Contribution of central neuroplasticity to pathological pain: review of clinical and experimental evidence

Spinal prodynorphin gene expression in collagen-induced arthritis: influence of the glucocorticosteroid budesonide

Changes in the spinal expression of the opioid precursor and prodynorphin, which has been implicated in the response to peripheral inflammation, were examined with semi-quantitative in situ hybridization histochemistry in rats subjected to collagen II-induced arthritis. The effects of glucocorticosteroid treatment on the basal and inflammation-induced prodynorphin expression were evaluated. Collagen II-induced arthritis caused a 16-fold increase in prodynorphin mRNA levels which comprised all neurons expressing low levels under normal conditions. In the superficial dorsal horn, one group of neurons of a large size reacted with a dramatic increase of prodynorphin mRNA, while another group of small neurons exhibited a moderate elevation of prodynorphin mRNA levels. In the deep dorsal horn of arthritic rats, most prodynorphin neurons were large and showed high prodynorphin mRNA levels. Systemic treatment with the glucocorticosteroid budesonide attenuated the arthritis-induced increase of prodynorphin mRNA expression in a topospecific manner. The budesonide-induced reduction of prodynorphin mRNA levels was more pronounced in the deep dorsal horn than in the superficial dorsal horn. Budesonide treatment of control animals caused a small, but significant increase in prodynorphin mRNA levels in the superficial laminae I/II without affecting prodynorphin mRNA levels in the deep dorsal horn. The degree of arthritis correlated closely with spinal prodynorphin mRNA levels. The tight correlation between severity of arthritis and prodynorphin mRNA levels in non-treated and corticosteroid-treated arthritic rats suggests that spinal prodynorphin expression is a good parameter for the evaluation of the influence of peripheral inflammation and of the efficacy of analgesic/anti-inflammatory drugs in its treatment. Opposite effects of budesonide on basal and inflammation-induced prodynorphin expression may involve a spinal site of action in addition to peripheral anti-inflammatory mechanisms. We suggest that the collagen II-induced arthritis in the rat is an excellent model for human rheumatoid arthritis allowing for the study of molecular plasticity of anti-inflammatory and anti-nociceptive drug action at different levels of the neuroaxis.

Local anesthetics as adjuvant analgesics

Local anesthetics administered to block nerve conduction for surgical anesthesia and to provide analgesia in management of acute pain have become a standard of anesthesiology practice. These drugs have had an important role in the multimodality management of chronic pain as well, and this role is expanding since the revival of systemic administration. Local anesthetics are analgesics, albeit not in the traditional clinical and pharmacologic sense. Evidence suggests that intravenous administration is an effective treatment in chronic neuropathic pain syndromes. There is also evidence that intravenous local anesthetics can relieve acute pain. Furthermore, the novel idea that acute procedural and postprocedural pain control with local anesthetics could prevent the development of chronic pain syndromes, including chronic neuropathic pain syndromes, adds another important potential dimension to the role of local anesthetics in pain management.

Quantitative sensory examination of epidural anaesthesia and analgesia in man: effects of pre- and post-traumatic morphine on hyperalgesia

The objectives of the study were: (1) comparison of hypoalgesic effects of pre- and post-traumatic epidural morphine (EM) on primary and secondary hyperalgesia, and (2) comparison of EM hypoalgesia in normal and injured skin. Burn injuries (25 x 50 mm rectangular thermode, 47 degrees C, 7 min) were produced on the calves of healthy volunteers, at 2 different days at least 1 week apart. In randomized order, the subjects received 4 mg of EM administered via the L2-L3 intervertebral space on one day and no treatment on the other day. One calf was injured 30 min prior to and the other calf 2.5 h after administration of morphine. Hence, the calf injured prior to morphine administration was a model of postinjury treatment, and the calf injured after morphine administration, a model of pretraumatic treatment. The timing of injuries was identical on the morphine treatment and control days. The injuries induced decrease in heat pain detection and tolerance thresholds within the area of injury (area of primary hyperalgesia) as well as reduction of areas of allodynia for brush and pinprick surrounding the injury (area of secondary hyperalgesia). Both pre- and post-traumatic administration of EM increased heat pain detection and tolerance thresholds, and decreased by approximately 50% the areas of secondary hyperalgesia 2.5 h postinjury. The effects of morphine were naloxone (NAL)-reversible (0.1 mg/kg, i.v.). There was no significant difference between pre- and post-traumatic administration of morphine on the effect of either primary or secondary hyperalgesia. EM increased the heat pain detection threshold more within the injury than at a corresponding non-injured site. There was no significant difference in the effect of morphine on heat pain tolerance in injured and non-injured skin. Following NAL, the areas of secondary hyperalgesia expanded beyond control size. It is suggested that the major effect of EM on secondary hyperalgesia is inhibition of C fibre-mediated activity which maintains the altered response properties of central neurons responsible for secondary hyperalgesia. Possible mechanisms of action of NAL in enhancement of hyperalgesia are discussed.

