Post-injury repeated administrations of minocycline improve the antinociceptive effect of morphine in chronic constriction injury model of neuropathic pain in rat

It is confirmed that pharmacological attenuation of glial cells can alleviate neuropathic pain by lowering proinflammatory cytokine expression. The present study tries to confirm that post-injury administration of glia inhibitor, minocycline, can attenuate the neuropathic pain symptoms and improves the efficacy of morphine anti-nociception in chronic constriction injury (CCI). Male Wistar rats (230-270 g) underwent surgery for induction CCI model of neuropathy. For assessment of the thermal hyperalgesia and mechanical allodynia after CCI induction, morphine (2.5, 5, 7.5, 10 and 15 mg/kg; s.c.) and saline were administered on post-operative days (PODs) 0, 6 and 14. Hargreaves and Von-Frey tests were performed before and 30 min after morphine administration, respectively. The results showed significant decrease in antinociceptive effect of morphine on POD 6 compared to POD 0 only at the dose of 5 mg/kg. On the other hand, on POD 14 the antinociceptive effect of morphine (5, 7.5, 10 and 15 mg/kg) significantly decreased in comparison with POD 0. In another set of experiments, animals received minocycline (10, 20 and 40 mg/kg; i.p.) for eight days from POD 6 to 13 and then the antinociceptive effect of single dose of morphine 5 mg/kg was tested on POD 14. Behavioral tests showed that minocycline (40 mg/kg) could effectively attenuate the thermal hyperalgesia and mechanical allodynia on POD 13. Moreover, minocycline (40, 20 mg/kg) improved the anti-hyperalgesic, and minocycline (40 mg/kg) improved the anti-allodynic effects of morphine 5 mg/kg on POD 14. It seems that the reduction of antinociceptive effect of morphine after CCI may be mediated through glia activation. Modulation of glial activity by minocycline can attenuate CCI-induced neuropathic pain. It is also shown that repeated post-injury administration of minocycline improves the antinociceptive effect of morphine in neuropathic pain.     

鞘内注射米诺四环素对慢性坐骨神经结扎大鼠机械痛敏和热痛敏的影响

目的观察鞘内注射小胶质细胞抑制剂米诺四环素对慢性坐骨神经结扎大鼠机械痛敏和热痛敏的影响。方法所有大鼠术前8d鞘内置管,用机械缩足反射阈值和热缩足潜伏期来分别评价大鼠机械痛敏和热痛敏。前给药组:生理盐水10μl或米诺四环素50μg,于坐骨神经结扎前1d开始持续到术后1d(每天2次)鞘内注射,机械缩足反射阈值和热缩足潜伏期分别于术前2d,术后1,3,5,7,14d测定;后给药组:坐骨神经结扎后7d,鞘内注射1次生理盐水10μl或米诺四环素50μg,其对机械缩足反射阈值和热缩足潜伏期的影响分别于给药后0.5、1、2、4、8h测定。结果CCI大鼠从术后1d形成稳定的热痛敏和机械痛敏,前鞘内注射米诺四环素明显增加CCI大鼠MWT和TWL(P<0.05,P<0.01),相反,后鞘内注射米诺四环素对CCI大鼠MWT和TWL无明显影响。结论前鞘内注射米诺四环素明显抑制CCI大鼠机械痛敏和热痛敏,提示小胶质细胞的活化参与慢性坐骨神经结扎引发神经病理痛的形成。     

Ethosuximide reduces allodynia and hyperalgesia and potentiates morphine effects in the chronic constriction injury model of neuropathic pain

