Excitotoxicity in the pathogenesis of neurological and psychiatric disorders: Therapeutic implications

Structural Evaluation and Electrophysiological Effects of Some Kynurenic Acid Analogs

Kynurenic acid (KYNA), a metabolite of tryptophan, as an excitatory amino acid receptor antagonist is an effective neuroprotective agent in case of excitotoxicity, which is the hallmark of brain ischemia and several neurodegenerative processes. Therefore, kynurenine pathway, KYNA itself, and its derivatives came into the focus of research. During the past fifteen years, our research group has developed several neuroactive KYNA derivatives, some of which proved to be neuroprotective in preclinical studies. In this study, the synthesis of these KYNA derivatives and their evaluation with divergent molecular characteristics are presented together with their most typical effects on the monosynaptic transmission in CA1 region of the hippocampus of the rat. Their effects on the basic neuronal activity (on the field excitatory postsynaptic potentials: fEPSP) were studied in in vitro hippocampal slices in 1 and 200 μM concentrations. KYNA and its derivative 4 in both 1 and 200 μM concentrations proved to be inhibitory, while derivative 8 only in 200 μM decreased the amplitudes of fEPSPs. Derivative 5 facilitated the fEPSPs in 200 μM concentration. This is the first comparative study which evaluates the structural and functional differences of formerly and newly developed KYNA analogs. Considerations on possible relations between molecular structures and their physiological effects are presented.

Glutamate/GABA co-release selectively influences postsynaptic glutamate receptors in mouse cortical neurons

Cultured rat cortical neurons co-expressing VGLUT1 and VGAT (mixed synapses) co-release Glu and GABA. Here, mixed synapses were studied in cultured mouse cortical neurons to verify whether in mice mixed synapses co-release Glu and GABA, and to gain insight into how they may influence excitation/inhibition balance. Results showed the existence of synapses and autapses that co-release Glu and GABA in cultured mouse cortical neurons, and the ability of both neurotransmitters to evoke postsynaptic responses mediated by ionotropic receptors. We studied the short-term plasticity of glutamatergic, GABAergic, and mixed responses and we found that the kinetics of mixPSC amplitude depression was similar to that observed in EPSCs, but it was different from that of IPSCs. We found similar presynaptic release characteristics in glutamatergic and mixed synapses. Analysis of postsynaptic features, obtained by measuring AMPAR- and NMDAR-mediated currents, showed that AMPAR-mediated currents were significantly higher in pure glutamatergic than in mixed synapses, whereas NMDAR-mediated currents were not significantly different from those measured in mixed synapses. Overall, our findings demonstrate that glutamatergic and mixed synapses share similar electrophysiological properties. However, co-release of GABA and Glu influences postsynaptic ionotropic glutamatergic receptor subtypes, thus selectively influencing AMPAR-mediated currents. These findings strengthen the view that mixed neurons can play a key role in CNS development and in maintaining the excitation-inhibition balance.

Neuroprotective effects of anthocyanins and its major component cyanidin-3-O-glucoside (C3G) in the central nervous system: An outlined review

Anthocyanins, a class of water soluble flavonoids extracted from plants like berries and soybean seed, have been shown to display obvious anti-oxidative, anti-inflammatory, and anti-apoptotic activities. They are recommended as a supplementation for prevention and/or treatment of disorders ranging from cardiovascular disease, metabolic syndrome, and cancer. In the central nervous system (CNS), anthocyanins and its major component cyanidin-3-O-glucoside (C3G) have been reported to produce preventive and/or therapeutic activities in a wide range of disorders, such as cerebral ischemia, Alzheimer's disease, Parkinson's disease, multiple sclerosis, and glioblastoma. Both anthocyanins and C3G can also affect some important processes in aging, including neuronal apoptosis and death as well as learning and memory impairment. Further, the anthocyanins and C3G have been shown to prevent neuro-toxicities induced by different toxic factors, such as lipopolysaccharide, hydrogen peroxide, ethanol, kainic acid, acrolein, glutamate, and scopolamine. Mechanistic studies have shown that inhibition of oxidative stress and neuroinflammation are two critical mechanisms by which anthocyanins and C3G produce protective effects in CNS disorder prevention and/or treatment. Other mechanisms, including suppression of c-Jun N-terminal kinase (JNK) activation, amelioration of cellular degeneration, activation of the brain-derived neurotrophic factor (BDNF) signaling, and restoration of Ca²⁺ and Zn²⁺ homeostasis, may also mediate the neuroprotective effects of anthocyanins and C3G. In this review, we summarize the pharmacological effects of anthocyanins and C3G in CNS disorders as well as their possible mechanisms, aiming to get a clear insight into the role of anthocyanins in the CNS.