Diazepam attenuates hyperexcitability and mechanical hypersensitivity of dorsal horn convergent neurones during reperfusion of the rat's tail following ischaemia

We have investigated the involvement of the GABAA-benzodiazepine receptor complex in nociceptive activity of convergent neurones in the spinal cord during ischaemia and reperfusion of their receptive fields on the rat's tail. In enflurane anaesthetized rats, extracellular recordings were made from convergent neurones located throughout the dorsal horn before, during and after 30 min of ischaemia. Following intrathecal saline pretreatment, there was a significant increase in spontaneous firing rate during ischaemia (219 +/- 21%, P < 0.02, n = 10) which persisted during reperfusion. After 10 min of reperfusion, the neurones exhibited a greater response than before ischaemia to both innocuous brush (54 +/- 11%, P < 0.05, n = 10) and noxious pinch (72 +/- 14%, P < 0.02, n = 10) and the enhanced sensitivity persisted over the 60-min reperfusion period. During reperfusion, receptive field size increased in most neurones tested. Intrathecal diazepam (100 and 500 micrograms) abolished the hyperexcitability and the hypersensitivity to both innocuous and noxious mechanical stimulation during reperfusion. The highest dose of diazepam (500 micrograms) also attenuated the increase in spontaneous firing rate during ischaemia. Diazepam, at the doses tested, had no effect on receptive field enlargements during reperfusion. The effect of 100 micrograms of diazepam was partially reversed by flumazenil (1 mg/kg i.p.) but not by naloxone (1 mg/kg i.p.). In the absence of ischaemia, diazepam had no effect on spontaneous firing rate nor on the responses to innocuous or noxious mechanical stimulation. Our results support an antinociceptive role for benzodiazepines in the dorsal horn elements responsible for reperfusion hyperalgesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Are ventromedial medulla neuronal properties modified by chronic peripheral inflammation? A single-unit study in the awake, freely moving polyarthritic rat

In the present work, we recorded the neuronal properties of the ventromedial medulla, a brainstem structure involved in the descending spinal control systems related to nociception, in awake, freely moving healthy and polyarthritic rats. These animals were rendered polyarthritic with a subcutaneous administration of the Freund's adjuvant into the tail, and studied at 20 and 30 days post-inoculation. At the ventromedial medulla level, the single-unit activities were recorded by means of a chronically implanted device supporting a 50 microns platinum-iridium wire as the recording electrode. With a total of 308 recorded neurons, we determined that in both healthy rats, i.e. animals having received mineral oil only and arthritic rats, there were ventromedial medulla units with common physiological properties, but also changes. In agreement with the results from anesthetized arthritic rats at spinal and thalamic levels, the systematic analysis of the responses to light touch and mechanical shock revealed that the 'multimodal, multireceptive' units, excited by innocuous and noxious stimuli, were much more responsive to both modalities in arthritic rats. Approximately 7% of these neurons displayed a 'paroxysmal' spontaneous activity, also reported in the literature for other structures. In addition, we recorded a significant number of neurons inhibited or excited-inhibited by innocuous and noxious cutaneous stimulations, and a few with a regular spontaneous activity, also responding, which has never been the case in healthy rats. We conclude that a peripheral chronic inflammation, such as arthritis, can produce changes of the ventromedial medulla neuronal properties, as compared to healthy animals. Consequently, in addition to its classical role in the spinal control of nociception, the ventromedial medulla is able to develop some form of plasticity in the case of persistent pain of peripheral origin.

Post-traumatic fibromyalgia: a case report narrated by the patient

This report describes a case of fibromyalgia developing following a workplace injury, but in which the issues of compensation and work disability were not relevant. A previously healthy 37-year-old woman developed back and groin pain after lifting a heavy box. Over the next months, pain and allodynia gradually spread over her body, and headaches, sleep disturbance, paresthesias, and bowel symptoms developed for the first time. The pain was constant and severe, invading and interfering with every area of daily function. Surprisingly, no previous case reports or definition of post-traumatic fibromyalgia could be found. This case report, narrated by the patient, suggests that there is such an entity as post-traumatic fibromyalgia, and that central nervous system plasticity plays a central role.

Relief of post-herpetic neuralgia with the N-methyl-D-aspartic acid receptor antagonist ketamine: a double-blind, cross-over comparison with morphine and placebo

Pain and sensory thresholds were examined before and after intravenous administration of ketamine (0.15 mg/kg), morphine (0.075 mg/kg) or saline in 8 patients with post-herpetic neuralgia. A randomized, double-blind, cross-over study design was used. Post-herpetic neuralgia was associated with impaired sensory function, as shown by reduced tactile and warm sensation in the affected compared with the contralateral non-affected skin area. Neither ketamine nor morphine changed significantly the thresholds for warm, cold, heat pain or tactile sensation. However, ketamine normalized abnormal heat pain sensations in 4 patients, probably due to a central effect. Ketamine, but not morphine, produced significant relief of pain. Pain evoked by non-noxious stimulation of the skin (allodynia) was significantly inhibited by ketamine as well as by morphine. Wind-up-like pain (i.e., pain evoked by repeatedly pricking the affected skin area) was significantly inhibited by ketamine, but significantly aggravated by morphine. Side effects were observed in all the 8 patients after injection of ketamine and in 6 patients after injection of morphine. The present results support the hypothesis that the N-methyl-D-aspartic acid (NMDA) receptors are involved in the control of post-herpetic neuralgia including allodynia and wind-up-like pain. The NMDA receptors also may play a role in the modulation of thermal perception.

