The calcitonin gene-related peptide antagonist CGRP8-37 increases the latency to withdrawal responses bilaterally in rats with unilateral experimental mononeuropathy, an effect reversed by naloxone

The Role of Inflammation, Hypoxia, and Opioid Receptor Expression in Pain Modulation in Patients Suffering from Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is a relatively common disease in the general population. Besides its interaction with many comorbidities, it can also interact with potentially painful conditions and modulate its course. The association between OSA and pain modulation has recently been a topic of concern for many scientists. The mechanism underlying OSA-related pain connection has been linked with different pathophysiological changes in OSA and various pain mechanisms. Furthermore, it may cause both chronic and acute pain aggravation as well as potentially influencing the antinociceptive mechanism. Characteristic changes in OSA such as nocturnal hypoxemia, sleep fragmentation, and systemic inflammation are considered to have a curtailing impact on pain perception. Hypoxemia in OSA has been proven to have a significant impact on increased expression of proinflammatory cytokines influencing the hyperalgesic priming of nociceptors. Moreover, hypoxia markers by themselves are hypothesized to modulate intracellular signal transduction in neurons and have an impact on nociceptive sensitization. Pain management in patients with OSA may create problems arousing from alterations in neuropeptide systems and overexpression of opioid receptors in hypoxia conditions, leading to intensification of side effects, e.g., respiratory depression and increased opioid sensitivity for analgesic effects. In this paper, we summarize the current knowledge regarding pain and pain treatment in OSA with a focus on molecular mechanisms leading to nociceptive modulation.

Phenotypic change in trigeminal ganglion neurons associated with satellite cell activation via extracellular signal-regulated kinase phosphorylation is involved in lingual neuropathic pain

Iatrogenic trigeminal nerve injuries remain a common and complex clinical problem. Satellite glial cell (SGC) activation, associated phosphorylation of extracellular signal-regulated kinase (ERK), and neuropeptide expression in the trigeminal ganglion (TG) are known to be involved in trigeminal neuropathic pain related to trigeminal nerve injury. However, the involvement of these molecules in orofacial neuropathic pain mechanisms is still unknown. Phosphorylation of ERK1/2 in lingual nerve crush (LNC) rats was observed in SGCs. To evaluate the role of neuron-SGC interactions under neuropathic pain, calcitonin gene-related peptide (CGRP)-immunoreactive (IR), phosphorylated ERK1/2 (pERK1/2)-IR and glial fibrillary acidic protein (GFAP)-IR cells in the TG were studied in LNC rats. The number of CGRP-IR neurons and neurons encircled with pERK1/2-IR SGCs was significantly larger in LNC rats compared with sham rats. The percentage of large-sized CGRP-IR neurons was significantly higher in LNC rats. The number of CGRP-IR neurons, neurons encircled with pERK1/2-IR SGCs, and neurons encircled with GFAP-IR SGCs was decreased following CGRP receptor blocker CGRP_8-37 or mitogen-activated protein kinase/ERK kinase 1 inhibitor PD98059 administration into the TG after LNC. Reduced thresholds to mechanical and heat stimulation to the tongue in LNC rats were also significantly recovered following CGRP_8-37 or PD98059 administration. The present findings suggest that CGRP released from TG neurons activates SGCs through ERK1/2 phosphorylation and TG neuronal activity is enhanced, resulting in the tongue hypersensitivity associated with lingual nerve injury. The phenotypic switching of large myelinated TG neurons expressing CGRP may account for the pathogenesis of tongue neuropathic pain.

Calcitonin gene-related peptide contributes to peripheral nerve injury-induced mechanical hypersensitivity through CCL5 and p38 pathways

The role of calcitonin gene related peptide (CGRP) in neuropathic pain was investigated in a mouse model of neuropathic pain, spinal nerve L5 transection (L5Tx). Intrathecal injection (i.t.) of CGRP8-37, a CGRP antagonist, significantly reduced L5Tx-induced mechanical hypersensitivity and lumbar spinal cord CCL5 expression. i.t. injection of a CCL5 neutralizing antibody significantly inhibited L5Tx-induced mechanical hypersensitivity. Further, pre-treatment with a p38-inhibitor, SB203580, was able to reduce CGRP-induced mechanical hypersensitivity, but not CGRP-induced CCL5 production. Our data indicate that CGRP can play its pro-nociceptive role through both a spinal cord CCL5-dependent, p38-independent pathway, and a p38-depenented, CCL5-independent pathway.

Involvements of galanin and its receptors in antinociception in nucleus accumbens of rats with inflammatory pain

This study tested the hypothesis that antinociceptive effects of galanin and its receptors in nucleus accumbens (NAc) of rats with inflammatory pain provoked by subcutaneous injection of 0.1 ml of 2% carrageenin into the sole of the rat's left hindpaw. The hindpaw withdrawal latencies (HWLs) in response to thermal and mechanical stimulation significantly decreased in bilateral hindpaws at 3 and 4 hour after a subcutaneous injection of carrageenin. However intra-NAc injection of 2 and 3 nmol, but not 1 nmol of galanin markedly induced an increase in the HWLs in a dose-dependent way. Western blot also showed, that the expression of galanin receptor 1 (GalR1) and galanin receptor 2 (GalR2) were significantly upregulated in NAc at 3 hour after a subcutaneous injection of carrageenin. In addition, the rats were intra-NAc injected galanin, 5 min later following by intra-NAc injection of galanin receptor antagonist galantide, the galanin-induce antinociceptive effects were suppressed by galantide. The results demonstrated that galanin and its receptors might be involved in antinociception in the NAc of rats with inflammatory pain.

Calcitonin gene-related peptide: physiology and pathophysiology

Calcitonin gene-related peptide (CGRP) is a 37-amino acid neuropeptide. Discovered 30 years ago, it is produced as a consequence of alternative RNA processing of the calcitonin gene. CGRP has two major forms (α and β). It belongs to a group of peptides that all act on an unusual receptor family. These receptors consist of calcitonin receptor-like receptor (CLR) linked to an essential receptor activity modifying protein (RAMP) that is necessary for full functionality. CGRP is a highly potent vasodilator and, partly as a consequence, possesses protective mechanisms that are important for physiological and pathological conditions involving the cardiovascular system and wound healing. CGRP is primarily released from sensory nerves and thus is implicated in pain pathways. The proven ability of CGRP antagonists to alleviate migraine has been of most interest in terms of drug development, and knowledge to date concerning this potential therapeutic area is discussed. Other areas covered, where there is less information known on CGRP, include arthritis, skin conditions, diabetes, and obesity. It is concluded that CGRP is an important peptide in mammalian biology, but it is too early at present to know if new medicines for disease treatment will emerge from our knowledge concerning this molecule.

Influences of calcitonin gene-related peptide on mu opioid receptors in nucleus accumbens neurons of rats

The Mu opioid receptor (MOR) has been shown to participate in the analgesic effect of the calcitonin gene-related peptide (CGRP) in the nucleus accumbens (NAc) of adult rats. However, it is not clear whether and how CGRP regulates the MOR at the molecular levels. In the present study, it is found that the level of MORs on the cell membrane of NAc neurons was increased twice more than the control level following CGRP treatment (1μM, 30min), which is a phenomenon that was blocked by the peptidergic antagonist CGRP8-37. No direct physical interaction was observed between MORs and CGRP receptors, and neither brefeldin A nor dynosore preincubation affected such effects of CGRP. However, addition of 20μM monensin 1h before CGRP treatment significantly blocked the action of CGRP on surface MORs. In living animals, microinjection of CGRP (1nmol in 1μl) into the NAc partially restored morphine antinociception in morphine-tolerant rats, and the effect of CGRP on surface MORs extended beyond normal NAc neurons to chronic morphine-treated NAc neurons. To conclude, these results demonstrate that CGRP can act on MORs and increase the number of surface MORs in NAc neurons, partially explaining the involvement of opioid receptors in CGRP-induced antinociception in the rat NAc.

