A nitric oxide synthesis inhibitor (L-NAME) reduces licking behavior and Fos-labeling in the spinal cord of rats during formalin-induced inflammation

Antinociceptive effects of vitamin B-complex: A behavioral and histochemical study in rats

B-vitamins have been evaluated as a useful adjuvant therapy to treat pain. In spite of clinical and experimental evidence indicating the analgesic effect of B-vitamins, few studies have investigated their effect on aspects of the inflammatory pain response. In the present study, we investigated the analgesic effect of chronic application of B-complex vitamins (Neurobion) using an inflammatory experimental pain model in rats. Nociceptive behavioral responses were evaluated in male Wistar rats after plantar injection of formalin, comparing the treatment group (TG) with Neurobion pretreatment to the control group (CG) without the pretreatment. In addition, neuronal activity in the central pain pathway was evaluated using c-Fos immunohistochemical reactivity and NADPH-d histochemistry. A highly significant reduction of painful behaviors such as licking and flinching were observed in TG, especially during the secondary phase of the formalin test compared to CG. Results suggest that long-term pre-treatment using Neurobion can have a beneficial effect in reducing the chronic phase of pain. In addition, we observed a downregulation of c-Fos and NADPH-d in dorsal spinal neurons, suggesting that the antinociceptive effect induced by Neurobion could be due to a suppression of nociceptive transmission at the spinal level, particularly in the afferent regions of the dorsal spinal horn, which these neurons utilizing nitric oxide at least as one of their pain neurotransmitters.

Medial prefrontal cortex nitric oxide modulates neuropathic pain behavior through mu opioid receptors in rats

Background

The neocortex, including the medial prefrontal cortex (mPFC), contains many neurons expressing nitric oxide synthase (NOS). In addition, increasing evidence shows that the nitric oxide (NO) and opioid systems interact in the brain. However, there have been no studies on the interaction of the opioid and NO systems in the mPFC. The objective of this study was to investigate the effects of administrating L-arginine (L-Arg, a precursor of NO) and N(gamma)-nitro-L-arginine methyl ester (L-NAME, an inhibitor of NOS) into the mPFC for neuropathic pain in rats. Also, we used selective opioid receptor antagonists to clarify the possible participation of the opioid mechanism.

Methods

Complete transection of the peroneal and tibial branches of the sciatic nerve was applied to induce neuropathic pain, and seven days later, the mPFC was cannulated bilaterally. The paw withdrawal threshold fifty percent (50% PWT) was recorded on the 14th day.

Results

Microinjection of L-Arg (2.87, 11.5 and 45.92 nmol per 0.25 µL) increased 50% PWT. L-NAME (17.15 nmol per 0.25 µL) and naloxonazine (an antagonist of mu opioid receptors, 1.54 nmol per 0.25 µL) inhibited anti-allodynia induced by L-Arg (45.92 nmol per 0.25 µL). Naltrindole (a delta opioid receptor antagonist, 2.45 nmol per 0.25 µL) and nor-binaltorphimine (a kappa opioid receptor antagonist, 1.36 nmol per 0.25 µL) were unable to prevent L-Arg (45.92 nmol per 0.25 µL)-induced antiallodynia.

Conclusions

Our results indicate that the NO system in the mPFC regulates neuropathic pain. Mu opioid receptors of this area might participate in pain relief caused by L-Arg.

Prospects for the Personalized Multimodal Therapy Approach to Pain Management via Action on NO and NOS

Chronic pain syndromes are an important medical problem generated by various molecular, genetic, and pathophysiologic mechanisms. Back pain, neuropathic pain, and posttraumatic pain are the most important pathological processes associated with chronic pain in adults. Standard approaches to the treatment of them do not solve the problem of pain chronicity. This is the reason for the search for new personalized strategies for the prevention and treatment of chronic pain. The nitric oxide (NO) system can play one of the key roles in the development of peripheral pain and its chronicity. The purpose of the study is to review publications devoted to changes in the NO system in patients with peripheral chronical pain syndromes. We have carried out a search for the articles published in e-Library, PubMed, Oxford Press, Clinical Case, Springer, Elsevier, and Google Scholar databases. The search was carried out using keywords and their combinations. The role of NO and NO synthases (NOS) isoforms in peripheral pain development and chronicity was demonstrated primarily from animal models to humans. The most studied is the neuronal NOS (nNOS). The role of inducible NOS (iNOS) and endothelial NOS (eNOS) is still under investigation. Associative genetic studies have shown that single nucleotide variants (SNVs) of NOS1, NOS2, and NOS3 genes encoding nNOS, iNOS, and eNOS may be associated with acute and chronic peripheral pain. Prospects for the use of NOS inhibitors to modulate the effect of drugs used to treat peripheral pain syndrome are discussed. Associative genetic studies of SNVs NOS1, NOS2, and NOS3 genes are important for understanding genetic predictors of peripheral pain chronicity and development of new personalized pharmacotherapy strategies.

Chronic stress influences nociceptive sensitivity of female rats in an estrous cycle-dependent manner

Exposure to chronic stress can influence nociception and further induce hyperalgesia. Whether stress modulation of pain in female animals occurs in an estrous cycle-specific manner is still unclear. We profiled the changes in nociception (thermal, mechanical, formalin-evoked acute and inflammatory pain) of female Sprague-Dawley rats after treatment with chronic unpredictable mild stress (CUMS) and investigated whether these changes occur in an estrous cycle-dependent manner. The results showed that CUMS female rats exhibited a lower mechanical withdrawal threshold in proestrus and estrus, a longer formalin-evoked licking time in metestrus and diestrus, but no changes in the latency time on the tail-flick test. The present study findings suggest that chronic stress induces mechanical and formalin-evoked acute hyperalgesia of female rats in an estrous cycle-dependent manner.SUMMARYOur studies showed that chronic stress increased nociceptive sensitivity of female rats. Furthermore females had different stress-induced pain responses in different estrous phases: mechanical hyperalgesia in proestrus and estrus, formalin-evoked acute hyperalgesia in metestrus and diestrus.

Gabexate mesilate ameliorates the neuropathic pain in a rat model by inhibition of proinflammatory cytokines and nitric oxide pathway via suppression of nuclear factor-κB

Background

This study examined the effects of gabexate mesilate on spinal nerve ligation (SNL)-induced neuropathic pain. To confirm the involvement of gabexate mesilate on neuroinflammation, we focused on the activation of nuclear factor-κB (NF-κB) and consequent the expression of proinflammatory cytokines and inducible nitric oxide synthase (iNOS).

Methods

Male Sprague-Dawley rats were used for the study. After randomization into three groups: the sham-operation group, vehicle-treated group (administered normal saline as a control), and the gabexate group (administered gabexate mesilate 20 mg/kg), SNL was performed. At the 3rd day, mechanical allodynia was confirmed using von Frey filaments, and drugs were administered intraperitoneally daily according to the group. The paw withdrawal threshold (PWT) was examined on the 3rd, 7th, and 14th day. The expressions of p65 subunit of NF-κB, interleukin (IL)-1, IL-6, tumor necrosis factor-α, and iNOS were evaluated on the 7th and 14th day following SNL.

Results

The PWT was significantly higher in the gabexate group compared with the vehicle-treated group (P < 0.05). The expressions of p65, proinflammatory cytokines, and iNOS significantly decreased in the gabexate group compared with the vehicle-treated group (P < 0.05) on the 7th day. On the 14th day, the expressions of p65 and iNOS showed lower levels, but those of the proinflammatory cytokines showed no significant differences.

Conclusions

Gabexate mesilate increased PWT after SNL and attenuate the progress of mechanical allodynia. These results seem to be involved with the anti-inflammatory effect of gabexate mesilate via inhibition of NF-κB, proinflammatory cytokines, and nitric oxide.

