Intracerebroventricular injection of phospholipases A2 inhibitors modulates allodynia after facial carrageenan injection in mice

The present study was carried out, using inhibitors to secretory phospholipase A2 (sPLA2, 12-epi-scalaradial), cytosolic phospholipase A2 (cPLA2, AACOCF3), or calcium-independent phospholipase A2 (iPLA2, bromoenol lactone), to compare possible contributions of central nervous PLA2 isoforms to the development of allodynia after facial carrageenan injection in mice. C57BL/6J (B6) mice showed increased responses to facial stimulation using a von Frey hair (1 g force), at 8 h, 1 day, and 3 days after facial carrageenan injection. On the other hand, BALB/c mice did not show increased responses at any of the time points. In both B6 and BALB/c mice, intracerebroventricular injection of inhibitors to each of the three PLA2 isoforms significantly reduced responses to von Frey hair stimulation at 8 h and 1 day after facial carrageenan injection, but at 3 days after injection, only the sPLA2 inhibitor had an effect. Since BALB/c mice did not show increased responses after facial carrageenan injection, the reduction in responses actually indicates that there is loss of normal sensitivity to von Frey hair stimulation after intracerebroventricular injection of each of these inhibitors, in this strain of mice. The effects of PLA2 inhibitors are unlikely to be due simply to inhibition of arachidonic acid generation, since intracerebroventricular injection of arachidonic acid also had an anti-nociceptive effect. The above results support an important role of central nervous PLA2s in neurotransmission and pain transmission.

Nociception and the differential expression of cyclooxygenase-1 (COX-1), the COX-1 variant retaining intron-1 (COX-1v), and COX-2 in mouse dorsal root ganglia (DRG)

Prostaglandins (PGs) formed via the cyclooxygenase (COX) pathway mediate hyperalgesia in sensory nerve endings. To investigate the role of the COX isoforms in pain transmission we recently studied nociception in COX-isozyme-deficient mice using models of "sharp" rapidly transmitted pain (hot-plate) and slowly developing, diffuse pain (writhing) [Ballou L, Botting RM, Goorha S, Zhang J, Vane JR. Nociception in cyclooxygenase isozyme-deficient mice. Proc Natl Acad Sci USA 2000;97:10272]. Our results demonstrated that COX-1 (and not COX-2) was the primary isoform involved in nociception in both model systems. Given the importance of dorsal root ganglia (DRG) in pain transmission we examined the expression patterns of COX-1, -2 and the recently described variant of COX-1 retaining intron-1, originally referred to as "COX-3" but hereafter referred to as COX-1 variant (COX-1v), in mouse L4 or L5 DRG taken from normal and COX-isozyme-deficient mice. Messenger RNA and protein for COX isoforms from DRG, spinal cord as well as, heart, brain, kidney, spleen and skin of adult mice were isolated and analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis, respectively. Patterns of COX-isoform expression were determined using immunohistochemical techniques. We found that COX-1 and COX-1v were both expressed in neurons while COX-2 expression was completely undetectable in the DRG. Immunohistochemical analysis of COX expression in DRG of mice exhibiting the chronic pain and inflammation associated with collagen-induced arthritis (CIA) expressed COX-1 and COX-1v while no COX-2 could be detected. For purposes of comparison, COX-1v mRNA was also expressed in heart, brain, spinal cord, kidney, spleen and skin. Together, these data support a role for COX-1 and perhaps COX-1v, not COX-2, as the primary producers of PGs in mouse DRG in normal and in mice subject to chronic pain and inflammation. These data also suggest potential alternative analgesic mechanisms of action for the newly developed, COX-2 selective inhibitors and the nonsteroidal anti-inflammatory drugs (NSAIDs) in pain transmission in the peripheral nervous system.

Proteinase-activated receptor 2-mediated potentiation of transient receptor potential vanilloid subfamily 1 activity reveals a mechanism for proteinase-induced inflammatory pain

Proteinase-activated receptor (PAR) 2 is expressed on a subset of primary afferent neurons and involved in inflammatory nociception. Transient receptor potential vanilloid subfamily 1 (TRPV1) is a sensory neuron-specific cation channel that responds to capsaicin, protons, or heat stimulus. Here, we show that TRPV1 is coexpressed with PAR2 but not with PAR1 or PAR3, and that TRPV1 can functionally interact with PAR2. In human embryonic kidney 293 cells expressing TRPV1 and PAR2, PAR2 agonists increased capsaicin- or proton-evoked TRPV1 currents through a PKC-dependent pathway. After application of PAR2 agonists, temperature threshold for TRPV1 activation was reduced from 42 degrees C to well below the body temperature. PAR2-mediated Fos expression in spinal cord was decreased in TRPV1-deficient mice. The functional interaction was also observed in mouse DRG neurons and proved at a behavioral level. These represent a novel mechanism through which trypsin or tryptase released in response to tissue inflammation might trigger the sensation of pain by PAR2 activation.

Chronic morphine-induced loss of the facilitative interaction between vasoactive intestinal polypeptide and delta-opioid: involvement of protein kinase C and phospholipase Cbetas

This laboratory recently demonstrated a multiplicative interaction between the pelvic visceral afferent transmitter vasoactive intestinal polypeptide (VIP) and the delta-opioid receptor (DOR)-selective agonist [D-Pen2,5] enkephalin (DPDPE) to regulate cAMP levels in spinal cord [Brain Res. 959 (2003) 103]. Although DOR activation is required for the manifestation of the VIP-DPDPE facilitative interaction, its relevance to opioid antinociception remains unclear. The current study investigates whether or not the VIP-DPDPE facilitation of cAMP formation is subject to tolerance formation, a hallmark characteristic of opioid antinociception. Chronic morphine exposure abolishes the VIP-DPDPE facilitative interaction, consistent with its relevance to DOR antinociception. However, acute in vitro inhibition of protein kinase C (PKC) reinstates the VIP-DPDPE multiplicative interaction characteristic of opioid naïve spinal tissue. This suggests that its chronic morphine-induced loss requires a PKC phosphorylation. PKC phosphorylation negatively modulates phospholipase C (PLC)beta, enzymes intimately associated with phosphoinositide turnover and calcium trafficking. These are essential determinants of acute and chronic opioid effects. Accordingly, the effect of chronic morphine on their state of phosphorylation was also investigated. Central nervous system opioid tolerance is associated with the reciprocal phosphorylation (regulation) of two PLCbeta isoforms, PLCbeta1 and PLCbeta3. However, although chelerythrine reinstates the chronic morphine-induced loss of the multiplicative VIP-DPDPE interaction, it does not alter the associated changes in PLCbeta phosphorylation, possibly indicating different time courses of restitution of function and/or involvement of different kinases for different components of tolerance. These results could provide a mechanistic rubric for understanding positive modulation of opioid antinociception by afferent transmission.

Impact of neuropathic pain on the gene expression and activity of cytochrome P450side-chain-cleavage in sensory neural networks

Development of efficient therapy against chronic and stubborn pains requires fundamental identification of adequate cellular and molecular targets. This study combined cellular, molecular and biochemical approaches to investigate the gene expression and enzymatic activity of cytochrome P450side-chain-cleavage (P450scc) in spinal neural networks under normal and neuropathic pain states. P450scc is the key onset enzyme for steroidogenesis in endocrine glands and for neurosteroid biosynthesis in nerve cells. The P450scc gene was over-expressed in spinal and supra-spinal networks during neuropathic pain provoked by sciatic nerve ligature. Plasticity was observed in P450scc cellular distribution in pain circuits and its activity also increased inducing in vivo, hyper-secretion of pregnenolone and allopregnanolone which strongly stimulates type A receptors for g-aminobutyric acid, a pivotal neurotransmitter involved in pain modulation. These results, by establishing a direct link between neuropathic pain and neuroactive steroid formation in the nervous system, open new perspectives for chronic-pain modulation by endogenous neurosteroids.

