Mechanical Hyperalgesia In Rats Research Articles

Delayed onset muscle soreness: Involvement of neurotrophic factors.

Delayed-onset muscle soreness (DOMS) is quite a common consequence of unaccustomed strenuous exercise, especially exercise containing eccentric contraction (lengthening contraction, LC). Its typical sign is mechanical hyperalgesia (tenderness and movement related pain). Its cause has been commonly believed to be micro-damage of the muscle and subsequent inflammation. Here we present a brief historical overview of the damage-inflammation theory followed by a discussion of our new findings. Different from previous observations, we have observed mechanical hyperalgesia in rats 1-3 days after LC without any apparent microscopic damage of the muscle or signs of inflammation. With our model we have found that two pathways are involved in inducing mechanical hyperalgesia after LC: activation of the B2 bradykinin receptor-nerve growth factor (NGF) pathway and activation of the COX-2-glial cell line-derived neurotrophic factor (GDNF) pathway. These neurotrophic factors were produced by muscle fibers and/or satellite cells. This means that muscle fiber damage is not essential, although it is sufficient, for induction of DOMS, instead, NGF and GDNF produced by muscle fibers/satellite cells play crucial roles in DOMS.

Mechanical hyperalgesia in rats with diabetic polyneuropathy is selectively inhibited by local peripheral nociceptin/orphanin FQ receptor and µ-opioid receptor agonism

Peripheral receptors may contribute to the effects of systemically administered centrally available analgesics. In the present study, we analysed the effect of local peripheral injection of the nociceptin/orphanin FQ peptide (NOP) receptor agonist Ro65-6570 and compared it to the µ-opioid peptide (MOP) receptor agonist morphine in streptozotocin-induced diabetic polyneuropathy in rats. Ro65-6570 and morphine were injected intraplantarly into the hind paw of diabetic rats, and mechanical withdrawal thresholds were determined in both paws (ipsi- and contralateral to the injection site). Ro65-6570 in the dose range of 7.1–71.4nmol/animal showed antihyperalgesic effects with maximal efficacy of 57.1±15.4% maximal possible effect (MPE) at the dose of 23.8nmol/animal. Intraplantar administration of morphine showed dose-dependent antihyperalgesic effects in the dose range of 25.8–257.8nmol/animal in a similar efficacy range with a maximal efficacy of 76.0±12.1% MPE at the dose of 257.8nmol/animal. Both compounds did not induce overt confounding side effects across the tested dose range. The NOP receptor antagonist J-113397 and the MOP receptor antagonist naloxone, intraplantarly co-administered with the respective agonists, selectively and completely prevented the antihyperalgesic action of the respective NOP and MOP receptor agonist. These results indicate that the activation of peripheral NOP and MOP receptors by Ro65-6570 and morphine, respectively, mediated antihyperalgesic effects in rats with diabetic polyneuropathy.

Reactive Oxygen Species Donors Increase the Responsiveness of Dorsal Horn Neurons and Induce Mechanical Hyperalgesia in Rats.

Our previous studies suggest that reactive oxygen species (ROS) scavengers have analgesic effect on neuropathic pain through spinal mechanisms in the rat. The studies suggest that superoxide in spinal cord is one of important mediators of persistent pain. To test the hypothesis that increase of superoxide-derived intermediates leads to central sensitization and pain, the effects of an intrathecal injection of chemical ROS donors releasing either OH∙, OCl−, or H2O2 were examined on pain behaviors. Following treatment with t-BOOH (OH∙ donor), dorsal horn neuron responses to mechanical stimuli in normal rats and the changes of neuronal excitability were explored on substantia gelatinosa (SG) neurons using whole-cell patch clamping recordings. Intrathecal administration of t-BOOH or NaOCl (OCl− donor), but not H2O2, significantly decreased mechanical thresholds of hind paws. The responses of wide dynamic range neurons to mechanical stimuli increased after a local application of t-BOOH. The t-BOOH increased the frequency and the amplitude of excitatory postsynaptic potentials, depolarized membrane potential in SG neurons, and increased the frequency of action potentials evoked by depolarizing current pulses. These results suggest that elevated ROS, especially OH∙, in the spinal cord sensitized dorsal horn neurons and produced hyperalgesia in normal rats.

