Trigeminal pain transmission requires reactive oxygen species production

Oxidative stress plays an important role in the central regulatory mechanism of orofacial hyperalgesia under low estrogen conditions

Hyperalgesia occurs in the orofacial region of rats when estrogen levels are low, although the specific mechanism needs to be investigated further. Furthermore, oxidative stress plays an important role in the transmission of pain signals. This study aimed to explore the role of oxidative stress in orofacial hyperalgesia under low estrogen conditions. We firstly found an imbalance between oxidative and antioxidant capacity within the spinal trigeminal subnucleus caudalis (SP5C) of rats after ovariectomy (OVX), resulting in oxidative stress and then a decrease in the orofacial pain threshold. To investigate the mechanism by which oxidative stress occurs, we used virus as a tool to silence or overexpress the excitatory amino acid transporter 3 (EAAT3) gene. Further investigation revealed that the regulation of glutathione (GSH) and reactive oxygen species (ROS) can be achieved by regulating EAAT3, which in turn impacts the occurrence of oxidative stress. In summary, our findings suggest that reduced expression of EAAT3 within the SP5C of rats in the low estrogen state may decrease GSH content and increase ROS levels, resulting in oxidative stress and ultimately lead to orofacial hyperalgesia. This suggests that antioxidants could be a potential therapeutic direction for orofacial hyperalgesia under low estrogen conditions, though more research is needed to understand its mechanism.

Lumbosacral spinal proteomic changes during PAR4-induced persistent bladder pain

Repeated intravesical activation of protease-activated receptor-4 (PAR4) in mice results in persistent bladder hyperalgesia (BHA). We investigated spinal proteomic changes associated with persistent BHA. Persistent BHA was induced in female mice by repeated (3x; days 0,2,4; n = 9) intravesical instillation of PAR4 activating peptide (PAR4-AP) while scrambled peptide served as the control (no pain; n = 9) group. The threshold to lower abdominal von Frey stimulation was recorded prior to and during treatment. On day 7, L6-S1 spinal segments were excised and examined for proteomic changes using LC-MS/MS. In-depth, unbiased proteomic tandem-mass tag (TMT) analysis identified and relatively quantified 6739 proteins. We identified significant changes with 29 decreasing and 51 increasing proteins in the persistent BHA group and they were associated with neuroprotection, redox modulation, mitochondrial factors, and neuronal-related proteins. In an additional experiment, decreases in protein levels were confirmed by immunohistochemistry for metallothionein 1/2. Our results show that persistent bladder pain is associated with central (spinal) protein changes. Previous work showed that PAR4-induced bladder pain is mediated, at least in part by spinal MIF. Further functional studies of these top changing proteins may lead to the discovery of novel potential therapeutic targets at the spinal level to modulate persistent bladder pain. Future studies will examine the effect of spinal MIF antagonism on PAR4-induced spinal proteomics associated with persistent bladder pain.

Vitamin C Acutely Affects Brain Perfusion and Mastication-Induced Perfusion Asymmetry in the Principal Trigeminal Nucleus

Prolonged mastication may induce an asymmetric modification of the local perfusion of the trigeminal principal nucleus. The aim of the present study was to evaluate the possible influence of vitamin C (vit. C) on such effect. Four groups of healthy volunteers underwent arterial spin labeling magnetic resonance imaging (ASL-MRI) to evaluate the local perfusion of the trigeminal nuclei after a vit. C-enriched lunch or a control lunch. Two ASL-MRI scans were acquired, respectively, before and after a 1 h-long masticating exercise or a 1 h long resting period. The results showed (i) an increased global perfusion of the brain in the vit. C-enriched lunch groups, (ii) an increased local perfusion of the right principal trigeminal nucleus (Vp) due to mastication, and (iii) a reduction of the rightward asymmetry of the Vp perfusion, due to mastication, after the vit C-enriched meal compared to the control meal. These results confirmed a long-lasting effect of prolonged mastication on Vp perfusion and also suggest a possible effect of vit. C on cerebral vascular tone regulation. Moreover, the data strongly draw attention on the side-to-side relation in Vp perfusion as a possible physiological parameter to be considered to understand the origin of pathological conditions like migraine.

Increased CSF Levels of Apolipoproteins and Complement Factors in Trigeminal Neuralgia Patients-In Depth Proteomic Analysis Using Mass Spectrometry

The main cause of trigeminal neuralgia (TN) is compression of a blood vessel at the root entry zone of the trigeminal nerve. However, a neurovascular conflict does not seem to be the only etiology and other mechanisms are implicated in the development of the disease. We hypothesized that TN patients may have distinct protein expression in the CSF. In this study, lumbar CSF from TN patients (n = 17), scheduled to undergo microvascular decompression, and from controls (n = 20) was analyzed and compared with in depth mass spectrometry TMTbased quantitative proteomics. We identified 2552 unique proteins, of which 46 were significantly altered (26 increased, and 20 decreased, q-value < .05) in TN patients compared with controls. An over-representation analysis showed proteins involved in high-density lipoprotein, such as Apolipoprotein A4, Apolipoprotein M, and Apolipoprotein A1, and the extracellular region, including proteins involved in the complement cascade to be over-represented. We conclude that TN patients have distinct protein expression in the CSF compared to controls. The pathophysiological background of the protein alterations found in this study warrants further investigation in future studies. PERSPECTIVE: In this article, cerebrospinal fluid from patients with trigeminal neuralgia was analyzed using in depth shotgun proteomics, revealing 46 differentially expressed proteins compared to controls. Among these, apolipoproteins and proteins involved in the complement system were elevated and significantly over-represented, implying an inflammatory component in the pathophysiology of the disease.

Reduction in Migraine and Headache Frequency and Intensity With Combined Antioxidant Prophylaxis (N-acetylcysteine, Vitamin E, and Vitamin C): A Randomized Sham-Controlled Pilot Study

OBJECTIVE

To investigate the preventive effects of a combined antioxidant drug (N-acetylcysteine, vitamin E, and vitamin C [NEC]) on migraine outcomes. Migraine is characterized by increased oxidative stress and neurogenic inflammation in the brain; therefore, antioxidants may have a migraine preventive effect.

DESIGN

Randomized, double-blind, sham-controlled pilot study.

SETTING

Australian community.

