Spinal cord transection-induced allodynia in rats--behavioral, physiopathological and pharmacological characterization.

In humans, spinal cord lesions induce not only major motor and neurovegetative deficits but also severe neuropathic pain which is mostly resistant to classical analgesics. Better treatments can be expected from precise characterization of underlying physiopathological mechanisms. This led us to thoroughly investigate (i) mechanical and thermal sensory alterations, (ii) responses to acute treatments with drugs having patent or potential anti-allodynic properties and (iii) the spinal/ganglion expression of transcripts encoding markers of neuronal injury, microglia and astrocyte activation in rats that underwent complete spinal cord transection (SCT). SCT was performed at thoracic T8-T9 level under deep isoflurane anaesthesia, and SCT rats were examined for up to two months post surgery. SCT induced a marked hyper-reflexia at hindpaws and strong mechanical and cold allodynia in a limited (6 cm2) cutaneous territory just rostral to the lesion site. At this level, pressure threshold value to trigger nocifensive reactions to locally applied von Frey filaments was 100-fold lower in SCT- versus sham-operated rats. A marked up-regulation of mRNAs encoding ATF3 (neuronal injury) and glial activation markers (OX-42, GFAP, P2×4, P2×7, TLR4) was observed in spinal cord and/or dorsal root ganglia at T6-T11 levels from day 2 up to day 60 post surgery. Transcripts encoding the proinflammatory cytokines IL-1ß, IL-6 and TNF-α were also markedly but differentially up-regulated at T6-T11 levels in SCT rats. Acute treatment with ketamine (50 mg/kg i.p.), morphine (3-10 mg/kg s.c.) and tapentadol (10-20 mg/kg i.p.) significantly increased pressure threshold to trigger nocifensive reaction in the von Frey filaments test, whereas amitriptyline, pregabalin, gabapentin and clonazepam were ineffective. Because all SCT rats developed long lasting, reproducible and stable allodynia, which could be alleviated by drugs effective in humans, thoracic cord transection might be a reliable model for testing innovative therapies aimed at reducing spinal cord lesion-induced central neuropathic pain.

Differential pharmacological alleviation of oxaliplatin-induced hyperalgesia/allodynia at cephalic versus extra-cephalic level in rodents.

Previous data showed that neuropathic pain induced by mechanical lesion of peripheral nerves responds differently to alleviating drugs at cephalic versus extracephalic level. Because neuropathic pain evoked by anti-cancer drugs differs from that triggered by mechanical nerve lesion, we investigated whether differences between cephalic and extracephalic levels could also be characterized in rodents rendered neuropathic by treatment with the anti-cancer platinum derivative oxaliplatin. C57BL/6J mice received two injections and Sprague-Dawley rats three injections of oxaliplatin (10 mg/kg, i.p.) or its vehicle, with three days intervals. Supersensitivity to mechanical (von Frey filaments), cold (acetone drop) and chemical/inflammatory (formalin) stimulations was assessed in vibrissae and hindpaw territories. Transcripts of neuroinflammatory markers were quantified by real-time RT-qPCR in rat ganglia and central tissues. Oxaliplatin induced mechanical allodynia, cold hyperalgesia and chemical/inflammatory supersensitivity at both hindpaw and vibrissal levels in mice and rats. Acute treatment with gabapentin (30 mg/kg i.p.), morphine (3 mg/kg s.c.) or the 5-HT1A receptor agonist 8-OH-DPAT (0.16 mg/kg s.c.) significantly reduced oxaliplatin-induced supersensitivity in hindpaw but not vibrissal territory. In contrast, the antimigraine drugs naratriptan (0.1 mg/kg s.c.) and olcegepant (0.6 mg/kg i.v.) decreased oxaliplatin-induced supersensitivity in vibrissal territory only. Among the various markers investigated, only TRPA1 transcript was upregulated in ganglia of oxaliplatin-treated rats. These data showed that oxaliplatin induced supersensitivity to various stimuli in both cephalic and extra-cephalic territories in rodents. Regional differences in the efficacy of drugs to alleviate oxaliplatin-induced allodynia/hyperalgesia further support the idea that mechanisms underlying neuropathic pain have peculiarities at cephalic versus extra-cephalic level.

GABA, but not opioids, mediates the anti-hyperalgesic effects of 5-HT7 receptor activation in rats suffering from neuropathic pain.

