Demonstration of low flow push-pull perfusion

Microdialysis coupled with droplet microfluidics and mass spectrometry for determination of neurotransmitters in vivo with high temporal resolution

Monitoring the concentration fluctuations of neurotransmitters in vivo is valuable for elucidating the chemical signals that underlie brain functions. Microdialysis sampling is a widely used tool for monitoring neurochemicals in vivo. The volume requirements of most techniques that have been coupled to microdialysis, such as HPLC, result in fraction collection times of minutes, thus limiting the temporal resolution possible. Further the time of analysis can become long for cases where many fractions are collected. Previously we have used direct analysis of dialysate by low-flow electrospray ionization-tandem mass spectrometry (ESI-MS/MS) on a triple quadrupole mass spectrometer to monitor acetylcholine, glutamate, and γ-amino-butyric acid to achieve multiplexed in vivo monitoring with temporal resolution of seconds. Here, we have expanded this approach to adenosine, dopamine, and serotonin. The method achieved limits of detection down to 2 nM, enabling basal concentrations of all these compounds, except serotonin, to be measured in vivo. Comparative analysis with LC-MS/MS showed accurate results for all compounds except for glutamate, possibly due to interference for this compound in vivo. Pairing this analysis with droplet microfluidics yields 11 s temporal resolution and can generate dialysate fractions down to 3 nL at rates up to 3 fractions per s from a microdialysis probe. The system is applied to multiplexed monitoring of neurotransmitter dynamics in response to stimulation by 100 mM K+ and amphetamine. These applications demonstrate the suitability of the droplet ESI-MS/MS method for monitoring short-term dynamics of up to six neurotransmitters simultaneously.

Electroosmotic Perfusion, External Microdialysis: Simulation and Experiment

Information about the rates of hydrolysis of neuropeptides by extracellular peptidases can lead to a quantitative understanding of how the steady-state and transient concentrations of neuropeptides are controlled. We have created a small microfluidic device that electroosmotically infuses peptides into, through, and out of the tissue to a microdialysis probe outside the head. The device is created by two-photon polymerization (Nanoscribe). Inferring quantitative estimates of a rate process from the change in concentration of a substrate that has passed through tissue is challenging for two reasons. One is that diffusion is significant, so there is a distribution of peptide substrate residence times in the tissue. This affects the product yield. The other is that there are multiple paths taken by the substrate as it passes through tissue, so there is a distribution of residence times and thus reaction times. Simulation of the process is essential. The simulations presented here imply that a range of first order rate constants of more than 3 orders of magnitude is measurable and that 5-10 min is required to reach a steady state value of product concentration following initiation of substrate infusion. Experiments using a peptidase-resistant d-amino acid pentapeptide, yaGfl, agree with simulations.

Progress in on-line, at-line, and in-line coupling of sample treatment with capillary and microchip electrophoresis over the past 10 years: A review

The review presents an evaluation of the development of on-line, at-line and in-line sample treatment coupled with capillary and microchip electrophoresis over the last 10 years. In the first part, it describes different types of flow-gating interfaces (FGI) such as cross-FGI, coaxial-FGI, sheet-flow-FGI, and air-assisted-FGI and their fabrication using molding into polydimethylsiloxane and commercially available fittings. The second part deals with the coupling of capillary and microchip electrophoresis with microdialysis, solid-phase, liquid-phase, and membrane based extraction techniques. It mainly focuses on modern techniques such as extraction across supported liquid membrane, electroextraction, single drop microextraction, head space microextraction, and microdialysis with high spatial and temporal resolution. Finally, the design of sequential electrophoretic analysers and fabrication of SPE microcartridges with monolithic and molecularly imprinted polymeric sorbents are discussed. Applications include the monitoring of metabolites, neurotransmitters, peptides and proteins in body fluids and tissues to study processes in living organisms, as well as the monitoring of nutrients, minerals and waste compounds in food, natural and wastewater.

