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

Nitric Oxide in the Spinal Cord Is Involved in the Hyperalgesia Induced by Tetrahydrobiopterin in Chronic Restraint Stress Rats

It has been well recognized that exposure to chronic stress could increase pain responding and exacerbate pain symptoms, resulting in stress-induced hyperalgesia. However, the mechanisms underlying stress-induced hyperalgesia are not yet fully elucidated. To this end, we observed that restraint as a stressful event exacerbated mechanical and thermal hyperalgesia, accompanied with up-regulation of nitric oxide (NO) (P < 0.001), GTP cyclohydrolase 1 (GCH1) (GCH1 mRNA: P = 0.001; GCH1 protein: P = 0.001), and tetrahydrobiopterin (BH4) concentration (plasma BH4: P < 0.001; spinal BH4: P < 0.001) on Day 7 in restraint stress (RS) rats. Intrathecal injection of N ^ω-nitro-L-arginine methyl ester (L-NAME), a non-specific NO synthase inhibitor, or N-([3-(aminomethyl)phenyl]methyl) ethanimidamide, a special inhibitor of inducible NO synthase (iNOS), for seven consecutive days attenuated stress-induced hyperalgesia and decreased the production of NO (P < 0.001). Interestingly, 7-nitro indazole, a special inhibitor of neuronal NO synthase, alleviated stress-induced hyperalgesia but did not affect spinal NO synthesis. Furthermore, intrathecal injection of BH4 not only aggravated stress-induced hyperalgesia but also up-regulated the expression of spinal iNOS (iNOS mRNA: P = 0.015; iNOS protein: P < 0.001) and NO production (P < 0.001). These findings suggest that hyperalgesia induced by RS is associated with the modulation of the GCH1-BH4 system and constitutively expressed spinal iNOS. Thus, the GCH1-BH4-iNOS signaling pathway may be a new novel therapeutic target for pain relief in the spinal cord.

Oral Ultramicronized Palmitoylethanolamide: Plasma and Tissue Levels and Spinal Anti-hyperalgesic Effect

Palmitoylethanolamide (PEA) is a pleiotropic lipid mediator with established anti-inflammatory and anti-hyperalgesic activity. Ultramicronized PEA (PEA-um) has superior oral efficacy compared to naïve (non-micronized) PEA. The aim of the present study was two-fold: (1) to evaluate whether oral PEA-um has greater absorbability compared to naïve PEA, and its ability to reach peripheral and central tissues under healthy and local inflammatory conditions (carrageenan paw edema); (2) to better characterize the molecular pathways involved in PEA-um action, particularly at the spinal level. Rats were dosed with 30 mg/kg of [13C]4-PEA-um or naïve [13C]4-PEA by oral gavage, and [13C]4-PEA levels quantified, as a function of time, by liquid chromatography/atmospheric pressure chemical ionization/mass spectrometry. Overall plasma levels were higher in both healthy and carrageenan-injected rats administered [13C]4-PEA-um as compared to those receiving naïve [13C]4-PEA, indicating the greater absorbability of PEA-um. Furthermore, carrageenan injection markedly favored an increase in levels of [13C]4-PEA in plasma, paw and spinal cord. Oral treatment of carrageenan-injected rats with PEA-um (10 mg/kg) confirmed beneficial peripheral effects on paw inflammation, thermal hyperalgesia and tissue damage. Notably, PEA-um down-regulated distinct spinal inflammatory and oxidative pathways. These last findings instruct on spinal mechanisms involved in the anti-hyperalgesic effect of PEA-um in inflammatory pain.

Laser acupuncture-induced analgesic effect and molecular alterations in an incision pain model: a comparison with electroacupuncture-induced effects

Low-level laser acupuncture (LLLA) produces photobiomodulation through acupuncture point and is an alternative to low-level laser therapy. Although the analgesic effect of LLLA on chronic pain has been proven, its effect on acute postincisional pain has yet to be investigated. A plantar incision (PI) model was used to mimic human postsurgical pain. Male adult rats received GaAlAs laser irradiation at the right ST36 acupoint immediately after operation and on the following 4 days. Three laser treatment groups (two red laser groups with a 30- or 15-min treatment duration and one 30-min near-infrared laser group) were compared with sham LLLA and naive groups and an electroacupuncture (EA) group (separate study). Behavioral withdrawal thresholds of both hind paws were measured before and after incision. Expression of mitogen-activated protein kinases (p-ERK and p-p38), inducible nitric oxide synthase (iNOS), and tumor necrosis factor (TNF) in the spinal cord was analyzed. All three LLLA treatments attenuated post-PI tactile allodynia in the ipsilateral paw, but only the 30-min red laser treatment affected the contralateral paw and had similar efficacy to that of EA. All laser treatments barely reduced heat hyperalgesia in both hind paws. At 3 days after PI, the 30-min red laser group showed reversed increases of PI-induced p-ERK, p-p38, and iNOS but not TNF expression in the spinal cord. Repetitive LLLA treatments ameliorated PI-induced mechanical pain. The inhibition of multiple sensitization signals highlights the unique clinical role of LLLA. Thus, LLLA is an alternative to EA as an adjuvant for postoperative pain control.

Endogenous nitric oxide inhibits spinal NMDA receptor activity and pain hypersensitivity induced by nerve injury

The role of nitric oxide (NO) in nociceptive transmission at the spinal cord level remains uncertain. Increased activity of spinal N-methyl-d-aspartate (NMDA) receptors contributes to development of chronic pain induced by peripheral nerve injury. In this study, we determined how endogenous NO affects NMDA receptor activity of spinal cord dorsal horn neurons in control and spinal nerve-ligated rats. Bath application of the NO precursor l-arginine or the NO donor S-nitroso-N-acetylpenicillamine (SNAP) significantly inhibited NMDA receptor currents of spinal dorsal horn neurons in both sham control and nerve-injured rats. Inhibition of neuronal nitric oxide synthase (nNOS) or blocking the S-nitrosylation reaction with N-ethylmaleimide abolished the inhibitory effects of l-arginine on NMDA receptor currents recorded from spinal dorsal horn neurons in sham control and nerve-injured rats. However, bath application of the cGMP analog 8-bromo-cGMP had no significant effects on spinal NMDA receptor currents. Inhibition of soluble guanylyl cyclase also did not alter the inhibitory effect of l-arginine on spinal NMDA receptor activity. Furthermore, knockdown of nNOS with siRNA abolished the inhibitory effects of l-arginine, but not SNAP, on spinal NMDA receptor activity in both groups of rats. Additionally, intrathecal injection of l-arginine significantly attenuated mechanical or thermal hyperalgesia induced by nerve injury, and the l-arginine effect was diminished in rats treated with a nNOS inhibitor or nNOS-specific siRNA. These findings suggest that endogenous NO inhibits spinal NMDA receptor activity through S-nitrosylation. NO derived from nNOS attenuates spinal nociceptive transmission and neuropathic pain induced by nerve injury.

Spinal AMPA Receptor GluA1 Ser831 Phosphorylation Controls Chronic Alcohol Consumption-Produced Prolongation of Postsurgical Pain

Previous studies have shown that excessive alcohol drinking is associated with chronic pain development; however, the molecular mechanism underlying this association is poorly understood. In this study, we investigated the effect of chronic alcohol consumption on plantar incision-induced postsurgical pain. We observed that 4-week ethanol drinking significantly prolonged plantar incision-induced mechanical pain, but not thermal pain. The chronic alcohol consumption enhanced plantar incision-produced α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluA1 phosphorylation at the Ser831 site in the spinal cord. The targeted mutation of the GluA1 phosphorylation site in GluA1 S831A mutant mice significantly inhibited the incisional pain prolongation produced by chronic alcohol consumption. Moreover, chronic alcohol consumption combined with plantar incision markedly increased AMPA receptor-mediated miniature excitatory postsynaptic currents in the spinal dorsal horn neurons, and this effect was diminished significantly in the GluA1 S831A mutant mice. Our results suggest that chronic alcohol consumption may promote the development of persistent postsurgical pain by enhancing AMPA receptor GluA1 Ser831 phosphorylation. We identified chronic alcohol consumption as a risk factor for pain chronification after surgery.