Among receptors mediating serotonin actions in pain control, the 5-HT(7)R is of special interest because it is expressed by primary afferent fibers and intrinsic GABAergic and opioidergic interneurons within the spinal dorsal horn. Herein, we investigated whether GABA and/or opioids contribute to 5-HT(7)R-mediated control of neuropathic pain caused by nerve ligation. Acute administration of 5-HT(7)R agonists (AS-19, MSD-5a, E-55888) was found to markedly reduce mechanical and thermal hyperalgesia in rats with unilateral constriction injury to the sciatic nerve (CCI-SN). In contrast, mechanical hypersensitivity caused by unilateral constriction injury to the infraorbital nerve was essentially unaffected by these ligands. Further characterization of the anti-hyperalgesic effect of 5-HT(7)R activation by the selective agonist E-55888 showed that it was associated with a decrease in IL-1ß mRNA overexpression in ipsilateral L4-L6 dorsal root ganglia and lumbar dorsal horn in CCI-SN rats. In addition, E-55888 diminished CCI-SN-associated increase in c-Fos immunolabeling in superficial laminae of the lumbar dorsal horn and the locus coeruleus, but increased c-Fos immunolabeling in the nucleus tractus solitarius and the parabrachial area in both control and CCI-SN rats. When injected intrathecally (i.t.), bicuculline (3 µg i.t.), but neither phaclofen (5 µg i.t.) nor naloxone (10 µg i.t.), significantly reduced the anti-hyperalgesic effects of 5-HT(7)R activation (E-55888, 10 mg/kg s.c.) in CCI-SN rats. These data support the idea that 5-HT(7)R-mediated inhibitory control of neuropathic pain is underlain by excitation of GABAergic interneurons within the dorsal horn. In addition, 5-HT(7)R activation-induced c-Fos increase in the nucleus tractus solitarius and the parabrachial area suggests that supraspinal mechanisms might also be involved.

Differential effects of calcitonin gene-related peptide receptor blockade by olcegepant on mechanical allodynia induced by ligation of the infraorbital nerve vs the sciatic nerve in the rat.

Previous studies showed that 5-hydroxytryptamine (5-HT)(1B/1D) receptor stimulation by triptans alleviates neuropathic pain caused by chronic constriction injury to the infraorbital nerve (CCI-ION) but not the sciatic nerve (CCI-SN) in rats. To assess whether such differential effects in the cephalic vs extracephalic territories is a property shared by other antimigraine drugs, we used the same models to investigate the effects of olcegepant, which has an antimigraine action mediated through calcitonin gene-related peptide (CGRP) receptor blockade. Adult male rats underwent unilateral CCI to the ION or the SN, and subsequent allodynia and/or hyperalgesia were assessed in ipsilateral vibrissal territory or hindpaw, respectively, using von Frey filaments and validated nociceptive tests. c-Fos expression was quantified by immunohistochemistry and interleukin 6 and activating transcription factor 3 (ATF3) mRNAs by real-time quantitative reverse transcriptase-polymerase chain reaction. Like naratriptan (0.1 to 0.3mg/kg, subcutaneously), olcegepant (0.3 to 0.9mg/kg, intravenously) markedly reduced mechanical allodynia in CCI-ION rats. In contrast, in CCI-SN rats, mechanical allodynia was completely unaffected and hyperalgesia was only marginally reduced by these drugs. A supra-additive antiallodynic effect was observed in CCI-ION rats treated with olcegepant (0.3mg/kg intravenously) plus naratriptan (0.1mg/kg subcutaneously), whereas this drug combination remained inactive in CCI-SN rats. Olcegepant (0.6mg/kg, intravenously) significantly reduced the number of c-Fos immunolabeled cells in spinal nucleus of the trigeminal nerve and upregulation of ATF3 transcript (a marker of neuron injury) but not that of interleukin-6 in trigeminal ganglion of CCI-ION rats. These findings suggest that CGRP receptor blockade might be of potential interest to alleviate trigeminal neuropathic pain.

N-methyl-D-aspartate receptor-mediated modulations of the anti-allodynic effects of 5-HT1B/1D receptor stimulation in a rat model of trigeminal neuropathic pain.

