Intrathecal CGRP8-37-induced bilateral increase in hindpaw withdrawal latency in rats with unilateral inflammation

Activation of Transient Receptor Potential Vanilloid 1 Is Involved in Both Pain and Tumor Growth in a Mouse Model of Cancer Pain

Activation of calcitonin gene-related peptide (CGRP)-positive sensory neurons in the tumor microenvironment has been shown to be involved in tumor growth. However, how CGRP-positive sensory neurons are activated requires elucidation. In this study, we focused on transient receptor potential vanilloid 1 (TRPV1) and examined the contribution of TRPV1 to tumor growth and cancer pain in a mouse cancer model in which Lewis lung carcinoma was subcutaneously inoculated in the left plantar region. Tumor inoculation gradually increased the volumes of the hind paws of wild type (WT) mice over time, but those of both αCGRP knockout mice and TRPV1 knockout mice were significantly smaller than those of WT mice after tumor inoculation. Both TRPV1 and CGRP are therefore suggested to be involved in tumor growth. In an immunohistochemical study, the percentage of phosphorylated cyclic adenosine monophosphate response element-binding protein (p-CREB)-positive profiles in CGRP-positive dorsal root ganglion (DRG) neurons in WT mice was significantly increased after tumor inoculation. The percentage of p-CREB-positive profiles in CGRP-positive DRG neurons in TRPV1 knockout mice was also increased after tumor inoculation, but was significantly lower than that in WT mice, indicating the contribution of TRPV1 to activation of CGRP-positive DRG neurons. Cancer pain in TRPV1 knockout mice was significantly lower than that in WT mice. In conclusion, TRPV1 is involved in both tumor growth and cancer pain, potentially leading to a novel strategy for the treatment of cancer pain and cancer development. Cancer pain is also suggested to facilitate tumor growth.

IUPHAR review: Navigating the role of preclinical models in pain research

Chronic pain is a complex and challenging medical condition that affects millions of people worldwide. Understanding the underlying mechanisms of chronic pain is a key goal of preclinical pain research so that more effective treatment strategies can be developed. In this review, we explore nociception, pain, and the multifaceted factors that lead to chronic pain by focusing on preclinical models. We provide a detailed look into inflammatory and neuropathic pain models and discuss the most used animal models for studying the mechanisms behind these conditions. Additionally, we emphasize the vital role of these preclinical models in developing new pain-relief drugs, focusing on biologics and the therapeutic potential of NMDA and cannabinoid receptor antagonists. We also discuss the challenges of TRPV1 modulation for pain treatment, the clinical failures of neurokinin (NK)- 1 receptor antagonists, and the partial success story of Ziconotide to provide valuable lessons for preclinical pain models. Finally, we highlight the overall success and limitations of current treatments for chronic pain while providing critical insights into the development of more effective therapies to alleviate the burden of chronic pain.

A narrative review of the calcitonin peptide family and associated receptors as migraine targets: Calcitonin gene-related peptide and beyond

OBJECTIVE

To summarize the pharmacology of the calcitonin peptide family of receptors and explore their relationship to migraine and current migraine therapies.

BACKGROUND

Therapeutics that dampen calcitonin gene-related peptide (CGRP) signaling are now in clinical use to prevent or treat migraine. However, CGRP belongs to a broader peptide family, including the peptides amylin and adrenomedullin. Receptors for this family are complex, displaying overlapping pharmacologic profiles. Despite the focus on CGRP and the CGRP receptor in migraine research, recent evidence implicates related peptides and receptors in migraine.

METHODS

This narrative review summarizes literature encompassing the current pharmacologic understanding of the calcitonin peptide family, and the evidence that links specific members of this family to migraine and migraine-like behaviors.

RESULTS

Recent work links amylin and adrenomedullin to migraine-like behavior in rodent models and migraine-like attacks in individuals with migraine. We collate novel information that suggests females may be more sensitive to amylin and CGRP in the context of migraine-like behaviors. We report that drugs designed to antagonize the canonical CGRP receptor also antagonize a second CGRP-responsive receptor and speculate as to whether this influences therapeutic efficacy. We also discuss the specificity of current drugs with regards to CGRP isoforms and how this may influence therapeutic profiles. Lastly, we emphasize that receptors related to, but distinct from, the canonical CGRP receptor may represent underappreciated and novel drug targets.

CONCLUSION

Multiple peptides within the calcitonin family have been linked to migraine. The current focus on CGRP and its canonical receptor may be obscuring pathways to further therapeutics. Drug discovery schemes that take a wider view of the receptor family may lead to the development of new anti-migraine drugs with favorable clinical profiles. We also propose that understanding these related peptides and receptors may improve our interpretation regarding the mechanism of action of current drugs.

Role of calcitonin gene-related peptide in pain regulation in the parabrachial nucleus of naive rats and rats with neuropathic pain

Researches have shown that calcitonin gene-related peptide (CGRP) plays a pivotal role in pain modulation. Nociceptive information from the periphery is relayed from parabrachial nucleus (PBN) to brain regions implicated involved in pain. This study investigated the effects and mechanisms of CGRP and CGRP receptors in pain regulation in the PBN of naive and neuropathic pain rats. Chronic sciatic nerve ligation was used to model neuropathic pain, CGRP and CGRP 8-37 were injected into the PBN of the rats, and calcitonin receptor-like receptor (CLR), a main structure of CGRP receptor, was knocked down by lentivirus-coated CLR siRNA. The hot plate test (HPT) and the Randall Selitto Test (RST) was used to determine the latency of the rat hindpaw response. The expression of CLR was detected with RT-PCR and western blotting. We found that intra-PBN injecting of CGRP induced an obvious anti-nociceptive effect in naive and neuropathic pain rats in a dose-dependent manner, the CGRP-induced antinociception was significantly reduced after injection of CGRP 8-37, Moreover, the mRNA and protein levels of CLR, in PBN decreased significantly and the antinociception CGRP-induced was also significantly lower in neuropathic pain rats than that in naive rats. Knockdown CLR in PBN decreased the expression of CLR and the antinociception induced by CGRP was observably decreased. Our results demonstrate that CGRP induced antinociception in PBN of naive or neuropathic pain rats, CGRP receptor mediates this effect. Neuropathic pain induced decreases in the expression of CGRP receptor, as well as in CGRP-induced antinociception in PBN.