Glial Cell AMPA Receptors in Nervous System Health, Injury and Disease

Glia form a central component of the nervous system whose varied activities sustain an environment that is optimised for healthy development and neuronal function. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA)-type glutamate receptors (AMPAR) are a central mediator of glutamatergic excitatory synaptic transmission, yet they are also expressed in a wide range of glial cells where they influence a variety of important cellular functions. AMPAR enable glial cells to sense the activity of neighbouring axons and synapses, and as such many aspects of glial cell development and function are influenced by the activity of neural circuits. However, these AMPAR also render glia sensitive to elevations of the extracellular concentration of glutamate, which are associated with a broad range of pathological conditions. Excessive activation of AMPAR under these conditions may induce excitotoxic injury in glial cells, and trigger pathophysiological responses threatening other neural cells and amplifying ongoing disease processes. The aim of this review is to gather information on AMPAR function from across the broad diversity of glial cells, identify their contribution to pathophysiological processes, and highlight new areas of research whose progress may increase our understanding of nervous system dysfunction and disease.

Carnosic Acid Pretreatment Attenuates Mitochondrial Dysfunction in SH-SY5Y Cells in an Experimental Model of Glutamate-Induced Excitotoxicity

Mitochondria are the major site of adenosine triphosphate (ATP) production in mammalian cells. Moreover, mitochondria produce most of the reactive oxygen species (ROS) in nucleated cells. Redox and bioenergetic abnormalities have been seen in mitochondria during the onset and progression of neurodegenerative diseases. In that context, excitotoxicity induced by glutamate (GLU) plays an important role in mediating neurotoxicity. Several drugs have been used in the treatment of diseases involving excitotoxicity. Nonetheless, some patients (20-30%) present drug resistance. Thus, it is necessary to find chemicals able to attenuate mitochondrial dysfunction in the case of excitotoxicity. In this work, we treated the human neuroblastoma SH-SY5Y cell line with the diterpene carnosic acid (CA) at 1 μM for 12 h prior to the exposure to GLU for further 24 h. We found that CA prevented the GLU-induced mitochondrion-related redox impairment and bioenergetic decline in SH-SY5Y cells. CA also downregulated the pro-apoptotic stimulus elicited by GLU in this experimental model. CA exerted mitochondrial protection by a mechanism associated with the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2), since silencing of this protein with small interfering RNA (siRNA) suppressed the CA-induced protective effects. Future directions include investigating whether CA would be able to modulate mitochondrial function and/or dynamics in in vivo experimental models of excitotoxicity.

The association of excitation and inhibition signaling with the relative symptom expression of autism and psychosis-proneness: Implications for psychopharmacology