On-Demand Nanoliter Sampling Probe for the Collection of Brain Fluid

Continuous fluidic sampling systems allow collection of brain biomarkers in vivo. Here, we propose a new sequential and intermittent sampling paradigm using droplets, called Droplet on Demand (DoD). It is implemented in a microfabricated neural probe and alternates phases of analyte removal from the tissue and phases of equilibration of the concentration in the tissue. It allows sampling droplets loaded with molecules from the brain extracellular fluid punctually, without the long transient equilibration periods typical of continuous methods. It uses an accurately defined fluidic sequence with controlled timings, volumes, and flow rates, and correct operation is verified by the embedded electrodes and a flow sensor. As a proof of concept, we demonstrated the application of this novel approach in vitro and in vivo, to collect glucose in the brain of mice, with a temporal resolution of 1-2 min and without transient regime. Absolute quantification of the glucose level in the samples was performed by direct infusion nanoelectrospray ionization Fourier transform mass spectrometry (nanoESI-FTMS). By adjusting the diffusion time and the perfusion volume of DoD, the fraction of molecules recovered in the samples can be tuned to mirror the tissue concentration at accurate points in time. Moreover, this makes quantification of biomarkers in the brain possible within acute experiments of only 20-120 min. DoD provides a complementary tool to continuous microdialysis and push-pull sampling probes. Thus, the advances allowed by DoD will benefit quantitative molecular studies in the brain, i.e., for molecules involved in volume transmission or for protein aggregates that form in neurodegenerative diseases over long periods.

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.

Microdialysis and microperfusion electrodes in neurologic disease monitoring

Contemporary biomarker collection techniques in blood and cerebrospinal fluid have to date offered only modest clinical insights into neurologic diseases such as epilepsy and glioma. Conversely, the collection of human electroencephalography (EEG) data has long been the standard of care in these patients, enabling individualized insights for therapy and revealing fundamental principles of human neurophysiology. Increasing interest exists in simultaneously measuring neurochemical biomarkers and electrophysiological data to enhance our understanding of human disease mechanisms. This review compares microdialysis, microperfusion, and implanted EEG probe architectures and performance parameters. Invasive consequences of probe implantation are also investigated along with the functional impact of biofouling. Finally, previously developed microdialysis electrodes and microperfusion electrodes are reviewed in preclinical and clinical settings. Critically, current and precedent microdialysis and microperfusion probes lack the ability to collect neurochemical data that is spatially and temporally coincident with EEG data derived from depth electrodes. This ultimately limits diagnostic and therapeutic progress in epilepsy and glioma research. However, this gap also provides a unique opportunity to create a dual-sensing technology that will provide unprecedented insights into the pathogenic mechanisms of human neurologic disease.

Bimodal neural probe for highly co-localized chemical and electrical monitoring of neural activities in vivo

Investigation of the chemical and electrical signals of cells in vivo is critical for studying functional connectivity and brain diseases. Most previous studies have observed either the electrical signals or the chemical signals of cells because recording electrical signals and neurochemicals are done by fundamentally different methods. Herein, we present a bimodal MEMS neural probe that is monolithically integrated with an array of microelectrodes for recording electrical activity, microfluidic channels for sampling extracellular fluid, and a microfluidic interface chip for multiple drug delivery and sample isolation from the localized region at the cellular level. In this work, we successfully demonstrated the functionality of our probe by monitoring and modulating bimodal (electrical and chemical) neural activities through the delivery of chemicals in a co-localized brain region in vivo. We expect our bimodal probe to provide opportunities for a variety of in-depth studies of brain functions as well as for the investigation of neural circuits related to brain diseases.

Collecting antibodies and large molecule biomarkers in mouse interstitial brain fluid: a comparison of microdialysis and cerebral open flow microperfusion