Function of neuronal nitric oxide synthase enzyme in temozolomide-induced damage of astrocytic tumor cells

Astrocytic tumors, including astrocytomas and glioblastomas, are the most common type of primary brain tumors. Treatment for glioblastomas includes radiotherapy, chemotherapy with temozolomide (TMZ) and surgical ablation. Despite certain therapeutic advances, the survival time of patients is no longer than 12-14 months. Cancer cells overexpress the neuronal isoform of nitric oxide synthase (nNOS). In the present study, it was examined whether the nNOS enzyme serves a role in the damage of astrocytoma (U251MG and U138MG) and glioblastoma (U87MG) cells caused by TMZ. First, TMZ (250 µM) triggered an increase in oxidative stress at 2, 48 and 72 h in the U87MG, U251MG and U138MG cell lines, as revealed by 2',7'-dichlorofluorescin-diacetate assay. The drug also reduced cell viability, as measured by MTT assay. U87MG cells presented a more linear decline in cell viability at time-points 2, 48 and 72 h, compared with the U251MG and U138MG cell lines. The peak of oxidative stress occurred at 48 h. To examine the role of NOS enzymes in the cell damage caused by TMZ, N(ω)-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI) were used. L-NAME increased the cell damage caused by TMZ while reducing the oxidative stress at 48 h. The preferential nNOS inhibitor 7-NI also improved the TMZ effects. It caused a 12.8% decrease in the viability of TMZ-injured cells. Indeed, 7-NI was more effective than L-NAME in restraining the increase in oxidative stress triggered by TMZ. Silencing nNOS with a synthetic small interfering (si)RNA (siRNAnNOShum_4400) increased by 20% the effects of 250 µM of TMZ on cell viability (P<0.05). Hoechst 33342 nuclear staining confirmed that nNOS knock-down enhanced TMZ injury. In conclusion, our data reveal that nNOS enzymes serve a role in the damage produced by TMZ on astrocytoma and glioblastoma cells. RNA interference with nNOS merits further studies in animal models to disclose its potential use in brain tumor anticancer therapy.

Dimethylarginine dimethylaminohydrolase 1 is involved in spinal nociceptive plasticity

Supplemental Digital Content is Available in the Text.

Inhibition of dimethylarginine dimethylaminohydrolase 1 attenuates pain-related behavior and hyperexcitability in pain conditions associated with excessive nitric oxide production, representing a novel therapeutic target.

Activation of neuronal nitric oxide synthase, and consequent production of nitric oxide (NO), contributes to spinal hyperexcitability and enhanced pain sensation. All NOS isoforms are inhibited endogenously by asymmetric dimethylarginine, which itself is metabolised by dimethylarginine dimethylaminohydrolase (DDAH). Inhibition of DDAH can indirectly attenuate NO production by elevating asymmetric dimethylarginine concentrations. Here, we show that the DDAH-1 isoform is constitutively active in the nervous system, specifically in the spinal dorsal horn. DDAH-1 was found to be expressed in sensory neurons within both the dorsal root ganglia and spinal dorsal horn; L-291 (N^G–[2-Methoxyethyl]-l-arginine methyl ester), a DDAH-1 inhibitor, reduced NO synthesis in cultured dorsal root ganglia neurons. Spinal application of L-291 decreased N-methyl-d-aspartate–dependent postdischarge and windup of dorsal horn sensory neurons—2 measures of spinal hyperexcitability. Finally, spinal application of L-291 reduced both neuronal and behavioral measures of formalin-induced central sensitization. Thus, DDAH-1 may be a potential therapeutic target in neuronal disorders, such as chronic pain, where elevated NO is a contributing factor.

Involvement of PKA-dependent upregulation of nNOS-CGRP in adrenomedullin-initiated mechanistic pathway underlying CFA-induced response in rats

We have previously shown that intrathecal administration of the adrenomedullin (AM) receptor antagonist AM(22-52) produces a long-lasting anti-hyperalgesia effect. This study examined the hypothesis that AM recruits other pronociceptive mediators in complete Freund's adjuvant (CFA)-induced inflammation. Injection of CFA in the hindpaw of rat produced an increase in the expression of nNOS in dorsal root ganglion (DRG) and the spinal dorsal horn. An intrathecal administration of AM(22-52), but not the CGRP antagonist BIBN4096BS, abolished the CFA-induced increase of nNOS. Moreover, AM-induced increase of CGRP was inhibited by the nNOS inhibitors L-NAME and 7-nitroindazole in cultured ganglion explants. Addition of AM to ganglion cultures induced an increase in nNOS protein, which was attenuated by the PKA inhibitor H-89. Treatment with AM also concentration-dependently increased cAMP content and pPKA protein level, but not its non-phosphorylated form, in cultured ganglia. In addition, nNOS was shown to be co-localized with the AM receptor components calcitonin receptor-like receptor and receptor activity-modifying protein 2- and 3 in DRG neurons. The present study suggests that the enhanced activity of nitric oxide (NO) mediates the biological action of AM at the spinal level and that AM recruits NO-CGRP via cAMP/PKA signaling in a mechanistic pathway underlying CFA-induced hyperalgesia.

Activation of the cGMP/Protein Kinase G Pathway by Nitric Oxide Can Decrease TRPV1 Activity in Cultured Rat Dorsal Root Ganglion Neurons

Recent studies have demonstrated that nitric oxide (NO) activates transient receptor potential vanilloid subtype 1 (TRPV1) via S-nitrosylation of the channel protein. NO also modulates various cellular functions via activation of the soluble guanylyl cyclase (sGC)/protein kinase G (PKG) pathway and the direct modification of proteins. Thus, in the present study, we investigated whether NO could indirectly modulate the activity of TRPV1 via a cGMP/PKG-dependent pathway in cultured rat dorsal root ganglion (DRG) neurons. NO donors, sodium nitroprusside (SNP) and S-nitro-N-acetylpenicillamine (SNAP), decreased capsaicin-evoked currents (I_cap). NO scavengers, hemoglobin and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (CPTIO), prevented the inhibitory effect of SNP on I_cap. Membrane-permeable cGMP analogs, 8-bromoguanosine 3', 5'-cyclic monophosphate (8bromo-cGMP) and 8-(4chlorophenylthio)-guanosine 3',5'-cyclic monophosphate (8-pCPT-cGMP), and the guanylyl cyclase stimulator YC-1 mimicked the effect of SNP on I_cap. The PKG inhibitor KT5823 prevented the inhibition of I_cap by SNP. These results suggest that NO can downregulate the function of TRPV1 through activation of the cGMP/PKG pathway in peripheral sensory neurons.

Antinociceptive activity of the selective iNOS inhibitor AR-C102222 in rodent models of inflammatory, neuropathic and post-operative pain

Nitric oxide generated by the nitric oxide synthase (NOS) isoforms contributes to pain processing. The selective inhibition of iNOS might represent a novel, therapeutic target for the development of antinociceptive compounds. However, few isoform-selective inhibitors of NOS have been developed. The present experiments examined the anti-inflammatory and antinociceptive activity of a selective inducible nitric oxide (iNOS) inhibitor, AR-C102222, on arachidonic acid-induced ear inflammation, Freund's complete adjuvant (FCA)-induced hyperalgesia, acetic acid-induced writhing, and tactile allodynia produced by L5 spinal nerve ligation (L5 SNL) or hindpaw incision (INC). AR-C102222 at a dose of 100mg/kg p.o., significantly reduced inflammation produced by the application of arachidonic acid to the ear, attenuated FCA-induced mechanical hyperalgesia, and attenuated acetic acid-induced writhing. In the L5 SNL and INC surgical procedures, tactile allodynia produced by both procedures was significantly reduced by 30mg/kg i.p. of AR-C102222. These data demonstrate that the selective inhibition of iNOS produces antinociception in different models of pain and suggest that the iNOS-NO system plays a role in pain processing.

