RESUMO
Local microvascular dysfunction and consequent tissue ischemia/hypoxia contribute to the symptoms of complex regional pain syndrome (CRPS) and peripheral neuropathic pain. As nitric oxide (NO) is a key regulator of microvascular blood flow, compounds that increase it are potentially therapeutic for these pain conditions. This led us to hypothesize that the topical administration of drugs that modulate local tissue NO levels can alleviate the pain of CRPS and peripheral neuropathic pain. We investigated the anti-allodynic effect of a combination of two NO-modulating drugs: meldonium and N-acetylcysteine (NAC). An equimolar topical formulation of the two drugs was tested on chronic post-ischemic pain (CPIP), a rat model of CRPS, as well as chronic constriction injury (CCI) of the sciatic nerve and chemotherapy-induced painful neuropathy (CIPN), rat models of peripheral neuropathic pain. Topical meldonium-NAC produced significant anti-allodynia in CPIP, CCI, and CIPN rats. Moreover repeated application of topical meldonium-NAC produced an increase in the duration of anti-allodynia in the CPIP and CCI rats. While pre-treatment with an NO synthase inhibitor attenuated the anti-allodynic effects of meldonium-NAC, 30-min hyperbaric oxygen treatment combined with a non-effective dose of meldonium-NAC produced significant anti-allodynic effects in CPIP rats. Both experiments implicated NO in the drug combination's anti-allodynic effects. To ascertain the role played by changes in local tissue NO, we performed a quantification of plantar muscle NO in CPIP rats after hind paw topical treatment with meldonium-NAC and revealed significantly increased plantar muscle NO levels in drug-treated rats. The drug combination also reversed the reduction in tissue oxygenation normally observed in CPIP hind paws. In addition to introducing a novel topical treatment for mechanical allodynia in CRPS and peripheral neuropathic pain, this work showcases the analgesic potential of locally targeting microvascular dysfunction and tissue ischemia/hypoxia in these conditions, with emphasis on the role of NO.
Assuntos
Acetilcisteína/administração & dosagem , Metilidrazinas/administração & dosagem , Neuralgia/metabolismo , Óxido Nítrico/metabolismo , Distrofia Simpática Reflexa/metabolismo , Administração Tópica , Animais , Modelos Animais de Doenças , Hiperalgesia/metabolismo , Masculino , Ratos , Ratos Long-Evans , Ratos Sprague-DawleyRESUMO
This paper examines the development of and some logistical and methodological issues surrounding the use of animal models of chronic pain. The first section addresses the emergent move towards mechanism-based and disease-related animal models of chronic pain that has accelerated since the late 1980s following publication of Bennett and Xie's (Pain 33:87-107, 1998) paper on chronic constriction injury of the sciatic nerve and Stein et al.'s (Pharmacol Biochem Behav 31:445-451, 1988) paper on unilateral hind paw inflammation with complete Freund's adjuvant. The discussion covers vast areas of chronic pain models developed over the past 50 years, starting with the numerous neuropathic, inflammatory and central pain models, as well as the growing number of models developed to study various forms of chronic pain from chronic back pain to visceral pain. It also examines the advantages and disadvantages of tonic pain models, mechanism-based and disease-related models of chronic pain, including issues related to the novel discovery of injury- or disease-related pathophysiological processes, the expansion of testing repertoires, and the successes and failures in the translation of analgesic development from animal preclinical models to human chronic pain conditions. The second section addresses experimental design considerations in the implementation of one of the 3Rs for the use of animal models of chronic pain; that is methods employed to reduce the number of animals used. The discussion covers various issues including the advantages and disadvantages of repeated dose designs and within-group drug testing, including incremental dosing schedules, and crossover designs. It also examines concerns surrounding the stability of symptoms and measures, including varying durations of multiple symptoms and the potential development of nociceptive sensitization, as well as possible use-dependent alterations in drug sensitivity and time-dependent changes in pain processes in specific animal models.
Assuntos
Dor Crônica , Modelos Animais de Doenças , Projetos de Pesquisa/normas , Pesquisa Translacional Biomédica/normas , AnimaisRESUMO
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.