Differential suppression of the ipsi- and contralateral nociceptive reflexes in the neonatal rat spinal cord by agonists of µ-, δ- and κ-opioid receptors

Nociceptive discharges caused by the unilateral tissue damage are processed in the spinal cord by both ipsi- and contralateral neuronal circuits. The mechanisms of the neurotransmitter control of this bilateral excitation spread is poorly understood. Spinally administered opiates are known to suppress nociceptive transmission and nociceptive withdrawal reflexes. Here we investigated whether three major types of opioid receptors are involved in the bilateral control of the spinal nociceptive sensorimotor processing. Effects of the µ-, δ- and κ-opioid receptor agonists on the ipsi- and contralateral nociceptive reflexes were studied by recording slow ventral root potentials in an isolated spinal cord preparation of the new-born rat. Absolute levels of expression of the opioid genes were analyzed by the droplet digital PCR. Ipsi- and contralateral slow ventral root potentials were most strongly suppressed by the µ-opioid receptor agonist DAMGO, by 63% and 85%, followed by the κ-opioid receptor agonist U-50488H, by 44% and 73%, and δ-opioid receptor agonist leucine-enkephalin, by 27% and 49%, respectively. All these agonists suppressed stronger contra- than ipsilateral responses. Naloxone prevented effects of the agonists indicating that they act through opioid receptors, which, as we show, are expressed in the neonatal spinal cord at the levels similar to those in adults. Thus, opioid receptor agonists suppress the segmental nociceptive reflexes. Stronger contralateral effects suggest that the endogenous opioid system regulates sensorimotor processing in the spinal commissural pathways. These effects of opioids may be relevant for treatment of symmetric clinical pain symptoms caused by unilateral tissue injury.

Contribution of CD137L to Sensory Hypersensitivity in a Murine Model of Neuropathic Pain

CD137L (4-1BBL) is a costimulatory molecule whose signaling can promote monocyte/macrophage functions; however, CD137L-mediated microglial response and its role in neuropathic pain remain unknown. We investigated CD137L following peripheral nerve injury-induced neuropathic pain using a spinal nerve L5 transection (L5Tx) murine model in both sexes. First, C57BL/6_CD137L knock-out (KO) mice displayed decreased mechanical and diminished heat hypersensitivity compared to wild-type (WT) controls, beginning on day 3 to up to day 35 post-L5Tx. Purified anti-mouse CD137L neutralizing monoclonal antibody (0.1 or 0.5 µg) was also used to identify CD137L’s window of action in BALB/c mice. Anti-CD137L antibody was intrathecally administered either from day 0 (before surgery) to day 7 (early treatment), or from day 6 to 13 post-L5Tx (late treatment), and nociceptive thresholds were assessed before surgery to up to day 35 post-surgery. Early treatment with anti-CD137L reduced L5Tx-induced mechanical but not heat hypersensitivity, while later treatment did not alter either sensitivity. Pro- versus anti-inflammatory responses within the lumbar spinal cord following L5Tx were further evaluated via quantitative real-time PCR (qRT-PCR) and immunohistochemistry (IHC) in time-course studies. Following L5Tx, female CD137L KO mice did not show increased iNOS mRNA and had reduced numbers of IL-1β⁺ cells compared to WT. At 21 d post-surgery, CD137L KO mice had higher total numbers of arginase (Arg)-1⁺ cells and Arg-1⁺ microglia. Altogether, results indicate that spinal cord CD137L contributes to the development of peripheral nerve injury-induced neuropathic pain, which may be in part mediated through CD137L’s modulation of the pro- and anti-inflammatory balance within the spinal cord.

Essential role of endogenous calcitonin gene-related peptide in pain-associated plasticity in the central amygdala

The role of the neuropeptide calcitonin gene‐related peptide (CGRP) is well established in nociceptive behaviors. CGRP is highly expressed in the projection pathway from the parabrachial nucleus to the laterocapsular region of the central amygdala (CeC), which plays a critical role in relaying nociceptive information. The CeC is a key structure in pain behavior because it integrates and modulates nociceptive information along with other sensory signals. Previous studies have demonstrated that blockade of the amygdalar CGRP‐signaling cascade attenuates nociceptive behaviors in pain models, while CGRP application facilitates amygdalar synaptic transmission and induces pain behaviors. Despite these lines of evidence, it remains unclear whether endogenous CGRP is involved in the development of nociceptive behaviors accompanied with amygdalar plasticity in a peripheral inflammation model in vivo. To directly address this, we utilized a previously generated CGRP knockout (KO) mouse to longitudinally study formalin‐induced plasticity and nociceptive behavior. We found that synaptic potentiation in the right PB‐CeC pathway that was observed in wild‐type mice was drastically attenuated in the CGRP KO mice 6 h post‐inflammation, when acute nociceptive behavior was no longer observed. Furthermore, the bilateral tactile allodynia 6 h post‐inflammation was significantly decreased in the CGRP KO mice. In contrast, the acute nociceptive behavior immediately after the formalin injection was reduced only at 20–25 min post‐injection in the CGRP KO mice. These results suggest that endogenous CGRP contributes to peripheral inflammation‐induced synaptic plasticity in the amygdala, and this plasticity may underlie the exaggerated nociception–emotion linkage in pain chronification.

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.

