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Aims: This study investigated the relationship between the analgesic efficacy of acetaminophen and the descending noradrenergic systems using rodent models of inflammatory pain., Main Methods: Inflammatory pain models were established by carrageenan injection into rats' paws. The models were defined as acute (4 h after carrageenan injection), subacute (24 h after carrageenan injection), and late (1 week after carrageenan injection) phase. To evaluate intravenous acetaminophen treatment, the withdrawal threshold to mechanical stimuli was assessed simultaneously with in vivo microdialysis assay of noradrenaline levels in the locus coeruleus (LC). Further analyses were performed to observe the effect of yohimbine on the treatment and the impact of AM404 treatment, a metabolite of acetaminophen, on noradrenaline levels in the LC., Key Findings: In all phases, intravenous acetaminophen had a significant anti-hyperalgesic effect (p < 0.05). There was a significant time-dependent increase in the noradrenaline concentration within the LC (acetaminophen versus saline treatment; at 30 min, p < 0.001; 60 min, p < 0.01) in the subacute pain model, but not in the acute and late phase pain models. Intrathecal pre-injection of yohimbine attenuated the anti-hyperalgesic effect after acetaminophen injection only in the subacute model (p < 0.05). In the subacute pain model, intracerebroventricular administration of AM404 showed the same trend in noradrenaline levels as acetaminophen administration (AM404 versus vehicle group at 30 min, p < 0.001)., Significance: We found the descending noradrenergic inhibitory system is involved in the antinociceptive action of acetaminophen in the subacute phase of inflammatory pain., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Oxylipins are lipid peroxidation products that participate in nociceptive, inflammatory, and vascular responses to injury. Effects of oxylipins depend on tissue-specific differences in accumulation of precursor polyunsaturated fatty acids and the expression of specific enzymes to transform the precursors. The study of oxylipins in nociception has presented technical challenges leading to critical knowledge gaps in the way these molecules operate in nociception. We applied a systems-based approach to characterize oxylipin precursor fatty acids, and expression of genes coding for proteins involved in biosynthesis, transport, signaling and inactivation of pro- and antinociceptive oxylipins in pain circuit tissues. We further linked these pathways to nociception by demonstrating intraplantar carrageenan injection induced gene expression changes in oxylipin biosynthetic pathways. We determined functional-biochemical relevance of the proposed pathways in rat hind paw and dorsal spinal cord by measuring basal and stimulated levels of oxylipins throughout the time-course of carrageenan-induced inflammation. Finally, when oxylipins were administered by intradermal injection we observed modulation of nociceptive thermal hypersensitivity, providing a functional-behavioral link between oxylipins, their molecular biosynthetic pathways, and involvement in pain and nociception. Together, these findings advance our understanding of molecular lipidomic systems linking oxylipins and their precursors to nociceptive and inflammatory signaling pathways in rats. PERSPECTIVE: We applied a systems approach to characterize molecular pathways linking precursor lipids and oxylipins to nociceptive signaling. This systematic, quantitative evaluation of the molecular pathways linking oxylipins to nociception provides a framework for future basic and clinical research investigating the role of oxylipins in pain., (Published by Elsevier Inc.)