Evidence for anti-inflammatory and putative analgesic effects of a monoclonal antibody to calcitonin gene-related peptide

BACKGROUND

Calcitonin gene-related peptide (CGRP) is a powerful pro-inflammatory mediator thought to play a significant role in the development of inflammation and pain. We investigated the role of CGRP in trigeminal inflammatory pain by determining the ability of a monoclonal antibody to CGRP to modify central Fos expression in response to stimulation of the inflamed ferret tooth pulp. We also assessed the effect of the antibody on pulpal inflammation.

METHODS

Ten adult ferrets were prepared under anaesthesia to allow stimulation of the upper and lower left canine pulps, recording from the digastric muscle and intravenous injections at subsequent experiments. In all animals, pulpal inflammation was induced by introducing human caries into a deep buccal cavity. Four days later animals were treated intravenously with either CGRP antibody (n=5) or vehicle (n=5). After a further 2 days animals were re-anaesthetised and the tooth pulps stimulated at 10 times jaw-opening reflex threshold. Brainstems and tooth pulps were processed immunohistochemically for Fos and the common leucocyte marker CD45, respectively.

RESULTS

Fos was expressed in ipsilateral trigeminal subnuclei caudalis (Vc) and oralis (Vo). Significantly fewer Fos-positive nuclei were present within Vc of CGRP antibody-treated animals (p=0.003 vs vehicle-treated). Mean percentage area of staining for CD45 was significantly less in antibody-treated animals (p=0.04 vs vehicle-treated).

CONCLUSIONS

This is the first direct evidence that sequestration of CGRP has anti-inflammatory and putative analgesic effects. Previous studies using this Fos model have demonstrated that it is able to predict clinical analgesic efficacy. Thus these data indicate that this antibody may have analgesic effects in dental pain and other types of inflammatory-mediated transmission, and suggest that this is in part due to peripheral anti-inflammatory effects.

Involvement of calcitonin gene-related peptide and CCL2 production in CD40-mediated behavioral hypersensitivity in a model of neuropathic pain

The neuropeptide calcitonin gene-related peptide (CGRP) is known to play a pro-nociceptive role after peripheral nerve injury upon its release from primary afferent neurons in preclinical models of neuropathic pain. We previously demonstrated a critical role for spinal cord microglial CD40 in the development of spinal nerve L5 transection (L5Tx)-induced mechanical hypersensitivity. Herein, we investigated whether CGRP is involved in the CD40-mediated behavioral hypersensitivity. First, L5Tx was found to significantly induce CGRP expression in wild-type (WT) mice up to 14 days post-L5Tx. This increase in CGRP expression was reduced in CD40 knockout (KO) mice at day 14 post-L5Tx. Intrathecal injection of the CGRP antagonist CGRP8-37 significantly blocked L5Tx-induced mechanical hypersensitivity. In vitro, CGRP induced glial IL-6 and CCL2 production, and CD40 stimulation added to the effects of CGRP in neonatal glia. Further, there was decreased CCL2 production in CD40 KO mice compared to WT mice 21 days post-L5Tx. However, CGRP8-37 did not significantly affect spinal cord CCL2 production following L5Tx in WT mice. Altogether, these data suggest that CD40 contributes to the maintenance of behavioral hypersensitivity following peripheral nerve injury in part through two distinct pathways, the enhancement of CGRP expression and spinal cord CCL2 production.

Delayed functional expression of neuronal chemokine receptors following focal nerve demyelination in the rat: a mechanism for the development of chronic sensitization of peripheral nociceptors

BACKGROUND

Animal and clinical studies have revealed that focal peripheral nerve axon demyelination is accompanied by nociceptive pain behavior. C-C and C-X-C chemokines and their receptors have been strongly implicated in demyelinating polyneuropathies and persistent pain syndromes. Herein, we studied the degree to which chronic nociceptive pain behavior is correlated with the neuronal expression of chemokines and their receptors following unilateral lysophosphatidylcholine (LPC)-induced focal demyelination of the sciatic nerve in rats.

RESULTS

Focal nerve demyelination increased behavioral reflex responsiveness to mechanical stimuli between postoperative day (POD) 3 and POD28 in both the hindpaw ipsilateral and contralateral to the nerve injury. This behavior was accompanied by a bilateral increase in the numbers of primary sensory neurons expressing the chemokine receptors CCR2, CCR5, and CXCR4 by POD14, with no change in the pattern of CXCR3 expression. Significant increases in the numbers of neurons expressing the chemokines monocyte chemoattractant protein-1 (MCP-1/CCL2), Regulated on Activation, Normal T Expressed and Secreted (RANTES/CCL5) and interferon gamma-inducing protein-10 (IP-10/CXCL10) were also evident following nerve injury, although neuronal expression pattern of stromal cell derived factor-1alpha (SDF1/CXCL12) did not change. Functional studies demonstrated that acutely dissociated sensory neurons derived from LPC-injured animals responded with increased [Ca2+]i following exposure to MCP-1, IP-10, SDF1 and RANTES on POD 14 and 28, but these responses were largely absent by POD35. On days 14 and 28, rats received either saline or a CCR2 receptor antagonist isomer (CCR2 RA-[R]) or its inactive enantiomer (CCR2 RA-[S]) by intraperitoneal (i.p.) injection. CCR2 RA-[R] treatment of nerve-injured rats produced stereospecific bilateral reversal of tactile hyperalgesia.

CONCLUSION

These results suggest that the presence of chemokine signaling by both injured and adjacent, uninjured sensory neurons is correlated with the maintenance phase of a persistent pain state, suggesting that chemokine receptor antagonists may be an important therapeutic intervention for chronic pain.

Involvement of galanin in nociceptive regulation in the arcuate nucleus of hypothalamus in rats with mononeuropathy

The hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulation increased significantly after intra-hypothalamic arcuate nucleus (ARC) injection of galanin in mononeuropathic rats, while intra-ARC injection of the putative antagonist of galanin receptors markedly reduced the HWLs. The number of galaninergic neurons in the ARC increased in rats with mononeuropathy than that in normal rats. The results demonstrated that both endogenous and exogenous galanin were involved in the regulation of nociception in the ARC of rats with peripheral nerve injury.

Molecular mechanisms of neuropathic pain–phenotypic switch and initiation mechanisms

Many known painkillers are not always effective in the therapy of chronic neuropathic pain manifested by hyperalgesia and tactile allodynia. The mechanisms underlying neuropathic pain appear to be complicated and to differ from acute and inflammatory pain. Recent advances in pain research provide us with a clear picture for the molecular mechanisms of acute pain, and substantial information is available concerning the plasticity that occurs under conditions of neuropathic pain. The most important changes responsible for the mechanisms of neuropathic pain are found in the altered gene/protein expression in primary sensory neurons. After damage to peripheral sensory fibers, up-regulated expression of the Ca(v)alpha(2)delta-(1) channel subunit, the Na(v)1.3 sodium channel, and bradykinin (BK) B1 and capsaicin TRPV1 receptors in myelinated neurons contribute to hyperalgesia; while the down-regulation of the Na(v)1.8 sodium channel, B2 receptor, substance P (SP), and even mu-opioid receptors in unmyelinated neurons is responsible for the phenotypic switch in pain transmission. Clarification of the molecular mechanisms for such complicated plasticity would be extremely valuable when considering the therapeutic design of pain relieving drugs. Although many reports deal with the changes in expression of key molecules related to neuropathic pain, the initiation and the mechanisms that follow remain to be determined. The current study using lysophosphatidic acid (LPA) receptor knockout mice revealed that LPA produced by nerve injury initiates neuropathic pain and demyelination following partial sciatic nerve ligation (PSNL). A single injection of LPA was found to mimic PSNL in terms of neuropathic pain and its underlying mechanisms. This discovery may lead to the subsequent discovery of LPA-induced secondary genes, which would be therapeutic targets for neuropathic pain.