The effects of intrathecal morphine and clonidine on the prevention and reversal of spinal cord hyperexcitability following sciatic nerve section in the rat

We have previously shown that intrathecal (i.t.) morphine, but not the alpha 2-adrenoreceptor agonist clonidine, administered prior to sciatic nerve section, reduced the level of autotomy in rats, which is a behavioural model of neuropathic pain. Neither drug was effective when administered 15 min after nerve section. We now examined the effects of i.t. morphine and clonidine on the development of flexor reflex hyperexcitability following sciatic nerve section in acute physiological experiments. The flexor reflex was recorded from the hamstring muscles in decerebrate, spinalized, unanesthetized rats. The effect of sciatic nerve section on the flexor reflex without drugs was compared with axotomy performed 60 min after i.t. injection of 3 micrograms or 30 micrograms morphine, as well as 50 micrograms clonidine. The effect of these drugs on reversing reflex hyperexcitability was also examined. Both doses of morphine administered prior to sciatic nerve section profoundly depressed the baseline reflex and the higher dose almost completely abolished reflex hyperexcitability following nerve section. In contrast, clonidine pre-administration was less effective in depressing the baseline reflex and blocked reflex hyperexcitability less than morphine. Both morphine and clonidine administered 15 min after nerve section reversed spinal hyperexcitability. Thus, the ability of morphine to prevent the occurrence of autotomy may be related to its effectiveness in blocking axotomy-induced hyperexcitability. These physiological data suggest that even a short period of spinal cord hyperexcitability following nerve injury may lead to the development of neuropathic pain.

Immediate and prolonged effects of pre- versus postoperative epidural analgesia with bupivacaine and morphine on pain at rest and during mobilisation after total knee arthroplasty

Thirty-two patients scheduled for total knee arthroplasty were randomized to receive an identical epidural blockade initiated 30 min before surgical incision (N = 16), or at closure of the surgical wound (N = 16). Before induction of general anaesthesia the epidural catheter was tested with bupivacaine 7.5 mg.ml-1, 2 ml. General anaesthesia was induced with thiopentone, pancuronium or atracurium, and fentanyl 0.1-0.3 mg, and maintained with N2O/O2 and enflurane. The epidural regimen consisted of a bolus of 16 ml of bupivacaine 7.5 mg.ml-1 plus morphine 2 mg, and continuous infusion of bupivacaine 1.25 mg.ml-1 plus morphine 0.05 mg.ml-1, 4 ml.h-1 for the first 24 h, and bupivacaine 0.625 mg.ml-1 plus morphine 0.05 mg.ml-1, 4 ml.h-1, for the next 24 h after operation. Additional morphine 2.5-5 mg was administered i.v. or i.m. for the first 24 h postoperatively, and ketobemidone or morphine 5-10 mg orally or rectally from 24 h to 7 d postoperatively, on request. Paracetamol 1000 mg every 8 h was administered from 48 h to 7 days postoperatively. No significant differences were observed in request for additional opioids, or in pain scores at rest or during mobilisation of the operated limb, during or after cessation of the epidural regimen. These results do not suggest timing of analgesia with a conventional, continuous epidural regimen to be of major clinical importance in patients undergoing total knee arthroplasty.

Subcutaneous formalin produces centrifugal hyperalgesia at a non-injected site via the NMDA-nitric oxide cascade

Previous work has demonstrated that pain facilitation can occur following injection of subcutaneous irritants, such as formalin. Such studies have focused on apparent pain facilitation induced at the site of irritant injection. Changes in processing of incoming pain information have typically been assumed to result from activation of neurocircuitry intrinsic to the spinal cord. The present series of studies have examined hyperalgesia exhibited at a site distant from the site of irritant injection and have begun to define the neurocircuitry and neuropharmacology underlying this pain enhancement. This work demonstrates that s.c. formalin injected into the dorsum of one hindpaw in rats produces prolonged hyperalgesia as measured by the tailflick test. Hyperalgesia is not mediated solely by circuitry intrinsic to the spinal cord, but rather involves activation of centrifugal pathways originating within the brain and descending to the spinal cord via pathway(s) outside of the dorsolateral funiculus. At the level of the spinal cord, this hyperalgesic state is mediated by an NMDA-nitric oxide cascade, since hyperalgesia can be abolished by administration of either an NMDA antagonist (APV) or a nitric oxide synthesis inhibitor (L-NAME).