Previous studies showed that triptans and other 5-HT(1B/1D)-receptor agonists attenuate hyper-responsiveness to mechanical stimulation of the face in a rat model of trigeminal neuropathic pain, probably by activating 5-HT(1B/1D)-receptors on primary afferent nociceptive fibers. We now tested whether blockade of post-synaptic receptors for the excitatory amino acid glutamate released by these fibers would increase this action. We thus evaluated whether (±)1-hydroxy-3-aminopyrrolidine-2-one (HA-966), an antagonist at the glycine/D-serine site of N-methyl-D-aspartate (NMDA)-receptors, would potentiate the anti-allodynic action of dihydroergotamine and zolmitriptan in rats with chronic constriction injury to the infraorbital nerve (CCI-ION). Complementary studies were performed with other NMDA-receptor ligands and in rats with chronic constriction injury to the sciatic nerve (CCI-SN) for comparison. Injury was produced by loose ligatures of the nerves. Responsiveness to mechanical stimulation (vibrissae or hindpaw territories) with von Frey filaments was used to evaluate allodynia 2 weeks after nerve ligature. Rats received NMDA-receptor ligands or saline 20 min before dihydroergotamine (25-100 µg/kg, i.v.) or zolmitriptan (25-100 µg/kg, s.c.). HA-966 (2.5mg/kg, s.c.), inactive on its own, enhanced the anti-allodynic effects of dihydroergotamine (eightfold increase) and zolmitriptan (threefold increase) in CCI-ION rats, but these drugs exerted no effects in allodynic CCI-SN rats. NMDA-receptor blockade by memantine (5mg/kg, i.p.) also enhanced, whereas activation at glycine/NMDA site by D-cycloserine (3mg/kg, i.p.) reduced the anti-allodynic properties of zolmitriptan in CCI-ION rats. Combined administration of NMDA-receptor antagonist and 5-HT(1B/1D)-receptor agonist may be a promising approach for alleviating trigeminal neuropathic pain.

Differential anti-neuropathic pain effects of tetrodotoxin in sciatic nerve- versus infraorbital nerve-ligated rats--behavioral, pharmacological and immunohistochemical investigations.

Several voltage-gated sodium channels are expressed in primary sensory neurons where they control excitability and participate in the generation and propagation of action potentials. Peripheral nerve injury-induced alterations in both tetrodotoxin (TTX)-sensitive and TTX-resistant sodium channels have been proposed to contribute to neuropathic pain caused by such lesion. We herein investigated whether the blockade of TTX-sensitive channels could reduce pain-related behaviors and evoked c-Fos immunoreactivity in rats with neuropathic pain produced by chronic unilateral constriction injury to either the sciatic nerve or the infraorbital nerve. Acute as well as subchronic administration of TTX (1-6 mug/kg s.c.) was found to suppress for up to 3 h allodynia and hyperalgesia in sciatic nerve-ligated rats. In contrast, TTX was only moderately effective in rats with ligated infraorbital nerve. In sciatic nerve-ligated rats, TTX administration prevented the increased c-Fos immunoreactivity occurring in the dorsal horn of the lumbar cord and some supraspinal areas in response to light mechanical stimulation of the nerve-injured hindpaw. The anti-allodynia/antihyperalgesia caused by TTX in these neuropathic rats was promoted by combined treatment with naloxone (0.5 mg/kg s.c.) but unaffected by the 5-HT(1B) receptor antagonist F11648 (0.5 mg/kg s.c.) and the alpha(2)-adrenergic receptor antagonist idazoxan (0.5 mg/kg i.v.). In contrast, the anti-allodynic and anti-hyperalgesic effects of TTX were significantly attenuated by co-administration of morphine (3 mg/kg s.c.) or the cholecystokinin(2)-receptor antagonist CI-1015 (0.1 mg/kg i.p.). These results indicate that TTX alleviates pain-related behaviors in sciatic nerve-lesioned rats through mechanisms that involve complex interactions with opioidergic systems.

Differential implication of proinflammatory cytokine interleukin-6 in the development of cephalic versus extracephalic neuropathic pain in rats.

Responses resulting from injury to the trigeminal nerve exhibit differences compared with those caused by lesion of other peripheral nerves. With the aim of elucidating the physiopathological mechanisms underlying cephalic versus extracephalic neuropathic pain, we determined the time course expression of proinflammatory cytokines interleukin-6 (IL-6) and IL-1beta, neuronal injury (ATF3), macrophage/microglial (OX-42), and satellite cells/astrocyte (GFAP) markers in central and ganglion tissues in rats that underwent unilateral chronic constriction injury (CCI) to either infraorbital nerve (IoN) (cephalic area) or sciatic nerve (SN) (extracephalic area). Whereas CCI induced microglial activation in both models, we observed a concomitant upregulation of IL-6 and ATF3 in the ipsilateral dorsal horn of the lumbar cord in SN-CCI rats but not in the ipsilateral spinal nucleus of the trigeminal nerve (Sp5c) in IoN-CCI rats. Preemptive treatment with minocycline (daily administration of 20 mg/kg, i.p., for 2 weeks) partially prevented pain behavior and microglial activation in SN-CCI rats but was ineffective in IoN-CCI rats. We show that IL-6 can upregulate OX-42 and ATF3 expression in cultured microglia and neurons from spinal cord, respectively, as well as in the dorsal horn after acute intrathecal administration of the cytokine. We propose that IL-6 could be one of the promoters of the signaling cascade leading to abnormal pain behavior in SN-CCI but not IoN-CCI rats. Our data further support the idea that different pathophysiological mechanisms contribute to the development of cephalic versus extracephalic neuropathic pain.