Roles for the kallikrein-kinin system in inflammatory exudation and pain: lessons from studies on kininogen-deficient rats

Roles for the kallikrein-kinin system in inflammation have been investigated extensively, and many reviews on this topic have been published during the 50 years since the discovery of bradykinin in 1949. Recent progress in the field has been remarkable with the help of experiments using gene-targetted transgenic or knockout mice, which have added further valuable information in addition to previous results obtained from pharmacological and biochemical studies using purified and isolated components of the system. Furthermore, much knowledge has been accumulated as a result of the development of various bradykinin agonists and antagonists. In this review, we focused on the data obtained from the kininogen-deficient rat, which is a natural mutant, and discuss the results in comparison with those from bradykinin receptor knockout mice. These data have clarified that endogenous bradykinin exerts a most important role in inflammatory exudation along with prostanoids, preferentially to histamine, serotonin, or neuropeptides. In inflammatory pain perception also, bradykinin produced in the local perivascular spaces stimulates polymodal pain receptors in conjunction with co-helpers such as prostanoids, vanilloids, and neuropeptides. These important roles are concluded based on consistent results obtained from experiments using several antagonists of bradykinin, kininogen-deficient rats, and bradykinin receptor knockout mice.

Mice lacking fatty acid amide hydrolase exhibit a cannabinoid receptor-mediated phenotypic hypoalgesia

Although the N-arachidonoyl ethanolamine (anandamide) binds to cannabinoid receptors and has been implicated in the suppression of pain, its rapid catabolism in vivo by fatty acid amide hydrolase (FAAH) has presented a challenge in investigating the physiological functions of this endogenous cannabinoid. In order to test whether anandamide and other non-cannabinoid fatty amides modulate nociception, we compared FAAH (+/+) and (-/-) mice in the tail immersion, hot plate, and formalin tests, as well as for thermal hyperalgesia in the carrageenan and the chronic constriction injury (CCI) models. FAAH (-/-) mice exhibited a CB1 receptor-mediated phenotypic hypoalgesia in thermal nociceptive tests. These mice also exhibited CB1 receptor-mediated hypoalgesia in both phases of the formalin test accompanied with a phenotypic anti-edema effect, which was not blocked by either CB1 or CB2 antagonists. Additionally, FAAH (-/-) mice displayed thermal anti-hyperalgesic and anti-inflammatory effects in the carrageenan model that were mediated, in part, by CB2, but not CB1 receptors. In contrast, no genotype differences in pain behavior were evident following CCI, which was instead found to obliterate the phenotypic hypoalgesia displayed by FAAH (-/-) mice in the tail immersion and hot plate tests, suggesting that nerve injury may promote adaptive changes in these animals. Collectively, these findings demonstrate a cannabinoid receptor-mediated analgesic phenotype in FAAH (-/-) mice. In more general terms, these findings suggest that selective inhibitors of FAAH might represent a viable pharmacological approach for the clinical treatment of pain disorders.

[Effect of a single emotional-pain stress on the basic carboxypeptidases activity in the rat brain regions and adrenals]

Effect of a single emotional-algesic stress on the carboxypeptidase H and PMSF-inhibited carboxypeptidase activities (phenylmethylsulphonilfluoride-inhibited carboxypeptidase) taking part in the final stage of formation of biologically active neuropeptides from precursors, was studied. Activity of the enzymes depended on duration of the stress and time after the stress in pituitary and adrenal glands. Differences in changes of carboxypeptidase H and PMSF-inhibited carboxypeptidase activities was found, especially in adrenals. The role of enzymes in emotional-algesic stress development and in metabolism of regulatory peptides by the stress was discussed.

Modulation of visceral pain and inflammation by protease-activated receptors

The gastrointestinal (GI) tract is exposed to a large array of proteases, under both physiological and pathophysiological conditions. The discovery of G protein-coupled receptors activated by proteases, the protease-activated receptors (PARs), has highlighted new signaling functions for proteases in the GI tract, particularly in the domains of inflammation and pain mechanisms. Activation of PARs by selective peptidic agonists in the intestine or the pancreas leads to inflammatory events and changes in visceral nociception, suggesting that PARs could be involved in the modulation of visceral pain and inflammation. PARs are present in most of the cells that are potentially actors in the generation of irritable bowel syndrome (IBS) symptoms. Activation of PARs interferes with several pathophysiological factors that are involved in the generation of IBS symptoms, such as altered motility patterns, inflammatory mediator release, altered epithelial functions (immune, permeability and secretory) and altered visceral nociceptive functions. Although definitive studies using genetically modified animals, and, when available, pharmacological tools, in different IBS and inflammatory models have not yet confirmed a role for PARs in those pathologies, PARs appear as promising targets for therapeutic intervention in visceral pain and inflammation processes.

Nonnucleoside Inhibitors of Adenosine Kinase

Adenosine (ADO) is an endogenous inhibitory neuromodulator that increases nociceptive thresholds in response to tissue trauma and inflammation. Adenosine kinase (AK) is a key intracellular enzyme regulating intra- and extracellular concentrations of ADO. AK inhibition selectively amplifies extracellular ADO levels at cell and tissue sites where accelerated release of ADO occurs. AK inhibitors have been shown to provide effective antinociceptive, antiinflammatory and anticonvulsant activity in animal models, thus suggesting their potential therapeutic utility for pain, inflammation, epilepsy and possibly other central and peripheral nervous system diseases associated with cellular trauma and inflammation. This beneficial outcome may potentially lack nonspecific effects associated with the systemic administration of ADO receptor agonists. Until recently all of the reported AK inhibitors contained adenosine-like structural motif. The present review will discuss design, synthesis and analgesic and antiinflammatory properties of the novel nonnucleoside AK inhibitors that do not have close structural resemblance with the natural substrate ADO. Two classes of the nonnucleoside AK inhibitors are built on pyridopyrimidine and alkynylpyrimidine cores.

Genetic Testing for Enzymes of Drug Metabolism: Does It Have Clinical Utility for Pain Medicine at the Present Time? A Structured Review

STUDY DESIGN

This is a structured review of genomic (genetic) testing for enzymes of drug metabolism.

OBJECTIVES

Recently, industry began offering genomic testing for enzymes of drug metabolism. As such, the objective of this review was to determine if genomic testing for enzymes of drug metabolism has any imminent clinical relevance for the practice of pain medicine.

METHODS

Relevant references relating to pharmacogenetics, pharmacogenomics, and the metabolizing of drugs used in pain medicine by cytochrome P-450 enzymes were located and reviewed in detail. The P-450 enzymes that metabolize each drug and whether that drug had been identified as being subject to a clinical consequence of a genetic polymorphism of the P-450 enzyme involved in its metabolism were placed into tabular form.

RESULTS OF DATA SYNTHESIS

1) For a large number of drugs, we do not yet know which cytochrome P-450 enzymes are involved in their metabolism; 2) For a large number of drugs, the consequences of a P-450 genetic polymorphism have yet to be determined; 3) Genetic polymorphism can lead to important potential clinical consequences for some opioids, anticonvulsants (phenytoin), benzodiazepines (diazepam), muscle relaxants (succinylcholine), antidepressants (imipramine, nortriptyline, venlafaxine), typical neuroleptics, alcohol, antihypertensives (propranolol, timolol), local anesthetics (procainamide), L-dopa, nicotine, and warfarin. Based on these results, factors for and against using genomic testing were reviewed.

CONCLUSIONS/RECOMMENDATIONS

It was concluded that genomic testing for enzymes of drug metabolism has significant potential for improving the efficacy of drug treatment and reducing adverse drug reactions. Recommendations for when such testing would be useful are outlined.

