Long-term dynamic changes of NMDA receptors following an excitotoxic challenge

Excitotoxicity is a form of neuronal death characterized by the sustained activation of N-methyl-D-aspartate receptors (NMDARs) triggered by the excitatory neurotransmitter glutamate. NADPH-diaphorase neurons [also known as nNOS (+) neurons] are a subpopulation of aspiny interneurons, largely spared following excitotoxic challenges. Unlike nNOS (-) cells, nNOS (+) neurons fail to generate reactive oxygen species in response to NMDAR activation, a key divergent step in the excitotoxic cascade. However, additional mechanisms underlying the reduced vulnerability of nNOS (+) neurons to NMDAR-driven neuronal death have not been explored. Using functional, genetic, and molecular analysis in striatal cultures, we demonstrate that nNOS (+) neurons possess distinct NMDAR properties. These specific features are primarily driven by the peculiar redox milieu of this subpopulation. In addition, we found that nNOS (+) neurons exposed to a pharmacological maneuver set to mimic chronic excitotoxicity alter their responses to NMDAR-mediated challenges. These findings suggest the presence of mechanisms providing long-term dynamic regulation of NMDARs that can have critical implications in neurotoxic settings.

Glutamatergic Mechanisms in Glioblastoma and Tumor-Associated Epilepsy

The progression of glioblastomas is associated with a variety of neurological impairments, such as tumor-related epileptic seizures. Seizures are not only a common comorbidity of glioblastoma but often an initial clinical symptom of this cancer entity. Both, glioblastoma and tumor-associated epilepsy are closely linked to one another through several pathophysiological mechanisms, with the neurotransmitter glutamate playing a key role. Glutamate interacts with its ionotropic and metabotropic receptors to promote both tumor progression and excitotoxicity. In this review, based on its physiological functions, our current understanding of glutamate receptors and glutamatergic signaling will be discussed in detail. Furthermore, preclinical models to study glutamatergic interactions between glioma cells and the tumor-surrounding microenvironment will be presented. Finally, current studies addressing glutamate receptors in glioma and tumor-related epilepsy will be highlighted and future approaches to interfere with the glutamatergic network are discussed.

Liposomal Encapsulated FSC231, a PICK1 Inhibitor, Prevents the Ischemia/Reperfusion-Induced Degradation of GluA2-Containing AMPA Receptors

Strokes remain one of the leading causes of disability within the United States. Despite an enormous amount of research effort within the scientific community, very few therapeutics are available for stroke patients. Cytotoxic accumulation of intracellular calcium is a well-studied phenomenon that occurs following ischemic stroke. This intracellular calcium overload results from excessive release of the excitatory neurotransmitter glutamate, a process known as excitotoxicity. Calcium-permeable AMPA receptors (AMPARs), lacking the GluA2 subunit, contribute to calcium cytotoxicity and subsequent neuronal death. The internalization and subsequent degradation of GluA2 AMPAR subunits following oxygen–glucose deprivation/reperfusion (OGD/R) is, at least in part, mediated by protein-interacting with C kinase-1 (PICK1). The purpose of the present study is to evaluate whether treatment with a PICK1 inhibitor, FSC231, prevents the OGD/R-induced degradation of the GluA2 AMPAR subunit. Utilizing an acute rodent hippocampal slice model system, we determined that pretreatment with FSC231 prevented the OGD/R-induced association of PICK1–GluA2. FSC231 treatment during OGD/R rescues total GluA2 AMPAR subunit protein levels. This suggests that the interaction between GluA2 and PICK1 serves as an important step in the ischemic/reperfusion-induced reduction in total GluA2 levels.

Leaky wiring of the brain: local cluster of coupled synapses and extracellular signal integration

The wiring scheme is key to the function of the brain. Neurons are structurally wired by synapses and it is a long-held view that most synapses in the CNS are sufficiently isolated to avoid cross-talk to AMPA receptors of neighboring synapses. On the contrary, we show here with optical reporter proteins that the neurotransmitter glutamate regularly spreads far into the extracellular space (>1μm) after vesicular release. Together with 2P-glutamate uncaging our data suggest that multi-vesicular release rather regularly generates crosstalk responses at AMPA receptors of ~2-4 adjacent synapses (>70 synapses for NMDA receptors). Extracellular spread of glutamate is cooperative and coincident synaptic release events show enhanced spread and cause supra-additive activation of postsynapses. Thus, synaptic wiring of the brain seems to deviate more from point-to-point communication than previously reported and involves broadcasting information to very local neighborhoods which can stabilize learning performance and allow for integration of synaptic activity within the extracellular space.

Impaired Oligodendrocyte Development Following Preterm Birth: Promoting GABAergic Action to Improve Outcomes

Preterm birth is associated with poor long-term neurodevelopmental and behavioral outcomes, even in the absence of obvious brain injury at the time of birth. In particular, behavioral disorders characterized by inattention, social difficulties and anxiety are common among children and adolescents who were born moderately to late preterm (32–37 weeks' gestation). Diffuse deficits in white matter microstructure are thought to play a role in these poor outcomes with evidence suggesting that a failure of oligodendrocytes to mature and myelinate axons is responsible. However, there remains a major knowledge gap over the mechanisms by which preterm birth interrupts normal oligodendrocyte development. In utero neurodevelopment occurs in an inhibitory-dominant environment due to the action of placentally derived neurosteroids on the GABAA receptor, thus promoting GABAergic inhibitory activity and maintaining the fetal behavioral state. Following preterm birth, and the subsequent premature exposure to the ex utero environment, this action of neurosteroids on GABAA receptors is greatly reduced. Coinciding with a reduction in GABAergic inhibition, the preterm neonatal brain is also exposed to ex utero environmental insults such as periods of hypoxia and excessive glucocorticoid concentrations. Together, these insults may increase levels of the excitatory neurotransmitter glutamate in the developing brain and result in a shift in the balance of inhibitory: excitatory activity toward excitatory. This review will outline the normal development of oligodendrocytes, how it is disrupted under excitation-dominated conditions and highlight how shifting the balance back toward an inhibitory-dominated environment may improve outcomes.