PKMζ Is Not Required for Development of Postsurgical Pain

Previous studies have shown that protein kinase M zeta (PKMζ), a brain-specific isoform of protein kinase C, is involved in the central processing of nociception in several pain models by using a synthetic zeta inhibitory peptide. In the present study, we investigated whether PKMζ contributes to the pathogenesis of postsurgical pain using both conditional and conventional PKMζ knockout mice. Our results showed that the expression of PKMζ in anterior cingulate cortex, but not spinal cord, of the conditional PKMζ knockout mice was inhibited following tamoxifen injection. And the conditional PKMζ knockout mice displayed similar plantar incision-produced postsurgical pain responses as those in wild-type mice. Moreover, the expression of PKMζ was inhibited in both anterior cingulate cortex and spinal cord of the conventional PKMζ knockout mice. And there were no significant differences in the development of postsurgical pain among wild-type, heterozygous, and homozygous conventional PKMζ knockout mice. These data suggest that PKMζ is not required for the development of postsurgical pain after plantar incision.

Changes in nitric oxide synthase isoforms in the trigeminal ganglion of rat following chronic tooth pulp inflammation

Nitric oxide (NO) possibly plays an important role in the events resulting in hyperalgesia. NO synthase (NOS) is a key enzyme in the production of NO. Changes in NOS expression in primary sensory neurons may be involved in the persistent sensory abnormalities that can be induced by inflammation. To assess the possible roles of NOS in trigeminal sensory system, we studied changes in the expression of NOS isoforms in the trigeminal ganglion (TG) following chronic inflammation after pulp exposure (PX) in rats. The neurons innervating injured tooth in the TG were labeled by fluoro-gold (FG). Immunohistochemical staining was used to reveal the presence of NOS. The results showed that within the FG-labeled population, neuron counts revealed a significant increase in the proportion of NOS neurons following PX, in which the frequency of iNOS and nNOS-positive neurons started to increase at 3 and 7day, respectively, and peaked at 28day. There was no eNOS expression observed in the control group and PX-treated groups. The results demonstrate that PX-induced chronic pulpal inflammation results in significant increase of nNOS and iNOS in the TG. It suggests that nNOS and iNOS could be involved in mediation of peripheral processing of nociceptive information following chronic tooth pulp inflammation.

Nitric oxide-mediated pain processing in the spinal cord

A large body of evidence indicates that nitric oxide (NO) plays an important role in the processing of persistent inflammatory and neuropathic pain in the spinal cord. Several animal studies revealed that inhibition or knockout of NO synthesis ameliorates persistent pain. However, spinal delivery of NO donors caused dual pronociceptive and antinociceptive effects, pointing to multiple downstream signaling mechanisms of NO. This review summarizes the localization and function of NO-dependent signaling mechanisms in the spinal cord, taking account of the recent progress made in this field.

Involvement of NO-cGMP pathway in anti-hyperalgesic effect of PDE5 inhibitor tadalafil in experimental hyperalgesia

The association of elevated level of cyclic guanosine monophosphate (cGMP) with inhibition of hyperalgesia and involvement of nitric oxide (NO)-cGMP pathway in the modulation of pain perception was previously reported. Phosphodiesterases 5 (PDE5) inhibitors, sildenafil and tadalafil (TAD) used in erectile dysfunction, are known to act via the NO-cGMP pathway. TAD exerts its action by increasing the levels of intracellular cGMP. Hence, the present study investigated the effect of TAD 5, 10, or 20 mg/kg, per os (p.o.) or L-NAME 20 mg/kg, intraperitoneally (i.p.) and TAD (20 mg/kg, p.o.) in carrageenan- and diabetes-induced hyperalgesia in rats using hot plate test at 55 ± 2 °C. In carrageenan- and diabetes-induced hyperalgesia, TAD (10 and 20 mg/kg, p.o.) significantly increased paw withdrawal latencies (PWLs) as compared to the control group. L-NAME significantly decreased PWLs as compared to the normal group and aggravated the hyperalgesia. Moreover, significant difference in PWLs of L-NAME and TAD 20 was evident. Co-administration of L-NAME (20 mg/kg) with TAD (20 mg/kg) showed significant difference in PWLs as compared to the TAD (20 mg/kg), indicating L-NAME reversed and antagonized TAD-induced anti-hyperalgesia. This suggested an important role of NO-cGMP pathway in TAD-induced anti-hyperalgesic effect.

The relationship between opioids and immune signalling in the spinal cord

Opioids are considered the gold standard for the treatment of moderate to severe pain. However, heterogeneity in analgesic efficacy, poor potency and side effects are associated with opioid use, resulting in dose limitations and suboptimal pain management. Traditionally thought to exhibit their analgesic actions via the activation of the neuronal G-protein-coupled opioid receptors, it is now widely accepted that neuronal activity of opioids cannot fully explain the initiation and maintenance of opioid tolerance, hyperalgesia and allodynia. In this review we will highlight the evidence supporting the role of non-neuronal mechanisms in opioid signalling, paying particular attention to the relationship of opioids and immune signalling.

Treatment with a carbon monoxide-releasing molecule inhibits chronic inflammatory pain in mice: nitric oxide contribution

RATIONALE

Carbon monoxide synthetized by inducible heme oxygenase (HO-1) exerts potent anti-inflammatory and antinociceptive effects during acute and neuropathic pain, but its role in the modulation of chronic inflammatory pain and the possible involvement of nitric oxide in this action remain unknown.

OBJECTIVES AND METHODS

The antiallodynic and antihyperalgesic effects of a carbon monoxide releasing molecule, tricarbonyldichlororuthenium(II) dimer (CORM-2), daily administered from days 4 to 14 after complete Freund's adjuvant (CFA) injection in wild-type (WT), neuronal (NOS1-KO), and inducible (NOS2-KO) nitric oxide synthases knockout mice, were evaluated using von Frey filaments and plantar tests. Effects of CORM-2 treatment on the expression of HO-1, NOS1, and NOS2 at 14 days after inflammation induction were assessed by Western blot.

RESULTS

Main inflammatory pain symptoms induced by CFA in WT, NOS1-KO, and NOS2-KO mice were significantly reduced in a time-dependent manner by CORM-2 treatment. In all genotypes, inflammation increased the dorsal root ganglia and paw expression of HO-1, but CORM-2 treatment only over-expressed this enzyme in the paw of all genotypes. The increased NOS1 expression induced by inflammation in WT mice was abolished by CORM-2 treatment, while there was no effect of the inflammation in neither CORM-2 treatment in the expression of NOS2 in WT and NOS1-KO mice.

CONCLUSIONS

CORM-2 treatment inhibits inflammatory pain through enhancing HO-1 paw expression in all genotypes and reducing NOS1 over-expression in WT mice. An interaction between HO-1/carbon monoxide and NOS1/nitric oxide systems was also demonstrated. CORM-2 treatment may represent a new approach for management chronic inflammatory pain.

Regulation of nociceptive transduction and transmission by nitric oxide

The potential involvement of nitric oxide (NO), a diffusible gaseous signaling messenger, in nociceptive transduction and transmission has been extensively investigated. However, there is no consistent and convincing evidence supporting the pronociceptive action of NO at the physiological concentration, and the discrepancies are possibly due to the nonspecificity of nitric oxide synthase inhibitors and different concentrations of NO donors used in various studies. At the spinal cord level, NO predominantly reduces synaptic transmission by inhibiting the activity of NMDA receptors and glutamate release from primary afferent terminals through S-nitrosylation of voltage-activated calcium channels. NO also promotes synaptic glycine release from inhibitory interneurons through the cyclic guanosine monophosphate/protein kinase G signaling pathway. Thus, NO probably functions as a negative feedback regulator to reduce nociceptive transmission in the spinal dorsal horn during painful conditions.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Lack of effect of chronic pre-treatment with the FAAH inhibitor URB597 on inflammatory pain behaviour: evidence for plastic changes in the endocannabinoid system

BACKGROUND AND PURPOSE

Elevating levels of endocannabinoids with inhibitors of fatty acid amide hydrolase (FAAH) is a major focus of pain research, purported to be a safer approach devoid of cannabinoid receptor-mediated side effects. Here, we have determined the effects of sustained pharmacological inhibition of FAAH on inflammatory pain behaviour and if pharmacological inhibition of FAAH was as effective as genetic deletion of FAAH on pain behaviour.