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.

Protein kinases regulate the phosphorylation of the GluR1 subunit of AMPA receptors of spinal cord in rats following noxious stimulation

The present project was designed to investigate the role of protein kinase A (PKA) and protein kinase C (PKC) in the regulation of phosphorylation of the GluR1 subunits of AMPA receptors in the spinal cord of rats after capsaicin injection. We found that after capsaicin injection, a significant upregulation of phosphorylated GluR1 both at Ser(831) and Ser(845) was detected on the side ipsilateral to the injection. Intrathecal treatment with a PKA inhibitor, H89 ([N-[2-((3-bromophenyl)-2-propenyl)amino)ethyl]-5-isoquinoline sulfonamide, HCl), or a PKC inhibitor, NPC15473 (2,6-diamino-N-([1-oxotridecyl)-2-piperidinyl]methyl)hexanamide), significantly blocked the increased phosphorylation at different serine sites without affecting the GluR1 protein itself. Our results suggest that increased phosphorylation of the GluR1 subunit of AMPA receptors contributes to central sensitization following acute peripheral inflammation, and the effect may occur at different phosphorylation sites through the activation of the PKA or PKC protein kinase cascades.

A Newly Identified Role for Superoxide in Inflammatory Pain

Novel classes of pain-relieving molecules are needed to fill the void between nonsteroidal anti-inflammatory agents and narcotics. Our studies have identified superoxide as a novel mediator of hyperalgesia (clinically defined as an augmented sensitivity to painful stimuli) and have exposed potential pathways through which this radical modulates the hyperalgesic response. The role of superoxide in pain was elucidated using a superoxide dismutase mimetic, M40403 [a manganese(II) complex with a bis(cyclo-hexylpyridine-substituted) macrocyclic ligand]. Intraplantar injection of carrageenan in rats led to time-dependent development of peripheral inflammation [measured parameters of inflammation included paw edema, cytokine release in the paw exudates, nitrotyrosine formation (a marker of peroxynitrite formation and oxidative stress), and poly-ADP-ribose-polymerase activation (the nuclear enzyme activated by superoxide/peroxynitrite)] and hyperalgesia. M40403 blocked all measured parameters of inflammation and hyperalgesia. Furthermore, when given therapeutically (2 h after the induction of hyperalgesia) either by intravenous or intrathecal administration, M40403 but not its inactive congener M40404 inhibited hyperalgesia with a rapid onset of action. Our results also show that, at the level of the spinal cord and time of peak hyperalgesia, endogenous manganese superoxide dismutase was nitrated and subsequently deactivated, losing its capacity to remove superoxide. The antihyperalgesic effects of M40403 were not reversed by naloxone excluding the potential involvement of an opiate pathway. Collectively, these studies have unraveled a critical role for superoxide in the nociceptive signaling cascade both peripherally and centrally. The discovery of this pathway opens a new therapeutic strategy for the development of novel nonnarcotic antihyperalgesic agents.

The effects of protein phosphatase inhibitors on nociceptive behavioral responses of rats following intradermal injection of capsaicin

The functions of crucial proteins in the nervous system are modulated by kinases and phosphatases which catalyze opposing reactions of phosphorylation and dephosphorylation. During spinal cord central sensitization, serine/threonine protein phosphatase 2A (PP2A) may play an important role in determining the excitability of nociceptive neurons in the spinal cord by modulating the phosphorylation state of some critical proteins. The effects of a general inhibitor of PP2A, okadaic acid (OA), and a specific inhibitor, fostriecin, on the behavioral responses of rats following capsaicin injection were investigated in this study. Hyperalgesia was initiated by injection of capsaicin into the plantar surface of the hindpaw of rats. An intrathecal catheter was previously implanted into the subarachnoid space of the spinal cord for the administration of a variety of drugs. Rats were tested for responses to mechanical stimuli using von Frey filaments of different bending forces applied at a site outside the area of injection. Responses to heat stimuli were detected from a site near the injection area. The responses were recorded before and after injection of capsaicin with the perfusion of ACSF, OA negative control, OA or fostriecin at different time points. The results demonstrated that secondary mechanical hyperalgesia and allodynia can be induced by the intradermal injection of capsaicin. Compared to administration of ACSF or the OA negative control, infusion of the phosphatase inhibitor OA or of fostriecin into the subarachnoid space enhanced the secondary mechanical hyperalgesia and allodynia by making the intradermal capsaicin-induced hyperalgesia and allodynia last longer.

Activation of p38 mitogen-activated protein kinase in spinal hyperactive microglia contributes to pain hypersensitivity following peripheral nerve injury

Neuropathic pain is an expression of pathological operation of the nervous system, which commonly results from nerve injury and is characterized by pain hypersensitivity to innocuous stimuli, a phenomenon known as tactile allodynia. The mechanisms by which nerve injury creates tactile allodynia have remained largely unknown. We report that the development of tactile allodynia following nerve injury requires activation of p38 mitogen-activated protein kinase (p38MAPK), a member of the MAPK family, in spinal microglia. We found that immunofluorescence and protein levels of the dually phosphorylated active form of p38MAPK (phospho-p38MAPK) were increased in the dorsal horn ipsilateral to spinal nerve injury. Interestingly, the phospho-p38MAPK immunofluorescence in the dorsal horn was found exclusively in microglia, but not in neurons or astrocytes. The level of phospho-p38MAPK immunofluorescence in individual microglial cells was much higher in the hyperactive phenotype in the ipsilateral dorsal horn than the resting one in the contralateral side. Intrathecal administration of the p38MAPK inhibitor, 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580), suppresses development of the nerve injury-induced tactile allodynia. Taken together, our results demonstrate that nerve injury-induced pain hypersensitivity depends on activation of the p38MAPK signaling pathway in hyperactive microglia in the dorsal horn following peripheral nerve injury.

Protein kinase C epsilon and gamma: involvement in formalin-induced nociception in neonatal rats

The central nervous system undergoes dynamic changes as it matures. However, until recently, very little was known about the impact of these changes on pain and analgesia. This study tested the hypothesis that the epsilon and gamma isozymes of protein kinase C (PKC) contribute to formalin-induced nociception in an age-dependent manner. Expression of epsilon and gamma PKC and the contributions of these isozymes in formalin-induced nociception was examined in postnatal day 7, 15, and 21 rats. epsilonPKC expression in dorsal root ganglion neurons and gammaPKC expression in lamina II of the spinal cord increased from the first to the third postnatal week. Coupling immunohistochemical and Western analysis, translocation of epsilonPKC followed intraplantar formalin in all ages. In contrast, formalin-induced gammaPKC translocation was observed only in postnatal day 21 rats. Behaviorally, intrathecal administration of the epsilonPKC-specific inhibitor (epsilonV1-2) attenuated phase 1 and phase 2 formalin behaviors at all ages. In contrast, intrathecal administration of the gammaPKC-specific inhibitor (gammaV5-3) attenuated only phase 2 responses in postnatal day 15 and 21 rats. Functionally, inhibition of epsilonPKC decreased capsaicin-stimulated release of glutamate and calcitonin gene-related peptide in spinal cords isolated from postnatal day 7 rats. These results suggest that epsilonPKC age independently mediates inflammatory pain produced by intraplantar formalin. In contrast, gammaPKC contributes to formalin-induced nociception in an age-dependent manner. Identifying the molecular mechanisms responsible for age-specific patterns of nociception is necessary for the rational development of novel therapeutic strategies for treating pediatric pain.