SUBJECTS

Adults reporting 2 to 8 migraines per month for at least a year.

METHODS

After a 1-month baseline period, 35 subjects completed 3 months of treatment with NEC (n = 19) or sham (n = 16) capsules. The primary outcome was the difference in mean number of headaches per month between baseline and final month of the trial for NEC and sham groups; secondary outcomes are listed below.

RESULTS

For NEC there was a significant decrease in mean number of headaches by 3.0 per month (P = 0.004) compared with 1.4 for sham (P = 0.073); there was no significant difference in these changes between the 2 groups (P = 0.052). Average monthly headache (P = 0.041) and migraine frequency (P = 0.018) were significantly less for NEC vs. sham. In NEC subjects, there was a significant decrease in average monthly migraine days (-3.1), moderate/severe headache days (-3.2), migraine duration, headache pain scores, and acute headache medication use.

CONCLUSIONS

This is the first randomized controlled trial to find that combined antioxidant therapy with NEC reduces headaches and migraines in adult migraineurs. Given the limitations of this pilot study, an adequately powered randomized controlled trial is planned to further investigate antioxidant prophylaxis in migraine.

A study of chewing muscles: Age-related changes in type I collagen and matrix metalloproteinase-2 expression

OBJECTIVE

In this study, the aim was to investigate the biochemical, physiological and histological changes that occur in masticatory muscles of the masticatory system with aging.

DESIGN

In this study, 14 BALB/c mice were used. Animals were divided into two equal groups of seven. Group I was organized as the group of young animals (n = 7) and Group II as the group of adult animals (n = 7). After routine histological follow-up was performed, the tissues were embedded in paraffin. 4-5 μm thick cross-sections were taken from paraffin-embedded tissues and they were stained with Haemotoxylin and Eosin Type I collagen and Matrix metalloproteinase-2 (MMP-2) immunohistochemically.

RESULTS

It was observed that there was a decrease and shrinking in blood vessels due to aging. In young mice, Type I collagen and MMP-2 immunoreactivity in the masseter muscle tissue showed low staining, while Type I collagen and MMP-2 immunoreactivity in the temporal muscle tissue showed moderate staining. Type I collagen and MMP-2 immunoreactivity were significantly higher in the masseter and temporal muscles of elderly mice (p = 0.001). In the H-score evaluation, MMP-2 immune reactivity was significantly lower in young mice than in older mice (p = 0.001).

CONCLUSION

It was determined that severe pain complications and functional losses are likely to occur with the increase of degeneration due to aging of masticator muscles.

Spinal Reactive Oxygen Species and Oxidative Damage Mediate Chronic Pain in Lame Dairy Cows

Simple Summary

Chronic inflammatory diseases could impact central nervous system homeostasis, being oxidative damage of the dorsal horn, a relevant mechanism mediating central sensitization. Chronic inflammatory lameness in dairy cows is a painful condition that affects animal welfare, affecting dairy production worldwide. This study reveals increased levels of reactive oxygen species, malondialdehyde, and carbonyl groups, indicating lipid and protein damage in the spinal cord of cows with chronic lameness. Moreover, antioxidant system activity was similar between lame and non-lame cows which suggests that antioxidant dysregulation was not the cause of oxidative damage, as has been proposed previously. Based on the fact that nociceptive pathways are strongly conserved between species, there is no reason to neglect that chronic pain in cows promotes Central Nervous System (CNS) alterations, such as oxidative damage. Moreover, lame cows develop central sensitization, as allodynia and hyperalgesia are centrally and not peripherally mediated. Our results support the current assumption that chronic pain is a central nervous system disease and lameness in dairy cows is far beyond an inflammation of the hoof.

Abstract

Lameness in dairy cows is a worldwide prevalent disease with a negative impact on animal welfare and herd economy. Oxidative damage and antioxidant system dysfunction are common features of many CNS diseases, including chronic pain. The aim of this study was to evaluate the levels of reactive oxygen species (ROS) and oxidative damage markers in the spinal cord of dairy cows with chronic inflammatory lameness. Locomotion score was performed in order to select cows with chronic lameness. Dorsal horn spinal cord samples were obtained post mortem from lumbar segments (L2–L5), and ROS, malondialdehyde (MDA), and carbonyl groups were measured along with the activity of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and total antioxidant response (TAR). Lame cows had increased levels of ROS, MDA, and carbonyl groups, while no differences were observed between lame and non-lame cows in SOD, GPx, CAT, and TAR activity. We conclude that painful chronic inflammatory lameness in dairy cows is associated with an increase in ROS, MDA, and carbonyl groups. Nonetheless, an association between ROS generation and dysfunction of the antioxidant system, as previously proposed, could not be established.

Orofacial antinociceptive effect of Mimosa tenuiflora (Willd.) Poiret

Mimosa tenuiflora (Willd.) Poiret, popularly known in Brazil as "jurema-preta" is widely used against bronchitis, fever, headache and inflammation. Its antioxidant, anti-inflammatory and antinociceptive potential has already been reported. To assess the orofacial antinociceptive effect of M. tenuiflora, ethanolic extracts of M. tenuiflora (leaves, twigs, barks and roots) were submitted to in vitro tests of antioxidant activity. The extract with the highest antioxidant potential was partitioned and subjected to preliminary chemical prospecting, GC-MS, measurement of phenolic content and cytotoxicity tests of the fraction with the highest antioxidant activity. The nontoxic fraction with the highest antioxidant activity (FATEM) was subjected to tests of acute and chronic orofacial nociception and locomotor activity. The possible mechanisms of neuromodulation were also assessed. The EtOAc fraction, obtained from the ethanolic extract of M. tenuiflora barks, was the one with the highest antioxidant potential and nontoxic (FATEM), and Benzyloxyamine was the major constituent (34.27%). FATEM did not alter the locomotor system of mice and reduced significantly the orofacial nociceptive behavior induced by formalin, glutamate, capsaicin, cinnamaldehyde or acidic saline compared to the control group. FATEM also inhibited formalin- or mustard oil-induced temporomandibular nociception. In addition, it also reduced mustard oil-induced orofacial muscle nociception. However, FATEM did not alter hypertonic saline-induced corneal nociception. Neuropathic nociception was reversed by treatment with FATEM. The antinociceptive effect of FATEM was inhibited by naloxone, L-NAME and glibenclamide. FATEM has pharmacological potential for the treatment of acute and neuropathic orofacial pain and this effect is modulated by the opioid system, nitric oxide and ATP-sensitive potassium channels. These results lead us to studies of isolation and characterization of bioactive principles.

