Contribution of microvascular dysfunction to chronic pain

There is growing evidence that microvascular dysfunction is a pathology accompanying various injuries and conditions that produce chronic pain and may represent a significant contributing factor. Dysfunction that occurs within each component of the microvasculature, including arterioles, capillaries and venules impacts the health of surrounding tissue and produces pathology that can both initiate pain and influence pain sensitivity. This mini review will discuss evidence for a critical role of microvascular dysfunction or injury in pathologies that contribute to chronic pain conditions such as complex regional pain syndrome (CRPS) and fibromyalgia.

Novel Co-crystal of Pentoxifylline and Protocatechuic Acid Relieves Allodynia in Rat Models of Peripheral Neuropathic Pain and CRPS by Alleviating Local Tissue Hypoxia

Local tissue ischemic hypoxia is a peripheral process that can be targeted with topical treatment to alleviate pain under chronic pain conditions such as complex regional pain syndrome (CRPS) and peripheral neuropathic pain. We recently reported three novel salts and a co-crystal composed of vasoactive agents and antioxidant nutraceuticals, all of which produced potent topical anti-allodynic effects in the chronic postischemic pain (CPIP) rat model of CRPS. One of the products, pentx-pca, is a co-crystal synthesized from pentoxifylline (pentx) and protocatechuic acid (pca). Pentx-pca exhibited potent topical anti-allodynic effects in CPIP and rats with chronic constriction injury of the sciatic nerve exceeding effects produced individually by pentx and pca. We hypothesized that the anti-allodynic effects of pentx-pca in CPIP rats were due to its impact on local tissue oxygenation and subsequent oxygen-dependent mitochondrial respiration. Percutaneous tissue oxygen saturation (SaO₂) measurements taken from the hind paw of the CPIP rats revealed that anti-allodynic doses of topical pentx-pca increased local tissue SaO₂. Moreover, assessment of the oxygen-dependent mitochondrial function using a triphenyl tetrazolium chloride assay revealed that mitochondrial dysfunction significantly declined in the plantar muscle collected from CPIP rats topically treated with anti-allodynic doses of pentx-pca as compared to vehicle-treated CPIP rats. Furthermore, time-dependent resolution of plantar muscle mitochondrial dysfunction, that occurred in the CPIP rats at 6-week post procedure, paralleled the loss of the anti-allodynic response to topical treatment with pentx-pca. Our results indicated that pentx-pca produced potent anti-allodynic effects in the CPIP rat model of CRPS by alleviating peripheral tissue ischemia/hypoxia and downstream hypoxia-driven mitochondrial dysfunction.

The effect of a topical combination of clonidine and pentoxifylline on post-traumatic neuropathic pain patients: study protocol for a randomized, double-blind placebo-controlled trial

BACKGROUND

First-line pharmacotherapy for neuropathic pain entails the use of systemic antidepressants and anticonvulsants. These drugs are not optimally effective and poorly tolerated, especially for older patients with comorbid conditions. Given the high number of such patients, there is a need for a greater repertoire of safer and more effective analgesics. Clonidine and pentoxifylline are vasodilator agents that work synergistically to enhance tissue perfusion and oxygenation. The topical administration of these drugs, individually and in combination, has shown anti-nociceptive properties in rodent models of neuropathic pain. A topically-administered combination of clonidine and pentoxifylline also effectively reduced the intensity of both spontaneous and evoked pain in healthy volunteers with experimentally-induced neuropathic pain. The next step in advancing this formulation to clinical use is the undertaking of a phase II clinical study to assess its efficacy and safety in neuropathic pain patients.

METHODS/DESIGN

This is a study protocol for a randomized, double-blind, placebo-controlled, phase II clinical trial with a cross-over design. It is a single-centered, 5-week study that will enroll a total of 32 patients with post-traumatic peripheral neuropathic pain. Patients will be treated topically with either a combination of clonidine and pentoxifylline or placebo for a period of 2 weeks each, in randomly assigned order across patients, with an intervening washout period of 1 week. The primary outcome measures of the study are the intensity of spontaneous pain recorded daily in a pain diary with a visual analog scale, and the degree of mechanical allodynia evoked by a brush stimulus. The secondary outcome measures of the study include scores of pain relief and change in the area of punctate hyperalgesia. This trial has been prospectively registered with ClinicalTrials.gov on November 1, 2017. ClinicalTrials.gov Identifier: NCT03342950 .

DISCUSSION

The analgesic use of topical treatment with clonidine and pentoxifylline in combination has not been investigated in post-traumatic neuropathic pain. This study could generate the first evidence for the efficacy and safety of the formulation in alleviating pain in patients with neuropathic pain. Furthermore, this trial will provide objective grounds for the investigation of other agents that enhance tissue oxygenation in the topical treatment of peripheral neuropathic pain.

TRIAL REGISTRATION

This trial has been registered with ClinicalTrials.gov owned by NIH's US National Library of Medicine. ClinicalTrials.gov NCT03342950 . Registered on November 1, 2017 (trial was prospectively registered).

PROTOCOL VERSION AND IDENTIFIERS

This is protocol version 5, dated June 2018. McGill University Health Center (MUHC) Reaseach Ethics Board (REB) identification number: TTNP 2018-3906.

Sex differences in the contributions of spinal atypical PKCs and downstream targets to the maintenance of nociceptive sensitization

Background

Chronic pain has been shown to depend on nociceptive sensitization in the spinal cord, and while multiple mechanisms involved in the initiation of plastic changes have been established, the molecular targets which maintain spinal nociceptive sensitization are still largely unknown. Building upon the established neurobiology underlying the maintenance of long-term potentiation in the hippocampus, this present study investigated the contributions of spinal atypical protein kinase C (PKC) isoforms PKCι/λ and PKMζ and their downstream targets (p62/GluA1 and NSF/GluA2 interactions, respectively) to the maintenance of spinal nociceptive sensitization in male and female rats.

Results

Pharmacological inhibition of atypical PKCs by ZIP reversed established allodynia produced by repeated intramuscular acidic saline injections in male animals only, replicating previously demonstrated sex differences. Inhibition of both PKCι/λ and downstream substrates p62/GluA1 resulted in male-specific reversals of intramuscular acidic saline-induced allodynia, while female animals continued to display allodynia. Inhibition of NSF/GluA2, the downstream target to PKMζ, reversed allodynia induced by intramuscular acidic saline in both sexes. Neither PKCι/λ, p62/GluA1 or NSF/GluA2 inhibition had any effect on formalin response for either sex.

Conclusion

This study provides novel behavioural evidence for the male-specific role of PKCι/λ and downstream target p62/GluA1, highlighting the potential influence of ongoing afferent input. The sexually divergent pathways underlying persistent pain are shown here to converge at the interaction between NSF and the GluA2 subunit of the AMPA receptor. Although this interaction is thought to be downstream of PKMζ in males, these findings and previous work suggest that females may rely on a factor independent of atypical PKCs for the maintenance of spinal nociceptive sensitization.

Topical drug therapeutics for neuropathic pain

INTRODUCTION

Neuropathic pain (NP) is a particularly severe and intractable chronic condition that is not well treated by commonly recommended systemic pharmacological therapies, partly due to dose-limiting side effects or adverse events. The use of topical therapeutics for NP is growing and benefits from the reduced potential for adverse effects, as well as the ability to directly target peripheral pathological processes.

AREAS COVERED

The current review defines and describes the limitations of various commonly prescribed systemic pharmacological therapies for NP. It also provides a justification for increased research aimed at developing topical therapeutics for NP, particularly localized and peripheral NP. The review discusses the various classes of topical treatments used for NP, including agents that: block sensory inputs; activate inhibitory systems; provide mechanism-based therapeutics; are used in mucosal tissues; and include combinations that produce multimodal therapeutic effects.

EXPERT OPINION

There are arguments that the current topical therapeutics for NP rely too heavily on the use of local anesthetics and capsaicinoids, and more research is certainly needed on topical therapies that are multimodal and/or are targeted at the peripheral sources of pathology. The potential for novel topical therapeutics may be enhanced by further research on topical co-drugs, drug-drug salts, co-crystals and hydrates, and ionic liquids.

Mild Social Stress in Mice Produces Opioid-Mediated Analgesia in Visceral but Not Somatic Pain States

Visceral pain has a greater emotional component than somatic pain. To determine if the stress-induced analgesic response is differentially expressed in visceral versus somatic pain states, we studied the effects of a mild social stressor in either acute visceral or somatic pain states in mice. We show that the presence of an unfamiliar conspecific mouse (stranger) in an adjacent cubicle of a standard transparent observation box produced elevated plasma corticosterone levels compared with mice tested alone, suggesting that the mere presence of a stranger is stressful. We then observed noxious visceral or somatic stimulation-induced nociceptive behavior in mice tested alone or in mildly stressful conditions (ie, beside an unfamiliar stranger). Compared with mice tested alone, the presence of a stranger produced a dramatic opioid-dependent reduction in pain behavior associated with visceral but not somatic pain. This social stress-induced reduction of visceral pain behavior relied on visual but not auditory/olfactory cues. These findings suggest that visceral pain states may provoke heightened responsiveness to mild stressors, an effect that could interfere with testing outcomes during simultaneous behavioral testing of multiple rodents.