Essential role of endogenous calcitonin gene-related peptide in pain-associated plasticity in the central amygdala

The role of the neuropeptide calcitonin gene-related peptide (CGRP) is well established in nociceptive behaviors. CGRP is highly expressed in the projection pathway from the parabrachial nucleus to the laterocapsular region of the central amygdala (CeC), which plays a critical role in relaying nociceptive information. The CeC is a key structure in pain behavior because it integrates and modulates nociceptive information along with other sensory signals. Previous studies have demonstrated that blockade of the amygdalar CGRP-signaling cascade attenuates nociceptive behaviors in pain models, while CGRP application facilitates amygdalar synaptic transmission and induces pain behaviors. Despite these lines of evidence, it remains unclear whether endogenous CGRP is involved in the development of nociceptive behaviors accompanied with amygdalar plasticity in a peripheral inflammation model in vivo. To directly address this, we utilized a previously generated CGRP knockout (KO) mouse to longitudinally study formalin-induced plasticity and nociceptive behavior. We found that synaptic potentiation in the right PB-CeC pathway that was observed in wild-type mice was drastically attenuated in the CGRP KO mice 6 h post-inflammation, when acute nociceptive behavior was no longer observed. Furthermore, the bilateral tactile allodynia 6 h post-inflammation was significantly decreased in the CGRP KO mice. In contrast, the acute nociceptive behavior immediately after the formalin injection was reduced only at 20-25 min post-injection in the CGRP KO mice. These results suggest that endogenous CGRP contributes to peripheral inflammation-induced synaptic plasticity in the amygdala, and this plasticity may underlie the exaggerated nociception-emotion linkage in pain chronification.

Metformin Synergizes With Conventional and Adjuvant Analgesic Drugs to Reduce Inflammatory Hyperalgesia in Rats

BACKGROUND

Metformin is a widely used and safe antidiabetic drug that has recently been shown to possess analgesic properties in models of inflammatory pain. Because various arthritic inflammatory disorders are highly prevalent in diabetic patients, we aimed to examine the type of interaction between metformin and several conventional and adjuvant analgesic drugs (ibuprofen, aspirin, tramadol, and pregabalin) in a rat model of somatic inflammatory hyperalgesia.

METHODS

Inflammation of the rat hind paw was induced by an intraplantar injection of carrageenan (0.1 mL, 1%). The antihyperalgesic effects of metformin (intraperitoneally), analgesics (orally or intraperitoneally), and 2-drug metformin-analgesic combinations were assessed with an electronic Von Frey anesthesiometer, by measuring the change in paw withdrawal thresholds induced by carrageenan (n = 6 rats in drug/drug combination-treated groups). First, we determined the doses of individual drugs needed to produce an antihyperalgesic effect of 50% (ED50 values). In combination experiments, drugs were coadministered in fixed-dose fractions (1/16, 1/8, 1/4, and 1/2) of their individual ED50 values and the type of interaction between components was determined by isobolographic analysis.

RESULTS

Metformin (50-200 mg/kg) significantly and dose-dependently reduced carrageenan-induced hyperalgesia with a maximal antihyperalgesic effect (mean ± SEM) of 62 ± 6% (all P ≤ .024). Ibuprofen (25-150 mg/kg), aspirin (100-400 mg/kg), tramadol (0.5-5 mg/kg), and pregabalin (2.5-20 mg/kg) also produced significant and dose-dependent antihyperalgesic effects (all P ≤ .042) of similar magnitude to metformin (the maximal antihyperalgesic effects were 73 ± 4% for ibuprofen, 62 ± 4.2% for aspirin, 69 ± 5.9% for tramadol, and 56 ± 3.9% for pregabalin). In combination experiments, administration of 2-drug metformin-analgesic combinations led to a significant and dose-dependent reduction of carrageenan-induced hyperalgesia (all P ≤ .027). The isobolographic analysis revealed that metformin interacted synergistically with the examined analgesics (experimental ED50 values of 2-drug combinations were significantly lower than theoretical additive ED50 values; all P < .05) and that there was a similar, approximately 5-fold, reduction of doses of both drugs in all tested combinations.

CONCLUSIONS

Our results suggest that in patients who are already receiving metformin therapy, lower doses of ibuprofen/aspirin/tramadol/pregabalin might be sufficient for achieving satisfactory pain relief. Metformin-aspirin combination might be particularly useful because it may achieve multiple therapeutic goals (glucoregulation, pain relief, and cardioprotection).

Analgesic Microneedle Patch for Neuropathic Pain Therapy

Neuropathic pain caused by nerve injury is debilitating and difficult to treat. Current systemic pharmacological therapeutics for neuropathic pain produce limited pain relief and have undesirable side effects, while current local anesthetics tend to nonspecifically block both sensory and motor functions. Calcitonin gene related peptide (CGRP), a neuropeptide released from sensory nerve endings, appears to play a significant role in chronic neuropathic pain. In this study, an analgesic microneedle (AMN) patch was developed using dissolvable microneedles to transdermally deliver selective CGRP antagonist peptide in a painless manner for the treatment of localized neuropathic pain. Local analgesic effects were evaluated in rats by testing behavioral pain sensitivity in response to thermal and mechanical stimuli using neuropathic pain models such as spared-nerve injury and diabetic neuropathy pain, as well as neurogenic inflammatory pain model induced by ultraviolet B (UVB) radiation. Unlike several conventional therapies, the AMN patches produced effective analgesia on neuropathic pain without disturbing the normal nociception and motor function of the rat, resulting from the high specificity of the delivered peptide against CGRP receptors. The AMN patches did not cause skin irritation or systemic side effects. These results demonstrate that dissolvable microneedle patches delivering CGRP antagonist peptide provide an effective, safe, and simple approach to mitigate neuropathic pain with significant advantages over current treatments.

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.