GCH1, BH4 and pain

Understanding and consequently treating neuropathic pain effectively is a challenge for modern medicine, as unlike inflammation, which can be controlled relatively well, chronic pain due to nerve injury is refractory to most current therapeutics. Here we define a target pathway for a new class of analgesics, tetrahydrobiopterin (BH4) synthesis and metabolism. BH4 is an essential co-factor in the synthesis of serotonin, dopamine, epinephrine, norepinephrine and nitric oxide and as a result, its availability influences many systems, including neurons. Following peripheral nerve damage, levels of BH4 are dramatically increased in sensory neurons, consequently this has a profound effect on the physiology of these cells, causing increased activity and pain hypersensitivity. These changes are principally due to the upregulation of the rate limiting enzyme for BH4 synthesis GTP Cyclohydrolase 1 (GCH1). A GCH1 pain-protective haplotype which decreases pain levels in a variety of settings, by reducing the levels of endogenous activation of this enzyme, has been characterized in humans. Here we define the control of BH4 homeostasis and discuss the consequences of large perturbations within this system, both negatively via genetic mutations and after pathological increases in the production of this cofactor that result in chronic pain. We explain the nature of the GCH1 reduced-function haplotype and set out the potential for a ' BH4 blocking' drug as a novel analgesic.

Nitric oxide implicates c-Fos expression in the cuneate nucleus following electrical stimulation of the transected median nerve

In this study, we investigated whether nitric oxide (NO) modulated injury-induced neuropeptide Y (NPY) releasing and c-Fos expression in the cuneate nucleus (CN) after median nerve transection (MNT). We first examined the temporal changes of neuronal nitric oxide synthase (nNOS) expression in the dorsal root ganglion (DRG) and CN after MNT. Following MNT, the amounts of nNOS-like immunoreactive (nNOS-LI) neurons in the DRG and CN significantly increased as compared with those of the sham-operated rats. Furthermore, 4 weeks after MNT, the increases of nNOS-LI neurons in the DRG and CN were attenuated by pre-emptive lidocaine treatment in a dose-dependent manner. Finally, 4 weeks after MNT, pre-stimulation administration of L-NAME (N (ω)-Nitro-L: -arginine methyl ester) or 7-NI (7-nitroindazole) suppressed the amount of NPY release from the stimulated terminals and thus attenuated c-Fos expression in the CN. Our data implied that NO would modulate neuronal activity in the DRG and CN both after MNT.

NADPH-d and Fos reactivity in the rat spinal cord following experimental spinal cord injury and embryonic neural stem cell transplantation

AIMS

The aim of this study is to determine the role of nitric oxide (NO) in neuropathic pain and the effect of embryonic neural stem cell (ENSC) transplantation on NO content in rat spinal cord neurons following spinal cord injury (SCI).

MAIN METHODS

Ninety adult male Sprague-Dawley rats were divided into 3 groups (n=30, each): control (laminectomy), SCI (hemisection at T12-T13 segments) and SCI+ENSC. Each group was further divided into sub-groups (n=5 each) based on the treatment substance (L-NAME, 75 mg/kg/i.p.; L-arginine, 225 mg/kg/i.p.; physiological saline, SF) and duration (2h for acute and 28 days for chronic groups). Pain was assessed by tail flick and Randall-Selitto tests. Fos immunohistochemistry and NADPH-d histochemistry were performed in segments 2 cm rostral and caudal to SCI.

KEY FINDINGS

Tail-flick latency time increased in both acute and chronic L-NAME groups and increased in acute and decreased in chronic L-arginine groups. The number of Fos (+) neurons decreased in acute and chronic L-NAME and decreased in acute L-arginine groups. Following ENSC, Fos (+) neurons did not change in acute L-NAME but decreased in the chronic L-NAME groups, and decreased in both acute and chronic L-arginine groups. NADPH-d (+) neurons decreased in acute L-NAME and increased in L-arginine groups with and without ENSC transplantation.

SIGNIFICANCE

This study confirms the role of NO in neuropathic pain and shows an improvement following ENSC transplantation in the acute phase, observed as a decrease in Fos(+) and NADPH-d (+) neurons in spinal cord segments rostral and caudal to injury.

The nociceptive and anti-nociceptive effects of bee venom injection and therapy: a double-edged sword

Bee venom injection as a therapy, like many other complementary and alternative medicine approaches, has been used for thousands of years to attempt to alleviate a range of diseases including arthritis. More recently, additional theraupeutic goals have been added to the list of diseases making this a critical time to evaluate the evidence for the beneficial and adverse effects of bee venom injection. Although reports of pain reduction (analgesic and antinociceptive) and anti-inflammatory effects of bee venom injection are accumulating in the literature, it is common knowledge that bee venom stings are painful and produce inflammation. In addition, a significant number of studies have been performed in the past decade highlighting that injection of bee venom and components of bee venom produce significant signs of pain or nociception, inflammation and many effects at multiple levels of immediate, acute and prolonged pain processes. This report reviews the extensive new data regarding the deleterious effects of bee venom injection in people and animals, our current understanding of the responsible underlying mechanisms and critical venom components, and provides a critical evaluation of reports of the beneficial effects of bee venom injection in people and animals and the proposed underlying mechanisms. Although further studies are required to make firm conclusions, therapeutic bee venom injection may be beneficial for some patients, but may also be harmful. This report highlights key patterns of results, critical shortcomings, and essential areas requiring further study.

Nitric oxide and pain: 'Something old, something new'

Challenges have emerged following the revival of nitric oxide (NO) from 'something old', a simple gas derived from nitrogen and oxygen with a role in the early stages of evolution, into 'something new', an endogenously formed biological mediator regulating a wide variety of physiological functions. Although pain is a common sensation, it encompasses multiple neurobiologic components, of which NO is only one. In pain research, the study of NO is complicated by convoluted problems related mostly to the effects of NO, which are pro- or anti-nociceptive depending on the circumstances. This dual function reflects the multi-faceted roles of the NO molecule described in physiology. This review covers current information about NO and its implications in pain mechanisms. In addition, it follows the pain pathways, demonstrating the role of NO in peripheral nociceptive transmission as well in central sensitization. This knowledge may provide the scientific basis for developing new drugs that are indicated for different types of pain, drugs that may be related to the chemical links of NO. A comprehensive approach to understanding the effects of NO will help clinicians identify novel agents that combine the pharmacological profile of native drugs with a controllable manner of NO release. Inhibitors of NO synthesis may have analgesic effects and would be of interest for treating inflammatory and neuropathic pain. Unfortunately, only a few of these compounds have reached the stage of clinical pain trials.

Opposing actions of neuronal nitric oxide synthase isoforms in formalin-induced pain in mice

The role of central and peripheral neuronal nitric oxide synthase (nNOS) splice variants in the development of inflammatory hyperalgesia was investigated using the formalin test. Supraspinal administration of the NOS inhibitor NOArg lowered both the first and second phase of the formalin response. An oligodeoxynucleotide targeting four nNOS isoforms given supraspinally also reduced the formalin response of both phases. Supraspinal antisense mapping suggested that this effect results from the nNOS-1 splice variant, implying that nNOS-1 is important in mediating formalin pain. At the spinal level, antisense mapping suggested a role of both the nNOS-1 and the nNOS-beta variants in producing formalin pain. Conversely, an antisense selective against nNOS-2 had an opposing effect against the first phase, increasing its intensity. This result, which was similar to prior studies examining opioid actions, implies that endogenous nNOS-2 activity acted to minimize pain perception. Locally in the foot, arginine, the precursor for NO, increased the phase II response at low doses while higher doses reduced the response. This complex biphasic response suggested opposing NOS actions. Local antisense mapping again showed that nNOS-1 is involved in producing phase II of the formalin response while nNOS-2 had an opposite effect similar to that seen spinally. Finally, downregulation of nNOS-1 by antisense prevented tolerance to morphine in both the tail-flick and the formalin test. Together, these observations illustrate the complexity of nNOS in pain perception and the existence of opposing nNOS systems likely due to splice variants of nNOS.