EXPERIMENTAL APPROACH

Effects of pre-treatment with a single dose, versus 4 day repeated dosing with the selective FAAH inhibitor, URB597 (i.p. 0.3 mg·kg⁻¹), on carrageenan-induced inflammatory pain behaviour and spinal pro-inflammatory gene induction were determined in rats. Effects of pain induction and of the drug treatments on levels of arachidonoyl ethanolamide (AEA), palmitoyl ethanolamide (PEA) and oleolyl ethanolamide (OEA) in the spinal cord were determined.

KEY RESULTS

Single, but not repeated, URB597 treatment significantly attenuated the development of inflammatory hyperalgesia (P < 0.001, vs. vehicle-treated animals). Neither mode of URB597 treatment altered levels of AEA, PEA and OEA in the hind paw, or carrageenan-induced paw oedema. Single URB597 treatment produced larger increases in AEA, PEA and OEA in the spinal cord, compared with those after repeated administration. Single and repeated URB597 treatment decreased levels of immunoreactive N-acylphosphatidylethanolamine phospholipase D (NAPE-PLD) in the spinal cord and attenuated carrageenan-induced spinal pro-inflammatory gene induction.

CONCLUSION AND IMPLICATIONS

Changes in the endocannabinoid system may contribute to the loss of analgesic effects following repeated administration of low dose URB597 in this model of inflammatory pain.

Sinularin from indigenous soft coral attenuates nociceptive responses and spinal neuroinflammation in carrageenan-induced inflammatory rat model

Three decades ago, the marine-derived compound sinularin was shown to have anti-edematous effects on paw edema induced by carrageenan or adjuvant. To the best of our knowledge, no new studies were conducted to explore the bioactivity of sinularin until we reported the analgesic properties of sinularin based on in vivo experiments. In the present study, we found that sinularin significantly inhibits the upregulation of proinflammatory proteins, inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) and upregulates the production of transforming growth factor-β (TGF-β) in lipopolysaccharide (LPS)-stimulated murine macrophage RAW 264.7 cells according to western blot analysis. We found that subcutaneous (s.c.) administration of sinularin (80 mg/kg) 1 h before carrageenan injection significantly inhibited carrageenan-induced nociceptive behaviors, including thermal hyperalgesia, mechanical allodynia, cold allodynia, and hindpaw weight-bearing deficits. Further, s.c. sinularin (80 mg/kg) significantly inhibited carrageenan-induced microglial and astrocyte activation as well as upregulation of iNOS in the dorsal horn of the lumbar spinal cord. Moreover, s.c. sinularin (80 mg/kg) inhibited carrageenan-induced tissue inflammatory responses, redness and edema of the paw, and leukocyte infiltration. The results of immunohistochemical studies indicate that s.c. sinularin (80 mg/kg) could upregulate production of TGF-β1 in carrageenan-induced inflamed paw tissue. The present results demonstrate that systemic sinularin exerts analgesic effects at the behavioral and spinal levels, which are associated with both inhibition of leukocyte infiltration and upregulation of TGF-β1.

Discovery of a potent, orally bioavailable and highly selective human neuronal nitric oxide synthase (nNOS) inhibitor, N-(1-(piperidin-4-yl)indolin-5-yl)thiophene-2-carboximidamide as a pre-clinical development candidate for the treatment of migraine

We recently reported a series of 1,6-disubstituted indoline-based thiophene amidine compounds (5) as selective neuronal nitric oxide synthase (nNOS) inhibitors to mitigate the cardiovascular liabilities associated with hERG K(+) channel inhibition (IC(50) = 4.7 μM) with previously reported tetrahydroquinoline-based selective nNOS inhibitors (4). The extended structure-activity relationship studies within the indoline core led to the identification of 43 as a selection candidate for further evaluations. The in vivo activity in two different pain (spinal nerve ligation and migraine pain) models, the excellent physicochemical and pharmacokinetic properties, oral bioavailability (F(po) = 91%), and the in vitro safety profile disclosed in this report make 43 an ideal candidate for further evaluation in clinical applications related to migraine pain.

Masseter inflammation differentially regulates three nitric oxide synthases in the rat trigeminal subnucleus caudalis

OBJECTIVE

The aim of the present study was to evaluate changes in expression levels of three nitric oxide synthases (NOSs), namely inducible NOS (iNOS), neuronal NOS (nNOS) and endothelial NOS (eNOS), in the subnucleus caudalis of the trigeminal sensory nuclear complex (Vc) under experimental myositis conditions.

DESIGN

Male Sprague Dawley rats were injected with an inflammatory agent, complete Freund's adjuvant (CFA), or capsaicin in the masseter muscle. The brainstem region containing the Vc was extracted at both immediate (30 and 60 min) and longer (1, 3, 7 days) time points to examine the changes in the three NOS protein levels via the Western blot technique. Subsequently, the RT-PCR experiments were carried out to verify the changes in iNOS mRNA.

RESULTS

Following the injections of CFA, there were no significant changes in the level of the three NOS proteins at the immediate time points. However, there was a significant upregulation of iNOS mRNA and protein 3 days after CFA-induced inflammation. Neither nNOS nor eNOS showed significant changes in the protein level at any of the longer time points. Capsaicin injection in the masseter, which we recently reported to upregulate all three NOS at the immediate time points, did not result in significant changes at longer time points.

CONCLUSION

Acute and chronic muscle inflammation differentially modulates the expression of the three NOS in the Vc. These data suggest that the contribution of each NOS in craniofacial muscle pain processing under inflammatory conditions may be anticipated with distinct temporal profiles.

NOpiates: Novel Dual Action Neuronal Nitric Oxide Synthase Inhibitors with μ-Opioid Agonist Activity

A novel series of benzimidazole designed multiple ligands (DMLs) with activity at the neuronal nitric oxide synthase (nNOS) enzyme and the μ-opioid receptor was developed. Targeting of the structurally dissimilar heme-containing enzyme and the μ-opioid GPCR was predicated on the modulatory role of nitric oxide on μ-opioid receptor function. Structure-activity relationship studies yielded lead compound 24 with excellent nNOS inhibitory activity (IC50 = 0.44 μM), selectivity over both endothelial nitric oxide synthase (10-fold) and inducible nitric oxide synthase (125-fold), and potent μ-opioid binding affinity, K i = 5.4 nM. The functional activity as measured in the cyclic adenosine monosphospate secondary messenger assay resulted in full agonist activity (EC50 = 0.34 μM). This work represents a novel approach in the development of new analgesics for the treatment of pain.

The three isoforms of nitric oxide synthase distinctively affect mouse nocifensive behavior

Nitric oxide synthases (NOSs) have been shown to modulate thermal hyperalgesia and mechanical hypersensitivity in inflammatory and neuropathic pain. However, little is known about the effect of NOSs on baseline function of sensory nerve fibers. Using genetic deficiency and pharmacologic inhibition of NOSs, we examined the impact of the three isoforms NOS1, NOS2, and NOS3 on baseline nocifensive behavior by measuring current vocalization threshold in response to electrical stimulation at 5, 250, 2000 Hz that preferentially stimulate C, Aδ, and Aβ fibers. In response to 5, 250 and 2000 Hz, NOS1-deficient animals had significantly higher current vocalization thresholds compared with wild-type. Genetic deficiency of NOS2 was associated with higher current vocalization thresholds in response to 5 Hz (C-fiber) stimulation. In contrast, NOS3-deficient animals had an overall weak trend toward lower current vocalization thresholds at 5 Hz and significantly lower current vocalization threshold compared with wild-type animals at 250 and 2000 Hz. Therefore, NOSs distinctively affect baseline mouse current vocalization threshold and appear to play a role on nocifensive response to electrical stimulation of sensory nerve fibers.