Neuropathic pain is caused by a lesion or disease of the somatosensory nervous system and treatment of neuropathic pain remains a challenge. The purpose of the present study was to examine the effect of ethosuximide, an anti-epileptic and relatively selective T-type calcium blocker and morphine, a prototypical opioid in the behavioral responses following the chronic constriction injury (CCI) model of neuropathic pain. Experiments were performed on eight groups (n=8) of male Sprague-Dawley rats (230-280 g). The animals were injected with saline, ethosuximide (100, 200, 300 mg/kg), morphine (4 mg/kg), and a combination of morphine (4 mg/kg) plus ethosuximide (100mg/kg, i.p.). The cold-and mechano-allodynia and thermal hyperalgesia were measured prior to surgery (the day 0) and 3, 5, 7, 14 and 21 days post surgery. Ethosuximide and morphine significantly decreased cold and mechano allodynia and thermal hyperalgesia. However, the co-administration of both drugs seems to be more effective than the ethosuximide or morphine alone on cold and mechano allodynia and thermal hyperalgesia .Our results suggest that ethosuximide block tactile and thermal hypersensitivity after the CCI model, also, ethosuximide potentiates the analgesic effects of morphine in neuropathic pain conditions and behavioral responses.     

Disability of development of tolerance to morphine and U-50,488H, a selective kappa-opioid receptor agonist, in neuropathic pain model mice

We examined the analgesic and anti-allodynic effects of morphine and U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)-benzeneacetamide methanesulfonate salt), a selective kappa-opioid receptor agonist, and the development of tolerance to their effects in neuropathic pain model mice induced by sciatic nerve ligation (SNL). In the tail-pinch method, morphine at 10 mg/kg, s.c. produced a weak analgesic effect in SNL mice; however, U-50,488H at 5 mg/kg, s.c. produced an analgesic effect equipotent to that in normal mice. In contrast, morphine produced an adequate analgesic effect when given either intracerebroventricularly (i.c.v.) or intrathecally (i.t.), but U-50,488H only produced analgesia when given i.t. Repeated administration of morphine (either i.c.v. or i.t.) or U-50,488H (either s.c. or i.t.), did not induce tolerance to the effect. In the static allodynia test with an application of von Frey filaments, both compounds given s.c. suppressed the allodynic effect, but in the dynamic allodynia test involving lightly stroking the plantar surface with a cotton bud, only U-50,488H produced an anti-allodynic effect. Repeated administrations of both compounds did not develop tolerance to these anti-allodynic effects. Thus, U-50,488H was found to be a highly effective at blocking hyperalgesia and allodynia in nerve injury, and these findings suggest that kappa-opioid receptor agonists are attractive pharmacological targets for the control of patients with neuropathic pain.     

Centrally mediated antihyperalgesic and antiallodynic effects of zonisamide following partial nerve injury in the mouse

Some antiepileptic drugs are used clinically to relieve neuropathic pain. We have evaluated the effects and investigated the possible mechanisms of action of zonisamide, an antiepileptic drug, on thermal hyperalgesia and tactile allodynia in a murine chronic pain model that was prepared by partial ligation of the sciatic nerve. Subcutaneously administered zonisamide (10 and 30 mg/kg) produced antihyperalgesic and antiallodynic effects in a dose-dependent manner; these effects were manifested by elevation of the withdrawal threshold in response to a thermal (plantar test) or mechanical (von Frey) stimulus, respectively. Similar analgesic effects were obtained in both the plantar and von Frey tests when zonisamide was injected either intracerebroventricularly (i.c.v., 10 and 30 μg) or intrathecally (i.t., 10 and 30 μg). It is thought that this elevation of the thermal and mechanical withdrawal thresholds after local injection of zonisamide is not generated secondarily via impaired motor activity, since zonisamide (30 μg, i.c.v. or i.t.) did not affect locomotor activity, as assessed in sciatic-nerve-ligated mice. Moreover, the nitric oxide synthase inhibitor L-NAME, when injected either i.c.v. or i.t., potentiated the analgesic effects of zonisamide. In contrast, neither i.c.v. nor i.t. zonisamide produced antinociceptive effects against acute thermal and mechanical nociception in non-ligated mice. Together, following peripheral nerve injury, it appears that zonisamide produces centrally mediated antihyperalgesic and antiallodynic effects partly via the blockade of nitric oxide synthesis.     

Guggulipid of Commiphora mukul,with antiallodynic and antihyperalgesic activities in both sciatic nerve and spinal nerve ligation models of neuropathic pain

Abstract

Context: Guggulipid is a neutral fraction of ethyl acetate extract of gum resin of the tree Commiphora mukul Engl. (Burseraceae) and used in Ayurvedic medicine for treatment of neurological disorders.