Assuntos
Isoenzimas/metabolismo , Nociceptividade , Proteína Quinase C/metabolismo , Caracteres Sexuais , Medula Espinal/enzimologia , Animais , Peptídeo Relacionado com Gene de Calcitonina/farmacologia , Peptídeos Penetradores de Células , Feminino , Formaldeído , Hiperalgesia/enzimologia , Imidazóis/farmacologia , Lipopeptídeos/farmacologia , Masculino , Nociceptividade/efeitos dos fármacos , Organofosfatos/farmacologia , Ratos Long-Evans , Receptores de AMPA/metabolismo , Medula Espinal/efeitos dos fármacosRESUMO
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.
Assuntos
Neuralgia/tratamento farmacológico , Neuralgia/genética , Proteína Quinase C/deficiência , Caracteres Sexuais , Animais , Capsaicina/toxicidade , Peptídeos Penetradores de Células , Modelos Animais de Doenças , Relação Dose-Resposta a Droga , Feminino , Adjuvante de Freund/toxicidade , Inflamação/induzido quimicamente , Inflamação/complicações , Lipopeptídeos/uso terapêutico , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Transgênicos , Neuralgia/induzido quimicamente , Neuralgia/patologia , Limiar da Dor/efeitos dos fármacos , Piperidinas/uso terapêutico , Proteína Quinase C/genética , Ratos , Ratos Long-Evans , Medula Espinal/metabolismo , Medula Espinal/patologiaRESUMO
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.
Assuntos
Capilares/efeitos dos fármacos , Dor Crônica/tratamento farmacológico , Dor Crônica/fisiopatologia , Isquemia/complicações , Administração Tópica , Agonistas de Receptores Adrenérgicos alfa 2/administração & dosagem , Agonistas de Receptores Adrenérgicos alfa 2/farmacologia , Animais , Química Farmacêutica , Clonidina/análogos & derivados , Clonidina/farmacologia , Relação Dose-Resposta a Droga , Combinação de Medicamentos , Hiperalgesia/tratamento farmacológico , Hiperemia/fisiopatologia , Fluxometria por Laser-Doppler , Masculino , Molsidomina/análogos & derivados , Molsidomina/uso terapêutico , Doadores de Óxido Nítrico/administração & dosagem , Doadores de Óxido Nítrico/farmacologia , Pomadas , Medição da Dor/efeitos dos fármacos , Ácidos Fosfatídicos/farmacologia , Inibidores de Fosfodiesterase/farmacologia , Ratos , Ratos Long-Evans , Fluxo Sanguíneo Regional/efeitos dos fármacosRESUMO
BACKGROUND: Ischemia-reperfusion injury causes chronic postischemia pain (CPIP), and rats with higher glycemia during ischemia-reperfusion injury exhibit increased allodynia. Glycemia-induced elevation of nuclear factor κB (NFκB) may contribute to increased allodynia. METHODS: Glycemia during a 3-h ischemia-reperfusion injury was manipulated by: normal feeding; or normal feeding with administration of insulin; dextrose; or insulin/dextrose. In these groups, NFκB was measured in ipsilateral hind paw muscle and spinal dorsal horn by enzyme-linked immunosorbent assay (ELISA), and SN50, an NFκB inhibitor, was administered to determine its differential antiallodynic effects depending on glycemia. RESULTS: CPIP fed/insulin rats (12.03 ± 4.9 g, N = 6) had less allodynia than fed, fed/insulin/dextrose, and fed/dextrose rats (6.29 ± 3.37 g, N = 7; 4.57 ± 3.03 g, N = 6; 2.95 ± 1.10 g, N = 9), respectively. Compared with fed rats (0.209 ± 0.022 AU, N = 7), NFκB in ipsilateral plantar muscles was significantly lower for fed/insulin rats, and significantly higher for fed/dextrose rats (0.152 ± 0.053 AU, N = 6; 0.240 ± 0.057 AU, N = 7, respectively). Furthermore, NFκB in the dorsal horn of fed, fed/insulin/dextrose, and fed/dextrose rats (0.293 ± 0.049 AU; 0.267 ± 0.037 AU; 0.315 ± 0.015 AU, respectively, N = 6 for each) was significantly higher than in fed/insulin animals (0.267 ± 0.037 AU, N = 6). The antiallodynic SN50 dose-response curves of CPIP rats in the fed/insulin/dextrose, fed/dextrose, and fed conditions exhibited a rightward shift compared with the fed/insulin group. The threshold SN50 dose of CPIP fed/dextrose, fed/insulin/dextrose, and fed rats (328.94 ± 92.4 ng, 77.80 ± 44.50 ng, and 24.89 ± 17.20 ng, respectively) was higher than that for fed/insulin rats (4.06 ± 7.04 ng). CONCLUSIONS: NFκB was activated in a glycemia-dependent manner in CPIP rats. Hypoglycemic rats were more sensitive to SN50 than rats with higher glycemia. The finding that SN50 reduces mechanical allodynia suggests that NFκB inhibitors might be useful for treating postischemia pain.
