The role of heme oxygenase in neuropathic and incisional pain

The role of carbon monoxide, heme oxygenase 1, and the Nrf2 transcription factor in the modulation of chronic pain and their interactions with opioids and cannabinoids

Chronic pain and its associated comorbidities are difficult to treat, even when the most potent analgesic compounds are used. Thus, research on new strategies to effectively relieve nociceptive and/or emotional disorders accompanying chronic pain is essential. Several studies have demonstrated the anti-inflammatory and antinociceptive effects of different carbon monoxide-releasing molecules (CO-RMs), inducible heme oxygenase 1 (HO-1), and nuclear factor-2 erythroid factor-2 (Nrf2) transcription factor activators in several models of acute and chronic pain caused by inflammation, nerve injury or diabetes. More recently, the antidepressant and/or anxiolytic effects of several Nrf2 transcription factor inducers were demonstrated in a model of chronic neuropathic pain. These effects are mainly produced by inhibition of oxidative stress, inflammation, glial activation, mitogen-activated protein kinases and/or phosphoinositide 3-kinase/phospho-protein kinase B phosphorylation in the peripheral and/or central nervous system. Other studies also demonstrated that the analgesic effects of opioids and cannabinoids are improved when these drugs are coadministered with CO-RMs, HO-1 or Nrf2 activators in different preclinical pain models and that these improvements are generally mediated by upregulation or prevention of the downregulation of µ-opioid receptors, δ-opioid receptors and/or cannabinoid 2 receptors in the setting of chronic pain. We reviewed all these studies as well as studies on the mechanisms of action underlying the effects of CO-RMs, HO-1, and Nrf2 activators in chronic pain. In summary, activation of the Nrf2/HO-1/carbon monoxide signaling pathway alone and/or in combination with the administration of specific analgesics is a valid strategy for the treatment of chronic pain and some associated emotional disorders.

Carbon monoxide contributes to the constipating effects of granisetron in rat colon

AIM

To investigate the mechanisms underlying the potential contribution of the heme oxygenase/carbon monoxide (HO/CO) pathway in the constipating effects of granisetron.

METHODS

For in vivo studies, gastrointestinal motility was evaluated in male rats acutely treated with granisetron [25, 50, 75 μg/kg/subcutaneous (sc)], zinc protoporphyrin IX [ZnPPIX, 50 μg/kg/intraperitoneal (ip)] and hemin (50 μmol/L/kg/ip), alone or in combination. For in vitro studies, the contractile neurogenic response to electrical field stimulation (EFS, 3, 5, 10 Hz, 14 V, 1 ms, pulse trains lasting 10 s), as well as the contractile myogenic response to acetylcholine (ACh, 0.1-100 μmol/L) were evaluated on colon specimens incubated with granisetron (3 μmol/L, 15 min), ZnPPIX (10 μmol/L, 60 min) or CO-releasing molecule-3 (CORM-3, 100, 200, 400 μmol/L) alone or in combination. These experiments were performed under co-treatment with or without atropine (3 μmol/L, a muscarinic receptor antagonist) or N^G-nitro-L-Arginine (L-NNA, 100 μmol/L, a nitric oxide synthase inhibitor).

RESULTS

Administration of granisetron (50, 75 μg/kg) in vivo significantly increased the time to first defecation (P = 0.045 vs vehicle-treated rats), clearly suggesting a constipating effect of this drug. Although administration of ZnPPIX or hemin alone had no effect on this gastrointestinal motility parameter, ZnPPIX co-administered with granisetron abolished the granisetron-induced constipation. On the other hand, co-administration of hemin and granisetron did not modify the increased constipation observed under granisetron alone. When administered in vitro, granisetron alone (3 μmol/L) did not significantly modify the colon’s contractile response to either EFS or ACh. Incubation with ZnPPIX alone (10 μmol/L) significantly reduced the colon’s contractile response to EFS (P = 0.016) but had no effect on contractile response to ACh. Co-administration of ZnPPIX and atropine (3 μmol/L) abolished the ZnPPIX-mediated decrease in contractile response to EFS. Conversely, incubation with CORM-3 (400 μmol/L) alone increased both the contractile response to EFS at 10 Hz (10 Hz: 71.02 ± 19.16 vs 116.25 ± 53.70, P = 0.01) and the contractile response to ACh (100 μmol/L) (P = 0.012). Co-administration of atropine abolished the CORM-3-mediated effects on the EFS-mediated response. When granisetron was co-incubated in vitro with ZnPPIX, the ZnPPIX-mediated decrease in colon contractile response to EFS was lost. On the other hand, co-incubation of granisetron and CORM-3 (400 μmol/L) further increased the colon’s contractile response to EFS (at 5 Hz: P = 0.007; at 10 Hz: P = 0.001) and to ACh (ACh 10 μmol/L: P = 0.001; ACh 100 μmol/L: P = 0.001) elicited by CORM-3 alone. L-NNA co-administered with granisetron and CORM-3 abolished the potentiating effect of CORM-3 on granisetron on both the EFS-induced and ACh-induced contractile response.