[Neuropathic pain. Physiopathological mechanisms and therapeutic perspectives]. / Douleurs neuropathiques. Physiopathologie et perspectives thérapeutiques.

Neuropathic pain is generally resistant to "classical" analgesic drugs, including opioids, and there is still an urgent need for really effective treatments to alleviate pain caused by lesions of the peripheral and/or central nervous system. The pathophysiological mechanisms underlying neuropathic pain are still poorly known, and treatments are mainly empirical. Antidepressant drugs are generally prescribed first, with positive but limited results in a significant proportion of patients. Anticonvulsant drugs (carbamazepine, phenytoin, lamotrigine) are also used but are often poorly tolerated. Clinical studies and preclinical investigations support the idea that the nature of neuropathic pain, and the underlying mechanisms, are different in the cephalic (trigeminal) territories and the extracephalic (spinal) territories. In order to further investigate these regional differences, we used rat nerve ligature models. Comparison of allodynia/hyperalgesia in the vibrissal territory caused by unilateral ligature of the infraorbital nerve (2nd branch of the trigeminal nerve) with those in the hindpaw ipsilateral to unilateral ligature of the sciatic nerve revealed marked differences in their responses to sodium channel blockers (such as tetrodotoxin), serotonin (5-HT) receptor agonists and calcitonin gene-related peptide (CGRP) receptor antagonists. In particular, 5-HT7 receptor agonists were particularly effective at reducing allodynia in sciatic nerve-ligated rats, but were completely ineffective in infraorbital nerve-ligated rats. Conversely, triptans (5-HT1B/1D receptor agonists) and CGRP-receptor antagonists markedly inhibited cephalic allodynia in infraorbital nerve-ligated rats but failed to relieve neuropathic pain in sciatic nerve-ligated animals. Interestingly, ligature-induced expression of the proinflammatory cytokine interleukin-6 in central tissues showed marked differences in sciatic nerve- and infraorbital nerve-ligated rats, providing direct evidence of differences in the mechanisms underlying extra-cephalic- and cephalic neuropathic pain. Such preclinical studies should contribute to the design of innovative strategies for more effective and well-tolerated treatments for neuropathic pain in cephalic and extra-cephalic territories.

Mechanical, thermal and formalin-induced nociception is differentially altered in 5-HT1A-/-, 5-HT1B-/-, 5-HT2A-/-, 5-HT3A-/- and 5-HTT-/- knock-out male mice.

Extensive studies in rodents suggest that serotonin (5-HT) modulates nociceptive responses through the stimulation of several receptor types. However, it remains to demonstrate that these receptors participate in the control of nociception under physiological conditions. Pain behaviors of mutants which do not express 5-HT1A, 5-HT1B, 5-HT2A or 5-HT3A receptors, or lacking the 5-HT transporter, compared to paired wild-type mice of the same genetic background, were examined using validated tests based on different sensory modalities. Mechanical (von Frey filaments, tail pressure, tail clip tests), thermal (radiant heat, 46 degrees C water bath, hot-plate test) and formalin-induced nociception were determined in 2- to 3-month-old males. 5-HT1A knock-out mice differed from wild-types by higher thermal sensitivity (hot-plate test only), and 5-HT1B knock-out mice by higher thermal and formalin sensitivity. Both 5-HT2A and 5-HT3A knock-out mice differed from wild-types by a dramatic decrease in the formalin-induced nociceptive responses for phase II (16-45 min after injection/inflammatory phase). In contrast, neither mechanical, thermal nor formalin-induced nociception differed between mutants lacking the 5-HT transporter and paired wild-type mice. Although differences in spontaneous locomotor activity in 5-HT1B-/- (increase) and 5-HT3A-/- (decrease) knock-out mice versus paired wild-types might have confounded differences in nociception, acute 5-HT receptor blockade by selective antagonists was found to replicate in wild-type mice the effects on pain behavior, but not on locomotor activity, of the respective gene knock-out in mutants. These results support the conclusion that the complex control of pain mechanisms by 5-HT, acting at multiple receptors, is physiologically relevant in mice.

Evidence for adenosine- and serotonin-mediated antihyperalgesic effects of cizolirtine in rats suffering from diabetic neuropathy.