Pre-emptive intrathecal administration of an NMDA receptor antagonist (AP-5) prevents hyper-reflexia in a model of persistent visceral pain

Dorsal horn sensitization following somatic noxious stimuli is partly mediated by the N-methyl-D-aspartate (NMDA) sub-type of glutamate receptor. This phenomenon has been comparatively sparsely investigated in the area of visceral pain. We have therefore investigated the role of spinal NMDA receptors in central sensitization in an animal model of persistent visceral pain. In anaesthetized rats the lumbosacral spinal cord was exposed by laminectomy and the pre-emptive effect of intrathecal AP-5 upon the hyper-reflexia associated with chemical inflammation of the bladder was investigated. The effect of intrathecal AP-5 (an NMDA receptor antagonist) upon the normal cystometrogram (CMG) was also measured. AP-5 (125-1000 micrograms) prevented the hyper-flexia associated with bladder inflammation in a dose-dependant fashion. In general, within the dose range 62.5-1000 micrograms, AP-5 had no significant effect upon the normal micturition reflex. However, at the top of this dose range a minor non-significant depression of this reflex was noted. NMDA receptors do not appear to mediate the micturition reflex at a spinal cord level. However, they are involved in the induction of hyper-reflexia following urinary bladder inflammation, this hyper-reflexia can be prevented by pre-emptive intrathecal administration of AP-5.

The role of nitric oxide in the development and maintenance of the hyperalgesia produced by intraplantar injection of carrageenan in the rat

Activation of the N-methyl-D-aspartate (NMDA) receptor has been reported to be involved in the mechanisms that underlie thermal hyperalgesia produced by the intraplantar injection of carrageenan. As NMDA-mediated thermal hyperalgesia produced in models of acute and persistent pain have been reported to involve production of nitric oxide, we examined the role of nitric oxide in both the development and maintenance of the thermal hyperalgesia produced by the intraplantar injection of carrageenan. In addition, we examined the role of nitric oxide in the maintenance of the mechanical hyperalgesia produced by intraplantar injection of carrageenan. Prior to the intraplantar injection of carrageenan (2 mg in 100 microliters) there was no significant difference in thermal withdrawal latencies or mechanical withdrawal thresholds between the left and right hindpaws. Three hours after injection of carrageenan into the left hindpaw, rats showed evidence of a significantly faster thermal withdrawal latency and lower mechanical withdrawal threshold of the left hindpaw compared to the right hindpaw. In addition, the left hindpaw was significantly increased in size (diameter) compared with the right hindpaw. In these same rats, the intrathecal administration of saline, NG-nitro-L-arginine methyl ester (L-NAME; 2-200 nmol) or the inactive enantiomer, NG-nitro-D-arginine methyl ester (D-NAME; 200 nmol) did not produce any significant change in thermal nociceptive withdrawal latencies in the non-injected paw. However, administration of L-NAME (2-20 nmol), but not saline or D-NAME produced a dose dependent and reversible block of the thermal hyperalgesia for a period of up to 3 h.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of balanced analgesia on early convalescence after major orthopaedic surgery

Forty-two patients scheduled for total knee arthroplasty (n = 20) or hip arthroplasty (n = 22) were randomly allocated to receive either continuous epidural bupivacaine/morphine for 48 h postoperatively plus oral piroxicam, or general anaesthesia followed by a conventional intramuscular opioid and acetaminophen regimen. Patients undergoing knee- or hip arthroplasty treated with epidural analgesia had significantly lower pain scores during mobilization under the 48 h epidural infusion compared with patients receiving conventional treatment, while no important differences were observed after cessation of the epidural regimen. However, the achieved pain relief had no impact on postoperative convalescence parameters, such as ambulation, patient activity including need for nursing care, fatigue or hospital stay. Late postoperative pain, fatigue and conservative attitudes and routines in the postoperative care, were the most important reasons limiting mobilization and activity. We conclude that effective early (48 h) postoperative pain relief with balanced analgesia does not per se lead to important improvements in convalescence and hospital stay.

Differential action of morphine and various opioid agonists on thermal allodynia and hyperalgesia in mononeuropathic rats