Objectives: The present study was undertaken to assess the antiallodynic and antihyperalgesic activities of guggulipid in rats.

Materials and methods: The screening study included the CCI and L5–L6 SNL models of neuropathic pain. Guggulipid (100 and 50?mg/kg) or saline was administered intraperitoneally in a blinded, randomized manner from postoperative day (POD) 7 to 13. Paw withdrawal duration (PWD) to spontaneous pain, chemical allodynia and mechanical hyperalgesia and paw withdrawal latency (PWL) to mechanical allodynia and thermal hyperalgesia were tested before surgery, before and after guggulipid or saline administration (from POD7 to 13) and after the withdrawal of treatment (from POD14 to 20).

Results: The activity profiles of the different doses of guggulipid were found to vary with time. In CCI rats, guggulipid (100 and 50?mg/kg) significantly (p?<?0.05) reduced the spontaneous pain, mechanical allodynia and mechanical and thermal hyperalgesia responses and the LD₅₀ of guggulipid was 1600?mg/kg. In SNL rats, both doses of guggulipid were found to be ineffective in reversing the spontaneous pain but showing antiallodynic and antihyperalgesic activity.

Discussion and conclusion: The results demonstrated that guggulipid produce antinociception in the peripheral nerve injury (CCI and SNL) models of neuropathic pain. The underlying mechanisms are expected to be modulating microglial activation occurring due to peripheral nerve injury.     

3-[4-(3-Trifluoromethyl-phenyl)-piperazin-1-yl]-dihydrofuran-2-one and pregabalin attenuate tactile allodynia in the mouse model of chronic constriction injury

Abstract

Purpose: There is a strong medical demand to search for novel, more efficacious and safer than available, analgesics for the treatment of neuropathic pain. This study investigated antinociceptive activity of intraperitoneally administered 3-[4-(3-trifluoromethyl-phenyl)-piperazin-1-yl]-dihydrofuran-2-one (LPP1) and pregabalin in the chronic constriction injury (CCI) model of neuropathic pain in mice and evaluated these drugs’ influence on motor coordination. In addition, microscopic examinations of the sciatic nerve were performed to assess, if a surgical method or drug treatment caused changes in the structure of this nerve. Moreover, the alterations of nerve growth factor (NGF) content after drug treatment were assessed.

Methods: Antiallodynic and antihyperalgesic activities of LPP1 and pregabalin were assessed in the von Frey and hot plate tests. Motor-impairing properties were evaluated in the rotarod test. Microscopic examinations of the sciatic nerve were performed using electron microscope. In immunohistochemical assays the content of NGF in the sciatic nerve after single or repeated administration of test drugs was assessed.

Results: Microscopic examinations of the sciatic nerve revealed ultrastructural changes in nerve fibers indicating for neurodegenerative processes induced by CCI. Seven days after CCI surgery LPP1 and pregabalin reduced tactile allodynia in von Frey test (ED₅₀ values were 1.5 and 15.4?mg/kg, respectively). None of the test drugs at dose range 0.5–100?mg/kg induced motor deficits in the rotarod test. In immunohistochemical assays repeated doses of pregabalin and LPP1 elevated NGF content.

Conclusions: LPP1 has antiallodynic properties and is an interesting lead structure in the search for novel analgesics used in neuropathic pain.     

Inhibition of microglial activity alters spinal wide dynamic range neuron discharge and reduces microglial Toll‐like receptor 4 expression in neuropathic rats