Pain-relieving effect of CGRP antagonism on inflammatory pain

The neuropeptide calcitonin gene-related peptide (CGRP) is known to play a major role in the pathogenesis of pain syndromes, in particular migraine pain; however, its implication in inflammatory processes is not well known. The CGRP receptor antagonist BIBN4096BS was shown to reduce migraine pain and trigeminal neuronal activity. An analgesic action of this compound can also be found in rats with induced acute inflammation by injection of complete Freund's adjuvant (CFA) in one hindpaw. In this model the compound reduced inflammatory pain and spinal neuronal activity. Behavioral experiments (Randall-Selitto test) revealed a reversal of the CFA-induced mechanical hyperalgesia in rats after systemic drug administration. In vivo electrophysiological studies performed in rats injected with CFA using recordings of wide dynamic range neurons in deep dorsal horn layers of the lumbar spinal cord, confirmed a reduction of neuronal activity after systemic drug administration. The same considerable amount of reduction occurred after topical administration onto the paw with resulting systemic plasma concentrations in the low nanomolar range. Spinal administration of BIBN4096BS did not modify the neuronal activity in the CFA model which suggests that peripheral blockade of CGRP receptors by BIBN4096BS significantly alleviates inflammatory pain.

Acid-induced hyperalgesia and anxio-depressive comorbidity in rats

Fibromyalgia is a prevalent disorder characterized by chronic widespread pain (CWP) and complex comorbid symptoms. A CWP model is developed through repeated unilateral intramuscular injections of acid saline resulting in bilateral mechanical hyperalgesia in rats. The present study aims to evaluate whether both anxious and depressive comorbidities exist in this acid-induced pain model, similarly to patients with CWP syndromes. The anxiety-like behaviors were evaluated using the open field and elevated plus maze tests, and depression-like behaviors were measured by the forced swimming, sucrose consumption, and sucrose preference tests. The pain group receiving acidic saline displayed significantly lower paw withdrawal thresholds for 4weeks than animals in the vehicle group after repetitive intramuscular injections. The pain group showed a significantly shorter duration of exploring the central zone of the open field and the open arms of the elevated plus maze compared to the vehicle group. The pain group had a significantly lower preference for and consumption of the hedonic sucrose. Moreover, rats with chronic pain showed significantly longer immobility than the vehicle group in the forced swimming test. The results indicate that psychiatric behaviors are exacerbated in the CWP model. This study provides evidence for the validity of the acid-induced pain model analogous to patients with CWP syndromes.

Activated Spinal Astrocytes are Involved in the Maintenance of Chronic Widespread Mechanical Hyperalgesia after Cast Immobilization

BackgroundIn the present study, we examined spinal glial cell activation as a central nervous system mechanism of widespread mechanical hyperalgesia in rats that experienced chronic post-cast pain (CPCP) 2 weeks after cast immobilization. Activated spinal microglia and astrocytes were investigated immunohistologically in lumbar and coccygeal spinal cord segments 1 day, 5 weeks, and 13 weeks following cast removal.ResultsIn the lumbar cord, astrocytes were activated after microglia. Astrocytes also were activated after microglia in the coccygeal cord, but with a delay that was longer than that observed in the lumbar cord. This activation pattern paralleled the observation that mechanical hyperalgesia occurred in the hindleg or the hindpaw before the tail. The activating transcription factor 3 (ATF3) immune response in dorsal root ganglia (DRG) on the last day of cast immobilization suggested that nerve damage might not occur in CPCP rats. The neural activation assessed by the phosphorylated extracellular signal-regulated kinase (pERK) immune response in DRG arose 1 day after cast removal. In addition, L-α-aminoadipate (L-α-AA), an inhibitor of astrocyte activation administered intrathecally 5 weeks after cast removal, inhibited mechanical hyperalgesia in several body parts including the lower leg skin and muscles bilaterally, hindpaws, and tail.ConclusionsThese findings suggest that activation of lumbar cord astrocytes is an important factor in widespread mechanical hyperalgesia in CPCP.