Discovery of N-(3-(1-methyl-1,2,3,6-tetrahydropyridin-4-yl)-1H-indol-6-yl) thiophene-2-carboximidamide as a selective inhibitor of human neuronal nitric oxide synthase (nNOS) for the treatment of pain

3,6-Disubstituted indole derivatives were designed, synthesized, and evaluated as inhibitors of human nitric oxide synthase (NOS). Bulky amine containing substitution on the 3-position of the indole ring such as an azabicyclic system showed better selectivity over 5- and 6-membered cyclic amine substitutions. Compound (-)-19 showed the best selectivity for neuronal NOS over endothelial NOS (90-fold) and inducible NOS (309-fold) among the current series. Compounds 16 and (-)-19 were shown to be either inactive or very weak inhibitors of human cytochrome P450 enzymes, indicating a low potential for drug-drug interactions. Compound 16 was shown to reverse thermal hyperalgesia in vivo in the Chung model of neuropathic pain. Compound 16 was also devoid of any significant vasoconstrictive effect in human coronary arteries, associated with the inhibition of human eNOS. These results suggest that 16 may be a useful tool for evaluating the potential role of selective nNOS inhibitors in the treatment of pain such as migraine and CTTH.

Systemic inhibition of the mammalian target of rapamycin (mTOR) pathway reduces neuropathic pain in mice

The management of neuropathic pain is unsatisfactory, and new treatments are required. Because the sensitivity of a subset of fast-conducting primary afferent nociceptors is thought to be regulated by the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway, selectively targeting mTORC1 represents a new strategy for the control of chronic pain. Here we show that activated mTOR was expressed largely in myelinated sensory fibers in mouse and that inhibiting the mTORC1 pathway systemically alleviated mechanical hypersensitivity in mouse models of inflammatory and neuropathic pain. Specifically, systemic administration of mTORC1 inhibitor temsirolimus (CCI-779), both acutely (25 mg/kg i.p.) and chronically (4 daily 25 mg/kg i.p.), inhibited the mTORC1 pathway in sensory axons and the spinal dorsal horn and reduced mechanical and cold hypersensitivity induced by nerve injury. Moreover, systemic treatment with CCI-779 also reduced mechanical but not heat hypersensitivity in an inflammatory pain state. This treatment did not influence nociceptive thresholds in naive or sham-treated control animals. Also, there was no evidence for neuronal toxicity after repeated systemic treatment with CCI-779. Additionally, we show that acute and chronic i.p. administration of Torin1 (20 mg/kg), a novel ATP-competitive inhibitor targeting both mTORC1 and mTORC2 pathways, reduced the response to mechanical and cold stimuli in neuropathic mice. Our findings emphasize the importance of the mTORC1 pathway as a regulator of nociceptor sensitivity and therefore as a potential target for therapeutic intervention, particularly in chronic pain.

Nitric oxide inhibits nociceptive transmission by differentially regulating glutamate and glycine release to spinal dorsal horn neurons

Nitric oxide (NO) is involved in many physiological functions, but its role in pain signaling remains uncertain. Surprisingly, little is known about how endogenous NO affects excitatory and inhibitory synaptic transmission at the spinal level. Here we determined how NO affects excitatory and inhibitory synaptic inputs to dorsal horn neurons using whole-cell recordings in rat spinal cord slices. The NO precursor L-arginine or the NO donor SNAP significantly increased the frequency of glycinergic spontaneous and miniature inhibitory postsynaptic currents (IPSCs) of lamina II neurons. However, neither L-arginine nor SNAP had any effect on GABAergic IPSCs. L-arginine and SNAP significantly reduced the amplitude of monosynaptic excitatory postsynaptic currents (EPSCs) evoked from the dorsal root with an increase in paired-pulse ratio. Inhibition of the soluble guanylyl cyclase abolished the effect of L-arginine on glycinergic IPSCs but not on evoked monosynaptic EPSCs. Also, inhibition of protein kinase G blocked the increase in glycinergic sIPSCs by the cGMP analog 8-bromo-cGMP. The inhibitory effects of L-arginine on evoked EPSCs and high voltage-activated Ca(2+) channels expressed in HEK293 cells and dorsal root ganglion neurons were abolished by blocking the S-nitrosylation reaction with N-ethylmaleimide. Intrathecal injection of L-arginine and SNAP significantly increased mechanical nociceptive thresholds. Our findings suggest that spinal endogenous NO enhances inhibitory glycinergic input to dorsal horn neurons through sGC-cGMP-protein kinase G. Furthermore, NO reduces glutamate release from primary afferent terminals through S-nitrosylation of voltage-activated Ca(2+) channels. Both of these actions probably contribute to inhibition of nociceptive transmission by NO at the spinal level.

Hospitalization for pain in patients with sickle cell disease treated with sildenafil for elevated TRV and low exercise capacity

In adults with sickle cell disease (SCD), an increased tricuspid regurgitation velocity (TRV) by Doppler echocardiography is associated with increased morbidity and mortality. Although sildenafil has been shown to improve exercise capacity in patients with pulmonary arterial hypertension, it has not been evaluated in SCD. We therefore sought to determine whether sildenafil could improve exercise capacity in SCD patients with increased TRV and a low exercise capacity. A TRV ≥ 2.7 m/s and a 6-minute walk distance (6MWD) between 150 and 500 m were required for enrollment in this 16-week, double-blind, placebo-controlled sildenafil trial. After 74 of the screened subjects were randomized, the study was stopped early due to a higher percentage of subjects experiencing serious adverse events in the sildenafil arm (45% of sildenafil, 22% of placebo, P = .022). Subject hospitalization for pain was the predominant cause for this difference: 35% with sildenafil compared with 14% with placebo (P = .029). There was no evidence of a treatment effect on 6MWD (placebo-corrected effect -9 m; 95% confidence interval [95% CI] -56-38; P = .703), TRV (P = .503), or N-terminal pro-brain natriuretic peptide (P = .410). Sildenafil appeared to increase hospitalization rates for pain in patients with SCD. This study is registered at www.clinicaltrials.gov as NCT00492531.

Pharmacological characterization of KLYP961, a dual inhibitor of inducible and neuronal nitric-oxide synthases

Nitric oxide (NO) derived from neuronal nitric-oxide synthase (nNOS) and inducible nitric-oxide synthase (iNOS) plays a key role in various pain and inflammatory states. KLYP961 (4-((2-cyclobutyl-1H-imidazo[4,5-b]pyrazin-1-yl)methyl)-7,8-difluoroquinolin-2(1H)-one) inhibits the dimerization, and hence the enzymatic activity of human, primate, and murine iNOS and nNOS (IC(50) values 50-400 nM), with marked selectivity against endothelial nitric-oxide synthase (IC(50) >15,000 nM). It has ideal drug like-properties, including excellent rodent and primate pharmacokinetics coupled with a minimal off-target activity profile. In mice, KLYP961 attenuated endotoxin-evoked increases in plasma nitrates, a surrogate marker of iNOS activity in vivo, in a sustained manner (ED(50) 1 mg/kg p.o.). KLYP961 attenuated pain behaviors in a mouse formalin model (ED(50) 13 mg/kg p.o.), cold allodynia in the chronic constriction injury model (ED(50) 25 mg/kg p.o.), or tactile allodynia in the spinal nerve ligation model (ED(50) 30 mg/kg p.o.) with similar efficacy, but superior potency relative to gabapentin, pregabalin, or duloxetine. Unlike morphine, the antiallodynic activity of KLYP961 did not diminish upon repeated dosing. KLYP961 also attenuated carrageenin-induced edema and inflammatory hyperalgesia and writhing response elicited by phenylbenzoquinone with efficacy and potency similar to those of celecoxib. In contrast to gabapentin, KLYP961 did not impair motor coordination at doses as high as 1000 mg/kg p.o. KLYP961 also attenuated capsaicin-induced thermal allodynia in rhesus primates in a dose-related manner with a minimal effective dose (≤ 10 mg/kg p.o.) and a greater potency than gabapentin. In summary, KLYP961 represents an ideal tool with which to probe the physiological role of NO derived from iNOS and nNOS in human pain and inflammatory states.