Aminooxadiazolyl kainic acid reveals that kainic acid receptors contribute to astrocytoma glutamate signaling

ABSTRACTThe excitatory neurotransmitter glutamate triggers a Ca2+ rise and the extension of processes in astrocytes. Our results suggest that kainic acid receptors (KAR) can independently initiate glutamate signaling in astrocytoma U118-MG cells. The natural product kainic acid triggered glioexcitablity in cells and was inhibited by the KAR antagonist CNQX, but its activity was lower than glutamate on KARs. We created a new heteroaryl kainoid based on rational design: aminooxadiazolyl kainic acid 1 (AODKA). AODKA induced a larger calcium influx and a faster processes extension than kainic acid in U118-MG cells. AODKA is a new tool to study KAR activity in the nervous system.Abstract Figure

A Neurotoxic Ménage-à-trois: Glutamate, Calcium, and Zinc in the Excitotoxic Cascade

Fifty years ago, the seminal work by John Olney provided the first evidence of the neurotoxic properties of the excitatory neurotransmitter glutamate. A process hereafter termed excitotoxicity. Since then, glutamate-driven neuronal death has been linked to several acute and chronic neurological conditions, like stroke, traumatic brain injury, Alzheimer’s, Parkinson’s, and Huntington’s diseases, and Amyotrophic Lateral Sclerosis. Mechanisms linked to the overactivation of glutamatergic receptors involve an aberrant cation influx, which produces the failure of the ionic neuronal milieu. In this context, zinc, the second most abundant metal ion in the brain, is a key but still somehow underappreciated player of the excitotoxic cascade. Zinc is an essential element for neuronal functioning, but when dysregulated acts as a potent neurotoxin. In this review, we discuss the ionic changes and downstream effects involved in the glutamate-driven neuronal loss, with a focus on the role exerted by zinc. Finally, we summarize our work on the fascinating distinct properties of NADPH-diaphorase neurons. This neuronal subpopulation is spared from excitotoxic insults and represents a powerful tool to understand mechanisms of resilience against excitotoxic processes.

L-type Voltage-gated Calcium Channel Modulators Inhibit Glutamate-induced Morphology in Astrocytoma Cells

ABSTRACTThe excitatory neurotransmitter glutamate evokes physiological responses within the astrocytic network that lead to fine morphological dynamics. However, the mechanism by which astrocytes couple glutamate sensing with cellular calcium rise remains unclear. Employing natural properties of U118-MG astrocytoma cells, we tested a possible connection between L-type voltage-gated calcium channels (Cav) and glutamate receptors. Using live confocal imaging and pharmacological inhibitors, the extension of U118-MG processes upon glutamate exposure are shown to depend mainly on extracellular calcium entry via L-type Cav’s. Inhibitors of the Cav α1 protein, decreased astrocytic filopodia extension; while, gabapentinoids, ligands of the Cav’s α2δ auxiliary subunit blocked all process growth. This study suggests that α2δ is the main contributor to Cav’s role in glutamate-dependent filopodiagenesis. It opens new avenues of research on the role of α2δ in neuron-astrocyte glutamate signaling and neurochemical signaling at tripartite synapses.

Carbon-Nanotube-Based Monolithic CMOS Platform for Electrochemical Detection of Neurotransmitter Glutamate

We present a monolithic biosensor platform, based on carbon-nanotube field-effect transistors (CNTFETs), for the detection of the neurotransmitter glutamate. We used an array of 9′216 CNTFET devices with 96 integrated readout and amplification channels that was realized in complementary metal-oxide semiconductor technology (CMOS). The detection principle is based on amperometry, where electrochemically active hydrogen peroxide, a product of the enzymatic reaction of the target analyte and an enzyme that was covalently bonded to the CNTFET, modulated the conductance of the CNTFET-based sensors. We assessed the performance of the CNTs as enzymatic sensors by evaluating the minimal resolvable concentration change of glutamate in aqueous solutions. The minimal resolvable concentration change amounted to 10 µM of glutamate, which was one of the best values reported for CMOS-based systems so far.

AMPA receptor localization in trigeminal ganglion and its upregulation in migraine

Migraine is characterized by headache due to imbalance between excitation and inhibition of neurons disabling normal day to day activities. The excitations of neurons are done by excitatory neurotransmitter glutamate which plays the key role in creating any pathology related to neurons. This study was done to identify GluR1 a subunit of AMPA glutamate receptor in the cells of trigeminal ganglion after inducing migraine and compare it with control rats. The GluR1 subunits were localized in the cytoplasm of neurons, and these subunits were up-regulated following a migraine. The GluR1 was also localized in satellite glial cells and nerve fibers, indicating these subunits expressed in neurons and migrate during nociceptive sensitization. This GluR1 expression in the cells of trigeminal ganglion may be crucial in nociceptive sensitization leading to migraine and other painful conditions like trigeminal neuralgia.