CONCLUSION

Taken together, findings from in vivo and in vitro studies suggest that the HO/CO pathway is involved in the constipating effects of granisetron.

Carbon monoxide may be an important molecule in migraine and other headaches

Introduction

Carbon monoxide was previously considered to just be a toxic gas. A wealth of recent information has, however, shown that it is also an important endogenously produced signalling molecule involved in multiple biological processes. Endogenously produced carbon monoxide may thus play an important role in nociceptive processing and in regulation of cerebral arterial tone.

Discussion

Carbon monoxide-induced headache shares many characteristics with migraine and other headaches. The mechanisms whereby carbon monoxide causes headache may include hypoxia, nitric oxide signalling and activation of cyclic guanosine monophosphate pathways. Here, we review the literature about carbon monoxide-induced headache and its possible mechanisms.

Conclusion

We suggest, for the first time, that carbon monoxide may play an important role in the mechanisms of migraine and other headaches.

Effects of treatment with a carbon monoxide-releasing molecule and a heme oxygenase 1 inducer in the antinociceptive effects of morphine in different models of acute and chronic pain in mice

RATIONALE

Treatment with a carbon monoxide-releasing molecule (tricarbonyldichlororuthenium(II) dimer, CORM-2) or a classical heme oxygenase 1 inducer (cobalt protoporphyrin IX, CoPP) has potent anti-inflammatory effects, but the role played by these treatments in the antinociceptive effects of morphine during acute and chronic pain was not evaluated.

OBJECTIVES

In wild type (WT), neuronal (NOS1-KO), or inducible (NOS2-KO) nitric oxide synthases knockout mice, we evaluated the effects of CORM-2 and CoPP treatments in the antinociceptive actions of morphine and their interaction with nitric oxide during acute, visceral, and chronic inflammatory or neuropathic pain.

METHODS

Acute and visceral pain was assessed through formalin and acid acetic writhing tests. Chronic inflammatory pain induced by the intra-articular administration of complete Freund's adjuvant and neuropathic pain by partial ligation of sciatic nerve were evaluated by measuring allodynia and hyperalgesia using the von Frey filaments, plantar, or cold plate tests.

RESULTS

While nitric oxide, synthetized by NOS1 and/or NOS2, increased the local antinociceptive effects of morphine during acute and chronic pain, it decreased the inhibitory effects of morphine after visceral pain. Moreover, while CORM-2 or CoPP treatments did not alter or reduced the antinociceptive effects of morphine during acute and visceral pain, both treatments improved the local antiallodynic and antihyperalgesic effects of morphine after chronic inflammatory or neuropathic pain in WT, but not in KO mice.

CONCLUSIONS

CORM-2 and CoPP treatments improved the local antinociceptive effects of morphine during chronic inflammatory and neuropathic pain by interaction with nitric oxide synthetized by NOS1 and NOS2 isoforms.

Treatment with Carbon Monoxide-releasing Molecules and an HO-1 Inducer Enhances the Effects and Expression of µ-Opioid Receptors during Neuropathic Pain

Background:

The administration of µ-opioid receptors (MOR) and δ-opioid receptors (DOR) as well as cannabinoid-2 receptor (CB2R) agonists attenuates neuropathic pain. We investigated if treatment with two carbon monoxide-releasing molecules (CORM-2 and CORM-3) or an inducible heme oxygenase inducer (cobalt protoporphyrin IX, CoPP) could modulate the local and systemic effects and expression of MOR, DOR, and CB2R during neuropathic pain.

Methods:

In C57BL/6 mice, at 10 days after the chronic constriction of sciatic nerve, we evaluated the effects of the intraperitoneal administration of 10 mg/kg of CORM-2, CORM-3, or CoPP on the antiallodynic and antihyperalgesic actions of a locally or systemically administered MOR (morphine), DOR ([d-Pen(2),d-Pen(5)]-enkephalin) or CB2R ((2-methyl-1-propyl-1H-indol-3-yl)-1-naphthalenylmethanone ) agonist. The effects of CORM-2 and CoPP treatments on the expression of MOR, DOR, CB2R, inducible and constitutive heme oxygenases, microglia activation marker (CD11b/c), and neuronal and inducible nitric oxide synthases were also assessed.

Results:

Treatments with CO-RMs and CoPP reduced the mechanical and thermal hypersensitivity induced by sciatic nerve injury, increased the local, but not systemic, antinociceptive effects of morphine, and decreased those produced by DPDPE and JWH-015. Both CORM-2 and CoPP treatments enhanced MOR and inducible heme oxygenase expression, unaltered DOR and constitutive heme oxygenase expression, and decreased the overexpression of CB2R, CD11b/c, and neuronal and inducible nitric oxide synthases induced by sciatic nerve injury.

