Nitric oxide mediates behavioral signs of neuropathic pain in an experimental rat model

Anti-allodynic and anti-hyperalgesic activity of (±)-licarin A in neuropathic rats via NO-cyclic-GMP-ATP-sensitive K+ channel pathway

The study aimed to examine the effect of intraperitoneal and intrathecal (±)-licarin A in neuropathic pain induced by L5 and L6 spinal nerve ligation (SNL) in male Wistar rats and the possible involvement of the NO-cGMP-ATP-sensitive K+ channel pathway. Neuropathic pain signs (allodynia and hyperalgesia) were evaluated on postoperative Day 14 using von Frey filaments. Single intraperitoneal (0.01, 0.1, 1, and 10 mg/kg) and intrathecal (0.01, 0.1, 1, and 10 µg/rat) administration of (±)-licarin A improved allodynia and hyperalgesia. The (±)-licarin A-induced anti-allodynic and anti-hyperalgesic activity was prevented by the intrathecal injection of  l-NAME (100 µg/rat; nonselective nitric oxide synthase inhibitor), ODQ (10 µg/rat; guanylate cyclase inhibitor), and glibenclamide (50 µg/rat; adenosine triphosphate (ATP)-sensitive K+ channel blocker). The data suggest that (±)-licarin A exerts its anti-allodynic and anti-hyperalgesic activity by activating the NO-cGMP-ATP-sensitive K+ channel pathway.

Brain nuclei and neural circuits in neuropathic pain and brain modulation mechanisms of acupuncture: a review on animal-based experimental research

Neuropathic pain (NP) is known to be associated with abnormal changes in specific brain regions, but the complex neural network behind it is vast and complex and lacks a systematic summary. With the help of various animal models of NP, a literature search on NP brain regions and circuits revealed that the related brain nuclei included the periaqueductal gray (PAG), lateral habenula (LHb), medial prefrontal cortex (mPFC), and anterior cingulate cortex (ACC); the related brain circuits included the PAG-LHb and mPFC-ACC. Moreover, acupuncture and injurious information can affect different brain regions and influence brain functions via multiple aspects to play an analgesic role and improve synaptic plasticity by regulating the morphology and structure of brain synapses and the expression of synapse-related proteins; maintain the balance of excitatory and inhibitory neurons by regulating the secretion of glutamate, γ-aminobutyric acid, 5-hydroxytryptamine, and other neurotransmitters and receptors in the brain tissues; inhibit the overactivation of glial cells and reduce the release of pro-inflammatory mediators such as interleukins to reduce neuroinflammation in brain regions; maintain homeostasis of glucose metabolism and regulate the metabolic connections in the brain; and play a role in analgesia through the mediation of signaling pathways and signal transduction molecules. These factors help to deepen the understanding of NP brain circuits and the brain mechanisms of acupuncture analgesia.

Nitric oxide as a messenger between neurons and satellite glial cells in dorsal root ganglia

Abnormal neuronal activity in sensory ganglia contributes to chronic pain. There is evidence that signals can spread between cells in these ganglia, which may contribute to this activity. Satellite glial cells (SGCs) in sensory ganglia undergo activation following peripheral injury and participate in cellular communication via gap junctions and chemical signaling. Nitric oxide (NO) is released from neurons in dorsal root ganglia (DRG) and induces cyclic GMP (cGMP) production in SCGs, but its role in SGC activation and neuronal excitability has not been explored. It was previously reported that induction of intestinal inflammation with dinitrobenzoate sulfonate (DNBS) increased gap junctional communications among SGCs, which contributed to neuronal excitability and pain. Here we show that DNBS induced SGC activation in mouse DRG, as assayed by glial fibrillary acidic protein upregulation. DNBS also upregulated cGMP level in SGCs, consistent with NO production. In vitro studies on intact ganglia from DNBS-treated mice showed that blocking NO synthesis inhibited both SGCs activation and cGMP upregulation, indicating an ongoing NO production. Application of NO donor in vitro induced SGC activation, augmented gap junctional communications, and raised neuronal excitability, as assessed by electrical recordings. The cGMP analog 8-Br-cGMP mimicked these actions, confirming the role of the NO-cGMP pathway in intraganglionic communications. NO also augmented Ca²⁺ waves propagation in DRG cultures. It is proposed that NO synthesis in DRG neurons increases after peripheral inflammation and that NO induces SGC activation, which in turn contributes to neuronal hyperexcitability. Thus, NO plays a major role in neuron-SGC communication.

Antiallodynic effect induced by [6]-gingerol in neuropathic rats is mediated by activation of the serotoninergic system and the nitric oxide-cyclic guanosine monophosphate-adenosine triphosphate-sensitive K+ channel pathway

The present study evaluated the possible antiallodynic effect induced by [6]-gingerol in rats with L5-L6 spinal nerve ligation (SNL). Moreover, we determined the possible mechanism underlying the antiallodynic effect induced by [6]-gingerol in neuropathic rats. The animals underwent L5-L6 SNL for the purpose of developing tactile allodynia. Tactile allodynia was measured with von Frey filaments. Intrathecal administration of [6]-gingerol reversed SNL-induced tactile allodynia. The [6]-gingerol-induced antiallodynic effect was prevented by the intrathecal administration of methiothepin (30 μg per rat; nonselective 5-hydroxytryptamine [5-HT] antagonist), WAY-100635 (6 μg per rat; selective 5-HT_1A receptor antagonist), SB-224289 (5 μg per rat; selective 5-HT_1B receptor antagonist), BRL-15572 (4 μg per rat; selective 5-HT_1D receptor antagonist), and SB-659551 (6 μg per rat; selective 5-HT_5A receptor antagonist), but naloxone (50 μg per rat; nonselective opioid receptor antagonist) did not prevent the [6]-gingerol-induced antiallodynic effect. Moreover, intrathecal administration of Nω-nitro-l-arginine methyl ester (100 μg per rat; nonselective nitric oxide [NO] synthase inhibitor), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 μg per rat; inhibitor of guanylate cyclase), and glibenclamide (50 μg per rat; channel blocker of adenosine triphosphate [ATP]-sensitive K⁺ channels) prevented the [6]-gingerol-induced antiallodynic effect. These data suggest that the antiallodynic effect induced by [6]-gingerol is mediated by the serotoninergic system involving the activation of 5-HT_1A/1B/1D/5A receptors, as well as the NO-cyclic guanosine monophosphate-ATP-sensitive K⁺ channel pathway but not by the opioidergic system.