Intranasally administered IGF-1 inhibits spreading depression in vivo

Spreading depression (SD) is a wave of cellular depolarization that travels slowly through susceptible gray matter brain areas. SD is the most likely cause of migraine aura and perhaps migraine pain, and is a well-accepted animal model of migraine. Identification of therapeutics that can prevent SD may have clinical relevance toward migraine treatment. Here we show that insulin-like growth factor-1 (IGF-1) significantly inhibited neocortical SD in vivo after intranasal delivery to rats. A single dose of IGF-1 inhibited SD within an hour, and continued to protect for at least seven days thereafter. A two-week course of IGF-1, administered every third day, further decreased SD susceptibility and showed no aberrant effects on glial activation, nasal mucosa, or serum markers of toxicity. SD begets SD in vitro by mechanisms that involve microglial activation. We add to this relationship by showing that recurrent SD in vivo increased susceptibility to subsequent SD, and that intervention with IGF-1 significantly interrupted this pathology. These findings support nasal administration of IGF-1 as a novel intervention capable of mitigating SD susceptibility, and as a result, potentially migraine.

Reactive oxygen species increase neuronal excitability via activation of nonspecific cation channel in rat medullary dorsal horn neurons

The caudal subnucleus of the spinal trigeminal nucleus (medullary dorsal horn; MDH) receives direct inputs from small diameter primary afferent fibers that predominantly transmit nociceptive information in the orofacial region. Recent studies indicate that reactive oxygen species (ROS) is involved in persistent pain, primarily through spinal mechanisms. In this study, we aimed to investigate the role of xanthine/xanthine oxidase (X/XO) system, a known generator of superoxide anion (O₂^·−), on membrane excitability in the rat MDH neurons. For this, we used patch clamp recording and confocal imaging. An application of X/XO (300 µM/30 mU) induced membrane depolarization and inward currents. When slices were pretreated with ROS scavengers, such as phenyl N-tert-butylnitrone (PBN), superoxide dismutase (SOD), and catalase, X/XO-induced responses decreased. Fluorescence intensity in the DCF-DA and DHE-loaded MDH cells increased on the application of X/XO. An anion channel blocker, 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS), significantly decreased X/XO-induced depolarization. X/XO elicited an inward current associated with a linear current-voltage relationship that reversed near −40 mV. X/XO-induced depolarization reduced in the presence of La³⁺, a nonselective cation channel (NSCC) blocker, and by lowering the external sodium concentration, indicating that membrane depolarization and inward current are induced by influx of Na⁺ ions. In conclusion, X/XO-induced ROS modulate the membrane excitability of MDH neurons, which was related to the activation of NSCC.

Mastication induces long-term increases in blood perfusion of the trigeminal principal nucleus

Understanding mechanisms for vessel tone regulation within the trigeminal nuclei is of great interest because some headache syndromes are due to dysregulation of such mechanisms. Previous experiments on animal models suggest that mastication may alter neuron metabolism and blood supply in these nuclei. To investigate this hypothesis in humans, arterial spin-labeling magnetic resonance imaging (MRI) was used to measure blood perfusion within the principal trigeminal nucleus (Vp) and in the dorsolateral-midbrain (DM, including the mesencephalic trigeminal nucleus) in healthy volunteers, before and immediately after a mastication exercise consisting of chewing a gum on one side of the mouth for 1 h at 1 bite/s. The side preference for masticating was evaluated with a chewing test and the volume of the masseter muscle was measured on T1-weighted MRI scans. The results demonstrated that the mastication exercise caused a perfusion increase within the Vp, but not in the DM. This change was correlated to the preference score for the side where the exercise took place. Moreover, the basal Vp perfusion was correlated to the masseter volume. These results indicate that the local vascular tone of the trigeminal nuclei can be constitutively altered by the chewing practice and by strong or sustained chewing.

Peripheral antinociceptive action of mangiferin in mouse models of experimental pain: role of endogenous opioids, K(ATP)-channels and adenosine

This study aimed to assess the possible systemic antinociceptive activity of mangiferin and to clarify the underlying mechanism, using the acute models of chemical (acetic acid, formalin, and capsaicin) and thermal (hot-plate and tail-flick) nociception in mice. Mangiferin at oral doses of 10 to 100 mg/kg evidenced significant antinociception against chemogenic pain in the test models of acetic acid-induced visceral pain and in formalin- and capsaicin-induced neuro-inflammatory pain, in a naloxone-sensitive manner, suggesting the participation of endogenous opiates in its mechanism. In capsaicin test, the antinociceptive effect of mangiferin (30 mg/kg) was not modified by respective competitive and non-competitive transient receptor potential vanilloid 1 (TRPV1) antagonists, capsazepine and ruthenium red, or by pretreatment with L-NAME, a non-selective nitric oxide synthase inhibitor, or by ODQ, an inhibitor of soluble guanylyl cyclase. However, mangiferin effect was significantly reversed by glibenclamide, a blocker of K(ATP) channels and in animals pretreated with 8-phenyltheophylline, an adenosine receptor antagonist. Mangiferin failed to modify the thermal nociception in hot-plate and tail-flick test models, suggesting that its analgesic effect is only peripheral but not central. The orally administered mangiferin (10-100 mg/kg) was well tolerated and did not impair the ambulation or the motor coordination of mice in respective open-field and rota-rod tests, indicating that the observed antinociception was unrelated to sedation or motor abnormality. The findings of this study suggest that mangiferin has a peripheral antinociceptive action through mechanisms that involve endogenous opioids, K(ATP)-channels and adenosine receptors.