It is believed that neuropathic pain results from aberrant neuronal discharges although some evidence suggests that the activation of glia cells contributes to pain after an injury to the nervous system. This study aimed to evaluate the role of microglial activation on the hyper‐responsiveness of wide dynamic range neurons (WDR) and Toll‐like receptor 4 (TLR4) expressions in a chronic constriction injury (CCI) model of neuropathic pain in rats. Adult male Wistar rats (230 ± 30 g) underwent surgery for induction of CCI neuropathy. Six days after surgery, administration of minocycline (10, 20, and 40 mg/kg, i.p.) was initiated and continued until day 14. After administration of the last dose of minocycline or saline, a behavioral test was conducted, then animals were sacrificed and lumbar segments of the spinal cord were collected for Western blot analysis of TLR4 expression. The electrophysiological properties of WDR neurons were investigated by single unit recordings in separate groups. The findings showed that after CCI, in parallel with thermal hyperalgesia, the expression of TLR4 in the spinal cord and the evoked response of the WDR neurons to electrical, mechanical, and thermal stimulation significantly increased. Post‐injury administration of minocycline effectively decreased thermal hyperalgesia, TLR4 expression, and hyper‐responsiveness of WDR neurons in CCI rats. The results of this study indicate that post‐injury, repeated administration of minocycline attenuated neuropathic pain by suppressing microglia activation and reducing WDR neuron hyper‐responsiveness. This study confirms that post‐injury modulation of microglial activity is a new strategy for treating neuropathic pain.     

Behavioural effects of the novel AMPA/GluR5 selective receptor antagonist NS1209 after systemic administration in animal models of experimental pain

The effects of systemic administration of the novel AMPA/GluR5 selective receptor antagonist NS1209 in animal models of experimental pain have been tested and compared with the AMPA receptor antagonist NBQX and the opiate morphine. In the mouse hot plate test, NS1209 (3-30 mg/kg, s.c. and i.p.) and morphine (3-30 mg/kg, s.c.) significantly increased the nociceptive response latency, whereas NBQX (3-30 mg/kg, i.p.) was ineffective. In the rat formalin test, a model of persistent pain, NS1209 (3 and 6 mg/kg, i.p.) and morphine (0.5 and 3 mg/kg, s.c.) produced dose-dependent reductions in second phase nociceptive behaviours, although NBQX (10 and 20 mg/kg, i.p.) was without effect. In the chronic constriction injury model of neuropathic pain, NS1209 (3 and 6 mg/kg, i.p.), NBQX (10 and 20 mg/kg, i.p.) and morphine (3 and 6 mg/kg, s.c.) all reduced mechanical allodynia and hyperalgesia responses to von Frey hair and pin prick stimulation of the injured hindpaw. NS1209 and morphine also reduced cold hypersensitivity in response to ethyl chloride stimulation of the injured hindpaw. At the doses associated with anti-nociceptive actions, no effects on motor performance as determined by the rotarod test were observed for any of the drugs tested. Thus, systemic administration of NS1209 at non-ataxic doses has marked analgesic actions comparable to those of morphine in a range of animal models of experimental pain.     

Minocycline can delay the development of morphine tolerance,but cannot reverse existing tolerance in the maintenance period of neuropathic pain in rats

Neuropathic pain is a challenge for physicians and basic science researchers, because it often does not respond to routine treatment. The administration of morphine has been considered one of the effective recommended treatments, but its wide application is limited because of the development of antinociceptive tolerance. In general, basic science studies focus on neuropathic pain and morphine tolerance separately. However, we tried to investigate the effect of microglial activation on morphine tolerance in spinal nerve ligation (SNL) rats during the maintenance period of neuropathic pain. This study produced the following results. The morphine tolerance model in neuropathic pain was established by repeated administration of morphine twice daily (10 mg/kg s.c) in the maintenance period of SNL rats. Minocycline, the microglial activation inhibitor, was given once daily (30 mg/kg, i.p.) at different time‐points. The CD11b protein level was measured by western blot to monitor microglial activation. Rats’ mechanical allodynia was assessed using the 50% paw withdrawal threshold, and the tail antinociception was determined using the percentage of the maximal possible antinociceptive effect. First, the repeated administration of morphine induced the development of antinociceptive tolerance during the maintenance period of neuropathic pain. Second, during the development of morphine tolerance, microglial activation, which is related to the analgesic effect of morphine, decreased in the first few days, but this pattern was reversed in the following days with the development of morphine tolerance. Third, the repeated administration of minocycline, a microglial activation inhibitor, did not influence the antinociceptive effect of a single dose of morphine. Fourth, the pre‐administration of minocycline can delay the development of morphine tolerance, but repeated minocycline administration cannot reverse existing morphine tolerance. We concluded that microglial activation contributes to the morphine tolerance of SNL rats in the maintenance period of neuropathic pain, and that minocycline delays the development of morphine tolerance, but does not reverse existing morphine tolerance during the maintenance period of neuropathic pain in rats. These findings might be useful for clinical pain management.     