Cyclooxygenase 2 in infiltrating inflammatory cells in injured nerve is universally up-regulated following various types of peripheral nerve injury

We previously reported the up-regulation of cyclooxygenase 2 (COX2) in injured sciatic nerve of rats with partial sciatic nerve ligation (PSNL) and the reversal of PSNL-elicited tactile allodynia by local injection of the COX inhibitor ketorolac [Eur J Neurosci 15 (2002) 1037]. We further asked whether COX2 up-regulation in injured nerve is a universal phenomenon following various types of nerve injury. In the current study, we observed that abundant COX2 immunoreactive (IR) cell profiles appeared in injured nerves of rats following spinal nerve ligation (SNL), chronic constriction injury (CCI) and complete sciatic nerve transection. Most COX2-IR cells were identified as infiltrating macrophages. Partial injury induced greater COX2 up-regulation than complete injury. COX2 up-regulation reached a peak at 2-4 weeks, evidently declined by 3 months and disappeared by 7 months postlesion. These findings suggest that up-regulation of COX2 in injured nerve is a common event during the initial several months after nerve injury. We observed that local ketorolac-elicited anti-allodynia was closely associated with the abundance of COX2-IR cells in injured nerve, varying with the type of injury and time after injury. The anti-allodynia lasted the longest when local ketorolac was given 2-4 weeks after PSNL, CCI and SNL. The duration of local ketorolac's anti-allodynia was the longest in CCI rats, which also exhibited the most abundant COX2 up-regulation. Local ketorolac's anti-allodynia lasted much shorter when given 2-3 months after lesion. Local ketorolac failed to induce anti-allodynia 7 months after lesion, a time when COX2-IR cells completely disappeared from the injured nerve except a few cells at the injury site. Our data strongly suggest that during the initial several months after nerve injury, peripherally over-produced prostaglandins play an important role in the maintenance of neuropathic pain.

Mitogen-activated protein kinases as potential targets for pain killers

Pathological pain, such as inflammatory and neuropathic pain, is an expression of neural plasticity. Mitogen-activated protein kinases (MAPKs) play an important role in neural plasticity via post-translational, translational and transcriptional regulation. Under conditions of tissue and nerve damage, extracellular signal-regulated kinase (ERK) and p38 MAPK can be activated by nociceptive activity and inflammatory mediators in primary sensory neurons in the peripheral nervous system, and spinal cord neurons and glia in the central nervous system. Activation of ERK in dorsal horn neurons is nociceptive-specific and suppressed by several analgesics, and therefore has potential for the development of an assay to test the efficacy of new analgesics. Inhibition of ERK or p38 alleviates inflammatory pain and neuropathic pain in animal models. Development of specific inhibitors for these two MAPKs may lead to new therapies for pathological pain.

[Use of non steroid anti-inflammatory drugs in modern clinical practice]

The article presents the review of current non steroid anti-inflammatory drugs, their mechanisms, principles of their modern classification and use in internal diseases practice. DICLOBERL has been found to be the drug of choice in acute and chronic pain management. A wide variety of the preparation of the medication allows us to have the most effective and safety form to guarantee good results.

Increased level of neuronal phosphoinositide 3-kinase γ by the activation of μ-opioid receptor in the mouse periaqueductal gray matter: further evidence for the implication in morphine-induced antinociception

It has been proposed that phosphoinositide 3-kinase (PI3K), one of the phosphatidylinositol kinases, can be regulated by G-protein-coupled receptor as well as nerve growth factor-associated receptors. The aim of the present study was to investigate whether in vivo treatment with morphine, a mu-opioid receptor (MOR) agonist, could directly regulate PI3Kgamma isoform in the mouse periaqueductal gray matter (PAG). Using the polyclonal antibody recognizing a p110gamma catalytic subunit of PI3Kgamma, PI3Kgamma-like immunoreactivity (IR) was mostly seen in the membrane of the cell labeled by anti-neuron-specific nuclear protein. A single s.c. injection of morphine caused a marked increase in the number of PI3Kgamma-IR expressing cells in the PAG. Double immunolabeling assay showed that MOR-IR was mostly overlapped with PI3Kgamma-IR on the cell surface in the PAG section. Additionally, phosphorylated-phospholipase Cgamma1 (PLCgamma1-IR) was highly detected in the membrane compartment of the increased PI3Kgamma-IR-positive cells of this region. Further pharmacological evidence for the critical role of PI3Kgamma in MOR-mediated antinociceptive response was provided by the warm-plate test. The dose-response lines for antinociceptive effects of morphine were significantly shifted to the right following i.c.v. treatment with PI3K inhibitors. These findings suggested that acute treatment with morphine may directly activate the PI3Kgamma/PLCgamma1 pathway in the PAG. This effect may, at least in part, result in the expression of morphine-induced pharmacological actions including antinociception in mice.

Nitric oxide synthase and soluble guanylate cyclase are involved in spinal cord wind-up activity of monoarthritic, but not of normal rats

While increasing evidence points to a role for the nitric oxide (NO)/cyclic guanosine 3,5-monophosphate (GMPc) cascade in hyperalgesia and allodynia, participation of the NO/GMPc pathway in synaptic processing in the spinal cord, i.e. wind-up activity, is less clear. We studied the effects of intrathecal administration of Nomega-nitro-L-arginine methyl ester (L-NAME) and methylene blue, inhibitors of NO synthase and guanylate cyclase respectively, on wind-up activity developed in a C-fiber reflex response paradigm. 5, 10 and 20 microg i.t. of L-NAME or methylene blue did not modify spinal wind-up in normal rats, while a dose-dependent inhibition of wind-up was observed in monoarthritic rats. Results suggest that the NO/GMPc pathway plays a non-significant role in wind-up activity evoked in normal animals, while it may be essential in chronic pain processing.

Spinal upregulation of the nitric oxide synthase-interacting protein NOSIP in a rat model of inflammatory pain

Nitric oxide and nitric oxide synthases are key players in synaptic plasticity events in the spinal cord, which underly the development of chronic pain states. To date, little is known about the molecular mechanisms regulating the activity of nitric oxide synthases in nociceptive systems. The present study was aimed at the immunohistochemical determination of the expression of a nitric oxide synthase-interacting protein (NOSIP) in the rat spinal cord and dorsal root ganglia and studying its regulation in states of nociceptive hypersensitivity in a rat model of post-inflammatory pain. NOSIP is predominantly expressed in nociceptive primary neurons and in neurons of the spinal dorsal horn and the number of NOSIP-positive spinal neurons increases significantly following induction of unilateral intraplantar injection of complete Freund's adjuvant. Thus, NOSIP may modulate nitric oxide homeostasis in physiological and pathological pain conditions.

Immunocytochemical localization of extracellular signal-regulated kinases 1 and 2 phosphorylated neurons in the brainstem of rat following visceral noxious stimulation

The aim of the present study was to determine the activation of the extracellular signal-regulated kinases (ERKs) 1 and 2 in brainstem neurons following noxious visceral stimulation. Ether and urethane anaesthetized rats received an intraperitoneal injection of acetic acid (ENS, UNS) or were left untreated (ECT, UCT). Paraffin embedded brain sections immunoreacted with an antibody specific for phosphorylated ERKs. In noxious stimulated rats ERKs activated neuron profiles in the periaqueductal gray matter, parabrachial, dorsal raphe, solitary tract nucleus, area postrema and superior colliculus suggest that ERKs activation takes place mainly in brainstem nuclei in which nociception and visceral activities interact. The comparison between ENS and UNS rats shows that the long acting anaesthetic urethane attenuates the number of the ERKs activated neurons compared to the short acting ether.