Cizolirtine is a novel non-opioid drug which demonstrated antinociceptive activity in numerous pain models in rodents. Yet, its mechanism of action remains unknown. Several lines of evidence support the idea that adenosine (ADO) and serotonin (5-HT) modulate nociceptive signaling. Our study aimed at investigating whether these neuroactive molecules could be implicated in the mechanism of action of cizolirtine. Cizolirtine-induced antihyperalgesia was compared before and after pretreatment with ADO A(1)-A(2A) and 5-HT(1B/1D) receptor ligands in rats rendered diabetic by streptozotocin pretreatment and suffering from neuropathic pain. Cizolirtine alone (30-80 mg/kg, i.p.) significantly increased mechanical nociceptive thresholds. Acute pretreatment with the A(1)-A(2A) receptor antagonist caffeine (5 mg/kg, i.p.) or the 5-HT(1B/1D) receptor antagonist GR-127,935 (3 mg/kg, i.p.) significantly reduced the antihyperalgesic effects of cizolirtine. Conversely, cizolirtine-induced antihyperalgesia was promoted by pretreatment with either the selective A(1) receptor agonist CPA (0.3 mg/kg, i.p.) or the selective 5-HT(1B) receptor agonist CP-94,253 (3mg/kg, i.p.), and this potentiation was totally prevented by acute pretreatment with respective antagonists. Interestingly, A(1) receptor blockade by DPCPX inhibited the promoting effect of CP-94,253 on cizolirtine-induced antihyperalgesia, suggesting that the adenosine A(1)-mediated step takes place downstream the serotonin 5-HT(1B)-mediated step in the neurobiological mechanisms underlying cizolirtine action.

Attenuation of pain-related behavior in a rat model of trigeminal neuropathic pain by viral-driven enkephalin overproduction in trigeminal ganglion neurons.

Trigeminal neuropathic pain represents a real challenge to therapy because commonly used drugs are devoid of real beneficial effect or patients frequently become intolerant or refractory to some of these compounds. In a rat model of trigeminal neuropathic pain, which shares numerous similarities with human trigeminal neuralgia and trigeminal neuropathic pain, we used a genomic herpes simplex virus-derived vector (HSVLatEnk) to examine the possible effect of a local overproduction of proenkephalin A (PA) targeted to the trigeminal primary sensory neurons. Unilateral peripheral inoculation of recombinant vectors on the vibrissal pad territory resulted in an about ninefold increase in proenkephalin A mRNA levels in trigeminal ganglion ipsilateral to the infected side. Transgene-derived met-enkephalin accumulated in numerous nerve cell bodies of trigeminal ganglion and was transported through the sensory nerve fibers located in the infraorbital nerve. Bilateral mechanical hyperresponsiveness, which developed 2 weeks after chronic constrictive injury of the left infraorbital nerve, was significantly attenuated in animals overproducing PA in the trigeminal ganglion ipsilateral to the lesioned infraorbital nerve. This antiallodynic effect was reversed by both the opioid receptor antagonist naloxone and the peripherally acting antagonist naloxone methiodide. Our data demonstrate that the local overproduction of PA-derived peptides in trigeminal ganglion sensory neurons evoked a potent antiallodynic effect through the stimulation of mainly peripherally located opioid receptors and suggest that targeted delivery of endogenous opioids may be of interest for the treatment of some severe forms of neuropathic pain.

Antihyperalgesic effects of cizolirtine in diabetic rats: behavioral and biochemical studies.

Although clinically well controlled at the metabolic level, type I diabetes resulting from an insufficient insulin secretion remains the cause of severe complications. In particular, diabetes can be associated with neuropathic pain which fails to be treated by classical analgesics. In this study, we investigated the efficacy of a novel non opioid analgesic, cizolirtine, to reduce mechanical hyperalgesia associated with streptozotocin (STZ)-induced diabetes, in the rat. Cizolirtine was compared to paroxetine, an antidepressant drug with proven efficacy to relieve painful diabetic neuropathy. Under acute conditions, cizolirtine (30 and 80 mg/kgi.p.) significantly increased paw withdrawal and vocalization thresholds in the paw pressure test in diabetic rats displaying mechanical hyperalgesia. The antihyperalgesic effects of cizolirtine persisted under chronic treatment conditions, since pre-diabetes thresholds were recovered after a two week-treatment with the drug (3 mg/kg/day, s.c.). In this respect, cizolirtine was as efficient as paroxetine (5 mg/kg per day, s.c.) which, however, was inactive under acute treatment conditions. Measurements of the spinal release of calcitonin gene-related peptide (CGRP) through intrathecal perfusion under halothane-anesthesia showed that acute administration of cizolirtine (80 mg/kg, i.p.) significantly diminished (-36%) the peptide outflow in diabetic rats suffering from neuropathic pain. This effect as well as the antihyperalgesic effect of cizolirtine were prevented by the alpha(2)-adrenoreceptor antagonist idazoxan (2 mg/kg, i.p.). These data suggest that the antihyperalgesic effect of cizolirtine in diabetic rats suffering from neuropathic pain implies an alpha(2)-adrenoceptor-dependent presynaptic inhibition of CGRP-containing primary afferent fibers.