Reduced GABAA receptor α6 expression in the trigeminal ganglion enhanced myofascial nociceptive response

Activation of the GABAA receptor results in inhibition of neuronal activity. One subunit of this multi-subunit receptor termed alpha 6 (Gabrα6) contributed to inflammatory temporomandibular joint (TMJ) nociception but TMJ disorders often include myofascial pain. To address Gabrα6 role in myofascial pain we hypothesized that Gabrα6 has an inhibitory role in myofascial nociceptive responses similar to inflammatory TMJ arthritis. To test this hypothesis a, myofascial nociceptive response was induced by placing a ligature bilaterally on the tendon attachment of the anterior superficial part of a male rat's masseter muscle. Four days after ligature placement Gabrα6 expression was reduced by infusing the trigeminal ganglia (TG) with small interfering RNA (siRNA) having homology to either the Gabrα6 gene (Gabrα6 siRNA) or no known gene (control siRNA). After siRNA infusion nociceptive behavioral responses were measured, i.e., feeding behavior and head withdrawal after pressing upon the region above the ligature with von Frey filaments. Neuronal activity in the TG and trigeminal nucleus caudalis and upper cervical region (Vc-C1) was measured by quantitating the amount of phosphorylated extracellular signal-regulated kinase (p-ERK). Total Gabrα6 and GABAA receptor contents in the TG and Vc-C1 were determined. Gabrα6 siRNA infusion reduced Gabrα6 and GABAA receptor expression and significantly increased the nociceptive response in both nociceptive assays. Gabrα6 siRNA infusion also significantly increased TG p-ERK expression of the ligated rats. From these results we conclude GABAA receptors consisting of the Gabrα6 subunit inhibit TG nociceptive sensory afferents in the trigeminal pathway and have an important role in the regulation of myofascial nociception.

Transition to persistent orofacial pain after nerve injury involves supraspinal serotonin mechanisms

The orofacial region is a major focus of chronic neuropathic pain conditions characterized by primary hyperalgesia at the site of injury and secondary hyperalgesia outside the injured zone. We have used a rat model of injury to the maxillary branch (V2) of the trigeminal nerve to produce constant and long-lasting primary hyperalgesia in the V2 territory and secondary hyperalgesia in territories innervated by the mandibular branch (V3). Our findings indicate that the induction of primary and secondary hyperalgesia depended on peripheral input from the injured nerve. In contrast, the maintenance of secondary hyperalgesia depended on central mechanisms. The centralization of the secondary hyperalgesia involved descending 5-HT drive from the rostral ventromedial medulla and the contribution of 5-HT3 receptors in the trigeminal nucleus caudalis (Vc), the homolog of the spinal dorsal horn. Electrophysiological studies further indicate that after nerve injury spontaneous responses and enhanced poststimulus discharges in Vc nociresponsive neurons were time-dependent on descending 5-HT drive and peripheral input. The induction phase of secondary hyperalgesia involved central sensitization mechanisms in Vc neurons that were dependent on peripheral input, whereas the maintenance phase of secondary hyperalgesia involved central sensitization in Vc neurons conducted by a delayed descending 5-HT drive and a persistence of peripheral inputs. Our results are the first to show that the maintenance of secondary hyperalgesia and underlying central sensitization associated with persistent pain depend on a transition to supraspinal mechanisms involving the serotonin system in rostral ventromedial medulla-dorsal horn circuits.

The Effects of Central Angiotensin II and Its Specific Blockers on Nociception. Possible Interactions with Oxidative Stress Status / Efekti Centralnog Angiotenzina II I Njegovih Specifičnih Blokatora Na Nocicepciju. Moguće Interakcije Sa Statusom Oksidativnog Stresa

It has already been demonstrated that a complete brain renin-angiotensin system (RAS) exists distinctly separate from the peripheral system and is implicated in complex functions such as memory, emotional responses and pain. Regarding the implications of angiotensin II (the main bioactive peptide of RAS) in pain, although there are many studies in this area of research, most of the results are controversial. Also, it seems that oxidative stress follows angiotensin II infusion, but the role of AT1 vs. AT2 receptors is not well established. In this context, we were interested in studying the effects of central RAS on nociception, through the intracerebroventricular administration of losartan and PD-123177 (antagonists for the AT1/AT2 receptors), as well as an ACE inhibitor (captopril) and also angiotensin II in rats, which were subsequently tested using the hot-plate task, a well known behavioral test for pain perception. We present here the analgesic effect of angiotensin II administration, as shown by in creased latency-time in the hot-plate, as well as a nociceptive effect of angiotensin II blockers like AT1 and AT2 specific antagonists (losartan and PD-123177) and an ACE inhibitor (captopril), as their administration resulted in decreased latency-time. Moreover, we demonstrated a significant correlation between the results of the nociceptive behavioral task and the levels of some main oxidative stress markers. This provides additional evidence for an analgesic effect of Ang II administration, as well as for a nociceptive effect of Ang II blockers. Moreover, a significant correlation between the nociception and angiotensin II-induced oxidative stress is presented.

D-Amino acid oxidase-mediated increase in spinal hydrogen peroxide is mainly responsible for formalin-induced tonic pain

BACKGROUND AND PURPOSE

Spinal reactive oxygen species (ROS) are critically involved in chronic pain. D-Amino acid oxidase (DAAO) oxidizes D-amino acids such as D-serine to form the byproduct hydrogen peroxide without producing other ROS. DAAO inhibitors are specifically analgesic in tonic pain, neuropathic pain and cancer pain. This study examined the role of spinal hydrogen peroxide in pain and the mechanism of the analgesic effects of DAAO inhibitors.

EXPERIMENTAL APPROACH

Formalin-induced pain behaviours and spinal hydrogen peroxide levels were measured in rodents.

KEY RESULTS

Formalin injected into the paw increased spinal hydrogen peroxide synchronously with enhanced tonic pain; both were effectively prevented by i.t. fluorocitrate, a selective astrocyte metabolic inhibitor. Given systemically, the potent DAAO inhibitor CBIO (5-chloro-benzo[d]isoxazol-3-ol) blocked spinal DAAO enzymatic activity and specifically prevented formalin-induced tonic pain in a dose-dependent manner. Although CBIO maximally inhibited tonic pain by 62%, it completely prevented the increase in spinal hydrogen peroxide. I.t. catalase, an enzyme specific for decomposition of hydrogen peroxide, completely depleted spinal hydrogen peroxide and prevented formalin-induced tonic pain by 65%. Given systemically, the ROS scavenger PBN (phenyl-N-tert-butylnitrone) also inhibited formalin-induced tonic pain and increase in spinal hydrogen peroxide. Formalin-induced tonic pain was potentiated by i.t. exogenous hydrogen peroxide. CBIO did not increase spinal D-serine level, and i.t. D-serine did not alter either formalin-induced tonic pain or CBIO's analgesic effect.