Osthole ameliorates neurogenic and inflammatory hyperalgesia by modulation of iNOS, COX-2, and inflammatory cytokines in mice

BACKGROUND

Osthole is a bioactive component reported in medicinal plants such as Angelica pubescens and Cnidium monnieri, known for analgesic activity. However, the toxicity, median effective dose (ED₅₀), and dual modulation of nitric oxide and cyclooxygenase pathways along with inflammatory cytokines of osthole are yet to be determined.

METHODS

The animals (mice) were assessed for general behaviour and mortality in varying doses (50, 300, and 2000 mg kg^-1) of osthole for acute toxicity over 14 days. The analgesic activity was investigated using acetic acid and formalin-induced hyperalgesia, and anti-inflammatory activity was explored in carrageenan-induced paw oedema. ED₅₀ of osthole was calculated using Design Expert software. Involvement of nitric oxide and cyclooxygenase pathways was investigated by agonist challenges with L-arginine and substance P, respectively. The expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) was determined in spinal sections by immunohistochemical analysis. Lipopolysaccharide (LPS) challenge was used to assess in vivo effect on inflammatory cytokines (TNFα and IL-6).

RESULTS

Acute toxicity studies revealed no behavioural abnormality or mortality on osthole treatment and unremarkable histological findings. Osthole was found to significantly decrease acetic acid and formalin-induced hyperalgesia (ED₅₀ = 5.43 mg kg^-1) and carrageenan-induced paw oedema with no toxicity symptoms. Osthole produced a marked decrease in iNOS and COX-2 expression as well as TNFα and IL-6. The findings corroborate to modulation of iNOS and COX-2 and inflammatory cytokines by osthole. This study provides promising insights and prospects for application of osthole in pain management.

Chronic Monosodium Glutamate Administration Induced Hyperalgesia in Mice

Monosodium glutamate (MSG) is a widely used food additive. Although it is generally considered safe, some questions regarding the impact of its use on general health have arisen. Several reports correlate MSG consumption with a series of unwanted reactions, including headaches and mechanical sensitivity in pericranial muscles. Endogenous glutamate plays a significant role in nociceptive processing, this neurotransmitter being associated with hyperalgesia and central sensitization. One of the mechanisms underlying these phenomena is the stimulation of Ca2+/calmodulin sensitive nitric oxide synthase, and a subsequent increase in nitric oxide production. This molecule is a key player in nociceptive processing, with implications in acute and chronic pain states. Our purpose was to investigate the effect of this food additive on the nociceptive threshold when given orally to mice. Hot-plate and formalin tests were used to assess nociceptive behaviour. We also tried to determine if a correlation between chronic administration of MSG and variations in central nitric oxide (NO) concentration could be established. We found that a dose of 300 mg/kg MSG given for 21 days reduces the pain threshold and is associated with a significant increase in brain NO level. The implications of these findings on food additive-drug interaction, and on pain perception in healthy humans, as well as in those suffering from affections involving chronic pain, are still to be investigated.

Neuronal P2X7 receptor-induced reactive oxygen species production contributes to nociceptive behavior in mice

ATP can activate a variety of pathways through P2 purinoreceptors, leading to neuroprotection and pathology in the CNS. Among all P2X receptors, the P2X7 receptor (P2X7R) is a well-defined therapeutic target for inflammatory and neuropathic pain. Activation of P2X7R can generate reactive oxygen species (ROS) in macrophages and microglia. However, the role of ROS in P2X7R–induced pain remains unexplored. Here, we investigated the downstream effects of neuronal P2X7R activation in the spinal cord. We found that ATP induces ROS production in spinal cord dorsal horn neurons, an effect eliminated by ROS scavenger N-tert-butyl-α-phenylnitrone (PBN) and P2X7R antagonist A438079. A similar effect was observed with a P2X7R agonist, BzATP, and was attenuated by a NADPH oxidase inhibitor apocynin. Intrathecal administration of BzATP resulted in ROS production in the spinal cord and oxidative DNA damage in dorsal horn neurons. BzATP also induced robust biphasic spontaneous nociceptive behavior. Pre-treatment with A438079 abolished all BzATP-induced nociceptive behaviors, while ROS scavengers dose-dependently attenuated the secondary response. Here, we provide evidence that neuronal P2X7R activation leads to ROS production and subsequent nociceptive pain in mice. Together, the data indicate that P2X7R-induced ROS play a critical role in the P2X7R signaling pathway of the CNS.

Anti-allodynic effect of mangiferin in neuropathic rats: Involvement of nitric oxide-cyclic GMP-ATP sensitive K+ channels pathway and serotoninergic system

The neurobiology of neuropathic pain is caused by injury in the central or peripheral nervous system. Recent evidence points out that mangiferin shows anti-nociceptive effect in inflammatory pain. However, its role in inflammatory and neuropathic pain and the possible mechanisms of action are not yet established. The purpose of this study was to determine the possible anti-allodynic effect of mangiferin in rats with spinal nerve ligation (SNL). Furthermore, we sought to investigate the possible mechanisms of action that contribute to these effects. Mechanical allodynia to stimulation with the von Frey filaments was measured by the up and down method. Intrathecal administration of mangiferin prevented, in a dose-dependent fashion, SNL-induced mechanical allodynia. Mangiferin-induced anti-allodynia was prevented by the intrathecal administration of L-NAME (100μg/rat, non-selective nitric oxide synthase inhibitor), ODQ (10μg/rat, inhibitor of guanylate-cyclase) and glibenclamide (50μg/rat, channel blocker of ATP-sensitive K⁺ channels). Moreover, methiothepin (30μg/rat, non-selective 5-HT receptor antagonist), WAY-100635 (6μg/rat, selective 5-HT_1A receptor antagonist), SB-224289 (5μg/rat, selective 5-HT_1B receptor antagonist), BRL-15572 (4μg/rat, selective 5-HT_1D receptor antagonist) and SB-659551 (6μg/rat, selective 5-HT_5A receptor antagonist), but not naloxone (50μg/rat, non-selective opioid receptor antagonist), were able to prevent mangiferin-induced anti-allodynic effect. These data suggest that the anti-allodynic effect induced by mangiferin is mediated at least in part by the serotoninergic system, involving the activation of 5-HT_1A/1B/1D/5A receptors, as well as the nitric oxide-cyclic GMP-ATP-sensitive K⁺ channels pathway, but not by the opioidergic system, in the SNL model of neuropathic pain in rats.