Conclusions:

This study shows that CO-RMs and CoPP treatments increase the local antinociceptive effects of morphine through enhancing MOR peripheral expression and inhibiting spinal microglial activation and overexpression of neuronal/inducible nitric oxide synthases.

Common trace elements alleviate pain in an experimental mouse model

Trace elements represent a group of essential metals or metaloids necessary for life, present in minute amounts. Analgesic adjuvants can enhance the effect of other pain drugs or be used for pain control themselves. Previous studies on the effects of trace elements on nociception and their potential use as analgesic adjuvants have yielded conflicting results. In this study, we tested the hypothesis that three vital trace elements (Zn²⁺, Mg²⁺, Cu²⁺) have direct antinociceptive effects. Groups of eight Swiss mice were intraperitoneally (i.p) injected with incremental concentrations of Zn²⁺ sulfate (0.5, 2.0 mg/kg), Zn²⁺ citrate (0.125, 0.5 mg/kg), Mg²⁺ chloride (37.5, 75, 150 mg/kg), Cu²⁺ chloride (0.5, 1.0, 2.0 mg/kg), and Cu²⁺ sulfate (0.5, 1.0 mg/kg) or saline (control). Evaluations were made by hot plate (HP) and tail flick (TF) tests for central antinociceptive effect, writhing test (WT) for visceral antinociceptive effect, and activity cage (AC) test for spontaneous behavior. Zn²⁺ induced pain inhibition in HP/TF tests (up to 17%) and WT (up to 25%), with no significant differences among the salts used. Mg²⁺ salts induced pain inhibition for all performed tests (up to 85% in WT). Cu²⁺ salts showed antinociceptive effects for HP/TF (up to 28.6%) and WT (57.28%). Only Mg²⁺ and Cu²⁺ salts have displayed significant effects in AC (Mg²⁺ anxiolytic/depressant effect; Cu²⁺ anxiolytic effect). We interpret these data to mean that all tested trace elements induced antinociceptive effects in central and visceral pain tests. Our data indicate the potential use of these cheap adjuvants in pain therapy.

Carbon monoxide reduces neuropathic pain and spinal microglial activation by inhibiting nitric oxide synthesis in mice

Background

Carbon monoxide (CO) synthesized by heme oxygenase 1 (HO-1) exerts antinociceptive effects during inflammation but its role during neuropathic pain remains unknown. Our objective is to investigate the exact contribution of CO derived from HO-1 in the modulation of neuropathic pain and the mechanisms implicated.

Methodology/Principal Findings

We evaluated the antiallodynic and antihyperalgesic effects of CO following sciatic nerve injury in wild type (WT) or inducible nitric oxide synthase knockout (NOS2-KO) mice using two carbon monoxide-releasing molecules (CORM-2 and CORM-3) and an HO-1 inducer (cobalt protoporphyrin IX, CoPP) daily administered from days 10 to 20 after injury. The effects of CORM-2 and CoPP on the expression of HO-1, heme oxygenase 2 (HO-2), neuronal nitric oxide synthase (NOS1) and NOS2 as well as a microglial marker (CD11b/c) were also assessed at day 20 after surgery in WT and NOS2-KO mice. In WT mice, the main neuropathic pain symptoms induced by nerve injury were significantly reduced in a time-dependent manner by treatment with CO-RMs or CoPP. Both CORM-2 and CoPP treatments increased HO-1 expression in WT mice, but only CoPP stimulated HO-1 in NOS2-KO animals. The increased expression of HO-2 induced by nerve injury in WT, but not in NOS2-KO mice, remains unaltered by CORM-2 or CoPP treatments. In contrast, the over-expression of CD11b/c, NOS1 and NOS2 induced by nerve injury in WT, but not in NOS2-KO mice, were significantly decreased by both CORM-2 and CoPP treatments. These data indicate that CO alleviates neuropathic pain through the reduction of spinal microglial activation and NOS1/NOS2 over-expression.

Conclusions/Significance

This study reports that an interaction between the CO and nitric oxide (NO) systems is taking place following sciatic nerve injury and reveals that increasing the exogenous (CO-RMs) or endogenous (CoPP) production of CO may represent a novel strategy for the treatment of neuropathic pain.

Hydrogen sulfide as a gasotransmitter

Nitric oxide (NO) and carbon monoxide (CO) are well established as messenger molecules throughout the body, gasotransmitters, based on striking alterations in mice lacking the appropriate biosynthetic enzymes. Hydrogen sulfide (H(2)S) is even more chemically reactive, but until recently there was little definitive evidence for its physiologic formation. Cystathionine beta-synthase (EC 4.2.1.22), and cystathionine gamma-lyase (CSE; EC 4.4.1.1), also known as cystathionine, can generate H(2)S from cyst(e)ine. Very recent studies with mice lacking these enzymes have established that CSE is responsible for H(2)S formation in the periphery, while in the brain cystathionine beta-synthase is the biosynthetic enzyme. Endothelial-derived relaxing factor activity is reduced 80% in the mesenteric artery of mice with deletion of CSE, establishing H(2)S as a major physiologic endothelial-derived relaxing factor. H(2)S appears to signal predominantly by S-sulfhydrating cysteines in its target proteins, analogous to S-nitrosylation by NO. Whereas S-nitrosylation typically inhibits enzymes, S-sulfhydration activates them. S-nitrosylation basally affects 1-2% of its target proteins, while 10-25% of H(2)S target proteins are S-sulfhydrated. In summary, H(2)S appears to be a physiologic gasotransmitter of comparable importance to NO and carbon monoxide.

