Simultaneous efflux of endogenous D-ser and L-glu from single acute hippocampus slices during oxygen glucose deprivation

Development of μ-Low-Flow-Push-Pull Perfusion Probes for Ex Vivo Sampling from Mouse Hippocampal Tissue Slices

This work demonstrates a reduced tip μ-low-flow-push-pull perfusion technique for ex vivo sampling of the extracellular space of mouse hippocampal brain slices. Concentric fused-silica capillary probes are pulled by an in-house gravity puller with a butane flame producing probe tips averaging an overall outer diameter of 30.3 ± 8 μm. The 10-30 nL/min perfusion flow rate through the probe generates an average recovery of 90%. Sampling was performed with mouse brain tissue slices to characterize basal neurotransmitter content in this model system. Samples were collected from hippocampal tissue slices at a volume of 200 nL per sample. Sample arginine, histamine, lysine, glycine, glutamate, and aspartate content was quantified by micellar electrokinetic chromatography with LED-induced fluorescence detection. Primary amine content was sampled over several hours to determine evidence for tissue damage and loss of extracellular content from the tissue slice. Overall, all amino acid concentrations trended lower as an effect of time relative to tissue slicing. There were significant concentration decreases seen for histamine, lysine, and aspartate between time points 0-2 and 2-6 h (p < 0.05) relative to tissue slicing. Analysis of averaged sampling experiments does not appear to reveal significant probe-insertion-related amino acid changes. The work presented shows the applicability of an 80% reduction of probe tip size relative to previous designs for the collection of extracellular content from thin tissue slices.

Arctic ground squirrel resist peroxynitrite-mediated cell death in response to oxygen glucose deprivation

Cerebral ischemia-reperfusion (I/R) injury initiates a cascade of events, generating nitric oxide (NO) and superoxide(O₂^•-) to form peroxynitrite (ONOO^-), a potent oxidant. Arctic ground squirrels (AGS; Urocitellus parryii) show high tolerance to I/R injury. However, the underlying mechanism remains elusive. We hypothesize that tolerance to I/R modeled in an acute hippocampal slice preparation in AGS is modulated by reduced oxidative and nitrative stress. Hippocampal slices (400µm) from rat and AGS were subjected to oxygen glucose deprivation (OGD) using a novel microperfusion technique. Slices were exposed to NO, O₂^.- donors with and without OGD; pretreatment with inhibitors of NO, O₂^.- and ONOO^- followed by OGD. Perfusates collected every 15min were analyzed for LDH release, a marker of cell death. 3-nitrotyrosine (3NT) and 4-hydroxynonenal (4HNE) were measured to assess oxidative and nitrative stress. Results show that NO/O₂^.- alone is not sufficient to cause ischemic-like cell death, but with OGD enhances cell death more in rat than in AGS. A NOS inhibitor, SOD mimetic and ONOO^- inhibitor attenuates OGD injury in rat but has no effect in AGS. Rats also show a higher level of 3NT and 4HNE with OGD than AGS suggesting the greater level of injury in rat is via formation of ONOO^-.

Acidotoxicity via ASIC1a Mediates Cell Death during Oxygen Glucose Deprivation and Abolishes Excitotoxicity

Ischemic reperfusion (I/R) injury is associated with a complex and multifactorial cascade of events involving excitotoxicity, acidotoxicity, and ionic imbalance. While it is known that acidosis occurs concomitantly with glutamate-mediated excitotoxicity during brain ischemia, it remains elusive how acidosis-mediated acidotoxicity interacts with glutamate-mediated excitotoxicity. Here, we investigated the effect of acidosis on glutamate-mediated excitotoxicity in acute hippocampal slices. We tested the hypothesis that mild acidosis protects against I/R injury via modulation of NMDAR, but produces injury via activation of acid sensing ion channels (ASIC1a). Using a novel microperfusion approach, we monitored time course of injury in acutely prepared, adult hippocampal slices. We varied the duration of insult to delay the return to preinsult conditions to determine if injury was caused by the primary insult or by the modeled reperfusion phase. We also manipulated pH in presence and absence of oxygen glucose deprivation (OGD). The role of ASIC1a and NMDAR was deciphered by treating the slices with and without an ASIC or NMDAR antagonist. Our results show that injury due to OGD or low pH occurs during the insult rather than the modeled reperfusion phase. Injury mediated by low pH or low pH OGD requires ASIC1a and is independent of NMDAR activation. These findings point to ASIC1a as a mediator of ischemic cell death caused by stroke and cardiac arrest.