Dual effect of exogenous nitric oxide on neuronal excitability in rat substantia gelatinosa neurons

Nitric oxide (NO) is an important signaling molecule involved in nociceptive transmission. It can induce analgesic and hyperalgesic effects in the central nervous system. In this study, patch-clamp recording was used to investigate the effect of NO on neuronal excitability in substantia gelatinosa (SG) neurons of the spinal cord. Different concentrations of sodium nitroprusside (SNP; NO donor) induced a dual effect on the excitability of neuronal membrane: 1 mM of SNP evoked membrane hyperpolarization and an outward current, whereas 10 µM induced depolarization of the membrane and an inward current. These effects were prevented by hemoglobin and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (c-PTIO) (NO scavengers), phenyl N-tert-butylnitrone (PBN; nonspecific reactive oxygen species scavenger), and through inhibition of soluble guanylyl cyclase (sGC). Pretreatment with n-ethylmaleimide (NEM; thiol-alkylating agent) also decreased effects of both 1 mM and 10 µM SNP, suggesting that these responses were mediated by direct S-nitrosylation. Charybdotoxin (CTX) and tetraethylammonium (TEA) (large-conductance Ca²⁺-activated K⁺ channel blockers) and glybenclamide (ATP-sensitive K⁺ channel blocker) decreased SNP-induced hyperpolarization. La³⁺ (nonspecific cation channel blocker), but not Cs⁺ (hyperpolarization-activated K⁺ channel blocker), blocked SNP-induced membrane depolarization. In conclusion, NO dually affects neuronal excitability in a concentration-dependent manner via modification of various K⁺ channels.

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.

NXN-188, a selective nNOS inhibitor and a 5-HT1B/1D receptor agonist, inhibits CGRP release in preclinical migraine models

BACKGROUND

NXN-188 is a combined neuronal nitric oxide synthase (nNOS) inhibitor and 5-hydroxytryptamine 1B/1D (5-HT1B/1D) receptor agonist. Using preclinical models, we evaluated whether these two unique therapeutic principles have a synergistic effect in attenuating stimulated calcitonin gene-related peptide (CGRP) release, a marker of trigeminal activation.

METHODS

We examined the effect of NXN-188 on: (1) KCl-, capsaicin- and resiniferatoxin (RTX)-induced immunoreactive CGRP (iCGRP) release from isolated preparation of rat dura mater, trigeminal ganglion (TG) and trigeminal nucleus caudalis (TNC); and (2) capsaicin- and electrical stimulation (ES)-induced middle meningeal artery (MMA) dilation in a rat closed-cranial window.

RESULTS

NXN-188 inhibited: (1) KCl-stimulated iCGRP release from dura mater (% decrease mean ± SEM, lowest effective concentration) (35 ± 6%, 30 µM), TG (24 ± 11%, 10 µM) and TNC (40 ± 8%, 10 µM); (2) capsaicin- and RTX-induced iCGRP release from dura mater; and (3) capsaicin- and ES-induced increase in dural artery diameter (32 ± 5%, 3 mg kg(-1) intravenous (i.v.) and 36 ± 1%, 10 mg kg(-1) i.v.).

CONCLUSIONS

NXN-188 inhibits CGRP release from migraine-relevant cephalic tissues. Its effect is most likely mediated via a combination of nNOS-inhibition and 5-HT1B/1D receptor agonism in dura mater while the mechanisms of action for inhibition of CGRP release from TG and TNC have to be investigated further.

Changes in the nitric oxide system in the shore crab Hemigrapsus sanguineus (Crustacea, decapoda) CNS induced by a nociceptive stimulus

Using NADPH-diaphorase (NADPH-d) histochemistry, inducible nitric oxide synthase (iNOS)-immunohistochemistry and immunoblotting, we characterized the nitric oxide (NO)-producing neurons in the brain and thoracic ganglion of a shore crab subjected to a nociceptive chemical stimulus. Formalin injection into the cheliped evoked specific nociceptive behavior and neurochemical responses in the brain and thoracic ganglion of experimental animals. Within 5–10 min of injury, the NADPH-d activity increased mainly in the neuropils of the olfactory lobes and the lateral antenna I neuropil on the side of injury. Later, the noxious-induced expression of NADPH-d and iNOS was detected in neurons of the brain, as well as in segmental motoneurons and interneurons of the thoracic ganglion. Western blotting analysis showed that an iNOS antiserum recognized a band at 120 kDa, in agreement with the expected molecular mass of the protein. The increase in nitrergic activity induced by nociceptive stimulation suggests that the NO signaling system may modulate nociceptive behavior in crabs.

[Effects of nitric oxide synthase inhibitor on hindlimb muscles in rats with neuropathic pain induced by unilateral peripheral nerve injury]

PURPOSE

The purpose of this study was to examine effects of nitric oxide synthase (NOS) inhibitor on muscle weight and myofibrillar protein content of affected and unaffected hindlimb muscles in rats with neuropathic pain induced by unilateral peripheral nerve injury.

METHODS

Neuropathic pain was induced by ligation and cutting of the left L5 spinal nerve. Adult male Sprague-Dawley rats were randomly assigned to one of two groups: The NOSI group (n=19) had NOS inhibitor (L-NAME) injections daily for 14 days, and the Vehicle group (n=20) had vehicle injections daily for 14 days. Withdrawal threshold, body weight, food intake and activity were measured every day. At 15 days all rats were anesthetized and soleus, plantaris and gastrocnemius muscles were dissected from hindlimbs. Muscle weight and myofibrillar protein content of the dissected muscles were determined.

RESULTS

The NOSI group showed significant increases as compared to the Vehicle group for body weight at 15 days, muscle weight and myofibrillar protein content of the unaffected soleus and gastrocnemius. The NOSI group demonstrated a higher pain threshold than the vehicle group.

CONCLUSION

NOSI for 14 days attenuates unaffected soleus and gastrocnemius muscle atrophy in neuropathic pain model.

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.

The selective neuronal nitric oxide synthase inhibitor 7-nitroindazole has acute analgesic but not cumulative effects in a rat model of peripheral neuropathy

Chronic neuropathic pain that may arise from various nerve injuries or insults remains notoriously difficult to manage. The neuronal isoform of the enzyme nitric oxide synthase (nNOS) has been shown to be involved in the spinal transmission of nociception in animal models of chronic pain. The aim of this study is to evaluate the effect of single dose and repeated administration of a selective nNOS inhibitor. Rats were unilaterally implanted with a 2-mm polyethylene cuff around the sciatic nerve. Paw withdrawal thresholds were measured using von Frey filament stimulation. Rats were given 10, 20, or 30 mg/kg of 7-nitroindazole (7-NI), or vehicle, on days 2, 5, and 7 after model induction, respectively. Paw withdrawal thresholds were measured before and at 30 and 60 min after injection. 7-NI significantly increased paw withdrawal thresholds at 60 min at the 20 and 30 mg/kg dosages. In the second part of this study, rats were given 20 mg/kg 7-NI daily for five days starting immediately after cuff implantation (days 0 to 4), and the cuff was removed on day 4. Withdrawal thresholds were measured intermittently over a 24-day observation period. No differences in withdrawal thresholds were observed between drug and vehicle-treated rats. Therefore, early and repeated administration of 7-NI did not affect the development or progression of the model. In conclusion, inhibition of nNOS had an analgesic but not a pre-emptive effect in this model of peripheral neuropathic pain.