PERSPECTIVE

In mice, mild social stress due to the presence of an unfamiliar conspecific mouse reduces pain behavior associated with noxious visceral but not somatic stimulation, suggesting that stress responsiveness may be enhanced in visceral pain versus somatic pain states.

Consistent sex-dependent effects of PKMζ gene ablation and pharmacological inhibition on the maintenance of referred pain

Background

Persistently active PKMζ has been implicated in maintaining spinal nociceptive sensitization that underlies pain hypersensitivity. However, evidence for PKMζ in the maintenance of pain hypersensitivity comes exclusively from short-term studies in males using pharmacological agents of questionable selectivity. The present study examines the contribution of PKMζ to long-lasting allodynia associated with neuropathic, inflammatory, or referred visceral and muscle pain in males and females using pharmacological inhibition or genetic ablation.

Results

Pharmacological inhibition or genetic ablation of PKMζ reduced mild formalin pain and slowly developing contralateral allodynia in nerve-injured rats, but not moderate formalin pain or ipsilateral allodynia in models of neuropathic and inflammatory pain. Pharmacological inhibition or genetic ablation of PKMζ also effectively reduced referred visceral and muscle pain in male, but not in female mice and rats.

Conclusion

We show pharmacological inhibition and genetic ablation of PKMζ consistently attenuate long-lasting pain hypersensitivity. However, differential effects in models of referred versus inflammatory and neuropathic pain, and in males versus females, highlight the roles of afferent input-dependent masking and sex differences in the maintenance of pain hypersensitivity.

A Cyclic Tetrapeptide ("Cyclodal") and Its Mirror-Image Isomer Are Both High-Affinity μ Opioid Receptor Antagonists

Head-to-tail cyclization of the μ opioid receptor (MOR) agonist [Dmt¹]DALDA (H-Dmt-d-Arg-Phe-Lys-NH₂ (9; Dmt = 2',6'-dimethyltyrosine) resulted in a highly active, selective MOR antagonist, c[-d-Arg-Phe-Lys-Dmt-] (1) ("cyclodal"), with subnanomolar binding affinity. A docking study of cyclodal using the crystal structure of MOR in the inactive form showed a unique binding mode with the two basic residues of the ligand forming salt bridges with the Asp¹²⁷ and Glu²²⁹ receptor residues. Cyclodal showed high plasma stability and was able to cross the blood-brain barrier to reverse morphine-induced, centrally mediated analgesia when given intravenously. Surprisingly, the mirror-image isomer (optical antipode) of cyclodal, c[-Arg-d-Phe-d-Lys-d-Dmt-] (2), also turned out to be a selective MOR antagonist with 1 nM binding affinity, and thus, these two compounds represent the first example of mirror image opioid receptor ligands with both optical antipodes having high binding affinity. Reduction of the Lys-Dmt peptide bond in cyclodal resulted in an analogue, c[-d-Arg-Phe-LysΨ[CH₂NH]Dmt-] (8), with MOR agonist activity.

The bifunctional μ opioid agonist/antioxidant [Dmt(1)]DALDA is a superior analgesic in an animal model of complex regional pain syndrome-type i

Reactive oxygen species (ROS) play an important role in the development of complex regional pain syndrome-Type I (CRPS-I), as also demonstrated with the chronic post ischemia pain (CPIP) animal model of CRPS-I. We show that morphine and the antioxidant N-acetylcysteine (NAC) act synergistically to reduce mechanical allodynia in CPIP rats. The tetrapeptide amide [Dmt(1)]DALDA (H-Dmt-d-Arg-Phe-Lys-NH2) is a potent and selective μ opioid receptor (MOR) agonist with favorable pharmacokinetic properties and with antioxidant activity due to its N-terminal Dmt (2',6'-dimethyltyrosine) residue. In the CPIP model, [Dmt(1)]DALDA was 15-fold more potent than morphine in reversing mechanical allodynia and 4.5-fold more potent as analgesic in the heat algesia test. The results indicate that bifunctional compounds with MOR agonist/antioxidant activity have therapeutic potential for the treatment of CRPS-I.

Topical combinations to treat microvascular dysfunction of chronic postischemia pain

BACKGROUND

Growing evidence indicates that patients with complex regional pain syndrome (CRPS) exhibit tissue abnormalities caused by microvascular dysfunction in the blood vessels of skin, muscle, and nerve. We tested whether topical combinations aimed at improving microvascular function would relieve allodynia in an animal model of CRPS. We hypothesized that topical administration of either α2-adrenergic (α2A) receptor agonists or nitric oxide (NO) donors given to increase arterial blood flow, combined with either phosphatidic acid (PA) or phosphodiesterase (PDE) inhibitors to increase capillary blood flow, would effectively reduce allodynia and signs of microvascular dysfunction in the animal model of chronic pain.

METHODS

Mechanical allodynia was induced in the hindpaws of rats with chronic postischemia pain (CPIP). Allodynia was assessed before and after topical application of vehicle, single drugs or combinations of an α2A receptor agonist (apraclonidine) or an NO donor (linsidomine), with PA or PDE inhibitors (lisofylline, pentoxifylline). A topical combination of apraclonidine + lisofylline was also evaluated for its effects on a measure of microvascular function (postocclusive reactive hyperemia) and tissue oxidative capacity (formazan production by tetrazolium reduction) in CPIP rats.

RESULTS

Each of the single topical drugs produced significant dose-dependent antiallodynic effects compared with vehicle in CPIP rats (N = 30), and the antiallodynic dose-response curves of either PA or PDE inhibitors were shifted 5- to 10-fold to the left when combined with nonanalgesic doses of α2A receptor agonists or NO donors (N = 28). The potent antiallodynic effects of ipsilateral treatment with combinations of α2A receptor agonists or NO donors with PA or PDE inhibitors were not reproduced by the same treatment of the contralateral hindpaw (N = 28). Topical combinations produced antiallodynic effects lasting up to 6 hours (N = 15) and were significantly enhanced by low-dose systemic pregabalin in early, but not late, CPIP rats (N = 18). An antiallodynic topical combination of apraclonidine + lisofylline was also found to effectively relieve depressed postocclusive reactive hyperemia in CPIP rats (N = 61) and to increase formazan production in postischemic tissues (skin and muscle) (N = 56).

CONCLUSIONS

The present results support the hypothesis that allodynia in an animal model of CRPS is effectively relieved by topical combinations of α2A receptor agonists or NO donors with PA or PDE inhibitors. This suggests that topical treatments aimed at improving microvascular function by increasing both arterial and capillary blood flow produce effective analgesia for CRPS.

Pentoxifylline reduces chronic post-ischaemia pain by alleviating microvascular dysfunction

BACKGROUND

Microvascular dysfunction and ischaemia in muscle play a role in the development of cutaneous tactile allodynia in chronic post-ischaemia pain (CPIP). Hence, studies were designed to assess whether pentoxifylline (PTX), a vasodilator and haemorrheologic agent, relieves allodynia in CPIP rats by alleviating microvascular dysfunction.

METHODS

Laser Doppler flowmetry of plantar blood flow was used to examine the effects of PTX on CPIP-induced alterations in post-occlusive reactive hyperaemia (reflecting microvascular dysfunction), and von Frey testing was used to examine its effects on CPIP-induced allodynia. Time-course effects of PTX on allodynia and microvascular dysfunction were assessed early (2-8 days) and late (18-25 days) post-ischaemia/reperfusion (I/R) injury, and its effects on allodynia were also tested at 30 days post-I/R injury.

RESULTS

PTX (25 mg/kg) produced significant anti-allodynic effects throughout the 21-day time course, but was not effective 30 days post-I/R injury. In laser Doppler studies, the reduced reactive hyperaemia in early CPIP rats was significantly improved by PTX (25 mg/kg). Conversely, treatment with PTX at the same dose did not affect reactive hyperaemia in late CPIP rats, likely since reactive hyperaemia was not significantly reduced pre-drug in these animals.

CONCLUSION

Since poor tissue perfusion underlies early stages of CPIP pain, the ameliorative effect of PTX on microvascular dysfunction might account for its anti-allodynic effect in our experimental model of complex regional pain syndrome type I.

Systemic pregabalin attenuates facial hypersensitivity and noxious stimulus-evoked release of glutamate in medullary dorsal horn in a rodent model of trigeminal neuropathic pain

Pregabalin is effective in treating many neuropathic pain conditions. However, the mechanisms of its analgesic effects remain poorly understood. The aim of the present study was to determine whether pregabalin suppresses facial mechanical hypersensitivity and evoked glutamate release in the medullary dorsal horn (MDH) in a rodent model of trigeminal neuropathic pain. Nociceptive mechanical sensitivity was assessed pre-operatively, and then post-operatively 1h following pregabalin or vehicle (saline) treatment on post-operative days 2 and 5 following infraorbital nerve transection (IONX). In addition, an in vivo microdialysis probe was inserted into the exposed medulla post-operatively and dialysate samples were collected. Glutamate release was then evoked by mustard oil (MO) application to the tooth pulp, and the effects of pregabalin or vehicle were examined on the MDH glutamate release. Glutamate concentrations in the dialysated samples were determined by HPLC, and data analyzed by ANOVA. IONX animals (but not control animals) showed facial mechanical hypersensitivity for several days post-operatively. In addition, tooth pulp stimulation with MO evoked a transient release of glutamate in the MDH of IONX animals. Compared to vehicle, administration of pregabalin significantly attenuated the facial mechanical hypersensitivity as well as the MO-evoked glutamate release in MDH. This study provides evidence in support of recent findings pointing to the usefulness of pregabalin in the treatment of orofacial neuropathic pain.