Attenuation of inflammatory and neuropathic pain behaviors in mice through activation of free fatty acid receptor GPR40

Background

The G-protein-coupled receptor 40 (GPR40) is suggested to function as a transmembrane receptor for medium- to long-chain free fatty acids and is implicated to play a role in free fatty acids-mediated enhancement of glucose-stimulated insulin secretion from pancreas. However, the functional role of GPR40 in nervous system including somatosensory pain signaling has not been fully examined yet.

Results

Intrathecal injection of GPR40 agonist (MEDICA16 or GW9508) dose-dependently reduced ipsilateral mechanical allodynia in CFA and SNL models and thermal hyperalgesia in carrageenan model. These anti-allodynic and anti-hyperalgesic effects were almost completely reversed by a GPR40 antagonist, GW1100. Immunohistochemical analysis revealed that GPR40 is expressed in spinal dorsal horn and dorsal root ganglion neurons, and immunoblot analysis showed that carrageenan or CFA inflammation or spinal nerve injury resulted in increased expression of GPR40 in these areas. Patch-clamp recordings from spinal cord slices exhibited that bath-application of either MEDICA16 or GW9508 significantly decreased the frequency of spontaneous excitatory postsynaptic currents in the substantia gelatinosa neurons of the three pain models.

Conclusions

Our results indicate that GPR40 signaling pathway plays an important suppressive role in spinal nociceptive processing after inflammation or nerve injury, and that GPR40 agonists might serve as a new class of analgesics for treating inflammatory and neuropathic pain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0003-8) contains supplementary material, which is available to authorized users.

Calcitonin gene-related peptide is involved in inflammatory pain but not in postoperative pain

BACKGROUND

The aim of this study was to clarify the roles of calcitonin gene-related peptide (CGRP) in postoperative pain and inflammatory pain.

METHODS

αCGRP knockout mice that the authors have developed and wild-type mice were used. Pain behaviors were assessed after incision and complete Freund's adjuvant (CFA) injection. Changes in CGRP and c-Fos expression in the dorsal horn were also examined.

RESULTS

Guarding pain scores in αCGRP knockout mice were lower than those in wild-type mice at 24 h (3.8 ± 1.6 vs. 6.8 ± 1.5, P = 0.044) and 48 h (1.8 ± 1.7 vs. 6.0 ± 1.5, P = 0.001) after CFA injection (n = 8 to 9). Withdrawal latencies to heat stimulation in αCGRP knockout mice were higher than those in wild-type mice at 24 to 72 h after CFA injection (4.9 ± 1.0 vs. 3.4 ± 0.8 at 24 h, P = 0.04; 5.1 ± 0.3 vs. 3.2 ± 0.9 at 48 h, P = 0.047; and 5.4 ± 1.6 vs. 3.5 ± 0.5 s at 72 h, P = 0.045) (n = 11 to 13), but withdrawal thresholds to mechanical stimulation were comparable. CGRP expression was increased at 24 h after CFA injection in wild-type mice, and the c-Fos-positive profile was increased at 4 h after CFA injection (ipsilateral vs. contralateral: 12.3 ± 4.6 vs. 1.3 ± 1.9, P < 0.0001) and maintained at 24 h (10.0 ± 4.1 vs. 0.8 ± 1.3, P < 0.0001) (n = 4 to 6).

CONCLUSION

These results suggest that contribution of the αCGRP system depends on the modality of pain and the stage of inflammation.

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.

Bilateral elevation of interleukin-6 protein and mRNA in both lumbar and cervical dorsal root ganglia following unilateral chronic compression injury of the sciatic nerve

Background

Current research implicates interleukin (IL)-6 as a key component of the nervous-system response to injury with various effects.

Methods

We used unilateral chronic constriction injury (CCI) of rat sciatic nerve as a model for neuropathic pain. Immunofluorescence, ELISA, western blotting and in situ hybridization were used to investigate bilateral changes in IL-6 protein and mRNA in both lumbar (L4-L5) and cervical (C7-C8) dorsal root ganglia (DRG) following CCI. The operated (CCI) and sham-operated (sham) rats were assessed after 1, 3, 7, and 14 days. Withdrawal thresholds for mechanical hyperalgesia and latencies for thermal hyperalgesia were measured in both ipsilateral and contralateral hind and fore paws.

Results

The ipsilateral hind paws of all CCI rats displayed a decreased threshold of mechanical hyperalgesia and withdrawal latency of thermal hyperalgesia, while the contralateral hind and fore paws of both sides exhibited no significant changes in mechanical or thermal sensitivity. No significant behavioral changes were found in the hind and fore paws on either side of the sham rats, except for thermal hypersensitivity, which was present bilaterally at 3 days. Unilateral CCI of the sciatic nerve induced a bilateral increase in IL-6 immunostaining in the neuronal bodies and satellite glial cells (SGC) surrounding neurons of both lumbar and cervical DRG, compared with those of naive control rats. This bilateral increase in IL-6 protein levels was confirmed by ELISA and western blotting. More intense staining for IL-6 mRNA was detected in lumbar and cervical DRG from both sides of rats following CCI. The DRG removed from sham rats displayed a similar pattern of staining for IL-6 protein and mRNA as found in naive DRG, but there was a higher staining intensity in SGC.

Conclusions

Bilateral elevation of IL-6 protein and mRNA is not limited to DRG homonymous to the injured nerve, but also extended to DRG that are heteronymous to the injured nerve. The results for IL-6 suggest that the neuroinflammatory reaction of DRG to nerve injury is propagated alongside the neuroaxis from the lumbar to the remote cervical segments. This is probably related to conditioning of cervical DRG neurons to injury.

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.

Activation of extracellular signal-regulated protein kinase 5 is essential for cystitis- and nerve growth factor-induced calcitonin gene-related peptide expression in sensory neurons

Background

Cystitis causes considerable neuronal plasticity in the primary afferent pathways. The molecular mechanism and signal transduction underlying cross talk between the inflamed urinary bladder and sensory sensitization has not been investigated.

