Involvement of reactive oxygen species in long-term potentiation in the spinal cord dorsal horn

Metabolic regulation of single synaptic vesicle exo- and endocytosis in hippocampal synapses

Glucose has long been considered a primary energy source for synaptic function. However, it remains unclear to what extent alternative fuels, such as lactate/pyruvate, contribute to powering synaptic transmission. By detecting individual release events in hippocampal synapses, we find that mitochondrial ATP production regulates basal vesicle release probability and release location within the active zone (AZ), evoked by single action potentials. Mitochondrial inhibition shifts vesicle release closer to the AZ center and alters the efficiency of vesicle retrieval by increasing the occurrence of ultrafast endocytosis. Furthermore, we uncover that terminals can use oxidative fuels to maintain the vesicle cycle during trains of activity. Mitochondria are sparsely distributed along hippocampal axons, and we find that terminals containing mitochondria display enhanced vesicle release and reuptake during high-frequency trains. Our findings suggest that mitochondria not only regulate several fundamental features of synaptic transmission but may also contribute to modulation of short-term synaptic plasticity.

Metabolic Regulation of Single Synaptic Vesicle Exo- and Endocytosis in Hippocampal Synapses

Glucose has long been considered a primary source of energy for synaptic function. However, it remains unclear under what conditions alternative fuels, such as lactate/pyruvate, contribute to powering synaptic transmission. By detecting individual release events in cultured hippocampal synapses, we found that mitochondrial ATP production from oxidation of lactate/pyruvate regulates basal vesicle release probability and release location within the active zone (AZ) evoked by single action potentials (APs). Mitochondrial inhibition shifted vesicle release closer to the AZ center, suggesting that the energetic barrier for vesicle release is lower in the AZ center that the periphery. Mitochondrial inhibition also altered the efficiency of single AP evoked vesicle retrieval by increasing occurrence of ultrafast endocytosis, while inhibition of glycolysis had no effect. Mitochondria are sparsely distributed along hippocampal axons and we found that nerve terminals containing mitochondria displayed enhanced vesicle release and reuptake during high-frequency trains, irrespective of whether neurons were supplied with glucose or lactate. Thus, synaptic terminals can entirely bypass glycolysis to robustly maintain the vesicle cycle using oxidative fuels in the absence of glucose. These observations further suggest that mitochondrial metabolic function not only regulates several fundamental features of synaptic transmission but may also contribute to modulation of short-term synaptic plasticity.

Influence of Oxidative Stress and Inflammation on Nutritional Status and Neural Plasticity: New Perspectives on Post-Stroke Neurorehabilitative Outcome

Beyond brain deficits caused by strokes, the effectiveness of neurorehabilitation is strongly influenced by the baseline clinical features of stroke patients, including a patient's current nutritional status. Malnutrition, either as a pre-stroke existing condition or occurring because of ischemic injury, predisposes patients to poor rehabilitation outcomes. On the other hand, a proper nutritional status compliant with the specific needs required by the process of brain recovery plays a key role in post-stroke rehabilitative outcome favoring neuroplasticity mechanisms. Oxidative stress and inflammation play a role in stroke-associated malnutrition, as well as in the cascade of ischemic events in the brain area, where ischemic damage leads to neuronal death and brain infarction, and, via cell-to-cell signaling, the alteration of neuroplasticity processes underlying functional recovery induced by multidisciplinary rehabilitative treatment. Nutrition strategies based on food components with oxidative and anti-inflammatory properties may help to reverse or stop malnutrition and may be a prerequisite for supporting the ability of neuronal plasticity to result in satisfactory rehabilitative outcome in stroke patients. To expand nutritional recommendations for functional rehabilitation recovery, studies considering the evolution of nutritional status changes in post-stroke patients over time are required. The assessment of nutritional status must be included as a routine tool in rehabilitation settings for the integrated care of stroke-patients.

Erythropoietin administration exerted neuroprotective effects against cardiac ischemia/reperfusion injury

Acute myocardial infarction (AMI) leads to cardiac dysfunction and also causes brain dysfunction and pathology. The neuroprotective effects of erythropoietin (EPO), the hormone controlling the production of red blood cells, have been shown in case of cerebral ischemic/reperfusion (I/R) injury. However, the effects of EPO on the brain pathologies induced by cardiac I/R injury have not been investigated. We hypothesized that the administration of EPO attenuates brain damage caused by cardiac I/R injury through decreasing peripheral and brain oxidative stress, preserving microglial morphology, attenuating hippocampal necroptosis, and decreasing hippocampal apoptosis, and hippocampal dysplasticity. Male Wistar rats (n ​= ​38) were divided into two groups, sham (n ​= ​6) and cardiac I/R (n ​= ​32). All rats being subjected to the cardiac I/R operation were randomly divided into 4 subgroups (n ​= ​8/group): vehicle, EPO pretreatment, EPO given during ischemia, and EPO given at the onset of reperfusion. The EPO was given at a dosage of 5000 units/kg via intravenous injection. Left ventricle function, oxidative stress, brain mitochondrial function, microglial morphology, hippocampal necroptosis, hippocampal apoptosis, and hippocampal plasticity were measured. EPO administration exerted beneficial anti-oxidative, anti-inflammatory, and anti-apoptotic effects on the brain against cardiac I/R. Giving EPO before cardiac ischemia conferred the greatest neuroprotection against cardiac I/R injury through the attenuation of LV dysfunction, decrease in peripheral and brain oxidative stress, and the attenuation of microglial activation, brain mitochondrial dysfunction, apoptosis, and necroptosis, leading to the improvement of hippocampal dysplasticity under cardiac I/R conditions. EPO pretreatment provided the greatest benefits on brain pathology induced by cardiac I/R.

TRPM3-mediated dynamic mitochondrial activity in NGF-induced latent sensitization of chronic low back pain

ABSTRACT

The transient receptor potential ion channel TRPM3 is highly prevalent on nociceptive dorsal root ganglion (DRG) neurons, but its functions in neuronal plasticity of chronic pain remain obscure. In an animal model of nonspecific low back pain (LBP), latent spinal sensitization known as nociceptive priming is induced by nerve growth factor (NGF) injection. Here we address the TRPM3-associated molecular basis of NGF-induced latent spinal sensitization at presynaptic level by studying TRPM3-mediated calcium transients in DRG neurons. By investigating TRPM3-expressing HEK cells, we further show the dynamic mitochondrial activity downstream of TRPM3 activation. NGF enhances TRPM3 function, attenuates TRPM3 tachyphylaxis, and slows intracellular calcium clearance; TRPM3 activation triggers more mitochondrial calcium loading than depolarization does, causing a steady-state mitochondrial calcium elevation and a delayed recovery of cytosolic calcium; mitochondrial calcium buffering accounts for approximately 40% of calcium influx subsequent to TRPM3 activation. TRPM3 activation provokes an outbreak of pulsatile superoxide production (mitoflash) that comes in the form of a surge in frequency being tunable. We suggest that mitoflash pulsations downstream of TRPM3 activation might be an early signaling event initiating pain sensitization. Tuning of mitoflash activity would be a novel bottom-up therapeutic strategy for chronic pain conditions such as LBP and beyond.

Nicotinamide Adenine Dinucleotide Phosphate Oxidases Are Everywhere in Brain Disease, but Not in Huntington's Disease?