In a rat model of mononeuropathy produced by 4 loose ligatures around the common sciatic nerve, the effects of 1 mg/kg morphine and mu-, delta- and kappa-agonists, DAMGO (2 and 3 mg/kg), BUBUC (3 and 6 mg/kg) and U-69,593 (1.5 mg/kg), were evaluated by measuring the struggle latency (SL in sec) to immersion of the paw on the nerve-injured side in a cold (10 degrees C) or hot (42 degrees C, 44 degrees C, and 46 degrees C) water bath. Experiments were performed 2 weeks after surgery. The agonists were used at doses that produced potent antinociceptive effects on the vocalization test in this model. At 46 degrees C (clearly in the noxious range), only morphine and DAMGO had significant effects. The effect of morphine lasted for more than 2 h with a maximum at 40 min (SL = 13.8 +/- 1.6 sec, 252% of control values). For 2 and 3 mg/kg DAMGO, the dose-related effect lasted for 120 min at least, with a maximum at 20-40 min (SL = 6.0 +/- 0.5 and 8.8 +/- 0.7 sec, 148% and 170% of control values, respectively). These effects were more potent and prolonged than in normal rats and were reversed by 0.1 mg/kg naloxone i.v. By contrast, morphine and all selective agonists failed to relieve the abnormal reactions to 10 degrees C, 42 degrees C (in the non-noxious range) and 44 degrees C (at the noxious threshold) stimuli. Our data illustrate a differential effect of opioids on nociceptive tests based on different stimulus modalities and intensities in this model of mononeuropathic pain.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence of a role for N-methyl-D-aspartate (NMDA) receptors in the facilitation of tail withdrawal after spinal transection

Peripheral injury produces a characteristic excitation of spinal cord dorsal horn cells (wind-up) which is associated with a facilitation of spinal nociceptive reflexes (hyperalgesia). These phenomena are believed to be mediated by a trauma-induced increase in the release of excitatory amino acids (EAAs). A similar increase in the activity of dorsal horn neurons and spinal reflexes occurs after spinal transection. Therefore, the present studies examined the possibility that EAAs, acting through the NMDA receptor, might also be involved in behavioral hyperalgesia produced by central injury. The first experiment assessed the effect of pretreatment with the NMDA antagonist, ketamine, on the facilitated tail flick (TF) response of spinally transected rats. Separate groups of animals were spinalized under isoflurane anesthesia alone, intramuscular ketamine anesthesia alone, or a combination of isoflurane and intrathecal ketamine. The TF was examined 24 h later, before and 30 min after an intrathecal injection of morphine. In the second experiment, the effect of intraperitoneal or intrathecal ketamine on the TF was assessed to separate groups of rats that underwent spinal transection or sham surgery under isoflurane anesthesia. Pretreatment with either systemic or intrathecal ketamine did not alter TF facilitation or morphine-induced antinociception in spinal rats. However, both systemic and intrathecal ketamine significantly increased TF latencies in spinal, relative to intact rats. These results indicate that ketamine did not prevent the development of spinal reflex facilitation, but it selectively reduced this reaction once it was established in spinal rats. The data support an involvement of EAAs in reflex facilitation produced by spinal transection.

The involvement of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the responsiveness of rat spinal neurons with input from the chronically inflamed ankle

Unilateral adjuvant inflammation was induced at the rat ankle 2 or 20 days before an evaluation of the contribution of N-methyl-D-aspartate (NMDA) and non-NMDA receptors to the processing of nociceptive information by wide dynamic range neurons in the spinal cord. Microionophoretic application of either the NMDA receptor antagonists ketamine and DL-2-amino-5-phosphonovalerate (AP5) or the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) reduced the responses to innocuous and noxious mechanical stimulation of the inflamed ankle. The pattern of these effects was comparable to that in rats with acute inflammation suggesting that non-NMDA and NMDA receptors are similarly involved in acute, prolonged acute and chronic inflammation-evoked activity.

Pentobarbital prevents the development of C-fiber-induced hyperalgesia in the rat

Noxious stimuli applied to the skin can produce long-lasting, C-fiber-dependent, secondary hyperalgesia that is mediated by central mechanisms. NMDA receptor antagonists and low doses of morphine can preferentially block the development of hyperalgesia without significantly altering unpotentiated responses to nociceptive stimuli. The aim of our study was to determine if low doses of pentobarbital can also preferentially alter either hyperalgesic or unpotentiated responses to nociceptive heat stimuli in spinalized and intact rats. Our results demonstrate the following. (1) Mustard oil applied above the ankle joint or electrical stimulation of the sciatic nerve at C-fiber intensity in spinalized, unanesthetized rats decreased the latency to withdrawal of the foot from water maintained at 47-49 degrees C. This secondary hyperalgesia to thermal stimulation persisted for at least 1 h and was most likely mediated by central mechanisms. (2) Pentobarbital in both spinalized and spinal cord-intact rats prevented the development of the late component (42-120 min) but only partially decreased the early (2-6 min) component of hyperalgesia. In contrast, pentobarbital had relatively minimal effects on unpotentiated withdrawal responses. Thus, pentobarbital is similar to morphine in its ability to prevent hyperalgesia, but may differ from the anesthetic isoflurane, which does not interfere with the development of hyperalgesia.