Chronic ibuprofen administration reduces neuropathic pain but does not exert neuroprotection after spinal cord injury in adult rats

Ibuprofen is commonly used as an anti-inflammatory analgesic drug, although it is not amongst the first-line treatments for neuropathic pain. Its main effects are mediated by non-specific inhibition of COX enzymes, but it also exerts some COX-independent effects, such as the inhibition of RhoA signaling and the modulation of glial activity. These effects have boosted the use of ibuprofen as a tool to promote axonal regeneration and to increase functional recovery after neural injuries, although with controversial results showing positive and negative outcomes of ibuprofen treatment in several experimental models. We have evaluated the effects of ibuprofen administered at 60mg/kg twice a day to rats subjected to a mild spinal cord contusion. Our results indicate that ibuprofen ameliorates mechanical hyperalgesia in rats by reducing central hyperexcitability, but failed to produce improvements in the recovery of locomotion. Despite an early effect on reducing microglial reactivity, the ibuprofen treatment did not provide histological evidence of neuroprotection; indeed the volume of cord tissue spared rostral to the lesion was decreased in ibuprofen treated rats. In summary, the early modulation of neuroinflammation produced by the administration of ibuprofen seems to eventually lead to a worse resolution of detrimental events occurring in the secondary injury phase, but also to reduce the development of neuropathic pain.

The K + channel GIRK2 is both necessary and sufficient for peripheral opioid‐mediated analgesia

The use of opioid agonists acting outside the central nervous system (CNS) is a promising therapeutic strategy for pain control that avoids deleterious central side effects such as apnea and addiction. In human clinical trials and rat models of inflammatory pain, peripherally restricted opioids have repeatedly shown powerful analgesic effects; in some mouse models however, their actions remain unclear. Here, we investigated opioid receptor coupling to K+ channels as a mechanism to explain such discrepancies. We found that GIRK channels, major effectors for opioid signalling in the CNS, are absent from mouse peripheral sensory neurons but present in human and rat. In vivo transgenic expression of GIRK channels in mouse nociceptors established peripheral opioid signalling and local analgesia. We further identified a regulatory element in the rat GIRK2 gene that accounts for differential expression in rodents. Thus, GIRK channels are indispensable for peripheral opioid analgesia, and their absence in mice has profound consequences for GPCR signalling in peripheral sensory neurons.GIRK channels are indispensable for peripheral opioid analgesia. The absence of GIRK channels from mouse dorsal root ganglion neurons questions the predictive validity of mice as a model organism for investigating peripheral GPCRmediated analgesia.

AKAP‐dependent sensitization ofCav3.2 channels via theEP4receptor/cAMPpathway mediatesPGE2‐induced mechanical hyperalgesia

Background and PurposeTheCav3.2 isoform ofT‐typeCa2+channels (Tchannels) is sensitized by hydrogen sulfide, a pro‐nociceptive gasotransmitter, and also byPKAthat mediatesPGE2‐induced hyperalgesia. Here we examined and analysedCav3.2 sensitization via thePGE2/cAMPpathway inNG108‐15 cells that expressCav3.2 and produce cAMPin response toPGE2, and its impact on mechanical nociceptive processing in rats.Experimental ApproachInNG108‐15 cells and rat dorsal root ganglion (DRG) neurons,T‐channel‐dependent currents (Tcurrents) were measured with the whole‐cell patch‐clamp technique. The molecular interaction ofCav3.2 with A‐kinase anchoring protein 150 (AKAP150) and its phosphorylation were analysed by immunoprecipitation/immunoblotting inNG108‐15 cells. Mechanical nociceptive threshold was determined by the paw pressure test in rats.Key ResultsInNG108‐15 cells and/or ratDRGneurons, dibutyryl cAMP(db‐cAMP) orPGE2increasedTcurrents, an effect blocked byAKAPSt‐Ht31 inhibitor peptide (AKAPI) orKT5720, aPKAinhibitor. The effect ofPGE2was abolished byRQ‐00015986‐00, anEP4receptor antagonist.AKAP150 was co‐immunoprecipitated withCav3.2, regardless of stimulation with db‐cAMP, andCav3.2 was phosphorylated by db‐cAMPorPGE2. In rats, intraplantar (i.pl.) administration of db‐cAMPorPGE2caused mechanical hyperalgesia, an effect suppressed byAKAPI, two distinctT‐channel blockers,NNC55‐0396 and ethosuximide, or ZnCl2, known to inhibitCav3.2 amongTchannels. Oral administration ofRQ‐00015986‐00 suppressed thePGE2‐induced mechanical hyperalgesia.Conclusion and ImplicationsOur findings suggest thatPGE2causesAKAP‐dependent phosphorylation and sensitization ofCav3.2 through theEP4receptor/cAMP/PKApathway, leading to mechanical hyperalgesia in rats.