Assuntos
Glicemia/análise , Dor Crônica/etiologia , Hiperalgesia/etiologia , NF-kappa B/metabolismo , Traumatismo por Reperfusão/complicações , Animais , Dor Crônica/sangue , Masculino , Ratos , Ratos Long-Evans , Traumatismo por Reperfusão/sangue , Teste de Desempenho do Rota-RodRESUMO
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.
RESUMO
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.
Assuntos
Dor Crônica/metabolismo , Proteína Quinase C/genética , Medula Espinal/metabolismo , Animais , Dor Crônica/genética , Dor Crônica/fisiopatologia , Masculino , Plasticidade Neuronal/fisiologia , Nociceptores/metabolismo , Medição da Dor , Células do Corno Posterior/metabolismo , Células do Corno Posterior/fisiopatologia , Proteína Quinase C/metabolismo , Ratos , Ratos Long-Evans , Medula Espinal/fisiopatologiaRESUMO
BACKGROUND: Ischemia-reperfusion (I/R) injuries consist of enhanced oxidative and inflammatory responses along with microvascular dysfunction after prolonged ischemia and reperfusion. Because I/R injuries induce chronic postischemia pain (CPIP) in laboratory animals, it is possible that surgical procedures using prolonged ischemia may result in chronic postoperative pain. Glycemic modulation during ischemia and reperfusion could affect pain after I/R injury because glucose triggers oxidative, inflammatory, and thrombotic reactions, whereas insulin has antioxidative, antiinflammatory, and vasodilatory properties. METHODS: One hundred ten rats underwent a 3-h period of ischemia followed by reperfusion to produce CPIP. Rats with CPIP had previously been divided into six groups with differing glycemic modulation paradigms: normal feeding; fasting; fasting with normal saline administration; fasting with dextrose administration; normal feeding with insulin administration; and normal feeding with insulin and dextrose administration. Blood glucose concentration was assessed during I/R in these separate groups of rats, and these rats were tested for mechanical and cold allodynia over the 21 days afterward (on days 2, 5, 7, 9, 12, and 21 after I/R injury). RESULTS: I/R injury in rats with normoglycemia or relative hyperglycemia (normal feeding and fasting with dextrose administration groups) led to significant mechanical and cold allodynia; conversely, relative hypoglycemia associated with insulin treatment or fasting (fasting, fasting with normal saline administration, and normal feeding with insulin administration groups) reduced allodynia induced by I/R injury. Importantly, insulin treatment did not reduce allodynia when administered to fed rats given dextrose (normal feeding with dextrose and insulin administration group). CONCLUSION: Study results suggest that glucose levels at the time of I/R injury significantly modulate postinjury pain thresholds in rats with CPIP. Strict glycemic control during I/R injury significantly reduces CPIP and, conversely, hyperglycemia significantly enhances it, which could have potential clinical applications especially in the surgical field.