Neurocircuitry of illness-induced hyperalgesia

We have previously demonstrated that illness-inducing agents such as lithium chloride (LiCl) and the bacterial cell wall endotoxin lipopolysaccharide (LPS) produce hyperalgesia on diverse pain measures. The present series of studies attempted to identify the neurocircuitry mediating these effects. These studies have demonstrated that illness-inducing agents produce hyperalgesia by activating: (a) peripheral nerves rather than by generating a blood-borne mediator (Expt. 1); (b) vagal afferents, specifically afferents within the hepatic branch of the vagus (Expt. 2); (c) as yet unidentified brain site(s) rostral to the mid-mesencephalon (Expt. 6); (d) a centrifugal pathway that arises from the nucleus raphe magnus, and not from the adjacent nucleus reticularis paragigantocellularis pars alpha (Expts. 4 and 5); (e) a centrifugal pathway in the dorsolateral funiculus of the spinal cord (Expt. 3); and (f) the same centrifugal pathways for diverse illness inducing agents (Expts. 3, 7 and 8). These data call for the re-evaluation of a number of assumptions inherent in previous studies of hyperalgesia.

NMDA causes release of nitric oxide from rat spinal cord in vitro

Anatomic studies have localized nitric oxide synthase (NOS) activity in the rat spinal cord dorsal horn and intermediolateral cell column. Behavioral and electrophysiologic studies suggest that N-methyl-D-aspartate (NMDA) stimulates nitric oxide synthesis in the dorsal horn. This report describes a novel bioassay to determine directly in vitro whether NMDA causes release of nitric oxide from rat spinal cord. Modified Krebs-Henseleit solution at 26 degrees C was perfused over spinal cord slices from adult male rats, then dropped onto a ring of endothelium-denuded rat aorta. Following preconstriction with phenylephrine, NMDA (10(-10) to 10(-3) M) alone or with other drugs was added to the perfusion solution and vascular tension measured. NMDA-containing solutions applied directly on the preconstricted vessels without exposure to spinal cord tissue had no effect on vessel tone. In contrast, NMDA via the spinal cord perfusion caused concentration-dependent vascular relaxation, which was blocked by MK-801, hemoglobin, methylene blue, and several arginine analogues which inhibit NOS. [14C]citrulline assay suggested NOS in rat spinal cord was non-endothelial in nature. NMDA perfusion of spinal cord slices in vitro causes vascular relaxation in this bioassay due to actions on NMDA receptors and which is consistent with release of nitric oxide. These results support previous anatomical, behavioral, and electrophysiologic studies in rat spinal cord and describe a novel, sensitive, and simple bioassay for nitric oxide release from neural tissue in vitro.

Effects of dizocilpine on autotomy behavior after nerve section in mice

The effects of acute administration of dizocilpine (MK-801) at different perioperative times on autotomy behavior after sciatic and saphenous nerve transection were studied in the mouse. Control mice developed a severe self-mutilating behavior starting 1-3 days postoperation and reaching a maximum by 11 days. Mice injected with a single dose of dizocilpine (0.4 mg/kg i.p.) before operation, the 1st or 3rd postoperative day autotomized significantly less than controls. An 1-wk treatment with the same dose once a day did not show further benefit. A single administration of dizocilpine the 5th day after surgery slightly halted further progression of autotomy. Dizocilpine did not have any deleterious effect on normal peripheral nerve function. These results suggest that NMDA receptor blockade prevents development of hyperalgesia and neuropathic pain after peripheral nerve injuries but only when it is administered before or during the first 3 days after injury.

Acute and conditioned hyperalgesic responses to illness

It has been argued that pain functions to facilitate recovery from injury and/or illness by stimulating recuperative behaviors. If this is the case, then hyperalgesia might be expected to be part of the constellation of adaptations that occur during sickness. The present series of studies tested two agents that induce illness (lithium chloride and bacterial cell-wall endotoxin (lipopolysaccharide)) to determine their acute effects on pain responsivity in rats. Both agents produced hyperalgesia as measured by the tail-flick and formalin tests. This enhanced responsivity appears to be specific to pain since (a) no enhanced response was observed to a non-painful stimulus (6 g von Frey hairs), and (b) the effect could not be accounted for by changes in tail skin temperature. In addition, a conditioned taste aversion paradigm was used to examine the possibility that illness-induced hyperalgesia could be conditioned to a novel taste (saccharine). This procedure was successful in producing a conditioned hyperalgesia which was comparable in magnitude and duration to acute illness induced pain facilitation. Taken together, this series of studies suggests that such pain facilitation might have adaptive functions similar to those ascribed to other illness-induced behaviors.

Topical application of capsaicin for treatment of oral neuropathic pain and trigeminal neuralgia

Neuropathic pain may be a major cause of pain in the head and neck. Trigeminal neuralgia may appear as intraoral pain. This article reviews a series of 24 consecutive cases of oral pain treated with topical capsaicin. Complete remission of neuropathic pain was seen in 31.6% of patients; partial remission was achieved in 31.6% of patients. Trigeminal neuralgia with an intraoral trigger was less responsive to topical therapy than neuropathic pain. Further study is needed to clarify the efficacy of topical capsaicin in neuropathic and neuralgic pain and the effect of differing dosages and frequency of application. On the basis of the findings in this open-label clinical trial, controlled clinical study of capsaicin in neuropathic oral pain states appears warranted.