Background: Peripheral inflammation leads to the development of persistent thermal hyperalgesia and mechanical allodynia associated with increased expression of interleukin-1β (IL-1β) in the spinal cord. The aim of the present study was to investigate the effects of thiazolidine derivatives, 1b ([2-(2-hydroxyphenyl)-1,3-thiazolidin-4-yl](morpholin-4-yl)methanone) and 1d (2-hydroxy-4-{[2-(2-hydroxyphenyl)-1,3-thiazolidine-4-carbonyl]amino}benzoic acid), on thermal hyperalgesia, mechanical allodynia and on IL-1β expression during carrageenan-induced inflammation in the spinal cord in mice. Inflammatory pain was induced by injecting 1% carrageenan into the right hind paw of the mice., Methods: The animals were administered thiazolidine derivatives, 1b and 1d (1 mg/kg, 3 mg/kg, or 10 mg/kg), intraperitoneally 30 minutes before carrageenan administration. The animals' behavior was evaluated by measuring thermal hyperalgesia, mechanical allodynia, and motor coordination. The IL-1β expression was measured by enzyme-linked immunosorbent assay. Acute and sub-acute toxicity studies were conducted to evaluate the toxicity profile of compounds., Results: Treatment with the thiazolidine derivative, 1b and 1d, attenuated carrageenan-induced thermal hyperalgesia and mechanical allodynia at doses of 1 mg/kg, 3 mg/kg, and 10 mg/kg. No motor coordination deficits were observed in animals. The compounds also reduced IL-1β expression in the spinal cord of mice. Acute and sub-acute toxicity studies revealed that both compounds were safe., Conclusion: The compounds exhibit promising activity against inflammatory pain due to their ability to produce anti-hyperalgesic and anti-allodynic effects and to inhibit IL-1β expression in the spinal cord., Competing Interests: The authors declare no conflicts of interest for this work., (© 2021 Malik et al.)

Introduction: To better understand the pathophysiology of chronic muscle pain, there are multiple animal models that mimic different acute/chronic pain conditions, such as carrageenan injection. Our previous studies demonstrated differences between muscles of different innervation in acute pain. In this study we characterized the effect of carrageenan in 2 muscles: masseter (trigeminal innervation) and gastrocnemius (spinal innervation)., Methods: Carrageenan (3%, 6%, and 9%) was injected into the masseter and gastrocnemius of rats. Mechanical, heat, and chemical nociceptive thresholds were measured for 14 days., Results: Carrageenan did not induce mechanical allodynia or thermal hypersensitivity in either muscle. Instead, it induced a short-lasting mechanical hyperalgesia, greater in the masseter than in the gastrocnemius., Conclusion: Carrageenan injected into the masseter and gastrocnemius induces a short-lasting hyperalgesia. These results could indicate a higher susceptibility of orofacial muscles to this type of insult and, consequently, a difference between trigeminal and spinal innervation. Muscle Nerve 56: 804-813, 2017., (© 2016 Wiley Periodicals, Inc.)

