Intrathecal CGRP(8-37) results in a bilateral increase in hindpaw withdrawal latency in rats with a unilateral thermal injury

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.

Calcitonin gene-related peptide receptor antagonists: beyond migraine pain--a possible analgesic strategy for osteoarthritis?

Osteoarthritis (OA) pain is poorly understood and managed, as current analgesics have only limited efficacy and unwanted side effect profiles. A broader understanding of the pathological mechanisms driving OA joint pain is vital for the development of improved analgesics. Both clinical and preclinical data suggest an association between joint levels of the sensory neuropeptide calcitonin gene-related peptide (CGRP) and pain during OA. Whether a direct causative link exists remains an important unanswered question. Given the recent development of small molecule CGRP receptor antagonists with clinical efficacy against migraine pain, the interrogation of the role of CGRP in OA pain mechanisms is extremely timely. In this article, we provide the background to the importance of CGRP in pain mechanisms and review the emerging clinical and preclinical evidence implicating a role for CGRP in OA pain. We suggest that the CGRP receptor antagonists developed for migraine pain warrant further investigation in OA.

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.

Morphine exposure in early life increases nociceptive behavior in a rat formalin tonic pain model in adult life

Considering the importance of a deeper understanding of the effect throughout life of opioid analgesia at birth, our objective was to determine whether morphine administration in early life, once a day for 7 days in 8-day-old rats, alters the nociceptive response over the short (P16), medium (P30), and long term (P60) and to evaluate which system is involved in the altered nociceptive response. The nociceptive responses were assessed by the formalin test, and the behavior analyzed was the total time spent in biting and flicking of the formalin-injected hindpaw, recorded during the first 5 min (phase I) and from 15-30 min (phase II). The morphine group showed no change in nociceptive response at P16, but at P30 and P60, the nociceptive response was increased in phase I, and in both phases, respectively. At P30 and P60, the animals received a non-steroidal anti-inflammatory drug (indomethacin) or NMDA receptor antagonist (ketamine) 30 min before the formalin test. The increase in the nociceptive response was completely reversed by ketamine, and partially by indomethacin. These results indicate that early morphine exposure causes an increase in the nociceptive response in adult life. It is possible that this lower nociception threshold is due to neuroadaptations in nociceptive circuits, such as the glutamatergic system. Thus, this work demonstrates the importance of evaluating clinical consequences related to early opioid administration and suggests a need for a novel design of agents that may counteract opiate-induced neuroplastic changes.

The role of peptides in central sensitization

Peptides released in the spinal cord from the central terminals of nociceptors contribute to the persistent hyperalgesia that defines the clinical experience of chronic pain. Using substance P (SP) and calcitonin gene-related peptide (CGRP) as examples, this review addresses the multiple mechanisms through which peptidergic neurotransmission contributes to the development and maintenance of chronic pain. Activation of CGRP receptors on terminals of primary afferent neurons facilitates transmitter release and receptors on spinal neurons increases glutamate activation of AMPA receptors. Both effects are mediated by cAMP-dependent mechanisms. Substance P activates neurokinin receptors (3 subtypes) which couple to phospholipase C and the generation of the intracellular messengers whose downstream effects include depolarizing the membrane and facilitating the function of AMPA and NMDA receptors. Activation of neurokinin-1 receptors also increases the synthesis of prostaglandins whereas activation of neurokinin-3 receptors increases the synthesis of nitric oxide. Both products act as retrograde messengers across synapses and facilitate nociceptive signaling in the spinal cord. Whereas these cellular effects of CGRP and SP at the level of the spinal cord contribute to the development of increased synaptic strength between nociceptors and spinal neurons in the pathway for pain, the different intracellular signaling pathways also activate different transcription factors. The activated transcription factors initiate changes in the expression of genes that contribute to long-term changes in the excitability of spinal and maintain hyperalgesia.

Neuropeptide and kinin antagonists

Neuropeptides and kinins are important messengers in the nervous system and--on the basis of their anatomical localisation and the effects produced when the substances themselves are administered, to animals or to human subjects-a significant number of them have been suggested to have a role in pain and inflammation. Experiments in gene deletion (knock-out or null mutant) mice and parallel experiments with pharmacological receptor antagonists in a variety of species have strengthened the evidence that a number of peptides, notably substance P and calcitonin gene-related peptide (CGRP), and the kinins have a pathophysiological role in nociception. Clinical studies with non-peptide pharmacological antagonists are now in progress to determine if blocking the action of these peptides might have utility in the treatment of 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.

Direct evidence of primary afferent sprouting in distant segments following spinal cord injury in the rat: colocalization of GAP-43 and CGRP

Mechanical and thermal allodynia develops after spinal cord injury in three areas relative to the lesion: below level, at level, and above level. The present study tests colocalization of CGRP, associated with nociceptive neurons, with growth-associated protein (GAP-43), expressed in growing neurites, to test for neurite sprouting as a mechanism for reorganization of pain pathways at the level of the lesion and distant segments. Male Sprague-Dawley rats were divided into three groups: sham control (N = 10), hemisected at T13 and sacrificed at 3 days (N = 5) and at 30 days (N = 5) following surgery, the spinal cord tissue was prepared for standard fluorescent immunocytochemistry using mouse monoclonal anti-GAP-43 (1:200) and/or rabbit polyclonal anti-CGRP (1:200), density of immunoreaction product (IR) was quantified using the Bioquant software and values from the hemisected group were compared to similar regions from the sham control. We report significant increases at C8 and L5, in CGRP-IR in lamina III compared to control tissue (P < 0.05). We report significant bilateral increases in GAP-43-IR at C8, T13, and L5 segments in lamina I through IV, at 3 days post hemisection, compared to control tissue (P < 0.05), some of which is colocalized with alpha-CGRP. The increased area and density of GAP-43-IR is consistent with neurite sprouting, and the colocalization with alpha-CGRP indicates that some of the sprouting neurites are nociceptive primary afferents. These data are consistent with endogenous regenerative neurite growth mechanisms that occur near and several segments from a spinal lesion, that provide one of many substrates for the development and maintenance of the dysfunctional state of allodynia after spinal cord injury.