Effects of the novel analgesic, cizolirtine, in a rat model of neuropathic pain.

Cizolirtine (5-9[(N,N-dimethylaminoethoxy)phenyl]methyl0-1-methyl-1H-pyrazol citrate) is a centrally acting analgesic with a currently unknown mechanism of action, whose efficacy has been demonstrated in various models of acute and inflammatory pain in rodents. Further studies were performed in order to assess its potential antinociceptive action in a well-validated model of neuropathic pain, i.e. that produced by unilateral sciatic nerve constriction in rats. Animals were subjected to relevant behavioural tests based on mechanical (vocalization threshold to paw pressure) and thermal (struggle latency to paw immersion in a cold (10 degrees C) water bath) stimuli, 2 weeks after sciatic nerve constriction, when pain-related behaviour was fully developed. Acute pretreatment with 2.5-10 mg/kg p.o. of cizolirtine reversed both mechanical and thermal allodynia. These effects were antagonized by prior injection of the alpha(2)-adrenoceptor antagonist idazoxan (0.5 mg/kg i.v.), but not the opioid receptor antagonist naloxone (0.1 mg/kg i.v.). On the other hand, cizolirtine (10 mg/kg p.o.) produced no motor deficits in animals using the rotarod test. Our study showed that cizolirtine suppressed pain-related behavioural responses to mechanical and cold stimuli in neuropathic rats, probably via an alpha(2)-adrenoceptor-dependent mechanism. These results suggest that cizolirtine may be useful for alleviating some neuropathic somatosensory disorders, in particular cold allodynia, with a reduced risk of undesirable side effects.

The antimigraine 5-HT 1B/1D receptor agonists, sumatriptan, zolmitriptan and dihydroergotamine, attenuate pain-related behaviour in a rat model of trigeminal neuropathic pain.

1. Peripheral lesion to the trigeminal nerve may induce severe pain states. Several lines of evidence have suggested that the antimigraine effect of the triptans with 5-HT(1B/1D) receptor agonist properties may result from inhibition of nociceptive transmission in the spinal nucleus of the trigeminal nerve by these drugs. On this basis, we have assessed the potential antinociceptive effects of sumatriptan and zolmitriptan, compared to dihydroergotamine (DHE), in a rat model of trigeminal neuropathic pain. 2. Chronic constriction injury was produced by two loose ligatures of the infraorbital nerve on the right side. Responsiveness to von Frey filament stimulation of the vibrissal pad was used to evaluate allodynia. 3. Two weeks after ligatures, rats with a chronic constriction of the right infraorbital nerve displayed bilateral mechanical hyper-responsiveness to von Frey filament stimulation of the vibrissal pad with a mean threshold of 0.38+/-0.04 g on the injured side and of 0.43+/-0.04 g on the contralateral (left) side (versus > or =12.5 g on both sides in the same rats prior to nerve constriction injury). 4. Sumatriptan at a clinically relevant dose (100 microg kg(-1), s.c.) led to a significant reduction of the mechanical allodynia-like behaviour on both the injured and the contralateral sides (peak-effects 6.3+/-1.1 g and 4.4+/-0.7 g, respectively). A more pronounced effect was obtained with zolmitriptan (100 microg kg(-1), s.c.) (peak-effects: 7.4+/-0.9 g and 3.2+/-1.3 g) whereas DHE (50-100 microg kg(-1), i.v.) was less active (peak-effect approximately 1.5 g). 5. Subcutaneous pretreatment with the 5-HT(1B/1D) receptor antagonist, GR 127935 (3 mg kg(-1)), prevented the anti-allodynia-like effects of triptans and DHE. Pretreatment with the 5-HT(1A) receptor antagonist, WAY 100635 (2 mg kg(-1), s.c.), did not alter the effect of triptans but significantly enhanced that of DHE (peak effect 4.3+/-0.5 g). 6. In a rat model of peripheral neuropathic pain, which consisted of a unilateral loose constriction of the sciatic nerve, neither sumatriptan (50-300 microg kg(-1)) nor zolmitriptan (50-300 microg kg(-1)) modified the thresholds for paw withdrawal and vocalization in response to noxious mechanical stimulation. 7. These results support the rationale for exploring the clinical efficacy of brain penetrant 5-HT(1B/1D) receptor agonists as analgesics to reduce certain types of trigeminal neuropathic pain in humans.

The glycine/NMDA receptor antagonist (+)-HA966 enhances the peripheral effect of morphine in neuropathic rats.