Although interleukin-1β (IL-1β) is a prototypical pro-inflammatory cytokine, the specific mechanisms underlying the role of its cognate receptor, the interleukin-1 receptor (IL-1R) in peripheral sensitization remain to be investigated. Since emerging evidence in the literature indicates that IL-1β can modulate membrane-bound receptors, we decided to examine the involvement of P2Y1 receptor (P2Y1R) in IL-1β induced pain and the potential interaction of P2Y1Rs and IL-1Rs in both naïve and carrageenan injected rats. Intraplantar (i.pl) injection of IL-1β dose-dependently produced mechanical and thermal hypersensitivity in naïve rats. Pre-treatment with IL-1ra (i.pl, 30 and 100ng), an endogenous IL-1R antagonist, prevented the IL-1β induced mechanical and thermal hypersensitivity. Pre-treatment with MRS2500 (i.pl, 1 and 3nmol), a specific P2Y1R antagonist, dose-dependently reduced IL-1β induced thermal hypersensitivity, but did not affect the development of mechanical hypersensitivity. Conversely coadministration of MRS2500 (i.pl, 0.1nmol, sub-effective dose) together with IL-1ra (10nmol, sub-effective dose) significantly reduced IL-1β induced thermal, but not mechanical hypersensitivity. We next used immunohistochemistry to demonstrate that P2Y1 and IL-1 type I receptors co-localize predominantly in small diameter neurons in the dorsal root ganglion. We also performed experiments to examine the interaction of P2Y1Rs and IL-1Rs under the inflammatory conditions induced by 2% carrageenan. Intraplantar coadministration of MRS2500 (3nmol, sub-effective dose) and IL-1ra (30ng, sub-effective dose) significantly reduced inflammatory thermal, but not mechanical, hypersensitivity. These data indicate the involvement of P2Y1Rs in IL-1β mediated pain in both naive and carrageenan injected rats. There is a positive interaction between peripheral P2Y1Rs and IL-1Rs in both IL-1β and carrageenan-induced thermal hypersensitivity., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Various studies have demonstrated the role of the nitric oxide (NO)/cGMP pathway in pain processing. Our group has also shown that this system participates in opioid-induced antinociception during peripheral inflammation. We have previously observed that inflammation mobilizes an endogenous opioidergic system to control hyperalgesia. Here, we investigated whether the NO/cGMP pathway underlies peripheral endogenous nociception control during inflammation. In this study, a pharmacological approach was used in conjunction with the rat paw pressure test to assess the effects of intraplantar NO synthase inhibitor NG-Nitro-l-arginine (NOArg), guanylyl cyclase inhibitor methylene blue (MB), phosphodiesterase-5 inhibitor zaprinast (ZP), or NO precursor l-arginine injection on carrageenan-induced hyperalgesia, which mimics an inflammatory process, or by prostaglandin E(2) (PGE(2)), which directly sensitizes nociceptors. Intraplantar carrageenan (62.5, 125, 250 or 500μg) or PGE(2) (0.1, 0.5 or 2μg) administration produced hyperalgesia, which manifested as a reduction in the rat nociceptive threshold to mechanical stimuli. NOArg (25, 50 or 100μg/paw) and MB (125, 250 or 500μg/paw) induced significant and dose-dependent reductions in the nociceptive threshold of carrageenan-induced (125μg/paw) hyperalgesia, but not PGE(2)-induced (0.5μg/paw) hyperalgesia. This was a local effect because it did not produce any modifications in the contralateral paw. Both Zaprinast (100, 200 or 400μg/paw) and l-arginine (100, 200 or 400μg/paw) significantly counteracted carrageenan-induced hyperalgesia (250μg/paw), yielding an increase in the nociceptive threshold compared with the control. Zaprinast (200μg/paw) or l-arginine (400μg/paw) did not produce an antinociceptive effect in the contralateral paw, indicating local action. In addition, at the same dose that was able to modify carrageenan-induced hyperalgesia, neither zaprinast nor l-arginine modified PGE(2) (2μg) injection-induced hyperalgesia of the rat paw. Taken together, these results indicate that the l-arginine/NO/cGMP pathway functions as an endogenous modulator of peripheral inflammatory hyperalgesia., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Novel classes of pain-relieving molecules are needed to fill the void between non-steroidal anti-inflammatory agents and narcotics. We have recently shown that intraplantar administration of sphingosine 1-phosphate (S1P) in rats causes peripheral sensitization and hyperalgesia through the S1P(1) receptor subtype (S1PR(1)): the mechanism(s) involved are largely unknown and were thus explored in the present study. Intraplantar injection of carrageenan in rats led to a time-dependent development of thermal hyperalgesia that was associated with pronounced edema and infiltration of neutrophils in paw tissues. Inhibition of 1) S1P formation with SK-I, a sphingosine kinase inhibitor, 2) S1P bioavailability with the S1P blocking antibody Sphingomab, LT1002 (but not its negative control, LT1017) or 3) S1P actions through S1PR(1) with the selective S1PR(1) antagonist, W146 (but not its inactive enantiomer, W140) blocked thermal hyperalgesia and infiltration of neutrophils. Taken together, these findings identify S1P as an important contributor to inflammatory pain acting through S1PR(1) to elicit hyperalgesia in a neutrophil-dependant manner. In addition and in further support, we demonstrate that the development of thermal hyperalgesia following intraplantar injection of S1P or SEW2871 (an S1PR(1) agonist) was also associated with neutrophilic infiltration in paw tissues as these events were attenuated by fucoidan, an inhibitor of neutrophilic infiltration. Importantly, FTY720, an FDA-approved S1P receptor modulator known to block S1P-S1PR(1) signaling, attenuated carrageenan-induced thermal hyperalgesia and associated neutrophil infiltration. Targeting the S1P/S1PR(1) axis opens a therapeutic strategy for the development of novel non-narcotic anti-hyperalgesic agents.