The galactosylation of N(ω)-nitro-L-arginine enhances its anti-nocifensive or anti-allodynic effects by targeting glia in healthy and neuropathic mice

This study has investigated whether the galactosyl ester prodrug of N(ω)-nitro-L-arginine (NAGAL), shows enhanced analgesic efficacy in healthy mice and in models of visceral and neuropathic pain: the writhing test and the spared nerve injury (SNI), respectively. NAGAL was compared to methyl ester pro-drug of N(ω)-nitro-l-arginine (L-NAME), a widely exploited non-specific nitric oxide synthase (NOS) inhibitor, for analgesic potential. The writhing test revealed that the ED(50) value, along with the 95% confidence limit (CL) was 3.82 (1.77-6.04) mg/kg for NAGAL and, 36.75 (20.07-68.37) mg/kg for L-NAME. Notably, NAGAL elicited a greater anti-allodynic effect than L-NAME did in neuropathic mice. Biomolecular and morphological studies revealed that spared nerve injury increased the expressions of pro-inflammatory enzymes (caspase-1) and two glial cell biomarkers: integrin alpha M (ITGAM) and glial fibrillary acidic protein (GFAP) in the spinal cord. Finally, GLUT-3, an isoform of the hexose transporters capable to bind NAGAL and inducible NOS (iNOS), were found to be over-expressed in the activated astrocytes of the spinal cord of neuropathic mice. NAGAL administration normalized expression levels of these biomarkers. NAGAL showed a greater efficacy in inhibiting visceral pain and allodynia than L-NAME possibly by a greater cell permeation through the hexose transporter which is highly over-expressed by activated glia.

Safety and pharmacokinetics of NXN-188 after single and multiple doses in five phase I, randomized, double-blind, parallel studies in healthy adult volunteers

BACKGROUND

NXN-188 is a dual-action oral therapeutic being developed for the treatment of acute migraine. The mechanism of action of NXN-188 involves inhibition of both the neuronal nitric oxide synthase enzyme isoform and affinity for serotonin (5-hydroxytryptamine1B/D) receptors.

OBJECTIVES

The aims of the initial Phase I clinical studies were to compare the pharmacokinetic (PK) properties of NXN-188 administered as a single dose or multiple twice-daily doses to healthy adult volunteers and to determine the tolerability of NXN-188 in these individuals.

METHODS

Healthy adult male and female subjects were enrolled in 5 Phase I, randomized, double-blind studies, all of which (except for a fed/fasted trial) were placebo controlled. In the 4 single-dose studies, which differed with respect to feeding status and the formulation used (capsules or solution), subjects received NXN-188 at doses of 2 to 800 mg (0.027-11.2 mg/kg). In the repeat-dose study, subjects received 50-mg (0.71 mg/kg) doses twice daily for 4 days. Serum samples were analyzed for NXN-188 using validated HPLC-MS/MS methods. Standard clinical laboratory analyses (chemistry, hematology, and urinalysis) and measurements of serum creatine kinase and myoglobin levels were conducted at screening, admission, discharge, and follow-up. Baseline and postexposure values were compared to assess tolerability. Electrocardiography and physical examination were conducted at screening and at discharge and follow-up if any negative change occurred from the previous findings. Vital signs (heart rate, blood pressure, respiration), including assessment for orthostatic changes, were measured at screening, check-in, and follow-up visits (1 hour before dosing, every 30 minutes for the first 4 hours, then every hour for the next 4 hours, then every 4 hours for the remainder of the 24-hour study). Adverse events were recorded, reviewed, and monitored throughout the study.

RESULTS

Two hundred three subjects (102 women, 101 men) 18 to 50 years of age were enrolled in the 5 studies; 168 subjects received NXN-188 and 35 received placebo. Most (91%) of the subjects were white; weight ranged from 69.3 to 71.8 kg (body mass index, 24.5-25.8 kg/m(2)). The initial absorption phase of orally administered NXN-188 peaked at approximately 1 hour, followed by a second absorption phase with a T(max) of approximately 4 to 5 hours. Exposure (C(max) and AUC) increased in a slightly greater than dose-proportional manner across a dose range of 2 to 800 mg (0.027-11.2 mg/kg). Elimination was multiexponential, with an initial rapid plasma drug elimination (plasma concentrations decreased approximately 70%-90% from Cmax within 24 hours after dosing), followed by a prolonged clearance phase of very low NXN-188 concentrations ( approximately 1%-5% of Cmax) that persisted for several weeks. Clearance ranged from 70 to 130 L/h, and the NXN-188 halflife ranged from 11 to 178 hours. Neither food nor gender had any measurable effect on the PK properties of NXN-188. Overall, dizziness was reported more often in the NXN-188 groups than in the placebo groups (6.3% vs 2.9%, respectively). Frequently reported adverse events that occurred more often in the placebo groups than in the NXN-188 groups were somnolence (11.4% vs 6.3%, respectively), and headache (8.6% vs 6.9%). Incidences of orthostatic hypotension (6.3% vs 5.7%) and postural (orthostatic) tachycardia syndrome (6.3% vs 5.7%) were comparable in the NXN-188 and placebo groups, respectively. No serious adverse events were reported at any dose of NXN-188 up to the current maximum dose (800 mg or 11.2 mg/kg).

CONCLUSION

NXN-188 exhibited linear pharmaco-kinetics over the dose range studied and appeared to be well tolerated in these healthy volunteers.