A series of D-amino acid oxidase inhibitors specifically prevents and reverses formalin-induced tonic pain in rats

We have found that mutation of D-amino acid oxidase (DAO) diminished formalin-induced tonic pain. The present research further studied the analgesic effects of a series of DAO inhibitors in this model. 5-Chlorobenzo[d]isoxazol-3-ol (CBIO), 4H-thieno[3,2-b]pyrrole-5-carboxylic acid (compound 8), 5-methylpyrazole-3-carboxylic acid (AS057278), sodium benzoate, and 4-nitro-3-pyrazole carboxylic acid (NPCA) inhibited rat spinal cord-derived DAO activity in a concentration-dependent manner, with maximal inhibition of 100% and potency rank of CBIO > compound 8 > AS057278 > sodium benzoate > NPCA. In rats, intrathecal injections of CBIO, compound 8, AS057278, and sodium benzoate but not NPCA specifically prevented formalin-induced tonic pain but not acute nociception, with the same potency order as in the DAO activity assay. The highly potent analgesia of DAO inhibitors was evidenced by CBIO, which prevented 50% pain at 0.06 μg, approximately 5-fold the potency of morphine. CBIO given after formalin challenge also reversed the established pain state to the same degree as prevention. The antihyperalgesic potencies of these DAO inhibitors were highly correlated to their inhibitions of spinal DAO activity. Maximum inhibition of pain by these compounds was approximately 60%, comparable with that of the N-methyl-D-aspartic acid receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), suggesting that a larger portion of formalin-induced tonic pain is "DAO-sensitive," whereas the remaining 40% of tonic pain and acute nociception is "DAO-insensitive." These findings, combined with our previous DAO gene mutation and induction results, indicate spinal DAO mediates both induction and maintenance of formalin-induced tonic pain and further validate spinal DAO as a novel and efficacious target molecule for the treatment of chronic pain.

The spinal cord expression of neuronal and inducible nitric oxide synthases and their contribution in the maintenance of neuropathic pain in mice

Background

Nitric oxide generated by neuronal (NOS1), inducible (NOS2) or endothelial (NOS3) nitric oxide synthases contributes to pain processing, but the exact role of NOS1 and NOS2 in the maintenance of chronic peripheral neuropathic pain as well as the possible compensatory changes in their expression in the spinal cord of wild type (WT) and NOS knockout (KO) mice at 21 days after total sciatic nerve ligation remains unknown.

Methodology/Principal Findings

The mechanical and thermal allodynia as well as thermal hyperalgesia induced by sciatic nerve injury was evaluated in WT, NOS1-KO and NOS2-KO mice from 1 to 21 days after surgery. The mRNA and protein levels of NOS1, NOS2 and NOS3 in the spinal cord of WT and KO mice, at 21 days after surgery, were also assessed. Sciatic nerve injury led to a neuropathic syndrome in WT mice, in contrast to the abolished mechanical allodynia and thermal hyperalgesia as well as the decreased or suppressed thermal allodynia observed in NOS1-KO and NOS2-KO animals, respectively. Sciatic nerve injury also increases the spinal cord expression of NOS1 and NOS2 isoforms, but not of NOS3, in WT and NOS1-KO mice respectively. Moreover, the presence of NOS2 is required to increase the spinal cord expression of NOS1 whereas an increased NOS1 expression might avoid the up-regulation of NOS2 in the spinal cord of nerve injured WT mice.

Conclusions/Significance

These data suggest that the increased spinal cord expression of NOS1, regulated by NOS2, might be responsible for the maintenance of chronic peripheral neuropathic pain in mice and propose these enzymes as interesting therapeutic targets for their treatment.

Nitric oxide is negatively correlated to pain during acute inflammation

BACKGROUND

The role that nitric oxide (NO) plays in modulating pain in the periphery is unclear. We show here, the results of two independent clinical studies (microdialysis and gene expression studies) and a pilot dose finding study (glyceryl trinitrate study), to study the role of NO in the early phase of acute inflammatory pain following oral surgery. The effect of ketorolac on NO production and nitric oxide synthase (NOS) gene expression was also studied.

RESULTS

Microdialysis samples showed significantly higher levels of NO at the first 100 min compared to the last 80 minutes in the placebo treated group. In the ketorolac group, on the other hand, NO levels gradually decreased over the first 60 min but were similar to placebo over the later 100-180 min, with no significant change in NO level over time. The levels of NO were negatively correlated to pain intensity scores. Local infusion of the NO donor glyceryl trinitrate at the site of surgery, showed a small analgesic effect that did not reach statistical significance in the sample size used. While the gene expression of iNOS and eNOS were not up-regulated, 3 hours after surgery, nNOS was downregulated in both treatment groups and eNOS gene expression was significantly lower in the ketorolac group compared to the placebo group. Further, there was a positive correlation between the change in gene expression of nNOS and eNOS in the placebo group but not in the ketorolac group.

CONCLUSION

We suggest that at this early stage of inflammatory pain in man, NO is analgesic in the periphery. Further, ketorolac down-regulates eNOS gene expression.

Reactive nitroxidative species and nociceptive processing: determining the roles for nitric oxide, superoxide, and peroxynitrite in pain

Pain is a multidimensional perception and is modified at distinct regions of the neuroaxis. During enhanced pain, neuroplastic changes occur in the spinal and supraspinal nociceptive modulating centers and may result in a hypersensitive state termed central sensitization, which is thought to contribute to chronic pain states. Central sensitization culminates in hyperexcitability of dorsal horn nociceptive neurons resulting in increased nociceptive transmission and pain perception. This state is associated with enhanced nociceptive signaling, spinal glutamate-mediated N-methyl-D: -aspartate receptor activation, neuroimmune activation, nitroxidative stress, and supraspinal descending facilitation. The nitroxidative species considered for their role in nociception and central sensitization include nitric oxide (NO), superoxide ([Formula: see text]), and peroxynitrite (ONOO(-)). Nitroxidative species are implicated during persistent but not normal nociceptive processing. This review examines the role of nitroxidative species in pain through a discussion of their contributions to central sensitization and the underlying mechanisms. Future directions for nitroxidative pain research are also addressed. As more selective pharmacologic agents are developed to target nitroxidative species, the exact role of nitroxidative species in pain states will be better characterized and should offer promising alternatives to available pain management options.

Nitric oxide synthase modulates CFA-induced thermal hyperalgesia through cytokine regulation in mice

Background

Although it has been largely demonstrated that nitric oxide synthase (NOS), a key enzyme for nitric oxide (NO) production, modulates inflammatory pain, the molecular mechanisms underlying these effects remain to be clarified. Here we asked whether cytokines, which have well-described roles in inflammatory pain, are downstream targets of NO in inflammatory pain and which of the isoforms of NOS are involved in this process.

Results

Intraperitoneal (i.p.) pretreatment with 7-nitroindazole sodium salt (7-NINA, a selective neuronal NOS inhibitor), aminoguanidine hydrochloride (AG, a selective inducible NOS inhibitor), L-N(G)-nitroarginine methyl ester (L-NAME, a non-selective NOS inhibitor), but not L-N(5)-(1-iminoethyl)-ornithine (L-NIO, a selective endothelial NOS inhibitor), significantly attenuated thermal hyperalgesia induced by intraplantar (i.pl.) injection of complete Freund's adjuvant (CFA). Real-time reverse transcription-polymerase chain reaction (RT-PCR) revealed a significant increase of nNOS, iNOS, and eNOS gene expression, as well as tumor necrosis factor-alpha (TNF), interleukin-1 beta (IL-1β), and interleukin-10 (IL-10) gene expression in plantar skin, following CFA. Pretreatment with the NOS inhibitors prevented the CFA-induced increase of the pro-inflammatory cytokines TNF and IL-1β. The increase of the anti-inflammatory cytokine IL-10 was augmented in mice pretreated with 7-NINA or L-NAME, but reduced in mice receiving AG or L-NIO. NNOS-, iNOS- or eNOS-knockout (KO) mice had lower gene expression of TNF, IL-1β, and IL-10 following CFA, overall corroborating the inhibitor data.