Involvement of NOS/NO in the development of chronic dental inflammatory pain in rats

Nitric oxide (NO) is believed to be an important messenger molecule in nociceptive transmission. To assess the possible roles of NO in trigeminal sensory system, we examined the distribution and density of histochemical staining for nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), a marker for nitric oxide synthase (NOS), and immunohistochemical staining for c-Fos, a neuronal activity marker, in the trigeminal ganglion (TG) and trigeminal nucleus caudalis (Vc) following pulp exposure (PX) injured rats. The neurons innervating injured tooth in TG were labeled by the retrograde transport of fluoro-gold (FG). Teeth were processed for H&E staining. We found that NADPH-d activity increased significantly in the TG and Vc following PX pretreatment (7-28 days, especially in 21-28 days). Such changes were closely corresponding to the pattern of c-Fos detected by immunocytochemistry. The results demonstrate that PX-induced chronic pulpal inflammation results in significant alterations in the TG cells and in the Vc, and such changes may underlie the observed NADPH-d activity. It suggests that NOS/NO may play an active role in both peripheral and central processing of nociceptive information following chronic tooth inflammation.

Involvement of spinal nitric oxide (NO) in rat pain-related behaviors induced by the venom of scorpion Buthus martensi Karsch

In the present study, we investigated the role of spinal nitric oxide (NO) in rat pain-related behaviors induced by the venom of scorpion Buthus martensi Karsch (BmK). The results showed that the number of neuronal NO synthase (nNOS) positive neurons significantly increased in superficial (I-II), deep (V-VI) dorsal horn laminae and the ventral gray laminae (VII-X), but not in the nucleus proprius (III and IV) of bilateral L4-L5 lumbar spinal cord after unilateral intraplantar injection of BmK venom from 2h to 7d. This increase on the ipsilateral side to BmK venom injection was always greater than that on the contralateral side. Western blotting analysis confirmed that spinal nNOS expression was significantly up-regulated following BmK venom administration. In addition, intrathecal delivery of N(omega)-nitro-l-arginine methyl ester hydrochloride (l-NAME; a NOS inhibitor) before intraplantar injection of BmK venom by 10 min significantly attenuated spontaneous nociceptive responses and prevented the development of primary thermal hyperalgesia as well as bilateral mechanical hyperalgesia. Intrathecal injection of l-NAME could also partially inhibit BmK venom-induced c-Fos expression in lumbar spinal cord at 2 h. Thus, the results suggest that spinal NO as a critical mediator is involved in various pain-related behaviors and c-Fos expression induced by BmK venom in rats.

Genetic knockout and pharmacologic inhibition of neuronal nitric oxide synthase attenuate nerve injury-induced mechanical hypersensitivity in mice

Neuronal nitric oxide synthase (nNOS) is a key enzyme for nitric oxide production in neuronal tissues and contributes to the spinal central sensitization in inflammatory pain. However, the role of nNOS in neuropathic pain remains unclear. The present study combined a genetic strategy with a pharmacologic approach to examine the effects of genetic knockout and pharmacologic inhibition of nNOS on neuropathic pain induced by unilateral fifth lumbar spinal nerve injury in mice. In contrast to wildtype mice, nNOS knockout mice failed to display nerve injury-induced mechanical hypersensitivity. Furthermore, either intraperitoneal (100 mg/kg) or intrathecal (30 microg/5 microl) administration of L-NG-nitro-arginine methyl ester, a nonspecific NOS inhibitor, significantly reversed nerve injury-induced mechanical hypersensitivity on day 7 post-nerve injury in wildtype mice. Intrathecal injection of 7-nitroindazole (8.15 microg/5 microl), a selective nNOS inhibitor, also dramatically attenuated nerve injury-induced mechanical hypersensitivity. Western blot analysis showed that the expression of nNOS protein was significantly increased in ipsilateral L5 dorsal root ganglion but not in ipsilateral L5 lumbar spinal cord on day 7 post-nerve injury. The expression of inducible NOS and endothelial NOS proteins was not markedly altered after nerve injury in either the dorsal root ganglion or spinal cord. Our findings suggest that nNOS, especially in the dorsal root ganglion, may participate in the development and/or maintenance of mechanical hypersensitivity after nerve injury.

NO pain: potential roles of nitric oxide in neuropathic pain

The disabling human syndrome of "neuropathic pain" is an intractable complication of peripheral nerve injury or degeneration. A complex interaction between injured peripheral axons, sensory neurons and central nervous system signaling is thought to account for it. In this brief review, we present evidence that the free radical signaling molecule, nitric oxide (NO) may act at several levels of the nervous system during the development of experimental neuropathic pain. For example, NO may directly influence injured axons in the periphery, may indirectly influence pain by its role in the process of Wallerian degeneration, and may signal in the dorsal horn of the spinal cord. While it is premature to argue for therapeutic approaches that alter NO actions, it may be an important player in the cascade of events that generate neuropathic pain.

Increased spinal c-Fos expression with noxious and non-noxious peripheral stimulation after severe spinal contusion

The effects of severe contusive spinal cord injury (SCI), at thoracic level 8 (T8), on lumbar c-Fos expression in the spinal cord was investigated. As hypothesized, chronic SCI has a significant effect on expression of c-Fos in the dorsal spinal sensory areas with noxious and innocuous peripheral stimulation of the sciatic nerve. This alteration to stimulation effects was measured using counts of c-Fos immunoreactive cells in the dorsal horn of the L5 lumbar spinal cord in injured animals at 90 days post-injury and in uninjured controls. The number of c-Fos immunoreactive cells increased in SCI rats only after noxious peripheral stimulation (electrical and chemical) suggesting a general increase in excitability in spinal pathways (central sensitization) associated with chronic SCI. These altered responses may represent a functional anatomical reorganization of spinal cord circuitry leading to increased dorsal horn c-Fos expression as a response to severe chronic contusive damage to the spinal cord sensory pathways.

Endogenous morphine: opening new doors for the treatment of pain and addiction

Nitric oxide (NO) signalling is at the forefront of intense research interest because its many effects remain controversial and seemingly contradictory. This paper examines its role as a potential mediator of pain and tolerance. Within this context discussion covers endogenous morphine, documenting its ability to be made in animal tissues, including nervous tissue, and in diverse animal phyla. Supporting morphine as an endogenous signalling molecule is the presence of the newly cloned mu3 opiate receptor subtype found in animal (including human) immune, vascular and neural tissues, which is coupled to NO release. Importantly, this mu opiate receptor subtype is morphine-selective and opioid peptide-insensitive, further highlighting the presence of morphinergic signalling coupled to NO release. These findings provide novel insights into pain and tolerance as morphinergic signalling exhibits many similarities with NO actions. Taken together, a select morphinergic signalling system utilising NO opens the gate for the development of novel pharmaceuticals and/or the use of old pharmaceuticals in new ways.

Prostaglandins, glutamate and nitric oxide synthase mediate nitroglycerin-induced hyperalgesia in the formalin test

Increasing evidence supports a possible role for nitric oxide (NO) in the transmission of pain signals and in the development of central mechanisms of hyperalgesia. Previously, we have shown that nitroglycerin, an NO donor, is able to induce a long-lasting hyperalgesic state in rats. Nitroglycerin-induced hyperalgesia can be detected as an increase in the nociceptive behavior evoked by the formalin test. In the present study we investigated the possible mediators in the nitroglycerin-induced hyperalgesic state. Male Sprague-Dawley rats were injected with nitroglycerin and pretreated with indomethacin, 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine hydrogen maleate (MK-801) or N(omega)-nitro-L-arginine methyl ester (L-NAME). The results obtained showed that inhibition of prostaglandins or NO synthesis prevents nitroglycerin-induced hyperalgesia in Phase II of the formalin test. A similar inhibitory effect was also observed following pretreatment with the glutamate antagonist MK801. The present findings point to the role of prostaglandins, NO synthesis and glutamate activity in the induction of nitroglycerin-induced hyperalgesia.