Superoxide signaling in pain is independent of nitric oxide signaling

Two reactive oxygen species (ROS), nitric oxide (NO(.)) and superoxide ((.)O2), contribute to persistent pain. Using three different animal models where ROS mediate pain, this study examined whether NO(.) and (.)O2 converge to peroxynitrite (ONOO(-)) or whether each has an independent signaling pathway to produce hyperalgesia. The hyperalgesia after spinal nerve ligation was attenuated by removing (.)O2 by TEMPOL or inhibiting NO(.) production by L-NAME, but not by removing peroxynitrite with FeTMPyP. Nitric oxide-induced hyperalgesia was not affected by removing (.)O2 but was reduced by a guanyl cyclase inhibitor. Superoxide-induced hyperalgesia was not affected by inhibiting NO(.) production but was suppressed by a protein kinase C inhibitor. The data suggest that NO(.) and (.)O2 operate independently to generate pain.

Disruption of nNOS-PSD95 protein-protein interaction inhibits acute thermal hyperalgesia and chronic mechanical allodynia in rodents

BACKGROUND AND PURPOSE

Post-synaptic density protein 95 (PSD95) contains three PSD95/Dosophilia disc large/ZO-1 homology domains and links neuronal nitric oxide synthase (nNOS) with the N-methyl-D-aspartic acid (NMDA) receptor. This report assesses the effects of disruption of the protein-protein interaction between nNOS and PSD95 on pain sensitivity in rodent models of hyperalgesia and neuropathic pain.

EXPERIMENTAL APPROACH

We generated two molecules that interfered with the nNOS-PSD95 interaction: IC87201, a small molecule inhibitor; and tat-nNOS (residues 1-299), a cell permeable fusion protein containing the PSD95 binding domain of nNOS. We then characterized these inhibitors using in vitro and in vivo models of acute hyperalgesia and chronic allodynia, both of which are thought to require nNOS activation.

KEY RESULTS

IC87201 and tat-nNOS (1-299) inhibited the in vitro binding of nNOS with PSD95, without inhibiting nNOS catalytic activity. Both inhibitors also blocked NMDA-induced 3',5'-cyclic guanosine monophosphate (cGMP) production in primary hippocampal cultures. Intrathecal administration of either inhibitor potently reversed NMDA-induced thermal hyperalgesia in mice. At anti-hyperalgesic doses, there was no effect on acute pain thresholds or motor coordination. Intrathecal administration of IC87201 and tat-nNOS also reversed mechanical allodynia induced by chronic constriction of the sciatic nerve.

CONCLUSIONS AND IMPLICATIONS

nNOS-PSD95 interaction is important in maintaining hypersensitivity in acute and chronic pain. Disruption of the nNOS-PSD95 interaction provides a novel approach to obtain selective anti-hyperalgesic compounds.

Differential astroglial responses in the spinal cord of rats submitted to a sciatic nerve double crush treated with local injection of cultured Schwann cell suspension or lesioned spinal cord extract: implications on cell therapy for nerve repair

PURPOSE

Reactive astrocytes are implicated in several mechanisms after central or peripheral nervous system lesion, including neuroprotection, neuronal sprouting, neurotransmission and neuropathic pain. Schwann cells (SC), a peripheral glia, also react after nerve lesion favoring wound/repair, fiber outgrowth and neuronal regeneration. We investigated herein whether cell therapy for repair of lesioned sciatic nerve may change the pattern of astroglial activation in the spinal cord ventral or dorsal horn of the rat.

METHODS

Injections of a cultured SC suspension or a lesioned spinal cord homogenized extract were made in a reservoir promoted by a contiguous double crush of the rat sciatic nerve. Local injection of phosphate buffered saline (PBS) served as control. One week later, rats were euthanized and spinal cord astrocytes were labeled by immunohistochemistry and quantified by means of quantitative image analysis.

RESULTS

In the ipsilateral ventral horn, slight astroglial activations were seen after PBS or SC injections, however, a substantial activation was achieved after cord extract injection in the sciatic nerve reservoir. Moreover, SC suspension and cord extract injections were able to promote astroglial reaction in the spinal cord dorsal horn bilaterally.

CONCLUSION

Spinal cord astrocytes react according to repair processes of axotomized nerve, which may influence the functional outcome. The event should be considered during the neurosurgery strategies.

Nitric oxide activates ATP-sensitive potassium channels in mammalian sensory neurons: action by direct S-nitrosylation

Background

ATP-sensitive potassium (K_ATP) channels in neurons regulate excitability, neurotransmitter release and mediate protection from cell-death. Furthermore, activation of K_ATP channels is suppressed in DRG neurons after painful-like nerve injury. NO-dependent mechanisms modulate both K_ATP channels and participate in the pathophysiology and pharmacology of neuropathic pain. Therefore, we investigated NO modulation of K_ATP channels in control and axotomized DRG neurons.

Results

Cell-attached and cell-free recordings of K_ATP currents in large DRG neurons from control rats (sham surgery, SS) revealed activation of K_ATP channels by NO exogenously released by the NO donor SNAP, through decreased sensitivity to [ATP]i.

This NO-induced K_ATP channel activation was not altered in ganglia from animals that demonstrated sustained hyperalgesia-type response to nociceptive stimulation following spinal nerve ligation. However, baseline opening of K_ATP channels and their activation induced by metabolic inhibition was suppressed by axotomy. Failure to block the NO-mediated amplification of K_ATP currents with specific inhibitors of sGC and PKG indicated that the classical sGC/cGMP/PKG signaling pathway was not involved in the activation by SNAP. NO-induced activation of K_ATP channels remained intact in cell-free patches, was reversed by DTT, a thiol-reducing agent, and prevented by NEM, a thiol-alkylating agent. Other findings indicated that the mechanisms by which NO activates K_ATP channels involve direct S-nitrosylation of cysteine residues in the SUR1 subunit. Specifically, current through recombinant wild-type SUR1/Kir6.2 channels expressed in COS7 cells was activated by NO, but channels formed only from truncated isoform Kir6.2 subunits without SUR1 subunits were insensitive to NO. Further, mutagenesis of SUR1 indicated that NO-induced K_ATP channel activation involves interaction of NO with residues in the NBD1 of the SUR1 subunit.

Conclusion

NO activates K_ATP channels in large DRG neurons via direct S-nitrosylation of cysteine residues in the SUR1 subunit. The capacity of NO to activate K_ATP channels via this mechanism remains intact even after spinal nerve ligation, thus providing opportunities for selective pharmacological enhancement of K_ATP current even after decrease of this current by painful-like nerve injury.

Relationship between neuronal nitric oxide synthase and NADPH-diaphorase in the dorsal root ganglia during neuropathic pain

BACKGROUND

Changes in nitric oxide (NO) production in the dorsal root ganglia (DRG) may contribute to allodynia after nerve injury. It is known that the histochemistry of NADPH-diaphorase (NADPH-d) is known to be not always coincident with NOS. This study was conducted to investigate the relationship between nNOS and NADPH-d expression in the DRG in a spinal nerve injury model of neuropathic pain, and to elucidate role that NO plays in neuropathic pain.