Involvement of ATP in noxious stimulus-evoked release of glutamate in rat medullary dorsal horn: a microdialysis study

Our electrophysiological studies have shown that both purinergic and glutamatergic receptors are involved in central sensitization of nociceptive neurons in the medullary dorsal horn (MDH). Here we assessed the effects of intrathecal administration of apyrase (a nucleotide degrading enzyme of endogenous adenosine 5-triphosphate [ATP]), a combination of apyrase and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, an adenosine A1 receptor antagonist), or 2,3-O-2,4,6-trinitrophenyl-adenosine triphosphate (TNP-ATP, a P2X1, P2X3, P2X2/3 receptor antagonist) on the release of glutamate in the rat MDH evoked by application of mustard oil (MO) to the molar tooth pulp. In vivo microdialysis was used to dialyse the MDH every 5 min, and included 3 basal samples, 6 samples after drug treatment and 12 samples following application of MO. Tooth pulp application of MO induced a significant increase in glutamate release in the MDH. Superfusion of apyrase or TNP-ATP alone significantly reduced the MO-induced glutamate release in the MDH, as compared to vehicle. Furthermore, the suppressive effects of apyrase on glutamate release were reduced by combining it with DPCPX. This study demonstrates that application of an inflammatory irritant to the tooth pulp induces glutamate release in the rat MDH in vivo that may be reduced by processes involving endogenous ATP and adenosine.

Systemic pregabalin attenuates sensorimotor responses and medullary glutamate release in inflammatory tooth pain model

Our previous studies have demonstrated that application of inflammatory irritant mustard oil (MO) to the tooth pulp induces medullary glutamate release and central sensitization in the rat medullary dorsal horn (MDH), as well as nociceptive sensorimotor responses in craniofacial muscles in rats. There is recent evidence that anticonvulsant drugs such as pregabalin that influence glutamatergic neurotransmission are effective in several pain states. The aim of this study was to examine whether systemic administration of pregabalin attenuated glutamate release in the medulla as well as these nociceptive effects reflected in increased electromyographic (EMG) activity induced by MO application to the tooth pulp. Male adult rats were anesthetized with isofluorane (1.0-1.2%), and jaw and tongue muscle EMG activities were recorded by needle electrodes inserted bilaterally into masseter and anterior digastric muscles and into the genioglossus muscle, and also the medullary release of glutamate was assessed by in vivo microdialysis. Pregabalin or vehicle control (isotonic saline) was administered 30 min before the pulpal application of MO or vehicle control (mineral oil). Application of mineral oil to the maxillary first molar tooth pulp produced no change in baseline EMG activity and glutamate release. However, application of MO to the pulp significantly increased both the medullary release of glutamate and EMG activity in the jaw and tongue muscles for several minutes. In contrast, pre-medication with pregabalin, but not vehicle control, significantly and dose-dependently attenuated the medullary glutamate release and EMG activity in these muscles after MO application to the tooth pulp (analysis of variance (ANOVA), p<0.05). These results suggest that pregabalin may attenuate the medullary release of glutamate and associated nociceptive sensorimotor responses in this acute inflammatory pulpal pain model, and that it may prove useful for the treatment of orofacial inflammatory pain states.

PKMζ is essential for spinal plasticity underlying the maintenance of persistent pain

BACKGROUND

Chronic pain occurs when normally protective acute pain becomes pathologically persistent. We examined here whether an isoform of protein kinase C (PKC), PKMζ, that underlies long-term memory storage in various brain regions, also sustains nociceptive plasticity in spinal cord dorsal horn (SCDH) mediating persistent pain.

RESULTS

Cutaneous injury or spinal stimulation produced persistent increases of PKMζ, but not other atypical PKCs in SCDH. Inhibiting spinal PKMζ, but not full-length PKCs, reversed plasticity-dependent persistent painful responses to hind paw formalin and secondary mechanical hypersensitivity and SCDH neuron sensitization after hind paw capsaicin, without affecting peripheral sensitization-dependent primary heat hypersensitivity after hind paw capsaicin. Inhibiting spinal PKMζ, but not full-length PKCs, also reversed mechanical hypersensitivity in the rat hind paw induced by spinal stimulation with intrathecal dihydroxyphenylglycine. Spinal PKMζ inhibition also alleviated allodynia 3 weeks after ischemic injury in rats with chronic post-ischemia pain (CPIP), at a point when allodynia depends on spinal changes. In contrast, spinal PKMζ inhibition did not affect allodynia in rats with chronic contriction injury (CCI) of the sciatic nerve, or CPIP rats early after ischemic injury, when allodynia depends on ongoing peripheral inputs.

CONCLUSIONS

These results suggest spinal PKMζ is essential for the maintenance of persistent pain by sustaining spinal nociceptive plasticity.

A hypothesis for the cause of complex regional pain syndrome-type I (reflex sympathetic dystrophy): pain due to deep-tissue microvascular pathology

Complex regional pain syndrome-type I (CRPS-I; reflex sympathetic dystrophy) is a chronic pain condition that usually follows a deep-tissue injury such as fracture or sprain. The cause of the pain is unknown. We have developed an animal model (chronic post-ischemia pain) that creates CRPS-I-like symptomatology. The model is produced by occluding the blood flow to one hind paw for 3 hours under general anesthesia. Following reperfusion, the treated hind paw exhibits an initial phase of hyperemia and edema. This is followed by mechano-hyperalgesia, mechano-allodynia, and cold-allodynia that lasted for at least 1 month. Light microscopic analyses and electron microscopic analyses of the nerves at the site of the tourniquet show that the majority of these animals have no sign of injury to myelinated or unmyelinated axons. However, electron microscopy shows that the ischemia-reperfusion injury produces a microvascular injury, slow-flow/no-reflow, in the capillaries of the hind paw muscle and digital nerves. We propose that the slow-flow/no-reflow phenomenon initiates and maintains deep-tissue ischemia and inflammation, leading to the activation of muscle nociceptors, and the ectopic activation of sensory afferent axons due to endoneurial ischemia and inflammation. These data, and a large body of clinical evidence, suggest that in at least a subset of CRPS-I patients, the fundamental cause of the abnormal pain sensations is ischemia and inflammation due to microvascular pathology in deep tissues, leading to a combination of inflammatory and neuropathic pain processes. Moreover, we suggest a unifying idea that relates the pathogenesis of CRPS-I to that of CRPS-II. Lastly, our hypothesis suggests that the role of the sympathetic nervous system in CRPS-I is a factor that is not fundamentally causative, but may have an important contributory role in early-stage disease.

Role of peripheral endothelin receptors in an animal model of complex regional pain syndrome type 1 (CRPS-I)

Chronic post-ischemic pain (CPIP) is an animal model of CRPS-I developed using a 3-h ischemia-reperfusion injury of the rodent hind paw. The contribution of local endothelin to nociception has been evaluated in CPIP mice by measuring sustained nociceptive behaviors (SNBs) following intraplantar injection of endothelin-1 or -2 (ET-1, ET-2). The effects of local BQ-123 (ETA-R antagonist), BQ-788 (ETB-R antagonist), IRL-1620 (ETB-R agonist) and naloxone (opioid antagonist) were assessed on ET-induced SNBs and/or mechanical and cold allodynia in CPIP mice. ETA-R and ETB-R expression was assessed using immunohistochemistry and Western blot analysis. Compared to shams, CPIP mice exhibited hypersensitivity to local ET-1 and ET-2. BQ-123 reduced ET-1- and ET-2-induced SNBs in both sham and CPIP animals, but not mechanical or cold allodynia. BQ-788 enhanced ET-1- and ET-2-induced SNBs in both sham and CPIP mice, and cold allodynia in CPIP mice. IRL-1620 displayed a non-opioid anti-nociceptive effect on ET-1- and ET-2-induced SNBs and mechanical allodynia in CPIP mice. The distribution of ETA-R and ETB-R was similar in plantar skin of sham and CPIP mice, but both receptors were over-expressed in plantar muscles of CPIP mice. This study shows that ETA-R and ETB-R have differing roles in nociception for sham and CPIP mice. CPIP mice exhibit more local endothelin-induced SNBs, develop a novel local ETB-R agonist-induced (non-opioid) analgesia, and exhibit over-expression of both receptors in plantar muscles, but not skin. The effectiveness of local ETB-R agonists as anti-allodynic treatments in CPIP mice holds promise for novel therapies in CRPS-I patients.

Complex regional pain syndrome: what's in a name?

