociceptive potency. Here, the possibility of similar mechanisms in the antinociception of spinal cannabinoids was explored. Response thresholds to non-noxious mechanical and noxious thermal stimuli were assessed. Antinociception was determined using the 52°C tail-flick test. Mice received repeated WIN 55,212-2, its inactive enantiomer, WIN 55,212-3 or vehicle (i.th., bid, 5 days). WIN 55,212-2, but not WIN 55,212-3 or vehicle, produced a time-related increased sensitivity to non-noxious and noxious stimuli. WIN 55,212-2, but not WIN 55,212-3 or vehicle, elicited a significant increase in lumbar spinal dynorphin content at treatment day 5. Increased sensitivity to mechanical and thermal stimuli produced by WIN 55,212-2 was reversed to baseline levels by i.th. MK-801 or dynorphin antiserum; control serum had no effect. WIN 55,212-2, but not WIN 55,212-3 or vehicle, produced dose-related antinociception and repeated administration resulted in antinociceptive tolerance. While MK-801 and dynorphin antiserum did not alter acute antinociception produced by WIN 55,212-2, these substances significantly blocked antinociceptive tolerance when given immediately prior to WIN 55,212-2 challenge on day 5. Daily MK-801 pretreatments, prior to WIN 55,212-2 injection, also produced a significant block of antinociceptive tolerance. These data suggest that like opioids, repeated spinal administration of a cannabinoid CB1 agonist elicits abnormal pain, which results in increased expression of spinal dynorphin. Manipulations that block cannabinoid-induced pain also block the behavioral manifestation of cannabinoid tolerance.

Behavioural assessment of dimethylsulfoxide neurotoxicity in rats

Dimethylsulfoxide (DMSO) is a solvent used to dissolve hydrophobic drugs. Recent studies have demonstrated that repeated administration of DMSO induces significant disorders of the peripheral nervous system. To address this issue, we have studied the behavioural effects of repeated intraperitoneal injections of various concentrations of DMSO (1.8–3.6–7.2%) in male Sprague–Dawley rats. Behavioural effects were assessed with a commonly used battery of sensory and motor tests. The motor tests used were actimeter and grip strength test. Sensory test used noxious and non-noxious mechanical (paw pressure test and von Frey hairs test) and thermal (plantar test and tail immersion test) stimuli. Clinical status of the animals was good throughout the experiment and no motor deficits were observed. Nevertheless, sensory assessment displayed a mechanical allodynia of short duration.

Local infusion of nerve growth factor attenuates myelinated nerve fiber sprouting into lamina II of the spinal dorsal horn and reduces the increased responsiveness to mechanical stimuli in rats with chronic constriction nerve injury.

To clarify the relationship between allodynia and the sprouting of myelinated fibers, we examined whether the administration of nerve growth factor (NGF) affected the paw withdrawal response to non-noxious mechanical stimuli and the sprouting of myelinated fibers into lamina II of the spinal dorsal horn, using a chronic constriction injury model of the sciatic nerve. Mechanical allodynia was determined as the threshold of the withdrawal response stimulated by von Frey filaments. Sprouting was examined using horseradish peroxidase conjugated to the B fragment of cholera toxin (B-HRP). NGF was continuously infused into the site of nerve injury for 14 days after nerve ligation. With vehicle infusion, significantly increased responsiveness to mechanical stimuli was observed on postoperative days (PODs) 5, 7, and 14 after ligation, compared with before surgery, and B-HRP-positive fibers were newly localized in lamina II on PODs 7 and 14. Infusion of NGF reduced the responsiveness to mechanical stimuli on 5, 7, and 14 PODs and B-HRP-positive fibers in lamina II on PODs 7 and 14. We propose that the suppression of the increased responsiveness to mechanical stimuli produced by NGF could be related to the disappearance of B-HRP-positive fibers in lamina II.