METHODS

nNOS immunohistochemistry and/or NADHP-d histochemistry were conducted in the DRG of a spinal nerve transection model of neuropathic pain, and the pain behavior was then measured by a von Frey filament test of the hindpaws of wild type and nNOS knock-out mice.

RESULTS

nNOS immunoreactive neurons and NADPH-d stained neurons were not always identical. Additionally NADPH-d increased, but nNOS did not increase significantly in the DRG after spinal nerve transection. Neuropathic pain behavior increased in the hindpaw of nNOS(-/-) mice after spinal nerve transection, but was lower than that of wild type mice after spinal nerve transection.

CONCLUSIONS

nNOS immunoreactive neurons and NADPH-d stained neurons were not always identical in the DRG, and a novel NADPH-d positive source may be involved in neuropathic pain after spinal nerve transection. Changes in nNOS expression in the DRG were not the primary cause of neuropathic pain behavior in a spinal nerve transection model of neuropathic pain.

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 the nitric oxide-cyclic GMP-protein kinase G-K+ channel pathway in the antihyperalgesic effects of bovine lactoferrin in a model of neuropathic pain

The possible involvement of the nitric oxide (NO)-cyclic GMP (cGMP)-protein kinase G (PKG) pathway on bovine lactoferrin (BLF)-induced spinal antihyperalgesic activity was elucidated in sciatic nerve injured rats. Intrathecal BLF reduced thermal hyperalgesia in a dose-dependent manner. Pretreatment with NG-L-nitro-arginine methyl ester (L-NAME, non-specific inhibitor of NO synthase), 7-nitroindazole (7-NI, neuronal NO synthase inhibitor), 1H-[1,2,4]-oxadiazolo [4,3-a] quinoxalin-1-one (ODQ, guanylyl-cyclase inhibitor), (9S, 10R, 12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2, 9-dimethyl-1-oxo-9, 12-epoxy-1H-diindolo-[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester (KT-5823, specific PKG inhibitor) or glybenclamide (ATP-sensitive K+ channel blocker), but not NG-D-nitro-arginine methyl ester (D-NAME, an inactive enantiomer of l-NAME), d-Phe-Cys-Tyr-d-Trp-Orn-Thr-NH2 (CTOP, selective mu-opioid receptor antagonist) or naloxone (nonselective opioid receptor antagonist) prevented BLF-induced antihyperalgesia. Data suggest that BLF-induced spinal antihyperalgesia could be due to activation of the NO-cGMP-PKG-K+ channel pathway and it is not mediated by mu-opioid receptor in a model of neuropathic pain.

Different roles of nitric oxide synthase-1 and -2 between herpetic and postherpetic allodynia in mice

We investigated using the mice role of nitric oxide synthase (NOS) in the spinal dorsal horn in herpetic and postherpetic pain, especially allodynia, which was induced by transdermal inoculation of the hind paw with herpes simplex virus type-1 (HSV-1). The virus inoculation induced NOS2 expression in the lumbar dorsal horn of mice with herpetic allodynia, but not postherpetic allodynia. There were no substantial alternations in the expression level of NOS1 at the herpetic and postherpetic stages. Herpetic allodynia was significantly inhibited by i.p. administration of the selective NOS2 inhibitor S-methylisothiourea, but not the selective NOS1 inhibitor 7-nitroindazole. NOS2 expression was observed around HSV-1 antigen-immunoreactive cells. On the other hand, postherpetic allodynia was significantly inhibited by i.p. administration of 7-nitroindazole, but not S-methylisothiourea. The activity of reduced nicotinamide adenine dinucleotide phosphate diaphorase, an index of NOS1 activity, significantly increased in the laminae I and II of the lumbar dorsal horn of mice with postherpetic allodynia, but not mice without postherpetic allodynia. The expression level of NOS1 mRNA in the dorsal root ganglia was similar between mice with and without postherpetic allodynia. The results suggest that herpetic and postherpetic allodynia is mediated by nitric oxide in the dorsal horn and that NOS2 and NOS1 are responsible for herpetic and postherpetic allodynia, respectively. It may be worth testing the effects of NOS2 and NOS1 inhibitors on herpetic pain and postherpetic neuralgia in human subjects, respectively.

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.

Pain-related behavior following REM sleep deprivation in the rat: influence of peripheral nerve injury, spinal glutamatergic receptors and nitric oxide

We assessed whether pain-related behavior in neuropathic or control rats is changed following rapid eye movement sleep deprivation (REMSD). Furthermore, we determined the contribution of spinal glutamatergic receptors and nitric oxide to sensitivity changes following REMSD versus peripheral nerve injury. Pain behavior was assessed in Sprague-Dawley (SD) and Hannover-Wistar (HW) rats with a spinal nerve ligation or a sham operation. Nerve ligation produced mechanical hypersensitivity of the injured dermatome in all animals. Baseline sensitivity to mechanical stimulation was higher in the HW than the SD group, independent of nerve injury. In both strains, mechanical sensitivity of neuropathic and sham-operated animals was increased following 48 h of REMSD. Heat sensitivity of an uninjured dermatome was not different among experimental conditions. Reversal of mechanical hypersensitivity was attempted in HW rats by spinal administration of an antagonist of the metabotropic glutamate receptor 5 (mGluR(5)) or the NMDA receptor and a nitric oxide synthase (NOS) inhibitor. Mechanical hypersensitivity induced by REMSD in unoperated rats was attenuated by all three drugs, while in neuropathic animals the mechanical anti-hypersensitive effect was most pronounced with the antagonist of the mGluR(5) or a NOS inhibitor. The results indicate that the strain of the animals markedly influences baseline withdrawal threshold to mechanical stimulation. Mechanical hypersensitivity following REMSD, however, is similarly increased in HW and SD strains, and the REMSD-associated increase in mechanical sensitivity is independent of nerve injury. Furthermore, mechanical hypersensitivities following REMSD and peripheral nerve injury share common spinal mechanisms involving, at least, the mGluR(5) and nitric oxide.