The underlying mechanisms of autism and schizophrenia are poorly understood, partly due to a lack of dimension-specific research. Aberrant excitatory and inhibitory neurotransmission are implicated in both conditions, particularly in social dysfunction. This study investigates the extent to which the degree of autistic tendency and psychosis-proneness exclusively and interactively predict excitatory and inhibitory neurotransmitter concentrations in the superior temporal cortex (STC). In 38 adults (18 male, 18-40 years), we obtained autistic tendencies (Autism-Spectrum Quotient [AQ]) and psychosis-proneness scores (Schizotypal Personality Questionnaire [PP]); magnetic resonance spectroscopy (MRS) quantified glutamate and GABA+ concentrations from the STC. Results demonstrated a negative AQ/PP interaction with glutamate concentration for the left STC voxel, where PP increased with glutamate for average AQ, while AQ decreased with glutamate for average-high PP. There was a negative AQ/PP interaction with glutamate/GABA+ ratio for the right STC, AQ increasing with glutamate/GABA+ for low-average PP, while PP decreased with glutamate/GABA+ for high AQ. Consistent with animal studies, we also reveal that overall reduced glutamate/GABA+ ratio might be precipitated by increased right hemisphere GABA+ concentrations. These findings illustrate the importance of considering the concurrent effects of autism and psychosis dimensions on understanding the pathophysiological mechanisms implicated in either condition, and can advance psychopharmacological research into better treatment options for patients.

Cognitive neuropsychological theory: Reconciliation of psychological and biological approaches for depression

New antidepressants and individualized approaches to treatment, matching specific therapies to individual patients, are urgently needed. For this, a better understanding of processes underpinning the development of depressive symptoms and response to medications are required. The cognitive neuropsychological model offers a novel approach uniquely combining biological and psychological approaches to explain how antidepressants exert their effect, why there is a delay in the onset of their clinical effect, and how changes in emotional processing are an essential step for a clinical antidepressant effect to take place. The paper presents the model and its underpinnings in the form of research in both healthy and depressed individuals, as well as the potential for its practical use.

The effect of glutamate-induced excitotoxicity on DNA methylation in astrocytes in a new in vitro neuron-astrocyte-endothelium co-culture system

Glutamate-induced excitotoxicity is a contributer to many neurological diseases. Astrocytes may represent a new target for treating glutamate-induced excitotoxicity. However, the in vitro culture system that mimics the in vivo microenvironment is lacking. This study aimed to establish a new in vitro co-culture system including neurons, astrocytes, and endothelial cells (NAE), and to investigate the effect of glutamate-induced excitotoxicity on DNA methylation in astrocytes. A NAE co-culture method was created using a Transwell chamber, in which neurons were seeded on the bottom of the lower chamber, endothelial cells were plated on the top membrane, and astrocytes were plated on the bottom membrane of the insert. Glutamate-induced toxicity was induced using glutamate and glycine, and examined using immunofluorescence and lactate dehydrogenase release assay. Global methylation in astrocytes was analyzed, and the expression of DNMT1 and DNMT3a was examined using Western blot analysis. Glutamate treatment induced less neuronal damage in the NAE system compared with the control group in which neurons and astrocytes were cultured alone. Global DNA methylation was increased and the expression of DNMT1 and DNMT3a in astrocytes was increased after glutamate treatment, which was blocked by application of the NMDAR inhibitor MK-801 and the DNMT inhibitor 5-azaC from the endothelial cells. The in vitro ANE culture system is effective for studying glutamate-induced excitotoxicity, and may be used for testing the passage of drugs across the blood-brain barrier. Inhibition of DNA methylation in astrocytes may be a new therapeutic strategy for treating glutamate-induced excitotoxicity.

N-Acetylcysteine Prevents the Spatial Memory Deficits and the Redox-Dependent RyR2 Decrease Displayed by an Alzheimer's Disease Rat Model