Exacerbated mechanical hyperalgesia in rats with genetically predisposed depressive behavior: Role of melatonin and NMDA receptors

A connection between pain and depression has long been recognized in the clinical setting; however, its mechanism remains unclear. This study showed that mechanical hyperalgesia induced by unilateral temporomandibular joint (TMJ) inflammation was exacerbated in Wistar-Kyoto (WKY) rats with genetically predisposed depressive behavior. Reciprocally, TMJ inflammation enhanced depressive behavior such that a lower nociceptive threshold correlated with a higher score of depressive behavior in the same WKY rats. As compared with Wistar rats, WKY rats showed a lower plasma melatonin level, downregulation of the melatonin MT1 receptor, but upregulation of the NR1 subunit of the NMDA receptor in the ipsilateral trigeminal subnucleus caudalis (Sp5C). Intracisternal administration of 6-chloromelatonin (250μg, twice daily for 7days) concurrently attenuated mechanical hyperalgesia and depressive behavior in WKY rats as well as downregulated the NR1 expression in the ipsilateral Sp5C. In patch-clamp recordings, melatonin dose-dependently decreased NMDA-induced currents in spinal cord dorsal horn substantia gelatinosa neurons. These results demonstrate a reciprocal relationship between TMJ inflammation-induced mechanical hyperalgesia and depressive behavior and suggest that the central melatoninergic system, through modulation of the NMDA receptor expression and activity, may play a role in the mechanisms of the comorbidity between pain and depression.

Analgesic properties of loperamide differ following systemic and local administration to rats after spinal nerve injury

The analgesic properties and mechanisms of loperamide hydrochloride, a peripherally acting opioid receptor agonist, in neuropathic pain warrant further investigation. We examined the effects of systemic or local administration of loperamide on heat and mechanical hyperalgesia in rats after an L5 spinal nerve ligation (SNL). (1) Systemic loperamide (0.3-10 mg/kg, subcutaneous in the back) dose dependently reversed heat hyperalgesia in SNL rats, but did not produce thermal analgesia. Systemic loperamide (3 mg/kg) did not induce thermal antinociception in naïve rats; (2) systemic loperamide-induced anti-heat hyperalgesia was blocked by pretreatment with intraperitoneal naloxone methiodide (5 mg/kg), but not by intraperitoneal naltrindole (5 mg/kg) or intrathecal naltrexone (20 μg/10 μL); (3) local administration of loperamide (150 μg), but not vehicle, into plantar or dorsal hind paw tissue induced thermal analgesia in SNL rats and thermal antinociception in naïve rats; (4) the analgesic effect of intraplantar loperamide (150 μg/15 μL) in SNL rats at 45 min, but not 10 min, post-injection was blocked by pretreatment with an intraplantar injection of naltrexone (75 μg/10 μL); (5) systemic (3.0 mg/kg) and local (150 μg) loperamide reduced the exaggerated duration of hind paw elevation to noxious pinprick stimuli in SNL rats. Intraplantar injection of loperamide also decreased the frequency of pinprick-evoked response in naïve rats. These findings suggest that both systemic and local administration of loperamide induce an opioid receptor-dependent inhibition of heat and mechanical hyperalgesia in nerve-injured rats, but that local paw administration of loperamide also induces thermal and mechanical antinociception.