The ERK/MAPK pathway, as a target for the treatment of neuropathic pain

Peripheral nerve injury produces neuropathic pain as well as phosphorylation of mitogen activated protein kinase (MAPK) family in dorsal root ganglia (DRG) and dorsal horn. Following nerve injury, phosphorylation of extracellular signal-regulated protein kinase (ERK), an important member of this family, is sequentially increased in neurons, microglia and astrocytes of the dorsal horn and gracile nucleus, and in injured large DRG neurons. Nerve injury-induced phosphorylation of ERK occurs early and is long-lasting. In several animal models of neuropathic pain, MEK inhibitors, known to suppress the synthesis of ERK, have proven effective to alleviate pain at various time points. Thus, the regulation of ERK/MAPK can be considered as a promising therapeutic target for the treatment of neuropathic pain.

Clonidine depresses LTP of C-fiber evoked field potentials in spinal dorsal horn via NO-cGMP pathway

Clonidine, a specific alpha2-adrenergic receptor agonist, has been found to be effective for the treatment of neuropathic pain, the mechanism underlying the effect is, however, not well understood. Here, the effect of clonidine on long-term potentiation (LTP) of C-fiber evoked field potentials in spinal dorsal horn, which is a synaptic model of injury-induced hyperalgesia, was investigated. LTP of C-fiber evoked field potentials was recorded in the superficial layers of spinal dorsal horn in anesthetized adult Sprague-Dawley rats. Clonidine and other substances were applied locally at the recording spinal segments before or after LTP induction by tetanic stimulation. We found that (1) Clonidine completely blocked LTP induction, when applied 30 min before tetanic stimulation and depressed spinal LTP, when applied 30 min and 3 h after LTP induction. (2) The inhibitory effect of clonidine on spinal LTP had two phases: a fast phase lasting for about 3.5 h and a slow phase persisting for the rest time of experiments (up to 8 h after drug). (3) Spinal clonidine at low dose (10.7 micro g/100 micro l) depressed spinal LTP but not C-fiber baseline response and at higher dose (107 micro g/100 micro l) depressed both of them. (4) Pretreatment with alpha2-adrenergic receptor antagonist yohimbine completely blocked the inhibitory effect of clonidine. (5) Pretreatment with muscarinic receptor antagonist atropine, nitric oxide synthesis inhibitor l-NNA or cGMP inhibitor ODQ depressed the fast phase inhibition significantly and abolished the slow phase inhibition completely. These results suggest that clonidine may exert analgesic effect by depressing the synaptic plasticity in spinal dorsal horn, via activation of muscarinic receptor-NO-cGMP pathway.

Pharmacological evidence for the participation of NO-cyclic GMP-PKG-K+ channel pathway in the antiallodynic action of resveratrol

The possible participation of the nitric oxide (NO)-cyclic GMP-protein kinase G (PKG)-K+ channels pathway in the antiallodynic action of resveratrol and YC-1 in spinal nerve injured rats was assessed. Ligation of L5/L6 spinal nerves produced a clear-cut tactile allodynia in the rats. Intrathecal administration of resveratrol (100-600 microg) and 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (0.1-2.7 microg, YC-1, a soluble guanylyl cyclase activator) decreased tactile allodynia induced by ligation of L5/L6 spinal nerves. Intrathecal treatment with NG-L-nitro-arginine methyl ester (10-100 microg, L-NAME, a NO synthase inhibitor), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (1-10 microg, ODQ, a soluble guanylyl cyclase inhibitor), KT-5823 (5-500 ng, a PKG inhibitor) and iberiotoxin (5-500 ng, a large-conductance Ca2+ -activated K+ channel blocker), but not NG-D-nitro-arginine methyl ester (100 microg, D-NAME, an inactive isomer of L-NAME), glibenclamide (12.5-50 microg, ATP-sensitive K+ channel blocker) or vehicle, significantly diminished resveratrol (300 microg)- and YC-1 (2.7 microg)-induced spinal antiallodynia. These effects were independent of prostaglandin synthesis inhibition as indomethacin did not affect resveratrol-induced antiallodynia. Results suggest that resveratrol and YC-1 could activate the proteins of the NO-cyclic GMP-PKG spinal pathway or large-conductance Ca2+ -activated, but not ATP-sensitive, K+ channels at the spinal cord in order to produce at least part of their antiallodynic effect in this model of neuropathy.

PDZ domains at excitatory synapses: potential molecular targets for persistent pain treatment

Persistent pain, a common clinical condition, could be caused by inflammation, tissue injury secondary to trauma or surgery, and nerve injuries. It is often inadequately controlled by current treatments, such as opioids and nonsteroidal anti-inflammatory drugs. The PDZ (Postsynaptic density 95, Discs large, and Zonula occludens-1) domains are ubiquitous protein interaction modules often found among multi-protein signaling complexes at neuronal synapses. Recent preclinical research shows that targeted disruption of PDZ domain-mediated protein interaction among N-methyl-Daspartate (NMDA) receptor signaling complexes significantly attenuates the development and maintenance of persistent pain without affecting nociceptive responsiveness to acute pain. PDZ domains at excitatory synapses may be new molecular targets for prevention and treatment of persistent pain. Here, we illustrate expression and distribution of the PDZ domain-containing proteins associated with NMDA receptors in the pain-related regions of the central nervous system, review the evidence for their roles in persistent pain states, and discuss potential mechanisms by which these PDZ domain-containing proteins are involved in persistent pain.

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.

Experiments with nitric oxide synthase inhibitors in spinal nerve ligated rats provide no evidence of a role for nitric oxide in neuropathic mechanical allodynia

We have investigated the effect of treatment with N(omega)-nitro-l-arginine methylester (l-NAME), a non-selective nitric oxide synthase inhibitor (NOS), both before and after the induction of mechanical allodynia by tight ligation of the left L5 and L6 spinal nerves in rats (SNL rats). The degree of mechanical allodynia was measured by tactile threshold for paw flinching with von Frey filaments. Intraperitoneal (i.p.) administration of l-NAME (3-30 mg/kg) 1 week after the spinal nerve ligation produced a dose-dependent reduction of the behavioral signs of mechanical allodynia, but the effect was not reversed by pretreatment with l-arginine (300 mg/kg). N(omega)-Nitro-l-arginine (l-NNA, i.p., 30 mg/kg), aminoguanidine (AG, i.p., 30 mg/kg) and a potent neuronal NOS inhibitor (LY457963, i.p., 30 mg/kg) did not reduce mechanical sensitivity in the SNL rats. Furthermore, using an ex vivo NOS activity assay, l-NAME partially inhibited the spinal NOS activity, whereas LY457963 almost completely inhibited the spinal NOS activity. Prior administration of l-NAME (i.p., 30 mg/kg) or of MK-801 (0.5 mg/kg), an NMDA antagonist, 30 min before the spinal nerve ligation significantly prevented the development of mechanical allodynia after spinal nerve ligation for an extended period of time. High doses of l-arginine (100 mg/kg or 300 mg/kg, i.p.), however, did not reverse the preemptive effect of l-NAME. These results suggest that neither the anti-allodynic nor the preemptive effects of l-NAME are mediated by NOS inhibition.