Haeme oxygenase-1 overexpression via nAChRs and the transcription factor Nrf2 has antinociceptive effects in the formalin test

Epibatidine has shown antinociceptive effects in various pain models, being 200-fold more potent than morphine. Previous results from our laboratory demonstrated that HO-1 overexpression has an antinociceptive effect in the formalin test. Furthermore, epibatidine was able to induce haeme oxygenase-1 (HO-1). So, the aim of this study was to investigate the effect of HO-1 overexpression induced by epibatidine in nociception elicited by formalin injection in the mice hindpaw. Administration of epibatidine (4 microg/kg) 24h before the test reduced the nociceptive response during the first phase and second phase of the formalin test. This effect was prevented by treatment with tin protoporphyrin (SnPP, an inhibitor of HO-1 activity) administered via intraplantar 5min before the test, suggesting a main role of HO-1. Western blot analysis revealed that epibatidine treatment increased by 2-fold HO-1 expression in the paw; this effect was lost in knockout mice for nuclear factor-erythroid 2-related factor 2 (Nrf2) and was accompanied by the loss of its antinociceptive effect. Furthermore, the antinociceptive effect of epibatidine was related to the activation of alpha7 and/or alpha9 nAChRs since methyllycaconitine (MLA) and mecamylamine but not dihydro-beta-erythroidine (DHbetaE) reverted this effect. Finally, we showed by flow cytometry and by immunofluorescence that white blood cells of the animals injected with epibatidine expressed more HO-1 than control animals, and this expression was also reverted by MLA pre-treatment. These findings demonstrate that HO-1 induction by epibatidine has antinociceptive and anti-inflammatory effects by the activation of MLA-sensitive nAChRs.

Regulation and expression of heme oxygenase enzymes in aged-rat brain: age related depression in HO-1 and HO-2 expression and altered stress-response

The heme oxygenase isozymes, HO-1 and HO-2, oxidatively cleave the heme molecule to produce biliverdin and the gaseous messenger, CO. The cleavage results in the release of iron, a regulator of transferrin, ferritin, and nitric oxide (NO) synthase gene expression. Biliverdin reductase (BVR) then catalyzes the reduction of biliverdin, generating the potent intracellular antioxidant, bilirubin. We report an age-related decrease in HO-1 and HO-2 expression present in select brain regions including the hippocampus and the substantia nigra, that are involved in the high order cognitive processes of learning and memory. The age-related loss of monoxide-producing potential in select regions of the brain was not specific to the HO system but was also observed in neuronal NO-generating system. Furthermore, compared to 2-month old rats, the ability of aged brain tissue to respond to hypoxic/hyperthermia was compromised at both the protein and the transcription levels as judged by attenuated induction of HO-1 immunoreactive protein and its 1.8 Kb transcript. Neotrofin (AIT), a cognitive-enhancing and neuroprotective drug, caused a robust increase in HO-1 immunoreactive protein in select neuronal regions and increased the expression of HO-2 transcripts. The potential interplay between regulation of HO-2 gene expression and the serum levels of the adrenal steroids is discussed. We suggest the search for therapeutic agents that reverse the decline and aberrant stress response of HO enzymes may lead to effective treatment regimens for age-associated neuronal deficits.

Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications

Over the last decade, studies have unraveled many aspects of endogenous production and physiological functions of carbon monoxide (CO). The majority of endogenous CO is produced in a reaction catalyzed by the enzyme heme oxygenase (HO). Inducible HO (HO-1) and constitutive HO (HO-2) are mostly recognized for their roles in the oxidation of heme and production of CO and biliverdin, whereas the biological function of the third HO isoform, HO-3, is still unclear. The tissue type-specific distribution of these HO isoforms is largely linked to the specific biological actions of CO on different systems. CO functions as a signaling molecule in the neuronal system, involving the regulation of neurotransmitters and neuropeptide release, learning and memory, and odor response adaptation and many other neuronal activities. The vasorelaxant property and cardiac protection effect of CO have been documented. A plethora of studies have also shown the importance of the roles of CO in the immune, respiratory, reproductive, gastrointestinal, kidney, and liver systems. Our understanding of the cellular and molecular mechanisms that regulate the production and mediate the physiological actions of CO has greatly advanced. Many diseases, including neurodegenerations, hypertension, heart failure, and inflammation, have been linked to the abnormality in CO metabolism and function. Enhancement of endogenous CO production and direct delivery of exogenous CO have found their applications in many health research fields and clinical settings. Future studies will further clarify the gasotransmitter role of CO, provide insight into the pathogenic mechanisms of many CO abnormality-related diseases, and pave the way for innovative preventive and therapeutic strategies based on the physiologic effects of CO.