F166 A NEW METHOD FOR MEASURING CFA-INDUCED MECHANICAL HYPERALGESIA IN RAT

European Journal of Pain SupplementsVolume 5, Issue S1 p. 122-122 F166 A NEW METHOD FOR MEASURING CFA-INDUCED MECHANICAL HYPERALGESIA IN RAT T. Schwagarus, T. Schwagarus In Vivo Neuropharmacology, Evotec AG, Hamburg, GermanySearch for more papers by this authorN. Hoeschen, N. Hoeschen In Vivo Neuropharmacology, Evotec AG, Hamburg, GermanySearch for more papers by this authorF. Machet, F. Machet In Vivo Neuropharmacology, Evotec AG, Hamburg, GermanySearch for more papers by this authorS. Harich, S. Harich In Vivo Neuropharmacology, Evotec AG, Hamburg, GermanySearch for more papers by this authorS. Boyce, S. Boyce In Vivo Neuropharmacology, Evotec AG, Hamburg, GermanySearch for more papers by this author T. Schwagarus, T. Schwagarus In Vivo Neuropharmacology, Evotec AG, Hamburg, GermanySearch for more papers by this authorN. Hoeschen, N. Hoeschen In Vivo Neuropharmacology, Evotec AG, Hamburg, GermanySearch for more papers by this authorF. Machet, F. Machet In Vivo Neuropharmacology, Evotec AG, Hamburg, GermanySearch for more papers by this authorS. Harich, S. Harich In Vivo Neuropharmacology, Evotec AG, Hamburg, GermanySearch for more papers by this authorS. Boyce, S. Boyce In Vivo Neuropharmacology, Evotec AG, Hamburg, GermanySearch for more papers by this author First published: 30 January 2012 https://doi.org/10.1016/S1754-3207(11)70415-1Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume5, IssueS1September 2011Pages 122-122 RelatedInformation

Repeated Cisplatin Treatments inhibit Gastrointestinal Motility and Induces Baroreflex Changes and Mechanical Hyperalgesia in Rats

Cisplatin is a chemotherapy agent known for its neurotoxicity. We evaluated the effect of cisplatin on the gastric emptying (GE), gastrointestinal (GI) transit of liquid, baroreflex function, thermal, and mechanical withdrawal latencies in rats. Cisplatin increased the GE of liquid with doses ≥2 mg.kg−1 by 59.7–77.4%. This GE delay was not present two weeks after the treatment with five doses of cisplatin at 1 mg.kg−1. Cisplatin also enhanced baroreflex gain possibly by increasing sympathetic activity. Our results demonstrated that cisplatin (2–10 mg.kg−1) causes autonomic neuropathy with GI and baroreflex changes and mechanical but not thermal hyperalgesia in rats.

Ameliorative effect of caffeic acid against inflammatory pain in rodents

The present study was designed to evaluate the antinociceptive profile of caffeic acid in mice and rats. Caffeic acid (5–100 mg/kg, p.o.), in a dose dependent manner inhibited acetic acid-induced writhing and late phase of formalin-induced pain in mice, with an ED 50 of 22.38 and 10.92 mg/kg, respectively. However, caffeic acid was ineffective in the hot plate and tail flick tests. Analgesic activity was also examined in carrageenan and lipopolysaccharide (LPS)-induced mechanical hyperalgesia in rats, where locally induced myeloperoxidase (MPO), malondialdehyde (MDA) and nitrite levels in foot pad were estimated by colorimetric assay. Oral administration of caffeic acid (200 mg/kg, p.o.) showed analgesic activity similar to nimesulide (4 mg/kg, p.o.) and inhibited MPO, MDA and nitrite generation in the inflamed paw. Histological examination revealed reduction in neutrophil infiltration and protection of tissue damage by caffeic acid. These results suggest that caffeic acid exhibits peripheral analgesic effect in mice and rats and could be further examined for the treatment of chronic painful episodes.

Mu and delta opioid receptors on nociceptors attenuate mechanical hyperalgesia in rat

Sensitization to mechanical stimuli is important in most pain syndromes. We evaluated the populations of nociceptors mediating mechanical hyperalgesia and those mediating mu-opioid receptor (MOR) and delta-opioid receptor (DOR) agonist-induced inhibition of hyperalgesia, in the rat. We found that: (1) intradermal injection of both the endogenous ligand for the Ret receptor, glia-derived growth factor (GDNF), and the ligand for the tropomyosin receptor kinase A (TrkA) receptor, nerve growth factor (NGF)—which are present on distinct populations of nociceptors—both produce mechanical hyperalgesia; (2) DOR agonist 4-[(R)-[(2S,5R)-4-allyl-2,5-dimethylpiperazin-1-yl](3-methoxyphenyl)methyl]-N,N-diethylbenzamide (SNC) but not MOR agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) inhibit GDNF-induced hyperalgesia; (3) both DAMGO and SNC inhibit NGF hyperalgesia, even in rats pretreated with isolectin B4 (IB4)-saporin, a toxin that destroys IB4-binding neurons; (4) co-administration of low doses of DAMGO and SNC produce enhanced analgesia, and; (5) repeated administration of DAMGO produces cross-tolerance to the analgesic effect of SNC. These findings demonstrate that, most nociceptors have a role in mechanical hyperalgesia, only the DOR agonist inhibits GDNF hyperalgesia, and MOR and DOR are co-localized on a functionally important population of TrkA-positive nociceptors.