Non-noxious A fiber afferent input enhances capsaicin-induced mechanical hyperalgesia in the rat

Intradermal injection of capsaicin induces primary hyperalgesia at the injection site and secondary hyperalgesia in the surrounding undamaged skin. The secondary hyperalgesia is thought to be due to central sensitization of the dorsal horn neurons while primary hyperalgesia is caused by sensitization of nociceptors in the damaged skin. In this study, we asked if additional non-noxious afferent input from the undamaged skin influences the already developed secondary hyperalgesia, which follows an intradermal injection of capsaicin. Capsaicin dissolved in olive oil was injected into the middle of the hind paw of male Sprague–Dawley rats (250–300 g) under gaseous anesthesia. This produced a decrease in the mechanical threshold at the base of the toes for hind limb withdrawals lasting for 1–2 h, thus showing a short-lasting (hours) secondary hyperalgesia. When the capsaicin injection was immediately followed by repeated non-noxious mechanical stimuli or weak electrical stimuli (A fiber strength) applied to the area of secondary hyperalgesia (toes) for 30 min, the reduction of the mechanical threshold lasted longer than 24 h. These results suggest that non-noxious A fiber afferent input can powerfully modulate central sensitization in the spinal dorsal horn, causing the duration of the secondary hyperalgesia to be greatly extended.

Analgesia induced by chronic nicotine infusion in rats

Acute administration of nicotine induces analgesia with subsequent development of tolerance. In human studies, females are less sensitive to the analgesic effects of nicotine than males. Few previous animal studies have investigated analgesic effects of chronic nicotine administration or addressed gender differences. To investigate whether chronic administration of nicotine induces analgesia in male and female rats as assessed by a battery of standard pain assays, if tolerance develops, and if hyperalgesia occurs following cessation of nicotine. Nicotine (free base; 6 mg/kg/day i.v.) or saline was administered for 2 weeks via implanted osmotic pumps. Pain behavior was assessed before, during, and for 3 weeks after nicotine infusion by measuring tail flick latency, hot-plate latency, and thermal paw withdrawal latency. The paw-withdrawal threshold to non-noxious mechanical stimuli was also measured. Effects of nicotine infusion, gender, and time were assessed by three-way analyses of variance. Both male and female rats exhibited a comparable degree of analgesia in the hot-plate test with development of tolerance during the 2-week infusion period. Males, but not females, showed analgesia in the tail flick test. Analgesia was not observed for thermally evoked paw withdrawal in either males or females, nor did nicotine affect non-noxious mechanically evoked paw withdrawals. Males and females showed cessation of weight gain during the first week of nicotine infusion. Chronic nicotine-induced analgesia was confirmed in both male and female rats as assessed using the hot-plate test which reflects integrated pain behavior. Males, but not females, exhibited analgesia in a nociceptive withdrawal reflex test (tail flick), indicating that nicotine-induced analgesia may depend on both the type of pain test and gender. The lack of nicotine-induced analgesia assessed by the tail flick reflex test in female rats is consistent with recent human studies showing that nicotine reduces pain elicited by brief noxious cutaneous stimulation in male but not female subjects.

Inhibition of Neuropathic Pain by Selective Ablation of Brainstem Medullary Cells Expressing the μ-Opioid Receptor

Neurons in the rostroventromedial medulla (RVM) project to spinal loci where the neurons inhibit or facilitate pain transmission. Abnormal activity of facilitatory processes may thus represent a mechanism of chronic pain. This possibility and the phenotype of RVM cells that might underlie experimental neuropathic pain were investigated. Cells expressing mu-opioid receptors were targeted with a single microinjection of saporin conjugated to the mu-opioid agonist dermorphin; unconjugated saporin and dermorphin were used as controls. RVM dermorphin-saporin, but not dermorphin or saporin, significantly decreased cells expressing mu-opioid receptor transcript. RVM dermorphin, saporin, or dermorphin-saporin did not change baseline hindpaw sensitivity to non-noxious or noxious stimuli. Spinal nerve ligation (SNL) injury in rats pretreated with RVM dermorphin-saporin failed to elicit the expected increase in sensitivity to non-noxious mechanical or noxious thermal stimuli applied to the paw. RVM dermorphin or saporin did not alter SNL-induced experimental pain, and no pretreatment affected the responses of sham-operated groups. This protective effect of dermorphin-saporin against SNL-induced pain was blocked by beta-funaltrexamine, a selective mu-opioid receptor antagonist, indicating specific interaction of dermorphin-saporin with the mu-opioid receptor. RVM microinjection of dermorphin-saporin, but not of dermorphin or saporin, in animals previously undergoing SNL showed a time-related reversal of the SNL-induced experimental pain to preinjury baseline levels. Thus, loss of RVM mu receptor-expressing cells both prevents and reverses experimental neuropathic pain. The data support the hypothesis that inappropriate tonic-descending facilitation may underlie some chronic pain states and offer new possibilities for the design of therapeutic strategies.