Possible involvement of the spinal nitric oxide/cGMP pathway in vincristine-induced painful neuropathy in mice

The mechanisms that underlie the development of vincristine-induced painful neuropathy are poorly understood. The nitric oxide (NO)-cGMP pathway has been reported to be involved in the spinal transmission of nociceptive information. In the present study, we examined whether alterations in spinal nociceptive processing via the NO-cGMP pathway contribute to vincristine-induced painful neuropathy in mice. Mice were intraperitoneally treated with vincristine at a dose of 0.05 mg/kg 1 day after the measurement of pre-drug latency in the tail-flick test, and then treated with a dose of 0.125 mg/kg twice a week for 6 weeks. In vincristine-treated mice, a significant decrease in tail-flick latencies developed at 4 weeks after treatment. Pretreatment with L-arginine (30-300 mg/kg, s.c.), a substrate of NO synthase (NOS), dose-dependently increased the tail-flick latencies in vincristine-treated mice. The L-arginine-induced increase in tail-flick latencies in vincristine-treated mice was completely reversed by i.t. pretreatment with NG-nitro-L-arginine methyl ester (L-NAME, 3-30 nmol), a NOS inhibitor. Furthermore, i.t. pretreatment with 8-bromoguanosine 3', 5'-cyclic monophosphate (8-Br-cGMP, 0.3-3.0 nmol), a membrane-permeable cGMP analog, dose-dependently increased the tail-flick latencies in vincristine-treated mice. The contents of NO metabolites, cGMP and protein levels of neuronal NOS in the spinal cord in vincristine-treated mice were significantly reduced compared to those in vehicle-treated naive mice. These results indicate that dysfunction of the L-arginine/NO/cGMP cascade in the spinal cord may trigger vincristine-induced thermal hyperalgesia in mice.

GW274150, a novel and highly selective inhibitor of the inducible isoform of nitric oxide synthase (iNOS), shows analgesic effects in rat models of inflammatory and neuropathic pain

Nitric oxide (NO), synthesised by different isoforms of nitric oxide synthase (NOS), has been linked with the development and maintenance of nociception. We studied the role of the inducible isoform, iNOS, in two different rat pain models with an inflammatory component. iNOS was immunohistochemically detected locally in the paw 6h after Freund's Complete Adjuvant (FCA) injection, showing a plateau at 24-72 h and falling slowly in the following weeks. This correlated with the late phase of the hypersensitivity to pain revealed in the behavioural tests. A highly selective iNOS inhibitor GW274150 (1-30 mg/kg orally, 24h after FCA) suppressed the accumulation of nitrite in the inflamed paw indicating substantial iNOS inhibition. At the same time it partially reversed FCA-induced hypersensitivity to pain and edema in a dose-dependent manner. After Chronic Constriction Injury (CCI) surgery to the sciatic nerve, iNOS presence was only detected locally in the region of the nerve (inflammatory cells). GW274150 (3-30 mg/kg orally, 21 days after surgery) also reversed significantly the CCI-associated hypersensitivity to pain. No iNOS was detectable in dorsal root ganglia, spinal cord or brain in either model. This study demonstrates a role for peripherally-expressed iNOS in pain conditions with an inflammatory component and the potential value of iNOS inhibitors in such conditions.

Effect of genetic knockout or pharmacologic inhibition of neuronal nitric oxide synthase on complete Freund's adjuvant-induced persistent pain

Nitric oxide (NO) acts as a neurotransmitter or neuromodulator involving in the modulation of thermal and/or inflammatory hyperalgesia. The neuronal nitric oxide synthase (nNOS) is a key enzyme for NO production in normal neuronal tissues, but its functional role in chronic pain remains unclear. The present study combined a genetic strategy with a pharmacologic approach to address the role of nNOS in the central mechanism of complete Freund's adjuvant (CFA)-induced chronic inflammatory pain. Targeted disruption of the nNOS gene significantly reduced CFA-induced mechanical pain hypersensitivity during the maintenance (but not the development) of inflammatory pain, while it failed to attenuate either development or maintenance of CFA-induced thermal pain hypersensitivity. Intraperitoneal administration of L-N(G)-nitro-arginine methyl ester (L-NAME), a non-specific NOS inhibitor, blocked CFA-evoked thermal and mechanical pain hypersensitivity at both development (2h) and maintenance (24h) phase in wild type mice, but had no effect in the knockout mice. Furthermore, intrathecal injection of either L-NAME or 7-nitroindazole, a selective nNOS inhibitor, markedly attenuated mechanical pain hypersensitivity at both 2 and 24h after CFA injection. Finally, spinal cord nNOS (but not endothelial NOS or inducible NOS) expression was up-regulated at 24h after CFA injection, occurring mainly in the ipsilateral superficial dorsal horn. Together, these data indicate that spinal cord nNOS may be essential for the maintenance of mechanical pain hypersensitivity and that it may also be sufficient for the development of mechanical pain hypersensitivity and for the development and maintenance of thermal pain hypersensitivity after chronic inflammation. Our findings suggest that spinal cord nNOS might play a critical role in central mechanisms of the development and/or maintenance of chronic inflammatory pain.

Inhibition of p38 mitogen-activated protein kinase attenuates interleukin-1beta-induced thermal hyperalgesia and inducible nitric oxide synthase expression in the spinal cord

We have reported recently that intrathecal (i.t.) injection of interleukin-1beta (IL-1beta), at a dose of 100 ng, induces inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) production in the spinal cord and results in thermal hyperalgesia in rats. This study further examines the role of mitogen-activated protein kinase (MAPK) in i.t. IL-1beta-mediated iNOS-NO cascade in spinal nociceptive signal transduction. All rats were implanted with an i.t. catheter either with or without an additional microdialysis probe. Paw withdrawal latency to radiant heat is used to assess thermal hyperalgesia. The iNOS and MAPK protein expression in the spinal cord dorsal horn were examined by western blot. The [NO] in CSF dialysates were also measured. Intrathecal IL-1beta leads to a time-dependent up-regulation of phosphorylated p38 (p-p38) MAPK protein expression in the spinal cord 30-240 min following IL-1beta injection (i.t.). However, neither the phosphorylated extracellular signal-regulated kinase (p-ERK) nor phosphorylated c-Jun NH2-terminal kinase (p-JNK) was affected. The total amount of p38, ERK, and JNK MAPK proteins were not affected following IL-1beta injection. Intrathecal administration of either selective p38 MAPK, or JNK, or ERK inhibitor alone did not affect the thermal nociceptive threshold or iNOS protein expression in the spinal cord. However, pretreatment with a p38 MAPK inhibitor significantly reduced the IL-1beta-induced p-p38 MAPK expression by 38-49%, and nearly completely blocked the subsequent iNOS expression (reduction by 86.6%), NO production, and thermal hyperalgesia. In contrast, both ERK and JNK inhibitor pretreatments only partially (approximately 50%) inhibited the IL-1beta-induced iNOS expression in the spinal cord. Our results suggest that p38 MAPK plays a pivotal role in i.t. IL-1beta-induced spinal sensitization and nociceptive signal transduction.