Within a 2-year period in the 1940s, 2 Boston physicians published dramatically opposing views on the underlying nature of a syndrome now known as complex regional pain syndrome (CRPS). Evans suggested, in several papers in 1946-1947, that sympathetic reflexes maintain pain and dystrophy in affected limbs. Foisie, in 1947, suggested arterial vasospasms were key in the etiology of this pain syndrome. Evans' hypothesis established the nomenclature for this syndrome for 60 years, and his term, "reflex sympathetic dystrophy," guided clinical treatment and research activities over the same period. Foisie's proposed nomenclature was unrecognized, and had virtually no impact on the field. Recent evidence suggests that Evans' contribution to the field may have in fact led clinicians and researchers astray all those years. This focus article on CRPS compares recent observations with these 2 earlier theories and asks the question-what if we had adopted Foisie's nomenclature from the beginning?

PERSPECTIVE

This article discusses 2 opposing historical views on the etiology of what is now known as CRPS, and how they affected nomenclature, research, and clinical therapy in subsequent decades. This focus article may help researchers and clinicians realize the importance of syndrome names, and how they may inadvertently misdirect research and treatment.

Metabotropic glutamate receptors (mGluRs) regulate noxious stimulus-induced glutamate release in the spinal cord dorsal horn of rats with neuropathic and inflammatory pain

In rats with persistent pain, spinal group I metabotropic glutamate receptor (mGluR) activity has been shown to be pronociceptive, whereas spinal group II/III activity is anti-nociceptive. In brain, group I mGluR activity produces positive feedback effects on glutamate release, whereas group II/III activity produces negative feedback effects. It is unknown whether the nociceptive versus anti-nociceptive effects of spinal group I versus group II/III mGluR activity depend on differential regulation of spinal glutamate release. Here, we used behavioral nociceptive testing and in vivo microdialysis to assess the effect of intrathecal treatment with group I mGluR antagonists [cyclopropan[b] chromen-1a-carboxylate, (CPCCOEt), 2-methyl-6-(phenylethynyl) pyridine (MPEP)] or groups II [aminopyrrolidine-2R,4R-dicarboxylate (APDC)] and III [l-2-amino-4-phosphonobutyrate (l-AP4)] mGluR agonists or vehicle, on nociception and noxious stimulus-induced increases in glutamate release in the spinal cord dorsal horn of rats with a chronic constriction injury (CCI) of the sciatic nerve or hind paw injection of complete Freund's adjuvant (CFA). None of the treatments significantly influenced basal spinal glutamate concentrations in either CCI or CFA rats. In CCI rats, formalin-induced nociception and increases in spinal glutamate concentrations were significantly attenuated by pre-treatment with CPCCOEt, MPEP, APDC, or l-AP4. In CFA rats, capsaicin-induced increases in nociception and spinal glutamate concentrations were significantly attenuated by pre-treatment with CPCCOEt, MPEP, or APDC, but not l-AP4. This study demonstrates that group I antagonists and group II/III mGluR agonists attenuated the enhanced nociception and noxious stimulus-induced glutamate release in spinal cord dorsal horn of CCI and/or CFA rats in vivo, and suggests a possible mechanism for their anti-hyperalgesic effects.

Evidence that pregabalin reduces neuropathic pain by inhibiting the spinal release of glutamate

Pregabalin is an anti-convulsant that successfully treats many neuropathic pain syndromes, although the mechanism of its anti-hyperalgesic action remains elusive. This study aims to help delineate pregabalin's anti-hyperalgesic mechanisms. We assessed the effectiveness of pregabalin at decreasing mechanical and cold hypersensitivity induced in a rat model of neuropathic pain. Thus, we compared the effectiveness of pre- or post-treatment with systemic or intrathecal (i.t.) pregabalin at reducing the development and maintenance of the neuropathic pain symptoms. Pregabalin successfully decreased mechanical and cold hypersensitivity, as a pre-treatment, but was less effective at suppressing cold hypersensitivity when administered as a post-treatment. Furthermore, both i.t. and systemic administration of pregabalin were effective in reducing the behavioral hypersensitivity, with the exception of systemic post-treatment on cold hypersensitivity. We also examined pregabalin's effects at inhibiting hind paw formalin-induced nociception in naïve rats and formalin-induced release of excitatory amino acids in the spinal cord dorsal horn (SCDH) both in naïve rats and in rats with neuropathic pain. Pregabalin dose-dependently reduced nociceptive scores in the formalin test. We also present the first evidence that pregabalin reduces the formalin-induced release of glutamate in SCDH. Furthermore, i.t. pregabalin reduces the enhanced noxious stimulus-induced spinal release of glutamate seen in neuropathic rats. These data suggest that pregabalin reduces neuropathic pain symptoms by inhibiting the release of glutamate in the SCDH.

Role of NFkappaB in an animal model of complex regional pain syndrome-type I (CRPS-I)

NFkappaB is involved in several pathogenic mechanisms that are believed to underlie the complex regional pain syndrome (CRPS), including ischemia, inflammation and sensitization. Chronic postischemia pain (CPIP) has been developed as an animal model that mimics the symptoms of CRPS-I. The possible involvement of NFkappaB in CRPS-I was studied using CPIP rats. Under sodium pentobarbital anesthesia, a tourniquet was placed around the rat left ankle joint, producing 3 hours of ischemia, followed by rapid reperfusion (IR injury). NFkappaB was measured in nuclear extracts of muscle and spinal cord tissue using ELISA. Moreover, the anti-allodynic (mechanical and cold) effect was tested for systemic, intrathecal, or intraplantar treatment with the NFkappaB inhibitor pyrrolidine dithiocarbamate (PDTC). At 2 and 48 hours after IR injury, NFkappaB was elevated in muscle and spinal cord of CPIP rats compared to shams. At 7 days, NFkappaB levels were normalized in muscle, but still elevated in spinal cord tissue. Systemic PDTC treatment relieved mechanical and cold allodynia in a dose-dependent manner, lasting for at least 3 hours. Intrathecal-but not intraplantar-administration also relieved mechanical allodynia. The results suggest that muscle and spinal NFkappaB plays a role in the pathogenesis of CPIP and potentially of human CRPS.

PERSPECTIVE

Using the CPIP model, we demonstrate that NFkappaB is involved in the development of allodynia after a physical injury (ischemia and reperfusion) without direct nerve trauma. Since CPIP animals exhibit many features of human CRPS-I, this observation indicates a potential role for NFkappaB in human CRPS.

Regulation of peripheral blood flow in complex regional pain syndrome: clinical implication for symptomatic relief and pain management

BACKGROUND

During the chronic stage of Complex Regional Pain Syndrome (CRPS), impaired microcirculation is related to increased vasoconstriction, tissue hypoxia, and metabolic tissue acidosis in the affected limb. Several mechanisms may be responsible for the ischemia and pain in chronic cold CPRS.

DISCUSSION

The diminished blood flow may be caused by either sympathetic dysfunction, hypersensitivity to circulating catecholamines, or endothelial dysfunction. The pain may be of neuropathic, inflammatory, nociceptive, or functional nature, or of mixed origin.

SUMMARY

The origin of the pain should be the basis of the symptomatic therapy. Since the difference in temperature between both hands fluctuates over time in cold CRPS, when in doubt, the clinician should prioritize the patient's report of a persistent cold extremity over clinical tests that show no difference. Future research should focus on developing easily applied methods for clinical use to differentiate between central and peripheral blood flow regulation disorders in individual patients.

MRI structural brain changes associated with sensory and emotional function in a rat model of long-term neuropathic pain

In human conditions, chronic pain is associated with widespread anatomical changes in the brain. Nevertheless, little is known about the time course of these changes or the relationship of anatomical changes to perception and behaviour. In the present study, we use a rat model of neuropathic pain (spared nerve injury, SNI) and 7 T MRI to determine the longitudinal supraspinal changes associated with pain-like and anxiety-like behaviours. SNI rats and sham controls were scanned at seven time points, 1 week before surgery, 2 weeks after, and then once a month for 5 months. At each time point we performed behavioural tests, including thermal and mechanical sensitivity, and tests of locomotion and exploratory behaviour (open field and elevated plus maze). We found that SNI rats had early and sustained thermal and mechanical hyperalgesia, and developed anxiety-like behaviours several months after injury. Compared to sham controls, SNI rats had decreased frontal cortex volumes several months after surgery, coincident with the onset of anxiety-like behaviours. There was also decreased volume in retrosplenial and entorhinal cortices. We also explored areas that correlated with mechanical hyperalgesia and found that increased hyperalgesia was associated with decreased volumes in bilateral S1 hindlimb area, anterior cingulate cortex (ACC, areas 32 and 24), and insula. Overall, our results suggest that long-term neuropathic pain has widespread effects on brain anatomy related to the duration and magnitude of the pain.