Involvement of neuropeptide Y and Y1 receptor in antinociception in the arcuate nucleus of hypothalamus, an immunohistochemical and pharmacological study in intact rats and rats with inflammation

Neuropeptide Y (NPY) plays an important role in pain modulation at different levels in the central nervous system. In the brain, NPY and NPY receptors distribute abundantly in the arcuate nucleus of hypothalamus (ARC), a structure involved in pain processing. The present study was undertaken to investigate the role of NPY in nociceptive modulation in the ARC of intact rats and rats with carrageenan-induced inflammation. Intra-ARC administration of NPY induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation in intact rats, which was attenuated by the Y1 receptor antagonist NPY28-36. Intra-ARC administration of NPY also induced dose-dependent increases in HWLs to noxious stimulation in rats with inflammation. Furthermore, intra-ARC injection of either the antiserum against NPY or NPY28-36 induced decreases in HWLs in rats with inflammation, while both of them produced no effects in intact ones. Additionally, there were marked increases of Y1 receptor in the bilateral ARC of rats with inflammation tested by immunohistochemistry, while no significant changes of NPY were observed, implicating that the increased Y1 receptor has an important effect in the NPY-induced antinociception. We also found that intra-ARC injection of Y2 receptor agonist NPY3-36 produced no significant antinociception in either intact rats or rats with inflammation. Together, we demonstrate that NPY exerts an antinociceptive effect in the ARC of intact rats and rats with inflammation. Both Y1 receptor and endogenous released NPY in the ARC are involved in the nociceptive modulation during inflammation.

Effects of amitriptyline and gabapentin on bilateral hyperalgesia observed in an animal model of unilateral axotomy

Nociceptive responses in an animal model of peripheral nerve injury were studied. The left common sciatic nerve was exposed, tightly ligated at two locations and transected between the ligatures. A bilateral decrease in the nociceptive threshold to mechanical stimulation was observed within 3 h after the operation. The skin of the lateral dorsal part of the injured paw was hypoalgesic, while the medial dorsal paw innervated by the intact saphenous nerve and the contralateral dorsal paw exhibited hyperalgesia. Amitriptyline, an antidepressant, at 25, 50 and 100 mg/kg per day, p.o., and gabapentin, an anticonvulsant, at 30, 100 and 300 mg/kg per day, p.o., significantly inhibited the decrease in the mechanical nociceptive threshold in the injured and uninjured paws. The effects of amitriptyline at 25 and 50 mg/kg were evident at doses that did not cause neurologic deficits as assessed by the inclined screen test. Indomethacin, a cyclooxygenase inhibitor, and morphine (except at the highest dose of 30 mg/kg, s.c.) showed no analgesic effects in this model. The tail-flick latency was also significantly decreased compared with intact rats. Similar bilateral hyperalgesia was observed when axotomy was performed using silk thread instead of chromic gut. When this axotomy model was applied to mice, the nociceptive thresholds in both paws immediately showed a significant decrease in the same manner as in rats. The bilateral and systemic hyperalgesia observed in this axotomy model, which resembles the clinical features of chronic neuropathic pain, suggests the involvement of the central nervous system in the maintenance of the chronic pain state.

Alterations in the substance P-induced anti-nociception in the central nervous system of rats after morphine tolerance

The present study investigated the alteration in intracerebroventricular administration of substance P-induced anti-nociception after morphine tolerance in rats. The nociceptive response latencies to noxious thermal and mechanical stimulation increased significantly after intracerebroventricular administration of SP in normal rats. The SP-induced anti-nociception decreased significantly in morphine-tolerant rats than that in opioid naive rats. The results demonstrated that SP induced significant anti-nociceptive effects in the brain in normal rats, and the SP-induced anti-nociception was attenuated significantly in morphine-tolerant rats than that in opioid naive rats, indicating an alteration in SP-induced anti-nociception in the brain after chronic morphine treatments.

Contralateral, ipsilateral and bilateral treatments with the kappa-opioid receptor agonist U-50,488H in mononeuropathic rats

The effect of repeated contralateral administration of the kappa-opioid receptor agonist U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate) on nociceptive behaviour was investigated and compared with ipsilateral and bilateral treatments in a rat model of peripheral unilateral neuropathy (chronic constriction of the common sciatic nerve). Administration of 0.3 mg U-50,488H into the contralateral hindpaw on days 6 and 10 after induction of mononeuropathy increased hindpaw withdrawal latency to mechanical but not to thermal stimulation compared to saline-treated rats. No difference in pain-related behaviour was found between different peripheral (contralateral, ipsilateral and bilateral) treatments with 0.3 mg U-50,488H. Autotomy behaviour was reduced for 6 weeks after sciatic nerve ligation in rats treated contralaterally with the opioid receptor agonist. Antinociceptive effects of contralaterally administered U-50,488H were abolished by the peripherally acting opioid receptor antagonist naloxone methiodide. Our findings indicate that contralateral treatment with U-50,488H attenuates nociceptive behaviour in mononeuropathic rats. These antinociceptive effects are mediated via peripheral opioid receptors.

Interactions of galanin and opioids in nociceptive modulation in the arcuate nucleus of hypothalamus in rats

The fact that galanin, beta-endorphin and their receptors are present in the arcuate nucleus of hypothalamus (ARC), coupled with our previous observation that both beta-endorphin and galanin play antinociceptive roles in pain modulation in the ARC, made it of interest to study their interactions. The hindpaw withdrawal latency (HWL) in response to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. We showed that the antinociceptive effect induced by intra-ARC injection of galanin was dose-dependently attenuated by the following intra-ARC injection of naloxone. Furthermore, intra-ARC administration of the selective mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) attenuated the increased HWL induced by intra-ARC injection of galanin in a dose-dependent manner, while the delta-opioid receptor antagonist naltrindole or the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) did not. Moreover, intra-ARC injection of a galanin receptor antagonist galantide attenuated intraperitoneal morphine-induced increases in HWLs. These results demonstrate that the antinociceptive effect of galanin was related to the opioid system, especially mu-opioid receptor was involved in, and that systemic morphine induced antinociception involves galanin in the ARC.

Role of nociceptin in the modulation of nociception in the arcuate nucleus of rats

Neuropeptide nociceptin/orphanin FQ is the endogenous ligand for the opioid-receptor-like receptor 1 (ORL1), mediating essential functions in the central and peripheral nervous systems. The present study was performed to investigate the role of nociceptin and ORL1 receptor in nociception and morphine-induced antinociception in the arcuate nucleus of hypothalamus in rats. Hindpaw withdrawal latencies (HWL) were measured by hot-plate and Randall Selitto tests. The HWL to both thermal and mechanical stimulation decreased significantly after intra-arcuate nucleus injection of nociceptin in a dose-dependent manner. The effect of nociceptin was blocked significantly by subsequent intra-arcuate nucleus administration of [Nphe(1)]nociceptin(1-13)-NH(2), an ORL1 receptor antagonist. Furthermore, an intra-arcuate nucleus injection of nociceptin dramatically attenuated the antinociceptive effect induced by morphine either injected in the same site or applied intraperitoneally. These results suggest that nociceptin in the arcuate nucleus induces a hyperalgesic effect by acting on ORL1 receptors. The present study also demonstrates an interaction between nociceptin and opioids in the arcuate nucleus of the hypothalamus.