Formalin injection causes a coordinated spinal cord CO/NO-cGMP signaling system response

Background

The CO/NO-cGMP signalling system participates in the regulation of many physiological processes. The roles this system plays in spinal cord nociceptive signalling are particularly important. While individual components have been examined in isolation, little study has been dedicated to understanding the regulation and functioning of the system as a whole.

Results

In these studies we examined the time course of expression of 13 genes coding for components of this system including isoforms of the heme oxygenase (HO), nitric oxide synthase (NOS), soluble guanylate cyclase (sGC), cGMP dependent protein kinase (PKG) and phosphodiesterase (PDE) enzyme systems. Of the 13 genes studied, 11 had spinal cord mRNA levels elevated at one or more time points up to 48 hours after hindpaw formalin injection. Of the 11 with elevated mRNA, 8 had elevated protein levels 48 hours after formalin injection when mechanical allodynia was maximal. No component had an increased protein level which did not have an increased mRNA level at one or more time points. Injection of morphine 10 mg/kg prior to formalin completely abolished the acute nociceptive behaviours, but did not alter the degree of sensitivity which developed in the formalin treated hind paws during the subsequent 48 hours. Morphine treatment did, however, eliminate formalin induced increases in enzyme protein levels.

Conclusion

Our results indicate that the expression of the components of the CO/NO-cGMP signalling system seems to be coordinated in such a way that a generalized multi-level enhancement rather than a tightly limited step specific response occurs with noxious stimulation. Furthermore, the analgesic morphine administered prior to noxious stimulation can prevent long-term changes in gene expression though not necessarily nociceptive sensitisation.

Mechanisms of incisional pain

Postoperative, incisional pain is a unique but common form of acute pain. Because effective postoperative analgesia reduces morbidity following surgery, new treatments continue to be sought. It is through the development of investigational models and studies of the mechanisms that perioperative medicine can be advanced. This article reviews studies on a rat plantar hindpaw model for postoperative pain and proposes mechanisms for enhanced excitability of sensory neurons caused by incisions.

Alterations in spinal cord gene expression after hindpaw formalin injection

Heme oxygenase type 2 (HO-2) is an enzyme that uses heme as a substrate to produce iron, biliverdin, and carbon monoxide (CO). This enzyme participates in regulation of nociceptive signal transmission in spinal cord tissue. We set out to identify genes undergoing alterations in expression in a model of inflammatory pain and to determine whether HO-2 participates in that regulation. After the hindpaw injection of formalin in mice, we measured changes in expression of immediate early genes including c-fos, c-jun, jun B, nerve growth factor induced genes (NGFI-A and NGFI-B) and activity-related cytoskeletal protein (ARC) using real-time PCR. The mRNA corresponding to these genes increased in abundance in the first hour after formalin injection and then slowly declined. Changes in the abundance of prodynorphin, extracellular signal related kinases (ERK1 and ERK2) and N-methyl-D-aspartate (NMDA) receptor R1 subunit mRNA generally peaked between 8 and 12 hr after formalin injection. In HO-2 null mutant mice, the enhancement of expression was less for all genes studied. We went on to quantify gene expression in superficial dorsal horn tissue using laser capture microdissection followed by RNA amplification and real-time PCR. The results confirmed that the changes in gene expression were occurring in regions of the spinal cord involved in nociceptive processing. We conclude that the hindpaw injection of formalin leads to enhanced early and late expression of many genes in spinal cord dorsal horn tissue, and that this enhancement of expression relies to a degree on the presence of HO-2.

Formalin-induced spinal cord calcium/calmodulin-dependent protein kinase II alpha expression is modulated by heme oxygenase in mice

The injection of formalin into the hindpaws of rats and mice is widely used as a model of inflammatory pain. The allodynia observed in this model is due in part to sensitization of spinal cord dorsal horn neurons, a form of neuroplasticity similar to long-term potentiation in the hippocampus. Ca(2+)/calmodulin-dependent kinase type IIalpha (CaMKIIalpha) is a key component of long-term potentiation. Here we report alterations in CaMKIIalpha mRNA and protein expression in spinal cord tissue from wild-type and heme oxygenase type 2 (HO-2) null mutant mice after formalin injection. Behavioral experiments demonstrated a long lived allodynia in wild-type C57Bl/6J mice after hindpaw formalin injection, but less in null mutant mice. Both CaMKIIalpha mRNA and protein expression were increased in a time-dependent manner in the spinal cords of wild-type mice after formalin injection. Confocal microscopy localized the increased expression to the superficial laminae of the spinal cord dorsal horn. In the HO-2 null mutant mice no significant change in CaMKIIalpha mRNA expression and only a small increase in protein were noted. These findings suggest that time-dependent CaMKIIalpha expression may underlie central sensitization and allodynia induced by hindpaw formalin injection, and that this process is modulated by HO-2.