The determination of concentrations of large therapeutic molecules, like monoclonal antibodies (mAbs), in the interstitial brain fluid (ISF) is one of the cornerstones for the translation from preclinical species to humans of treatments for neurodegenerative diseases. Microdialysis (MD) and cerebral open flow microperfusion (cOFM) are the only currently available methods for extracting ISF, and their use and characterization for the collection of large molecules in rodents have barely started. For the first time, we compared both methods at a technical and performance level for measuring ISF concentrations of a non-target-binding mAb, trastuzumab, in awake and freely moving mice. Without correction of the data for recovery, concentrations of samples are over 10-fold higher through cOFM compared to MD. The overall similar pharmacokinetic profile and ISF exposure between MD (corrected for recovery) and cOFM indicate an underestimation of the absolute concentrations calculated with in vitro recovery. In vivo recovery (zero-flow rate method) revealed an increased extraction of trastuzumab at low flow rates and a 6-fold higher absolute concentration at steady state than initially calculated with the in vitro recovery. Technical optimizations have significantly increased the performance of both systems, resulting in the possibility of sampling up to 12 mice simultaneously. Moreover, strict aseptic conditions have played an important role in improving data quality. The standardization of these complex methods makes the unraveling of ISF concentrations attainable for various diseases and modalities, starting in this study with mAbs, but extending further in the future to RNA therapeutics, antibody-drug conjugates, and even cell therapies.

Multimaterial and multifunctional neural interfaces: from surface-type and implantable electrodes to fiber-based devices

Development of neural interfaces from surface electrodes to fibers with various type, functionality, and materials.

Droplets for Sampling and Transport of Chemical Signals in Biosensing: A Review

The chemical, temporal, and spatial resolution of chemical signals that are sampled and transported with continuous flow is limited because of Taylor dispersion. Droplets have been used to solve this problem by digitizing chemical signals into discrete segments that can be transported for a long distance or a long time without loss of chemical, temporal or spatial precision. In this review, we describe Taylor dispersion, sampling theory, and Laplace pressure, and give examples of sampling probes that have used droplets to sample or/and transport fluid from a continuous medium, such as cell culture or nerve tissue, for external analysis. The examples are categorized, as follows: (1) Aqueous-phase sampling with downstream droplet formation; (2) preformed droplets for sampling; and (3) droplets formed near the analyte source. Finally, strategies for downstream sample recovery for conventional analysis are described.

Use and Future Prospects of in Vivo Microdialysis for Epilepsy Studies

Epilepsy is a common neurological disease characterized by recurrent unpredictable seizures. For the last 30 years, microdialysis sampling has been used to measure changes in excitatory and inhibitory neurotransmitter concentrations before, during, and after seizures. These advances have fostered breakthroughs in epilepsy research by identifying neurochemical changes associated with seizures and correlating them to electrophysiological data. Recent advances in methodology may be useful in further delineating the chemical underpinnings of seizures. A new model of ictogenesis has been developed that allows greater control over the timing of seizures that are similar to spontaneous seizures. This model will facilitate making chemical measurements before and during a seizure. Recent advancements in microdialysis sampling, including the use of segmented flow, "fast" liquid chromatography (LC), and capillary electrophoresis with laser-induced fluorescence (CE-LIF) have significantly improved temporal resolution to better than 1 min, which could be used to measure transient, spontaneous neurochemical changes associated with seizures. Microfabricated sampling probes that are markedly smaller than conventional probes and allow for a much greater spatial resolution have been developed. They may allow the targeting of specific brain regions important to epilepsy studies. Coupling microdialysis sampling to optogenetics and light-stimulated release of neurotransmitters may also prove useful for studying epileptic seizures.

A miniaturized push-pull-perfusion probe for few-second sampling of neurotransmitters in the mouse brain

Measuring biomolecule concentrations in the brain of living animals, in real time, is a challenging task, especially when detailed information at high temporal resolution is also required. Traditionally, microdialysis probes are used that generally have sampling areas in the order of about 1 mm2, and provide information on concentrations with a temporal resolution of at least several minutes. In this paper, we present a novel miniaturized push-pull perfusion sampling probe that uses an array of small 3 μm-wide sampling channels to sample neurotransmitters at a typical recovery rate of 61%, with a reduced risk of clogging. The added feature to segment the dialysate inside the probe into small water-in-decane droplets enables the detection of concentrations with a temporal resolution of a few seconds. Here we used the probe for in vivo recordings of neurotransmitter glutamate released upon electrical stimulation in the brain of a mouse to demonstrate the feasibility of the probe for real-time neurochemical brain analysis.