The neuropeptide Substance P (SP), that has a high affinity for the neurokinin 1 (NK1) receptor, is involved in modulation of pain transmission. Although SP is thought to have excitatory actions and promote nociception in the spinal cord, the peptide induces analgesia at the supraspinal level. The aim of this study was to evaluate the role of supraspinal SP and the NK1 receptor in inflammatory pain induced by injection of carrageenan in the hind paw of the rat. There are two nociceptive behavioral responses associated with this pain state: mechanical allodynia and heat hyperalgesia. Because the NK1 receptor colocalizes with the MOP receptor in supraspinal sites involved in pain modulation, we also decided to study the possible involvement of the opioid system on SP-induced analgesia. We found that treatment with SP, at doses of 3.5, 5 and 7 μg/5 μl/rat i.c.v., clearly showed inhibition of allodynia and hyperalgesia. Pretreatment with the selective NK1 antagonist L-733,060 (10mg/kg i.p.) blocked the SP-induced analgesia, suggesting the involvement of the NK1 receptor. This SP-induced analgesia was significantly reduced by administration of the opioid antagonist naloxone (3mg/kg s.c.). This reduction occurred when SP was administered either before or after the carrageenan injection. These results suggest a significant antinociceptive role for SP and the NK1 receptor in inflammatory pain at the supraspinal level, possibly through the release of endogenous opioids., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Pharmacological studies indicate that spinal p38 mitogen-activated protein kinase plays a role in the development of hyperalgesia. We investigated whether either the spinal isoform p38alpha or p38beta is involved in peripheral inflammation evoked pain state and increased expression of spinal COX-2. Using intrathecal antisense oligonucleotides, we show that hyperalgesia is prevented by downregulation of p38beta but not p38alpha, whereas increases in spinal COX-2 protein expression at 8 hours are mediated by both p38alpha and beta isoforms. These data suggest that early activation of spinal p38beta isoform may affect acute facilitatory processing, and both p38beta and alpha isoforms mediate temporally delayed upregulation of spinal COX-2.

The ability of an individual to sense pain is fundamental for its capacity to adapt to its environment and to avoid damage. The sensation of pain can be enhanced by acute or chronic inflammation. In the present study, we have investigated whether inflammatory pain, as measured by hypernociceptive responses, was modified in the absence of the microbiota. To this end, we evaluated mechanical nociceptive responses induced by a range of inflammatory stimuli in germ-free and conventional mice. Our experiments show that inflammatory hypernociception induced by carrageenan, lipopolysaccharide, TNF-alpha, IL-1beta, and the chemokine CXCL1 was reduced in germ-free mice. In contrast, hypernociception induced by prostaglandins and dopamine was similar in germ-free or conventional mice. Reduction of hypernociception induced by carrageenan was associated with reduced tissue inflammation and could be reversed by reposition of the microbiota or systemic administration of lipopolysaccharide. Significantly, decreased hypernociception in germ-free mice was accompanied by enhanced IL-10 expression upon stimulation and could be reversed by treatment with an anti-IL-10 antibody. Therefore, these results show that contact with commensal microbiota is necessary for mice to develop inflammatory hypernociception. These findings implicate an important role of the interaction between the commensal microbiota and the host in favoring adaptation to environmental stresses, including those that cause pain.