Huntington’s disease (HD) is an inherited neurodegenerative disorder characterized by neuronal loss and tissue atrophy mainly in the striatum and cortex. In the early stages of the disease, impairment of neuronal function, synaptic dysfunction and white matter loss precedes neuronal death itself. Relative to other neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease and Amyotrophic Lateral Sclerosis, where the effects of either microglia or NADPH oxidases (NOXs) are recognized as important contributors to disease pathogenesis and progression, there is a pronounced lack of information in HD. This information void contrasts with evidence from human HD patients where blood monocytes and microglia are activated well before HD clinical symptoms (PET scans), and the clear signs of oxidative stress and inflammation in post mortem HD brain. Habitually, NOX activity and oxidative stress in the central nervous system (CNS) are equated with microglia, but research of the last two decades has carved out important roles for NOX enzyme function in neurons. Here, we will convey recent information about the function of NOX enzymes in neurons, and contemplate on putative roles of neuronal NOX in HD. We will focus on NOX-produced reactive oxygen species (ROS) as redox signaling molecules in/among neurons, and the specific roles of NOXs in important processes such as neurogenesis and lineage specification, neurite outgrowth and growth cone dynamics, and synaptic plasticity where NMDAR-dependent signaling, and long-term depression/potentiation are redox-regulated phenomena. HD animal models and induced pluripotent stem cell (iPSC) studies have made it clear that the very same physiological processes are also affected in HD, and we will speculate on possible roles for NOX in the pathogenesis and development of disease. Finally, we also take into account the limited information on microglia in HD and relate this to any contribution of NOX enzymes.

Analgesic dorsal root ganglionic field stimulation blocks conduction of afferent impulse trains selectively in nociceptive sensory afferents

Increased excitability of primary sensory neurons after peripheral nerve injury may cause hyperalgesia and allodynia. Dorsal root ganglion field stimulation (GFS) is effective in relieving clinical pain associated with nerve injury and neuropathic pain in animal models. However, its mechanism has not been determined. We examined effects of GFS on transmission of action potentials (APs) from the peripheral to central processes by in vivo single-unit recording from lumbar dorsal roots in sham injured rats and rats with tibial nerve injury (TNI) in fiber types defined by conduction velocity. Transmission of APs directly generated by GFS (20 Hz) in C-type units progressively abated over 20 seconds, whereas GFS-induced Aβ activity persisted unabated, while Aδ showed an intermediate pattern. Activity generated peripherally by electrical stimulation of the sciatic nerve and punctate mechanical stimulation of the receptive field (glabrous skin) was likewise fully blocked by GFS within 20 seconds in C-type units, whereas Aβ units were minimally affected and a subpopulation of Aδ units was blocked. After TNI, the threshold to induce AP firing by punctate mechanical stimulation (von Frey) was reduced, which was reversed to normal during GFS. These results also suggest that C-type fibers, not Aβ, mainly contribute to mechanical and thermal hypersensitivity (von Frey, brush, acetone) after injury. Ganglion field stimulation produces use-dependent blocking of afferent AP trains, consistent with enhanced filtering of APs at the sensory neuron T-junction, particularly in nociceptive units.

The role of PPARγ in chemotherapy-evoked pain

Although millions of people are diagnosed with cancer each year, survival has never been greater thanks to early diagnosis and treatments. Powerful chemotherapeutic agents are highly toxic to cancer cells, but because they typically do not target cancer cells selectively, they are often toxic to other cells and produce a variety of side effects. In particular, many common chemotherapies damage the peripheral nervous system and produce neuropathy that includes a progressive degeneration of peripheral nerve fibers. Chemotherapy-induced peripheral neuropathy (CIPN) can affect all nerve fibers, but sensory neuropathies are the most common, initially affecting the distal extremities. Symptoms include impaired tactile sensitivity, tingling, numbness, paraesthesia, dysesthesia, and pain. Since neuropathic pain is difficult to manage, and because degenerated nerve fibers may not grow back and regain normal function, considerable research has focused on understanding how chemotherapy causes painful CIPN so it can be prevented. Due to the fact that both therapeutic and side effects of chemotherapy are primarily associated with the accumulation of reactive oxygen species (ROS) and oxidative stress, this review focuses on the activation of endogenous antioxidant pathways, especially PPARγ, in order to prevent the development of CIPN and associated pain. The use of synthetic and natural PPARγ agonists to prevent CIPN is discussed.

Mitochondrial hydrogen peroxide positively regulates neuropeptide secretion during diet-induced activation of the oxidative stress response

Mitochondria play a pivotal role in the generation of signals coupling metabolism with neurotransmitter release, but a role for mitochondrial-produced ROS in regulating neurosecretion has not been described. Here we show that endogenously produced hydrogen peroxide originating from axonal mitochondria (mtH2O2) functions as a signaling cue to selectively regulate the secretion of a FMRFamide-related neuropeptide (FLP-1) from a pair of interneurons (AIY) in C. elegans. We show that pharmacological or genetic manipulations that increase mtH2O2 levels lead to increased FLP-1 secretion that is dependent upon ROS dismutation, mitochondrial calcium influx, and cysteine sulfenylation of the calcium-independent PKC family member PKC-1. mtH2O2-induced FLP-1 secretion activates the oxidative stress response transcription factor SKN-1/Nrf2 in distal tissues and protects animals from ROS-mediated toxicity. mtH2O2 levels in AIY neurons, FLP-1 secretion and SKN-1 activity are rapidly and reversibly regulated by exposing animals to different bacterial food sources. These results reveal a previously unreported role for mtH2O2 in linking diet-induced changes in mitochondrial homeostasis with neuropeptide secretion.

Redox Imbalance Associates with Clinical Worsening in Spinocerebellar Ataxia Type 2

BACKGROUND

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disease presenting with redox imbalance. However, the nature and implications of redox imbalance in SCA2 physiopathology have not been fully understood.

OBJECTIVE

The objective of this study is to assess the redox imbalance and its association with disease severity in SCA2 mutation carriers.

METHODS

A case-control study was conducted involving molecularly confirmed SCA2 patients, presymptomatic individuals, and healthy controls. Several antioxidant parameters were assessed, including serum thiol concentration and the superoxide dismutase, catalase, and glutathione S-transferase enzymatic activities. Also, several prooxidant parameters were evaluated, including thiobarbituric acid-reactive species and protein carbonyl concentrations. Damage, protective, and OXY scores were computed. Clinical correlates were established.

RESULTS

Significant differences were found between comparison groups for redox markers, including protein carbonyl concentration (F = 3.30; p = 0.041), glutathione S-transferase activity (F = 4.88; p = 0.009), and damage (F = 3.20; p = 0.045), protection (F = 12.75; p < 0.001), and OXY (F = 7.29; p = 0.001) scores. Protein carbonyl concentration was positively correlated with CAG repeat length (r = 0.27; p = 0.022), while both protein carbonyl concentration (r = -0.27; p = 0.018) and OXY score (r = -0.25; p = 0.013) were inversely correlated to the disease duration. Increasing levels of antioxidants and decreasing levels of prooxidant parameters were associated with clinical worsening.

CONCLUSIONS

There is a disruption of redox balance in SCA2 mutation carriers which depends on the disease stage. Besides, redox changes associate with markers of disease severity, suggesting a link between disruption of redox balance and SCA2 physiopathology.

A combination of an antioxidant with a prebiotic exerts greater efficacy than either as a monotherapy on cognitive improvement in castrated-obese male rats

Previous studies by ourselves and others have demonstrated that both obesity and testosterone deprivation have been related to cognitive decline. We have also shown that a prebiotic and n-acetyl cysteine (NAC) improved cognitive dysfunction in obese rats and castrated-male rats. However, the effects of NAC, a prebiotic (inulin), and a combination of the two on cognition in castrated-obese rats has never been investigated. The hypothesis was that NAC and inulin attenuated cognitive decline in castrated-obese rats by improving gut dysbiosis, and decreasing oxidative stress, glial activation and apoptosis. Male Wistar rats (n = 36) were fed with either a normal diet (ND: n = 6) or a high-fat diet (HFD: n = 30) for twenty-eight weeks. The resultant obese rats had a bilateral orchiectomy (ORX) and were randomly divided into five subgroups (n = 6/ subgroup). Each subgroup was treated with one of five therapies: a vehicle; testosterone replacement (2 mg/kg/day); NAC (100 mg/kg); inulin (10%, w/w), or a combination of the NAC and inulin for four weeks. The results demonstrated that castrated-obese rats developed gut dysbiosis, metabolic disturbance, brain pathologies, and cognitive decline. All of the pathological conditions in the brain were ameliorated to an equal extent by testosterone replacement, NAC, and inulin supplementation. Interestingly, a combination of NAC and inulin had the greatest beneficial effect on cognitive function by synergistically reducing hippocampal inflammation and ameliorating glial dysmorphology. These findings suggest that a combination of NAC and inulin may confer the greatest benefits in improving cognitive function in castrated-obese male rats.