Systemic opioid dosing until adequate analgesia in neuropathic pain may involve intolerable and untreatable side effects. Peripheral opioid receptor mechanisms may participate in the antinociceptive effect of systemic morphine. We evaluated the effect of peripherally injected morphine alone, and the ability of the functional antagonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex (+)-HA966 to modulate the antinociceptive effect of peripheral morphine in a rat model of neuropathic pain. Mononeuropathy was induced by placing four ligatures around the common sciatic nerve. Experiments were performed 2 weeks after the nerve ligature, when the pain-related behavior reached a stable maximum. Rats received injections of either subcutaneous (+)-HA966 (2.5mg/kg) or saline administered 20 min before morphine (50-150 microg injected into the nerve-injured hindpaw). The antinociceptive effect was tested against mechanical (vocalization threshold to hindpaw pressure) or thermal (struggle latency to hindpaw immersion into a 46 degrees C hot water bath) stimuli. In both tests, morphine alone (100-150 microg) produced antinociception. Pretreatment with (+)-HA966 did not potentiate the analgesic effectiveness of the two highest doses of morphine, but it did produce analgesia when combined with a low dose of morphine (50 microg), which did not produce analgesia by itself. These effects were reversed by intraplantar naloxone methiodide (50 microg injected into the nerve-injured hindpaw) indicating a peripherally opioid-mediated mechanism of action. The present studies suggested that combined administration of glycine/NMDA receptor antagonists, and peripherally acting morphine may be an interesting approach in the treatment of neuropathic pain.

Effect of gabapentin and lamotrigine on mechanical allodynia-like behaviour in a rat model of trigeminal neuropathic pain.

Injury to the trigeminal nervous system may induce severe pain states. This study examined the antinociceptive effect of the novel anticonvulsants, gabapentin and lamotrigine, in a rat model of trigeminal neuropathic pain produced by chronic constriction of one infraorbital nerve. Responsiveness to von Frey filament stimulation of the vibrissal pad was evaluated 2 weeks post-operation. Hyper-responsive rats received acute and repeated (five injections separated by the half-life of the compound) injections with gabapentin and lamotrigine. 76% of the nerve-injured rats displayed pronounced hyper-responsiveness (median 0.217 g (lower-upper percentiles 0.217-0.217) vs. 12.5 g pre-operative), that was resistant to both single (5-100 mg/kg) and repeated (5-30 mg/kg) injections with i.p. lamotrigine. Repeated (30 and 50 mg/kg), but not single (30-100 mg/kg) injections of i.p. gabapentin partially alleviated the mechanical allodynia-like behaviour. Repeated injections of gabapentin at 50 but not at 30 mg/kg produced motor deficits. The results indicate that gabapentin rather than lamotrigine may be a better therapeutic approach for the clinical management of some trigeminal neuropathic pain disorders.

Antinociceptive effect of systemic gabapentin in mononeuropathic rats, depends on stimulus characteristics and level of test integration.

The anticonvulsant gabapentin is effective against neuropathic pain, but the primary site(s) and mechanism(s) of action are unknown. In order to explore the relative contribution of spinal versus supra-spinal mechanisms to the antinociceptive effect of gabapentin, this study used two differentially integrated nociceptive tests. We systematically compared the effects of various doses of gabapentin on the paw withdrawal to pressure (PWTP), a spinally coordinated reflex and the vocalization threshold to paw pressure (VTPP), a supra-spinal integrated test in the sciatic nerve constriction rat model of neuropathic pain. In addition, we evaluated the effect of gabapentin on the struggle latency to paw immersion into a non-noxious cold (10 degrees C) water bath. Similar lower doses (1-30 mg/kg) of gabapentin produced potent antinociception in the VTPP test but were devoid of effects on the PWTP. The effect was observed not only on the nerve-injured side, but also, although less pronounced, on the contralateral side. Only the highest dose (100 mg/kg) of the anticonvulsant was able to induce an increase in the nerve-injured paw threshold in both tests. In the thermal test, gabapentin (3, 10 and 30 mg/kg i.p.) dose-dependently increased the response time to the 10 degrees C stimulus. Gabapentin at 100 mg/kg but not at 30 mg/kg produced motor deficits in animals using the rotarod test. Taken together, our findings suggest that low doses of gabapentin have a preferential action on the more integrated pain-related behaviour in neuropathic rats. The present results confirm that gabapentin may be a useful approach for the clinical management of several aspects of neuropathic pain.

The development of pain-related behaviour and opioid tolerance after neuropathy-inducing surgery and sham surgery.