CGRP Peptide and Regenerating Sensory Axons

CGRP peptide, a widely expressed constituent of sensory neurons, plays important roles in nerve function and repair when axons are severed. CGRP synthesis declines, yet peptide nonetheless accumulates in severed axon endbulbs. In this work we explore an apparent selective and ongoing expression of CGRP peptide in regenerative sensory axon sprouts. Following sural nerve crush in rats out to 14 days, regenerating and branching sensory axons had intense and selective expression of CGRP, not associated with endbulbs. Parent L4 and L5 perikarya and axons in the sural nerve proximal to crush, however, did not exhibit such heightened CGRP presence. Instead, back labeling of regenerating axons with fluorogold or diamidino yellow labeled perikarya with reduced CGRP expression. Similarly, ATF-3, a robust marker of axotomized neurons, was associated with reduced, rather than elevated expression of alphaCGRP mRNA. Unexpectedly, however, we identified an enlarged secondary population of intact uninjured neurons, frequently smaller and projecting to the dorsal horn with new and heightened intense CGRP expression but not ATF-3- or tracer-labeled. Distal regenerating sensory axons selectively express CGRP peptide despite reduced perikaryal content, a phenomenon not explained by simple accumulation. Having an injured neighbor neuron, however, may also paradoxically alter how CGRP is expressed in intact neurons.

Calcitonin gene-related peptide (CGRP) modulates nociceptive trigeminovascular transmission in the cat

Calcitonin gene-related peptide (CGRP) is released into the cranial circulation of humans during acute migraine. To determine whether CGRP is involved in neurotransmission in craniovascular nociceptive pathways, we microiontophoresed onto neurons in the trigeminocervical complex and intravenously administered the CGRP receptor antagonists alpha-CGRP-(8-37) and BIBN4096BS. Cats were anaesthetised with alpha-chloralose, and using halothane during surgical preparation. A craniotomy and C1/C2 laminectomy allowed access to the superior sagittal sinus (SSS) and recording site. Recordings of activity in the trigeminocervical complex evoked by electrical stimulation of the SSS were made. Multibarrelled micropipettes incorporating a recording electrode were used for microiontophoresis of test substances. Cells recorded received wide dynamic range (WDR) or nociceptive specific (NS) input from cutaneous receptive fields on the face or forepaws. Cell firing was increased to 25-30 Hz by microiontophoresis of L-glutamate (n = 43 cells). Microiontophoresis of alpha-CGRP excited seven of 17 tested neurons. BIBN4096BS inhibited the majority of units (26 of 38 cells) activated by l-glutamate, demonstrating a non-presynaptic site of action for CGRP. alpha-CGRP-(8-37) inhibited a similar proportion of units (five of nine cells). Intravenous BIBN4096BS resulted in a dose-dependent inhibition of trigeminocervical SSS-evoked activity (ED50 31 microg kg(-1)). The maximal effect observed within 30 min of administration. The data suggest that there are non-presynaptic CGRP receptors in the trigeminocervical complex that can be inhibited by CGRP receptor blockade and that a CGRP receptor antagonist would be effective in the acute treatment of migraine and cluster headache.

Involvement of opioid receptors in the oxytocin-induced antinociception in the central nervous system of rats

Recent studies showed that oxytocin and opioid peptides play important roles in pain modulation at different levels in the central nervous system. The present study was performed to explore whether opioid system is involved in the oxytocin-induced antinociception in the brain of rats. The results showed that: (1) intracerebroventricular injection of oxytocin induced dose-dependent increases in hindpaw withdrawal latencies (HWL) to noxious thermal and mechanical stimulation in rats. (2) The antinociceptive effect of oxytocin was attenuated dose-dependently by intracerebroventricular injection of naloxone, indicating an involvement of opioid system in the oxytocin-induced antinociception. (3) It is interesting that the antinociceptive effect of oxytocin was attenuated by subsequent intracerebroventricular injection of the mu-opioid antagonist beta-funaltrexamine (beta-FNA) and the kappa-opioid antagonist nor-binaltorphimine (nor-BNI), but not the delta-opioid antagonist naltrindole. The results indicate that oxytocin plays an antinociceptive role in the brain of rats; mu- and kappa-opioid receptors, not delta-receptors, are involved in the oxytocin-induced antinociception in the central nervous system of rats.

Substance P microinjected into the periaqueductal gray matter induces antinociception and is released following morphine administration

The aims of the present study were to investigate, in rats, the behavioral effects of substance P (SP) microinjected into the ventrolateral periaqueductal gray (PAG) and the effects of the neurokinin 1 (NK-1) receptor antagonist [d-Arg1, d-Trp7, 9, Leu11]-substance P (Spantide). The effect of morphine administration on the release of SP in the ventrolateral PAG was also investigated using microdialysis in awake rats. SP microinjected into the ventrolateral part of the PAG induced significant increases in the hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation as an antinociceptive response. The NK-1 receptor antagonist blocked these effects but exhibited no antinociceptive effect alone. Subcutaneous administration of morphine increased basal SP-like immunoreactivity (SP-LI) release in the microdialysate obtained from the ventrolateral PAG of freely moving rats. Our results demonstrate that SP injected into the ventrolateral PAG induces an antinociceptive effect via activation of NK-1 receptors. Morphine administered systemically induces the release of SP in the ventrolateral PAG. We suggest that an increased release of SP in the PAG may contribute to opioid antinociception.

Role of corticotropin-releasing factor and its receptor in nociceptive modulation in the central nucleus of amygdala in rats

Corticotropin-releasing factor (CRF) plays important physiological functions in the central nervous system. The present study was performed to investigate the role of CRF and CRF receptor in nociceptive modulation in the central nucleus of amygdala (CeA) of rats. The hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation increased significantly after intra-CeA administration of 0.1 and 0.01 nmol of CRF, but not 0.001 nmol, indicating that CRF induces antinociceptive effects in the CeA of rats. The antinociceptive effect may be due to the dose of CRF was attenuated by intra-CeA administration of 0.1 nmol alpha-hCRF9-41, a selective CRF receptor antagonist, suggesting that the CRF-induced antinociception is mediated by the CRF receptors in the CeA. Furthermore, the HWL to both thermal and mechanical stimulation decreased significantly after intra-CeA administration of alpha-hCRF9-41 alone, suggesting an involvement of endogenous CRF in the CeA in nociceptive modulation. The present study demonstrated that both exogenous and endogenous CRF plays an antinociceptive effect in the CeA, the effect is mediated by CRF receptor.

Involvement of opioid receptors in the CGRP8-37-induced inhibition of the activity of wide-dynamic-range neurons in the spinal dorsal horn of rats

The present study was performed to explore the involvement of opioid receptors in the calcitonin gene-related peptide 8-37 (CGRP8-37, an antagonist of CGRP receptor)-induced inhibition of the activity of wide-dynamic-range (WDR) neurons in the spinal dorsal horn of rats. Extracellular recording was performed with a multibarrelled glass micropipette, and the chemicals were delivered by micro-iontophoresis. The discharge frequency of WDR neurons was evoked by subcutaneous electrical stimulation applied to the ipsilateral hindpaw. Iontophoretic application of CGRP8-37 by an ejection current of 160 nA induced significant inhibition of the discharge frequency of WDR neurons. The inhibitory effect of CGRP8-37 on the activity of WDR neurons was attenuated by later iontophoretic application of the opioid antagonist naloxone. Furthermore, the effect of CGRP8-37 was attenuated by either iontophoretic application of the kappa-receptor antagonist nor-binaltorphimine (nor-BNI) or the mu-receptor antagonist beta-funaltrexamine (beta-FNA) but not by the delta-receptor antagonist naltrindole. The results indicate that kappa- and mu-opioid receptors on the membrane of WDR neurons are involved in the modulation of CGRP8-37-induced antinociception in dorsal horn of the spinal cord in rats.