Gene therapy for the management of pain: part II: molecular targets.

TREATMENT of chronic pain, particularly of neuropathic etiology, is extremely difficult and resistant to many available pharmacologic therapies. Current analgesic agents may be limited with regard to analgesic efficacy or side effects. Newer and experimental pharmacologic agents may also have significant limitations. By targeting a specific receptor or other specific protein targets, a gene therapy approach to the treatment of pain may provide greater analgesic efficacy without the limitations associated with current pharmacotherapy. Advances in the field of gene therapy, along with significant increases in our understanding of the neurobiology of nociception and knowledge of the fundamental genetic structure of many nociceptive targets, have made gene therapy for the management of pain a conceivable reality. In part I of this review, we introduced the basic concepts of gene therapy with an emphasis on the available tools (e.g., viral vectors and antisense oligonucleotides) and strategies for upregulating antinociceptive or downregulating pronociceptive targets. In part II, we summarize current knowledge regarding the nociceptive role, molecular biology, and antisense and knockout data of several novel nociceptive targets for gene therapy. We base our selection of the targets included in this review on the three aforementioned criteria. The targets selected are the best characterized and, in our opinion, most likely amenable to the gene therapeutic approach. A simple but feasible strategy and potential gene therapy targets for the management of pain are summarized in figure 1. However, the list is admittedly incomplete, and the readers are referred to other recent reviews cited in part I of this review for a broader perspective on potential targets for the management of pain.

Hyperalgesia and allodynia induced by intrathecal (RS)-dihydroxyphenylglycine in rats.

To investigate the role of Group I mGluRs in allodynia and hyperalgesia, we examined the behavioural responses of rats to noxious and non-noxious mechanical and thermal stimuli following intrathecal (i.t.) treatment (25 nmol) with the selective mGluR1/5 agonist, (RS)-dihydroxyphenylglycine ((RS)-DHPG). (RS)-DHPG administration produced a persistent decrease in response latency on a 48 degrees C hotplate, a reduction in the 50% response threshold to von Frey hairs, and an increase in responses to a tail pinch. These data suggest that activation of spinal mGluR1/5 receptors plays a role in the development of persistent allodynia and hyperalgesia associated with tissue or nerve injury.

Effects of cutaneous afferent input on fatigue-induced changes in fusimotor activity of decerebrate cats

Interaction of cutaneous and small-diameter, primarily fatigue-induced, muscle afferent inputs on fusimotor neurons has been studied in decerebrate cats. Spike discharges of fusimotor neurons to medial gastrocnemius were recorded from filaments dissected free from this muscle nerve. Non-noxious mechanical stimuli (10 Hz, 2 mm vibration) were applied to the skin area on the lateral side of the heel, innervated by sural nerve, during long-lasting (250 s) fatiguing contraction of lateral gastrocnemius and soleus muscles, elicited by electrical stimulation (40 Hz, 1.3×motor threshold) of the muscle nerves. In 15 units (58%) the pattern of responses to muscle contraction and/or fatigue (initial transient, and late long-lasting increase in firing rate, respectively) was preserved in the presence of skin vibration which, by itself, provoked either a slight increase or no changes in fusimotor discharge rate. Pattern of the response to skin vibration prevailed in the presence of muscle contraction and fatigue only if the vibration by itself induced marked increase in fusimotor discharge rate (three units). In the remaining eight units the responses to both stimuli applied simultaneously were dissimilar in pattern to the response to either stimulus applied alone: the initial, tension-related, increase in firing rate was prolonged, while the late, fatigue-induced one was attenuated and its post-contraction part almost abolished. Possible mechanisms and functional role of interaction between cutaneous and muscle afferent inflows are discussed.