Antihyperalgesic, but not antiallodynic, effect of melatonin in nerve-injured neuropathic mice: Possible involvements of the L-arginine-NO pathway and opioid system

The present study was undertaken to determine the effects of intracerebroventricular (i.c.v.) and intraperitoneal (i.p.) melatonin on mechanical allodynia and thermal hyperalgesia in mice with partial tight ligation of the sciatic nerve, and how the nitric oxide (NO) precursor l-arginine and the opiate antagonist naloxone influence this effect. A plantar analgesic meter was used to assess thermal hyperalgesia, and nerve injury-induced mechanical hyperalgesia was assessed with von Frey filaments. 1-5 weeks following the surgery, marked mechanical allodynia and thermal hyperalgesia developed in neuropathic mice. Intracerebroventricular and intraperitoneal melatonin, with its higher doses, produced a blockade of thermal hyperalgesia, but not mechanical allodynia. Administration of both l-arginine and naloxone, at doses which produced no effect on their own, partially reversed antihyperalgesic effect of melatonin. These results suggest that although it has different effects on neuropathic pain-related behaviors, melatonin may have clinical utility in neuropathic pain therapy in the future. It is also concluded that l-arginine-NO pathway and opioidergic system are involved in the antihyperalgesic effect of melatonin in nerve-injured mice.

Satellite glial cells in sensory ganglia: from form to function

Current information indicates that glial cells participate in all the normal and pathological processes of the central nervous system. Although much less is known about satellite glial cells (SGCs) in sensory ganglia, it appears that these cells share many characteristics with their central counterparts. This review presents information that has been accumulated recently on the physiology and pharmacology of SGCs. It appears that SGCs carry receptors for numerous neuroactive agents (e.g., ATP, bradykinin) and can therefore receive signals from other cells and respond to changes in their environment. Activation of SGCs might in turn influence neighboring neurons. Thus SGCs are likely to participate in signal processing and transmission in sensory ganglia. Damage to the axons of sensory ganglia is known to contribute to neuropathic pain. Such damage also affects SGCs, and it can be proposed that these cells have a role in pathological changes in the ganglia.

Reactive oxygen species (ROS) play an important role in a rat model of neuropathic pain

Reactive oxygen species (ROS) are free radicals produced in biological systems that are involved in various degenerative brain diseases. The present study tests the hypothesis that ROS also play an important role in neuropathic pain. In the rat spinal nerve ligation (SNL) model of neuropathic pain, mechanical allodynia develops fully 3 days after nerve ligation and persists for many weeks. Systemic injection of a ROS scavenger, phenyl-N-tert-butylnitrone (PBN), relieves SNL-induced mechanical allodynia in a dose-dependent manner. Repeated injections cause no development of tolerance or no loss of potency. Preemptive treatment with PBN is also effective in preventing full development of neuropathic pain behavior. Systemic injection was mimicked by intrathecal injection with a little less efficacy, while intracerebroventricular administration produced a much smaller effect. These data suggest that PBN exerts its anti-allodynic action mainly by spinal mechanisms. Systemic treatment with other spin-trap reagents, 5,5-dimethylpyrroline-N-oxide and nitrosobenzene, showed similar analgesic effects, suggesting that ROS are critically involved in the development and maintenance of neuropathic pain. Thus this study suggests that systemic administration of non-toxic doses of free radical scavengers could be useful for treatment of neuropathic pain.

nNOS expression following inferior alveolar nerve injury in the ferret

Damage to the inferior alveolar nerve (IAN) may result in permanent painful dysaesthesia, and there is compelling evidence to suggest that ectopic activity from the injury site plays a crucial role in the initiation of this disorder. The aim of this study was to determine whether neuronal nitric oxide synthase (nNOS), a regulator of neuronal excitability, could be involved in the development of the abnormal activity. In seven ferrets, the left IAN was exposed and a retrograde tracer, fluorogold, was applied to the nerve for the identification of cell bodies in the trigeminal ganglion with axons in the IAN. In four animals, the nerve was sectioned distal to the injection site, and three served as controls. After 3 days, the animals were perfused with fixative, and the left and right IANs and trigeminal ganglia were processed using indirect immunofluorescence for nNOS. Image analysis was used to quantify the percentage area of staining (PAS) at the injury site. In the ganglia, counts were made of positively labelled cells in the fluorogold population. At the injury site, PAS was significantly greater in injured nerves than in either contralateral or control nerves, and contralateral PAS was elevated compared to control. In the ganglia, the proportion of nNOS-labelled cells was significantly reduced following injury. These results indicate a possible translocation of the nNOS protein from the cell body to the site of nerve injury, where it accumulates. Thus, nNOS could play a role in the development of ectopic activity at a site of trigeminal nerve injury.

Are synaptic MAGUK proteins involved in chronic pain?

Chronic pain, particularly neuropathic pain, is notoriously difficult to treat. NMDA receptor antagonists are effective in reducing pain hypersensitivity in animal models and clinical settings but are associated with an unacceptable level of side-effects. Recent studies of the role of a family of synaptic membrane-associated guanylate kinase proteins in chronic pain provide new insights into central mechanisms of chronic pain that could result in new biochemical targets for its treatment.

Peripherally administered amitriptyline derivatives have differential anti-allodynic effects in a rat model of neuropathic pain

We assessed if derivatives of amitriptyline could alleviate mechanical allodynia in a rat model of neuropathic pain. N-methyl amitriptyline (NMA), and amitriptyline HCl (AHC) were compared to evaluate the antiallodynic effects produced by systemic and peripheral administration. Under general anesthesia with halothane, neuropathic injury was produced in rats by tightly ligating the left L5 and L6 spinal nerves. All animals developed neuropathic pain behaviors within 7 days after the surgery. For systemic delivery, three groups of rats were injected intraperitoneally with NMA (15 mg/kg), AHC (12.5 mg/kg), or saline. For peripheral routes of injection, rats were divided into 3 x 3 groups and injected with 100, 330, and 1000 nmol/kg doses of each NMA, AHC and 0.9% normal saline. NMA and AHC suppressed mechanical allodynia following peripheral administration but not following intraperitoneal injection. NMA was less potent but showed a longer-acting response than AHC. This study shows that NMA can be used to relieve mechanical allodynia and is a longer-acting agent than AHC.