Above-level mechanical hyperalgesia in rats develops after incomplete spinal cord injury but not after cord transection, and is reversed by amitriptyline, morphine and gabapentin

Spinal cord injury (SCI) is a major cause of persistent neuropathic pain of central origin. Recent evidence suggests neuropathic pain in clinically complete SCI patients correlates with limited sensory function below the lesion (sensory discomplete). On this basis we examined if the onset of mechanical hyperalgesia was different in rodents after a severe incomplete clip-compression SCI versus a complete spinal cord transection at thoracic segment T13. Above-level withdrawal behaviors evoked by forepaw stimulation provided evidence of mechanical hyperalgesia after incomplete but not complete SCI, whereas below-level responses evoked by hindpaw stimulation revealed hypersensitivity after both injuries. The latency of the above-level response was 4–5 wks but was longer after a moderate clip-compression injury. Mechanical hyperalgesia was fully reversed by three analgesic drugs used in treating neuropathic SCI pain, but their duration of action differed significantly, showing a rank order of amitriptyline (24–48 h) ≫ morphine (6 h) > gabapentin (2 h). Evidence of central sensitization in cervical spinal cord segments that receive sensory projections from the forelimbs was provided by immunohistochemistry for Zif268, a functional marker of neuroplasticity. Zif268-immunoreactive neurons in laminae I/II increased in response to repetitive noxious forepaw stimulation in the incomplete SCI group, and this response was reduced in the complete transection and sham-operated groups. These data are consistent with the hypothesis that neuropathic pain of cord origin is more likely to develop after SCI when there is an incomplete loss of axons traversing the lesion.

Peripheral μ-, κ- and δ-opioid receptors mediate the hypoalgesic effect of celecoxib in a rat model of thermal hyperalgesia

Aims The endogenous opioids mediate the analgesic effects of celecoxib in a model of mechanical hyperalgesia in rats. As responses to thermal stimuli may differ from those to mechanical stimuli, we have here assessed celecoxib in a rat model of thermal hyperalgesia and the possible involvement of endogenous opioids and their corresponding receptors in these effects. Main methods Injection of carrageenan (CG) into one hind paw induced a dose-related hyperalgesia (decreased time for paw withdrawal) to thermal stimuli (infra-red light beam), over 6 h. Key findings Celecoxib (sc) 30 min before CG (250 μg per paw) induced a dose-dependent reversal of hyperalgesia, with withdrawal times well above basal levels, characterizing development of hypoalgesia. Indomethacin (sc) reversed CG-induced hyperalgesia only to basal levels (an anti-hyperalgesic effect). Naltrexone (sc) prevented hypoalgesia after celecoxib but did not change the response to indomethacin. Local (intraplantar) injection of either a selective antagonist of μ-(beta-funaltrexamine), κ-(nor-binaltorphimine) or of δ-(naltrindole) opioid receptors also reversed the hypoalgesic effects of celecoxib, without modifying the hyperalgesia due to CG or affecting the nociceptive thresholds in the non-injected paw. Significance Our data show that celecoxib, unlike indomethacin, was hypoalgesic in this model of thermal hyperalgesia, and that this effect was mediated by peripheral μ-, κ- and δ-opioid receptors.