The roles of nerve growth factor and cholecystokinin in the enhancement of morphine analgesia in a rodent model of central nervous system inflammation

Animal models of inflammatory pain are characterized by the release of inflammatory mediators such as cytokines and neurotrophic factors, and enhanced analgesic sensitivity to opioids. In this study, we examine the mechanisms underlying this effect, in particular the roles of cholecystokinin (CCK) and nerve growth factor (NGF), in an animal model of central nervous system (CNS) inflammation induced by spinal administration of lipopolysaccharide (LPS). Although spinal administration of LY-225910 (25 ng), a CCK-B antagonist, enhanced morphine analgesia in naïve rats, it was unable to do so in LPS-treated animals. Conversely, spinal CCK-8S administration (1 ng) decreased morphine analgesia in LPS-treated rats, but not in naïve animals. Further, spinal anti-NGF (3 microg) was able to reduce morphine analgesia in LPS-treated rats, but not in naïve animals, an effect that was reversed by spinal administration of LY-225910. While CCK-8S concentration was increased in spinal cord extracts of LPS animals as compared to controls, morphine-induced spinal CCK release in the extracellular space, as measured by in-vivo spinal cord microdialysis was inhibited in LPS animals as compared to controls, and this was reversed by anti-NGF pretreatment. Finally, chronic spinal administration of beta-NGF (7 microg/day) for 7 days enhanced spinal morphine analgesia, possibly by mimicking a CNS inflammatory state. We suggest that in intrathecally LPS-treated rats, spinal CCK release is altered resulting in enhanced morphine analgesia, and that this mechanism may be regulated to an important extent by NGF.

Cutaneous tactile allodynia associated with microvascular dysfunction in muscle

BACKGROUND

Cutaneous tactile allodynia, or painful hypersensitivity to mechanical stimulation of the skin, is typically associated with neuropathic pain, although also present in chronic pain patients who do not have evidence of nerve injury. We examine whether deep tissue microvascular dysfunction, a feature common in chronic non-neuropathic pain, contributes to allodynia.

RESULTS

Persistent cutaneous allodynia is produced in rats following a hind paw ischemia-reperfusion injury that induces microvascular dysfunction, including arterial vasospasms and capillary slow flow/no-reflow, in muscle. Microvascular dysfunction leads to persistent muscle ischemia, a reduction of intraepidermal nerve fibers, and allodynia correlated with muscle ischemia, but not with skin nerve loss. The affected hind paw muscle shows lipid peroxidation, an upregulation of nuclear factor kappa B, and enhanced pro-inflammatory cytokines, while allodynia is relieved by agents that inhibit these alterations. Allodynia is increased, along with hind paw muscle lactate, when these rats exercise, and is reduced by an acid sensing ion channel antagonist.

CONCLUSION

Our results demonstrate how microvascular dysfunction and ischemia in muscle can play a critical role in the development of cutaneous allodynia, and encourage the study of how these mechanisms contribute to chronic pain. We anticipate that focus on the pain mechanisms associated with microvascular dysfunction in muscle will provide new effective treatments for chronic pain patients with cutaneous tactile allodynia.

Sympathetic vasoconstrictor antagonism and vasodilatation relieve mechanical allodynia in rats with chronic postischemia pain

Chronic pain that responds to antisympathetic treatments and alpha-adrenergic antagonists is clinically referred to as sympathetically maintained pain. Animal models of neuropathic pain have shown mixed results in terms of antinociceptive effectiveness of antisympathetic agents. The effectiveness of these agents have not been yet investigated in animal models of complex regional pain syndrome-type 1 (CRPS-I). In this study, we examined the effectiveness of antisympathetic agents and sympathetic vasoconstrictor antagonists, as well as agents that are vasodilators, in relieving mechanical allodynia in a recently developed animal model of CRPS-I (chronic postischemia pain or CPIP) produced by 3 hours of hind paw ischemia-reperfusion injury. Systemic guanethidine, phentolamine, clonidine, and prazosin are effective in reducing mechanical allodynia particularly at 2 days after reperfusion, and less so at 7 days after reperfusion. A nitric oxide donor vasodilator, SIN-1, also reduces mechanical allodynia more effectively at 2 days after reperfusion, but not at 7 days after reperfusion. These results suggest that the pain of CPIP, and possibly also CRPS-I, is relieved by reducing sympathetically mediated vasoconstriction, or enhancing vasodilatation.

PERSPECTIVE

The results of this study indicate that sympathetic block, or administration of alpha(1)-adrenergic antagonists, clonidine, or a nitric oxide donor, relieve allodynia in an animal model of CRPS-I. Thus, the pain of CRPS-I may depend on enhanced vasoconstrictor responsiveness, which may be relieved by blocking sympathetic efferent-dependent vasoconstriction, or by enhancing nitric oxide-dependent vasodilatation.

Rats with chronic post-ischemia pain exhibit an analgesic sensitivity profile similar to human patients with complex regional pain syndrome--type I

Chronic post-ischemia pain was induced in anesthetized rats by placing a tourniquet at the ankle joint for 3 h, and removing it to allow reperfusion. The effectiveness of standard analgesic drugs to attenuate mechanical allodynia was assessed 2 and 7 days after ischemia/reperfusion. Only high doses of morphine, dexamethasone and pregabalin partially reduced mechanical allodynia 2 days post-ischemia/reperfusion, while other treatments (ibuprofen, acetaminophen, amitriptyline) were not effective. Furthermore, only the highest dose of pregabalin reduced mechanical allodynia 7 days post-ischemia/reperfusion. These results are consistent with findings that complex region pain syndrome-I pain is refractory to most standard analgesic treatments.

Effects of inflammation on the ultrastructural localization of spinal cord dorsal horn group I metabotropic glutamate receptors

Inflammatory pain is thought to induce functional plasticity of spinal dorsal horn neurons and may produce changes in glutamate receptor expression. Plasticity of group I metabotropic glutamate receptors (mGluR1 and mGluR5) is important in various neuronal systems, and these receptors are also known to modulate nociceptive neurotransmission in the spinal dorsal horn. The present study aimed at determining whether persistent inflammatory pain produces alterations in intracellular and plasma membrane-associated mGluR1alpha and mGluR5 in spinal cord dorsal horn. Persistent inflammation was induced in male Long Evans rats by a unilateral intraplantar injection of 100 muL of complete Freund's adjuvant (CFA). Three days after the CFA injection thermal withdrawal latencies were obtained prior to processing of transverse spinal cord sections for preembedding immunogold labeling after incubation in primary antibody for mGluR1alpha or mGluR5. Using electron microscopy, we quantified immunogold-labeled mGluR1alpha and mGluR5 profiles, located in lamina V and I-II, respectively, of both CFA-treated rats and untreated control rats. Compared to untreated rats, CFA-treated rats had a significant increase in the number of plasma membrane-associated mGluR5 immunogold-labeled particles in lamina I-II neurons of the spinal cord. Although no changes to mGluR1alpha expression were found in CFA-treated rats, plasma membrane-associated mGluR1alpha was significantly closer to the synapse. Therefore, in CFA-treated rats there was a specific increase in the ratio of plasma membrane-associated versus intracellular immunogold-labeled particles for mGluR5, and lateral movement of mGluR1alpha toward the synapse, indicating that peripheral inflammation-induced trafficking of group I mGluRs in spinal dorsal horn neurons may be an important factor in the development of plastic changes associated with inflammation-induced chronic pain.

Enhanced 3,5-dihydroxyphenylglycine-induced sustained nociceptive behaviors in rats with neuropathy or chronic inflammation

Sustained nociceptive behaviors (SNBs) are an important but under-studied component of chronic pain conditions. The group I metabotropic glutamate receptor (mGluR) agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) produces SNBs when injected intrathecally, and group I mGluR antagonists are effective at reducing symptoms of neuropathic and inflammatory pain. The present experiments examined whether rats with sciatic nerve injury or persistent inflammation exhibit greater SNBs following intrathecal DHPG compared with control animals. SNBs were observed following intrathecal injection of DHPG (25 nmol) between the L4 and L5 vertebrae. We used a behavioral observation scoring system that allowed for assessment of specific behaviors in the hind paws. When DHPG was injected intrathecally in rats with chronic constriction injury (CCI) of the sciatic nerve, they showed increased paw stamping behavior compared to DHPG-injected sham controls. Rats treated with complete Freund's adjuvant (CFA)-induced inflammation failed to demonstrate a significant increase in paw stamping behavior. However, both CCI and CFA rats showed increased paw licking and biting of the neuropathic/inflamed hind paw after intrathecal DHPG injection. These results provide evidence for behaviorally relevant contributions of group I mGluRs to SNBs in models of neuropathic and inflammatory pain.

Emerging drugs in neuropathic pain

Neuropathic pain is a personally devastating and costly condition affecting 3-8% of the population. Existing treatments have limited effectiveness and produce relatively frequent adverse effects. Preclinical research has identified many promising pharmacological targets; however, reliable predictors of success in humans remain elusive. At least 50 new molecular entities have reached clinical development including: glutamate antagonists, cytokine inhibitors, vanilloid-receptor agonists, catecholamine modulators, ion-channel blockers, anticonvulsants, opioids, cannabinoids, COX inhibitors, acteylcholine modulators, adenosine receptor agonists and several miscellaneous drugs. Eight drugs are in Phase III trials at present. Strategies that may show promise over existing treatments include topical therapies, analgesic combinations and, in future, gene-related therapies. Recent years have heralded an explosion of pharmaceutical development in neuropathic pain, reflecting advanced knowledge of neurobiology and a heightened perception of the commercial value of neuropathic pain therapeutics. In the interest of improving patient care, the authors recommend implementing comparative studies throughout the development process in order to demonstrate the increased value of novel agents.