We have previously reported that primary hippocampal neurons exposed to synaptotoxic amyloid beta oligomers (AβOs), which are likely causative agents of Alzheimer’s disease (AD), exhibit abnormal Ca²⁺ signals, mitochondrial dysfunction and defective structural plasticity. Additionally, AβOs-exposed neurons exhibit a decrease in the protein content of type-2 ryanodine receptor (RyR2) Ca²⁺ channels, which exert critical roles in hippocampal synaptic plasticity and spatial memory processes. The antioxidant N-acetylcysteine (NAC) prevents these deleterious effects of AβOs in vitro. The main contribution of the present work is to show that AβOs injections directly into the hippocampus, by engaging oxidation-mediated reversible pathways significantly decreased RyR2 protein content but increased single RyR2 channel activation by Ca²⁺ and caused considerable spatial memory deficits. AβOs injections into the CA3 hippocampal region impaired rat performance in the Oasis maze spatial memory task, decreased hippocampal glutathione levels and overall content of plasticity-related proteins (c-Fos, Arc, and RyR2) and increased ERK1/2 phosphorylation. In contrast, in hippocampus-derived mitochondria-associated membranes (MAM) AβOs injections increased RyR2 levels. Rats fed with NAC for 3-weeks prior to AβOs injections displayed comparable redox potential, RyR2 and Arc protein contents, similar ERK1/2 phosphorylation and RyR2 single channel activation by Ca²⁺ as saline-injected (control) rats. NAC-fed rats subsequently injected with AβOs displayed the same behavior in the spatial memory task as control rats. Based on the present in vivo results, we propose that redox-sensitive neuronal RyR2 channels partake in the mechanism underlying AβOs-induced memory disruption in rodents.

An outline for the pharmacological effect of icariin in the nervous system

Icariin is a major active component of the traditional herb Epimedium, also known as Horny Goat Weed. It has been extensively studied throughout the past several years and is known to exert anti-oxidative, anti-neuroinflammatory, and anti-apoptotic effects. It is now being considered as a potential therapeutic agent for a wide variety of disorders, ranging from neoplasm to cardiovascular disease. More recent studies have shown that icariin exhibits potential preventive and/or therapeutic effects in the nervous system. For example, icariin can prevent the production of amyloid β (1-42) and inhibit the expression of amyloid precursor protein (APP) and β-site APP cleaving enzyme 1 (BACE-1) in animal models of Alzheimer's disease (AD). Icariin has been shown to mitigate pro-inflammatory responses of microglia in culture and in animal models of cerebral ischemia, depression, Parkinson's disease (PD), and multiple sclerosis (MS). Icariin also prevents the neurotoxicity induced by hydrogen peroxide (H₂O₂), endoplasmic reticulum (ER) stress, ibotenic acid, and homocysteine. In addition, icariin is implicated in facilitating learning and memory in both normal aging animals and disease models. To date, we still have no consolidated source of knowledge about the pharmacological effects of icariin in the nervous system, though its roles in other tissues have been reviewed in recent years. Here, we summarize the pharmacological development of icariin as well as its possible mechanisms in prevention and/or therapy of disorders afflicting the nervous system in hope of expanding the knowledge about the preventive and/or therapeutic effect of icariin in brain disorders.

Effect of Cholesterol on Membrane Fluidity and Association of Aβ Oligomers and Subsequent Neuronal Damage: A Double-Edged Sword

Background: The beta-amyloid peptide (Aβ) involved in Alzheimer's disease (AD) has been described to associate/aggregate on the cell surface disrupting the membrane through pore formation and breakage. However, molecular determinants involved for this interaction (e.g., some physicochemical properties of the cell membrane) are largely unknown. Since cholesterol is an important molecule for membrane structure and fluidity, we examined the effect of varying cholesterol content with the association and membrane perforation by Aβ in cultured hippocampal neurons. Methods: To decrease or increase the levels of cholesterol in the membrane we used methyl-β-cyclodextrin (MβCD) and MβCD/cholesterol, respectively. We analyzed if membrane fluidity was affected using generalized polarization (GP) imaging and the fluorescent dye di-4-ANEPPDHQ. Additionally membrane association and perforation was assessed using immunocytochemistry and electrophysiological techniques, respectively. Results: The results showed that cholesterol removal decreased the macroscopic association of Aβ to neuronal membranes (fluorescent-puncta/20 μm: control = 18 ± 2 vs. MβCD = 10 ± 1, p < 0.05) and induced a facilitation of the membrane perforation by Aβ with respect to control cells (half-time for maximal charge transferred: control = 7.2 vs. MβCD = 4.4). Under this condition, we found an increase in membrane fluidity (46 ± 3.3% decrease in GP value, p < 0.001). On the contrary, increasing cholesterol levels incremented membrane rigidity (38 ± 2.7% increase in GP value, p < 0.001) and enhanced the association and clustering of Aβ (fluorescent-puncta/20 μm: control = 18 ± 2 vs. MβCD = 10 ± 1, p < 0.01), but inhibited membrane disruption. Conclusion: Our results strongly support the significance of plasma membrane organization in the toxic effects of Aβ in hippocampal neurons, since fluidity can regulate distribution and insertion of the Aβ peptide in the neuronal membrane.