Neuroinflammation, oxidative stress and their interplay in neuropathic pain: Focus on specialized pro-resolving mediators and NADPH oxidase inhibitors as potential therapeutic strategies

Neuropathic pain (NP) is a chronic condition that results from a lesion or disease of the nervous system, greatly impacting patients' quality of life. Current pharmacotherapy options deliver inadequate and/or insufficient responses and thus a significant unmet clinical need remains for alternative treatments in NP. Neuroinflammation, oxidative stress and their reciprocal relationship are critically involved in NP pathophysiology. In this context, new pharmacological approaches, aiming at enhancing the resolution phase of inflammation and/or restoring redox balance by targeting specific reactive oxygen species (ROS) sources, are emerging as potential therapeutic strategies for NP, with improved efficacy and safety profiles. Several reports have demonstrated that administration of exogenous specialized pro-resolving mediators (SPMs) ameliorates NP pathophysiology. Likewise, deletion or inhibition of the ROS-generating enzyme NADPH oxidase (NOX), particularly its isoforms 2 and 4, results in beneficial effects in NP models. Notably, SPMs also modulate oxidative stress and NOX also regulates neuroinflammation. By targeting neuroinflammatory and oxidative pathways, both SPMs analogues and isoform-specific NOX inhibitors are promising therapeutic strategies for NP.

Genetic and molecular basis of epilepsy-related cognitive dysfunction

Epilepsy is a common neurological disease characterized by recurrent seizures. About 70 million people were affected by epilepsy or epileptic seizures. Epilepsy is a complicated complex or symptomatic syndromes induced by structural, functional, and genetic causes. Meanwhile, several comorbidities are accompanied by epileptic seizures. Cognitive dysfunction is a long-standing complication associated with epileptic seizures, which severely impairs quality of life. Although the definitive pathogenic mechanisms underlying epilepsy-related cognitive dysfunction remain unclear, accumulating evidence indicates that multiple risk factors are probably involved in the development and progression of cognitive dysfunction in patients with epilepsy. These factors include the underlying etiology, recurrent seizures or status epilepticus, structural damage that induced secondary epilepsy, genetic variants, and molecular alterations. In this review, we summarize several theories that may explain the genetic and molecular basis of epilepsy-related cognitive dysfunction.

Gut dysbiosis develops before metabolic disturbance and cognitive decline in high-fat diet-induced obese condition

OBJECTIVES

High-fat diet (HFD) consumption caused metabolic disturbance, gut dysbiosis, brain pathology, microglia hyperactivity, and cognitive decline. However, the exact timeline of these abnormalities following HFD consumption is still elusive. Therefore, the aim of this study was to test the hypothesis that gut dysbiosis, peripheral inflammation, and peripheral insulin resistance occur before the brain inflammatory response, hippocampal synaptic dysplasticity, oxidative stress, apoptosis, and cognitive impairment in HFD-fed rats.

METHODS

Male Wistar rats received either a normal diet or an HFD for 2, 8, 12, 20, or 40 wk. At the end of each time point, cognitive functions and metabolic parameters were determined. Gut microbiota, brain immune cell activity, amyloid-β level, microglia morphology, hippocampal reactive oxygen species and apoptosis, hippocampal synaptic plasticity, and dendritic spine density were measured.

RESULTS

We found that HFD-fed rats developed gut dysbiosis at week 2 and peripheral insulin resistance at week 8. Rats fed an HFD for 12 wk displayed hippocampal synaptic dysplasticity, decreased dendritic spine density, an elevation of ionized calcium-binding adapter molecule 1⁺ cells, increased hippocampal reactive oxygen species levels and hippocampal apoptosis with cognitive decline. The decreased percentage of resident microglia and increased percentage of infiltrated macrophage were observed at weeks 20 and 40. Surprisingly, brain amyloid-β levels were increased after 40 wk of an HFD diet.

CONCLUSIONS

These findings demonstrated that gut dysbiosis develops in the earliest phase of consumption of an HFD, followed by brain pathology, which leads to cognitive decline in obese insulin-resistant rats. Therefore, an improvement in gut dysbiosis should provide beneficial effects in the prevention of neuropathology and cognitive decline in the obese.

Peripheral and central oxidative stress in chemotherapy-induced neuropathic pain

Chemotherapy-induced peripheral neuropathy (CIPN) is an adverse side effect of many anti-cancer chemotherapeutic treatments. CIPN often causes neuropathic pain in extremities, and oxidative stress has been shown to be a major contributing factor to this pain. In this study, we determined the site of oxidative stress associated with pain (specifically, mechanical hypersensitivity) in cisplatin- and paclitaxel-treated mouse models of CIPN and investigated the neurophysiological mechanisms accounting for the pain. C57BL/6N mice that received either cisplatin or paclitaxel (2 mg/kg, once daily on four alternate days) developed mechanical hypersensitivity to von Frey filament stimulations of their hindpaws. Cisplatin-induced mechanical hypersensitivity was inhibited by silencing of Transient Receptor Potential channels V1 (TRPV1)- or TRPA1-expressing afferents, whereas paclitaxel-induced mechanical hypersensitivity was attenuated by silencing of Aβ fibers. Although systemic delivery of phenyl N-tert-butylnitrone, a reactive oxygen species scavenger, alleviated mechanical hypersensitivity in both cisplatin- and paclitaxel-treated mice, intraplantar phenyl N-tert-butylnitrone was effective only in cisplatin-treated mice, and intrathecal phenyl N-tert-butylnitrone, only in paclitaxel-treated mice. In a reactive oxygen species-dependent manner, the mechanosensitivity of Aδ/C fiber endings in the hindpaw skin was increased in cisplatin-treated mice, and the excitatory synaptic strength in the spinal dorsal horn was potentiated in paclitaxel-treated mice. Collectively, these results suggest that cisplatin-induced mechanical hypersensitivity is attributed to peripheral oxidative stress sensitizing mechanical nociceptors, whereas paclitaxel-induced mechanical hypersensitivity is due to central (spinal) oxidative stress maintaining central sensitization that abnormally produces pain in response to Aβ fiber inputs.

Reactive Oxygen Species in the Regulation of the GABA Mediated Inhibitory Neurotransmission

Reactive oxygen species (ROS) are best known for being involved in cellular metabolism and oxidative stress, but also play important roles in cell communication. ROS signaling has become increasingly recognized as a mechanism implicated in the regulation of synaptic neurotransmission, under both physiological and pathological conditions. Hydrogen peroxide (H₂O₂) and superoxide anion are the main biologically relevant endogenous ROS in the nervous system. They are predominantly produced in the mitochondria of neurons and glial cells and their levels are tightly regulated by the antioxidant cell machinery, which allows for dynamic signaling through these agents. Physicochemical and biological properties of H₂O₂ enable it to effectively play an important role in signaling. This review brings up some or the most significant evidence supporting ROS as signaling agents in the nervous system and summarizes data showing that ROS modulate γ-aminobutyric acid (GABA)-mediated neurotransmission by pre- and postsynaptic mechanisms. ROS induce changes on both, the activity of phasic and tonic GABA_A receptors and GABA release from presynaptic terminals. Based on these facts, ROS signaling is discussed as a possible selective mechanism linking cellular metabolism to inhibitory neurotransmission through the direct or indirect modulation of the GABA_A receptor function. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Testosterone deprivation intensifies cognitive decline in obese male rats via glial hyperactivity, increased oxidative stress, and apoptosis in both hippocampus and cortex

AIM

The study hypothesized that testosterone deprivation aggravates cognitive decline in obesity through increasing oxidative stress, glial activation, and apoptosis.