Kinin receptors are involved in the genesis of inflammatory pain. However, there is controversy concerning the mechanism by which B(1) and B(2) kinin receptors mediate inflammatory hypernociception. In the present study, the role of these receptors on inflammatory hypernociception in mice was addressed. Mechanical hypernociception was detected with an electronic pressure meter paw test in mice and cytokines were measured by ELISA. It was observed that in naïve mice a B(2) (d-Arg-Hyp(3), d-Phe(7)-bradykinin) but not a B(1) kinin receptor antagonist (des-Arg(9)-[Leu(8)]-bradykinin, DALBK) inhibited bradykinin- and carrageenin-induced hypernociception. Bradykinin-induced hypernociception was inhibited by indomethacin (5 mg/kg) and guanethidine (30 mg/kg), while not affected by IL-1ra (10 mg/kg) or antibody against keratinocyte-derived chemokine (KC/CXCL-1, 500 ng/paw) or in TNFR1 knockout mice. By contrast, in previously lipopolysaccharide (LPS)-primed mouse paw, B(1) but not B(2) kinin receptor antagonist inhibited bradykinin hypernociception. Furthermore, B(1) kinin receptor agonist induced mechanical hypernociception in LPS-primed mice, which was inhibited by indomethacin, guanethidine, antiserum against TNF-alpha or IL-1ra. This was corroborated by the induction of TNF-alpha and IL-1beta release by B(1) kinin receptor agonist in LPS-primed mouse paws. Moreover, B(1) but not B(2) kinin receptor antagonist inhibited carrageenin-induced hypernociception, and TNF-alpha and IL-1beta release as well, in LPS-primed mice. These results suggest that in naïve mice the B(2) kinin receptor mediates inflammatory hypernociception dependent on prostanoids and sympathetic amines, through a cytokine-independent mechanism. On the other hand, in LPS-primed mice, the B(1) kinin receptor mediates hypernociception by a mechanism dependent on TNF-alpha and IL-1beta, which could stimulate prostanoid and sympathetic amine production.

Background and Purpose: Studies in vitro suggest that the standardised extract of Ginkgo biloba, EGb-761 has anti-inflammatory properties and modulatory effects on key pain-related molecules. This study investigated the analgesic and anti-inflammatory effects of EGb-761 on carrageenan-induced inflammatory and hindpaw incisional pain., Experimental Approach: Adult male Wistar rats (n=6-10/group; 250-420 g) were injected intradermally with carrageenan into the left hindpaw or anaesthetised with isoflurane (2%) and a longitudinal 1 cm incision was made through the skin, fascia and plantaris muscle of the hindpaw. EGb-761 (3, 10, 30, 100 or 300 mg kg(-1)), diclofenac (5 mg kg(-1)) or drug-vehicle was administered 3 h post-carrageenan/post-surgery. Hindpaw withdrawal latency (in seconds) to thermal stimulation, response threshold (in grams) to mechanical stimulation and paw volume were measured., Key Results: Carrageenan induced significant mechanical allodynia, thermal hyperalgesia and paw oedema at 6 h post-carrageenan, while paw incision surgery induced significant mechanical allodynia and thermal hyperalgesia at 6 and 24 h post-surgery. Administration of EGb-761 dose-dependently inhibited thermal hyperalgesia and was equally effective as diclofenac (5 mg kg(-1)) in both the carrageenan and hindpaw incision model. EGb-761 had no effect on carrageenan- or incision-induced mechanical allodynia or paw oedema. Diclofenac significantly reduced mechanical allodynia in both models and carrageenan-induced paw oedema., Conclusions and Implications: EGb-761 dose-dependently alleviates acute inflammatory and surgically induced thermal hyperalgesia and is comparable to diclofenac, a commonly prescribed non-steroidal anti-inflammatory drug. This indicates that EGb-761 has analgesic potential in acute inflammatory pain.