CONCLUSIONS AND IMPLICATIONS

Spinal hydrogen peroxide is specifically and largely responsible for formalin-induced pain, and DAAO inhibitors produce analgesia by blocking spinal hydrogen peroxide production rather than interacting with spinal D-serine.

Assessment of oxidative parameters in rat spinal cord after chronic constriction of the sciatic nerve

Although reactive oxygen species (ROS) are involved in neuropathic pain, the direct relationship between these species and chronic constriction of sciatic nerve (CCI) has not been studied in spinal cord. Thus, this study induced CCI in rats and these animals were sacrificed 3 and 10 days after the surgical procedure to determine the superoxide dismutase (SOD) and catalase activities, as well as ascorbic acid, hydrogen peroxide (H(2)O(2)) and lipid hydroperoxide levels in lumbosacral spinal cord. Von Frey Hair and hot plate tests were performed to assess the degree of mechanical and thermal hyperalgesia at days 0, 3 and 10. The results showed that CCI significantly induced mechanical and thermal hyperalgesia at days 3 and 10. Parallel there was increase in spinal cord lipid hydroperoxide at days 3 and 10 in rats submitted to CCI. In Sham rats a significant increase in this parameter occurred at day 10. H(2)O(2) decreased at day 10 only in CCI group. SOD activity was decreased in Sham and CCI groups at day 3, while catalase activity was increased in CCI rats at days 3 and 10. Ascorbic acid levels were reduced only in CCI rats at day 3. Although the role of such changes is unclear, many were not specific to neuropathic pain and the differences could be related to different degrees of central sensitization in Sham and CCI rats.

Roles of reactive oxygen and nitrogen species in pain

Peroxynitrite (PN; ONOO⁻) and its reactive oxygen precursor superoxide (SO; O₂•⁻) are critically important in the development of pain of several etiologies including pain associated with chronic use of opiates such as morphine (also known as opiate-induced hyperalgesia and antinociceptive tolerance). This is now an emerging field in which considerable progress has been made in terms of understanding the relative contributions of SO, PN, and nitroxidative stress in pain signaling at the molecular and biochemical levels. Aggressive research in this area is poised to provide the pharmacological basis for development of novel nonnarcotic analgesics that are based upon the unique ability to selectively eliminate SO and/or PN. As we have a better understanding of the roles of SO and PN in pathophysiological settings, targeting PN may be a better therapeutic strategy than targeting SO. This is because, unlike PN, which has no currently known beneficial role, SO may play a significant role in learning and memory. Thus, the best approach may be to spare SO while directly targeting its downstream product, PN. Over the past 15 years, our team has spearheaded research concerning the roles of SO and PN in pain and these results are currently leading to the development of solid therapeutic strategies in this important area.

Cortical spreading depression affects reactive oxygen species production

Cortical spreading depression (CSD) is a reversible electrophysiological phenomenon that is not associated with tissue damage under normal blood supply. The induction of CSD during ischemia causes an increase in tissue damage, while pre-conditioning with CSD decreases the damage induced by a subsequent episode of ischemia. The mechanisms underlying these effects are not clear. Because the production of reactive oxygen species (ROS) is involved in tissue damage during ischemia-reperfusion, the aim of the present study was to evaluate the effects of CSD on superoxide production (O(2)(-)), on hydrogen peroxide (H(2)O(2)) production and on superoxide dismutase (SOD) activity in the cerebral cortex. CSD was induced by KCl application on the cortical surface in rats. O(2)(-) production was evaluated using dihydroethidium (DHE) that is oxidized to the fluorescent dye ethidium (HEt) by O(2)(-). The extracellular level of H(2)O(2) was evaluated by microdialysis sampling and HPLC analysis. SOD activity was evaluated with a histochemical assay. The results showed an increase in H(2)O(2) production, an increase in SOD activity and a decrease in O(2)(-) concentration 1h after CSD induction.

Persistent facial pain increases superoxide anion production in the spinal trigeminal nucleus

Previous studies have demonstrated that there is an increase in oxidative stress in the cerebral cortex of rats after repeated painful stimulation and that long-lasting pain increases the production of superoxide ion (O(2) (-)), nitric oxide and peroxynitrite due to the activation of AMPA and NMDA receptors. The purpose of the present study was to evaluate the possible role of O(2) (-) in the transmission of oro-facial pain. Formaldehyde 1% was injected subcutaneously into one vibrissal pad of adult male Sprague-Dawley rats as a model of persistent pain, then O(2) (-) production and superoxide dismutase (SOD) activity were evaluated in the left and right spinal trigeminal nuclei. O(2) (-) production was revealed using dihidroetidium (DHE) injected at 10 or 45 min after the formalin injection in conscious or anaesthetized rats. A histochemical assay for SOD was performed to evaluate the activity of SOD at 10 min after the formalin injection. The results showed a significant increase in O(2) (-) production in the homolateral nucleus at 45 min. However, there was no significant difference between the two sides at 10 min after the formalin injection. No significant difference was observed in SOD activity between the two sides of the spinal trigeminal nucleus. This study demonstrated that there is an increased production of O(2) (-) in the second phase but not in the first phase of the formalin test; thus O(2) (-) is involved in pain induced by inflammation, but not in acute pain.

Mastication overload causes an increase in O2- production into the subnucleus oralis of the spinal trigeminal nucleus

The aim of the present study was to evaluate the production of superoxide anion (O(2)(-)) in the trigeminal complex nuclei after a functional mechanical overload of the teeth due to the preference for masticating on one side in rats. The preference for masticating on one side was induced by the discomfort due to a small abrasion of one molar; such lateralisation in mastication was confirmed by electromyography. The production of O(2)(-) was evaluated in the trigeminal nuclei by fluorescence microscopy after an injection of dihydroethidium. The results showed that there was an increased production of O(2)(-) in the subnucleus oralis of the spinal trigeminal nucleus in the same side where the mastication was preferred. This result demonstrates that an increased activity of non-painful sensory neurons can enhance the production of reactive oxygen species within the central second order sensory nuclei.