Results

In a rat cystitis model induced by cyclophosphamide (CYP) for 48 h, the mRNA and protein levels of the excitatory neurotransmitter calcitonin gene-related peptide (CGRP) are increased in the L6 dorsal root ganglia (DRG) in response to bladder inflammation. Cystitis-induced CGRP expression in L6 DRG is triggered by endogenous nerve growth factor (NGF) because neutralization of NGF with a specific NGF antibody reverses CGRP up-regulation during cystitis. CGRP expression in the L6 DRG neurons is also enhanced by retrograde NGF signaling when NGF is applied to the nerve terminals of the ganglion-nerve two-compartmented preparation. Characterization of the signaling pathways in cystitis- or NGF-induced CGRP expression reveals that the activation (phosphorylation) of extracellular signal-regulated protein kinase (ERK)5 but not Akt is involved. In L6 DRG during cystitis, CGRP is co-localized with phospho-ERK5 but not phospho-Akt. NGF-evoked CGRP up-regulation is also blocked by inhibition of the MEK/ERK pathway with specific MEK inhibitors U0126 and PD98059, but not by inhibition of the PI3K/Akt pathway with inhibitor LY294002. Further examination shows that cystitis-induced cAMP-responsive element binding protein (CREB) activity is expressed in CGRP bladder afferent neurons and is co-localized with phospho-ERK5 but not phospho-Akt. Blockade of NGF action in vivo reduces the number of DRG neurons co-expressing CGRP and phospho-CREB, and reverses cystitis-induced increases in micturition frequency.

Conclusions

A specific pathway involving NGF-ERK5-CREB axis plays an essential role in cystitis-induced sensory activation.

Interaction of histamine and calcitonin gene-related peptide in the formalin induced pain perception in rats

Histamine and calcitonin gene-related peptide (CGRP) contribute to the pain perception. The aim of the present study is to clarify the interaction of histamine and CGRP in the perception of inflammatory pain. The effects of a histamine H1 receptor antagonist (pyrilamine, i.p.), an H2 receptor antagonist (ranitidine, i.p.) and a CGRP antagonist (CGRP 8-37, i.t.) on the formalininduced pain was studied in rats. Pyrilamine and ranitidine produced a dose-dependent antinociceptive response in the first and the second phases of the formalin test. A single administration of pyrilamine (1 mg/kg, i.p.), ranitidine (10 mg/kg, i.p.) or CGRP 8-37 (10 µg/µL, i.t.) had no significant effects on the pain perception in the second phase. A combination of CGRP 8-37 and pyrilamine or ranitidine at these sub-effective doses, however, showed nociceptive response in the second phase. Moreover, a histamine (i.t.)-induced hyperalgesia was completely prevented by treatment with GGRP 8-37 at this dose. Our findings have raised the possibility that the CGRP system has interaction with histamine in the perception of inflammatory pain.

Mechanical hyperalgesia is attenuated by local administration of octreotide in pristane-induced arthritis in Dark-Agouti rats

AIMS

The Dark-Agouti (DA) rat is very susceptible to pristane-induced arthritis (PIA) and represents a suitable model for rheumatoid arthritis. In the present study, we examined the pain sensitivity and the effect of local administration of octreotide (OCT) on mechanical hyperalgesia in PIA DA rats.

MAIN METHODS

Arthritis was induced by intradermal injection of pristane (300 microl). The mechanical withdrawal threshold (MWT) and heat withdrawal latency (HWL) were used to evaluate the pain sensitivity. In addition, we recorded the discharge firings in the tibial nerve sensory C-fibers innervating the inflamed toe joints of arthritic DA rats.

KEY FINDINGS

Two weeks after injection of pristane, all DA rats developed severe arthritis. This symptom was associated with a decreased MWT (78.50+/-5.68 mN before pristane injection, 19.50+/-6.27 mN on day 14 after pristane injection), indicating a mechanical hyperalgesia in PIA. In contrast, HWL was comparable before and after pristane injection (10.25+/-0.70 s before injection; 9.45+/-1.23 s on day 14 after injection). Local injection of OCT markedly increased MWT and relieved the hyperalgesia in PIA. In addition, OCT significantly decreased the discharge rate of afferent C units evoked by both non-noxious and noxious joint movements.

SIGNIFICANCE

Taken together, the results demonstrate that mechanical hyperalgesia, but not thermal hyperalgesia is associated with PIA and that the mechanical hyperalgesia and the discharge of afferent C units are attenuated by local administration of OCT. These observations provide evidence for a novel therapeutic strategy for pain control in rheumatoid arthritis.

Spinal CGRP1 receptors contribute to supraspinally organized pain behavior and pain-related sensitization of amygdala neurons

CGRP receptor activation has been implicated in peripheral and central sensitization. The role of spinal CGRP receptors in supraspinal pain processing and higher integrated pain behavior is not known. Here we studied the effect of spinal inhibition of CGRP1 receptors on supraspinally organized vocalizations and activity of amygdala neurons. Our previous studies showed that pain-related audible and ultrasonic vocalizations are modulated by the central nucleus of the amygdala (CeA). Vocalizations in the audible and ultrasonic range and hindlimb withdrawal thresholds were measured in awake adult rats before and 5-6h after induction of arthritis by intra-articular injections of kaolin and carrageenan into one knee. Extracellular single-unit recordings were made from neurons in the latero-capsular division of the CeA (CeLC) in anesthetized rats before and after arthritis induction. CGRP1 receptor antagonists were applied to the lumbar spinal cord intrathecally (5 microl/min) 6h postinduction of arthritis. Spinal administration of peptide (CGRP8-37, 1 microM) and non-peptide (BIBN4096BS, 1 microM) CGRP1 receptor antagonists significantly inhibited the increased responses of CeLC neurons to mechanical stimulation of the arthritic knee but had no effect under normal conditions. In arthritic rats, the antagonists also inhibited the audible and ultrasonic components of vocalizations evoked by noxious stimuli and increased the threshold of hindlimb withdrawal reflexes. The antagonists had no effect on vocalizations and spinal reflexes in normal rats. These data suggest that spinal CGRP1 receptors are not only important for spinal pain mechanisms but also contribute significantly to the transmission of nociceptive information to the amygdala and to higher integrated behavior.