Assuntos
Glicemia/metabolismo , Hiperglicemia/tratamento farmacológico , Hiperglicemia/etiologia , Dor/tratamento farmacológico , Dor/etiologia , Traumatismo por Reperfusão/complicações , Animais , Doença Crônica , Temperatura Baixa , Jejum/fisiologia , Pé/irrigação sanguínea , Pé/patologia , Lateralidade Funcional/efeitos dos fármacos , Glucose/farmacologia , Temperatura Alta , Hiperglicemia/complicações , Hipoglicemiantes/uso terapêutico , Insulina/uso terapêutico , Masculino , Medição da Dor , Estimulação Física , Ratos , Ratos Long-Evans , Fluxo Sanguíneo Regional/fisiologiaRESUMO
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.
Assuntos
Síndromes da Dor Regional Complexa , Neuralgia , Pentoxifilina , Animais , Modelos Animais de Doenças , Hidroxibenzoatos , Hiperalgesia/tratamento farmacológico , Hipóxia , Neuralgia/tratamento farmacológico , Pentoxifilina/farmacologia , RatosRESUMO
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.
Assuntos
Neuralgia , Pentoxifilina , Analgésicos/efeitos adversos , Clonidina/efeitos adversos , Método Duplo-Cego , Humanos , Neuralgia/diagnóstico , Neuralgia/tratamento farmacológico , Neuralgia/etiologia , Medição da Dor , Pentoxifilina/efeitos adversos , Ensaios Clínicos Controlados Aleatórios como AssuntoRESUMO
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.
Assuntos
Ácido Glutâmico/metabolismo , Dor/metabolismo , Doenças do Sistema Nervoso Periférico/metabolismo , Células do Corno Posterior/metabolismo , Receptores de Glutamato Metabotrópico/fisiologia , Animais , Capsaicina/farmacologia , Inflamação/metabolismo , Masculino , Medição da Dor , Células do Corno Posterior/efeitos dos fármacos , Ratos , Ratos Long-Evans , Receptores de Glutamato Metabotrópico/agonistas , Receptores de Glutamato Metabotrópico/antagonistas & inibidoresRESUMO
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.
Assuntos
Analgésicos/uso terapêutico , Ácido Glutâmico/metabolismo , Ciática/tratamento farmacológico , Ciática/patologia , Medula Espinal/metabolismo , Ácido gama-Aminobutírico/análogos & derivados , Analgésicos/farmacologia , Animais , Cromatografia Líquida de Alta Pressão/métodos , Modelos Animais de Doenças , Relação Dose-Resposta a Droga , Vias de Administração de Medicamentos , Esquema de Medicação , Eletroquímica/métodos , Hiperalgesia/tratamento farmacológico , Hiperalgesia/etiologia , Masculino , Microdiálise/métodos , Medição da Dor/métodos , Limiar da Dor/efeitos dos fármacos , Estimulação Física/efeitos adversos , Pregabalina , Ratos , Ratos Long-Evans , Medula Espinal/efeitos dos fármacos , Traumatismos da Medula Espinal/complicações , Estatísticas não Paramétricas , Fatores de Tempo , Ácido gama-Aminobutírico/farmacologia , Ácido gama-Aminobutírico/uso terapêuticoRESUMO
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.
Assuntos
Capilares/patologia , Microcirculação , Animais , Síndromes Compartimentais/complicações , Síndromes Compartimentais/patologia , Síndromes Compartimentais/fisiopatologia , Modelos Animais de Doenças , Humanos , Inflamação/patologia , Inflamação/fisiopatologia , Isquemia/patologia , Isquemia/fisiopatologia , Distrofia Simpática Reflexa/etiologia , Distrofia Simpática Reflexa/fisiopatologia , Traumatismo por Reperfusão/patologia , Traumatismo por Reperfusão/fisiopatologiaRESUMO
The discovery and development of effective analgesics is greatly lagging behind the steadily rising prevalence of chronic pain. Currently prescribed analgesics for chronic pain are lacking in efficacy mainly due to their narrowly-targeted mechanism of action. Driving neuronal hyperexcitability that underlies symptoms of chronic pain are multiple non-neuronal processes, among which are tissue hypoxia and oxidative stress. Here we demonstrate the design, synthesis, and activity of new multi-component bi-functional analgesic crystalline solids, co-crystals, and salts, based on pairing of vasodilatory anti-hypoxic drugs pentoxifylline, clonidine and linsidomine with antioxidant nutraceuticals protocatechuic acid, α-lipoic acid, and caffeic acid. After validation, chemical and structural characterization of these novel salts and co-crystals, topical formulations of the products were tested in a rat model of complex regional pain syndrome. Analgesic effects achieved with the salts and co-crystal exceeded the efficacy and/or potency of constituent compounds indicating that more effective, advanced analgesics can readily be developed by careful pairing of compounds that simultaneously target multiple neural and non-neural processes driving chronic pain.