Arctic ground squirrel hippocampus tolerates oxygen glucose deprivation independent of hibernation season even when not hibernating and after ATP depletion, acidosis, and glutamate efflux

Cerebral ischemia/reperfusion (I/R) triggers a cascade of uncontrolled cellular processes that perturb cell homeostasis. The arctic ground squirrel (AGS), a seasonal hibernator resists brain damage following cerebral I/R caused by cardiac arrest and resuscitation. However, it remains unclear if tolerance to I/R injury in AGS depends on the hibernation season. Moreover, it is also not clear if events such as depletion of ATP, acidosis, and glutamate efflux that are associated with anoxic depolarization are attenuated in AGS. Here, we employ a novel microperfusion technique to test the hypothesis that tolerance to I/R injury modeled in an acute hippocampal slice preparation in AGS is independent of the hibernation season and persists even after glutamate efflux. Acute hippocampal slices were harvested from summer euthermic AGS, hibernating AGS, and interbout euthermic AGS. Slices were subjected to oxygen glucose deprivation (OGD), an in vitro model of I/R injury to determine cell death marked by lactate dehydrogenase (LDH) release. ATP was assayed using ENLITEN ATP assay. Glutamate and aspartate efflux was measured using capillary electrophoresis. For acidosis, slices were subjected to pH 6.4 or ischemic shift solution (ISS). Acute hippocampal slices from rats were used as a positive control, susceptible to I/R injury. Our results indicate that when tissue temperature is maintained at 36°C, hibernation season has no influence on OGD-induced cell death in AGS hippocampal slices. Our data also show that tolerance to OGD in AGS hippocampal slices occurs despite loss of ATP and glutamate release, and persists during conditions that mimic acidosis and ionic shifts, characteristic of cerebral I/R. Read the Editorial Comment for this article on page 10.

Bioanalytical Application of Amino Acid Detection by Capillary Electrophoresis

This chapter illustrates the usefulness of capillary electrophoresis (CE) for the analysis of amino acids, and both normal and chiral separations are covered. In order to provide a general description of the main results and challenges in the biomedical field, some relevant applications and reviews on CE of amino acids are tabulated. Furthermore, some detailed experimental procedures are shown, regarding the CE analysis of amino acids in body fluids, in microdialysate, and released upon hydrolysis of proteins. In particular, the protocols will deal with the following compounds: (1) underivatized aminoacids in blood; (2) γ-Aminobutyric acid, glutamate, and L-Aspartate derivatized with Naphthalene-2,3-dicarboxaldehyde; (3) hydrolysate from bovine serum albumine derivatized with phenylisothiocyanate. By examining these applications on real matrices, the capillary electrophoresis efficiency as tool for Amino Acid analysis can be ascertained.

In vivo D-serine hetero-exchange through alanine-serine-cysteine (ASC) transporters detected by microelectrode biosensors

D-serine, a co-agonist of N-methyl D-aspartate (NMDA) receptors, has been implicated in neurological and psychiatric disorders such as cerebral ischemia, lateral amyotrophic sclerosis, or schizophrenia. D-serine signaling represents an important pharmacological target for treating these diseases; however, the biochemical mechanisms controlling extracellular D-serine levels in vivo are still unclear. D-serine heteroexchange through small neutral amino acid transporters has been shown in cell cultures and brain slices and could provide a biochemical mechanism for the control of D-serine extracellular concentration in vivo. Alternatively, exocytotic D-serine release has also been proposed. In this study, the dynamics of D-serine release and clearance were explored in vivo on a second-by-second time scale using microelectrode biosensors. The rate of D-serine clearance in the rat frontal cortex after a microionophoretic injection revealed a transporter-mediated uptake mechanism. D-serine uptake was blocked by small neutral l-amino acids, implicating alanine-serine-cysteine (ASC) transporters, in particular high affinity Asc-1 and low affinity ASCT2 transporters. Interestingly, changes in alanine, serine, or threonine levels resulted in D-serine release through ASC transporters. Asc-1, but not ASCT2, appeared to release D-serine in response to changes in amino acid concentrations. Finally, neuronal silencing by tetrodotoxin increased D-serine extracellular concentration by an ASC-transporter-dependent mechanism. Together, these results indicate that D-serine heteroexchange through ASC transporters is present in vivo and may constitute a key component in the regulation of D-serine extracellular concentration.