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.

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

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

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

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

Inhibitory effects of tachykinin receptor antagonists on thermally induced inflammatory reactions in a rat model

Recent studies have proposed that activation of the sensory nervous system after thermal injury induces the release of vasoactive neuropeptides, including tachykinins which contribute to the local inflammatory reaction as well as to the nociceptive transmission at the spinal cord level. Effects of the tachykinins substance P and neurokinin A are mediated by the neurokinin 1 and 2 (NK1, NK2) receptors. The aim of the present study was to investigate the modulatory role of NK1 and NK2 antagonists on edema formation, and on hindpaw withdrawal latency to experimentally asses nociception. Thermal injury was inflicted on the anaesthetized rat by dipping the right hindpaw into hot water at 60 degrees C for 20 s. The amount of edema formation was calculated by measuring the hindpaw volume with a plethysmograph before and during 420 min after scalding. In other studies scalding was inflicted under brief anesthesia, and hindpaw withdrawal latencies (HWL) to mechanical stimulation were recorded before injury and at 180 min after. The effect on edemic reactions of rats treated locally with NK1 and NK2 receptor antagonist were studied, as well as the effect of the same compounds on HWL after intrathecal injection. Scalding induced a progressive edema formation which was reduced significantly in rats treated with local injection of 100 nmol of NK1 and NK2 antagonists 45 min after the injury. The thermally induced inflammation was followed by significant decrease of the latency of hindpaw withdrawal response to mechanical stimulation. Intrathecal injection of 30 nmol of the same drugs 180 min after scalding was followed by significant increase in HWL. The results indicate that SP and NKA contribute to the inflammatory reactions after thermal injury and that the tachykinin receptor antagonists possess the ability to reduce both the local edemic reaction as well as the nociceptive transmission at the spinal cord level.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Antagonists of sensory neuropeptides inhibit the secondary phase of increased circulation following thermally induced inflammation

A model of thermally induced inflammation in the anesthetized rat was used to measure acute microcirculatory reactions after heat exposure. The thermal injury was inflicted by dipping the right hindpaw into hot water at 60 degrees for 20 s. Local blood flow was recorded simultaneously in both hindpaws and continuously by laser Doppler flowmetry before, during and for 2 h after the thermal injury and the mean arterial blood pressure (MAP) was displayed on a chart recorder. To assess the contribution of the nervous system to the vascular changes seen, neuropeptide antagonists directed toward substance P (SP), neurokinin A (NKA), and calcitonin gene-related peptide (CGRP) were administered. The neurokinin antagonists (NK1, NK2) and the CGRP antagonist (CGRP8-37) were injected via a catheter into the jugular vein. During the first few minutes after thermal injury to the controls, an immediate increase in blood perfusion of about 351% was recorded, followed by a slow decrease of circulation. At 30 min after thermal injury, there was a secondary phase of increased microcirculation of approximately 329%. A slow decline of cutaneous circulation then followed and, after another 30 min, the value stabilized at a level about 100% above the level before injury. Pretreatment with intravenous injections of the NK1 antagonist, NK2 antagonist, and CGRP8-37 attenuated the first phase and almost abolished the secondary phase. No significant change of perfusion was observed on the unscalded paw. The MAP remained at a stable level throughout the experiment and was not affected by the thermal injury or by the administration of the antagonists as compared to controls. Our results show that sensory neuropeptides play a significant role in the blood flow increase seen following thermal injury.

Opioids modulate the calcitonin gene-related peptide8-37-mediated hindpaw withdrawal latency increase in thermally injured rats

The present study was performed to explore the modulatory potential of different endogenous opioid systems on transmission of presumed nociceptive information at the spinal cord level in thermally injured rats. Thermal injury was performed by dipping the left paw into water 60 degrees C for 20 s. This induced a significant bilateral decrease in hindpaw withdrawal latency HWL to pressure. Intrathecal administration of 10 nmol of CGRP8-37 induced a significant bilateral increase in HWL in the thermally injured group and in the intact controls. The effect of different opioid receptor antagonists on the increased latency to withdrawal response induced by intrathecal injection of 10 nmol of CGRP8-37 was explored in the thermally injured rats. The effect was reversed by intrathecal injection of 40 and 80 nmol of: b-funaltrexamine (mu opioid receptor antagonist) and naltrindole (delta opioid receptor antagonist), but not by norbinaltorphimine (kappa opioid receptor antagonist). The results of the present study show that intrathecal CGRP8-37 increases hindpaw withdrawal latency in thermally injured rats, an effect reduced by a mu as well as by a delta opioid receptor antagonist.