Targeting invading macrophage-derived PGE2, IL-6 and calcitonin gene-related peptide in injured nerve to treat neuropathic pain

Immune and inflammatory responses occurring in an injured nerve have been generally believed to contribute to the generation and maintenance of neuropathic pain. In this review, the authors demonstrate the upregulation of COX-2/prostaglandin E2, IL-6 and calcitonin gene-related peptide in invading macrophages and discuss possible mechanisms involved in their upregulation and how they contribute to the maintenance of neuropathic pain. By acting on nociceptors in dorsal root ganglion and local inflammatory cells via autocrine or paracrine pathways, these inflammatory mediators facilitate spontaneous ectopic activity and sustain nociceptive responses, an important mechanism underlying both ongoing and evoked neuropathic pain state. Targeting these mediators in injured nerve may provide novel therapeutic avenues to more successfully treat nerve injury-associated neuropathic pain.

Pain-related synaptic plasticity in spinal dorsal horn neurons: role of CGRP

Background

The synaptic and cellular mechanisms of pain-related central sensitization in the spinal cord are not fully understood yet. Calcitonin gene-related peptide (CGRP) has been identified as an important molecule in spinal nociceptive processing and ensuing behavioral responses, but its contribution to synaptic plasticity, cellular mechanisms and site of action in the spinal cord remain to be determined. Here we address the role of CGRP in synaptic plasticity in the spinal dorsal horn in a model of arthritic pain.

Results

Whole-cell current- and voltage-clamp recordings were made from substantia gelatinosa (SG) neurons in spinal cord slices from control rats and arthritic rats (> 6 h postinjection of kaolin/carrageenan into the knee). Monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of afferents in the dorsal root near the dorsal root entry zone. Neurons in slices from arthritic rats showed increased synaptic transmission and excitability compared to controls. A selective CGRP1 receptor antagonist (CGRP8-37) reversed synaptic plasticity in neurons from arthritic rats but had no significant effect on normal transmission. CGRP facilitated synaptic transmission in the arthritis pain model more strongly than under normal conditions where both facilitatory and inhibitory effects were observed. CGRP also increased neuronal excitability. Miniature EPSC analysis suggested a post- rather than pre-synaptic mechanism of CGRP action.

Conclusion

This study is the first to show synaptic plasticity in the spinal dorsal horn in a model of arthritic pain that involves a postsynaptic action of CGRP on SG neurons.

Increased calcitonin gene-related peptide in neuroma and invading macrophages is involved in the up-regulation of interleukin-6 and thermal hyperalgesia in a rat model of mononeuropathy

The pain related peptide, calcitonin gene-related peptide (CGRP), plays an important role in inflammatory pain and immune responses. However, its role in neuropathic pain is not established. Following nerve injury, CGRP and pro-inflammatory interleukin-6 (IL-6) are increased in injured nerves. The aim of this study was to determine if CGRP in injured nerves is involved in the up-regulation of IL-6 and in the maintenance of neuropathic pain. Perineural injection of a neutralizing IL-6 antiserum or CGRP receptor antagonists (CGRP8-37 and BIBN4096BS) effectively attenuated thermal hyperalgesia 4 weeks after partial sciatic nerve ligation. Perineural CGRP antagonists also dramatically reduced IL-6 level in injured nerves. CGRP release from injured sites was dramatically increased and CGRP immunoreactivity was localized in both neuroma and invading macrophages. CGRP receptor markers (CRLR and RAMP1) were expressed in invading macrophages. Both CGRP antagonists significantly reduced IL-6 release from injured nerve explants. In cell cultures derived from injured nerves, CGRP concentration-dependently increased IL-6 release, an effect also blocked by CGRP antagonists. Taken together, these data show that increased levels of CGRP in injured neuroma and invading macrophages are involved in the up-regulation of IL-6 in macrophages as well as in the maintenance of neuropathic pain.

The effects of enalapril maleate and cold stress exposure on tyrosine hydroxylase activity in some rat tissues

Enalapril is a highly specific and competitive inhibitor of angiotensin-I converting enzyme (ACE) and thus belongs to the category of ACE inhibitors. The beneficial effects of ACE inhibitors appear to result primarily from the suppression of the plasma renin-angiotensin-aldesterone system. This study was designed to detect the effects of enalapril maleate and cold stress on tyrosine hydroxylase (TH) activity in adrenal medulla, heart and hypothalamus in rat. In cold stress treatment (exposed to 8 degrees C cold for 48 h) TH activity was found to be raised significantly (p < 0.05) in adrenal medulla, hypothalamus and heart tissues. In the adrenal medulla, hypothalamus and heart tissues, TH activity of enalapril maleate treated rats (10 mg kg(-1) body weight) group was not raised significantly (p > 0.05). Following intraperitoneal injection of enalapril maleate (10 mg kg(-1) body weight) the rats were exposed to 8 degrees C cold for 48 h. After cold stress and enalapril maleate treatment no statistically significant change in tyrosine hydroxylase activity was detected in adrenal medulla, hypothalamus or heart (p > 0.05). The results of our studies show that enalapril maleate blocks the effect of cold stress on the regulation of TH activity.