Suppression of the morphine-induced rewarding effect and G-protein activation in the lower midbrain following nerve injury in the mouse: involvement of G-protein-coupled receptor kinase 2

The present study was designed to investigate whether a state of neuropathic pain induced by sciatic nerve ligation could alter the rewarding effect, antinociception, and G-protein activation induced by a prototype of mu-opioid receptor agonist morphine in the mouse. The sciatic nerve ligation caused a long-lasting and profound thermal hyperalgesia. Under this neuropathic pain-like state, an i.c.v. morphine-induced place preference was observed in sham-operated mice but not in sciatic nerve-ligated mice. However, no differences in the antinociceptive effect of i.c.v.-administered morphine were noted between the groups. The increases in the binding of guanosine-5'-o-(3-[(35)S]thio)triphosphate induced by morphine in lower midbrain membranes including the ventral tegmental area, which contributes to the expression of the rewarding effect of opioid, were significantly attenuated in sciatic nerve-ligated mice. On the other hand, there were no differences in the stimulation of guanosine-5'-o-(3-[(35)S]thio)triphosphate binding to pons/medulla membranes, which plays an important role in the antinociception of mu-opioid receptor agonists, between the groups. In addition, no changes in levels of guanosine-5'-o-(3-[(35)S]thio)triphosphate binding by either the selective delta- or kappa-opioid receptor agonists were noted in membrane of the lower midbrain and limbic forebrain membranes obtained from sciatic nerve-ligated mice. Reverse transcription-polymerase chain reaction analysis showed that sciatic nerve ligation did not alter the mRNA product of mu-opioid receptors in the lower midbrain, indicating that a decrease in some mu-opioid receptor functions may result from the uncoupling of mu-opioid receptors from G-proteins. We found a significant increase in protein levels of G-protein-coupled receptor kinase 2, which causes receptor phosphorylation in membranes of the lower midbrain but not in the pons/medulla, obtained from mice with nerve injury, whereas there were no changes in the protein level of phosphorylated-protein kinase C in the lower midbrain. These results suggest that the uncoupling of mu-opioid receptors from G-proteins by G-protein-coupled receptor kinase 2 in the lower midbrain may, at least in part, contribute to the suppression of the rewarding effect of morphine under neuropathic pain.

p38 mitogen-activated protein kinase is activated after a spinal nerve ligation in spinal cord microglia and dorsal root ganglion neurons and contributes to the generation of neuropathic pain

The possible involvement of p38 mitogen-activated protein kinase activation in spinal cord and dorsal root ganglion (DRG) cells in the development of peripheral neuropathic pain has been explored. Ligation of the L5 spinal nerve (SNL) on one side in adult rats produces an early onset and long-lasting mechanical allodynia. This lesion results in activation of p38 in the L5 segment of the spinal cord, most prominently in the ipsilateral dorsal horn, starting soon after the lesion (<1 d) and persisting for >3 weeks. The activated p38 in the spinal cord is restricted entirely to microglia; phospho-p38 colocalizes only with the microglial marker OX-42 and not with either the neuronal marker neuronal-specific nuclear protein or the astrocyte marker GFAP. In contrast, SNL induces a delayed (>3 d) activation of p38 in the L5 DRG that occurs predominantly in neurons. Continuous injection of the p38 inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580) via the intrathecal route, starting before the SNL surgery, reduces SNL-induced mechanical allodynia from day 1 to day 10, with maximal effects at early time points. Post-treatment with SB203580 starting on day 1 or on day 10 after surgery also reduces established mechanical allodynia. Because the reduction in neuropathic pain by p38 inhibition occurs before the appearance of p38 activation in DRG neurons, p38 activation in spinal cord microglia is likely to have a substantial role in the earliest phase of neuropathic pain. Coactivation of p38 in DRG neurons and spinal microglia may contribute to later phases of neuropathic pain.

Role of protein kinase A in phosphorylation of NMDA receptor 1 subunits in dorsal horn and spinothalamic tract neurons after intradermal injection of capsaicin in rats

Protein phosphorylation is a major mechanism for regulation of N-methyl-D-aspartate (NMDA) receptor function. The NMDA receptor 1 subunit (NR1) is phosphorylated by protein kinase A (PKA) on serine 890 and 897. We have recently reported that there is enhanced phosphorylation of NR1 on serine 897 in dorsal horn and spinothalamic tract (STT) neurons after intradermal injection of capsaicin (CAP) in rats [Zou et al. (2000) J. Neurosci. 20, 6989-6997]. Whether or not this phosphorylation, which develops during central sensitization following CAP injection, is mediated by PKA remains to be determined. In this study, western blots and immunofluorescence staining were employed to observe if pretreatment with a PKA inhibitor, N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, HCl (H89), blocks the enhanced phosphorylation of NR1 on serine 897 following injection of CAP into the glabrous skin of one hind paw of anesthetized rats. Western blots showed that pretreatment with H89 caused a decrease in CAP-induced phosphorylation of NR1 protein in spinal cord segments L(4)-S(1). In experiments using immunofluorescence staining, the numbers of phospho-NR1-like immunoreactive (p-NR1-LI) neurons seen after CAP injection were significantly decreased in the dorsal horn of the L(4)-L(5) segments on the side ipsilateral to the injection after PKA was inhibited. When STT cells were labeled by microinjection of the retrograde tracer, fluorogold, we found that the proportion of p-NR1-LI STT cells on the side ipsilateral to the injection in the superficial laminae of spinal cord segments L(4)-L(5) was markedly reduced when H89 was administered intrathecally before CAP injection. However, the proportion of p-NR1-LI STT cells in deep laminae was unchanged unless the PKC inhibitor, chelerythrine chloride, was co-administered with H89. Combined with our previous findings, the present results indicate that NR1 in spinal dorsal horn neurons, including the superficial dorsal horn STT cells, is phosphorylated following CAP injection and that this phosphorylation is due to the action of PKA. However, the phosphorylation of deep STT cells involves both PKA and PKC.

[Activity of DPP III in human cerebrospinal fluid derived from patients with pain]

BACKGROUND

In the course of elucidating physiological roles of spinorphin, an endogenous regulator of enkephalin-degrading enzyme, we found that dipeptidyl peptidase III (DPP III) was contained in human cerebrospinal fluid (CSF). In this study, we examined the activity of DPP III in human CSF derived from patients with pain.

METHODS

We used human CSF as specimen collected during operations under spinal anesthesia. Patients were divided into three groups, patients without pain (normal group), with acute pain, and with chronic pain. We measured DPP III activities in each group, using specific substrates, such as Arg-Arg-AMC.

RESULTS

DPP III activity for human CSF in patients with acute pain was significantly lower compared with that in patients without pain (P < 0.05). Furthermore, DPP III activity correlated with APN activity, another spinorphin-degrading enzyme, in patients with pain (r = 0.444).

CONCLUSION

These results indicate that DPP III may play a role in regulation of endogenous opioids, leading to pain modulation.

Effect of hyperbaric oxygen therapy on exercise-induced muscle soreness

The purpose of this study was to examine the effects of HBO2 therapy on exercise-induced muscle soreness. Subjects (n = 6 male and 10 female university student volunteers) were randomly divided into an experimental group that received HBO2 therapy and a control group that did not receive any treatments. HBO2 treatments consisted of 5 sessions of breathing 95% oxygen at 2.5 atm abs for 100 min. Temporary muscle soreness was created using a single-leg eccentric exercise task involving the quadriceps femoris. Over the next 14 days, measurements were obtained on muscle soreness, leg circumference, quadriceps peak torque, quadriceps average power, fatigue and plasma creatine kinase. After eccentric exercise, plasma creatine kinase (CK) levels and perceived muscle soreness were elevated but were not different between HBO2 and control groups. HBO2 therapy did not alter leg circumference, quadriceps peak torque, average power or fatigue compared to the control group. Faster recovery was observed in the HBO2 group on day 3 following the exercise protocol with perceived muscle soreness still elevated for the control group but not different from baseline for the HBO2 group. The data indicated that five HBO2 treatments did not speed recovery following eccentric exercise that induced temporary muscle soreness.