N-acetylcysteine inhibits hyperglycemia-induced oxidative stress and apoptosis markers in diabetic neuropathy

Several studies have indicated the involvement of oxidative stress in the development of diabetic neuropathy. In the present study, we have targeted oxidative stress mediated nerve damage in diabetic neuropathy using N-acetyl-l-cysteine (NAC), a potent antioxidant. After 8 weeks, streptozotocin-induced diabetic rats developed neuropathy which was evident from decreased tail-flick latency (thermal hyperalgesia). This was accompanied by decreased motor coordination as assessed by performance on rota-rod treadmill. Na(+) K(+) ATPase, a biochemical marker of development of diabetic neuropathy, was significantly inhibited in sciatic nerve of diabetic animals. NAC treatment at a daily dose between 1.4 and 1.5 g/kg body weight to diabetic animals for 7 weeks in drinking water ameliorated hyperalgesia, improved motor coordination and reversed reduction in Na(+) K(+) ATPase activity. There was an increase in lipid peroxidation in sciatic nerve of diabetic animals along with decrease in phospholipid levels, while NAC treatment attenuated lipid peroxidation and restored phospholipids to control levels. This was associated with decrease in glutathione and protein thiols. The activities of antioxidant enzymes; superoxide dismutase, catalase, glutathione reductase, glutathione peroxidase and glutathione-S-transferase were reduced in sciatic nerve of diabetic animals. Cytochrome c release and active caspase 3 were markedly increased in nerve from diabetic animals suggesting activation of apoptotic pathway. NAC treatment significantly ameliorated decrease in antioxidant defense and prevented cytochrome c release and caspase 3 activation. Electron microscopy revealed demyelination, Wallerian degeneration and onion-bulb formation in sciatic nerve of diabetic rats. NAC on the other hand was able to reverse structural deficits observed in sciatic nerve of diabetic rats. Our results clearly demonstrate protective effect of NAC is mediated through attenuation of oxidative stress and apoptosis, and suggest therapeutic potential of NAC in attenuation of diabetic neuropathy.

Spinal D-amino acid oxidase contributes to neuropathic pain in rats

D-amino acid oxidase (DAO) is an enzyme catalyzing oxidative deamination of neutral and polar d-amino acids and is expressed in the kidneys, liver, and central nervous system (CNS) including the spinal cord. We have previously demonstrated that DAO gene deletion/mutation by using mutant ddY/DAO(-/-) mice and systemic administration of the DAO inhibitor sodium benzoate blocked formalin-induced hyperalgesia in mice. In this study, we further investigated the potential role of DAO in neuropathic pain in a rat model of tight L(5)/L(6) spinal nerve ligation. After L(5)/L(6) spinal nerve ligation, the mRNA expression (measured by real-time quantitative polymerase chain reaction) and enzyme activity (measured by a colorimetric method) of DAO in the lumbar spinal cord were markedly increased, in agreement with the development of neuropathic pain (mechanical allodynia). Intraperitoneal injection of sodium benzoate (400 mg/kg) specifically blocked mechanical allodynia in neuropathic rats and formalin-induced hyperalgesia but did not suppress acute pain responses in the tail-flick test or formalin test. Systemic injection of sodium benzoate also inhibited DAO activity in the lumbar spinal cord of rats. Furthermore, direct intrathecal (spinal cord) injection of benzoate (30 mug/rat) specifically blocked spinal nerve ligation-induced mechanical allodynia in neuropathic rats and formalin-induced hyperalgesia (but not acute pain) in the formalin test. Based on the above results, we conclude that spinal DAO plays a pronociceptive (rather than an antinociceptive) role and might be a target molecule for the treatment of chronic pain of neuropathic origin.

Effects of systemic bicuculline or morphine on formalin-evoked pain-related behaviour and c-Fos expression in trigeminal nuclei after formalin injection into the lip or tongue in rats

This study examined differences in nociceptive responses between lip and tongue. Formalin-induced pain-related behaviour and c-Fos expression in the trigeminal caudal nucleus (Vc) with/without systemic preadministration of a gamma-aminobutyric acid (GABA) type A receptor antagonist, bicuculline (2 mg/kg, i.p., 10 min before formalin injection) or a micro-opioid receptor agonist, morphine (3 mg/kg, i.p., 10 min before formalin injection) have been studied. Formalin injection into the upper lip induced an immediate pain-related behaviour, mostly face-rubbing behaviour, for 15 min (phase 1, mean +/- SEM/5 min, 81.2 +/- 30.1), followed by a more increased activity for 15 min (phase 2, 205.4 +/- 43.6) and a decline to baseline for next 15 min (phase 3, 63.9 +/- 28.0). Formalin injection into the tongue induced similar amount of pain-related behaviour at phase 1 (67.9 +/- 16.7), followed by similar activity at phase 2 (48.6 +/- 6.2), and lesser behaviour at phase 3 (20.4 +/- 7.6). The behaviour at phase 2 decreased following preadministration of bicuculline or morphine when formalin was injected into the lip (b, 62.5 +/- 14.5; m, 95.8 +/- 10.0) but not into the tongue (b, 31.0 +/- 9.2; m, 77.4 +/- 27.0). A considerable numbers of c-Fos-immunoreactive (IR) cells were induced in the caudal and inter-medio-lateral center of superficial layers of the Vc (VcI/II; mean +/- SEM/section = 225.8 +/- 12.9) and magnocellular zone of the Vc (VcIII/IV; 67.1 +/- 4.7) 2 h after formalin injection into the lip. Much smaller numbers of c-Fos-IR cells were induced in the rostral and dorso-medial one-fourth of the VcI/II (72.6 +/- 3.7) and VcIII/IV (55.6 +/- 6.6) after formalin injection into the tongue. Following preadministration with systemic bicuculline or morphine, the formalin-induced c-Fos-IR cells were decreased more in the VcI/II when formalin was injected into the lip (VcI/II, 102.4 +/- 8.0; VcIII/IV, 32.8 +/- 1.4) than into the tongue (VcI/II, 49.5 +/- 8.1; VcIII/IV, 31.7 +/- 5.3). These results show that the lip is more sensitive to formalin-induced noxious stimulation and regulated more through GABA(A) and micro-opioid receptors than the tongue.