In Vivo Chemical Monitoring at High Spatiotemporal Resolution Using Microfabricated Sampling Probes and Droplet-Based Microfluidics Coupled to Mass Spectrometry

An essential approach for in vivo chemical monitoring is to use sampling probes coupled with analytical methods; however, this method traditionally has limited spatial and temporal resolution. To address this problem, we developed an analytical system that combines microfabricated push-pull sampling probes with droplet-based microfluidics. The microfabricated probe provides spatial resolution approximately 1000-fold better than that of common microdialysis probes. Microfabrication also facilitated integration of an extra channel into the probe for microinjection. We created microfluidic devices and interfaces that allowed manipulation of nanoliter droplet samples collected from the microfabricated probe at intervals of a few seconds. Use of droplet-based microfluidics prevented broadening of collected zones, yielding 6 s temporal resolution at 100 nL/min perfusion rates. Resulting droplets were analyzed by direct infusion nanoelectrospray ionization (nESI) mass spectrometry for simultaneous determination of glutamine, glutamate, γ-aminobutyric acid, and acetylcholine. Use of low infusion rates that enabled nESI (50 nL/min) was critical to allowing detection in the complex samples. Addition of ¹³C-labeled internal standards to the droplet samples was used for improved quantification. Utility of the overall system was demonstrated by monitoring dynamic chemical changes evoked by microinjection of high potassium concentrations into the brain of live rats. The results showed stimulated neurochemical release with rise times of 15 s. This work demonstrates the potential of coupling microfabricated sampling probes to droplet-based mass spectrometric assays for studying chemical dynamics in a complex microenvironment at high spatiotemporal resolution.

Microfabricated Probes for Studying Brain Chemistry: A Review

Probe techniques for monitoring in vivo chemistry (e.g., electrochemical sensors and microdialysis sampling probes) have significantly contributed to a better understanding of neurotransmission in correlation to behaviors and neurological disorders. Microfabrication allows construction of neural probes with high reproducibility, scalability, design flexibility, and multiplexed features. This technology has translated well into fabricating miniaturized neurochemical probes for electrochemical detection and sampling. Microfabricated electrochemical probes provide a better control of spatial resolution with multisite detection on a single compact platform. This development allows the observation of heterogeneity of neurochemical activity precisely within the brain region. Microfabricated sampling probes are starting to emerge that enable chemical measurements at high spatial resolution and potential for reducing tissue damage. Recent advancement in analytical methods also facilitates neurochemical monitoring at high temporal resolution. Furthermore, a positive feature of microfabricated probes is that they can be feasibly built with other sensing and stimulating platforms including optogenetics. Such integrated probes will empower researchers to precisely elucidate brain function and develop novel treatments for neurological disorders.

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.

In vivo neurochemical measurements in cerebral tissues using a droplet-based monitoring system

Direct collection of extracellular fluid (ECF) plays a central role in the monitoring of neurological disorders. Current approaches using microdialysis catheters are however drastically limited in term of temporal resolution. Here we show a functional in vivo validation of a droplet collection system included at the tip of a neural probe. The system comprises an advanced droplet formation mechanism which enables the collection of neurochemicals present in the brain ECF at high-temporal resolution. The probe was implanted in a rat brain and could successfully collect fluid samples organized in a train of droplets. A microfabricated target plate compatible with most of the surface-based detection methods was specifically developed for sample analysis. The time-resolved brain-fluid samples are analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The results provide a time evolution picture of the cerebral tissues neurochemical composition for selected elements known for their involvement in neurodegenerative diseases.

Numerical Modeling of Electroosmotic Push-Pull Perfusion and Assessment of Its Application to Quantitative Determination of Enzymatic Activity in the Extracellular Space of Mammalian Tissue

Many sampling methods have been developed to measure the extracellular concentrations of solutes in the extracellular space of mammalian tissue, e.g., brain. However, few have been used to quantitatively study the various processes, such as enzymatic degradation, that determines the fate of these solutes. For a method to be useful in this pursuit, it must be able to (1) perfuse tissue and collect the perfusate for quantitative analysis of the solutes introduced and reaction products produced, (2) control the average residence time of the active solutes, and (3) have the appropriate spatial resolution for the process of interest. Our lab previously developed a perfusion technique based on electroosmosis (EO), called EO push-pull perfusion (EOPPP), that is in principle suitable to meet these needs. However, much like the case for other sampling methods that came before, there are parameters that are needed for quantitative interpretation of data but that cannot be measured easily (or at all). In this paper, we present a robust finite element model that provides a deep understanding of fluid dynamics and mass transport in the EOPPP method, assesses the general applicability of EOPPP to studying enzyme activity in the ECS, and grants a simple approach to data treatment and interpretation to obtain, for example, Vmax and Km for an enzymatic reaction in the extracellular space of the tissue. This model is a valuable tool in optimizing and planning experiments without the need for costly experiments.