Simultaneous reduction in cancer pain, bone destruction, and tumor growth by selective inhibition of cyclooxygenase-2

More than half of all chronic cancer pain arises from metastases to bone, and bone cancer pain is one of the most difficult of all persistent pain states to fully control. Several tumor types including sarcomas and breast, prostate, and lung carcinomas grow in or preferentially metastasize to the skeleton where they proliferate, and induce significant bone remodeling, bone destruction, and cancer pain. Many of these tumors express the isoenzyme cycloxygenase-2 (COX-2), which is involved in the synthesis of prostaglandins. To begin to define the role COX-2 plays in driving bone cancer pain, we used an in vivo model where murine osteolytic 2472 sarcoma cells were injected and confined to the intramedullary space of the femur in male C3HHeJ mice. After tumor implantation, mice develop ongoing and movement-evoked bone cancer pain-related behaviors, extensive tumor-induced bone resorption, infiltration of the marrow space by tumor cells, and stereotypic neurochemical alterations in the spinal cord reflective of a persistent pain state. Thus, after injection of tumor cells, bone destruction is first evident at day 6, and pain-related behaviors are maximal at day 14. A selective COX-2 inhibitor was administered either acutely [NS398; 100 mg/kg, i.p.] on day 14 or chronically in chow [MF. tricyclic; 0.015%, p.o.] from day 6 to day 14 after tumor implantation. Acute administration of a selective COX-2 inhibitor attenuated both ongoing and movement-evoked bone cancer pain, whereas chronic inhibition of COX-2 significantly reduced ongoing and movement-evoked pain behaviors, and reduced tumor burden, osteoclastogenesis, and bone destruction by >50%. The present results suggest that chronic administration of a COX-2 inhibitor blocks prostaglandin synthesis at multiple sites, and may have significant clinical utility in the management of bone cancer and bone cancer pain.

Response to exercise of patients with idiopathic hyper-CK-emia

Patients with an idiopathic increase in serum creatine kinase (CK) levels (hyper-CK-emia) have a benign prognosis, but symptoms may be disabling in daily life. Previous studies have suggested that physical exercise increases the severity of complaints in these patients. We studied whether maximal and submaximal bouts of exercise on a cycle ergometer are harmful for patients with idiopathic hyper-CK-emia. Such dynamic exercise did not lead to larger increases in serum CK activity or more complaints in 11 patients with idiopathic hyper-CK-emia, compared with 11 age-matched healthy controls. Our data suggest that exercise does not result in more extensive muscle damage in patients with idiopathic hyper-CK-emia than in healthy subjects.

Genetic elimination of behavioral sensitization in mice lacking calmodulin-stimulated adenylyl cyclases

Adenylyl cyclase types 1 (AC1) and 8 (AC8), the two major calmodulin-stimulated adenylyl cyclases in the brain, couple NMDA receptor activation to cAMP signaling pathways. Cyclic AMP signaling pathways are important for many brain functions, such as learning and memory, drug addiction, and development. Here we show that wild-type, AC1, AC8, or AC1&8 double knockout (DKO) mice were indistinguishable in tests of acute pain, whereas behavioral responses to peripheral injection of two inflammatory stimuli, formalin and complete Freund's adjuvant, were reduced or abolished in AC1&8 DKO mice. AC1 and AC8 are highly expressed in the anterior cingulate cortex (ACC), and contribute to inflammation-induced activation of CREB. Intra-ACC administration of forskolin rescued behavioral allodynia defective in the AC1&8 DKO mice. Our studies suggest that AC1 and AC8 in the ACC selectively contribute to behavioral allodynia.

COX-2 inhibition and the control of pain

The discovery of the two isoenzymes of cyclooxygenase COX-1 and COX-2 and their separate functions, localization and regulation, has initiated the search for new and more selective inhibitors of prostaglandin biosynthesis. Selective COX-2 inhibitors were developed in order to improve an anti-inflammatory and analgesic specificity and potency. The role of inducible COX-2 at the peripheral site of inflammation is well known. The discovery of COX-2 in the spinal cord suggests that it is responsible for spinal prostaglandin release in nociceptive processes following a peripheral inflammatory stimulus. In the future, selective COX-2 inhibitors such as celecoxib (GD Searle & Co), rofecoxib (Merck & Co Inc) and the recently developed etoricoxib (Merck & Co Inc) may play an important role in the treatment of a wider range of pain conditions in addition to their present use as anti-inflammatory and analgesic drugs.

cAMP-dependent protein kinase regulates desensitization of the capsaicin receptor (VR1) by direct phosphorylation

The capsaicin receptor, VR1 (also known as TRPV1), is a ligand-gated ion channel expressed on nociceptive sensory neurons that responds to noxious thermal and chemical stimuli. Capsaicin responses in sensory neurons exhibit robust potentiation by cAMP-dependent protein kinase (PKA). In this study, we demonstrate that PKA reduces VR1 desensitization and directly phosphorylates VR1. In vitro phosphorylation, phosphopeptide mapping, and protein sequencing of VR1 cytoplasmic domains delineate several candidate PKA phosphorylation sites. Electrophysiological analysis of phosphorylation site mutants clearly pinpoints Ser116 as the residue responsible for PKA-dependent modulation of VR1. Given the significant roles of VR1 and PKA in inflammatory pain hypersensitivity, VR1 phosphorylation at Ser116 by PKA may represent an important molecular mechanism involved in the regulation of VR1 function after tissue injury.

Decreased levels of N-acetylaspartate in dorsolateral prefrontal cortex in a case of intractable severe sympathetically mediated chronic pain (complex regional pain syndrome, type I)

In our previous in vivo proton magnetic resonance spectroscopy ((1)H MRS) study we found reduced levels of N-acetylaspartate in dorsolateral prefrontal cortex of chronic back pain patients. This study tests whether these chemical abnormalities can be detected in other pain states. Using (1)H MRS, we measured levels for N-acetylaspartate and other identifiable chemicals relative to creatine in four bilateral brain regions, including dorsolateral prefrontal cortex, orbitofrontal cortex, cingulate, and thalamus, in a case of intractable severe sympathetically mediated chronic pain [complex regional pain syndrome (CRPS) type I]. The subject's chemical variations in the brain were compared to the same regional chemicals in 10 normal subjects (age- and sex-matched). Univariate statistics showed reduced levels of N-acetylaspartate in bilateral dorsolateral prefrontal cortex and increased levels of myo-inositol in left orbitofrontal cortex of the patient with intractable severe CRPS type I. These data support our original hypothesis that depletion of N-acetylaspartate in dorsolateral prefrontal cortex is a chemical marker of chronic pain, indicating for neuronal degeneration. Unpredicted changes of orbitofrontal myo-inositol may be related to the specific mood/affective state in an extreme pain perception. This is the first report, which identifies chemical markers in the prefrontal cortex for objective measurement and monitoring of CRPS type I. This information might lead to valuable insights into diagnosis and future effective interventions of CRPS type I (e.g., prefrontal brain stimulation).

Effect of repeated administration of morphine on the activity of extracellular signal regulated kinase in the mouse brain

The present study was designed to determine whether chronic morphine treatment could influence the activity of extracellular signal regulated kinase (ERK) in the mouse brain. The single subcutaneous injection of morphine produced profound antinociception and an increase in phosphorylated-ERK (p-ERK) immunoreactivity in the pons/medulla, and these effects were blocked by a mu-opioid receptor antagonist, naloxone. The potency of antinociception induced by the second morphine injection at 24 h after the first morphine injection was similar to that by the first morphine injection. The p-ERK immunoreactivity in the pons/medulla obtained at 24 h after a single injection of morphine was not different from the control level. Repeated morphine injection once a day for 7 days resulted in a marked reduction of antinociception by morphine. The p-ERK immunoreactivity in the pons/medulla increased remarkably after 7 days repeated morphine injection. These data suggest that the sustained activation of ERK activity be associated with the development of antinociceptive tolerance to morphine in mice.