A comparison of the glutamate release inhibition and anti-allodynic effects of gabapentin, lamotrigine, and riluzole in a model of neuropathic pain

The effects of treatment with the anti-convulsant agents, lamotrigine and riluzole were compared with gabapentin in a rat experimental model of neuropathic pain. Rats were treated intraperitoneally, with gabapentin (30, 100 and 300 mg/kg), lamotrigine (2, 10 and 50 mg/kg) or riluzole (6 and 12 mg/kg) prior to, and every 12 h for 4 days following chronic constriction injury (CCI) of the sciatic nerve. Mechanical and cold sensitivity were assessed prior to surgery (baseline) and then at 4, 8 and 12 days following CCI. The four-day treatment with each of the agents was effective at producing reductions in the development of mechanical and cold hypersensitivity for periods ranging from the fourth to 12th day. The highest doses of each of the agents were also assessed on formalin-induced nociceptive behaviors and on formalin-induced increases in extracellular glutamate (Glu) and aspartate (Asp) in the spinal cord dorsal horn (SCDH) of awake behaving rats using in vivo microdialysis. Nociceptive scores in formalin test were significantly decreased by gabapentin (300 mg/kg i.p.) and riluzole (12 mg/kg i.p.), but not by lamotrigine (50 mg/kg i.p.). Formalin-induced increases in glutamate levels in SCDH were lowered significantly, as compared with the controls, with all drugs both in the first phase and second phases, with the greatest effects for riluzole and gabapentin. Similar suppressive effects of the drugs were observed on formalin-induced increases in spinal aspartate, except that gabapentin and lamotrigine produced effects only during the second phase. Riluzole produced profound and prolonged reductions in the spinal levels of glutamate and aspartate both for basal and formalin-stimulated release. In conclusion, the results suggest that the anti-convulsant agents gabapentin, lamotrigine and riluzole may reduce the development of hyperalgesia in a rat model of neuropathic pain by reducing the spinal release of glutamate. Riluzole's pronounced suppressive effects on spinal EAA levels is attributed to its established role as a glutamate release inhibitor and an enhancer of glutamate transporter activity.

Intracellular messengers involved in spontaneous pain, heat hyperalgesia, and mechanical allodynia induced by intrathecal dihydroxyphenylglycine

We investigated the role of two intracellular second messengers, extracellular signal-regulated protein kinase (ERK) and protein kinase C (PKC), in a model of persistent pain using intrathecal (i.t.) (R,S)-3,5-dihydroxyphenylglycine (DHPG). Spontaneous nociceptive behaviours (SNBs), mechanical allodynia (von Frey thresholds) and heat hyperalgesia (plantar test latencies) induced by DHPG were measured in animals pretreated i.t. with membrane permeable inhibitors of ERK (PD 98059) and PKC (GF 109203X). Spinal administration of PD 98059 dose-dependently reduced SNBs, and attenuated both mechanical allodynia and heat hyperalgesia induced by DHPG. GF 109203X treatment also reduced SNBs and heat hyperalgesia, but did not affect mechanical allodynia induced by DHPG. Neither PD 98059, nor GF 109203X, altered mechanical or thermal thresholds in saline-injected control rats. These results suggest that both ERK and PKC are involved in persistent pain associated with the i.t. administration of DHPG.

Increased stress-induced analgesia in mice lacking the proneuropeptide convertase PC2

Many neuropeptides involved in pain perception are generated by endoproteolytic cleavages of their precursor proteins by the proprotein convertases PC1 and PC2. To investigate the role of PC2 in nociception and analgesia, we tested wild-type and PC2-null mice for their responses to mechanical and thermal nociceptive stimuli, before and after a short swim in cold or warm water. Basal responses and responses after a cold swim were similar between the two groups. However, after a short forced swim in warm water, PC2-null mice were significantly less responsive to the stimuli than wild-type mice, an indication of increased opioid-mediated stress-induced analgesia. The enhanced analgesia in PC2-null mice may be caused by an accumulation of opioid precursor processing intermediates with potent analgesic effects, or by loss of anti-opioid peptides.

Evidence that gabapentin reduces neuropathic pain by inhibiting the spinal release of glutamate

Gabapentin is an anticonvulsant that successfully treats many neuropathic pain syndromes, although the mechanism of its antihyperalgesic action remains elusive. This study aims to help delineate gabapentin's antihyperalgesic mechanisms. We assessed the effectiveness of gabapentin at decreasing mechanical and cold hypersensitivity induced in a rat model of neuropathic pain. Thus, we compared the effectiveness of pre- or post-treatment with systemic or intrathecal (i.t.) gabapentin at reducing the development and maintenance of the neuropathic pain symptoms. Gabapentin successfully decreased mechanical and cold hypersensitivity, both as a pretreatment and post-treatment. Furthermore, both i.t. and systemic administration of gabapentin were effective in reducing the behavioral hypersensitivity; however, the i.t. administration was superior to the systemic. We also examined gabapentin's effects at inhibiting hindpaw formalin-induced release of excitatory amino acids (EAAs) in the spinal cord dorsal horn (SCDH) both in naïve rats and in rats with neuropathic pain. We present the first evidence that gabapentin reduces the formalin-induced release of both glutamate and aspartate in SCDH. Furthermore, i.t. gabapentin reduces the enhanced noxious stimulus-induced spinal release of glutamate seen in neuropathic rats. These data suggest that gabapentin reduces neuropathic pain symptoms by inhibiting the release of glutamate in the SCDH.

mGlu and NMDA receptor contributions to capsaicin-induced thermal and mechanical hypersensitivity

Metabotropic glutamate (mGlu) receptors are G protein-coupled receptors, some of which are localized in the spinal cord dorsal horn, and are involved with pain perception. The anti-nociceptive effects of intrathecal (i.t.) pretreatment with various mGlu receptor agonists and antagonists were assessed in Long Evans rats with mechanical and thermal hypersensitivity after sub-dermal injection of capsaicin in the hindpaw. Selective group II (aminopyrrolidine-2R,4R-dicarboxylate, APDC) and group III (l-2-amino-4-phosphonobutyrate, L-AP4) agonists, as well as selective mGlu(1) (1-aminoindan-1,5(R,S)-dicarboxylic acid, AIDA) and mGlu(5) (2-methyl-6-(phenylethynyl)-pyridine, MPEP) receptor subtype antagonists were compared with that of an NMDA receptor antagonist (dizocilipine maleate, MK-801). The rats were observed for signs of capsaicin-induced mechanical and thermal hypersensitivity 15 min after capsaicin injection, and 20 min following i.t. drug administration. Results indicate there was a dose-dependent reduction in capsaicin-induced mechanical hypersensitivity for all mGlu receptor agents; with maximal increases in mechanical thresholds that were 7-fold for AIDA and APDC, 7.5-fold for L-AP4 and 5.6-fold for MPEP. However, only a weak reduction (often non-significant) in thermal hypersensitivity was observed with each of the mGlu receptor drugs; thermal latencies were maximally increased by 125% (AIDA), 0% (MPEP), 8% APDC and 205% (L-AP4). By contrast, the highest dose of MK-801 was able to significantly reduce both mechanical (maximal 6.67-fold increase in threshold) and thermal (maximal 3-fold increase in latencies) hyperalgesia. We conclude that mGlu receptors contribute to the development of mechanical allodynia, but not thermal hyperalgesia, following capsaicin injury; while iGluRs may contribute to both thermal and mechanical hypersensitivity.

Chronic post-ischemia pain (CPIP): a novel animal model of complex regional pain syndrome-type I (CRPS-I; reflex sympathetic dystrophy) produced by prolonged hindpaw ischemia and reperfusion in the rat

A neuropathic-like pain syndrome was produced in rats following prolonged hindpaw ischemia and reperfusion, creating an animal model of complex regional pain syndrome-Type I (CRPS-I; reflex sympathetic dystrophy) that we call chronic post-ischemia pain (CPIP). The method involves placing a tourniquet (a tight fitting O-ring) on one hindlimb of an anesthetized rat just proximal to the ankle joint for 3 h, and removing it to allow reperfusion prior to termination of the anesthesia. Rats exhibit hyperemia and edema/plasma extravasation of the ischemic hindpaw for a period of 2-4 h after reperfusion. Hyperalgesia to noxious mechanical stimulation (pin prick) and cold (acetone exposure), as well as mechanical allodynia to innocuous mechanical stimulation (von Frey hairs), are evident in the affected hindpaw as early as 8 h after reperfusion, and extend for at least 4 weeks in approximately 70% of the rats. The rats also exhibit spontaneous pain behaviors (hindpaw shaking, licking and favoring), and spread of hyperalgesia/allodynia to the uninjured contralateral hindpaw. Light-microscopic examination of the tibial nerve taken from the region just proximal to the tourniquet reveals no signs of nerve damage. Consistent with the hypothesis that the generation of free radicals may be partly responsible for CRPS-I and CPIP, two free radical scavengers, N-acetyl-L-cysteine (NAC) and 4-hydroxy-2,2,6,6-tetramethylpiperydine-1-oxyl (Tempol), were able to reduce signs of mechanical allodynia in this model.

