Sample collection and amino acids analysis of extracellular fluid of mouse brain slices with low flow push-pull perfusion

Interference with glutamate antiporter system xc - enables post-hypoxic long-term potentiation in hippocampus

Our group previously showed that genetic or pharmacological inhibition of the cystine/glutamate antiporter, system xc -, mitigates excitotoxicity after anoxia by increasing latency to anoxic depolarization, thus attenuating the ischaemic core. Hypoxia, however, which prevails in the ischaemic penumbra, is a condition where neurotransmission is altered, but excitotoxicity is not triggered. The present study employed mild hypoxia to further probe ischaemia-induced changes in neuronal responsiveness from wild-type and xCT KO (xCT-/-) mice. Synaptic transmission was monitored in hippocampal slices from both genotypes before, during and after a hypoxic episode. Although wild-type and xCT-/- slices showed equal suppression of synaptic transmission during hypoxia, mutant slices exhibited a persistent potentiation upon re-oxygenation, an effect we termed 'post-hypoxic long-term potentiation (LTP)'. Blocking synaptic suppression during hypoxia by antagonizing adenosine A1 receptors did not preclude post-hypoxic LTP. Further examination of the induction and expression mechanisms of this plasticity revealed that post-hypoxic LTP was driven by NMDA receptor activation, as well as increased calcium influx, with no change in paired-pulse facilitation. Hence, the observed phenomenon engaged similar mechanisms as classical LTP. This was a remarkable finding as theta-burst stimulation-induced LTP was equivalent between genotypes. Importantly, post-hypoxic LTP was generated in wild-type slices pretreated with system xc - inhibitor, S-4-carboxyphenylglycine, thereby confirming the antiporter's role in this phenomenon. Collectively, these data indicate that system xc - interference enables neuroplasticity in response to mild hypoxia, and, together with its regulation of cellular damage in the ischaemic core, suggest a role for the antiporter in post-ischaemic recovery of the penumbra.

Tonic extracellular glutamate and ischaemia: glutamate antiporter system xc - regulates anoxic depolarization in hippocampus

In stroke, the sudden deprivation of oxygen to neurons triggers a profuse release of glutamate that induces anoxic depolarization (AD) and leads to rapid cell death. Importantly, the latency of the glutamate-driven AD event largely dictates subsequent tissue damage. Although the contribution of synaptic glutamate during ischaemia is well-studied, the role of tonic (ambient) glutamate has received far less scrutiny. The majority of tonic, non-synaptic glutamate in the brain is governed by the cystine/glutamate antiporter, system x_c ^- . Employing hippocampal slice electrophysiology, we showed that transgenic mice lacking a functional system x_c ^- display longer latencies to AD and altered depolarizing waves compared to wild-type mice after total oxygen deprivation. Experiments which pharmacologically inhibited system x_c ^- , as well as those manipulating tonic glutamate levels and those antagonizing glutamate receptors, revealed that the antiporter's putative effect on ambient glutamate precipitates the ischaemic cascade. As such, the current study yields novel insight into the pathogenesis of acute stroke and may direct future therapeutic interventions. KEY POINTS: Ischaemic stroke remains the leading cause of adult disability in the world, but efforts to reduce stroke severity have been plagued by failed translational attempts to mitigate glutamate excitotoxicity. Elucidating the ischaemic cascade, which within minutes leads to irreversible tissue damage induced by anoxic depolarization, must be a principal focus. Data presented here show that tonic, extrasynaptic glutamate supplied by system x_c ^- synergizes with ischaemia-induced synaptic glutamate release to propagate AD and exacerbate depolarizing waves. Exploiting the role of system x_c ^- and its obligate release of ambient glutamate could, therefore, be a novel therapeutic direction to attenuate the deleterious effects of acute stroke.

Extracellular Fluid Collection and Analysis of Drosophila melanogaster Brain Tissue with μ-Low-Flow Push-Pull Perfusion (μLFPP)

The development of methods to generate quantitative chemical content information from precise tissue locations is needed to understand fundamental cellular and tissue physiology. This work describes a method to perfuse the extracellular fluid of fly brains in vivo using μ-low-flow push-pull perfusion (μLFPP) for quantitative chemical content determinations. Miniaturization of push-pull perfusion probe designs allowed the development of methods for probe tip placement into and sampling from the fruit fly's brain. Perfusate analysis identified and quantified arginine, octopamine, histidine, taurine, glycine, glutamate, and aspartate. The perfusate data did not exhibit any statistical differences based on sex. The perfusate analysis was compared to hemolymph samples to confirm probe placement in fly brain tissues. The appearance of probe placement into the brain space was confirmed with the following observations. Hemolymph and perfusate samples were found to contain analytes unique to each sample type. Quantitated levels of perfusate were not a simple dilution of hemolymph content. Further, the discovery of perfusates with composition similar to both hemolymph and brain perfusate when damage was intentionally inflicted supports the observation that perfusates are distinct from hemolymph. The analysis of perfusate collected for greater than an hour of sampling exhibits the possibility of monitoring applications. Altogether, this work demonstrates the viability of performing μ-low-flow push-pull perfusion for in vivo studies of fly brain tissues to identify and quantitate neurotransmitter content.

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.

Recent advances in amino acid analysis by capillary electromigration methods: June 2015-May 2017

In the tenth edition of this article focused on recent advances in amino acid analysis using capillary electrophoresis, we describe the most important research articles published on this topic during the period from June 2015 to May 2017. This article follows the format of the previous articles published in Electrophoresis. The new developments in amino acid analysis with CE mainly describe improvements in CE associated with mass spectrometry. Focusing on applications, we mostly describe clinical works, although metabolomics studies are also very important. Finally, works focusing on amino acids in food and agricultural applications are also described.