Validation of the digital pressure application measurement (PAM) device for detection of primary mechanical hyperalgesia in rat and mouse antigen-induced knee joint arthritis

Several tests have been developed to obtain mechanical nociceptive withdrawal thresholds for arthritis-associated pain research in preclinical animal models, which are routinely used for testing the efficacy of antinociceptive pharmaceutical candidates. Here, we aimed to validate a recently introduced and commercially available digital pressure application measurement (PAM) device for the detection of primary mechanical hyperalgesia in a model of antigen-induced knee joint arthritis (AIA) in rats and mice. Two particular advantages of the PAM device are visual feedback control of the force increase rate and the detection of the complete threshold range. Using PAM, we were able to quantify mechanical thresholds at the knee joint in rats and mice (400 and 350 g, respectively) before and during the time course of AIA (approximately 100 g for rats and mice in the acute phase). Inter-observer agreement was generally higher when using PAM instead of an analog dynamometer. In conclusion, the digital PAM device is a suitable apparatus to detect primary mechanical hyperalgesia in experimental knee joint arthritis in rats and mice. The use of this device allows visual feedback control of the stimulus rate, thus minimizing the chances of confounding factors arising from differences in ramp speed.

Sound Stress–Induced Long-Term Enhancement of Mechanical Hyperalgesia in Rats Is Maintained by Sympathoadrenal Catecholamines

Although stress plays an important role in chronic widespread pain syndromes, such as fibromyalgia, the underlying mechanism has remained elusive. We have recently demonstrated, in a model of chronic widespread pain, that prolonged enhancement of immune mediator hyperalgesia, induced by unpredictable sound stress, requires a contribution of both the sympathoadrenal (epinephrine) and the hypothalamic-pituitary adrenal (corticosterone) neuroendocrine stress axes. Because this stress protocol produced sustained elevation of plasma epinephrine, in the current study we tested the hypothesis that the sympathoadrenal axis also plays a role in maintenance of symptoms in this model of chronic widespread pain. After establishment, adrenal medullectomy abolished the enhancement of epinephrine-induced cutaneous and muscle hyperalgesia. Administration of stress levels of epinephrine to adrenal medullectomized rats reconstituted the pain phenotype. These observations suggest that the sympathoadrenal stress axis plays a major role in the induction as well as maintenance of stress-induced enhancement of mechanical hyperalgesia, mediated by prolonged elevation of circulating epinephrine. We present data showing mechanical hyperalgesia persisting for up to 28 days after exposure to sound stress, with evidence that the sympathoadrenal axis mediator epinephrine plays a major role. These findings could have clinical implications with regard to novel potential treatments for chronic widespread pain syndromes, such as fibromyalgia.

Intrathecal Administration of Proteinase-Activated Receptor-2 Agonists Produces Hyperalgesia by Exciting the Cell Bodies of Primary Sensory Neurons

Proteinase-activated receptors (PARs) are a family of G-protein-coupled receptors that are activated by endogenous serine proteinases that cleave the N-terminal domain of the receptor unmasking a "tethered ligand" sequence. Trypsin and other agonists at PAR(2) act on peripheral nerves to augment the transfer of nociceptive information. We tested whether PAR(2) agonists also exert a spinal pronociceptive effect by i.t. administering the selective ligand, Ser-Leu-Ile-Gly-Arg-Leu-NH(2) (SLI-GRL). This produced thermal and mechanical hyperalgesia in rats and mice and augmented mechanical and thermal hyperalgesia seen in the formalin inflammatory pain test. Effects of SLIGRL were abrogated in PAR(2)-deficient mice and were not seen with the inactive control peptide, Leu-Arg-Gly-Ile-Leu-Ser-NH(2). Surprisingly, electrophysiological studies, using whole-cell recording from rat substantia gelatinosa neurons, failed to demonstrate an increase in excitatory transmission or neuronal excitability following treatment with SLIGRL or trypsin. In fact, the actions of trypsin were consistent with a decrease in dorsal horn excitability. SLIGRL and trypsin did, however, depolarize and increase the excitability of large, medium and small primary afferent, dorsal root ganglion neurons. The effects were associated with an increase in conductance at hyperpolarized potentials and a decrease in conductance at depolarized potentials. PAR(2)-like immunoreactivity was found in DRG but not in spinal dorsal horn. These results suggest that activation of DRG neuron cell bodies may account for the pronociceptive actions of i.t. applied PAR(2) agonists. They also imply that pathophysiological release of PAR(2)-activating proteases in the vicinity of DRG neurons may produce profound effects on nociceptive processing in vivo.