Differential roles of neuronal and endothelial nitric oxide synthases during carrageenan-induced inflammatory hyperalgesia

The present study investigated the role of neuronal nitric oxide synthase (nNOS) in carrageenan-induced inflammatory pain by combining genomic and pharmacological strategies. Intrathecal injection of the nNOS inhibitor 7-nitroindazole dose-dependently inhibited carrageenan-induced thermal hyperalgesia in both early and late phases in wild-type mice. However in nNOS knockout mice, carrageenan-induced thermal hyperalgesia remained intact in the early phase but was reduced in the late phase. Spinal Ca2+ -dependent nitric oxide synthase (NOS) activity in nNOS knockout mice was significantly lower than that in wild-type mice. Following carrageenan injection, although the spinal Ca2+ -dependent NOS activity in both wild-type and knockout mice increased, the enzyme activity in nNOS knockout mice reached a level similar to that in wild-type mice. On the other hand, no significant difference in spinal Ca2+ -independent NOS activity was noted between wild-type and nNOS knockout mice before and after carrageenan injection. Furthermore, intrathecal administration of the endothelial NOS (eNOS) inhibitor L-N5-(1-iminoethyl)-ornithinein nNOS knockout mice inhibited the thermal hyperalgesia in both early and late phases, though this inhibitor had no effect in wild-type mice. Meanwhile, Western blot showed that eNOS expression in the spinal cord of nNOS knockout mice was up-regulated compared with wild-type mice; immunohistochemical staining showed that the spinal eNOS was mainly distributed in superficial laminae of the dorsal horn. Finally, double staining with confocal analysis showed that the enhanced spinal eNOS was expressed in astrocytes, but not in neurons. Our current results indicate that nNOS plays different roles in the two phases of carrageenan-induced inflammatory pain. In this model, enhanced spinal eNOS appears to compensate for the role of nNOS in nNOS knockout mice.

HIV-1 gp120 stimulates proinflammatory cytokine-mediated pain facilitation via activation of nitric oxide synthase-I (nNOS)

It has become clear that spinal cord glia (microglia and astrocytes) importantly contribute to the creation of exaggerated pain responses. One model used to study this is peri-spinal (intrathecal, i.t.) administration of gp120, an envelope protein of HIV-1 known to activate glia. Previous studies demonstrated that i.t. gp120 produces pain facilitation via the release of glial proinflammatory cytokines. The present series of studies tested whether spinal nitric oxide (NO) contributes to i.t. gp120-induced mechanical allodynia and, if so, what effect NO has on spinal proinflammatory cytokines. gp120 stimulation of acutely isolated lumbar dorsal spinal cords released NO as well as proinflammatory cytokines (tumor necrosis factor-alpha, interleukin-1beta (IL1), interleukin-6 (IL6)), thus identifying NO as a candidate mediator of gp120-induced behavioral effects. Behaviorally, identical effects were observed when gp120-induced mechanical allodynia was challenged by i.t. pre-treatment with either a broad-spectrum nitric oxide synthase (NOS) inhibitor (L-NAME) or 7-NINA, a selective inhibitor of NOS type-I (nNOS). Both abolished gp120-induced mechanical allodynia. While the literature pre-dominantly documents that proinflammatory cytokines stimulate the production of NO rather than the reverse, here we show that gp120-induced NO increases proinflammatory cytokine mRNA levels (RT-PCR) and both protein expression and protein release (serial ELISA). Furthermore, gp120 increases mRNA for IL1 converting enzyme and matrix metalloproteinase-9, enzymes responsible for activation and release of proinflammatory cytokines.

Intrathecal interleukin-1β administration induces thermal hyperalgesia by activating inducible nitric oxide synthase expression in the rat spinal cord

The effect of the pro-inflammatory cytokine interleukin-1beta (IL-1beta) on the inducible nitric oxide synthase-nitric oxide (iNOS-NO) cascade in nociceptive signal transduction was examined in the intact rat spinal cord. All rats were implanted with an intrathecal (i.t.) catheter; some were also implanted with an i.t. microdialysis probe. The paw withdrawal latency to radiant heat was used to assess thermal hyperalgesia. The iNOS protein expression in the spinal cord dorsal horn was examined by western blot analysis and NOS activity assay. NO production in the CSF dialysate was also measured. IL-1beta i.t. (100 ng) produced thermal hyperalgesia from 4 to 24 h after i.t. injection. The iNOS protein expression was induced at 4 h after i.t. IL-1beta injection, peaked at the 6th hour, and disappeared at 24 h. The iNOS activity showed a similar time-dependent change as the iNOS protein expression. NO release increased by 1.1- to 1.9-fold between 4 and 12 h, also with a peak at the 6th hour, after i.t. IL-1beta administration. Pretreatment with the iNOS inhibitor 1400W (10 microg, i.t.) 1 h before i.t. IL-1beta injection prevented all the responses of IL-1beta. Neither 1400W nor artificial CSF (aCSF) affected the thermal nociceptive threshold and NO production. These results demonstrate that i.t. administration of IL-1beta induced thermal hyperalgesia by activating the iNOS-NO cascade in the rat spinal cord. On the basis of the present findings, we suggest that i.t. administration of iNOS inhibitors may have potential in the treatment of inflammatory and neuropathic pain syndromes.

Expression of c-Fos and NADPH-d after peripheral noxious stimulation in the pedunculopontine tegmental nucleus

The participation of pedunculopontine tegmental nucleus in descending pain control mechanisms has been proposed. The aim of this study was to investigate NADPH-d and c-Fos reactivity in the pedunculopontine tegmental nucleus following noxious peripheral stimulation. Formalin induced hind paw inflammation in the rat resulted in bilateral increase of Fos and NADPH-d expression in the pedunculopontine tegmental nucleus. Approximately 30% of labelled cells were double-labelled. Close proximity to NADPH-d labelled neuronal processes with Fos-labelled nuclei were observed. This anatomical relationship supports the hypothesis that pedunculopontine tegmental nucleus is involved in pain modulation and proposes a new explanation for the presence of nitric oxide in this nucleus.

Snake venom phospholipase A2s (Asp49 and Lys49) induce mechanical allodynia upon peri-sciatic administration: involvement of spinal cord glia, proinflammatory cytokines and nitric oxide

Snakebites constitute a serious public health problem in Central and South America, where species of the lancehead pit vipers (genus Bothrops) cause the majority of accidents. Bothrops envenomations are very painful, and this effect is not neutralized by antivenom treatment. Two variants of secretory phospholipases A2 (sPLA2), corresponding to Asp49 and Lys49 PLA2s, have been isolated from Bothrops asper venom. These sPLA2s induce hyperalgesia in rats following subcutaneous injection. However, venom in natural Bothrops bites is frequently delivered intramuscularly, thereby potentially reaching peripheral nerve bundles. Thus, the present series of experiments tested whether these sPLA2s could exert pain-enhancing effects following administration around healthy sciatic nerve. Both were found to produce mechanical allodynia ipsilateral to the injection site; no thermal hyperalgesia was observed. As no prior study has examined potential spinal mechanisms underlying sPLA2 actions, a series of anatomical and pharmacological studies were performed. These demonstrated that both sPLA2s produce activation of dorsal horn astrocytes and microglia that is more prominent ipsilateral to the site of injection. As proinflammatory cytokines and nitric oxide have each been previously implicated in spinally mediated pain facilitation, the effect of pharmacological blockade of these substances was tested. The results demonstrate that mechanical allodynia induced by both sPLA2s is blocked by interleukin-1 receptor antagonist, anti-rat interleukin-6 neutralizing antibody, the anti-inflammatory cytokine interleukin-10, and a nitric oxide synthesis inhibitor (L-NAME). As a variety of immune cells also produce and release sPLA2s during inflammatory states, the data may have general implications for the understanding of inflammatory pain.