Brain slice on a chip: opportunities and challenges of applying microfluidic technology to intact tissues

Isolated brain tissue, especially brain slices, are valuable experimental tools for studying neuronal function at the network, cellular, synaptic, and single channel levels. Neuroscientists have refined the methods for preserving brain slice viability and function and converged on principles that strongly resemble the approach taken by engineers in developing microfluidic devices. With respect to brain slices, microfluidic technology may 1) overcome the traditional limitations of conventional interface and submerged slice chambers and improve oxygen/nutrient penetration into slices, 2) provide better spatiotemporal control over solution flow/drug delivery to specific slice regions, and 3) permit successful integration with modern optical and electrophysiological techniques. In this review, we highlight the unique advantages of microfluidic devices for in vitro brain slice research, describe recent advances in the integration of microfluidic devices with optical and electrophysiological instrumentation, and discuss clinical applications of microfluidic technology as applied to brain slices and other non-neuronal tissues. We hope that this review will serve as an interdisciplinary guide for both neuroscientists studying brain tissue in vitro and engineers as they further develop microfluidic chamber technology for neuroscience research.

d-Serine diffusion through the blood-brain barrier: effect on d-serine compartmentalization and storage

d-Serine is a co-agonist of N-methyl-d-aspartate (NMDA) receptors. It has been implicated in the etiology of schizophrenia and has shown efficacy as an adjuvant to reduce positive and negative symptoms of schizophrenia. In addition, d-serine can modulate cognition in animals when administered alone. However, the neurochemical effects of exogenous d-serine on extra- and intra-cellular d-serine brain levels are poorly understood. In this study, we used both high performance liquid chromatography (HPLC) and enzyme-based microelectrode biosensors to quantify d-serine in the rat brain. We demonstrated levels of 2.3-2.8μM in the extracellular medium, 4μM in plasma and 188pmol/mg in brain tissue samples. An intraperitoneal (i.p.) d-serine injection (1g/kg) produced a slow increase in extracellular d-serine concentration in the cortex despite a surge in d-serine up to 13mM in the plasma, indicating poor diffusion through the blood-brain barrier. Using the respective volume fractions of blood, extracellular and intracellular spaces published in the literature, we estimated that d-serine intracellular stores represented more than 99% of total d-serine. These intracellular stores almost doubled 3h after d-serine administration. Overall, our data indicate that d-serine administration increases brain extra- and intra-cellular concentrations despite weak diffusion through the blood-brain barrier. These results pave the way for a better understanding of the neurochemical mechanisms by which d-serine administration modulates cognition.

Recent advances in amino acid analysis by capillary electrophoresis

This paper describes the most important articles that have been published on amino acid analysis using CE during the period from June 2009 to May 2011 and follows the format of the previous articles of Smith (Electrophoresis 1999, 20, 3078-3083), Prata et al. (Electrophoresis 2001, 22, 4129-4138) and Poinsot et al. (Electrophoresis 2003, 24, 4047-4062; Electrophoresis 2006, 27, 176-194; Electrophoresis 2008, 29, 207-223; Electrophoresis 2010, 31, 105-121). We present new developments in amino acid analysis with CE, which are reported describing the use of lasers or light emitting diodes for fluorescence detection, conductimetry electrochemiluminescence detectors, mass spectrometry applications, and lab-on-a-chip applications using CE. In addition, we describe articles concerning clinical studies and neurochemical applications of these techniques.

Contributions of the D-serine pathway to schizophrenia

The glutamate neurotransmitter system is one of the major candidate pathways for the pathophysiology of schizophrenia, and increased understanding of the pharmacology, molecular biology and biochemistry of this system may lead to novel treatments. Glutamatergic hypofunction, particularly at the NMDA receptor, has been hypothesized to underlie many of the symptoms of schizophrenia, including psychosis, negative symptoms and cognitive impairment. This review will focus on D-serine, a co-agonist at the NMDA receptor that in combination with glutamate, is required for full activation of this ion channel receptor. Evidence implicating D-serine, NMDA receptors and related molecules, such as D-amino acid oxidase (DAO), G72 and serine racemase (SRR), in the etiology or pathophysiology of schizophrenia is discussed, including knowledge gained from mouse models with altered D-serine pathway genes and from preliminary clinical trials with D-serine itself or compounds modulating the D-serine pathway. Abnormalities in D-serine availability may underlie glutamatergic dysfunction in schizophrenia, and the development of new treatments acting through the D-serine pathway may significantly improve outcomes for many schizophrenia patients.

AMPA receptor mediated D-serine release from retinal glial cells

The N-methyl-D-aspartate receptor (NMDAR) co-agonist D-serine is important in a number of different processes in the CNS, ranging from synaptic plasticity to disease states, including schizophrenia. D-serine appears to be the major co-agonist acting on retinal ganglion cell NMDA receptors, but the cell type from which it originates and whether its release can be modulated by activity are unknown. In this study, we utilized a mutant mouse line with elevated d-serine to investigate this question. Direct measurements of extracellular D-serine using capillary electrophoresis demonstrate that D-serine can be released from the intact mouse retina through an α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor (AMPAR) dependent mechanism. α-Amino-3-hydroxyl-5-methyl-4-isoxazole-propionate-evoked D-serine release persisted in the presence of a cocktail of neural inhibitors but was abolished after administration of a glial toxin. These findings provide the first evidence that extracellular D-serine levels in the retina can be modulated, and that such modulation is contingent upon glial cell activity.