Neural probe combining microelectrodes and a droplet-based microdialysis collection system for high temporal resolution sampling

We propose a novel neural probe which combines microfluidic channels with recording and stimulation electrodes. The developed microfabrication approach enables the concentration of every active element such as electrodes and the sampling inlet in close proximity on the same surface. As a first approach, full functional validation is presented in this work (in vivo testing will be presented in the next study). Electrical characterization by impedance spectroscopy is performed in order to assess the electrode properties. An advanced experimental setup enabling the validation of the fluidic functions of the neural probe is also presented. It allowed the achievement of a high temporal resolution (170 ms) during sampling as a result of the integration of a T-junction droplet generator inside the probe. The droplets reached a volume of 0.84 nL and are separated by a non-aqueous phase (perfluoromethyldecalin, PFD). This probe represents an innovative tool for neuroscientists as it can be implanted in precise brain structures while combining electrical stimulation with sampling at a high temporal resolution.

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

Brain tissue slices are a common neuroscience model that allows relatively sophisticated analysis of neuronal networks in a simplified preparation. Most experimental methodology utilizes electrophysiological tools to probe these model systems. The work here demonstrates the adaptation of low-flow push-pull perfusion sampling (LFPS) to a brain slice system. LFPS is used to sample from the hippocampus of mouse brain slices. Perfusate amino acid levels are quantified following sampling with capillary electrophoresis. Glutamate was measured from the CA1 region of the hippocampus in slices taken from a cystine-glutamate transporter deletion mutant, xCT(-/-), and the background strain C57BL/6J. Sampling is performed over up to 6.5 h with standard tissue slice preparation and experimentation methods. Four amino acids were quantified to demonstrate the ability to perform LFPS and show good agreement with published literature. Perfusate glutamate levels are found to be significantly lower with xCT(-/-) slices (1.9(±0.5) μM) relative to controls (4.90(±1.1) μM). But, experiments with control slices show a significant decrease in glutamate over the 6 h sampling period that are not seen with xCT(-/-) slices. Increasing the LFPS sample collection rate during the first 90 min of sampling did not show a sampling artifact in perfusate glutamate content. Sampling immediately following slicing did not show an early increasing glutamate level that would be indicative of a significant contribution from blood or tissue damage. The data presented here show a complementarity to electrophysiological studies of tissue slices. The ability to characterize extracellular fluid chemical content with LFPS in these slices provides an alternative data stream for probing neurochemical signaling networks in brain tissue slices.

Experimental evaluation and computational modeling of tissue damage from low-flow push-pull perfusion sampling in vivo

BACKGROUND

Neurochemical monitoring via sampling probes is valuable for deciphering neurotransmission in vivo. Microdialysis is commonly used; however, the spatial resolution is poor.

NEW METHOD

Recently push-pull perfusion at low flow rates (50nL/min) has been proposed as a method for in vivo sampling from the central nervous system. Tissue damage from such probes has not been investigated in detail. In this work, we evaluated acute tissue response to low-flow push-pull perfusion by infusing the nuclear stains Sytox Orange and Hoechst 33342 through probes implanted in the striatum for 200min, to label damaged and total cells, respectively, in situ.

RESULTS

Using the damaged/total labeled cell ratio as a measure of tissue damage, we found that 33±8% were damaged within the dye region around a microdialysis probe. We found that low-flow push-pull perfusion probes damaged 24±4% of cells in the sampling area. Flow had no effect on the number of damaged cells for low-flow push-pull perfusion. Modeling revealed that shear stress and pressure gradients generated by the flow were lower than thresholds expected to cause damage. Comparison with existing methods.Push-pull perfusion caused less tissue damage but yielded 1500-fold better spatial resolution.