An antinociceptive role of galanin in the arcuate nucleus of hypothalamus in intact rats and rats with inflammation

In the arcuate nucleus of hypothalamus (ARC), galaninergic fibers form synaptic contacts with proopiomelanocortin neurons, which are involved in pain modulation. The present study assessed the role of exogenous and endogenous galanin in the modulation of nociception in the ARC of rats. The hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. Intra-ARC injection of galanin dose-dependently increased the HWLs in intact rats, indicating an antinociceptive role of exogenous galanin in the ARC. The antinociceptive effect of galanin was blocked by following intra-ARC injection of galantide, a putative galanin receptor antagonist, suggesting that the antinociceptive effect of galanin is mediated by galanin receptors. Moreover, intra-ARC injection of galanin increased the HWL in rats with inflammation. Intra-ARC administration of galantide alone reduced the HWLs in rats with inflammation, while there were no influences of galantide on the HWL in intact rats. Taken together, the results show that galanin has an antinociceptive role in the ARC of intact rats and rats with inflammation.

Plastic changes of calcitonin gene-related peptide in morphine tolerance: Behavioral and immunohistochemical study in rats

The present study was undertaken to investigate the plasticity of calcitonin gene-related peptide (CGRP) in antinociception after morphine tolerance in rats. The hindpaw withdrawal latencies (HWLs) to both thermal and mechanical stimulation increased significantly after intracerebroventricular injection of 2.5 nmol of CGRP in opioid-naive rats, indicating that CGRP produces an antinociceptive effect in the brain. Furthermore, there was an antinociceptive effect after intracerebroventricular injection of 2.5 nmol of CGRP in morphine-tolerant rats. Interestingly, the antinociceptive effect induced by intracerebroventricular injection of CGRP was lower in morphine-tolerant rats than that in opioid-naive rats at the same dose. At the same time, there was downregulation of CGRP-like immunoreactivity in both lateral septal nucleus and central nucleus of amygdala tested by immunohistochemical methods, whereas no significant changes were observed in arcuate nucleus of hypothalamus and periaqueductal gray after morphine treatment in rats. The present study demonstrates plastic changes in both CGRP-induced antinociception and CGRP-like immunoreactivity in rat brain after morphine tolerance, suggesting that CGRP may play an important role in morphine tolerance.

Antinociceptive role of oxytocin in the nucleus raphe magnus of rats, an involvement of μ-opioid receptor

Recent studies showed that oxytocin plays an important role in nociceptive modulation in the central nervous system. The present study was undertaken to investigate the role of oxytocin in antinociception in the nucleus raphe magnus (NRM) of rats and the possible interaction between oxytocin and the opioid systems. Intra-NRM injection of oxytocin induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulation in rats. The antinociceptive effect of oxytocin was significantly attenuated by subsequent intra-NRM injection of the oxytocin antagonist 1-deamino-2-D-Tyr-(Oet)-4-Thr-8-Orn-oxytocin. Intra-NRM injection of naloxone dose-dependently antagonized the increased HWLs induced by preceding intra-NRM injection of oxytocin, indicating an involvement of opioid receptors in oxytocin-induced antinociception in the NRM of rats. Furthermore, the antinociceptive effect of oxytocin was dose-dependently attenuated by subsequent intra-NRM injection of the mu-opioid antagonist beta-funaltrexamine (beta-FNA), but not by the kappa-opioid antagonist nor-binaltorphimine (nor-BNI) or the delta-opioid antagonist naltrindole. The results demonstrated that oxytocin plays an antinociceptive role in the NRM of rats through activating the oxytocin receptor. Moreover, mu-opioid receptors, not kappa and delta receptors, are involved in the oxytocin-induced antinociception in the NRM of rats.

Involvement of calcitonin gene-related peptide and its receptor in anti-nociception in the periaqueductal grey of rats

The present study investigated the role of calcitonin gene-related peptide (CGRP) in the modulation of nociception in periaqueductal grey (PAG) of rats. Hindpaw withdrawal latencies (HWLs) were tested by hot-plate and Randall Selitto tests. The HWLs to thermal and mechanical stimulation increased significantly after intra-PAG administration of 0.26 or 0.13 nmol of CGRP, but not 0.026 nmol of CGRP. The anti-nociceptive effects induced by CGRP were significantly blocked by intra-PAG administration of 0.026 or 0.26 nmol of the CGRP1 receptor antagonist CGRP8-37. Furthermore, administration of CGRP into the decussation of superior cerebellar peduncle, out of PAG, did not elicit anti-nociceptive effects during 60 min after the injection. The results demonstrated that CGRP plays an important role in anti-nociception in PAG of rats, and CGRP1 receptor is involved in the CGRP-induced anti-nociception.

Involvement of endogenous beta-endorphin in antinociception in the arcuate nucleus of hypothalamus in rats with inflammation

Although exogenous administration of beta-endorphin to the arcuate nucleus of hypothalamus (ARC) had been shown to produce antinociception, the role of endogenous beta-endorphin of the ARC in nociceptive processing has not been studied directly. The aim of the present study was to investigate the effect of endogenous beta-endorphin in the ARC on nociception in rats with carrageenan-induced inflammation. The hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. Intra-ARC injection of naloxone had no significant influence on the HWL to thermal and mechanical stimulation in intact rats. The HWL decreased significantly after intra-ARC injection of 1 or 10 microg of naloxone in rats with inflammation, but not with 0.1 microg of naloxone. Furthermore, intra-ARC administration of the selective mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) decreased the nociceptive response latencies to both stimulation in a dose-dependent manner in rats with inflammation, while intra-ARC administration of the selective delta-opioid receptor antagonist naltrindole or the selective kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) showed no influences on the nociceptive response latency. The antiserum against beta-endorphin, administered to the ARC, also dose-dependently reduced the HWL in rats with inflammation. The results indicate that endogenous beta-endorphin in the ARC plays an important role in the endogenous antinociceptive system in rats with inflammation, and that its effect is predominantly mediated by the mu-opioid receptor.

Neuronal plasticity in relation to nociception and healing of rat achilles tendon

Nerve regeneration and the occurrence of three neuropeptides; i.e. substance P (SP), calcitonin gene related peptide (CGRP) and galanin (GAL), were studied during healing of tendon rupture in the rat by semi-quantitative immunohistochemistry. The neuronal findings were related to nociception as assessed by hindpaw withdrawal latencies at thermal and mechanical tests. Experimental rupture of rat Achilles tendon--normally devoid of nerves--elicited extensive nerve ingrowth into the rupture site in the early phase of healing followed by almost complete fiber disappearance (weeks 12-16). The ingrowth of SP and CGRP positive fibers, seen already at weeks 1-2, was associated with increased nociception. Subsequently, the occurrence of GAL positive fibers at weeks 4-6 was associated with decreased nociception. An even stronger relationship to nociception during healing was observed when the rate of change in neuropeptide expression instead of the expression in absolute terms was considered, according to the "cascade" formula of SP(')+CGRP(')-GAL(').It may prove that the observed temporal occurrence of different neuropeptides reflects a role of the peripheral nervous system in regulating synchronously nociception and healing.