Vincristine hyperalgesia in the rat: a model of painful vincristine neuropathy in humans.

To investigate the mechanism of vincristine-induced pain in humans undergoing chemotherapy we have established a model of vincristine-induced hyperalgesia in rat. Vincristine (100 micrograms/kg) was administered daily over a period of two weeks. An acute dose-dependent decrease in mechanical nociceptive threshold and an increased response to non-noxious mechanical stimuli ("hyperalgesia") occurred after the second day of administration. Chronic lowered threshold and increased response to stimuli (determined 24 h after each injection) was first noted during the second week of vincristine administration. Responses gradually returned to baseline during the two weeks following discontinuation of treatment. Vincristine also increased sensitivity to heat stimulation. At a dose that produced hyperalgesia (100 micrograms/kg), vincristine did not cause a significant motor deficit. Peripheral administration of a mu-opioid agonist did not reduce vincristine-induced acute hyperalgesia. Hyperalgesia induced by vincristine in the rat provides a good model for the experimental study of painful peripheral neuropathies in human patients receiving vincristine as a chemotherapeutic agent.

Physiology of nociception

Nociception is related to the mechanisms elicited by stimuli threatening the integrity of the organism. At the peripheral level, unmyelinated C fibres (C polymodal nociceptores) or fine myelinated A delta fibres are excited by noxious stimulation, directly or indirectly by inflammatory processes. Nociceptive afferent fibres terminate in the superficial laminae of the dorsal horn of the spinal cord where informations are integrated and controlled. These first synapses are modulated by excitatory amino acids (glutamate and aspartate) and many peptides (substance P, CGRP, CCK, endogenous opiods). The majority of ascending pathways involved in nociception are located in the ventrolateral controlateral quadrant of the cord (spinorelicular and spinothalamic tracts). Many supraspinal sites are activated following nociceptive stimuli, with relays in the reticular formation of the brain stem (including the subnucleus reticularis dorsalis), the ponto-mesencephalic regions (periaqueducal gray matter and parabrachial area) and thalamic sites. Amygdala and hypothamic targets could be involved in motivational reactions and neuroendocrine adaptations to a noxious event. The cingular, insular and somatosensory cortices also receive nociceptive informations. Nociceptive signals are modulated at all levels of their transmission; the more extensively studied controls are located at the spinal level. Segmental controls are inhibitory effects produced by non-noxious mechanical stimuli. Spinal signals can also be inhibited following activation of bulbopinal descending inhibitor pathways and release of serotonin, norepinephrine and, indirectly, endogenous opiods. Inhibitory controls triggered by noxious stimuli could facilitate the extraction of the nociceptive tone of informations having priority over other stimuli.

Responses to mechanical stimulation of the upper limb in painful cervical radiculopathy

Clinical and electromyographic (EMG) responses to non-noxious mechanical stimuli were studied in four patients with painful cervical radiculopathy, and in two control subjects. In the symptomatic arm(s), palpation over one or more nerve trunks was painful and accompanied by EMG activity, whereas palpation of adjacent soft tissues was painless and unaccompanied by EMG activity. Electromyographic activity was widespread in three patients when myotatic reflexes were elicited in the symptomatic arm(s). In asymptomatic arms of patients and controls, EMG responses to the myotatic reflexes were more localised. Allodynic nerve trunks in cervical radiculopathy appear to be afferent correlates of central sensitisation; the accompanying EMG activity may represent a motor correlate of this same process.

Thermal, mechanical and chemical peripheral sensation in amphibians: Opioid and adrenergic effects

The acetic acid test (AAT) is a quantifiable assay of the response to noxious chemical stimuli on the hindlimb of the northern grass frog, Rana pipiens. AAT is sensitive to both opioid and adrenergic agonist modulation. The present study introduces the novel use of mechanical and thermal stimulus behavioral assays in comparison with the established acetic acid test in studying nociception in frogs. We evaluated mechanical and thermal responses and their sensitivity to systemic morphine (MOR) or dexmedetomidine (DEX) administrations with comparison to AAT. MOR produced dose-related elevations of response thresholds in all three sensory tests, whereas DEX produced elevations in the thermal and AAT assays but had no effect on sensitivity to non-noxious mechanical stimuli. The results suggest a distinct separation of sensory modalities in the frog similar to that observed in mammals and indicate the usefulness of this amphibian model in further studies of nociception.