Inhibition of opiate tolerance by non-competitive N-methyl-D-aspartate receptor antagonists

Our laboratory and others have previously reported that the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, interferes with the development of tolerance to the analgesic effects of morphine. The present studies were performed in order to further characterize the role of NMDA receptors in opiate tolerance. The results demonstrate that opiate tolerance is inhibited rapidly, and at low doses, by four different non-competitive NMDA receptor antagonists (MK-801, ketamine, dextrorphan and phencyclidine), suggesting that this inhibition results from blockade of NMDA receptors rather than from the 'side-effect' of a particular drug. The NMDA antagonists were found to inhibit the development but not the expression of opiate tolerance; i.e. they were able to prevent but not reverse tolerance. Finally, the results suggest that NMDA receptor antagonists do not interfere with associative tolerance; instead it appears that these drugs may specifically inhibit non-associative tolerance. It thus appears that NMDA receptors may have a fundamental role in the development of opiate tolerance, and that non-competitive NMDA receptor antagonists may be effective adjuncts to opiates in the treatment of chronic pain.

Effects of extracorporeal shock-wave lithotripsy on referred hyperalgesia from renal/ureteral calculosis

In patients suffering from colics due to calculosis of one upper urinary tract the evolution in time of referred parietal hyperalgesia after stone fragment elimination promoted by extracorporeal shock wave lithotripsy (ESWL) was studied. Before ESWL, all patients presented clinical evidence (positivity to dermographism and Head's procedure, pinch palpation, digital pressure and Giordano's manoeuver) and instrumental signs (significant lowering of pain threshold to electrical tissue stimulation) of cutaneous, subcutaneous and muscular tissue hyperalgesia in the lumbar region of the affected side. After ESWL, hyperalgesia decreased in the three tissues, as shown by progressive change in the clinical tests and an increase in pain threshold to electrical stimulation in relation to the extent of stone fragment expulsion. In the stone-free condition, hyperalgesia had disappeared in the skin but remained to a mild and moderate extent in the subcutaneous tissue and muscle respectively. It is concluded that the persistence in time of referred hyperalgesia is only in part linked to the continuing presence and activity of the stone in the urinary tract. To a certain extent, the phenomenon seems to become independent of the primary focus, possibly as a result of plastic neuronal changes in the central nervous system which, triggered by afferent visceral inputs, are maintained even after their removal.

Effects of Kelatorphan and morphine before and after noxious stimulation on immediate-early gene expression in rat spinal cord neurons

Expression of the immediate-early genes (IEG) c-FOS, NGF1-A and c-JUN was induced by noxious thermal stimulation in neurons of the rat spinal cord dorsal horn. Intravenous injection of Kelatorphan (5, 10 and 20 mg/kg), an inhibitor of multiple enkephalin-degrading enzymes, 20 min before noxious stimulation reduced the overall number of dorsal horn neurons expressing c-FOS and NGF1-A by up to 20-30%. While c-FOS expression was suppressed in superficial and deep laminae of the spinal cord, NGF1-A and c-JUN was only suppressed in superficial laminae. Morphine (5, 7.5 and 10 mg/kg) produced a dose-dependent reduction of c-FOS expression by up to 70% only when injected before noxious stimulation. Morphine injected 10 min after the noxious treatment was virtually ineffective. The depressant effect of Kelatorphan and morphine could be prevented by prior application of the opioid antagonist naloxone. Naloxone itself slightly increased the overall number of c-FOS-positive neurons in all laminae of the spinal cord. The present data support the existence of a tonic release of endogenous opioid peptides at the spinal level and show that inhibition of their peptidase-induced degradation modulates IEG expression in dorsal horn neurons of the rat. The finding that opioid agonists were ineffective when applied after stimulation underline the necessity of pre-emptive analgesia to prevent long-term activity-dependent changes in spinal cord neurons.

Application of morphine prior to noxious stimulation differentially modulates expression of Fos, Jun and Krox-24 proteins in rat spinal cord neurons