We investigated the anti-inflammatory effects of electroacupuncture (EA) on carrageenan-induced inflammatory model in association with peripheral and spinal COX-2 expression. EA with 2, 15 and 120 Hz, especially 2 Hz, had significant inhibitory effects on the developing of edema and hyperalgesia, which was measured in 30-min intervals after carrageenan injection. Therefore, we investigated whether the effect of 2 Hz EA on carrageenan-induced edema and hyperalgesia is associated with peripheral and spinal expression of inflammatory proteins. The expression of cyclooxygenase (COX)-1, COX-2, and inducible nitric oxide synthase (iNOS) was inhibited by 2 Hz EA in carrageenan-injected rat paws. Interestingly, we found that the mRNA of COX-1 and COX-2 expression in the spine was not induced by 2 Hz EA treatment after carrageenan-induced peripheral inflammation. In addition, synthesis of prostaglandin E(2) (PGE(2)) was partially inhibited by 2 Hz EA treatment in both peripheral and spinal nociceptive regions. In conclusion, EA treatment might be a useful therapy for mitigation of inflammatory edema and hyperalgesia through regulation of COX-2 expression in both peripheral and central nociceptive sites.

In male rats, carrageenan (CAR)-induced inflammation or exposure to a selective protein kinase C epsilon (PKC epsilon ) agonist (psi epsilon RACK) produces prolongation of the hyperalgesia induced by a subsequent exposure to an inflammatory mediator, a phenomenon referred to as hyperalgesic priming. Since many chronic inflammatory conditions are sexually dimorphic, we tested the hypothesis that hyperalgesic priming is sexually dimorphic. Prior injection of CAR or psi epsilon RACK produced a prolongation of the hyperalgesia induced by a subsequent injection of prostaglandin E(2), from less than 3 h to greater than 24 h, but only in male rats. In ovariectomized female rats priming with CAR and psi epsilon RACK produced hyperalgesic priming effects similar to that observed in the male rat, and this effect was reversed by estrogen replacement. While gonadectomy in males had no effect on CAR and psi epsilon RACK induced hyperalgesic priming, female phenotype was observed following implantation of estrogen in males. Thus, mechanisms mediating the development of hyperalgesic priming produced by inflammation are suppressed by estrogen. This regulation of priming by estrogen appears to occur at or downstream of the activation of PKC epsilon.

The purpose of the present study was to investigate the role of the alpha(2A)-adrenoceptor subtype in inflammatory hyperalgesia, and in adrenergic-mu-opioid interactions in acute pain and inflammatory hyperalgesia. Behavioral responses to mechanical and thermal stimuli were studied in alpha(2A)-adrenoceptor knockout mice and their wild-type controls. Thermal nociception was evaluated as paw withdrawal latencies to radiant heat applied to the hindpaws. Mechanical nociception was measured using von Frey monofilament applications to the hindpaws. Mechanical and thermal hyperalgesia, induced with intraplantar carrageenan (1 mg/40 microl) were compared in alpha(2A)-adrenoceptor knockout and wild-type mice. The effects of the systemically administered mu-opioid receptor agonist morphine (1-10 mg/kg) were evaluated on mechanical withdrawal responses under normal and inflammatory conditions in knockout and wild-type mice. Withdrawal responses to radiant heat and von Frey monofilaments were similar in alpha(2A)-adrenoceptor knockout and wild-type mice before and after the carrageenan-induced hindpaw inflammation. Also, the antinociceptive effects of morphine in mechanical nociceptive tests were similar before and after carrageenan-induced hindpaw inflammation. Our observations indicate that alpha(2A)-adrenoceptors are not tonically involved in the modulation of inflammation-induced mechanical and thermal hyperalgesia. In addition, alpha(2A)-adrenoceptors do not appear to play an important role in mu-opioid receptor-mediated antinociception or antihyperalgesia.

Phosphorylation of a subunit of N-methyl-D-aspartate (NMDA) receptor by protein tyrosine kinase (PTK) Src or Trk is known to enhance its channel activity. We examined whether a spinally administered selective PTK inhibitor, lavendustin A, which has high affinity for Src and Trk tyrosine kinases, could influence the development and maintenance of inflammatory hyperalgesia or NMDA-induced hyperalgesia. Inflammation was induced by injection of a mixture of carrageenan and kaolin into the tail base of rats. In another group of rats, hyperalgesia was induced by intrathecal administration of NMDA. Intrathecal administration of lavendustin A (1.0 microg) or NMDA receptor antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cycloheptane-5,10-iminemaleate, MK-801 (3.0 microg) before injection of a mixture of carrageenan and kaolin or after the development of inflammation inhibited carrageenan-kaolin-induced mechanical hyperalgesia. Intrathecal injection of 1.0 microg NMDA produced thermal and mechanical hyperalgesia. Co-administration of 1.0 microg lavendustin A with NMDA significantly reduced the duration of spontaneous pain behaviour and inhibited NMDA-induced hyperalgesia. Lavendustin A itself did not cause any sedation, motor impairment or analgesia. Our results suggest that inhibition of PTK could be therapeutically effective as an analgesic in some NMDA receptor-mediated hyperalgesic states.