Sciatic Cryoneurolysis in Rats

The purpose of this study was to evaluate the effects of preemptive and postlesion sympathectomy in the sciatic cryoneurolysis (SCN) model of neuropathic pain in rats.SCN in rats produces a prolonged significant mechanical allodynia (hypersensitivity to previously nonnoxious mechanical stimuli) with no thermal hyperalgesia. In at least two other models, sympathectomy is effective in attenuating existing mechanical allodynia and thermal hyperalgesia or deterring their development after nerve injury. These models appear to mimic the direct sympathetic involvement characteristic of the clinical syndrome termed sympathetically maintained pain (SMP). To investigate these concepts in the SCN model, sympathectomy was performed prior to SCN in animals with established SCN-induced allodynia. Sympathectomy did not alter the pattern of existing allodynia or its development in this model. The results suggest that SCN is a useful and easily reproducible model of sympathetically independent pain (SIP). (Anesth Analg 1995;81:544-8)

Sciatic Cryoneurolysis in Rats

The purpose of this study was to evaluate the effects of preemptive and postlesion sympathectomy in the sciatic cryoneurolysis (SCN) model of neuropathic pain in rats. SCN in rats produces a prolonged significant mechanical allodynia (hypersensitivity to previously non-noxious mechanical stimuli) with no thermal hyperalgesia. In at least two other models, sympathectomy is effective in attenuating existing mechanical allodynia and thermal hyperalgesia or deterring their development after nerve injury. These models appear to mimic the direct sympathetic involvement characteristic of the clinical syndrome termed sympathetically maintained pain (SMP). To investigate these concepts in the SCN model, sympathectomy was performed prior to SCN in animals with established SCN-induced allodynia. Sympathectomy did not alter the pattern of existing allodynia or its development in this model. The results suggest that SCN is a useful and easily reproducible model of sympathetically independent pain (SIP).

Differential effects of spinalization on discharge patterns and discharge rates of simultaneously recorded nociceptive and non-nociceptive spinal dorsal horn neurons

Recordings were made simultaneously from 2–5 neurons at the same site in the lumbar spinal dorsal horn of pentobarbital-anesthetized rats. Neurons were classified as low-threshold (LT) or multireceptive (MR) according to their responses to non-noxious mechanical or noxious radiant heat stimuli of the skin. At the same recording sites neurons could be encountered which belong to different classes and/or which had mechanoreceptive fields which did not overlap. Cold blocks of the upper or lower thoracic cord or transsections of the upper cervical cord were made to evaluate the effects of spinalization on both the rate and pattern of background activity and/or noxious heat-evoked responses of different dorsal horn neurons under identical experimental conditions. At 24 of 27 recording sites, spinalization had qualitatively or quantitatively different effects on the rate of background activity of simultaneously recorded neurons. Interspike interval (ISI) means of background activity were significantly reduced in 29 of 65 (44.6%) neurons, prolonged in 23 of 65 (35.4%) neurons, or unchanged in 13 of 65 (20%) neurons. MR neurons displayed a significantly higher incidence of decreased background activity 17 of 45 (37.8%) and a lower incidence of increased background activity (18 of 45, 40%) during spinalization that the LT neurons from which 1 of 12 (8.3%) decreased and 8 of 12 (66.6%) increased background activity. Almost all (95.4%) neurons changed their discharge patterns after spinalization. At 9 of 27 recording sites, the discharge patterns of simultaneously recorded neurons were affected differently by spinalization as revealed by the coefficient of dispersion of the interspike intervals (ISI), indicating changes in the tendency to discharge action potential in clusters (bursts). At the same recording sites the level of noxious heat-evoked responses of simultaneously recorded MR neurons was also differentially affected by spinalization. Nociceptive responses were significantly enhanced in 19 of 37 (51.4%) neurons (137.8 ± 142.6%) of control, mean ± SD), reduced in 13 of 37 neurons (35.1%) (by 58.9 ± 20.9%) and/or unchanged in 5 of 37 (13.5%) neurons. It is concluded that no general ‘tone’ of descending antinociception exists and that tonic descending excitatory and inhibitory systems may be active simultaneously modulating both the level and pattern of neuronal discharges.