CONCLUSIONS

Push-pull perfusion at low flow rates is a viable method for sampling from the brain with potential for high temporal and spatial resolution. Tissue damage is mostly caused by probe insertion. Smaller probes may yield even lower damage.

Electroosmotic perfusion of tissue: sampling the extracellular space and quantitative assessment of membrane-bound enzyme activity in organotypic hippocampal slice cultures

This review covers recent advances in sampling fluid from the extracellular space of brain tissue by electroosmosis (EO). Two techniques, EO sampling with a single fused-silica capillary and EO push-pull perfusion, have been developed. These tools were used to investigate the function of membrane-bound enzymes with outward-facing active sites, or ectoenzymes, in modulating the activity of the neuropeptides leu-enkephalin and galanin in organotypic-hippocampal-slice cultures (OHSCs). In addition, the approach was used to determine the endogenous concentration of a thiol, cysteamine, in OHSCs. We have also investigated the degradation of coenzyme A in the extracellular space. The approach provides information on ectoenzyme activity, including Michaelis constants, in tissue, which, as far as we are aware, has not been done before. On the basis of computational evidence, EO push-pull perfusion can distinguish ectoenzyme activity with a ~100 μm spatial resolution, which is important for studies of enzyme kinetics in adjacent regions of the rat hippocampus.

Stimulation and release from neurons via a dual capillary collection device interfaced to mass spectrometry

Neuropeptides are cell to cell signaling molecules that modulate a wide range of physiological processes. Neuropeptide release has been studied in sample sizes ranging from single cells and neuronal clusters, to defined brain nuclei and large brain regions. We have developed and optimized cell stimulation and collection approaches for the efficient measurement of neuropeptide release from neuronal samples using a dual capillary system. The defining feature is a capillary that contains octadecyl-modified silica nanoparticles on its inner wall to capture and extract releasates. This collection capillary is inserted into another capillary used to deliver solutions that chemically stimulate the cells, with solution flowing up the inner capillary to facilitate peptide collection. The efficiency of peptide collection was evaluated using six peptide standards mixed in physiological saline. The extracted peptides eluted from these capillaries were characterized via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) with low femtomole detection limits. Using the capillary collection system in small custom-fabricated culturing chambers, individual cultured neurons and neuronal clusters from the model animal Aplysia californica were stimulated with distinct neuronal secretagogues and the releasates were collected and characterized using MALDI-TOF MS.

In vivo measurement of extravasation of silver nanoparticles into liver extracellular space by push-pull-based continuous monitoring system

With the increasing prevalence of silver nanoparticles (AgNPs) in various products, whether such AgNPs will introduce new injury mechanisms from new pathologies remains to be determined. From the toxicokinetic viewpoint, it is vital to have in-depth knowledge of their in vivo transport kinetics and extravasation phenomenon. By combining push-pull perfusion sampling, in-tube solid phase extraction, and inductively coupled plasma mass spectrometry, we used an in vivo push-pull-based continuous monitoring system to investigate in vivo transport kinetics of extracellular AgNPs in living rat liver with a detection limit and temporal resolution of 0.64μgL(-1) and 10min, respectively. Before administration into living rats, the pre-incubation in DMEM with 10% FBS for 8h was adopted as the optimized exposure condition for the used AgNPs. After repeated-dose treatments, we observed a higher concentration of AgNPs in the liver extracellular space, suggesting that AgNP clearance by the reticuloendothelial system (RES) may be blocked by a prior administration of AgNPs. Future studies on AgNP distribution in different liver compartments (blood stream, extracellular space and Kupffer cells/hepatocytes) are necessary for defining the risks and benefits of AgNP applications.