The present experiment examined the effect of aspirin on the escape/avoidance behavioral response to a mechanical stimulus (476 mN von Frey monofilament) in the place escape avoidance paradigm (PEAP) following subcutaneous administration of carrageenan (CARR). Forty-one male Sprague-Dawley rats received subcutaneous injection of CARR or saline in the left hindpaw and 3 1/2 h later were administered aspirin (0, 50 or 150 mg/kg). Thirty minutes later, animals were tested in the PEAP and then the mechanical paw withdrawal threshold was measured. Compared with Saline vehicle-treated controls, all CARR-treated animals displayed hyperalgesia, as reflected by enhanced responding to mechanical stimulation applied to the CARR-injected paw. Mechanical hyperalgesia was significantly reduced by the pre-treatment of 150 mg/kg, but not 50 mg/kg aspirin. In the PEAP, CARR vehicle-treated animals avoided a preferred location of the test chamber that was associated with mechanical stimulation of the hyperalgesic paw. The shift from a preferred dark side of the chamber to the light side was attenuated by pre-treatment with both doses of aspirin (50 and 150 mg/kg). The lack of anti-hyperalgesia and avoidance behavior with 50 mg/kg aspirin suggests a decrease in the aversive nature of mechanical stimulation of the afflicted paw. It is suggested that the mechanisms underlying the affective/motivational dimension of nociception (escape/avoidance) can be dissociated from the processing of nociceptive information related to withdrawal responding.

We investigated if carrageenan-induced thermal hyperalgesia and mechanical allodynia are associated with a reduction in threshold and-or enhancement of suprathreshold nocifensive responses, using a method to measure the force of a hind limb wilhdrawal reflex elicited by graded noxious heat stimuli (36-52 degrees C, 5s) delivered by Peltier thermode tethered to the ventral hind paw of conscious rats. Withdrawal reflexes were recorded 2.5 h after intraplantar injection of carrageenan (1 or 0.1%) or saline vehicle in sessions >2 weeks apart: baseline reflexes were assessed the day before. Withdrawal reflex force increased linearly from 42-52 degrees C. Carrageenan 1% significantly enhanced withdrawal reflexes at 40-46 degrees C, reducing the slope and threshold of the stimulus-response function. This was associated with significant reductions in thermal paw withdrawal latency (Hargreaves test: by 50%), mechanical withdrawal threshold (by 82%) and weight bearing on the injected side (by 81%) measured with independent force plates. Smaller reductions in thermal paw withdrawal latency and mechanical withdrawal threshold, and smaller enhancement of withdrawal reflex force, were observed following 0.1% carrageenan. Intraplantar saline was ineffective. This method allows assessment of hyperalgesic changes in stimulus-response coding over a broad range of noxious stimulus intensities.