Anti-nociceptive effect induced by intrathecal injection of ATPA, an effect enhanced and prolonged by concanavalin A

The present study investigated the effect of intrathecal injection of (RS)-2-alpha-amino-3-(3-hydroxy-5-tbutylisoxazol-4-yl) propanoic acid (ATPA), a selective agonist to kainate receptor, on nociception in rats. Intrathecal administration of 1, 4 and 10 nmol of ATPA induced dose-dependent increases in the hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats. Pretreatment with intrathecal injection of 300 microg of concanavalin A (ConA) to block the desensitization of kainate receptors enhanced and prolonged the anti-nociceptive effect induced by intrathecal injection of ATPA. The results suggest that the pre-synaptic kainate receptor in the primary afferent terminals is involved in the transmission of nociceptive information in dorsal horn of the spinal cord in rats. Furthermore, blocking the desensitization of kainate receptor enhanced and prolonged the ATPA-induced anti-nociceptive effects.

Involvement of 5-hydroxytryptamine(1A) receptors in the descending anti-nociceptive pathway from periaqueductal gray to the spinal dorsal horn in intact rats, rats with nerve injury and rats with inflammation

Studies have shown that 5-hydroxytryptamine (5-HT) plays an important role in the descending pathway of pain modulation from brainstem to the spinal cord. Using selective 5-HT receptor antagonists, the present study investigated which type of 5-HT receptor(s) in the spinal cord was involved in the morphine-induced anti-nociception in intact rats, in rats with nerve injury and in rats with inflammation. The hindpaw withdrawal latencies decreased significantly after sciatic nerve injury and hindpaw inflammation compared with intact rats. Intrathecal administration of 25 or 10 microg of the selective 5-HT(1A) recepter antagonist spiroxatrine, but not 1 microg of spiroxatrine, significantly blocked the increased hindpaw withdrawal latencies to thermal and mechanical stimulation induced by intra-periaqueductal gray injection of 1 microg of morphine in intact rats. Intrathecal injection of the 5-HT(2) receptor antagonist RS 102221 and the 5-HT(3) receptor antagonist MDL 72222 had no significant effects on the increased hindpaw withdrawal latencies to both noxious stimulations induced by intra-periaqueductal gray injection of morphine. Furthermore, intrathecal administration of spiroxatrine, but not RS 102221 nor MDL 72222, significantly attenuated the increased hindpaw withdrawal latencies induced by intra-periaqueductal gray administration of morphine in rats with nerve injury and in rats with inflammation. The results demonstrate that the 5-HT(1A) receptor, not 5-HT(2) nor 5-HT(3) receptor, plays an important role in the descending pathway of anti-nociception from the brainstem to the spinal cord in intact rats, in rats with nerve injury and in rats with inflammation.

Repeated massage-like stimulation induces long-term effects on nociception: contribution of oxytocinergic mechanisms

Massage-like stroking induces acute antinociceptive effects that can be reversed by an oxytocin antagonist, indicating activation of oxytocin on endogenous pain controlling systems. We now demonstrate an increase in hindpaw withdrawal latencies (HWLs), in response to thermal and mechanical stimuli, which was present after six treatments of massage-like stroking every other day and which continued to increase through the remaining seven treatments. Repeated massage-like stroking also resulted in increased oxytocin-like immunoreactivity (oxytocin-LI) levels in plasma and periaquaductal grey matter (PAG). Furthermore, increases in HWLs were also present after injections of oxytocin into the PAG (0.1, 0.5 and 1.0 nmol). Intra-PAG oxytocin injection of 1 nmol followed by 1 or 20 nmol of naloxone attenuated the increments in HWL. Also, there was a dose-dependent attenuation of the oxytocin-induced antinociceptive effects following intra-PAG injection of the mu-opioid antagonist beta-funaltrexamine (beta-FNA) and the kappa-opioid antagonist nor-binaltorphimine (nor-BNI) but not the delta-antagonist naltrindole. The long-term antinociceptive effects of massage-like stroking may be attributed, at least partly, to the oxytocinergic system and its interaction with the opioid system, especially the mu- and the kappa-receptors in the PAG.

Blockade effect of mu and kappa opioid antagonists on the anti-nociception induced by intra-periaqueductal grey injection of oxytocin in rats

Intra-periaqueductal grey (PAG) injection of 1 nmol of oxytocin induced significant increases in hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats. The anti-nociceptive effect of oxytocin was attenuated significantly by subsequent intra-PAG injection of the mu opioid antagonist beta-funaltrexamine (beta-FNA) and the kappa opioid antagonist nor-binaltorphimine (nor-BNI), but not by the delta antagonist naltrindole. The results demonstrated that mu and kappa opioid receptors, not delta receptors, were involved in the oxytocin-induced anti-nociception in PAG of rats.

Role of calcitonin gene-related peptide and its antagonist on the evoked discharge frequency of wide dynamic range neurons in the dorsal horn of the spinal cord in rats

The present study was performed to explore the effect of calcitonin gene-related peptide (CGRP) and its antagonist CGRP8-37 on the evoked discharge frequency of wide dynamic range (WDR) neurons in the dorsal horn of the spinal cord in rats. Recording was performed with a multibarrelled glass micropipette and the chemicals were delivered by iontophoresis. The discharge of WDR neurons was evoked by transdermic electrical stimulation applied on the ipsilateral hindpaw. (1) Iontophoretic application of CGRP at an ejection current of 100 nA increased the discharge frequency of WDR neurons significantly. (2) Iontophoretic application of CGRP8-37 at an ejection current of 80 or 160 nA induced significant decreases in the discharge frequency of WDR neurons, but not at 40 nA. (3) Iontophoretic application of CGRP8-37 not only antagonized the CGRP-induced increase in the evoked discharge frequency of WDR neurons but also induced a significant decrease in the evoked discharge frequency of WDR neurons compared to basal levels. The results indicate that CGRP and its receptors play a facilitary role on the transmission and/or modulation of nociceptive information in the dorsal horn of the spinal cord in rats.

Blockade effects of (Nphe1)Nociceptin(1-13)-NH(2) on anti-nociception induced by intrathecal administration of nociceptin in rats

The present study investigated the roles of the opioid-receptor-like (ORL1) receptor and its endogenous ligand nociceptin on nociception in the spinal cord of rats. Intrathecal administration of 10 nmol of nociceptin produced significant increases in hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation. There were no significant changes of average maximum angles in inclined plane tests after intrathecal injection of 10 nmol of nociceptin in rats. The intrathecal nociceptin-induced increases in HWL were antagonized by intrathecal administration of (Nphe1)Nociceptin(1-13)-NH(2), a selective antagonist of ORL1 receptor, in a dose-dependent manner. The results demonstrated that ORL1 receptor is involved in the nociceptin-induced anti-nociceptive effect in the spinal cord of rats.

Involvement of CGRP and CGRP1 receptor in nociception in the nucleus accumbens of rats

The present study was performed to investigate the role of calcitonin gene-related peptide (CGRP) and its antagonist CGRP8-37 on nociception in the nucleus accumbens of rats. Hindpaw withdrawal latencies (HWLs) to noxious stimulation induced by hot plate and Randall Selitto tests were measured. The HWL to both thermal and mechanical stimulation increased significantly after intra-nucleus accumbens administration of 0.5 or 1 nmol of CGRP, but not 0.1 nmol, indicating that CGRP plays an anti-nociceptive effect in the nucleus accumbens of rats. The anti-nociceptive effect induced by intra-nucleus accumbens administration of 1 nmol of CGRP was blocked significantly by following intra-nucleus accumbens administration of 1 nmol of CGRP8-37, a selective antagonist of CGRP1 receptor. Furthermore, the HWLs to both thermal and mechanical stimulation decreased significantly after intra-nucleus accumbens administration of 0.02, 0.1 and 0.5 nmol of CGRP8-37 alone. The hyperalgesic effect of intra-nucleus accumbens administration of CGRP8-37 lasted for more than 60 min after the injection, suggesting that CGRP1 receptor is involved in anti-nociception in the nucleus accumbens of rats. The results indicate that CGRP and CGRP1 receptor have important roles in nociceptive modulation in the nucleus accumbens of rats.