The expression of Fos, Jun and Krox-24 proteins was investigated in spinal cord neurons of the rat 2, 4 and 8 h following noxious thermal stimulation of one hind-paw and pre-treatment with morphine. The number of neurons expressing c-Fos, c-Jun, Jun B and Krox-24 were maximal after 2 h and thereafter declined. The number of Fos B and Jun D immunoreactive neurons increased constantly for up to 8 h with Jun D showing expression above baseline only after 4 h following stimulation. Intravenous application of morphine (5 and 10 mg/kg) 20 min before noxious heat stimulation decreased the expression of all six proteins at any time-point with a predilective effect on neurons of deeper laminae of the dorsal horn. The suppressive effects of morphine were more pronounced with the higher dose of morphine and completely reversed by intravenous naloxone (1 and 10 mg/kg). The temporospatial patterns of expression following morphine were similar to those seen without morphine, but in a much smaller number of neurons and with a shorter time-course. However, despite the high dose of morphine and continuous halothane anaesthesia during the whole experimental procedures, a considerable number of neurons expressing the various genes remained in all laminae of the spinal cord. At 2 h following noxious heat stimulation morphine had decreased the number of labelled neurons for c-Fos, Fos B, Krox-24, c-Jun and Jun B to 30-60% of control levels in laminae I-II and to 10-30% in laminae III-VII,X of the spinal cord. At 4 h the level of reduction had further increased while Jun D was only moderately reduced to 75% in all laminae of the spinal cord. Eight hours following noxious heat plus morphine application we did not detect noxious evoked immunoreactivity for c-Fos, Krox-24, c-Jun and Jun B, while there was residual labelling for Fos B in the superficial dorsal horn and for Jun D in laminae I-VII and X of the spinal cord. The different temporospatial pattern of immediate early gene expression in neurons of the spinal cord dorsal horn following noxious stimulation suggest that variable transcription complexes may interact with DNA regulatory sequences and could thus activate alternative secondary response genes, even under protection of a high dosage of morphine applied before noxious stimulation.

NMDA receptor antagonists attenuate mechanical hyperalgesia in rats with unilateral inflammation of the hindpaw

The effects of N-methyl-D-aspartate (NMDA) receptor antagonists on mechanical hyperalgesia associated with tissue inflammation were studied. Following an injection of the inflammatory agent, complete Freund's adjuvant, into the rat hindpaw, there was a significant decrease in threshold and an increase in response duration to mechanical stimuli, suggesting that a state of mechanical hyperalgesia was induced. The intrathecal administration of the NMDA receptor antagonists, dizocilpine maleate and (+/-)-2-amino-5-phosphonopentanoic acid, significantly increased mechanical threshold and reduced response duration in the inflamed hindpaw, but had no effect on the non-injected paw. The results suggest that NMDA receptor activation may contribute to the mechanical hyperalgesia that follows peripheral tissue inflammation.

First onset of common pain symptoms: a prospective study of depression as a risk factor

This report examined first onset rates of 5 common pain symptoms and assessed, on a prospective basis, whether depressive symptoms at baseline were associated with onset risks. Adult Health Maintenance Organization (HMO) enrollees (n = 1016) were interviewed in 1986 about their history of each of 5 pain conditions (back pain, severe headache, chest pain, abdominal pain and temporomandibular disorder (TMD) pain). Three years later, this sample was re-interviewed (n = 803) to measure site-specific first onset rates. The Symptom Checklist 90-Revised (SCL-90-R) Depression scale was administered at baseline and at 3 year follow-up. Over the three year follow-up interval, rates of first onset were: 17.7% for back pain; 4.2% for severe headache; 3.0% for chest pain; 3.1% for abdominal pain; and 6.5% for TMD pain. Onset rates of persistent pain and of chronic pain dysfunction were substantially lower, but over 1% experienced onset of chronic pain dysfunction for back pain and for headache. There were not significant differences in onset rates of back pain, abdominal pain or TMD pain by severity or chronicity of depressive symptoms. Relative to the non-depressed, persons with moderate-to-severe depressive symptoms were more likely to develop headache and chest pain (adjusted odds ratios of 1.7 to 5.0). For headache and chest pain, onset risks were highest among the chronically depressed.(ABSTRACT TRUNCATED AT 250 WORDS)

Interleukin-1 mediates the behavioral hyperalgesia produced by lithium chloride and endotoxin

The sickness-inducing agents lithium chloride (LiCl) and lipopolysaccharide (LPS) produce a long-lasting facilitation of the nociceptive tailflick reflex. Many of the behavioral and physiological changes produced by illness are mediated by interleukin-1 (IL-1) released from monocytes stimulated by the pathogenic substance. Monocytes also produce an IL-1 receptor antagonist (IL-1ra) which has been sequenced and cloned. The present experiments report that IL-1 can itself produce hyperalgesia as assessed by tailflick to radiant heat, and that recombinant IL-1ra blocks the hyperalgesia produced by LiCl and LPS.

Central hyperexcitability triggered by noxious inputs

Repetitive activity in unmyelinated sensory afferent neurones, arising from electrical stimuli, tissue injury or nerve damage, can induce long-lasting sensitization in dorsal horn neurones. This process can be blocked by antagonists of the NMDA receptor. In the past year it has emerged that sensory neuropeptides and nitric oxide are also essential mediators of this phenomenon.

The pharmacology of pain signalling

Recent advances in pain research illustrate the analytical power of modern neurosciences in a field previously accessible only to methods of systems biology. Novel molecular and cellular biological techniques have changed the face of pain research by detailing the multiplicity of pain transducing and pain suppressive systems which involve neuronal and hormonal systems acting in concert to help the individual to cope with pain. The introduction of concepts of neuronal plasticity in this field has led to important therapeutical consequences. Novel compounds and new regimens for drug treatment to prevent activity-dependent long-term changes or to facilitate extinction in pain-related systems are emerging.