Attenuation of morphine tolerance and dependence by aminoguanidine in mice

The effect of aminoguanidine, an inducible nitric oxide synthase (iNOS) inhibitor, on morphine-induced tolerance and dependence in mice was investigated in this study. Acute administration of aminoguanidine (20 mg/kg, p.o.) did not affect the antinociceptive effect of morphine (10 mg/kg, s.c.) as measured by the hot plate test. Repeated administration of aminoguanidine along with morphine attenuated the development of tolerance to the antinociceptive effect of morphine. Also, the development of morphine dependence as assessed by naloxone-precipitated withdrawal manifestations was reduced by co-administration of aminoguanidine. The effect of aminoguanidine on naloxone-precipitated withdrawal was enhanced by concurrent administration of the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, dizocilpine (0.25 mg/kg, i.p.) or the non-specific nitric oxide synthase (NOS) inhibitor, l-N(G)-nitroarginine methyl ester (l-NAME; 5 mg/kg, i.p.) and antagonized by concurrent administration of the nitric oxide (NO) precursor, l-arginine (50 mg/kg, p.o.). Concomitantly, the progressive increase in NO production, but not in brain glutamate level, induced by morphine was inhibited by repeated administration of aminoguanidine along with morphine. Similarly, co-administration of aminoguanidine inhibited naloxone-induced NO overproduction, but it did not inhibit naloxone-induced elevation of brain glutamate level in morphine-dependent mice. The effect of aminoguanidine on naloxone-induced NO overproduction was potentiated by concurrent administration of dizocilpine or l-NAME and antagonized by concurrent administration of l-arginine. These results provide evidence that blockade of NO overproduction, the consequence of NMDA receptor activation, by aminoguanidine, via inhibition of iNOS, can attenuate the development of morphine tolerance and dependence.     

Curcumin exerts antinociceptive effects in a mouse model of neuropathic pain: descending monoamine system and opioid receptors are differentially involved

Curcumin, a phenolic compound present in Curcuma longa, has been reported to exert antinociceptive effects in some animal models, but the mechanisms remain to be elucidated. This work aimed to investigate the antinociceptive action of curcumin on neuropathic pain and the underlying mechanism(s). Chronic constriction injury (CCI), a canonical animal model of neuropathic pain, was produced by loosely ligating the sciatic nerve in mice and von Frey hair or hot plate test was used to assess mechanical allodynia or thermal hyperalgesia (to heat), respectively. Chronic, but not acute, curcumin treatment (5, 15 or 45 mg/kg, p.o., twice per day for three weeks) alleviated mechanical allodynia and thermal hyperalgesia in CCI mice, accompanied by increasing spinal monoamine (or metabolite) contents. Chemical ablation of descending noradrenaline (NA) by 6-hydroxydopamine (6-OHDA), or depletion of descending serotonin by p-chlorophenylalanine (PCPA), abolished curcumin's antinociceptive effect on mechanical allodynia or thermal hyperalgesia, respectively. The anti-allodynic action of curcumin on mechanical stimuli was totally blocked by chronic co-treatment with the β(2)-adrenoceptor antagonist ICI 118,551, or by acute co-treatment with the delta-opioid receptor antagonist naltrindole. Meanwhile, co-treatment with the 5-HT(1A) receptor antagonist WAY-100635 chronically, or with the irreversible mu-opioid receptor antangonist β-funaltrexamine acutely, completely abrogated the anti-hyperalgesic action of curcumin on thermal stimuli. Collectively, these findings indicate that the descending monoamine system (coupled with spinal β(2)-adrenoceptor and 5-HT(1A) receptor) is critical for the modality-specific antinociceptive effect of curcumin in neuropathic pain. Delta- and mu-opioid receptors are likely rendered as downstream targets, accordingly. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.     