Activation of spinal extracellular signaling-regulated kinase-1 and -2 by intraplantar carrageenan in rodents

We have examined the activation of spinal extracellular signaling-regulated kinase (ERK) in juvenile rats and adult mice after intraplantar carrageenan or saline and its relationship to pain behavior. In rats, intraplantar carrageenan evoked a peak five-fold activation of spinal ERK at 30 min measured by immunoblot. Saline injection resulted in a two-fold activation. This differential ERK activation correlated with a 2.5-fold greater pain response and the development of secondary hyperalgesia in carrageenan-injected rats, whereas both saline and carrageenan produced similar primary hyperalgesia. In mice, carrageenan injection produced a peak 3.5-fold activation of ERK, but saline was ineffective. We conclude that ERK activation may underlie spinal nociceptive processing and secondary hyperalgesia after carrageenan inflammation.

Neutral endopeptidase knockout induces hyperalgesia in a model of visceral pain, an effect related to bradykinin and nitric oxide

Neutral endopeptidase (EC3.4.24.11, NEP, enkephalinase) is a zinc-metalloendopeptidase, cleaving a variety of substrates like enkephalins, substance P, and bradykinin. In the brain, NEP is a key enzyme in the degradation of enkephalins. Pharmacological inhibition of NEP-activity causes analgesia resulting from enhanced extracellular enkephalin concentrations. Recently, transgenic mice lacking the enzyme NEP have been developed (Lu, 1995). The present study was designed to investigate the nociceptive behavior of these NEP-knockout mice. Interestingly, NEP-deficient mice did not respond with decreased pain perception, but exhibited hyperalgesia in the hot-plate jump, warm-water tail-withdrawal, and mostnotablyin theacetic-acid writhing test. Inhibition of aminopeptidase N by bestatin reduced writhing in both strains, whereas NEP-inhibition by thiorphan reduced writhing selectively in wild-type mice. Naloxone increased writhing in wild-type but not in knockouts, whereas the bradykinin B2-receptor antagonist HOE140 reduced writhing selectively in NEP-knockouts. Similarly, the nitric oxide synthase inhibitor L-NAME reduced writhing in NEP-knockouts. These results indicate that genetic elimination of NEP, in contrast to pharmacological inhibition, leads to bradykinin-induced hyperalgesia instead of enkephalin-mediated analgesia. Nitric oxide (NO) is suggested to be involved in this process.

Electroacupuncture-related changes of NADPH-diaphorase and neuronal nitric oxide synthase in the brainstem of spontaneously hypertensive rats

The aim of this study was to investigate the electroacupuncture-related changes of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and neuronal nitric oxide synthase (nNOS) in the brainstem of spontaneously hypertensive rats (SHR). We evaluated the changes of NADPH-d-positive neurons using a histochemical method and the changes of nNOS-positive neurons using an immunohistochemical method. The staining intensities of NADPH-d-positive neurons and nNOS-positive neurons were assessed in a quantitative fashion using a microdensitometrical method based on optical density by means of an image analyzer. The optical density of NADPH-d-positive neurons and nNOS-positive neurons of the Shinsu (BL23) and Choksamni (ST36) electroacupuncture groups were significantly decreased in most brainstem areas as compared to the normal and arbitrary groups, with the exception of the optical density of NADPH-d positive neurons in the prepositus nucleus as compared to the arbitrary group. The present results demonstrated that electroacupuncture changes the activity in the NO system in the brainstem of SHR and the site where electroacupuncture is administered is of importance for this effect.

Differential effects of NMDA and group I mGluR antagonists on both nociception and spinal cord protein kinase C translocation in the formalin test and a model of neuropathic pain in rats

Coincident with nociception, both noxious chemical stimulation of the hind paw and chronic constriction injury (CCI) of the sciatic nerve produce an increase in protein kinase C (PKC) translocation in the spinal cord of rats. Noxious stimulus-induced PKC translocation likely depends on glutamate activity at either N-methyl-D-aspartate (NMDA) receptors or group I metabotropic glutamate receptors (mGluR1/5) in the spinal cord dorsal horn. This study compares nociceptive responses to, and the alterations in membrane-associated PKC, induced by noxious chemical stimulation of the hindpaw and CCI of the sciatic nerve, as well as their modulation by both NMDA and mGluR1/5 receptor antagonists. Three groups of rats were given a single intrathecal (i.t.) injection of either vehicle, dizocilpine maleate (MK-801, 60 nmol), an NMDA receptor antagonist, or (S)-4-carboxyphenylglycine (S)-4CPG, (150 nmol), an mGluR1/5 antagonist, 10 min prior to a 50 microl of 2.5% formalin injection into the ventral surface of one hind paw. Another three groups of rats were given twice daily injections of either vehicle, MK-801 (30 nmol) or (S)-4CPG (90 nmol) i.t. for 5 days starting 30 min before CCI or sham injury of the sciatic nerve. Nociceptive responses were assessed for a 60 min period after the formalin injection in the first three groups, and tests of mechanical and cold allodynia were performed on days 4, 8, 12 and 16 after CCI for the latter three groups. Furthermore, changes in the levels of membrane-associated PKC, as assayed by quantitative autoradiography of the specific binding of [3H]-phorbol 12,13-dibutyrate ([3H]-PDBu) in the dorsal horn of the lumbar spinal cord sections, were assessed in formalin-injected rats (at 5, 25 and 60 min) and in neuropathic rats 5 days after CCI, treated (as above) with vehicle, MK-801 or (S)-4CPG. The results indicate that i.t. treatment with MK-801 significantly reduced nociceptive scores in the formalin test and also produced a significant suppression of formalin-induced increases in [3H]-PDBu binding in laminae I-II, III-VI and X of the lumbar spinal cord. In contrast, i.t. treatment with (S)-4CPG failed to significantly affect either nociceptive behaviours in the formalin test or formalin-induced increases in [3H]-PDBu binding in laminae I-II and III-VI of the lumbar spinal cord. On the other hand, i.t. treatment with either MK-801 or (S)-4CPG produced a significant reduction in mechanical and cold hypersensitivity, as well as [3H]-PDBu binding in laminae I-II and III-VI of the lumbar spinal cord, after CCI. These results suggest that while NMDA, but not mGluR1/5, receptors are involved in translocation of PKC and nociception in a model of persistent acute pain, both types of receptors influence the translocation of PKC in dorsal horn and mechanical and cold allodynia in a model of chronic neuropathic pain.

Protein kinase C gamma-like immunoreactivity of trigeminothalamic neurons in the medullary dorsal horn of the rat

We examined protein kinase C gamma-like immunoreactivity (PKCgamma-LI) of trigeminothalamic neurons in the rat medullary dorsal horn (MDH) after injecting a retrograde tracer, Fluoro-Gold (FG), into the thalamus. Over 90% of FG-labeled neurons in the marginal layer (lamina I) and a few FG-labeled neurons in the superficial part of the magnocellular layer (lamina III) showed PKCgamma-LI. No PKCgamma-neurons in the substantia gelatinosa (lamina II) were labeled with FG. PKCgamma-mediated regulation of trigeminothalamic neurons may contribute to the changes in MDH activity during persistent pain.