METHODS

Male Wistar rats (n = 24) were fed with either normal-diet (ND) or high-fat diet (HFD) for 24 weeks. At week 13, ND-fed rats and HFD-fed rats were randomly assigned to two subgroups to receive either a sham-operation or bilateral-orchiectomy (ORX). Rats were evaluated for metabolic parameters and cognition at 4, 8, and 12 weeks after the operation. At the end of protocol, the reactive oxygen species (ROS), glial morphology, and cell apoptosis were determined in hippocampus and cortex.

RESULTS

Both HFD-fed groups developed obese-insulin resistance, but ND-fed rats did not. HFD-fed rats with sham-operation showed cognitive decline, when compared to ND-fed rats with sham-operation at all time points. At 4- and 8-week after ORX, the cognitive impairment of ND-fed rats and both HFD-fed groups was not different. However, 12-week after ORX, cognitive decline and of glial hyperactivity of HFD-fed rats had the greatest increase among all groups. Hippocampal ROS levels and apoptotic cells in both HFD-fed groups were equally increased, but the cortical ROS levels and apoptotic cells of HFD-fed rats with ORX were the highest ones.

CONCLUSIONS

These findings suggest that testosterone deprivation aggravates cognitive decline in obesity via increasing oxidative stress, glial activity and apoptosis.

Microglial-driven changes in synaptic plasticity: A possible role in major depressive disorder

Recent data gathered from both in vitro and in vivo models of Major Depressive Disorder (MDD) have indicated that microglia play an active role in modifying some of the most important sources for neuronal plasticity, specifically long-term potentiation (LTP) and long-term depression (LTD). In addition, microglia have been implicated in neuro-immune interaction dysregulations, which are considered a core constituent of MDD pathology. While prior studies have investigated the diverse effects activated microglia can have in the context of depression, including regulation of inflammatory cytokine production and structural changes, recent evidence has revealed a more direct relationship between microglial activation and changes in synaptic function and plasticity, including LTP and LTD. Here we review these findings from animal models, as well as discuss how current preclinical evidence might shed light on novel therapeutic targets for patients with depressive disorder.

Dysfunctional Mitochondrial Bioenergetics and Synaptic Degeneration in Alzheimer Disease

Synapses are sites of high energy demand which are dependent on high levels of mitochondrial derived adenosine triphosphate. Mitochondria within synaptic structures are key for maintenance of functional neurotransmission and this critical biological process is modulated by energy metabolism, mitochondrial distribution, mitochondrial trafficking, and cellular synaptic calcium flux. Synapse loss is presumed to be an early yet progressive pathological event in Alzheimer disease (AD), resulting in impaired cognitive function and memory loss which is particularly prevalent at later stages of disease. Supporting evidence from AD patients and animal models suggests that pathological mitochondrial dynamics indeed occurs early and is highly associated with synaptic lesions and degeneration in AD neurons. This review comprehensively highlights recent findings that describe how synaptic mitochondria pathology involves dysfunctional trafficking of this organelle, to maladaptive epigenetic contributions affecting mitochondrial function in AD. We further discuss how these negative, dynamic alterations impact synaptic function associated with AD. Finally, this review explores how antioxidant therapeutic approaches targeting mitochondria in AD can further clinical research and basic science investigations to advance our in-depth understanding of the pathogenesis of AD.

A Targeted Mutation Disrupting Mitochondrial Complex IV Function in Primary Afferent Neurons Leads to Pain Hypersensitivity Through P2Y1 Receptor Activation

As mitochondrial dysfunction is evident in neurodegenerative disorders that are accompanied by pain, we generated inducible mutant mice with disruption of mitochondrial respiratory chain complex IV, by COX10 deletion limited to sensory afferent neurons through the use of an Advillin Cre-reporter. COX10 deletion results in a selective energy-deficiency phenotype with minimal production of reactive oxygen species. Mutant mice showed reduced activity of mitochondrial respiratory chain complex IV in many sensory neurons, increased ADP/ATP ratios in dorsal root ganglia and dorsal spinal cord synaptoneurosomes, as well as impaired mitochondrial membrane potential, in these synaptoneurosome preparations. These changes were accompanied by marked pain hypersensitivity in mechanical and thermal (hot and cold) tests without altered motor function. To address the underlying basis, we measured Ca²⁺ fluorescence responses of dorsal spinal cord synaptoneurosomes to activation of the GluK1 (kainate) receptor, which we showed to be widely expressed in small but not large nociceptive afferents, and is minimally expressed elsewhere in the spinal cord. Synaptoneurosomes from mutant mice showed greatly increased responses to GluK1 agonist. To explore whether altered nucleotide levels may play a part in this hypersensitivity, we pharmacologically interrogated potential roles of AMP-kinase and ADP-sensitive purinergic receptors. The ADP-sensitive P2Y₁ receptor was clearly implicated. Its expression in small nociceptive afferents was increased in mutants, whose in vivo pain hypersensitivity, in mechanical, thermal and cold tests, was reversed by a selective P2Y₁ antagonist. Energy depletion and ADP elevation in sensory afferents, due to mitochondrial respiratory chain complex IV deficiency, appear sufficient to induce pain hypersensitivity, by ADP activation of P2Y₁ receptors.

Reactive oxygen species regulate activity-dependent neuronal plasticity in Drosophila

Reactive oxygen species (ROS) have been extensively studied as damaging agents associated with ageing and neurodegenerative conditions. Their role in the nervous system under non-pathological conditions has remained poorly understood. Working with the Drosophila larval locomotor network, we show that in neurons ROS act as obligate signals required for neuronal activity-dependent structural plasticity, of both pre- and postsynaptic terminals. ROS signaling is also necessary for maintaining evoked synaptic transmission at the neuromuscular junction, and for activity-regulated homeostatic adjustment of motor network output, as measured by larval crawling behavior. We identified the highly conserved Parkinson’s disease-linked protein DJ-1β as a redox sensor in neurons where it regulates structural plasticity, in part via modulation of the PTEN-PI3Kinase pathway. This study provides a new conceptual framework of neuronal ROS as second messengers required for neuronal plasticity and for network tuning, whose dysregulation in the ageing brain and under neurodegenerative conditions may contribute to synaptic dysfunction.

Differential involvement of reactive oxygen species in a mouse model of capsaicin-induced secondary mechanical hyperalgesia and allodynia

Intradermally injected capsaicin induces secondary mechanical hyperalgesia and allodynia outside the primary (i.e., capsaicin-injected) site. This secondary mechanical hypersensitivity is attributed to central sensitization in which reactive oxygen species (ROS) play a key role. We examined whether ROS would be differentially involved in secondary mechanical hyperalgesia and allodynia using a mouse intraplantar capsaicin injection model. In mice, capsaicin-induced secondary mechanical hyperalgesia outlasted its allodynia counterpart. Unlike the hyperalgesia, the allodynia was temporarily abolished by an anesthetic given at the capsaicin-injected site. The ROS scavenger phenyl-N-tert-butylnitrone slowed the development of both secondary mechanical hyperalgesia and allodynia when administered before intraplantar capsaicin injection, whereas it inhibited only the allodynia when administered after capsaicin had already induced secondary mechanical hyperalgesia and allodynia. Intrathecal injection of the ROS donor KO₂ induced both mechanical hyperalgesia and allodynia with the former outlasting the latter. Metformin, an activator of redox-sensitive adenosine monophosphate-activated protein kinase, selectively inhibited capsaicin-induced secondary mechanical allodynia and intrathecal KO₂-induced mechanical allodynia. These results suggest that ROS is required for rapid activation of central sensitization mechanisms for both secondary mechanical hyperalgesia and allodynia after intraplantar capsaicin injection. Once activated, the mechanism for the hyperalgesia is long-lasting without being critically dependent on ongoing afferent activities arising from the capsaicin-injected site and the continuous presence of ROS. On the contrary, the ongoing afferent activities, ROS presence and adenosine monophosphate-activated protein kinase inhibition are indispensable for the maintenance mechanism for capsaicin-induced secondary mechanical allodynia.