1. The effect of interleukin-10 (IL-10) upon the hyperalgesic activities in rats of bradykinin, tumor necrosis factor alpha (TNF alpha), interleukin-1 beta (IL-1 beta), interleukin-6 (IL-6), interleukin-8 (IL-8), prostaglandin E2 (PGE2) and carrageenin were investigated in a model of mechanical hyperalgesia. 2. Hyperalgesic responses to bradykinin (1 micrograms) were inhibited in a dose-dependent manner by prior treatment with IL-10 (1-100 ng). 3. Hyperalgesic responses to TNF alpha (2.5 pg), IL-1 beta (0.5 pg) and IL-6 (1.0 ng) but not to IL-8 (0.1 ng) and PGE2 (50 ng and 100 ng) were inhibited by prior treatment with IL-10 (10 ng). 4. Hyperalgesic responses to carrageenin (100 micrograms) were inhibited by IL-10 (10 ng) when this cytokine was injected before but not after the carrageenin. 5. A monoclonal antibody to mouse IL-10 potentiated the hyperalgesic responses to carrageenin (10 micrograms) and TNF alpha (0.025 pg) but not that to IL-8 (0.01 ng). 6. In in vitro experiments in human peripheral blood mononuclear cells (MNCs), IL-10 (0.25-4.0 ng ml-1) inhibited in a dose-dependent manner PGE2 production by MNCs stimulated with IL-1 beta (1-64 ng ml-1) or endotoxin (lipopolysaccharide, LPS, 1 iu = 143 pg ml-1) but evoked only small increases in IL-1ra production. 7. These data suggest that IL-10 limits the inflammatory hyperalgesia evoked by carrageenin and bradykinin by two mechanisms: inhibition of cytokine production and inhibition of IL-1 beta evoked PGE2 production. Our data suggest that the latter effect is not mediated via IL-10 induced IL-Ira and may result from suppression by IL-10 of prostaglandin H synthase-2 (COX-2).

Changes in the thresholds to noxious mechanical and thermal stimulation were monitored in a group of normal sheep at different times (30 min-24 h) following intradermal injection of either low-dose (0.0625%) carrageenan, a widely used mucopolysaccharide irritant, or saline (0.9%) into the lateral aspect of the fore limb. Thresholds to noxious mechanical stimulation were measured on the ipsilateral limb whereas thresholds to noxious thermal stimulation were measured at a site distant to that of the injection, the ipsilateral ear. During the first 2 h after carrageenan injection the thresholds to noxious thermal stimulation fell, while thresholds to noxious mechanical stimulation were unaltered. The evoked hyperalgesia was reversed by the administration of non-steroidal anti-inflammatory agents. Hyperalgesia was not present 4 h after carrageenan injection in control or treated animals. This experiment was carried out on 3 further occasions and a diminishing response to injection of carrageenan was observed, suggesting development of tachyphylaxis to evoked thermal hyperalgesia.

Activation of the N-methyl-D-aspartate (NMDA) receptor has been reported to be involved in the mechanisms that underlie thermal hyperalgesia produced by the intraplantar injection of carrageenan. As NMDA-mediated thermal hyperalgesia produced in models of acute and persistent pain have been reported to involve production of nitric oxide, we examined the role of nitric oxide in both the development and maintenance of the thermal hyperalgesia produced by the intraplantar injection of carrageenan. In addition, we examined the role of nitric oxide in the maintenance of the mechanical hyperalgesia produced by intraplantar injection of carrageenan. Prior to the intraplantar injection of carrageenan (2 mg in 100 microliters) there was no significant difference in thermal withdrawal latencies or mechanical withdrawal thresholds between the left and right hindpaws. Three hours after injection of carrageenan into the left hindpaw, rats showed evidence of a significantly faster thermal withdrawal latency and lower mechanical withdrawal threshold of the left hindpaw compared to the right hindpaw. In addition, the left hindpaw was significantly increased in size (diameter) compared with the right hindpaw. In these same rats, the intrathecal administration of saline, NG-nitro-L-arginine methyl ester (L-NAME; 2-200 nmol) or the inactive enantiomer, NG-nitro-D-arginine methyl ester (D-NAME; 200 nmol) did not produce any significant change in thermal nociceptive withdrawal latencies in the non-injected paw. However, administration of L-NAME (2-20 nmol), but not saline or D-NAME produced a dose dependent and reversible block of the thermal hyperalgesia for a period of up to 3 h.(ABSTRACT TRUNCATED AT 250 WORDS)