Deep tissue inflammation upregulates neuropeptides and evokes nociceptive behaviors which are modulated by a neuropeptide antagonist

Promising recent developments in the therapeutic value of neuropeptide antagonists have generated renewed importance in understanding the functional role of neuropeptides in nociception and inflammation. To explore this relationship we examined behavioral changes and primary afferent neuronal plasticity following deep tissue inflammation. One hour following craniofacial muscle inflammation ipsilateral as well as contralateral head withdrawal thresholds and ipsi- and contralateral hindpaw withdrawal thresholds were lowered and remained reduced for 28 days. Elevated levels of calcitonin gene-related peptide (CGRP) within the trigeminal ganglion temporally correlated with this mechanical allodynia. Inflammation also induced an increase in the number of CGRP and substance P (SP)-immunopositive trigeminal ganglion neurons innervating inflamed muscle but did not evoke a shift in the size distribution of peptidergic muscle afferent neurons. Trigeminal proprioceptive muscle afferent neurons situated within the brainstem in the mesencephalic trigeminal nucleus did not express CGRP or SP prior to or following inflammation. Intravenous administration of CGRP receptor antagonist (8-37) two minutes prior to adjuvant injection blocked plasma extravasation and abolished both head and hindlimb mechanical allodynia. Local injection of CGRP antagonist directly into the masseter muscle prior to CFA produced similar, but less pronounced, effects. These findings indicate that unilateral craniofacial muscle inflammation produces mechanical allodynia at distant sites and upregulates CGRP and SP in primary afferent neurons innervating deep tissues. These data further implicate CGRP and SP in deep tissue nociceptive mechanisms and suggest that peptide antagonists may have therapeutic potential for musculoskeletal pain.

Anti-inflammatory effects of contralateral administration of the kappa-opioid agonist U-50,488H in rats with unilaterally induced adjuvant arthritis

OBJECTIVE

The effect of repeated contralateral treatment with the kappa-opioid receptor agonist U-50,488H {trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzene acetamide methanesulphonate} was investigated in rats with unilaterally induced adjuvant arthritis.

METHODS

Arthritis was induced by injection of Mycobacterium butyricum into the right hindpaw. Inflammatory parameters, nociceptive behaviour and cartilage turnover were evaluated up to 21 days after induction of arthritis.

RESULTS

Contralateral treatment with 0.3 mg U-50,488H into the left hindpaw twice per week reduced the hindpaw oedema, ankle joint inflammation, pain behaviour to mechanical stimuli and severity score of inflammation in the hindpaws of both sides as well as the systemic spread of inflammation to other areas, e.g. tail and/or forepaws, compared with saline-treated animals. Moreover, a significant decrease in the levels of cartilage oligomeric matrix protein was found in animals treated with U-50,488H, suggesting reduction of cartilage damage. The anti-inflammatory and chondroprotective effects of U-50,488H were abolished by administration of the peripheral opioid receptor antagonist naloxone methiodide.

CONCLUSIONS

This is the first report demonstrating that repeated contralateral administration of a kappa-opioid receptor agonist diminishes the development of a symmetrical joint disorder.

Increase in sensory neuropeptides surrounding the Achilles tendon in rats with adjuvant arthritis

The Achilles tendon in rats with adjuvant arthritis was analyzed by radioimmunoassay (RIA) and semi-quantitative immunohistochemistry for the occurrence of two sensory neuropeptides, substance P (SP) and calcitonin gene related peptide (CGRP), and a sensory modulating peptide, galanin (GAL). The tissue concentration of SP and CGRP in the Achilles tendon and its envelope, i.e. the paratenon and bony insertion, as assessed by RIA was increased by 22% and 71%, respectively, compared to normal controls, whereas the level of GAL was unchanged. Semi-quantitative immunohistochemistry applied to different regions of the tendon in arthritic rats disclosed an increased occurrence of SP and CGRP positive nerve fibers in the paratenon and bone tendinous junction, whereas GAL fibers were only increased at the bone tendinous junction. Notably, neither neuropeptides nor inflammatory cells were seen in the tendon proper. The increased occurrence of SP and CGRP in the tendon envelope presumably reflects inflammatory actions, whereas that of GAL implies an endogenous anti-inflammatory response. The observed SP and CGRP upregulation in the paratenon and bony insertion suggests a pathophysiological role in paratenonitis and enthesitis often seen in patients with rheumatoid arthritis. Presumably Achillodynia originates in the tendon envelope rather than the tendon proper. The observations could be used to define new pharmacological targets for mitigating symptoms from tendons in rheumatoid arthritis and possibly also in other disorders. Whether a neuronal pathogenic mechanism underlies tendon overuse disorders in non-arthritic tendinopathies and the development of degeneration, i.e. tendinosis, remains to be studied.

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.

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.

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.

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 oxytocin in spinal antinociception in rats with inflammation

The present study was conducted on rats with inflammation induced by subcutaneous injection of carrageenan into the left hindpaw. Intrathecal administration of oxytocin produced dose-dependent increases in the hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats with inflammation. The antinociceptive effect of oxytocin was blocked by intrathecal administration of atosiban, a selective oxytocin antagonist, indicating that oxytocin receptor mediates oxytocin-induced antinociception in the spinal cord. The oxytocin-induced antinociceptive effect was attenuated by intrathecal administration of the opioid antagonist naloxone, suggesting an involvement of the endogenous opioid system in oxytocin-induced antinociception in the spinal cord of rats with inflammation. Furthermore, the antinociceptive effect of oxytocin was attenuated by intrathecal injections of the mu-receptor antagonist beta-funaltrexamine and the kappa-receptor antagonist nor-binaltorphimine, but not by the delta-receptor antagonist naltrindole, illustrating that mu- and kappa-receptors, but not delta-receptor, are involved in oxytocin-induced antinociception in the spinal cord of rats with inflammation. Moreover, intrathecal administration of atosiban alone induced a hyperalgesia in rats with inflammation, indicating that endogenous oxytocin is involved in the transmission and regulation of nociceptive information in the spinal cord of rats with inflammation. The present study showed that both exogenous and endogenous oxytocin displayed antinociception in the spinal cord in rats with inflammation, and mu- and kappa-receptors were involved in oxytocin-induced antinociception.

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.

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.