Anti-nociceptive effect of neuropeptide Y in periaqueductal grey in rats with inflammation

Experimental inflammation was induced by subcutaneous injection of carrageenan into the left hindpaw of rats. Intra-periaqueductal grey (PAG) injection of 0.02 or 0.1 nmol of neuropeptide Y (NPY), but not 0.004 nmol, induced significant increases in hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats with inflammation. Furthermore, the anti-nociceptive effect of NPY was blocked partly by following intra-PAG injection of the Y1 receptor antagonist NPY28-36. The results demonstrated that NPY plays an anti-nociceptive role in PAG in rats with inflammation, in which Y1 receptor is involved.

4991W93 inhibits release of calcitonin gene-related peptide in the cat but only at doses with 5HT(1B/1D) receptor agonist activity?

Calcitonin gene-related peptide (CGRP) is a marker for trigeminovascular activation and is released during the headache phase of migraine and cluster headache. CGRP may have a role in migraine through its potent cranial vasodilator effects, or by an action on trigeminal nerve activity, both of which are targeted by 5HT(1B/1D) agonist drugs. CP122,288, a conformationally restricted analogue of sumatriptan that is a potent inhibitor of neurogenic plasma protein extravasation (PPE), was ineffective at inhibiting CGRP release at a single low dose; and is also ineffective as an acute anti-migraine compound. However, it remained unclear as to whether, as a class, the conformationally-restricted triptan analogues could have inhibitory effects on CGRP in higher doses. 4991W93, a conformationally restricted analogue of zolmitriptan, is also a potent inhibitor of PPE at doses without 5HT(1B/1D)-mediated effects, that was developed as an anti-migraine drug, and thus was suitable to test whether higher doses of such conformationally restricted triptan analogues could inhibit trigeminal-evoked CGRP release. The superior sagittal sinus (SSS) was stimulated in 14 anaesthetised cats and external jugular vein blood samples were analysed by radioimmunoassay for CGRP levels before, 1 min after SSS stimulation, and 1 min after SSS stimulation in the presence of 4991W93. Stimulation of the SSS resulted in release of CGRP from the external jugular vein. 4991W93 at a dose of 0.1 and 10 microg/kg, selected for maximal PPE blocking effects in rodents, was ineffective at inhibiting CGRP release, with an SSS stimulation level of 78+/-4 pmol/l compared to a post-4991W93 level of 79+/-3 pmol/l (n=4). In comparison CGRP release was inhibited after a dose of 100 microg/kg 4991W93 from 64+/-6 to 36+/-3 pmol/l (n=5). Given that 4991W93 is inactive clinically at non-vascular doses, it seems clear that the 5HT(1B/1D) agonist effects of the compound are necessary for blockade of CGRP release and thus any anti-migraine action. Taken with the clinical results, these data emphasise the importance of CGRP release in migraine, and suggest that other non-5HT-based pharmacological targets may account for PPE blockade in animal studies.

Involvement of neuropeptide Y and Y1 receptor in antinociception in nucleus raphe magnus of rats

The nociceptive response latencies increased significantly after intra-nucleus raphe magnus administration of 0.1 or 0.4 nmol of neuropeptide Y, but not 0.04 nmol, in rats. The neuropeptide Y-induced increases in hindpaw withdrawal latency were reversed by following injection of 0.42 nmol of the Y1 antagonist, NPY(28-36). The results indicate that NPY plays an antinociceptive role in nucleus raphe magnus in rats, which is mediated by the Y1 receptor. Furthermore, the neuropeptide Y-induced increases in hindpaw withdrawal latency were attenuated by following intra-nucleus raphe magnus injection of 6 nmol of the opioid antagonist naloxone, indicating that there is an interaction between NPY and opioids in nucleus raphe magnus.

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.

Anti-nociceptive effects of calcitonin gene-related peptide in nucleus raphe magnus of rats: an effect attenuated by naloxone

The present study investigated the role of calcitonin gene-related peptide (CGRP) on nociception in nucleus raphe magnus (NRM) and the interaction between CGRP and opioid peptides in NRM of rats. CGRP-like immunoreactivity was found at a concentration of 6.0+/-0. 77 pmol/g in NRM tissue of ten samples of rats, suggesting that it may contribute to physiological responses orchestrated by the NRM. The hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation increased significantly after intra-NRM administration of 0.5 or 1 nmol of CGRP in rats, but not 0.25 nmol. The anti-nociceptive effect induced by CGRP was antagonized by following intra-NRM injection of 1 nmol of the CGRP receptor antagonist CGRP8-37. Furthermore, the CGRP-induced anti-nociceptive effect was attenuated by following intra-NRM administration of 6 nmol of naloxone. The results indicate that CGRP and its receptors play an important role in anti-nociception, and there is a possible interaction between CGRP and opioid peptides in NRM of rats.

Antinociceptive role of galanin in periaqueductal grey of rats with experimentally induced mononeuropathy

The present study was performed in rats with experimentally induced mononeuropathy after left common sciatic nerve ligation. The hindpaw withdrawal latencies to thermal and mechanical stimulation increased significantly after intra-periaqueductal grey injection of 2 or 3nmol, but not 1nmol of galanin in rats with mononeuropathy. Intraperitoneal administration of 4.5mg/kg morphine induced significant increases in hindpaw withdrawal latencies to both noxious stimulation, which were attenuated by following intra-periaqueductal grey injection of 2nmol of the galanin antagonist galantide. Furthermore, the antinociceptive effect induced by intra-periaqueductal grey injection of 26.6nmol of morphine was attenuated significantly by following intra-periaqueductal gray administration of 2nmol of galantide. The results demonstrated that in periaqueductal grey galanin plays an antinociceptive role in rats with mononeuropathy and galanin is involved in the mechanisms of opioid-induced antinociception.

Alleviation of mechanical and thermal allodynia by CGRP(8-37) in a rodent model of chronic central pain

CGRP(8-37) is a truncated version of calcitonin gene-related peptide (CGRP) that binds to the CGRP receptor with similar affinity but does not activate the receptor and is a highly selective CGRP receptor antagonist. CGRP and activation of its receptor appear to play a role in peripheral inflammatory and neuropathic models of pain although there is considerable controversy. The aim of this study was to examine possible anti-nociceptive effects of CGRP(8-37) on a model of chronic central neuropathic pain known to develop weeks after spinal hemisection. Adult male Sprague-Dawley rats were given a spinal hemisection (N=34) or a sham surgery (N=10) at the T13 spinal segment. An externally accessible PE-10 intrathecal catheter that terminated at T13 was used for drug delivery. Animals were allowed to recover for 4 weeks at which time the hemisected animals displayed mechanical and thermal allodynia bilaterally, in both forelimbs and hindlimbs. CGRP(8-37) was delivered just prior to a testing session in 1, 5, 10, or 50 nM doses in artificial cerebral spinal fluid in 10 microl volumes. CGRP(8-37) was effective in alleviating mechanical and thermal allodynia in a dose-dependent manner (P<0.05). The 50 nM dose was most efficacious for both forelimb and hindlimb responses (P<0.05). The period of efficacy was 10 min to onset for a duration of 20 min. Post-drug washout responses were not statistically significant compared to pre-drug responses. The sham control groups demonstrated no statistically significant difference at any dose of CGRP(8-37) when compared to pre-surgical baseline values. In conclusion, CGRP(8-37) is effective in abolishing mechanical and thermal allodynia produced by spinal hemisection. Consequently, the CGRP receptor may play a role in chronic central neuropathic pain and offers a novel therapeutic approach to managing chronic central pain.