Heme oxygenase type 2 modulates behavioral and molecular changes during chronic exposure to morphine

The heme oxygenase (HO) enzyme system has been shown to participate in nociceptive signaling in a number of different models of pain. In these experiments we investigated the role of the HO type 2 (HO-2) isozyme in tolerance to the analgesic effects of morphine, and the hyperalgesia and allodynia which are measurable upon cessation of administration. Wild type C57Bl/6 wild type mice or HO-2 null mutants in that background strain were treated with morphine for 5 days. The morphine administration protocol consisted of either twice daily repeated s.c. boluses of 15 mg/kg or s.c. implantation of a morphine pellet. At the end of the treatment period wild type mice treated by either protocol exhibited tolerance, but the HO-2 null mutants did not. The HO-2 null mutants also exhibited less mechanical allodynia following cessation of morphine administration, though only modest differences in thermal hyperalgesia were noted. There was no correlation between the degree of tolerance obtained in the bolus and pellet protocols and the degree of hyperalgesia and allodynia observed after cessation of morphine administration in the wild type mice. Our final experiments analyzed increases in expression of mRNA for nitric oxide synthase type 1, N-methyl-D-aspartate (NMDA) receptor NMDAR1 subunit and prodynorphin in spinal cord tissue. In pellet-treated mice two- to three-fold increases were observed in the abundance of these species, but very little change was observed in the null-mutant mice. Taken together our results indicate that HO-2 participates in the acquisition of opioid tolerance, the expression of mechanical allodynia after cessation of opioid administration and in gene regulation occurring in the setting of treatment with morphine. Furthermore, these studies suggest that the mechanisms underlying analgesic tolerance and opioid-induced hypersensitivity are at least somewhat distinct.

Long-lasting inflammation and long-term hyperalgesia after subcutaneous formalin injection into the rat hindpaw

Subcutaneous formalin injection is widely used as a nociceptive stimulus in the rat. This procedure evokes overt behaviors that last about 90 minutes. However, little is known about the duration of paw inflammation and alterations in pain sensitivity to noxious stimuli after 2 hours. We studied the nociceptive responses to thermal and mechanical stimuli 2 hours to 4 weeks after formalin injection into the dorsal or plantar side of the hindpaw. Thirty-two adult male Sprague-Dawley rats were divided into 3 groups: In group I, 50 microL of 5% formalin was injected into the plantar side (n = 12); in group II, 50 microL of 5% formalin was injected into the dorsal side (n = 12); in group III, 50 microL saline was injected into the dorsal or plantar side of the hindpaw (n = 8). Nociceptive responses to thermal and mechanical stimuli applied to the dorsal or plantar surfaces of the injected and the contralateral hindpaws were recorded. The injection of formalin into the rat's hindpaw produced a hypoalgesic region around the injection site. In contrast, hyperalgesic responses to thermal and mechanical stimulation were induced on the opposite surface of the injected hindpaw as well as in the contralateral noninjected hindpaw. The hyperalgesic responses, which were observed 2 hours after formalin administration, were enhanced 1 to 3 days after injection and lasted 3 to 4 weeks. These results suggest that peripheral inflammation after subcutaneous formalin injection produces a long-lasting sensitization. Possible mechanisms for these changes in nociception are discussed.

Pharmacological characterisation of a rat model of incisional pain

1. Both clinical and preclinical models of postsurgical pain are being used more frequently in the early evaluation of new chemical entities. In order to assess the validity and reliability of a rat model of postincisional pain, the effects of different classes of clinically effective analgesic drugs were evaluated against multiple behavioural end points. 2. Following surgical incision, under general anaesthesia, of the plantar surface of the rat hind paw, we determined the time course of mechanical hyperalgesia, tactile allodynia and hind limb weight bearing using the Randall-Selitto (paw pressure) assay, electronic von Frey and dual channel weight averager, respectively. Behavioural evaluations began 24 h following surgery, and were continued for 9-14 days. 3. Mechanical hyperalgesia, tactile allodynia and a decrease in weight bearing were present on the affected limb within 1 day of surgery with maximum sensitivity 1-3 days postsurgery. Accordingly, we examined the effect of nonsteroidal antiinflammatory drugs (NSAIDs), morphine and gabapentin, on established hyperalgesia and allodynia, 1 day following plantar incision.4. In accordance with previous reports, both systemic morphine and gabapentin administration reversed mechanical hyperalgesia and tactile allodynia in the incised rat hind paw. Both drugs were more potent against mechanical hyperalgesia than tactile allodynia. 5. All of the NSAIDs tested, including cyclooxygenase 2 selective inhibitors, reversed mechanical hyperalgesia and tactile allodynia in the incised rat hind paw. The rank order of potency for both hyperalgesia and allodynia was indomethacin > celecoxib > etoricoxib > naproxen. 6. We have investigated the potency and efficacy of different classes of analgesic drugs in a rat model of postincisional pain. The rank order of potency for these drugs reflects their utility in treating postoperative pain in the clinic. As these compounds showed reliable efficacy across two different behavioural end points, the Randall-Selitto (paw pressure) assay and electronic von Frey, these methods may prove useful in the study of postsurgical pain and the assessment of novel treatments.