Suppression of interleukin-6 by minocycline in a rat model of neuropathic pain

Inflammatory mediators produced in the injured nerve have been proposed as contributing factors in the development of neuropathic pain. In this regard an important role is assigned to interleukin-6. The present study, evaluated the effect of pretreatment with minocycline, on pain behavior (hyperalgesia and allodynia) and serum level of interleukin-6 in chronic constriction injury (CCI) model of neuropathic pain in rat. Minocycline (5, 10, 20 and 40 mg/kg, i.p.) was injected 1 h before surgery and continued daily to day 14 post-ligation. Behavioral tests were recorded before surgery and on postoperative days 1, 3, 5, 7, 9, 10, 14, and the serum concentration of interleukin-6 was determined at day 14. We observed that minocycline which was reported to have a neuroprotective effect in some neurodegenerative diseases, reversed hyperalgesia and allodynia due to sciatic nerve ligation and inhibited the interleukin-6 production. It seems that minocycline could have an anti-inflammatory and analgesic effect in some chronic pain states.     

The α7 nicotinic acetylcholine receptor positive allosteric modulator prevents lipopolysaccharide-induced allodynia,hyperalgesia and TNF-α in the hippocampus in mice

BackgroundPrevious studies have shown that α7 nicotinic acetylcholine receptor (nAChR) has a critical role in the regulation of pain sensitivity and neuroinflammation. However, pharmacological effects of α7 nAChR activation in the hippocampus on neuroinflammatory mechanisms associated with allodynia and hyperalgesia remain unknown. We have determined the effects of 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS), an α7 nAChR positive allosteric modulator, on lipopolysaccharide (LPS)-induced allodynia and hyperalgesia in mice. We also evaluated the effects of TQS on immunoreactivity of microglial marker ionized-calcium binding adaptor molecule 1 (Iba-1), phospho-nuclear factor-κB (p-NF-κB p⁶⁵), tumor necrosis factor-alpha (TNF-α), and norepinephrine (NE) level.MethodsMice were treated with (0.25, 1 or 4 mg/kg, ip) followed by LPS (1 mg/kg, ip) administration. Allodynia and hyperalgesia were determined using von Frey filaments and hot plate respectively. Immunoreactivity of Iba-1, p-NF-κB p⁶⁵, and TNF-α, were measured in the hippocampus using immunofluorescence assay. Hippocampal NE level was evaluated using high performance liquid chromatography.ResultsLPS administration resulted in allodynia and hyperalgesia in mice after six h. Systemic administration of TQS prevented LPS-induced allodynia and hyperalgesia. TQS pretreatment significantly decreased the immunoreactivity of Iba-1, p-NF-κB, and TNF-α in CA1 and DG regions of the hippocampus. In addition, TQS reversed LPS-induced NE reduction in the hippocampus.ConclusionsTaken together, our results suggest that TQS prevented LPS-induced allodynia and hyperalgesia, upregulation of TNF-α expression and NE level reduction involving microglial α7 nAChR in part in the hippocampus. Therefore, these findings highlight the important effects of α7 nAChR allosteric modulator against symptoms of inflammatory pain.     

Antinociceptive efficacy of lacosamide in rat models for tumor- and chemotherapy-induced cancer pain

Pain is the most common physical symptom of cancer patients, with most patients experiencing more than one site of pain. Current treatments lack full efficacy. Based on the need for new approaches in that field the effect of systemic administration of lacosamide (SPM 927, (R)-2-acetamido-N-benzyl-3-methoxypropionamide, previously referred to as harkoseride or ADD 234037), a member of a series of functionalized amino acids that were specifically synthesized as anticonvulsive drug candidates, was examined in rats in a tumor-induced bone cancer pain model and in a chemotherapy-induced neuropathic pain model. Lacosamide inhibited tactile allodynia (20, 40 mg/kg, i.p.), thermal hyperalgesia (30 mg/kg) and reduced weight-bearing differences (40 mg/kg) in the rat model of bone cancer pain induced by injection of MRMT-1 cells into the tibia. Morphine (5 mg/kg, s.c) was effective inhibiting tactile allodynia and weight bearing but could not reduce thermal hyperalgesia. In the vincristine-induced neuropathic pain model, lacosamide attenuated thermal allodynia, on the cold plate (4 degrees C), at 10 and 30 mg/kg, and in the warm (38 degrees C) and hot plate (52 degrees C) even at 3 mg/kg. Tactile allodynia and mechanical hyperalgesia were inhibited by lacosamide at 10 and 30 mg/kg. In contrast to lacosamide, morphine (3 mg/kg, s.c.) had no effect on mechanical hyperalgesia. Lacosamide is effective as an analgesic in a bone cancer pain model as well as chemotherapy-induced neuropathic pain model in animals and even reduced hyperalgesia where morphine did not (3 or 5 mg/kg, s.c.).     