Monomethyl fumarate inhibits pain behaviors and amygdala activity in a rat arthritis model

Neuroplasticity in the amygdala, a brain center for emotions, leads to increased neuronal activity and output that can generate emotional-affective behaviors and modulate nocifensive responses. Mechanisms of increased activity in the amygdala output region (central nucleus, CeA) include increased reactive oxygen species, and so we explored beneficial effects of monomethyl fumarate (MMF), which can have neuroprotective effects through the nuclear factor (erythroid-derived 2)-like 2 (Nrf2) antioxidant response pathway. Systemic (intraperitoneal) MMF dose-dependently inhibited vocalizations and mechanosensitivity (hindlimb withdrawal reflexes) of rats in an arthritis pain model (kaolin-carrageenan-induced monoarthritis in the knee). Stereotaxic administration of MMF into the CeA by microdialysis also inhibited vocalizations but had a limited effect on mechanosensitivity, suggesting a differential contribution to emotional-affective vs sensory pain aspects. Extracellular single-unit recordings of CeA neurons in anesthetized rats showed that stereotaxic administration of MMF into the CeA by microdialysis inhibited background activity and responses of CeA neurons to knee joint stimulation in the arthritis pain model. Monomethyl fumarate had no effect on behaviors and neuronal activity under normal conditions. The results suggest that MMF can inhibit emotional-affective responses in an arthritis pain model through an action that involves the amygdala (CeA).

Reactive oxygen species affect spinal cell type-specific synaptic plasticity in a model of neuropathic pain

Spinal synaptic plasticity is believed to drive central sensitization that underlies the persistent nature of neuropathic pain. Our recent data showed that synaptic plasticity in the dorsal horn is cell type specific: intense afferent stimulation produced long-term potentiation (LTP) in excitatory spinothalamic tract neurons (STTn), whereas it produced long-term depression (LTD) in inhibitory GABAergic interneurons (GABAn). In addition, reactive oxygen species (ROS) were shown to be involved in LTP in STTn (STTn-LTP) and in LTD in GABAn (GABAn-LTD). This study examined the roles of 2 biologically important ROS--superoxide [·O2] and hydroxyl radicals [·OH]--in neuropathic mechanical hyperalgesia and cell type-specific spinal synaptic plasticity. The [·O2] donor induced stronger mechanical hyperalgesia than the [·OH] donor in naive mice. The [·O2] scavenger showed greater antihyperalgesic effect than [·OH] scavengers in the spinal nerve ligation (SNL) mouse model of neuropathic pain. In addition, the [·O2] donor induced both STTn-LTP and GABAn-LTD, but the [·OH] donor induced only GABAn-LTD. On the other hand, the [·O2] scavenger inhibited STTn-LTP and GABAn-LTD induction in naive mice and alleviated SNL-induced potentiation in STTn and depression in GABAn. The [·OH] scavenger, however, inhibited depression in GABAn but did not interfere with potentiation in STTn. These results indicate that mechanical hyperalgesia in SNL mice is the result of the combination of STTn-LTP and GABAn-LTD. Behavioral outcomes compliment electrophysiological results which suggest that [·O2] mediates both STTn-LTP and GABAn-LTD, whereas [·OH] is involved primarily in GABAn-LTD.

Regulation of neuronal development and function by ROS

Reactive oxygen species (ROS) have long been studied as destructive agents in the context of nervous system ageing, disease and degeneration. Their roles as signalling molecules under normal physiological conditions is less well understood. Recent studies have provided ample evidence of ROS-regulating neuronal development and function, from the establishment of neuronal polarity to growth cone pathfinding; from the regulation of connectivity and synaptic transmission to the tuning of neuronal networks. Appreciation of the varied processes that are subject to regulation by ROS might help us understand how changes in ROS metabolism and buffering could progressively impact on neuronal networks with age and disease.

Decreased microglial activation through gut-brain axis by prebiotics, probiotics, or synbiotics effectively restored cognitive function in obese-insulin resistant rats

BACKGROUND

Chronic high-fat diet (HFD) consumption caused not only obese-insulin resistance, but also cognitive decline and microglial hyperactivity. Modified gut microbiota by prebiotics and probiotics improved obese-insulin resistance. However, the effects of prebiotics, probiotics, and synbiotics on cognition and microglial activity in an obese-insulin resistant condition have not yet been investigated. We aimed to evaluate the effect of prebiotic (Xyloolidosaccharide), probiotic (Lactobacillus paracasei HII01), or synbiotics in male obese-insulin resistant rats induced by a HFD.

METHODS

Male Wistar rats were fed with either a normal diet or a HFD for 12 weeks. At week 13, the rats in each dietary group were randomly divided into four subgroups including vehicle group, prebiotics group, probiotics group, and synbiotics group. Rats received their assigned intervention for an additional 12 weeks. At the end of experimental protocol, the cognitive functioning of each rat was investigated; blood and brain samples were collected to determine metabolic parameters and investigate brain pathology.

RESULTS

We found that chronic HFD consumption leads to gut and systemic inflammation and impaired peripheral insulin sensitivity, which were improved by all treatments. Prebiotics, probiotics, or synbiotics also improved hippocampal plasticity and attenuated brain mitochondrial dysfunction in HFD-fed rats. Interestingly, hippocampal oxidative stress and apoptosis were significantly decreased in HFD-fed rats with all therapies, which also decreased microglial activation, leading to restored cognitive function.

CONCLUSIONS

These findings suggest that consumption of prebiotics, probiotics, and synbiotics restored cognition in obese-insulin resistant subjects through gut-brain axis, leading to improved hippocampal plasticity, brain mitochondrial function, and decreased microglial activation.

The volume-regulated anion channel (LRRC8) in nodose neurons is sensitive to acidic pH

The leucine rich repeat containing protein 8A (LRRC8A), or SWELL1, is an essential component of the volume-regulated anion channel (VRAC) that is activated by cell swelling and ionic strength. We report here for the first time to our knowledge its expression in a primary cell culture of nodose ganglia neurons and its localization in the soma, neurites, and neuronal membrane. We show that this neuronal VRAC/SWELL1 senses low external pH (pH_o) in addition to hypoosmolarity. A robust sustained chloride current is seen in 77% of isolated nodose neurons following brief exposures to extracellular acid pH. Its activation involves proton efflux, intracellular alkalinity, and an increase in NOX-derived H₂O_2. The molecular identity of both the hypoosmolarity-induced and acid pH_o-conditioned VRAC as LRRC8A (SWELL1) was confirmed by Cre-flox-mediated KO, shRNA-mediated knockdown, and CRISPR/Cas9-mediated LRRC8A deletion in HEK cells and in primary nodose neuronal cultures. Activation of VRAC by low pH_o reduces neuronal injury during simulated ischemia and N-methyl-D-aspartate-induced (NMDA-induced) apoptosis. These results identify the VRAC (LRRC8A) as a dual sensor of hypoosmolarity and low pH_o in vagal afferent neurons and define the mechanisms of its activation and its neuroprotective potential.

Mitochondrial Dysfunction in Depression

Abstract: Background

Depression is the most debilitating neuropsychiatric disorder with significant impact on socio-occupational and well being of individual. The exact pathophysiology of depression is still enigmatic though various theories have been put forwarded. There are evidences showing that mitochondrial dysfunction in various brain regions is associated with depression. Recent findings have sparked renewed appreciation for the role of mitochondria in many intracellular processes coupled to synaptic plasticity and cellular resilience. New insights in depression pathophysiology are revolving around the impairment of neuroplasticity. Mitochondria have potential role in ATP production, intracellular Ca²⁺ signalling to establish membrane stability, reactive oxygen species (ROS) balance and to execute the complex processes of neurotransmission and plasticity. So understanding the various concepts of mitochondrial dysfunction in pathogenesis of depression indubitably helps to generate novel and more targeted therapeutic approaches for depression treatment.