NOVELNEURALMODULATORS

The discovery that nitric oxide (NO) is produced by neurons and regulates synaptic activity has challenged the definition of a neurotransmitter. NO is not stored in synaptic vesicles and does not act at conventional receptors on the surface of adjacent neurons. The toxic gases carbon monoxide (CO) and hydrogen sulfide (H2S) are also produced by neurons and modulate synaptic activity. D-serine synthesis and release by astrocytes as an endogenous ligand for the "glycine" site of N-methyl D-aspartate (NMDA) receptors defy the concept that a neurotransmitter must be synthesized by neurons. We review the properties of these "atypical" neural modulators.

Spinal cord heme oxygenase participates in glutamate-induced pain-related behaviors

Heme oxygenase catalyzes the formation of CO, Fe(2+) and biliverdin from the substrate heme. In these studies, we attempted to define the roles heme oxygenase play in pain-related behaviors induced by intrathecal injection of the spinal neurotransmitter glutamate. The intrathecal injection of glutamate or the more selective agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) in C57Bl/6 mice lead to caudally directed pain behaviors which were sensitive to the heme oxygenase inhibitors tin protoporphyrin (Sn-protoporphyrin) and chromium mesoporphyrin (Cr-mesoporphyrin). Intrathecal injections of glutamate in heme oxygenase type 2 (HO-2) null-mutant animals resulted in reduced pain-related behaviors when compared with wild type animals. Glutamate, NMDA and AMPA stimulated cGMP accumulation in mouse spinal cord slices, which was blocked by heme oxygenase inhibitors. Glutamate did not stimulate cGMP production in HO-2 null-mutant animals. Our data are consistent with the hypothesis that pain-related behaviors induced by spinal glutamate rely on the activation of HO-2 and subsequent production of cGMP.

Heme oxygenase-1 expression after spinal cord injury: the induction in activated neutrophils

Tissue damage and neurological dysfunction after spinal cord injury may result, in part, from delayed or secondary mechanisms that appear to involve several endogenous factors. Among them, neutrophils are known to play important roles in the pathomechanisms of the secondary injury, that is, neutrophils are activated by an interaction with the endothelial cells, migrate into the damaged tissue and release several kinds of proteases or oxygen radicals. In the present study, we examined heme oxygenase-1 expression in the damaged spinal cord. The administration of an inhibitor of heme oxygenase-1 in vivo produced a delayed recovery of motor function after spinal cord injury, suggesting that heme oxygenase-1 may play roles as an endogenous anti-inflammatory enzyme and protective gene in the damaged and inflammatory tissue. We found that many neutrophils expressing heme oxygenase-1 mRNA and protein were recruited into the damaged spinal cord with extensive hemorrhages during early stage of spinal cord injury. In an in vitro study, neutrophils incubated with proinflammatory cytokines, such as interleukin-1, 6 or interferon-gamma, expressed heme oxygenase-1 mRNA and protein. Based on these findings we conclude that the activated neutrophils can express heme oxygenase-1 in the injured spinal cord tissue, perhaps expecting modulatory and neuroprotective actions in the inflammatory response to spinal cord injury.

Heme oxygenase inhibitors reduce formalin-induced Fos expression in mouse spinal cord tissue

Recent work from our laboratory and others supports a role for heme oxygenase in nociception and pain of several etiologies including inflammatory, incisional and neuropathic. Since it has been observed that heme oxygenase inhibitors reduce formalin-induced pain behaviors in mice and rats, we attempted to determine if this analgesic effect was reflected in a reduction in formalin-induced spinal cord Fos expression, an index of neuronal activation. To perform these studies, it was necessary to first examine the cytoarchitecture of the mouse lumbar spinal cord so that histological sections from known segmental levels could be chosen, and Fos-positive nuclei could be assigned to established dorsal horn laminae. After documenting the segmental and laminar distribution of Fos-positive nuclei following a 5% formalin injection, we went on to determine that the heme oxygenase inhibitor tin-protoporphyrin or morphine reduced this Fos expression as analyzed using confocal fluorescence microscopy. It was also observed that mice lacking expression of heme oxygenase type 2, an isozyme of heme oxygenase found in high abundance in the spinal cord, had lowered Fos expression after the formalin injection. Additional confocal microscopy studies demonstrated widespread expression of heme oxygenase type 2 in spinal cord neuron cell bodies. Double-labeling experiments showed that a high percentage of Fos-positive nuclei identified after administration of formalin were located within heme oxygenase type 2-positive cell profiles. Our studies support the hypothesis that heme oxygenase type 2 plays a role in formalin-induced nociception. Furthermore, from these results we suggest that the heme oxygenase type 2 located in spinal cord dorsal horn neurons participates in this nociceptive pathway.