虎杖提取物对神经病理性疼痛模型大鼠的镇痛作用研究

目的：研究虎杖提取物对神经病理性疼痛模型大鼠的镇痛作用,并探讨其作用机制。方法雄性SD大鼠,随机分为对照组和坐骨神经缩狭模型（chronic construction injury, CCI）组。CCI术后14 d,应用Von Frey法测定机械刺激缩足反射阈值（mechanical withdrawal threshold, MWT）,观察口服不同剂量的虎杖提取物对CCI大鼠机械性痛觉超敏的影响。并考察不同剂量虎杖提取物对CCI大鼠脊髓p-ERK、p-P38水平及小胶质细胞活化的影响。结果虎杖提取物可明显升高CCI大鼠术侧的MWT值（P〈0.05）,而对正常大鼠的MWT值没有显著影响（P〉0.05）。同时虎杖提取物可显著抑制CCI模型大鼠脊髓ERK和p38的磷酸化,并显著抑制IBA-1表达增加。结论虎杖提取物对CCI诱导的机械性痛觉超敏具有显著性的抑制作用,且镇痛作用维持时间长于吗啡,其机制可能是通过抑制脊髓水平ERK和p38的磷酸化,及通过抑制小胶质细胞活化而实现。     

The synthetic cannabinoids attenuate allodynia and hyperalgesia in a rat model of trigeminal neuropathic pain

Trigeminal neuralgia is a disorder of paroxysmal and severely disabling facial pain and continues to be a real therapeutic challenge. At present there are few effective drugs. Here we have evaluated the effects of the synthetic cannabinoid WIN 55,212-2 on mechanical allodynia and thermal hyperalgesia in a rat model of trigeminal neuropathic pain produced by a chronic constriction injury (CCI) of the infraorbital branch of the trigeminal nerve (ION). Relative to sham operation controls, rats with the CCI-ION consistently displayed hyperresponsiveness to von Frey filament and heat stimulation of the vibrissal pad. Both mechanical allodynia and thermal hyperalgesia are seen both ipsilateral and contralateral to the side of nerve injury, but is significantly more severe ipsilaterally. Administration of WIN 55,212-2 (0.3-5 mg/kg i.p.) dose-dependently increased the mechanical and heat withdrawal thresholds. WIN 55,212-2 (0.3-3 mg/kg i.p.) produced no significant motor deficits in animals using the rotarod test. The effect of WIN 55,212-2 was mimicked by cannabinoid CB1 receptor agonist HU 210 and was antagonized by CB1 receptor antagonist AM 251, but not by CB2 receptor antagonist AM 630 or vanilloid receptor 1 antagonist capsazepine, suggesting the involvement of CB1 receptors. CCI-ION also induced a time-dependent upregulation of CB1 receptors primarily within the ipsilateral superficial laminae of the trigeminal caudal nucleus revealed by both Western blot and immunohistochemistry. Taken together, these results suggest that cannabinoids may be a useful therapeutic approach for the clinical management of trigeminal neuropathic pain disorders.     

Nefopam potentiates morphine antinociception in allodynia and hyperalgesia in the rat