The Role of Magnesium in Neurological Disorders

Magnesium is well known for its diverse actions within the human body. From a neurological standpoint, magnesium plays an essential role in nerve transmission and neuromuscular conduction. It also functions in a protective role against excessive excitation that can lead to neuronal cell death (excitotoxicity), and has been implicated in multiple neurological disorders. Due to these important functions within the nervous system, magnesium is a mineral of intense interest for the potential prevention and treatment of neurological disorders. Current literature is reviewed for migraine, chronic pain, epilepsy, Alzheimer’s, Parkinson’s, and stroke, as well as the commonly comorbid conditions of anxiety and depression. Previous reviews and meta-analyses are used to set the scene for magnesium research across neurological conditions, while current research is reviewed in greater detail to update the literature and demonstrate the progress (or lack thereof) in the field. There is strong data to suggest a role for magnesium in migraine and depression, and emerging data to suggest a protective effect of magnesium for chronic pain, anxiety, and stroke. More research is needed on magnesium as an adjunct treatment in epilepsy, and to further clarify its role in Alzheimer’s and Parkinson’s. Overall, the mechanistic attributes of magnesium in neurological diseases connote the macromineral as a potential target for neurological disease prevention and treatment.

Effect of Cholesterol on Membrane Fluidity and Association of Aβ Oligomers and Subsequent Neuronal Damage: A Double-Edged Sword

Background: The beta-amyloid peptide (Aβ) involved in Alzheimer's disease (AD) has been described to associate/aggregate on the cell surface disrupting the membrane through pore formation and breakage. However, molecular determinants involved for this interaction (e.g., some physicochemical properties of the cell membrane) are largely unknown. Since cholesterol is an important molecule for membrane structure and fluidity, we examined the effect of varying cholesterol content with the association and membrane perforation by Aβ in cultured hippocampal neurons. Methods: To decrease or increase the levels of cholesterol in the membrane we used methyl-β-cyclodextrin (MβCD) and MβCD/cholesterol, respectively. We analyzed if membrane fluidity was affected using generalized polarization (GP) imaging and the fluorescent dye di-4-ANEPPDHQ. Additionally membrane association and perforation was assessed using immunocytochemistry and electrophysiological techniques, respectively. Results: The results showed that cholesterol removal decreased the macroscopic association of Aβ to neuronal membranes (fluorescent-puncta/20 μm: control = 18 ± 2 vs. MβCD = 10 ± 1, p < 0.05) and induced a facilitation of the membrane perforation by Aβ with respect to control cells (half-time for maximal charge transferred: control = 7.2 vs. MβCD = 4.4). Under this condition, we found an increase in membrane fluidity (46 ± 3.3% decrease in GP value, p < 0.001). On the contrary, increasing cholesterol levels incremented membrane rigidity (38 ± 2.7% increase in GP value, p < 0.001) and enhanced the association and clustering of Aβ (fluorescent-puncta/20 μm: control = 18 ± 2 vs. MβCD = 10 ± 1, p < 0.01), but inhibited membrane disruption. Conclusion: Our results strongly support the significance of plasma membrane organization in the toxic effects of Aβ in hippocampal neurons, since fluidity can regulate distribution and insertion of the Aβ peptide in the neuronal membrane.