Objective

The review was aimed to give a comprehensive insight on role of mitochondrial dysfunction in depression.

Result

Targeting mitochondrial dysfunction and enhancing the mitochondrial functions might act as potential target for the treatment of depression.

Conclusion

Literature cited in this review highly supports the role of mitochondrial dysfunction in depression. As impairment in the mitochondrial functions lead to the generation of various insults that exaggerate the pathogenesis of depression. So, it is useful to study mitochondrial dysfunction in relation to mood disorders, synaptic plasticity, neurogenesis and enhancing the functions of mitochondria might show promiscuous effects in the treatment of depressed patients.

Spinal CPEB-mtROS-CBP signaling pathway contributes to perineural HIV gp120 with ddC-related neuropathic pain in rats

Human immunodeficiency virus (HIV) patients treated with nucleoside reverse transcriptase inhibitors (NRTIs), have been known to develop neuropathic pain. While there has been a major shift away from some neurotoxic NRTIs in current antiretroviral therapy, a large number of HIV patients alive today have previously received them, and many have developed painful peripheral neuropathy. The exact mechanisms by which HIV with NRTIs contribute to the development of neuropathic pain are not known. Previous studies suggest that cytoplasmic polyadenylation element-binding protein (CPEB), reactive oxygen species (ROS), and cAMP-response element-binding protein (CREB)-binding protein (CBP), are involved in the neuroimmunological diseases including inflammatory/neuropathic pain. In this study, we investigated the role of CPEB, mitochondrial ROS (mtROS), or CBP in neuropathic pain induced by HIV envelope protein gp120 combined with antiretroviral drug. The application of recombinant gp120 into the sciatic nerve plus systemic ddC (one of NRTIs) induced mechanical allodynia. Knockdown of CPEB or CBP using intrathecal antisense oligodeoxynucleotide (AS-ODN) reduced mechanical allodynia. Intrathecal mitochondrial superoxide scavenger mito-tempol (Mito-T) increased mechanical withdrawal threshold. Knockdown of CPEB using intrathecal AS-ODN, reduced the up-regulated mitochondrial superoxide in the spinal dorsal horn in rats with gp120 combined with ddC. Intrathecal Mito-T lowered the increased expression of CBP in the spinal dorsal horn. Immunostaining studies showed that neuronal CPEB positive cells were co-localized with MitoSox positive profiles, and that MitoSox positive profiles were co-localized with neuronal CBP. Our studies suggest that neuronal CPEB-mtROS-CBP pathway in the spinal dorsal horn, plays an important role in the gp120/ddC-induced neuropathic pain in rats.

AMPKα1 knockout enhances nociceptive behaviors and spinal glutamatergic synaptic activities via production of reactive oxygen species in the spinal dorsal horn

Emerging studies have shown that pharmacological activation of adenosine monophosphate-activated protein kinase (AMPK) produces potent analgesic effects in different animal pain models. Currently, the spinal molecular and synaptic mechanism by which AMPK regulates the pain signaling system remains unclear. To address this issue, we utilized the Cre-LoxP system to conditionally knockout the AMPKα1 gene in the nervous system of mice. We demonstrated that AMPKα1 is imperative for maintaining normal nociception, and mice deficient for AMPKα1 exhibit mechanical allodynia. This is concomitantly associated with increased glutamatergic synaptic activities in neurons located in the superficial spinal dorsal horn, which results from the increased glutamate release from presynaptic terminals and function of ligand-gated glutamate receptors at the postsynaptic neurons. Additionally, AMPKα1 knockout mice have increased activities of extracellular signal-regulated kinases (ERK) and p38 mitogen-activated protein kinases (p38), as well as elevated levels of interleukin-1β (IL-1β), reactive oxygen species (ROS), and heme oxygenase 1 (HO-1) in the spinal dorsal horn. Systemic administration of a non-specific ROS scavenger (phenyl-N-tert-butylnitrone, PBN) or a HO-1 activator (Cobalt protoporphyrin IX, CoPP) attenuated allodynia in AMPKα1 knockout mice. Bath-perfusion of the ROS scavenger or HO-1 activator effectively attenuated the increased ROS levels and glutamatergic synaptic activities in the spinal dorsal horn. Our findings suggest that ROS are the key down-stream signaling molecules mediating the behavioral hypersensitivity in AMPKα1 knockout mice. Thus, targeting AMPKα1 may represent an effective approach for the treatment of pathological pain conditions associated with neuroinflammation at the spinal dorsal horn.

Reactive oxygen species mediate visceral pain-related amygdala plasticity and behaviors

Accumulating evidence suggests an important contribution of reactive oxygen species (ROS) to pain and neuropsychiatric disorders, but their role in pain-related plasticity in the brain is largely unknown. Neuroplasticity in the central nucleus of the amygdala (CeA) correlates positively with pain behaviors in different models. Little is known, however, about mechanisms of visceral pain-related amygdala changes. The electrophysiological and behavioral studies reported here addressed the role of ROS in the CeA in a visceral pain model induced by intracolonic zymosan. Vocalizations to colorectal distension and anxiety-like behavior increased after intracolonic zymosan and were inhibited by intra-CeA application of a ROS scavenger (tempol, a superoxide dismutase mimetic). Tempol also induced a place preference in zymosan-treated rats but not in controls. Single-unit recordings of CeA neurons in anesthetized rats showed increases of background activity and responses to visceral stimuli after intracolonic zymosan. Intra-CeA application of tempol inhibited the increased activity but had no effect under normal conditions. Whole-cell patch-clamp recordings of CeA neurons in brain slices from zymosan-treated rats showed that tempol decreased neuronal excitability and excitatory synaptic transmission of presumed nociceptive inputs from the brainstem (parabrachial area) through a combination of presynaptic and postsynaptic actions. Tempol had no effect in brain slices from sham controls. The results suggest that ROS contribute to visceral pain-related hyperactivity of amygdala neurons and amygdala-dependent behaviors through a mechanism that involves increased excitatory transmission and excitability of CeA neurons.

Induction of long-term potentiation and long-term depression is cell-type specific in the spinal cord

This study shows that the direction of synaptic plastic changes in the spinal cord is cell-type specific in response to nociceptive input.

The underlying mechanism of chronic pain is believed to be changes in excitability in spinal dorsal horn (DH) neurons that respond abnormally to peripheral input. Increased excitability in pain transmission neurons, and depression of inhibitory neurons, are widely recognized in the spinal cord of animal models of chronic pain. The possible occurrence of 2 parallel but opposing forms of synaptic plasticity, long-term potentiation (LTP) and long-term depression (LTD) was tested in 2 types of identified DH neurons using whole-cell patch-clamp recordings in mouse spinal cord slices. The test stimulus was applied to the sensory fibers to evoke excitatory postsynaptic currents in identified spinothalamic tract neurons (STTn) and GABAergic neurons (GABAn). Afferent conditioning stimulation (ACS) applied to primary afferent fibers with various stimulation parameters induced LTP in STTn but LTD in GABAn, regardless of stimulation parameters. These opposite responses were further confirmed by simultaneous dual patch-clamp recordings of STTn and GABAn from a single spinal cord slice. Both the LTP in STTn and the LTD in GABAn were blocked by an NMDA receptor antagonist, AP5, or an intracellular Ca²⁺ chelator, BAPTA. Both the pattern and magnitude of intracellular Ca²⁺ after ACS were almost identical between STTn and GABAn based on live-cell calcium imaging. The results suggest that the intense sensory input induces an NMDA receptor-dependent intracellular Ca²⁺ increase in both STTn and GABAn, but produces opposing synaptic plasticity. This study shows that there is cell type–specific synaptic plasticity in the spinal DH.