Long-term implanted cOFM probe causes minimal tissue reaction in the brain

This study investigated the histological tissue reaction to long-term implanted cerebral open flow microperfusion (cOFM) probes in the frontal lobe of the rat brain. Most probe-based cerebral fluid sampling techniques are limited in application time due to the formation of a glial scar that hinders substance exchange between brain tissue and the probe. A glial scar not only functions as a diffusion barrier but also alters metabolism and signaling in extracellular brain fluid. cOFM is a recently developed probe-based technique to continuously sample extracellular brain fluid with an intact blood-brain barrier. After probe implantation, a 2 week healing period is needed for blood-brain barrier reestablishment. Therefore, cOFM probes need to stay in place and functional for at least 15 days after implantation to ensure functionality. Probe design and probe materials are optimized to evoke minimal tissue reaction even after a long implantation period. Qualitative and quantitative histological tissue analysis revealed no continuous glial scar formation around the cOFM probe 30 days after implantation and only a minor tissue reaction regardless of perfusion of the probe.

Monitoring of in vivo manipulation of nitric oxide synthases at the rat retina using the push-pull perfusion sampling and capillary electrophoresis

Proteins play a variety of functional roles in tissues that underlie tissue health. The measurement of protein function is important to both understand normal and dysfunctional tissue states. Low-flow push-pull perfusion sampling (LFPS) has been used to collect submicroliter volumes of extracellular fluid which are well suited to capillary electrophoresis for compositional quantitative analysis. In this study, LFPS is used to deliver pharmacological agents to the in vivo retinal tissues at the probe sampling tip during sampling to measure protein function. Two native nitric oxide synthase enzymes were pharmacologically inhibited and the enzyme product NO metabolite, nitrate, was determined with capillary electrophoresis from the perfusates. LFPS delivered inhibitors including the non-selective N(G)-nitro-Larginine methyl ester (L-NAME), the nNOS selective 7-nitroindazole (7-NI), and eNOS N5-(1-imioethyl)-L-ornithine, dihydrochloride (L-NIO) were perfused to the sampling region either directly over a rat retina optic nerve head or 1-mm peripheral to the ONH. At the PONH, 65, 55 and 60% of baseline nitrate levels, respectively, were observed with inhibitor infusion. These are statistically significant (P<0.05) compared to saline drug infusion. However, infusion of the inhibitors to the ONH did lead to significant nitrate concentration decreases. This data suggests that the endogenous enzymes, nNOS and eNOS, are both spatially and functionally localized to the PONH at the in vivo rat retina.

Emerging trends in in vivo neurochemical monitoring by microdialysis

Mapping chemical dynamics in the brain of live subjects is a challenging but highly rewarding goal because it allows neurotransmitter fluctuations to be related to behavior, drug effects, and disease states. A popular method for such measurements is microdialysis sampling coupled to analytical measurements. This method has become well-established for monitoring low molecular weight neurotransmitters, metabolites, and drugs, especially in pharmacological and pharmacokinetic studies. Recent technological developments which improve the temporal and spatial resolution of the methods will enable it to be used for studying behavior and small brain nuclei. Better assays allow monitoring more neurotransmitters simultaneously. Extension to analysis of aggregating proteins like amyloid β is proving extremely useful for uncovering the roles of these molecules and how they contribute to neurodegenerative diseases.

Electroosmotic push-pull perfusion: description and application to qualitative analysis of the hydrolysis of exogenous galanin in organotypic hippocampal slice cultures

We demonstrate here a method that perfuses a small region of an organotypic hippocampal culture with a solution containing an enzyme substrate, a neuropeptide. Perfusate containing hydrolysis products is continually collected and subsequently analyzed for the products of the enzymatic degradation of the peptide substrate. The driving force for perfusion is an electric field. The fused silica capillaries used as "push" and "pull" or "source" and "collection" capillaries have a ζ-potential that is negative and greater in magnitude than the tissue's ζ-potential. Thus, depending on the magnitudes of particular dimensions, the electroosmotic flow in the capillaries augments the fluid velocity in the tissue. The flow rate is not directly measured; however, we determine it using a finite-element approach. We have determined the collection efficiency of the system using an all d-amino acid internal standard. The flow rates are low, in the nL/min range, and adjustable by controlling the current or voltage in the system. The collection efficiency of the d-amino acid peptide internal standard is variable, increasing with increased current and thus electroosmotic flow rate. The collection efficiency can be rationalized in the context of a Peclet number. Electroosmotic push-pull perfusion of the neuropeptide galanin (gal1-29) through the extracellular space of an organotypic hippocampal culture results in its hydrolysis by ectopeptidase reactions occurring in the extracellular space. The products of hydrolysis were identified by MALDI-MS. Experiments at two levels of current (8-12 μA and 19-40 μA) show that the probability of seeing hydrolysis products (apparently from aminopeptidases) is greater in the Cornu Ammonis area 3 (CA3) than in the Cornu Ammonis area 1 (CA1) in the higher current experiments. In the lower current experiments, shorter peptide products of aminopeptidases (gal13-29 to gal20-19) are seen with greater frequency in CA3 than in CA1 but there is no statistically significant difference for longer peptides (gal3-29 to gal12-29).