Intact carrageenan-induced thermal hyperalgesia in mice lacking inducible nitric oxide synthase

To date, the exact role of inducible nitric oxide synthase (iNOS) in inflammatory pain remains controversial. In the present study, we combined a pharmacological strategy (using a selective iNOS inhibitor) with a genomic strategy (using mice lacking the iNOS gene) to address the function of iNOS in the central mechanism of carrageenan-induced persistent inflammatory pain. In the wild type mice, intrathecal administration of L-N(6)-(1-iminoethyl)-lysine, a selective iNOS inhibitor, significantly inhibited thermal hyperalgesia in the late phase but not in the early phase of carrageenan inflammation. Moreover, iNOS mRNA expression in the lumbar enlargement segments of the spinal cord was dramatically induced at 24 h (late phase) after injection of carrageenan into a hind paw. Interestingly, targeted disruption of iNOS gene did not affect carrageenan-induced thermal hyperalgesia in either the early (2-6 h) or late phase. In the lumbar enlargement segments of iNOS knockout mice, nitric oxide synthase (NOS) enzyme activity remained at a similar level to that of the wild type mice at 24 h after carrageenan injection. We found that intrathecal administration of 7-nitroindazole (a selective neuronal NOS inhibitor), but not L-N(5)-(1-iminoethyl)-ornithine (a selective endothelial NOS inhibitor), significantly reduced carrageenan-induced thermal hyperalgesia in both the early phase and the late phase in iNOS knockout mice. We also found that expression of neuronal NOS but not endothelial NOS in the lumbar enlargement segments was significantly increased in iNOS knockout mice compared with wild type mice at 24 h after carrageenan injection. Our results indicate that neuronal NOS might compensate for the function of iNOS in the late phase of carrageenan-induced inflammatory pain in iNOS knockout mice. This suggests that iNOS may be sufficient, but not essential, for the late phase of the carrageenan-induced thermal hyperalgesia.

Spinal delivery of analgesics in experimental models of pain and analgesia

Systemic administration of analgesics can lead to serious adverse side effects compromising therapeutic benefit in some patients. Information coding pain transmits along an afferent neuronal network, the first synapses of which reside principally in the spinal cord. Delivery of compounds to spinal cord, the intended site of action for some analgesics, is potentially a more efficient and precise method for inhibiting the pain signal. Activation of specific proteins that reside in spinal neuronal membranes can result in hyperpolarization of secondary neurons, which can prevent transmission of the pain signal. This is one of the mechanisms by which opioids induce analgesia. The spinal cord is enriched in such molecular targets, the activation of which inhibit the transmission of the pain signal early in the afferent neuronal network. This review describes the pre-clinical models that enable new target discovery and development of novel analgesics for site-directed pain management.

Role of NO in modulating neuronal activity in superficial dorsal horn of spinal cord during exercise pressor reflex

Static contraction of hindlimb skeletal muscle in cats induces a reflex pressor response. The superficial dorsal horn of the spinal cord is the major site of the first synapse of this reflex. In this study, static contraction of the triceps surae muscle was evoked by electrical stimulation of the tibial nerve for 2 min in anesthetized cats (stimulus parameters: two times motor threshold at 30 Hz, 0.025-ms duration). Ten stimulations were performed and 1-min rest was allowed between stimulations. Muscle contraction caused a maximal increase of 32 +/- 5 mmHg in mean arterial pressure (MAP), which was obtained from the first three contractions. Activated neurons in the superficial dorsal horn were identified by c-Fos protein. Distinct c-Fos expression was present in the L6-S1 level of the superficial dorsal horn ipsilateral to the contracting leg (88 +/- 14 labeled cells per section at L7), whereas only scattered c-Fos expression was observed in the contralateral superficial dorsal horn (9 +/- 2 labeled cells per section, P < 0.05 compared with ipsilateral section). A few c-Fos-labeled cells were found in control animals (12 +/- 5 labeled cells per section, P < 0.05 compared with stimulated cats). Furthermore, double-labeling methods demonstrated that c-Fos protein coexisted with nitric oxide (NO) synthase (NOS) positive staining in the superficial dorsal horn. Finally, an intrathecal injection of an inhibitor of NOS, N-nitro-L-arginine methyl ester (5 mM), resulted in fewer c-Fos-labeled cells (58 +/- 12 labeled cells per section) and a reduced maximal MAP response (20 +/- 3 mmHg, P < 0.05). These results suggest that the exercise pressor reflex induced by static contraction is mediated by activation of neurons in the superficial dorsal horn and that formation of NO in this region is involved in modulating the activated neurons and the pressor response to contraction.

Activation and up-regulation of spinal cord nitric oxide receptor, soluble guanylate cyclase, after formalin injection into the rat hind paw

Nitric oxide synthase is expressed abundantly in the spinal cord, and nitric oxide (NO) has been shown to play important roles in the central mechanism of inflammatory hyperalgesia. However, the expression and function of the NO receptor, soluble guanylate cyclase, is not fully understood in this processing at the spinal cord level. In the present study, we report that the soluble guanylate cyclase alpha(1) subunit but not the beta(1) subunit was expressed in rat spinal cord, particularly in the dorsal horn. We showed that intrathecal administration of a selective inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, produced a significant anti-nociception demonstrated by the decrease in the number of flinches and shakes in the formalin-induced inflammatory pain model. This was accompanied by a marked reduction in formalin-induced c-fos expression in the spinal cord. During formalin-induced long-lasting inflammation, we found that the expression of the alpha(1) subunit of soluble guanylate cyclase was dramatically increased in the lumbar spinal cord on the second and fourth days after formalin injection into the dorsal side of a hind paw. Intraperitoneal pretreatment with an N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine maleate (MK-801), and a neuronal NO synthase inhibitor, 7-nitroindazole, not only significantly blocked formalin-induced secondary thermal hyperalgesia but also suppressed formalin-produced increase in the alpha(1) subunit of soluble guanylate cyclase in the spinal cord. The present results indicate that peripheral inflammation not only initially activates but also later up-regulates soluble guanylate cyclase expression via the NMDA receptor-NO signaling pathway, suggesting that soluble guanylate cyclase might be involved in the central mechanism of formalin-induced inflammatory hyperalgesia in the spinal cord.

Intrathecal clonidine decreases spinal nitric oxide release in a rat model of complete Freund's adjuvant induced inflammatory pain

A long-lasting antihyperalgesic effect has been demonstrated for intrathecal (IT) clonidine, an alpha2-adrenergic agonist. In the present study, the mechanism and antihyperalgesic effects of IT clonidine were examined post-treatment in a rat model of Complete Freund's Adjuvant (CFA)-induced inflammatory hyperalgesia. Using a chronic model of spinal cord dialysis, we examined the effect of the adjuvant-induced inflammation on spinal release of nitric oxide (NO) and the development of chronic pain and assessed the antinociceptive effects and mechanisms of the alpha2-adrenergic agonist, clonidine (IT). Chronic, persistent inflammatory pain was induced by left hind paw injection of 0.3 ml CFA prepared in a mixture with Mycobacterium butyricum. Rats were randomly assigned to groups receiving IT clonidine in discrete doses of 1, 10 or 50 microg, 3 or 24 hr post-inflammation. Measurement of total NOx (NO + NO2- + NO3-) was used to determine NO release into the cerebrospinal fluid. Rat thermal antinociception was assessed using a radiant heat thermal hyperalgesia model. CFA injection resulted in significant thermal hyperalgesia throughout the four days of observation. A dose-dependent suppression of thermal hyperalgesia and spinal NO release was observed after IT clonidine treatment. Evidence from this CFA-induced inflammatory pain model suggests that clonidine's spinal antihyperalgesic mechanisms act through inhibition of spinal NO release.