Conclusion

These findings lead us to propose that inhibition of NOS modulates inflammatory thermal hyperalgesia by regulating cytokine expression.

Postsynaptic density-93 deficiency protects cultured cortical neurons from N-methyl-D-aspartate receptor-triggered neurotoxicity

It has been reported that N-methyl-D-aspartate receptor (NMDAR)-triggered neurotoxicity is related to excessive Ca(2+) loading and an increase in nitric oxide (NO) concentration. However, the molecular mechanisms that underlie these events are not completely understood. NMDARs and neuronal NO synthase each binds to the scaffolding protein postsynaptic density (PSD)-93 through its PDZ domains. In this study, we determined whether PSD-93 plays a critical role in NMDAR/Ca(2+)/NO-mediated neurotoxicity. We found that the targeted disruption of the PSD-93 gene attenuated the neurotoxicity triggered by NMDAR activation, but not by non-NMDAR activation, in cultured mouse cortical neurons. PSD-93 deficiency reduced the amount of NMDAR subunits NR2A and NR2B in synaptosomal fractions from the cortical neurons and significantly prevented NMDA-stimulated increases in cyclic guanosine 3',5'-monophosphate and Ca(2+) loading in the cortical neurons. These findings indicate that PSD-93 deficiency could block NMDAR-triggered neurotoxicity by disrupting the NMDAR-Ca(2+)-NO signaling pathway and reducing expression of synaptic NR2A and NR2B. Since NMDARs, Ca(2+), and NO play a critical role during the development of brain trauma, seizures, and ischemia, the present work suggests that PSD-93 might contribute to molecular mechanisms of neuronal damage in these brain disorders.

The peripheral administration of a nitric oxide donor potentiates the local antinociceptive effects of a DOR agonist during chronic inflammatory pain in mice

Several works reveal that nitric oxide could enhance the peripheral antinociception induced by opioids during acute inflammation. Nonetheless, the role of nitric oxide in the local antinociceptive effects of delta-opioid receptor (DOR) agonists during chronic peripheral inflammation is not known. The aim of this study is to evaluate whether nitric oxide would enhance the local antinociceptive effects of a DOR agonist during chronic inflammatory pain in mice. Chronic inflammatory pain was induced by the subplantar administration of complete Freund's adjuvant (CFA; 30 microl) and thermal hyperalgesia assessed by plantar test. In C57BL/6J mice, we evaluated the local antinociceptive effects of a DOR agonist, [D-Pen2,5]-enkephalin (DPDPE) and a nitric oxide donor, DETA NONOate DETA/NO 2,2'-(hydroxynitrosohydrazino) Bis-Ethanamine (NOC-18) alone or combined (DPDPE plus NOC-18) at 1, 4, 7, and 10 days after CFA injection. The reversibility of the peripheral antinociceptive effects of DPDPE, alone or combined with NOC-18, was assessed with the local administration of selective (naltrindole) and non-selective (naloxone methiodide) DOR antagonists. The local administration of DPDPE or NOC-18 alone dose-dependently inhibited the thermal hyperalgesia induced by peripheral inflammation. Moreover, the co-administration of NOC-18 with DPDPE significantly increased the antinociceptive effects produced by DPDPE from 1 to 10 days of CFA-induced inflammatory pain (P < 0.05). These effects were completely blocked by naltrindole and naloxone methiodide. Our results demonstrate that nitric oxide might enhance the local antinociceptive effects of a DOR agonist during chronic inflammatory pain by interaction with peripheral DOR, representing a useful strategy for an efficient antinociceptive treatment of peripheral inflammatory pain.

No NO, no pain? The role of nitric oxide and cGMP in spinal pain processing

A large body of evidence indicates that nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) essentially contribute to the processing of nociceptive signals in the spinal cord. Many animal studies have unanimously shown that inhibition of NO or cGMP synthesis can considerably reduce both inflammatory and neuropathic pain. However, experiments with NO donors and cGMP analogs also caused conflicting results because dual pronociceptive and antinociceptive effects of these molecules have been observed. Here, we summarize the most recent advances in the understanding of NO- and cGMP-dependent signaling pathways in the spinal cord and further unravel the role of NO and cGMP in pain processing.

Pharmacological assessments of nitric oxide synthase isoforms and downstream diversity of NO signaling in the maintenance of thermal and mechanical hypersensitivity after peripheral nerve injury in mice

Nitric oxide synthase (NOS) isoforms and NO downstream signal pathways involved spinally in the maintenance of thermal and mechanical hypersensitivity were assessed in a mouse model of neuropathic pain developing after partial ligation of the sciatic nerve. Intrathecal injection of the NOS inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME), the highly selective neuronal NOS (nNOS) inhibitor N(omega)-propyl-l-arginine and the potent selective inducible NOS (iNOS) inhibitor 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine hydrochloride (AMT) exerted dose-dependent analgesic effects on thermal and mechanical hypersensitivity, which were assessed by the plantar and von Frey tests, respectively, suggesting that both nNOS and iNOS participate in producing NO to maintain neuropathic pain. Since the selective inhibitor of NO-sensitive guanylyl cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and the guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kinase (PKG) inhibitor Rp-8-pCPT-cGMPS intrathecally exerted dose-dependent analgesic effects on thermal and mechanical hypersensitivity, spinally released NO most likely stimulates the NO-cGMP-PKG pathway. Moreover, the superoxide dismutase mimetic 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), a potent superoxide scavenger, reduced thermal and mechanical hypersensitivity when administered intrathecally, suggesting that spinal release of superoxide, which can then react with NO to produce peroxynitrite, also appears to mediate neuropathic pain. Finally, intrathecal injection of phenyl-N-tert-butylnitrone (PBN), a reactive oxygen species (ROS) scavenger, ameliorated thermal and mechanical hypersensitivity, thus further confirming the importance of ROS including NO and superoxide in the maintenance of neuropathic pain. Together, the present results demonstrate that NO, produced presumably via nNOS and iNOS in the spinal cord, mediates the maintenance of neuropathic pain following peripheral nerve injury through both the NO-cGMP-PKG and the NO-peroxynitrite pathways.

Involvement of neuronal, inducible and endothelial nitric oxide synthases in capsaicin-induced muscle hypersensitivity

Nitric oxide, which has been implicated in the development of hyperalgesia in the spinal system, has not been systematically studied in the trigeminal system, especially in the context of inflammatory muscle pain condition. In this study, we investigated the functional role of centrally released nitric oxide in the pathogenesis of orofacial muscle pain. Specifically, we examined the contribution of neuronal, inducible and endothelial nitric oxide synthases, nNOS, iNOS and eNOS, respectively, in mediating masseter hypersensitivity under acute inflammatory condition. Time-dependent changes in nNOS, iNOS and eNOS protein expression in the subnucleus caudalis (Vc) were assessed following capsaicin injection in the masseter muscle of male Sprague Dawley rats. The expression of all three nitric oxide synthases was significantly up-regulated 30-60 min following capsaicin stimulation, which paralleled the time course of the development of capsaicin-induced masseter hypersensitivity. Pretreatment with each NOS inhibitor significantly attenuated the masseter hypersensitivity. These data showed that all three NOS in the Vc are functionally important for the development of craniofacial muscle hyperalgesia and suggest that the three NOS are closely orchestrated to regulate the level of nitric oxide under normal and pathologic conditions.

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.