The objective of this study was to resolve discrepancies regarding the possible antinociceptive synergy between morphine and nefopam in animal models of pain. Firstly, we have examined the antinociceptive activity of nefopam, a nonopioid antinociceptive compound that inhibits monoamine reuptake, in pain models of allodynia and hyperalgesia induced by carrageenan injection, or skin and muscle incision of the rat hind paw. Single subcutaneous administration of nefopam at 30 mg/kg blocked carrageenan- and incision-induced thermal hyperalgesia, and weakly but significantly diminished carrageenan-induced tactile allodynia. A weaker dose of nefopam (10 mg/kg) only reduced carrageenan-induced tactile allodynia and incision-induced thermal hyperalgesia. Secondly, we assessed the usefulness of the coadministration of nefopam with morphine. Combination of a nonanalgesic dose of nefopam (10 mg/kg) with a nonanalgesic dose of morphine (0.3 or 1.0 mg/kg) completely inhibited carrageenan- or incision-induced thermal hyperalgesia, respectively. In carrageenan-induced tactile allodynia, coadministration of weak analgesic doses of nefopam (10 and 30 mg/kg) with a nonanalgesic dose (1 mg/kg) or moderately analgesic dose (3 mg/kg) of morphine significantly reduced or reversed allodynia, respectively. In conclusion, coadministration of nefopam with morphine enhances the analgesic potency of morphine, indicating a morphine sparing effect of nefopam.     

Antihyperalgesic effects of imidazoline I2 receptor ligands in rat models of inflammatory and neuropathic pain

Background and Purpose

A new imidazoline I₂ receptor ligand, CR4056, is effective for chronic inflammatory pain and diabetic neuropathy. However, it is unclear whether other I₂ receptor ligands have similar effects and whether antinociceptive tolerance develops with repeated treatment.

Experimental Approach

The Von Frey filament test was used to measure mechanical hyperalgesia and the plantar test to measure thermal hyperalgesia in rats injected with complete Freund''s adjuvant (CFA) treatment or had undergone surgery to induce chronic constriction injury (CCI), models of inflammatory pain and peripheral neuropathic pain respectively. The effects of morphine and I₂ receptor ligands, 2-BFI, BU224, tracizoline and CR4056, 3.2–32 mg·kg−1, i.p., on hyperalgesia or affective pain (as measured by a place escape/avoidance paradigm) were studied in separate experiments.

Key Results

Morphine and the I₂ receptor ligands (2-BFI, BU224 and tracizoline) all dose-dependently attenuated mechanical and thermal hyperalgesia in CFA-treated rats. The anti-hyperalgesic effects of 2-BFI in CFA-treated and CCI rats were attenuated by the I₂ receptor antagonist idazoxan. The combination of 2-BFI and morphine produced additive effects against mechanical hyperalgesia in CFA-treated rats. Repeated treatment (daily for 7–9 days) with 2-BFI or CR4056 did not produce antinociceptive tolerance in CFA-treated or CCI rats. Morphine and the I₂ receptor ligands (2-BFI, BU224 and CR4056) were all effective at attenuating place escape/avoidance behaviour in CFA-treated rats.

Conclusions and Implications

Imidazoline I₂ receptor ligands have antihyperalgesic effects in rat models of inflammatory and neuropathic pain and may represent a new class of pharmacotherapeutics for the management of chronic pain.     

Development and expression of neuropathic pain in CB1 knockout mice

Neuropathic pain is a clinical manifestation characterized by the presence of spontaneous pain, allodynia and hyperalgesia. Here, we have evaluated the involvement of CB1 cannabinoid receptors in the development and expression of neuropathic pain. For this purpose, partial ligation of the sciatic nerve was performed in CB1 cannabinoid receptor knockout mice and their wild-type littermates. The development of mechanical and thermal allodynia, and thermal hyperalgesia was evaluated by using the von Frey filaments, cold-plate and plantar tests, respectively. Pre-surgical tactile and thermal withdrawal thresholds were similar in both genotypes. In wild-type mice, sciatic nerve injury led to a neuropathic pain syndrome characterized by a marked and long-lasting reduction of the paw withdrawal thresholds to mechanical and thermal stimuli. These manifestations developed similarly in mice lacking CB1 cannabinoid receptors. We have also investigated the consequences of gabapentin administration in these animals. Gabapentin (50 mg/kg/day, i.p.) induced a similar suppression of mechanical and thermal allodynia in both wild-type and CB1 knockout mice. Mild differences between genotypes were observed concerning the effect of gabapentin in the expression of thermal hyperalgesia. Taken together, our results indicate that CB1 cannabinoid receptors are not critically implicated in the development of neuropathic pain nor in the anti-allodynic and anti-hyperalgesic effects of gabapentin in this model.