The role of nitric oxide in the phosphorylation of cyclic adenosine monophosphate—responsive element-binding protein in the spinal cord after intradermal injection of capsaicin

We investigated the involvement of nitric oxide (NO) in the phosphorylation of cyclic adenosine monophosphate-responsive element-binding protein (CREB) in the spinal cord of rats during central sensitization after intradermal capsaicin injection. CREB and phosphorylated CREB (p-CREB) were measured by immunoblotting. The level of p-CREB increased by 20 minutes, peaked between 20 and 60 minutes after capsaicin injection, and started to decrease after 150 minutes. CREB itself did not show an obvious change after capsaicin injection. The p-CREB expression on the ipsilateral side of the spinal dorsal horn, but not on the contralateral side, increased significantly after capsaicin injection. The increase in p-CREB induced by capsaicin injection was partially blocked by pretreatment with N(G)-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, administered through a microdialysis fiber placed across the spinal cord. D-NAME, an inactive form of L-NAME, had no effect. CREB phosphorylation, not the level of CREB, was induced within 20 minutes by microdialysis administration of SIN-1, an NO donor. These results indicate that CREB phosphorylation in the spinal cord results from both endogenous and exogenous NO release and that p-CREB may play a role in central sensitization or in longer-term changes in gene expression induced by strong peripheral noxious stimulation.

Does nitric oxide play a role in orofacial pain transmission?

Although the synaptology, neural connectivity, and the roles played by nitric oxide (NO) and other neurotransmitters have been extensively studied in spinal pain, such information is rather scanty with respect to orofacial pain transmission. This paper presents the findings of several investigations carried out by the author and his colleagues on the roles of NO in orofacial pain transmission in male Wistar rats, using nicotinamide adenosine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry using light and electron microscopy; and NOS immunohistochemistry and immunofluorescence using both light and confocal laser scanning microscopy. The results revealed that (1) a complicated relation existed between the nitrergic axon terminals and dendrites in the caudal part of the spinal trigeminal nucleus (cSTN); (2) the nitrergic neuronal cells bodies were not projection neurons, but rather, local circuit neurons; (3) although the thalamus projecting neurons in the cSTN did not synthesize NO, they could be modulated by NO diffused from nitrergic neurons; (4) c-fos positive neurons in the superficial laminae of the cSTN, detected following subcutaneous injection of 0.5 ml of 4% formalin into the left lateral face of the rats, respond to the release of glutamate through activation of N-methyl-D-aspartate (NMDA), alpha-amine-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) and metabotropic glutamate (mGlu) receptors expressed by these c-fos neurons; and (5) NO might play a seemingly less important role than glutamate in neural transmission.

Subcutaneous formalin and intraplantar carrageenan increase nitric oxide release as measured by in vivo voltammetry in the spinal cord

The paper describes in vivo voltammetric detection of nitric oxide with carbon fibre microelectrodes at the lumbar spinal dorsal horn level of decerebrated-spinalized rats during peripheral noxious inflammatory processes. At the lumbar (L3-L4) dorsal horn level, a nitric oxide dependent peak of oxidation current (650 mV), remaining stable for up to 4h ((92 +/- 5)% of control) could be detected indicating that significant amounts of nitric oxide are produced continuously. Following subcutaneous injection in the hindpaw of 50 microl of 0.5% formalin the oxidation current rapidly increased ((115 +/- 5)% of control at 25 min) and reached (120 +/- 6)% of control 1h later. Subsequently the voltammograms stabilized for up to 90 min and decreased ((107 +/- 4)% at 124 min). After an injection in the hindpaw of 150 microl of 4% carrageenan, the voltammograms remained at control level for 1h and then the oxidation current increased continuously for up to 4h ((145 +/- 16)% of control at 240 min); such an increase was reversed by ketamine. In these two models of inflammation, the delay in onset and the duration of the increases in NO release within the dorsal horn relate, to some extent, to the time course of the peripheral inflammatory processes, since they are shorter after formalin than after carrageenan. The results provide a direct in vivo demonstration that the intercellular messenger nitric oxide participates in the transmission of noxious afferent messages within the dorsal horn of the spinal cord following peripheral inflammation.

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.

The role of CNS NMDA receptors and nitric oxide in visceral hyperalgesia

The studies summarized here document the role of NMDA receptors and nitric oxide in the lumbosacral spinal cord and rostral ventromedial medulla in the maintenance of visceral hyperalgesia. Experiments were conducted in rats in which drugs were administered into either the lumbosacral intrathecal space or directly into the rostral ventromedial medulla. The visceral stimulus was noxious colorectal distension, administered before and 3 h after intracolonic instillation of either saline or 25% zymosan. The visceromotor response to colonic distension was quantified and found to be significantly enhanced in rats in which the colon had previously been treated with zymosan. Enhanced responses to distension were attenuated dose-dependently by intrathecal administration of the NMDA receptor channel blocker MK-801 and by inhibition of the neuronal isoform of nitric oxide synthase (nNOS). In corresponding studies wherein drugs were administered directly into the rostral ventromedial medulla, NMDA receptor antagonism and NOS inhibition dose-dependently attenuated exaggerated responses to colonic distension. Taken together, these data suggest that zymosan-produced visceral hyperalgesia is influenced both at the level of the spinal cord and rostral ventromedial medulla, and that descending facilitatory influences from the rostral ventromedial medulla are important to the maintenance of visceral hyperalgesia.

Secondary somatosensory cortex stimulation facilitates the antinociceptive effect of the NO synthase inhibitor through suppression of spinal nociceptive neurons in the rat

Electrical stimulation of the secondary somatosensory cortex (S-II), which is clinically effective in some chronic pain patients, produces a weak antinociception by itself and also strongly facilitates the antinociceptive effect of the neuronal NO synthase inhibitor 7-nitro-indazole in laboratory animals (rats). The present study thus investigated the mechanisms by which S-II stimulation facilitates the 7-nitro-indazole-induced antinociception. S-II stimulation in combination with 7-nitro-indazole at a subeffective dose, 5 mg/kg, synergistically reduced the number of cells expressing c-Fos in response to intraplantar injection of formalin in the superficial regions (laminae I and II) of the L4 and L5 spinal dorsal horn in conscious rats, although each had no significant effect. A similar synergism produced by S-II stimulation and 7-nitro-indazole was also confirmed in both the first and second phases in the formalin-induced behavioral nociception test. The synergistic antinociception exerted by S-II stimulation in combination with 7-nitro-indazole was resistant to systemic administration of the opioid antagonist naloxone or the alpha-adrenoceptor antagonist phentolamine. In contrast, intrathecally administered methysergide, a serotonin receptor antagonist, at 20 microg/rat, abolished the first-phase, but not the second-phase, antinociception following S-II stimulation in combination with 7-nitro-indazole. These findings suggest that S-II stimulation, in combination with inhibition of neuronal NO synthase, can suppress spinal nociceptive neurons, at least in part through the descending spinal serotonergic pathway, resulting in antinociception.

Release of glutamate, nitric oxide and prostaglandin E2 and metabolic activity in the spinal cord of rats following peripheral nociceptive stimulation

Peripheral tissue injury and inflammation may result in a facilitated spinal nociceptive transmission and central sensitization. Particularly, nitric oxide (NO) and prostaglandins (PGs) have been shown to be key mediators involved in the induction and maintenance of this state. By means of spinal cord microdialysis we have determined interstitial glutamate, NO (NO2-/NO3-), PGE2, glycerol, glucose and lactate concentrations in the dorsal horns of the spinal cord following peripheral nociceptive stimulation to gain further insight into the link between excitatory neurotransmitters and metabolic functions in the spinal cord during nociception. Formalin and zymosan injection into one hind paw evoked a biphasic release of glutamate and NO with the glutamate peaks preceding those of NO. Moreover, zymosan induced a biphasic increase of interstitial glycerol concentrations accompanied by an increase of interstitial lactate indicating metabolic disturbances. In contrast, formalin injection led to an elevation of dialysate glucose concentrations which may be interpreted as an indication of enhanced metabolic activity. The sequential release of glutamate and NO in the dorsal horns of the spinal cord in response to peripheral nociceptive stimulation supports the theory that NO may act as a retrograde transmitter. The metabolic changes observed after formalin and zymosan injection suggest that an intense peripheral nociceptive stimulation may not only activate but also disturb metabolic activity and possibly membrane integrity in the spinal cord.