Pre-emptive analgesia using intravenous fentanyl plus low-dose ketamine for radical prostatectomy under general anesthesia does not produce short-term or long-term reductions in pain or analgesic use

The aim of the study was to evaluate post-operative pain and analgesic use after pre-operative or post-incisional i.v. fentanyl plus low dose i.v. ketamine vs. a standard treatment receiving i.v. fentanyl but not ketamine. Men undergoing radical prostatectomy under general anesthesia were randomly assigned in a double-blinded manner to one of three groups. Patients received i.v. fentanyl before incision followed by an i.v. bolus dose (0.2 ml kg(-1)) and an i.v. infusion (0.0025 ml kg(-1)min(-1)) of 1 mg ml(-1) ketamine (group 1) or normal saline (groups 2 and 3). Seventy minutes after incision, patients received i.v. fentanyl followed by an i.v. bolus dose (0.2 ml kg(-1)) and an i.v. infusion (0.0025 ml kg(-1)min(-1)) of saline (groups 1 and 3) or ketamine (group 2). Pain, von Frey pain thresholds, and cumulative morphine consumption using patient-controlled analgesia (PCA) were assessed up to 72 h after surgery. 143 patients completed the study (group 1, n = 47, group 2, n = 50, group 3, n = 46). Cumulative PCA morphine (mean+/-SD) did not differ significantly among groups (group 1, 92.3+/-45.9 mg; group 2, 107.2+/-58.4 mg; group 3, 103.6+/-50.4 mg; P = 0.08 for groups 1 vs. 2, and groups 1 vs. 3). On day 3, the hourly rate (mean+/-SEM) of morphine consumption was significantly lower (p < 0.0009) in group 1 (0.61+/-0.013 mg h(-1)) than group 2 (0.86+/-0.011 mg h(-1)) and group 3 (0.89+/-0.008 mg h(-1)). Pain scores and von Frey pain thresholds did not differ significantly among groups. Two-week and 6-month follow-ups did not reveal significant group differences in pain incidence, intensity, disability or mental health. Pre-operative, low-dose administration of i.v. ketamine did not result in a clinically meaningful reduction in pain or morphine consumption when compared with post-incisional administration of ketamine or a saline control condition.

Nociceptive effects of intrathecal administration of sulphur-containing amino acids

This study examined the nociceptive effects of the intrathecal administration of various doses of the following endogenous excitatory sulphur-containing amino acids (SAAs): L-cysteic acid (L-CA), L-cysteine sulfinic acid (L-CSA), L-homocysteic acid (L-HCA) and L-homocysteic sulfinic acid (L-HCSA). For a period of 10min, rats were observed for spontaneous nociceptive behaviours (SNBs), including: tail elevation, twitching or licking; hindpaw elevation, licking or shaking; and caudally directed biting or scratching. The amount of time each rat spent eliciting these individual behaviours was recorded and a total time (in seconds) spent exhibiting SNBs was then calculated. To determine which glutamate receptors are primarily responsible for these nociceptive behaviours, we pretreated additional groups of rats with selective antagonists for N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid/kainate (AMPA/KA) and group I metabotropic glutamate receptors (mGluR1 and 5). Results indicate that SAAs dose-dependently produce SNBs that are attenuated by NMDA receptor and group I mGluR antagonists.

Alterations in brain metabolism induced by chronic morphine treatment: NMR studies in rat CNS

High-resolution (500 MHz) multiresonance/multinuclear proton (1H) nuclear magnetic resonance (NMR) spectroscopy was used to detect metabolic changes and cellular injury in the rat brain stem and spinal cord following chronic morphine treatment. Compensatory changes were observed in glycine, glutamate, and inositols in the brain stem, but not the spinal cord, of chronic morphine-treated rats. In spinal cord, increases were detected in lactate and N-acetyl-aspartate (NAA), suggesting that there is anaerobic glycolysis, plasma membrane damage, and altered pH preferentially in the spinal cord of chronic morphine-treated rats.

Intrathecal nerve growth factor restores opioid effectiveness in an animal model of neuropathic pain

It is without dispute that the treatment of neuropathic pain is an area of largely unmet medical need. Available analgesics, such as morphine, either have minimal effects in neuropathic pain patients, or are not always well tolerated due to concurrent adverse effects. The chronicity of neuropathic pain is thought to be related to many neurochemical changes in the dorsal root ganglia (DRG) and spinal cord, including a reduction in the retrograde transport of nerve growth factor (NGF). In this study, we have determined the ability of chronic intrathecal (i.t.) infusion of NGF to reverse neuropathic pain symptoms and to restore morphine's effectiveness in an animal model of neuropathic pain. Seven days after sciatic nerve constriction injury, NGF was administered to the spinal cord by continuous infusion (125 ng/microl/h) via osmotic pumps attached to chronically implanted i.t. catheters. Spinal infusion of NGF did not affect the expression of tactile allodynia or thermal (hot) hyperalgesia in neuropathic rats, although it significantly increased cold water responses frequency at day 14. Following infusion of vehicle, i.t. morphine (20 microg) was ineffective in altering somatosensory thresholds in neuropathic rats. In contrast, morphine substantially attenuated the neuropathy-induced warm and cold hyperalgesia, as well as tactile allodynia, in neuropathic rats chronically infused with i.t. NGF. In addition, we demonstrate that i.t. morphine-induced antinociception was augmented by a cholecystokinin (CCK) antagonist in animals chronically infused with i.t. antibodies directed against NGF. We hypothesize that NGF is critical in maintaining neurochemical homeostasis in the spinal cord of nociceptive neurons, and that supplementation may be beneficial in restoring and/or maintaining opioid analgesia in chronic pain conditions resulting from traumatic nerve injury.

Enhanced thermal antinociceptive potency and anti-allodynic effects of morphine following spinal administration of endotoxin

Recently, an animal model of central inflammation characterized by widespread cutaneous hyperalgesia and allodynia following intracerebroventricular (i.c.v.) administration of lipopolysaccharide (LPS) was described. In the present study, we demonstrate that central administration of LPS via intrathecal (i.t.) injection produces bilateral tactile allodynia and thermal hyperalgesia in the rat. Also, the effects of morphine-induced antinociception were determined in this model. Here we demonstrate enhanced thermal antinociceptive potency of i.t. morphine in LPS-treated rats compared to controls. Intrathecal morphine was also effective in alleviating the tactile allodynia induced by LPS. Both the antinociceptive and anti-allodynic effects produced by i.t. morphine were completely antagonized by pretreatment with subcutaneous naloxone (1 mg x kg(-1)). This study demonstrates the presence of both heat hyperalgesia and mechanical allodynia following central administration of LPS, and an increased antinociceptive potency of i.t. morphine in this model.

Antinociceptive effects following intrathecal pretreatment with selective metabotropic glutamate receptor compounds in a rat model of neuropathic pain

In the present study, we examined the effects of intrathecal pretreatment (twice daily injections on postoperative (PO) days 0-3 with the selective Group I (mGluR1a) mGluR antagonist, (RS)-1-aminoindan-1,5-dicarboxylic acid ((RS)-AIDA), the selective Group I (mGluR5a) antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), the selective Group II mGluR agonist, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate ((2R,4R)-APDC) or the selective Group III mGluR agonist, L-2-amino-4-phosphonobutyrate (L-AP4), on mechanical and cold hypersensitivity associated with chronic constriction injury (CCI) of the sciatic nerve in rats. Mechanical and cold sensitivity was assessed prior to surgery (baseline) and then at 4, 8 and 12 days following CCI. Pretreatment with all of the mGluR agents produced reductions in the development of mechanical hypersensitivity. In addition, all the mGluR agents, except MPEP, were effective in reducing the development of cold hypersensitivity. This study demonstrates that spinal Group I mGluR antagonism, and Group II or III mGluR agonism, can effectively decrease the development of mechanical and cold hypersensitivity associated with CCI in rats. In addition, the results can be interpreted to suggest that activation of spinal Group I mGluRs contributes to spinal plasticity leading to the development of neuropathic pain, and that this effect is offset by activation of groups II and III mGluRs.

Antisense oligonucleotide knockdown of mGluR1 alleviates hyperalgesia and allodynia associated with chronic inflammation

Chronic inflammation induced by injection of complete Freund's adjuvant (CFA) into one hindpaw elicits thermal hyperalgesia and mechanical allodynia in the injected paw. Metabotropic glutamate receptors (mGluRs) have been implicated in dorsal horn neuronal nociceptive responses and pain associated with short-term inflammation. The goal of the present study was to assess the role of mGluR1 in the hyperalgesia and allodynia associated with the CFA model of chronic inflammation. Here we show that antisense (AS) oligonucleotide knockdown of spinal mGluR1 attenuates thermal hyperalgesia and mechanical allodynia in rats injected with CFA in one hindpaw. When intrathecal infusion of mGluR1 AS oligonucleotide (50 microg/day) began prior to CFA injection, mechanical allodynia was attenuated from Days 1 to 8 following CFA injection, whereas heat hyperalgesia was attenuated on Day 1 and then from Days 4 to 8. When intrathecal infusion of mGluR1 AS oligonucleotide was begun 2 days after CFA injection, both mechanical allodynia and heat hyperalgesia were attenuated at all time points following the oligonucleotide infusion. Thus, the present data suggest a role for mGluR1 in persistent inflammatory nociception.