RESUMO
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.
Assuntos
Encéfalo/patologia , Hiperalgesia/patologia , Neuralgia/patologia , Animais , Ansiedade/etiologia , Comportamento Animal , Encéfalo/fisiopatologia , Doença Crônica , Modelos Animais de Doenças , Hiperalgesia/fisiopatologia , Imageamento por Ressonância Magnética , Masculino , Neuralgia/complicações , Neuralgia/psicologia , Ratos , Ratos Long-EvansRESUMO
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.
Assuntos
Analgésicos/farmacologia , Doenças do Sistema Nervoso Central/tratamento farmacológico , Colecistocinina/fisiologia , Inflamação/tratamento farmacológico , Morfina/farmacologia , Fator de Crescimento Neural/fisiologia , Animais , Doenças do Sistema Nervoso Central/induzido quimicamente , Colecistocinina/efeitos dos fármacos , Modelos Animais de Doenças , Inflamação/induzido quimicamente , Injeções Espinhais , Lipopolissacarídeos/administração & dosagem , Masculino , Morfina/agonistas , Morfina/antagonistas & inibidores , Fator de Crescimento Neural/farmacologia , Ratos , Ratos Long-Evans , Sincalida/análogos & derivados , Sincalida/metabolismo , Sincalida/farmacologia , Medula Espinal/metabolismoRESUMO
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.
Assuntos
Síndromes da Dor Regional Complexa/fisiopatologia , Síndromes da Dor Regional Complexa/terapia , Fluxo Sanguíneo Regional/fisiologia , Animais , Gerenciamento Clínico , Humanos , Dor/fisiopatologia , Manejo da Dor , Medição da Dor/métodos , Vasoconstrição/fisiologiaRESUMO
The counterirritation phenomenon known as conditioned pain modulation, or diffuse noxious inhibitory control in animals, is of increasing interest due to its utility in predicting chronic pain and treatment response. It features considerable interindividual variability, with large subsets of pain patients and even normal volunteers exhibiting hyperalgesia rather than hypoalgesia during or immediately after receiving a conditioning stimulus. We observed that mice undergoing tonic inflammatory pain in the abdominal cavity (the conditioning stimulus) display hyperalgesia, not hypoalgesia, to noxious thermal stimulation (the test stimulus) applied to the hindpaw. In a series of parametric studies, we show that this hyperalgesia can be reliably observed using multiple conditioning stimuli (acetic acid and orofacial formalin), test stimuli (hindpaw and forepaw-withdrawal, tail-withdrawal, hot-plate, and von Frey tests) and genotypes (CD-1, DBA/2, and C57BL/6 mice and Sprague-Dawley rats). Although the magnitude of the hyperalgesia is dependent on the intensity of the conditioning stimulus, we find that the direction of effect is dependent on the effective test stimulus intensity, with lower-intensity stimuli leading to hyperalgesia and higher-intensity stimuli leading to hypoalgesia.
Assuntos
Dor Facial/complicações , Hiperalgesia/etiologia , Hipestesia/etiologia , Dor/complicações , Dor/etiologia , Ácido Acético/toxicidade , Animais , Modelos Animais de Doenças , Relação Dose-Resposta a Droga , Formaldeído/toxicidade , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Endogâmicos DBA , Medição da Dor , Traumatismos dos Nervos Periféricos/complicações , Estimulação Física/efeitos adversos , Psicofísica , Ratos , Ratos Sprague-Dawley , Especificidade da EspécieRESUMO
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.