Nociceptin/orphanin FQ into the rat periaqueductal gray decreases the withdrawal latency to heat and loading, an effect reversed by (Nphe(1))nociceptin(1-13)NH(2)

The present study investigated the effect of intraperiaqueductal grey injection of nociceptin/orphanin FQ (N/OFQ) and an antagonist (Nphe(1))nociceptin(1-13)NH(2) on the hindpaw withdrawal response to thermal and mechanical stimulation in rats. N/OFQ (5 nmol) significantly decreased the nociceptive thresholds in both tests and 1, 5 and 10 nmol of (Nphe(1))nociceptin(1-13)NH(2) significantly reversed this effect in a dose dependent way. Our results demonstrate, that N/OFQ has a nociceptive action, possibly through inhibition of PAG neurons. This effect is blocked by the antagonist (Nphe(1))nociceptin(1-13)NH(2) probably via ORL1 receptors in the periaqueductal grey.

Contralateral treatment with lidocaine reduces spinal neuronal activity in mononeuropathic rats

In anaesthetised and paralysed rats with chronic constriction of the sciatic nerve, the effects of subcutaneous contralateral lidocaine (100 microl) on the activity of lumbar (L(4)-L(5)) wide dynamic range neurons ipsilateral to the constriction have been investigated. The results show reduction of the spontaneous hyperactivity for 60 min; suppression or reduction of the responses to contralateral noxious stimulation for 60 min; lack of effect on the responses to ipsilateral noxious stimulation, except for the afterdischarge duration, reduced for 60 min. The finding that the altered neuronal activity following peripheral nerve injury associated to behavioural signs of neuropathic pain, can be reduced by contralateral treatment, may provide further suggestions to neuropathic pain mechanisms and management.

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.

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.

Differential effects of calcitonin gene-related peptide and calcitonin gene-related peptide 8-37 upon responses to N-methyl-D-aspartate or (R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate in spinal nociceptive neurons with knee joint input in the rat

Calcitonin gene-related peptide is involved in the spinal processing of nociceptive input from the knee joint and in the generation and maintenance of joint inflammation-evoked hyperexcitability of spinal cord neurons. The present study examined whether this peptide influences the excitation of nociceptive spinal cord neurons by agonists at the N-methyl-D-aspartate and the non-N-methyl-D-aspartate [(R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate] receptors, both of which are essential for the excitation and hyperexcitability of spinal cord neurons. In anaesthetized rats extracellular recordings were made from dorsal horn neurons with knee input, and compounds were administered ionophoretically close to the neurons recorded. When calcitonin gene-related peptide was administered the responses of the neurons to the application of both N-methyl-D-aspartate and AMPA were increased. The coadministration of the antagonist calcitonin gene-related peptide 8-37 had no effect on the responses to N-methyl-D-aspartate, but it prevented the enhancement of the responses to N-methyl-D-aspartate by calcitonin gene-related peptide. By contrast, the administration of calcitonin gene-related peptide 8-37 enhanced the responses of the neurons to AMPA, and it did not antagonize but rather increased the effects of calcitonin gene-related peptide on these responses. The data suggest that the facilitatory role of calcitonin gene-related peptide on the development and maintenance of inflammation-evoked hyperexcitability is caused at least in part by the modulation of the activation of the dorsal horn neurons through their N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors. The different effects of calcitonin gene-related peptide 8-37 on the respones to N-methyl-D-aspartate and AMPA suggest that different intracellular pathways may facilitate the activation of N-methyl-D-aspartate and ionotropic non-N-methyl-D-aspartate receptors.

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.

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.

Calcitonin gene-related peptide 8-37 inhibits the evoked discharge frequency of wide dynamic range neurons in dorsal horn of the spinal cord in rats

The present study was performed to explore the effect of calcitonin gene-related peptide 8-37 (CGRP8-37) on the electrical stimulation-evoked discharge frequency of wide dynamic range (WDR) neurons in the dorsal horn of the spinal cord in rats. The discharge frequencies of WDR neurons were evoked by transdermic electrical stimulation applied on the ipsilateral hindpaw. CGRP8-37 was applied directly on the dorsal surface of the L3 to L5 spinal cord. After the administration of 3 nmol of CGRP8-37, the evoked discharge frequency of WDR neurons decreased significantly, an effect lasting more than 30 min. The results indicate that CGRP receptors play an important role in the transmission of presumed nociceptive information in the dorsal horn of the spinal cord.

Receptor for calcitonin gene-related peptide: localization in the dorsal and ventral spinal cord

Although the distribution of calcitonin gene-related peptide has been extensively studied in the spinal cord, little is known about the precise subcellular localization of receptors for calcitonin gene-related peptide. The present study was undertaken to localize calcitonin gene-related peptide receptors in both the dorsal and ventral horns of the rat spinal cord. Immunocytochemical localization with specific monoclonal antibodies was performed at the light and electron microscopic levels. Calcitonin gene-related peptide receptor was expressed in neuronal but not glial elements. Discrete postsynaptic localization of receptor for the calcitonin gene-related peptide was evident in the cells and dendrites of the superficial dorsal horn. Some of the terminal endings apposing the stained synapses formed the central terminals of glomerular complexes. The endings were scallop shaped (Type I), typical of primary afferent terminations. Other dorsal horn structures with postsynaptic labeling were contacted by dome-shaped or elongated axonal endings. Presynaptic localization on some dorsal horn terminations may serve an autoreceptor function. Motoneurons, on the other hand, were contacted by axonal terminals with presynaptic calcitonin gene-related peptide receptors. These data suggest that (i) dorsal horn neurons are capable of direct primary afferent, calcitonin gene-related peptide receptor-mediated interactions and (ii) neuronal terminals contacting motor horn cells can be influenced through presynaptic paracrine-like calcitonin gene-related peptide receptor-mediated interactions. Thus, calcitonin gene-related peptide can have multiple modulatory effects on spinal cord neurons through site-specific receptors.