Reactive Oxygen Species Donors Increase the Responsiveness of Dorsal Horn Neurons and Induce Mechanical Hyperalgesia in Rats

Our previous studies suggest that reactive oxygen species (ROS) scavengers have analgesic effect on neuropathic pain through spinal mechanisms in the rat. The studies suggest that superoxide in spinal cord is one of important mediators of persistent pain. To test the hypothesis that increase of superoxide-derived intermediates leads to central sensitization and pain, the effects of an intrathecal injection of chemical ROS donors releasing either OH^∙, OCl⁻, or H₂O₂ were examined on pain behaviors. Following treatment with t-BOOH (OH^∙ donor), dorsal horn neuron responses to mechanical stimuli in normal rats and the changes of neuronal excitability were explored on substantia gelatinosa (SG) neurons using whole-cell patch clamping recordings. Intrathecal administration of t-BOOH or NaOCl (OCl⁻ donor), but not H₂O₂, significantly decreased mechanical thresholds of hind paws. The responses of wide dynamic range neurons to mechanical stimuli increased after a local application of t-BOOH. The t-BOOH increased the frequency and the amplitude of excitatory postsynaptic potentials, depolarized membrane potential in SG neurons, and increased the frequency of action potentials evoked by depolarizing current pulses. These results suggest that elevated ROS, especially OH^∙, in the spinal cord sensitized dorsal horn neurons and produced hyperalgesia in normal rats.

Oxidative stress induced by epileptic seizure and its attenuation by melatonin

An epileptic seizure and postictal period in addition to well-known features are also characterized by massive consumption of energy. This is thought to lead to oxidative stress and increased generation of free radicals, which is reflected by increased levels of oxidative products. Our previous work described the neuroprotective effects of melatonin in preventing cognitive worsening after a single epileptic seizure. This work was aimed on direct measurement of free radicals in brain tissue using the EPR method 1, 15 and 60 minutes after seizure. The measurement was performed in adult male Wistar rats at the mentioned intervals after a single tonic-clonic seizure induced by flurothyl. In comparison to control animals there was a significant increase in hydroxyl and nitroxyl radicals 60 minutes after the seizure. The levels of hydroxyl radicals were significantly lower in animals that received melatonin 60 minutes before seizure induction compared to animals without preventive treatment. Therefore, melatonin affected the generation of the measured free radicals differently. An important finding was the delayed increase in free radicals after a single seizure in the later phases of recovery.

Antiallodynic effect of intrathecal epigallocatechin-3-gallate due to suppression of reactive oxygen species

Background

Green tea modulates neuropathic pain. Reactive oxygen species (ROS) are suggested as a key molecule in the underlying mechanism of neuropathic pain in the spinal cord. We examined the effect of epigallocatechin-3-gallate (EGCG), the major catechin in green tea, in neuropathic pain and clarified the involvement of ROS on the activity of EGCG.

Methods

Neuropathic pain was induced in male Sprague-Dawley rats by spinal nerve ligation (SNL). A polyethylene tube was intrathecally located. Nociceptive degree was estimated by a von Frey filament and expressed as a paw withdrawal threshold (PWT). To determine the role of ROS on the effect of EGCG, a free radical donor (tert-BuOOH) was pretreated before administration of EGCG. ROS activity was assayed by xanthine oxidase (XO) and malondialdehyde (MDA).

Results

SNL decreased the PWT compared to sham rats. The decrease remained during the entire observation period. Intrathecal EGCG increased the PWT at the SNL site. Intrathecal tert-BuOOH significantly decreased the effect of EGCG. The levels of both XO and MDA in the spinal cord were increased in SNL rats compared to sham. Intrathecal EGCG decreased the level of XO and MDA.

Conclusions

EGCG may reduce neuropathic pain by SNL due to the suppression of ROS in the spinal cord.

Nox2-dependent signaling between macrophages and sensory neurons contributes to neuropathic pain hypersensitivity

Emerging lines of evidence indicate that production of reactive oxygen species (ROS) at distinct sites of the nociceptive system contributes to the processing of neuropathic pain. However, the mechanisms underlying ROS production during neuropathic pain processing are not fully understood. We here detected the ROS-generating nicotinamide adenine dinucleotide phosphate oxidase isoform Nox2 in macrophages of dorsal root ganglia (DRG) in mice. In response to peripheral nerve injury, Nox2-positive macrophages were recruited to DRG, and ROS production was increased in a Nox2-dependent manner. Nox2-deficient mice displayed reduced neuropathic pain behavior after peripheral nerve injury, whereas their immediate responses to noxious stimuli were normal. Moreover, injury-induced upregulation of tumor necrosis factor α was absent, and activating transcription factor 3 induction was reduced in DRG of Nox2-deficient mice, suggesting an attenuated macrophage-neuron signaling. These data suggest that Nox2-dependent ROS production in macrophages recruited to DRG contributes to neuropathic pain hypersensitivity, underlining the observation that Nox-derived ROS exert specific functions during the processing of pain.

Intrathecal infusion of hydrogen-rich normal saline attenuates neuropathic pain via inhibition of activation of spinal astrocytes and microglia in rats

BACKGROUND

Reactive oxygen and nitrogen species are key molecules that mediate neuropathic pain. Although hydrogen is an established antioxidant, its effect on chronic pain has not been characterized. This study was to investigate the efficacy and mechanisms of hydrogen-rich normal saline induced analgesia.

METHODOLOGY/PRINCIPAL FINDINGS

In a rat model of neuropathic pain induced by L5 spinal nerve ligation (L5 SNL), intrathecal injection of hydrogen-rich normal saline relieved L5 SNL-induced mechanical allodynia and thermal hyperalgesia. Importantly, repeated administration of hydrogen-rich normal saline did not lead to tolerance. Preemptive treatment with hydrogen-rich normal saline prevented development of neuropathic pain behavior. Immunofluorochrome analysis revealed that hydrogen-rich normal saline treatment significantly attenuated L5 SNL-induced increase of 8-hydroxyguanosine immunoreactive cells in the ipsilateral spinal dorsal horn. Western blot analysis of SDS/PAGE-fractionated tyrosine-nitrated proteins showed that L5 SNL led to increased expression of tyrosine-nitrated Mn-containing superoxide dismutase (MnSOD) in the spinal cord, and hydrogen-rich normal saline administration reversed the tyrosine-nitrated MnSOD overexpression. We also showed that the analgesic effect of hydrogen-rich normal saline was associated with decreased activation of astrocytes and microglia, attenuated expression of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in the spinal cord.

CONCLUSION/SIGNIFICANCE

Intrathecal injection of hydrogen-rich normal saline produced analgesic effect in neuropathic rat. Hydrogen-rich normal saline-induced analgesia in neuropathic rats is mediated by reducing the activation of spinal astrocytes and microglia, which is induced by overproduction of hydroxyl and peroxynitrite.

Effect of antioxidant treatment on spinal GABA neurons in a neuropathic pain model in the mouse

One feature of neuropathic pain is a reduced spinal gamma-aminobutyric acid (GABA)-ergic inhibitory function. However, the mechanisms behind this attenuation remain to be elucidated. This study investigated the involvement of reactive oxygen species in the spinal GABA neuron loss and reduced GABA neuron excitability in spinal nerve ligation (SNL) model of neuropathic pain in mice. The importance of spinal GABAergic inhibition in neuropathic pain was tested by examining the effects of intrathecally administered GABA receptor agonists and antagonists in SNL and naïve mice, respectively. The effects of SNL and antioxidant treatment on GABA neuron loss and functional changes were examined in transgenic GAD67-enhanced green fluorescent protein positive (EGFP+) mice. GABA receptor agonists transiently reversed mechanical hypersensitivity of the hind paw in SNL mice. On the other hand, GABA receptor antagonists made naïve mice mechanically hypersensitive. Stereological analysis showed that the numbers of enhanced green fluorescent protein positive (EGFP+) GABA neurons were significantly decreased in the lateral superficial laminae (I-II) on the ipsilateral L5 spinal cord after SNL. Repeated antioxidant treatments significantly reduced the pain behaviors and prevented the reduction in EGFP+ GABA neurons. The response rate of the tonic firing GABA neurons recorded from SNL mice increased with antioxidant treatment, whereas no change was seen in those recorded from naïve mice, which suggested that oxidative stress impaired some spinal GABA neuron activity in the neuropathic pain condition. Together the data suggest that neuropathic pain, at least partially, is attributed to oxidative stress, which induces both a GABA neuron loss and dysfunction of surviving GABA neurons.