Cell-permeable peptide Tat-PSD-95 PDZ2 inhibits chronic inflammatory pain behaviors in mice

Inflammatory conditions can lead to persistent debilitating pain, and the activation of N-methyl-D-aspartate receptors (NMDARs) has been shown to play an important role in the processing of inflammatory pain. Postsynaptic density protein-95 (PSD-95), a scaffolding protein, has been identified to interact with NMDARs at neuronal synapses of the central nervous system (CNS). However, the role of these interactions in the central sensitization of nociceptive processing has not been defined. In this study, we investigated the effect of disrupting NMDAR/PSD-95 interactions on chronic inflammatory pain behaviors. We constructed a fusion peptide, Tat-PSD-95 PDZ2, comprising the second PDZ domain of PSD-95, to disrupt specifically NMDARs/PSD-95 protein interactions. Western blot analysis showed that Tat-PSD-95 PDZ2 intraperitoneally injected into mice was delivered intracellularly into neurons in the CNS. By in vitro and in vivo binding assays, we found that the Tat-PSD-95 PDZ2 dose dependently inhibited the interactions between NMDARs and PSD-95. Furthermore, behavioral testing showed that mice given Tat-PSD-95 PDZ2 exhibited significantly reduced complete Freund's adjuvant (CFA)-induced chronic inflammatory pain behaviors compared to the vehicle-treated group. Our results indicate that by disrupting NMDAR/PSD-95 protein interactions, the cell-permeable fusion peptide Tat-PSD-95 PDZ2 provides a new target and approach for chronic inflammatory pain therapy.

Expression of nitric oxide synthase isoforms in the dorsal horn of monoarthritic rats: effects of competitive and uncompetitive N-methyl-D-aspartate antagonists

Chronic pain is associated with N-methyl-D-aspartate (NMDA) receptor activation and downstream production of nitric oxide, which has a pivotal role in multisynaptic local circuit nociceptive processing in the spinal cord. The formation of nitric oxide is catalyzed by three major nitric oxide synthase (NOS) isoforms (neuronal, nNOS; inducible, iNOS; endothelial, eNOS), which are increased in the spinal cord of rodents subjected to some tonic and chronic forms of experimental pain. Despite the important role of NOS in spinal cord nociceptive transmission, there have been no studies exploring the effect of NMDA receptor blockade on NOS expression in the dorsal horn during chronic pain. Furthermore, NOS isoforms have not been fully characterized in the dorsal horn of animals subjected to arthritic pain. The aim of this work was therefore to study the expression of nNOS, iNOS and eNOS in the dorsal horns of monoarthritic rats, and the modifications in NOS expression induced by pharmacological blockade of spinal cord NMDA receptors. Monoarthritis was produced by intra-articular injection of complete Freund's adjuvant into the right tibio-tarsal joint. At week 4, monoarthritic rats were given either the competitive NMDA antagonist (±)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or the uncompetitive NMDA antagonist ketamine. After 6 and 24 hours, animals were killed and posterior quadrants of the lumbar spinal cord were dissected. Sample tissues were homogenized and subjected to immunoblotting with anti-nNOS, anti-iNOS or anti-eNOS monoclonal antibodies. The nNOS isoform, but not the iNOS and eNOS isoforms, were detected in the dorsal horns of control rats. Monoarthritis increased the expression of nNOS, iNOS and eNOS in the dorsal horns ipsilateral and contralateral to the inflamed hindpaw. Intrathecal administration of CPP and ketamine reduced nNOS expression in monoarthritic rats but increased the expression of iNOS and eNOS. Results suggest that blockade of spinal cord NMDA receptors produces complex regulatory changes in the expression of NOS isoforms in monoarthritic rats that may be relevant for nitridergic neuronal/glial mechanisms involved in the pathophysiology of monoarthritis and in the pharmacological response to drugs interacting with NMDA receptors.

Role of soluble guanylate cyclase in the trigeminal subnucleus caudalis in capsaicin-induced muscle hypersensitivity

Nitric oxide (NO) produces its effects by activating soluble guanylate cyclase (sGC). In the present study, we investigated the potential role of sGC in the subnucleus caudalis (Vc) in mediating masseter hypersensitivity under acute inflammatory condition in male Sprague-Dawley rats. First, our Western blot analysis revealed that sGC protein is reliably detected in the Vc. Subsequent immunohistochemical studies demonstrated that neuronal cell bodies in the superficial laminae of the Vc positively stained for sGC. Astrocytes in deeper lamina of the Vc also showed sGC immunoreactivity. We then tested whether intrathecal administration of sGC inhibitors, methylene blue (MB), and ODQ, in the Vc, attenuates masseter hypersensitivity induced by intramuscular injection of capsaicin. Intrathecal MB or ODQ significantly blocked the capsaicin-induced reduction of mechanical threshold to noxious stimulation of the masseter. These data indicate that the NO-sGC pathway in the Vc is involved in mediating orofacial muscle hypersensitivity under acute inflammatory conditions.

Acute intracerebroventricular administration of palmitoylethanolamide, an endogenous peroxisome proliferator-activated receptor-alpha agonist, modulates carrageenan-induced paw edema in mice

Peroxisome proliferator-activated receptor (PPAR)-alpha is a nuclear transcription factor. Although the presence of this receptor in different areas of central nervous system (CNS) has been reported, its role remains unclear. Palmitoylethanolamide (PEA), a member of the fatty-acid ethanolamide family, acts peripherally as an endogenous PPAR-alpha ligand, exerting analgesic and anti-inflammatory effects. High levels of PEA in the CNS have been found, but the specific function of this lipid remains to be clarified. Using carrageenan-induced paw edema in mice, we show that i.c.v. administration of PEA may control peripheral inflammation through central PPAR-alpha activation. A single i.c.v. administration of 0.01 to 1 microg of PEA, 30 min before carrageenan injection, reduced edema formation in the mouse carrageenan test. This effect was mimicked by 0.01 to 1 microg of GW7647 [2-[[4-[2-[[(cyclohexylamino)carbonyl](4-cyclohexylbutyl)amino]ethyl]phenyl]thio]-2-methylpropanoic acid], a synthetic PPAR-alpha agonist. Moreover, central PEA administration significantly reduced the expression of the proinflammatory enzymes cyclooxygenase-2 and inducible nitric-oxide synthase, and it significantly restored carrageenan-induced PPAR-alpha reduction in the spinal cord. To investigate the mechanism by which i.c.v. PEA attenuated the development of carrageenan-induced paw edema, we evaluated inhibitor kappaB-alpha (I kappa B-alpha) degradation and nuclear factor-kappaB (NF-kappaB) p65 activation in the cytosolic or nuclear extracts from spinal cord tissue. PEA prevented IkB-alpha degradation and NF-kappaB nuclear translocation, confirming the involvement of this transcriptional factor in the control of peripheral inflammation. The obligatory role of PPAR-alpha in mediating the effects of PEA was confirmed by the lack of the compounds anti-inflammatory effects in mutant mice lacking PPAR-alpha. In conclusion, our data show for the first time that PPAR-alpha activation in the CNS can control peripheral inflammation.

Inhibition of spinal constitutive NOS-2 by 1400W attenuates tissue injury and inflammation-induced hyperalgesia and spinal p38 activation

Nitric oxide (NO) and its synthesizing enzymes, including NO synthase-2 (NOS-2, also called inducible NOS, iNOS), have been implicated in spinal nociception. 1400W is a highly selective NOS-2 inhibitor, as compared with either NOS-1 (neuronal NOS, nNOS) or NOS-3 (endothelial NOS). Here we examined the anti-nociceptive effects of intrathecal (IT) administration of 1400W in two experimental models of hyperalgesia (formalin and carrageenan models), in addition to the effect of 1400W on stimulation-induced activation of spinal p38 mitogen-activated protein kinase (p38). IT treatment of rats with 1400W produced a dose-dependent inhibition of paw formalin-induced phase II flinches, and attenuated carrageenan-induced thermal hyperalgesia. In contrast, IT injection of a selective inhibitor of NOS-1, nNOS inhibitor-I, had no effect in either model. Furthermore, 1400W at a dose that suppressed formalin-induced flinching behavior also blocked formalin-evoked p38 phosphorylation (activation) in the spinal cord, while nNOS inhibitor-I displayed no activity. The prompt effects of IT 1400W suggest involvement of constitutively expressed NOS-2 in spinal nociception. The NOS-2 protein in rat spinal cords was undetectable by Western blotting. However, when the protein was immunoprecipitated prior to Western blotting, NOS-2-immunoreactive bands were detected in the tissues, including naïve spinal cords. The presence of constitutive spinal NOS-2 was further confirmed by reverse transcriptase-polymerase chain reaction. Taken together, the present studies suggest that constitutively expressed spinal NOS-2 mediates tissue injury and inflammation-induced hyperalgesia, and that activation of p38 is one of the downstream factors in NO-mediated signaling in the initial processing of spinal nociception.