Attenuation of morphine tolerance after antisense oligonucleotide knock-down of spinal mGluR1

1. Chronic systemic treatment of rats with morphine leads to the development of opioid tolerance. This study was designed to examine the effects of intrathecal (i.t.) infusion of a metabotropic glutamate receptor 1 (mGluR1) antisense oligonucleotide, concomitant with chronic morphine treatment, on the development of tolerance to morphine's antinociceptive effects. 2. All rats received chronic (6 day) s.c. administration of morphine to induce opioid tolerance. Additionally, rats were treated with either mGluR1 antisense (AS), missense (MIS) or artificial cerebrospinal fluid (ACSF) by i.t. infusion via chronically implanted i.t. catheters connected to osmotic mini-pumps. The effects of acute i.t. or s.c. morphine on tail-flick latencies were assessed prior to and following chronic s.c. morphine treatment for all chronic i.t. infusion groups. mGluR1 protein level in the spinal cord was determined by Western blot analysis for all treatments, assessing the efficiency of knock-down with AS treatment. 3. Acute i.t. morphine dose-dependently produced antinociception in the tail-flick test in naïve rats. Systemic morphine-treated rats administered i.t. ACSF or MIS developed tolerance to i.t. morphine. Chronic i.t. infusion with mGluR1 AS significantly reduced the development of tolerance to i.t. morphine. 4. In contrast to i.t. morphine, tolerance developed to the antinociceptive effects of s.c. morphine, in all i.t. infusion groups, including the mGluR1 AS group. 5. The spinal mGluR1 protein level was dramatically decreased after mGluR1 AS infusion when compared to control animals (naïve and ACSF-treated animals). 6. These findings suggest that the spinal mGluR1 is involved in the development of tolerance to the antinociceptive effects of morphine. Selective blockade of mGluR1 may be beneficial in preventing the development of opioid analgesic tolerance.

Attenuation of hyperalgesia in a rat model of neuropathic pain after intrathecal pre- or post-treatment with a neurokinin-1 antagonist

Although many studies have demonstrated a role for substance P in pain, there have been conflicting reports implicating the involvement of substance P in neuropathic pain models. In this study, the non-peptide neurokinin-1 (NK-1) receptor antagonist, L-732,138 was chronically administered by intrathecal (i.t.) injection to rats with mono-neuropathy produced by sciatic nerve constriction. Rats exhibited tactile allodynia and cold hyperalgesia over a 16-day testing period. L-732,138 (5-200 nmol) administered i.t. prior to and for 3 consecutive days post-surgery attenuated the mechanical allodynia and cold hyperalgesia on days 4 and 8 post-surgery. The effects of i.t. L-732,138 were also determined in rats with established nerve injury-induced neuropathy. The NK-1 receptor antagonist was injected for 4 consecutive days starting on day 8 post-sciatic nerve injury. Administration of L-732,138 (5-200 nmol) i.t. produced both anti-allodynic and anti-hyperalgesic effects on day 12, but the effect was not permanent, as nociceptive thresholds were similar to controls by day 16. These results demonstrate that substance P is involved both in the induction and the maintenance of neuropathic pain and provides justification for the development and administration of substance P antagonists for the management of clinical neuropathic pain.

Differential effects of NMDA and group I mGluR antagonists on both nociception and spinal cord protein kinase C translocation in the formalin test and a model of neuropathic pain in rats

Coincident with nociception, both noxious chemical stimulation of the hind paw and chronic constriction injury (CCI) of the sciatic nerve produce an increase in protein kinase C (PKC) translocation in the spinal cord of rats. Noxious stimulus-induced PKC translocation likely depends on glutamate activity at either N-methyl-D-aspartate (NMDA) receptors or group I metabotropic glutamate receptors (mGluR1/5) in the spinal cord dorsal horn. This study compares nociceptive responses to, and the alterations in membrane-associated PKC, induced by noxious chemical stimulation of the hindpaw and CCI of the sciatic nerve, as well as their modulation by both NMDA and mGluR1/5 receptor antagonists. Three groups of rats were given a single intrathecal (i.t.) injection of either vehicle, dizocilpine maleate (MK-801, 60 nmol), an NMDA receptor antagonist, or (S)-4-carboxyphenylglycine (S)-4CPG, (150 nmol), an mGluR1/5 antagonist, 10 min prior to a 50 microl of 2.5% formalin injection into the ventral surface of one hind paw. Another three groups of rats were given twice daily injections of either vehicle, MK-801 (30 nmol) or (S)-4CPG (90 nmol) i.t. for 5 days starting 30 min before CCI or sham injury of the sciatic nerve. Nociceptive responses were assessed for a 60 min period after the formalin injection in the first three groups, and tests of mechanical and cold allodynia were performed on days 4, 8, 12 and 16 after CCI for the latter three groups. Furthermore, changes in the levels of membrane-associated PKC, as assayed by quantitative autoradiography of the specific binding of [3H]-phorbol 12,13-dibutyrate ([3H]-PDBu) in the dorsal horn of the lumbar spinal cord sections, were assessed in formalin-injected rats (at 5, 25 and 60 min) and in neuropathic rats 5 days after CCI, treated (as above) with vehicle, MK-801 or (S)-4CPG. The results indicate that i.t. treatment with MK-801 significantly reduced nociceptive scores in the formalin test and also produced a significant suppression of formalin-induced increases in [3H]-PDBu binding in laminae I-II, III-VI and X of the lumbar spinal cord. In contrast, i.t. treatment with (S)-4CPG failed to significantly affect either nociceptive behaviours in the formalin test or formalin-induced increases in [3H]-PDBu binding in laminae I-II and III-VI of the lumbar spinal cord. On the other hand, i.t. treatment with either MK-801 or (S)-4CPG produced a significant reduction in mechanical and cold hypersensitivity, as well as [3H]-PDBu binding in laminae I-II and III-VI of the lumbar spinal cord, after CCI. These results suggest that while NMDA, but not mGluR1/5, receptors are involved in translocation of PKC and nociception in a model of persistent acute pain, both types of receptors influence the translocation of PKC in dorsal horn and mechanical and cold allodynia in a model of chronic neuropathic pain.

Central neuroplasticity and pathological pain

The traditional specificity theory of pain perception holds that pain involves a direct transmission system from somatic receptors to the brain. The amount of pain perceived, moreover, is assumed to be directly proportional to the extent of injury. Recent research, however, indicates far more complex mechanisms. Clinical and experimental evidence shows that noxious stimuli may sensitize central neural structures involved in pain perception. Salient clinical examples of these effects include amputees with pains in a phantom limb that are similar or identical to those felt in the limb before it was amputated, and patients after surgery who have benefited from preemptive analgesia which blocks the surgery-induced afferent barrage and/or its central consequences. Experimental evidence of these changes is illustrated by the development of sensitization, wind-up, or expansion of receptive fields of CNS neurons, as well as by the enhancement of flexion reflexes and the persistence of pain or hyperalgesia after inputs from injured tissues are blocked. It is clear from the material presented that the perception of pain does not simply involve a moment-to-moment analysis of afferent noxious input, but rather involves a dynamic process that is influenced by the effects of past experiences. Sensory stimuli act on neural systems that have been modified by past inputs, and the behavioral output is significantly influenced by the "memory" of these prior events. An increased understanding of the central changes induced by peripheral injury or noxious stimulation should lead to new and improved clinical treatment for the relief and prevention of pathological pain.

Knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) alleviates pain and restores opioid efficacy after nerve injury in rats

1. Nerve injury often produces long-lasting spontaneous pain, hyperalgesia and allodynia that are refractory to treatment, being only partially relieved by clinical analgesics, and often insensitive to morphine. With the aim of assessing its therapeutic potential, we examined the effect of antisense oligonucleotide knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) in neuropathic rats. 2. We chronically infused rats intrathecally with either vehicle, or 50 microg day(-1) antisense or missense oligonucleotides beginning either 3 days prior to or 5 days after nerve injury. Cold, heat and mechanical sensitivity was assessed prior to any treatment and again every few days after nerve injury. 3. Here we show that knockdown of mGluR(1) significantly reduces cold hyperalgesia, heat hyperalgesia and mechanical allodynia in the ipsilateral (injured) hindpaw of neuropathic rats. 4. Moreover, we show that morphine analgesia is reduced in neuropathic rats, but not in sham-operated rats, and that knockdown of mGluR(1) restores the analgesic efficacy of morphine. 5. We also show that neuropathic rats are more sensitive to the excitatory effects of intrathecally injected N-methyl-D-aspartate (NMDA), and have elevated protein kinase C (PKC) activity in the spinal cord dorsal horn, two effects that are reversed by knockdown of mGluR(1). 6. These results suggest that activity at mGluR(1) contributes to neuropathic pain through interactions with spinal NMDA receptors and PKC, and that knockdown of mGluR(1) may be a useful therapy for neuropathic pain in humans, both to alleviate pain directly, and as an adjunct to opioid analgesic treatment.