Reactive oxygen species enhance excitatory synaptic transmission in rat spinal dorsal horn neurons by activating TRPA1 and TRPV1 channels

Central neuropathic pain (CNP) in the spinal cord, such as chronic pain after spinal cord injury (SCI), is an incurable ailment. However, little is known about the spinal cord mechanisms underlying CNP. Recently, reactive oxygen species (ROS) have been recognized to play an important role in CNP of the spinal cord. However, it is unclear how ROS affect synaptic transmission in the dorsal horn of the spinal cord. To clarify how ROS impact on synaptic transmission, we investigated the effects of ROS on synaptic transmission in rat spinal cord substantia gelatinosa (SG) neurons using whole-cell patch-clamp recordings. Administration of tert-butyl hydroperoxide (t-BOOH), an ROS donor, into the spinal cord markedly increased the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) in SG neurons. This t-BOOH-induced enhancement was not suppressed by the Na(+) channel blocker tetrodotoxin. However, in the presence of a non-N-methyl-D-aspartate glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, t-BOOH did not generate any sEPSCs. Furthermore, in the presence of a transient receptor potential ankyrin 1 (TRPA1) channel antagonist (HC-030031) or a transient receptor potential vanilloid 1 (TRPV1) channel antagonist (capsazepine or AMG9810), the t-BOOH-induced increase in the frequency of sEPSCs was inhibited. These results indicate that ROS enhance the spontaneous release of glutamate from presynaptic terminals onto SG neurons through TRPA1 and TRPV1 channel activation. Excessive activation of these ion channels by ROS may induce central sensitization in the spinal cord and result in chronic pain such as that following SCI.

Prooxidant-induced c-Src/nuclear factor kappa B-coupled signalling in sensory ganglia mediates cutaneous hyperalgesia

BACKGROUND

Persistent pain resulting from peripheral injury/inflammation is associated with altered sensitivity to cutaneous stimuli, which can manifest as hyperalgesia. The role of oxidant stress in the development, progression and maintenance of hyperalgesia is still not understood. Furthermore, there appears to be a relationship between c-Src kinase in the pain pathway and oxidative stress.

METHODS

We have used a novel prooxidant inflammatory pain model that involves potassium peroxychromate (PPC), a unique prooxidant that produces the same reactants as activated phagocytes. This model was used to investigate the role of oxidant-activated c-Src in mediating hyperalgesia. We compared the effects of PP2 (a Src family kinase inhibitor) and c-Src siRNA on behavioural hyperalgesia with sodium stibogluconate (SSG) (a non-receptor tyrosine phosphatase inhibitor) and AG 1478 (a receptor tyrosine kinase inhibitor).

RESULTS

PP2 and c-Src siRNA attenuated PPC-induced thermal hyperalgesia, while SSG enhanced it. AG 1478 had no effect. PP2 decreased the levels of IL-1β, c-Src/inhibitory kappa B kinase complex formed and prostaglandin E2 produced in the dorsal root ganglia (DRG) ipsilateral to the inflamed paw, while SSG increased the levels of these parameters. c-Src siRNA decreased Src expression and activity in the DRG ipsilateral to the inflamed paw.

CONCLUSIONS

These results confirm that prooxidant-activated c-Src plays a role in initiating and maintaining hyperalgesia by regulating a stimulus-response coupling between the inflamed tissue and the DRG in the pain pathway. Our data also suggest that oxidant-induced dysregulation of c-Src/nuclear factor kappa B coupling may contribute to our understanding of the transition from acute to chronic dysfunctional pain state seen in many human diseases.

Antinociceptive drug interaction between intrathecal vitamin E and gabapentin in the rat formalin test

Background

Gabapentin is thought to exert an effect through the voltage-dependent calcium channel. Vitamin E is a widely known antioxidant which neutralizes the harmful effect of ROS which is considered to play a prominent role in various painful conditions. This study was therefore conducted to assess the antinociceptive effects of gabapentin and vitamin E and the interaction of these drugs in the modulation of pain in rats subjected to a formalin test.

Methods

Sprague-Dawley rats with a lumbar intrathecal catheter were tested for their paw flinches by 5% formalin injection after intrathecal injection of gabapentin or vitamin E. After obtaining dose-response curves for each drug, the effect of the combination was tested by the total dose fraction value and isobolographic analysis.

Results

When a single drug was injected intrathecally, significant dose-dependent decreases in flinches were shown only in the late phase. ED₅₀ values of intrathecal gabapentin and vitamin E in the late phase were 75.3 ± 9.58 µg, and 17.56 ± 1.65 mg/kg respectively. The combination of gabapentin and vitamin E produced dose-dependent decreases in the number of flinches in both phases induced by the formalin test. The ED₅₀ value of the combination was lower than the theoretical additive values in the late phase, but did not show a significant difference with the theoretical additive value.

Conclusions

Gabapentin and vitamin E (by itself) have no antinociceptive effect in the early phase; however their combination has shown an antinociceptive effect. In addition, they show additive effects in the late phase of the formalin test.

NOXious signaling in pain processing

Chronic pain affects millions of people and often causes major health problems. Accumulating evidence indicates that the production of reactive oxygen species (ROS), such as superoxide anion or hydrogen peroxide, is increased in the nociceptive system during chronic inflammatory and neuropathic pain, and that ROS can act as specific signaling molecules in pain processing. Reduction of ROS levels by administration of scavengers or antioxidant compounds attenuated the nociceptive behavior in various animal models of chronic pain. However, the sources of increased ROS production during chronic pain and the role of ROS in pain processing are poorly understood. Current work revealed pain-relevant functions of the Nox family of NADPH oxidases, a group of electron-transporting transmembrane enzymes whose sole function seems to be the generation of ROS. In particular, significant expression of the Nox family members Nox1, Nox2, and Nox4 in various cells of the nociceptive system has been discovered. Studies using knockout mice suggest that these Nox enzymes specifically contribute to distinct signaling pathways in chronic inflammatory and/or neuropathic pain states. Accordingly, targeting Nox1, Nox2, and Nox4 could be a novel strategy for the treatment of chronic pain. Currently selective inhibitors of Nox enzymes are being developed. Here, we introduce the distinct roles of Nox enzymes in pain processing, we summarize recent findings in the understanding of ROS-dependent signaling pathways in the nociceptive system, and we discuss potential analgesic properties of currently available Nox inhibitors.

Antinociceptive effects of intraperitoneal and intrathecal vitamin e in the rat formalin test

Background

Vitamin E is widely known to be one of the reactive oxygen species (ROS) scavengers and a drug that can easily be obtained, and it has been shown to attenuate the pain responses induced by various causes in animal pain models. Thus, this experiment was conducted to assess the antinociceptive effects of vitamin E by comparing intraperitoneal and intrathecal injections in rats subjected to the formalin test.

Methods

After the intraperitoneal and intrathecal injections of vitamin E were carried out, respectively (IP: 500 mg/kg, 1 g/kg, and 2 g/kg, IT: 3 mg/kg, 10 mg/kg, and 30 mg/kg), the formalin test was perfumed. As soon as 5% formalin was injected into left hind paw, the number of flinches induced by pain was measured at 5-minute intervals for 1 hour.

Results

Formalin injected into the left hind paw induced biphasic nociceptive behavior in all animals. Intraperitoneal injection of vitamin E diminished the nociceptive behavior in a dose-dependent manner during the early and late phase. Intrathecal vitamin E diminished nociceptive behavior dose dependently during the late phase but showed no significant difference in the early phase.

Conclusions

Vitamin E attenuated acute nociception when it was injected systemically, while both systemic and intrathecal injection produced analgesia in a rat model of formalin-induced hyperalgesia.