The inability of opioids to control pain over time may be influenced by different factors such as drug tolerance, hyperalgesia due to repeated morphine administration or progression of the original disease. In addition, chronic pain may alter morphine tolerance development. This study examined whether chronic morphine exposure differently affects mechanical and thermal stimulus evoked pain-related behaviour in non-operated, nerve-injured and sham-operated rats. Further, we studied the effect of nerve injury and sham surgery on the development of tolerance to the analgesic effect of morphine. Vocalization thresholds to paw pressure and struggle latencies to hindpaw immersion into a 46 degrees C hot-water bath were determined in groups of non-operated rats, nerve-injured (chronic constriction of the sciatic nerve) and sham-operated rats. Immediately thereafter, pretreatment regimens with s.c. injections of either saline or morphine (10 mg/kg) were started. Injections were given twice daily on post-operative days 12-15, when the abnormal pain behaviour in nerve-injured rats is at a stable maximum. On day 16, the effect of an acute dose of i.v. morphine (1 mg/kg) was tested. On day 12 the baseline vocalization threshold and struggle latency were decreased in nerve-injured but not in non- and sham-operated rats. Morphine pretreatment further decreased the vocalization threshold in nerve-injured rats and induced threshold reductions in non- and sham-operated rats. In the thermal test, morphine pretreatment produced no change in baseline latencies in any of the groups. Following morphine pretreatment, acute i.v. morphine on day 16 remained effective against both mechanical and thermal stimuli in non-operated rats, but was strongly reduced in nerve-injured rats. Sham-operated rats displayed a tendency towards a reduced effect of i.v. morphine after morphine pretreatment in the mechanical but not in the thermal test. The results suggest that mechanical afferent systems may be more sensitive to hyperalgesia associated with repetitive morphine injections than thermal systems and that nerve injury facilitates the development of tolerance to morphine analgesia.

Combined systemic administration of the glycine/NMDA receptor antagonist, (+)-HA966 and morphine attenuates pain-related behaviour in a rat model of trigeminal neuropathic pain.

Chronic constriction injury to the infraorbital nerve (CCI-ION) by loose ligatures may represent a useful model for some trigeminal neuropathic pain disorders. Activation of the N-methyl-D-aspartate (NMDA) receptor is involved in the induction and maintenance of neuropathic pain and may contribute to the poor opioid sensitivity of this syndrome. We evaluated the effect of combined systemic administration of the functional antagonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex, (+)-(1-Hydroxy-3-aminopyrrolodine-2-one) ((+)-HA966) with morphine on mechanical allodynia-like behaviour in CCI-ION rats. Two weeks after surgery rats with a CCI-ION displayed mechanical hyperresponsiveness to von Frey filament stimulation of the vibrissal pad with a median at 0.217 g (95% confidence limits, 0. 217-0.224) versus > or = 12.5 g pre-operative. Administration of either (+)-HA966 (2.5 mg/kg s.c.) alone or morphine (1 mg/kg i.v.) alone was devoid of effects on the mechanical hyperresponsiveness. By contrast, combined administration of (+)-HA966 and morphine (0.25, 0. 5 and 1 mg/kg i.v.) dose-dependently increased the mechanical response thresholds (peak-effects 0.745 g (0.745-0.745), 4.64 (3.3-8. 7) and 12.5 g (8.4-12.5), respectively). This effect was prevented and reversed by naloxone (0.1 mg/kg i.v.). The drug combination produced no motor deficits in animals using the rotarod test. The present results indicate that combination therapy with NMDA/glycine receptor antagonists and morphine may be a useful approach for the clinical management of trigeminal neuropathic pain disorders.

Lack of cross-tolerance between the antinociceptive effects of systemic morphine and asimadoline, a peripherally-selective kappa-opioid agonist, in CCI-neuropathic rats.

The development of tolerance following repeated doses of morphine hinders the treatment of clinical pain. We have previously shown that morphine tolerance develops in neuropathic rats without cross-tolerance to a systemic kappa-opioid agonist; in the current work, using paw-pressure vocalization thresholds, we studied the antinociceptive effect of the peripherally-selective kappa (kappa)-opioid agonist, asimadoline, in both morphine-tolerant and opioid-naïve rats 2 weeks after sciatic nerve injury. In naïve rats, intraplantar (i.pl.) injection of asimadoline into the nerve-injured paw, at doses of 10, 15 and 20 (but not 30) microg, dose-dependently relieved the mechanical allodynia-like behaviour. The kappa-opioid antagonist, norbinaltorphimine, (30 microg, i.pl.) reversed this action; injection of asimadoline (15 microg) into the contralateral paw (i.pl.) or i.v., however, had no effect. These results confirm that at low doses, asimadoline exerts its action only in the periphery. In morphine-tolerant rats (after 10 mg/kg s.c. , twice daily for 4 days) and naïve, saline-pretreated rats, asimadoline (15 microg, i.pl.) relieved the mechanical allodynia-like behaviour to the same extent, indicating no cross-tolerance between morphine and the peripherally-selective drug. Our findings show promise for the treatment of neuropathic pain with low doses of peripherally-selective kappa-opioids.