Capsaicin-evoked release of immunoreactive calcitonin gene-related peptide from the spinal cord is mediated by nitric oxide but not by cyclic GMP

Recent data support a role for nitric oxide (NO) in pain processing at the level of the spinal cord, possibly via regulation of neuropeptide release. The goal of this study was to determine whether capsaicin, which selectively activates primary afferent neurons and evokes neuropeptide release, acts in an NO-dependent manner. Our results indicate that capsaicin (1 microM)-evoked release of immunoreactive calcitonin gene-related peptide (iCGRP) is significantly reduced in the presence of the NO synthase inhibitor, L-NAME (10-400 nM; F(3,45)=68.38; P<0.001) and, the selective nNOS inhibitor, 3-bromo-7-nitroindazole (170-680 nM; F(5,48)=56.2; P<0. 01). D-NAME (200 nM) had no effect on capsaicin-evoked iCGRP release. Hemoglobin (an extracellular scavenger of NO; 3 mg/ml) significantly reduced the effect of capsaicin on the release of iCGRP (F(1,8)=9.12; P<0.05). The NOS substrate, L-arginine, effectively reversed the inhibitory effect of 3-bromo-7-nitroindazole on capsaicin-evoked iCGRP release. To determine whether the NO-mediated release was NMDA-driven, we superfused spinal cord slices with competitive and non-competitive NMDA antagonists in the presence and absence of capsaicin. MK-801 (0. 1-10 microM; F(4,33)=8.49; P<0.0001) and AP-5 (0.01-10 microM; F(4, 38)=3.34; P<0.05) reduced capsaicin-evoked iCGRP release. CNQX, an AMPA/kainate antagonist (10 nM-10 microM), significantly decreased capsaicin-evoked release of iCGRP (F(6,42)=8.76; P<0.01) in a dose-dependent fashion. Additionally, our results demonstrate that while capsaicin-evoked release is significantly reduced in the presence of LY-83583 (10 microM; F(2,18)=3.46; P<0.01; a cyclic GMP lowering agent), there is no effect of ODQ (a potent and selective inhibitor of guanylate cyclase). Moreover, the application of a cell permeable analog of cyclic GMP (8-bromo-cGMP; 0.01-1000 microM) is without effect on both basal and evoked iCGRP release. Finally, we observed no colocalization of immunoreactive neuronal NOS (nNOS) with CGRP in the dorsal horn. In summary, these data indicate that capsaicin evokes the release of iCGRP, in part, via the production of NO which enters the extracellular space prior to having an effect. Moreover, iCGRP and nNOS are produced in distinct populations of neurons within the dorsal horn. We conclude that capsaicin-evoked release involves the activation of the NMDA receptor but is also modified by the activation of AMPA or kainate receptors. Finally, these data suggest that while capsaicin-evoked iCGRP release is modified by NO, this release does not require the activation of guanylate cyclase and subsequent production of cyclic GMP.

Antinociceptive effects of calcitonin gene-related peptide injected into periaqueductal grey of rats with mononeuropathy

Intra-periaqueductal grey (PAG) injection of calcitonin gene-related peptide (CGRP) induced dose-dependent increases in hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats with mononeuropathy. CGRP-induced increases in HWLs were blocked by intra-PAG injection of the CGRP antagonist CGRP8-37. The results demonstrated that CGRP and CGRP receptors in PAG play an important role in antinociception in rats with mononeuropathy.

Antinociceptive effects induced by intra-periaqueductal grey administration of neuropeptide Y in rats

Hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation increased dose-dependently after intra-periaqueductal grey (PAG) injection of neuropeptide Y (NPY). Furthermore, the NPY-induced increases in HWLs were attenuated by intra-PAG injection of the Y1 receptor antagonist NPY28-36. The results demonstrated that NPY plays an important role in antinociception in PAG, in which Y1 receptor is involved in.

Contralateral treatment with xylocaine reduces nociceptive behaviour in mononeuropathic rats

The aim of the present study was to establish whether contralateral treatment with local anaesthetics reduces nociceptive behaviour in mononeuropathic rats. Contralateral treatment with xylocaine on day 6 and 11 following sciatic nerve ligation increased withdrawal latencies to thermal but not to mechanical stimulation for 3-4 days. Rats who received contralateral treatment with xylocaine on day 6 and 11 showed a reduced autotomy score during the following 6 weeks. To compare ipsilateral and contralateral effects another set of experiments was performed. Rats treated contralaterally on day 11 had a significantly lower autotomy score at week 4 and 6. Our results demonstrate that contralateral treatment with xylocaine reduces nociceptive behaviour in mononeuropathic rats for days or weeks.

An interaction of opioids and galanin in dorsal horn of the spinal cord in mononeuropathic rats

The present study was performed in rats with experimentally induced mononeuropathy after common sciatic nerve ligation. The hind-paw withdrawal latencies to thermal and mechanical stimulation were increased significantly after intrathecal injection of 3 nmol of galanin. The increased hind-paw response latencies induced by galanin were attenuated by following intrathecal injection of 22 nmol, but not 11 or 2.75 nmol of the opioid receptor antagonist naloxone. Further, the increased hind-paw response latencies induced by galanin were prevented by following intrathecal injection of 10 nmol of mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA), but not by 10 nmol of delta-opioid receptor antagonist, natrindole or 10 nmol of kappa-opioid receptor antagonist, nor-binaltorphimine (nor-BNI). Intrathecal 10 nmol of beta-FNA alone had no significant effects on the hind-paw withdrawal responses. These results demonstrate the existence of a specific interaction between galanin and opioids in the transmission of presumed nociceptive information in the spinal cord of mononeuropathic rats. This interaction involves the activation of mu-opioid receptor.

Nerve constriction in the rat: model of neuropathic, surgical and central pain

In preparation for a series of electrophysiological experiments in a model of neuropathic pain, the present spinal reflex study was done to determine the optimal time after sciatic nerve constriction in the rat for tactile allodynia and to determine also the appropriate 'control' for the nerve constriction model. Therefore, this study focused on the magnitude and time course of change in paw withdrawal threshold following unilateral sciatic nerve constriction in the rat. Male Sprague-Dawley rats (375-425g) were used. Nerve constriction was done by placing a 2 mm polyethylene cuff (PE-90) around the left sciatic nerve (n=8). A second group of rats (n=8) received unilateral sham surgery and a third group (n=8) was unoperated. The ipsi- and contralateral hind paw withdrawal thresholds in each of the 3 groups were measured using von Frey hairs. In unoperated rats, the withdrawal threshold of each of the hind paws remained unchanged at approximately 50 g throughout the entire time course of the study, which lasted 145 days. However, in cuff-implanted rats, the withdrawal threshold of the nerve-injured hind paw decreased as soon as 1 day after surgery, reached as low as 1 to 2 g by 5 days and remained low throughout the test period. Threshold in sham-operated rats showed a bilateral decrease starting on days 1-3, which stabilised at about 30 g until about day 40, after which values returned gradually toward the unoperated withdrawal thresholds. In nerve-constricted rats the withdrawal threshold of the hind paw contralateral to the cuff followed the same change seen in sham-operated rats until about day 37, after which the withdrawal threshold matched that of the cuff-implanted hind paw. The data show that the cuff-induced sciatic nerve constriction produces a sustained hypersensitivity to normally innocuous tactile sensory input and that a relatively constant ipsilateral mechanical hyperalgesia can be found from days 5-27. It is also demonstrated that the contralateral hind paw and either hind paw in sham-operated rats are inappropriate as 'controls'. The data in this study suggest that three distinct types of allodynia are expressed. Ipsilateral allodynia may be representative of a model of neuropathic pain. The contralateral allodynia may be a model of central pain, as it likely arises from changes in central sensory processing. Allodynia in sham-operated rats was also expressed bilaterally and may be a model of long-term postoperative pain.