Activation of the nociceptin opioid system in rat sensory neurons produces antinociceptive effects in inflammatory pain: involvement of inflammatory mediators

Despite a large body of literature on the nociceptin (NC) opioid system in pain modulation, the mechanism of action of NC remains largely unexplored. Here, we investigated the role and mode of action of the spinal NC system in inflammatory pain. Preemptive intrathecal administration of NC attenuated thermal hyperalgesia and mechanical allodynia in rats with intraplantar complete Freund's adjuvant (CFA) injection. By using immunohistochemistry in L4 dorsal root ganglion (DRG) neurons, a marked increase of NC and ORL1 receptor immunoreactivity was detected following CFA. Intrathecal administration of NC attenuated the CFA-induced increases of calcitonin gene-related peptide, transient receptor potential vanilloid-1, and tumor necrosis factor-alpha in DRG neurons. Real-time reverse transcription-polymerase chain reaction showed that NC reduced the up-regulation of inducible nitric oxide synthase mRNA but not that of neuronal nitric oxide synthase mRNA in spinal cord segments after CFA. Furthermore, [Nphe1]NC(1-13)NH2, a selective opioid receptor-like 1 (ORL1) receptor antagonist, significantly antagonized the effects of NC on pain modulation and on the expression of inflammatory mediators, indicating a specific NC action through the ORL1 receptor. Together, these findings reveal novel mechanisms by which the NC system produces analgesia.

Differences in inflammatory pain in nNOS-, iNOS- and eNOS-deficient mice

To assess the relative importance of the isoforms of nitric oxide synthase (NOS) in inflammatory pain, we directly compared pain behaviour and paw thickness after intraplantar injection of complete Freund's adjuvant (CFA) in wild-type (WT) mice and in mice lacking either inducible (iNOS), endothelial (eNOS) or neuronal NOS (nNOS). In mice deficient for nNOS, thermal hyperalgesia was reduced by approximately 50% compared to wild type mice at 4 and 8h after CFA injection, and mechanical hypersensitivity was absent. The only change in pain behaviour in iNOS and eNOS deficient mice compared to WT mice was a more rapid recovery from thermal hyperalgesia. A compensatory up-regulation of nNOS in dorsal root ganglia (DRG) and spinal cords of iNOS and eNOS knockout mice was excluded using RT-PCR. However, an increase of iNOS gene expression was found in spinal cords of eNOS and nNOS deficient mice. To study the downstream effects of nNOS deficiency on DRG neurones, we assessed their immunoreactivity for calcitonin gene-related peptide (CGRP) and cytokines. We found a significant reduction in the CFA induced increase in CGRP immunoreactive neurones as well as in CGRP gene expression in nNOS deficient mice, whereas the percentage of cells immunopositive for tumour necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) was unchanged. These results support the proposed role of nNOS in sensitization of DRG neurones, and might indicate that CGRP is involved in this process.

Analgesic effects by electroacupuncture were decreased in inducible nitric oxide synthase knockout mice

BACKGROUND

This study was designed to investigate the involvement of inducible nitric oxide synthase (iNOS) in electroacupuncture (EA)-induced analgesia.

METHODS

Two and 100 Hz EA stimulation were applied at acupoint ST 36 (Zusanli) in iNOS knockout mice (n = 28). Needles were inserted 5 mm in depth in ST36. After insertion, the needles were fixed in situ with adhesive tape. EA was applied after the basal threshold determination. The EA parameters were set as follows: constant square wave current output (pulse width: 0.6 ms at 2 Hz and 0.2 ms at 100 Hz) and 2 mA intensities. EA was performed for 30 minutes and tail-flick latencies (TFLs) were evaluated every 15 minutes for 1 hour.

RESULTS

In 2 Hz EA stimulation, the tail-flick response (TFR) of wild-type mice for durations of 0, 15, 30, 45 and 60 minutes were 2.70 +/- 0.26, 4.19 +/- 0.37, 4.17 +/- 0.34, 3.57 +/- 0.27 and 3.39 +/- 0.32 seconds of TFLs. Meanwhile, iNOS -/- mice showed 4.10 +/- 0.33, 4.77 +/- 0.24, 5.26 +/- 0.30, 4.48 +/- 0.33 and 5.00 +/- 0.41 seconds of TFLs. In 100 Hz EA stimulation, the TFR of wild-type mice were 3.01 +/- 0.24, 4.67 +/- 0.31, 4.76 +/- 0.25, 4.04 +/- 0.45 and 4.26 +/- 0.30 seconds of TFLs. The iNOS -/- mice were 4.33 +/- 0.16, 5.29 +/- 0.28, 5.06 +/- 0.35, 4.52 +/- 0.17 and 4.80 +/- 0.28 seconds of TFLs. Wild-type mice exhibited 63.9% increase in TFL compared to the baseline after 2 Hz EA, whereas the iNOS knockout mice exhibited 32.9% increase in TFL. The TFL after 100 Hz EA showed similar trends: 66.5% increase in TFL in wildtype mice and 18.3% increase in the iNOS knockout mice.

CONCLUSION

The present findings suggested that iNOS may play a crucial role in both low- and high-frequency EA-induced analgesic effects.

Spinal alpha(2)-adrenergic and muscarinic receptors and the NO release cascade mediate supraspinally produced effectiveness of gabapentin at decreasing mechanical hypersensitivity in mice after partial nerve injury

After partial nerve injury, the central analgesic effect of systemically administered gabapentin is mediated by both supraspinal and spinal actions. We further evaluate the mechanisms related to the supraspinally mediated analgesic actions of gabapentin involving the descending noradrenergic system. Intracerebroventricularly (i.c.v.) administered gabapentin (100 microg) decreased thermal and mechanical hypersensitivity in a murine chronic pain model that was prepared by partial ligation of the sciatic nerve. These effects were abolished by intrathecal (i.t.) injection of either yohimbine (3 microg) or idazoxan (3 microg), alpha(2)-adrenergic receptor antagonists. Pretreatment with atropine (0.3 mg kg(-1), i.p. or 0.1 microg, i.t.), a muscarinic receptor antagonist, completely suppressed the effect of i.c.v.-injected gabapentin on mechanical hypersensitivity, whereas its effect on thermal hypersensitivity remained unchanged. Similar effects were obtained with pirenzepine (0.1 microg, i.t.), a selective M(1)-muscarinic receptor antagonist, but not with methoctramine (0.1 and 0.3 microg, i.t.), a selective M(2)-muscarinic receptor antagonist. The cholinesterase inhibitor neostigmine (0.3 ng, i.t.) potentiated only the analgesic effect of i.c.v. gabapentin on mechanical hypersensitivity, confirming spinal acetylcholine release downstream of the supraspinal action of gabapentin. Moreover, the effect of i.c.v. gabapentin on mechanical but not thermal hypersensitivity was reduced by i.t. injection of L-NAME (3 microg) or L-NMMA (10 microg), both of which are nitric oxide (NO) synthase inhibitors. Systemically administered naloxone (10 mg kg(-1), i.p.), an opioid receptor antagonist, failed to suppress the analgesic actions of i.c.v. gabapentin, indicating that opioid receptors are not involved in activation of the descending noradrenergic system by gabapentin. Thus, the supraspinally mediated effect of gabapentin on mechanical hypersensitivity involves activation of spinal alpha(2)-adrenergic receptors followed by muscarinic receptors (most likely M(1)) and the NO cascade. In contrast, the effect of supraspinal gabapentin on thermal hypersensitivity is independent of the spinal cholinergic-NO system.