Peroxynitrite: a strategic linchpin of opioid analgesic tolerance

Severe pain syndromes reduce quality of life in patients with inflammatory and neoplastic diseases, partly because the reduced analgesic effectiveness accompanying chronic opiate therapy (i.e. tolerance) leads to escalating doses and distressing side effects. Accordingly, there is major interest in new approaches to maintain opiate efficacy during repetitive dosing without engendering tolerance or causing unacceptable side effects. Recent mounting evidence implicates nitroxidative stress caused by the presence of superoxide (O(2*)(-)), nitric oxide (*NO) and subsequently peroxynitrite (ONOO(-)) in opiate analgesic tolerance. Here, we provide a pharmacological basis for developing inhibitors of ONOO(-) biosynthesis and/or ONOO(-) scavengers as potent adjuncts to opiates in the management of chronic pain, addressing an issue of major clinical and socio-economic importance while laying the basis for interventions with strong therapeutic potential.

Lipophilicity is a critical parameter that dominates the efficacy of metalloporphyrins in blocking the development of morphine antinociceptive tolerance through peroxynitrite-mediated pathways

Severe pain syndromes reduce the quality of life of patients with inflammatory and neoplastic diseases, partly because reduced analgesic effectiveness with chronic opiate therapy (i.e., tolerance) leads to escalating doses and distressing side effects. Peroxynitrite-mediated nitroxidative stress in the dorsal horn of the spinal cord plays a critical role in the induction and development of antinociceptive tolerance to morphine. This provides a valid pharmacological basis for developing peroxynitrite scavengers as potent adjuncts to opiates in the management of pain. The cationic Mn(III) ortho-N-alkylpyridylporphyrins MnTE-2-PyP(5+) and MnTnHex-2-PyP(5+) are among the most potent peroxynitrite scavengers, with nearly identical scavenging rate constants (approximately 10(7) M(-1) s(-1)). Yet, MnTnHex-2-PyP(5+) is significantly more lipophilic and more bioavailable and, in turn, was 30-fold more effective in blocking the development of morphine antinociceptive tolerance than MnTE-2-PyP(5+) using the hot-plate test in a well-characterized murine model. The hydrophilic MnTE-2-PyP(5+) and the lipophilic MnTnHex-2-PyP(5+) were 10- and 300-fold, respectively, more effective in inhibiting morphine tolerance than the hydrophilic Fe(III) porphyrin FeTM-4-PyP(5+). Both Mn porphyrins decreased levels of TNF-alpha, IL-1 beta, and IL-6 to normal values. Neither of them affected acute morphine antinociceptive effects nor caused motor function impairment. Also neither was able to reverse already established morphine tolerance. We have recently shown that the anionic porphyrin Mn(III) tetrakis(4-carboxylatophenyl)porphyrin is selective in removing ONOO(-) over O(2)(-), but at approximately 2 orders of magnitude lower efficacy than MnTE-2-PyP(5+) and MnTnHex-2-PyP(5+), which in turn parallels up to 100-fold lower ability to reverse morphine tolerance. These data (1) support the role of peroxynitrite rather than superoxide as a major mechanism in blocking the development of morphine tolerance and (2) show that lipophilicity is a critical parameter in enhancing the potency of such novel peroxynitrite scavengers.

Increase in reactive oxygen species and activation of Akt signaling pathway in neuropathic pain

Neuropathic pain occurs as a result of peripheral or central nervous system injury. Its pathophysiology involves mainly a central sensitization mechanism that may be correlated to many molecules acting in regions involved in pain processing, such as the spinal cord. It has been demonstrated that reactive oxygen species (ROS) and signaling molecules, such as the serine/threonine protein kinase Akt, are involved in neuropathic pain mechanisms. Thus, the aim of this study was to provide evidence of this relationship. Sciatic nerve transection (SNT) was used to induce neuropathic pain in rats. Western blot analysis of Akt and 4-hydroxy-2-nonenal (HNE)-Michael adducts, and measurement of hydrogen peroxide (H(2)O(2)) in the lumbosacral spinal cord were performed. The main findings were found seven days after SNT, when there was an increase in HNE-Michael adducts formation, total and p-Akt expression, and H(2)O(2) concentration. However, one and 15 days after SNT, H(2)O(2) concentration was raised in both sham (animals that were submitted to surgery without nerve injury) and SNT groups, showing the high sensibility of this ROS to nociceptive afferent stimuli, not only to neuropathic pain. p-Akt also increased in sham and SNT groups one day post injury, but at 3 and 7 days the increase occurred exclusively in SNT animals. Thus, there is crosstalk between intracellular signaling pathways and ROS, and these molecules can act as protective agents in acute pain situations or play a role in the development of chronic pain states.

Oxidative stress in the spinal cord is an important contributor in capsaicin-induced mechanical secondary hyperalgesia in mice

Recent studies indicate that reactive oxygen species (ROS) are critically involved in persistent pain primarily through spinal mechanisms, thus suggesting ROS involvement in central sensitization. To investigate ROS involvement in central sensitization, the effects of ROS scavengers and donors on pain behaviors were examined in mice. Capsaicin- induced hyperalgesia was used as a pain model since it has 2 distinctive pain components, primary and secondary hyperalgesia representing peripheral and central sensitization, respectively. Capsaicin (25 microg/5 microl) was injected intradermally into the left hind foot. Foot withdrawal frequencies in response to von Frey filament stimuli were measured and used as an indicator of mechanical hyperalgesia. The production of ROS was examined by using a ROS sensitive dye, MitoSox. Mice developed primary and secondary mechanical hyperalgesia after capsaicin injection. A systemic or intrathecal post-treatment with either phenyl-N-tert-butylnitrone (PBN) or 4-hydroxy-2,2,6,6-tetramethylpiperidine-1 oxyl (TEMPOL), ROS scavengers, significantly reduced secondary hyperalgesia, but not primary hyperalgesia, in a dose-dependent manner. Pretreatment with ROS scavengers also significantly reduced the magnitude and duration of capsaicin-induced secondary hyperalgesia. On the other hand, intrathecal injection of tert-butylhydroperoxide (t-BOOH, 5 microl), a ROS donor, produced a transient hyperalgesia in a dose-dependent manner. The number of MitoSox positive dorsal horn neurons was increased significantly after capsaicin treatment. This study suggests that ROS mediates the development and maintenance of capsaicin-induced hyperalgesia in mice, mainly through central sensitization and that the elevation of spinal ROS is most likely due to increased production of mitochondrial superoxides in the dorsal horn neurons.