Glucocorticoid effects on Fos immunoreactivity and NADPH-diaphorase histochemical staining following spinal cord injury

Glucocorticoids (GC) provide neuroprotection and early recovery after spinal cord injury (SCI). While several mechanisms were proposed to account for these effects, limited information exists regarding GC actions in sensory areas of the spinal cord. Presently, we studied the time course of Fos expression, and reduced nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemical staining to monitor neuronal responses to SCI with or without GC treatment. Rats with sham-operation or transection at the thoracic level (T7-T8) received vehicle or 5 mg/kg of the GC dexamethasone (DEX) at 5 min post-lesion and were sacrificed 2 or 4 h after surgery. Another group of SCI rats received vehicle or intensive DEX treatment (5 min, 6 h, 18 h and 46 h post-lesion) and were sacrificed 48 h after surgery. The number of NADPH-d positive neurons or Fos immunoreactive nuclei was studied by computer-assisted image analysis in superficial dorsal horn (Laminae I-III) and central canal area (Lamina X) below the lesion. While constitutive Fos immunoreactive nuclei were sparse in controls, SCI increased Fos expression at 2 and 4 h after injury. DEX treatment significantly enhanced the number of Fos positive nuclei in Laminae I-III by 4 h after transection, although the response was not maintained by intensive steroid treatment when tested at 48 h after SCI. NADPH-d positive neurons in Laminae I-III increased at 2 and 4 h after SCI while a delayed increased was found in central canal area (Lamina X). DEX treatment decreased NADPH-d positive neurons to sham-operated levels at all time points examined. Thus, while GC stimulation of Fos suggests activation of neurons involved in sympathetic outflow and/or pain, down-regulation of NADPH-d indicates attenuation of nociceptive outflow, considering the role of enzyme-derived nitric oxide in pain-related mechanisms. Differential hormonal effects on these molecules agree with their localization in different cell populations.

Therapeutic potential of PKC inhibitors in painful diabetic neuropathy

Diabetic neuropathy accompanied by anomalies in pain perception is one of the most frequent complications in insulin-dependent diabetes in humans. Many clinical and experimental studies have suggested that diabetes or hyperglycaemia alters pain sensitivity. In humans, diabetic neuropathy can be associated with burning, tactile hypersensitivity. Behavioural reactions of hyperalgesia in animal models of diabetes have been described. However, the aetiology of these disturbances is still unknown, although metabolic factors such as hyperglycaemia or neurotransmitter alteration may be involved. Activation of protein kinase C (PKC) has been implicated in changes in pain perception. Phorbol esters, which activate PKC, enhance the thermal hyperalgesia in diabetic mice and enhance nociceptive responses after tissue injury induced by formalin. Electrophysiological experiments have shown that activation of PKC leads to long-lasting enhancement of excitatory amino acid-mediated currents in dorsal horn neurones and trigeminal neurones. Thus, activation of PKC may underlie the neuronal sensitisation that produces hyperalgesia in diabetic neuropathy.

Changes in the levels of nitric oxide synthase and protein kinase C gamma following kainic acid receptor activation in the rat spinal cord

In this study, we evaluated the levels of nitric oxide synthase, both neuronal and induced (nNOS and iNOS, respectively), cyclooxygenase-1 and 2 (COX-1 and COX-2) and protein kinase C gamma (PKCgamma) and correlated these with algogenic behavior following spinal kainic acid (KA) receptor activation in rats. Thirty adult male Sprague-Dawley rats were randomly assigned into six groups (n=5). Groups A, B, and C received 0.5 g kainic acid intrathecally and were analyzed at 3, 6, 24 h after injection, respectively. Groups D, E, and F received saline and were analyzed at 3, 6, 24 h after injection, respectively. We observed for behavioral changes in the rats following intrathecal KA injection and analyzed the protein levels of NOS, COX and PKCgamma by Western blotting techniques. Importantly, we clarified the potential roles of PKCgamma in the regulation of nNOS and COX-2 following intrathecal injection with KA in the rat spinal cord. COX-2 protein was detected but not significantly changed in the lumbosacral spinal cord at 3, 6, and 24 h following intrathecal KA injection (P>0.05). In contrast, nNOS protein was detected at higher levels in comparison with normal spinal cord at 6 and 24 h after intrathecal administration of KA (P<0.05). PKCgamma also increased significantly at 3, 6, and 24 h after intrathecal KA injection when compared with the baseline level (P<0.05). On the other hand, COX-1 and iNOS were not detected in either normal or KA treated spinal cords. These results provide strong in vivo evidence to support the idea that nNOS but not COX-2, plays an important role in spinal KA receptor activation. Furthermore, up-regulation of PKCgamma is involved in KA induced algogenic behavior in rats.

Spinal N-acetyl-alpha-linked acidic dipeptidase (NAALADase) inhibition attenuates mechanical allodynia induced by paw carrageenan injection in the rat

N-Acetylated-alpha-linked acidic dipeptidase (NAALADase) hydrolyzes N-acetyl-aspartyl-glutamate (NAAG) to liberate N-acetyl-aspartate and glutamate. NAAG is a putative neurotransmitter and acts as a mixed agonist/antagonist on N-methyl-D-aspartate (NMDA) receptors and acts as an agonist on the metabotropic glutamate receptor 3 (mGluR3). In the present study, we examined the role of spinal NAALADase in the maintenance of mechanical allodynia induced by carrageenan injection, skin incision and mild thermal injury using 2-(phosphonomethyl)pentanedioic acid (2-PMPA), a specific NAALADase inhibitor, in rats. Mechanical allodynia was induced by injection of 2 mg carrageenan into the paw (carrageenan model), by creating a 1-cm longitudinal skin incision of the plantar aspect of the foot (post-operative model), or by application of thermal stimulation (52.5 degrees C) for 45 s to the hind paw (mild thermal injury model). 2-PMPA was administered intrathecally at the time when the maximum mechanical allodynia occurred. Mechanical allodynia was assessed by the measurement of mechanical threshold using von Frey filaments. The mechanical threshold was measured 5, 15, 30, 60 and 90 min after the drug administration. In the carrageenan model, 100 microg of 2-PMPA attenuated the level of mechanical allodynia. 2-PMPA had no effect on the level of mechanical allodynia in both the post-operative pain model and the mild thermal injury model. These data suggested that the inhibition of spinal NAALADase alleviated mechanical allodynia induced by paw carrageenan injection.

Neuronal nitric oxide synthase (nNOS) mRNA is down-regulated, and constitutive NOS enzymatic activity decreased, in thoracic dorsal root ganglia and spinal cord of the rat by a substance P N-terminal metabolite

Nitric oxide (NO) in the spinal cord plays a role in sensory and autonomic activity. Pain induced by acetic acid in the abdominal stretch (writhing) assay and hyperalgesia associated with chronic pain are highly sensitive to NO synthase (NOS) inhibitors. Because substance P (SP) is released and up-regulated in some models of chronic pain, we hypothesized that an accumulation of SP metabolites may influence NOS expression and activity. To test this hypothesis, we examined the effect of intrathecally (i.t.) injected substance P (1-7) [SP(1-7)], the major metabolite of SP in the rat, on neuronal NOS (nNOS) mRNA in the thoracic and lumbar spinal cord, dorsal root ganglia (DRG) and on the corresponding constitutive NOS (cNOS) enzyme activity. Detected using quantitative RT-PCR, nNOS mRNA content in the thoracic spinal cord was decreased 6 h after injection of 5 micromol of SP(1-7) and returned to control 2 days later. In thoracic DRG, nNOS mRNA was reduced 48 h after SP(1-7). The cNOS enzymatic activity in thoracic spinal tissue was gradually decreased to a minimum at 72 h. Down-regulation of NOS by SP(1-7) in the thoracic area appears to be highly associated with capsaicin-sensitive primary afferent neurons. No similar changes in either parameter were measured in the lumbar area after SP(1-7). These data suggest that N-terminal SP fragments, which are known to cause long-term antinociception in the writhing assay, may do so by their ability to down-regulate NO synthesis along nociceptive pathways.