The excitatory amino acid receptor antagonist MK-801 prevents the hypersensitivity induced by spinal cord ischemia in the rat

Protection by the NMDA receptor antagonist MK-801 against transient spinal cord ischemia-induced hypersensitivity was studied in rats. The spinal ischemia was initiated by vascular occlusion resulting from the interaction between the photosensitizing dye Erythrosin B and an argon laser beam. The hypersensitivity, termed allodynia, where the animals reacted by vocalization to nonnoxious mechanical stimuli in the flank area, was consistently observed during several days after induction of the ischemia. Pretreatment with MK-801 (1–5 mg/kg, iv) 10 min before laser irradiation dose dependently prevented the occurrence of allodynia. The neuroprotective effect of MK-801 was not reduced by maintaining normal body temperature during and after irradiation. There was a significant negative correlation between the delay in the administration of MK-801 after irradiation and the protective effect of the drug. Histological examination revealed slight morphological damage in the spinal cord in 38% of control rats after 1 min of laser irradiation without pretreatment with MK-801. No morphological abnormalities were observed in rats after pretreatment with MK-801 (0.5 mg/kg). The present results provide further evidence for the involvement of excitatory amino acids, through activation of the NMDA receptor, in the development of dysfunction following ischemic trauma to the spinal cord.

Beneficial effect of the opioid receptor antagonist naltrexone on hyper-sensitivity induced by spinal cord ischemia in rats: disassociation with MK-801

In this study we examined the effect of the long-acting opioid antagonist naltrexone on the allodynia-like effect of spinal ischemia in rats. The spinal cord ischemia was induced at midthoracic level by a recently developed photochemical technique using laser irradiation and photoactivatable intravascular dyes. An allodynia-like sensory disturbance, where the animals reacted by vocalization to non-noxious mechanical stimuli in the flank area, was consistently seen during several days after ischemia. Pretreatment with 10 and 20 mg/kg, but not 5 mg/kg naltrexone i.v. 10 min before irradiation decreased the incidence of allodynia. However, even the effect of the highest dose of naltrexone (20 mg/kg) was incomplete, which is in contrast to the effect of the NMDA receptor antagonist MK-801, which has been tested in the same model and found to completely prevent the incidence of allodynia at a dose of 0.5 mg/kg. Pretreatment with sub- or suprathreshold doses of naltrexone (5 and 20 mg/kg respectively) combined with a subthreshold dose of MK-801 (0.1 mg/kg) did not produce a synergistic effect. When naltrexone (20 mg/kg) was administered 10 min after induction of ischemia, it was totally ineffective in decreasing the occurrence and severity of allodynia. In contrast, MK-801 (0.5 mg/kg) still had a good protective effect when injected as this time. Histological examination showed slight morphological damage in the spinal cord in 38% of control rats after 1 min laser irradiation without pretreatment with naltrexone. No morphological abnormalities were observed in rats after pretreatment with naltrexone (20 mg/kg). The results suggest that opioid receptor antagonists and NMDA receptor antagonists prevent a consequence of transient spinal cord ischemia through different mechanisms. High doses of opioid antagonists may have anti-ischemic effects by improving local spinal cord microcirculation and therefore may have a role in preventing ischemia after traumatic spinal cord injury. On the other hand, the NMDA receptor may have a role in the secondary neuronal death resulting from ischemia. Thus, NMDA receptor antagonists may contribute to the prevention of tissue damage by antagonizing the excitotoxic action of glutamate and/or aspartate released by ischemia into the spinal cord. Finally, since only high doses of naltrexone had an effect in the present study, we cannot rule out the possiblity that this drug may act through non-opioid mechanisms.