Spinal cord nitric oxide synthase and heme oxygenase limit morphine induced analgesia

Spinal cord tissue contains two enzyme systems capable of producing monoxide gases which in turn are linked to the stimulation of soluble guanylate cyclase, nitric oxide synthase (NOS) which produces NO and heme oxygenase (HO) which produces CO. Reports from several laboratories link these two enzyme systems to pain of inflammatory and neuropathic etiologies. Additional studies have demonstrated that the activation of the NOS system by morphine limits the spinal analgesic action of this drug. In this study we first employed the hot plate model of pain to demonstrate that the NOS inhibitor L-NAME and the HO inhibitor Sn-P potentiate the analgesic actions of intrathecally administered morphine while having no intrinsic analgesic action at the doses used. We then determined that L-NAME loses its ability to potentiate morphine in nNOS null-mutant mice, while Sn-P no longer potentiates morphine in mice lacking a functional HO-2 gene. The intrathecal injection of the cGMP analog 8-Br cGMP caused hyperalgesia in the hot plate assay. Focusing on the possible involvement of cGMP metabolism, we documented that morphine stimulates cGMP production in a spinal cord slice model in a concentration dependent and naloxone reversible manner. Both L-NAME and Sn-P were potent inhibitors of morphine-stimulated cGMP production. Buffer containing either CO or the NO donor compound SNAP stimulated cGMP production as well. In spinal cord slices from either nNOS or HO-2 null-mutant animals morphine did not stimulate cGMP production. Taken together our data suggest that spinal monoxide generation modifies the acute analgesic actions of morphine.

Opioid-induced hyperalgesia and incisional pain

Opioids occupy a position of unsurpassed clinical utility in the treatment of pain of many etiologies. However, recent reports in laboratory animals and humans have documented the occurrence of hyperalgesia when the administration of opioids is abruptly tapered or discontinued, a condition known as opioid-induced hyperalgesia (OIH). In these studies we documented that rats administered morphine (40 mg. kg(-1). day(-1) for 6 days) via subcutaneous osmotic minipumps demonstrated thermal hyperalgesia and mechanical allodynia for several days after the cessation of morphine administration. Additional experiments using a rat model of incisional pain showed that that attributable to OIH were additive with the hyperalgesia and allodynia that resulted from incision. In our final experiments we observed that if naloxone is administered chronically before incision then discontinued (20 mg. kg(-1). day(-1) for 6 days), the hyperalgesia and allodynia that result from hind paw incision was markedly reduced. In contrast, naloxone 1 mg/kg administered acutely after hind paw incision increased hyperalgesia and allodynia. We conclude that the chronic administration of exogenous opioid receptor agonists and antagonists before incision can alter the hyperalgesia and allodynia observed in this pain model, perhaps by altering intrinsic opioidergic systems involved in setting thermal and mechanical nociceptive thresholds.

IMPLICATIONS

The chronic administration of opioids followed by abrupt cessation can lead to a state of hyperalgesia. In these studies we demonstrate that the hyperalgesia from opioid cessation and from hind paw incision are additive in rats. We suggest that failure to take into consideration preoperative opioid use can lead to excessive postoperative pain.

A murine model of opioid-induced hyperalgesia

Controversies surround the possible long-term physiological and psychological consequences of opioid use. Analgesic tolerance and addiction are commonly at the center of these controversies, but other concerns exist as well. A growing body of evidence suggests that hyperalgesia caused by the chronic administration of opioids can occur in laboratory animals and in humans. In these studies we describe a murine model of opioid-induced hyperalgesia (OIH). After the treatment of mice for 6 days with implanted morphine pellets followed by their removal, both thermal hyperalgesia and mechanical allodynia were documented. Additional experiments demonstrated that prior morphine treatment also increased formalin-induced licking behavior. These effects were intensified by intermittent abstinence accomplished through administration of naloxone during morphine treatment. Experiments designed to determine if the mu-opioid receptor mediated OLH in our model revealed that the relatively-selective mu-opioid receptor agonist fentanyl induced the thermal hyperalgesia and mechanical allodynia characteristic of OIH when administered in intermittent boluses over 6 days. In complimentary experiments we found that CXBK mice which have reduced mu-opioid receptor binding displayed no significant OIH after morphine treatment. Finally, we explored the pharmacological sensitivities of OIH. We found that the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) and the heme oxygenase (HO) inhibitor tin protoporphyrin (Sn-P) dose-dependently reduced OIH in this model while the NSAID indomethacin had no effect. Thus we have characterized a murine model of OIH which will be useful in the pursuit of the molecular mechanisms underlying this phenomenon.