The role of reactive oxygen species in capsaicin-induced mechanical hyperalgesia and in the activities of dorsal horn neurons

Previous findings that reactive oxygen species (ROS) are involved in neuropathic pain, mainly through spinal mechanisms, suggest that ROS may be involved in central sensitization. To investigate the possible role of ROS in central sensitization, we examined in rats the effects of ROS scavengers on capsaicin-induced secondary hyperalgesia, which is known to be mediated by central sensitization. We used two different ROS scavengers: phenyl N-tert-butylnitrone (PBN) and 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPOL). Intradermal capsaicin injection (20 microg in 20 microl olive oil) into the hind paw produced primary and secondary hyperalgesia. A systemic administration of PBN (100mg/kg, i.p.) or TEMPOL (200mg/kg, i.p.) alleviated capsaicin-induced secondary, but not primary, hyperalgesia. Intrathecal injection of PBN (1mg inof veterinary Surgery/anesthesiology, College of veterinary Medic 50 microl saline) greatly reduced hyperalgesia, whereas intracerebroventricular or intradermal injection of PBN produced only a minor analgesic effect, suggesting that PBN takes effect mainly through the spinal cord. Electrophysiological recordings from wide dynamic range (WDR) neurons in the dorsal horn showed that intradermal capsaicin enhanced the evoked responses to peripheral stimuli; systemic PBN or TEMPOL restored the responses to normal levels. Removal of ROS thus restored the responsiveness of spinal WDR neurons to normal levels, suggesting that ROS is involved in central sensitization, at least in part by sensitizing WDR neurons.

Hydrogen peroxide increases GABAergic mIPSC through presynaptic release of calcium from IP3 receptor-sensitive stores in spinal cord substantia gelatinosa neurons

GABAergic interneurons of the spinal cord substantia gelatinosa regulate the transmission of nociceptive information. Hydrogen peroxide (H2O2) is likely a diffusible messenger contributing to the development of long-lasting pathological pain states after nerve injury. In this study, we examined the presynaptic effects of H2O2 on the inhibitory interneurons of mouse substantia gelatinosa (SG) using whole-cell patch-clamp recordings from spinal cord slices. H2O2 increased the frequency of GABAergic miniature inhibitory postsynaptic current (mIPSC) in a concentration-dependent (10-1000 microM) manner. The profound increase in mIPSC frequency was diminished by thapsigargin or cyclopiazonic acid suggesting that the intracellular stored pool was the source of presynaptic calcium. Further examination revealed the 2-aminoethoxydiphenil borate blockable inositol-(1,4,5) trisphosphate receptor (IP3R) regulated pool of stored calcium as the likely source. The phospholipase C (PLC) blocker, 1-(6-[([17beta]-3-methoxyestra-1,3,5[10]-trien-17-yl)-amino]hexyl)-1H-pyrrole-2,5-dione (U73122), did not block the frequency increase, which suggested that the site of action of H2O2 lies downstream in the IP3 signalling pathway, and nifedipine-sensitivity of the frequency increase indicated a possible role of calcium-induced calcium-release. However, a direct examination of L-type voltage-gated calcium channels (VGCC) demonstrated that H2O2 did not increase the calcium influx through these channels. The H2O2 effect on mIPSC frequency was markedly reduced in the opisthotonus (Opt) mutant mice with a known deletion in the IP3R1 gene. We demonstrated that H2O2 increased presynaptic activity in the GABAergic interneurons by the release of calcium from the IP3R-regulated intracellular pool. The presynaptic IP3R could emerge as a novel target for preventing H2O2-induced synaptic plasticity in substantia gelatinosa leading to pathological pain states.

Superoxide, peroxynitrite and oxidative/nitrative stress in inflammation

A considerable body of evidence suggests that formation of potent reactive oxygen species and resulting oxidative/nitrative stress play a major role in acute and chronic inflammation and pain. Much of the knowledge in this field has been gathered by the use of pharmacological and genetic approaches. In this mini review, we will evaluate recent advances made towards understanding the roles of reactive oxygen species in inflammation, focusing in particular on superoxide and peroxynitrite. Given the limited space to cover this broad topic, here we will refer the reader to comprehensive review articles whenever possible.

Neurobiology of pain

The neurobiology of pain had a notable interest in research focused on the study of neuronal plasticity development, nociceptors, molecular identity, signaling mechanism, ionic channels involved in the generation, modulation and propagation of action potential in all type of excitable cells. All the findings open the possibility for developing new therapeutic treatment. Nociceptive/inflammatory pain and neuropathic pain represent two different kinds of persistent chronic pain. We have reviewed the different mechanism suggested for the maintenance of pain, like descending nociceptive mechanism and their changes after tissue damage, including suppression and facilitation of defence behavior during pain. The role of these changes in inducing NMDA and AMPA receptors gene expression, after prolonged inflammation is emphasized by several authors. Furthermore, a relation between a persistent pain and amygdale has been shown. Molecular biology is the new frontier in the study of neurobiology of pain. Since the entire genome has been studied, we will able to find new genes involved in specific condition such as pain, because an altered gene expression can regulate neuronal activity after inflammation or tissue damage.

Neuropathic pain modifies antioxidant activity in rat spinal cord

Oxidative stress is an important pathophysiological mechanism of many neurological diseases. Reactive oxygen and nitrogen species have been cited as molecules involved in the nociceptive process. In this study, rats were submitted to sciatic nerve transection (SNT) for induction of neuropathic pain, and enzyme activities of SOD and catalase as well as lipid peroxidation (LPO) were measured in the lumbosacral spinal cord. The results show that LPO was not changed after SNT. SOD activity was reduced 7 days after SNT, while the change in catalase activity occurred on the third and seventh days in both sham and SNT animals. Hyperalgesia in SNT group was detected at the same points in time. These results suggest that SNT was not a strong enough stimulus to deplete all antioxidant content in the spinal cord, since increase in LPO was not detected. However, the role of oxidative stress in nociception can not be excluded.