Assessment of tissue viability following electroosmotic push-pull perfusion from organotypic hippocampal slice cultures

We have developed a novel sampling technique that allows both introduction and removal of fluid from the extracellular space of living tissue. This method is based on the fluidics of push-pull perfusion but flow is driven by electroosmosis. We have applied this method to organotypic hippocampal cultures. A source capillary is inserted into the tissue and a collection capillary is in contact with the tissue surface through a thin layer of fluid. A voltage is applied across the proximal ends of source and collection capillary. In the applied field, fluid will move from source, into the tissue, and then be collected. In this process, damage to cells may occur. To understand better what sampling conditions influence damage most, we tested various sampling geometries and applied voltages, quantifying damage 16-24 h later using propidium iodide as a cell death marker. We found that damage correlates with both voltage drop and power dissipated in the tissue, but that voltage drop is a better indicator of damage when comparing models in which capillary arrangement and length are different.

Microfabricated sampling probes for in vivo monitoring of neurotransmitters

Microfabricated fluidic systems have emerged as a powerful approach for chemical analysis. Relatively unexplored is the use of microfabrication to create sampling probes. We have developed a sampling probe microfabricated in Si by bulk micromachining and lithography. The probe is 70 μm wide by 85 μm thick by 11 mm long and incorporates two buried channels that are 20 μm in diameter. The tip of the probe has two 20 μm holes where fluid is ejected or collected for sampling. Utility of the probe was demonstrated by sampling from the brain of live rats. For sampling, artificial cerebral spinal fluid was infused in through one channel at 50 nL/min while sample was withdrawn at the same flow rate from the other channel. Analysis of resulting fractions collected every 20 min from the striatum of rats by liquid chromatography with mass spectrometry demonstrated reliable detection of 17 neurotransmitters and metabolites. The small probe dimensions suggest it is less perturbing to tissue and can be used to sample smaller brain nuclei than larger sampling devices, such as microdialysis probes. This sampling probe may have other applications such as sampling from cells in culture. The use of microfabrication may also enable incorporation of electrodes for electrochemical or electrophysiological recording and other channels that enable more complex sample preparation on the device.

Chemical gradients within brain extracellular space measured using low flow push-pull perfusion sampling in vivo

Although populations of neurons are known to vary on the micrometer scale, little is known about whether basal concentrations of neurotransmitters also vary on this scale. We used low-flow push-pull perfusion to test if such chemical gradients exist between several small brain nuclei. A miniaturized polyimide-encased push-pull probe was developed and used to measure basal neurotransmitter spatial gradients within brain of live animals with 0.004 mm(3) resolution. We simultaneously measured dopamine (DA), norepinephrine, serotonin (5-HT), glutamate, γ-aminobutyric acid (GABA), aspartate (Asp), glycine (Gly), acetylcholine (ACh), and several neurotransmitter metabolites. Significant differences in basal concentrations between midbrain regions as little as 200 μm apart were observed. For example, dopamine in the ventral tegmental area (VTA) was 4.8 ± 1.5 nM but in the red nucleus was 0.5 ± 0.2 nM. Regions of high glutamate concentration and variability were found within the VTA of some individuals, suggesting hot spots of glutamatergic activity. Measurements were also made within the nucleus accumbens core and shell. Differences were not observed in dopamine and 5-HT in the core and shell; but their metabolites homovanillic acid (460 ± 60 nM and 130 ± 60 nM respectively) and 5-hydroxyindoleacetic acid (720 ± 200 nM and 220 ± 50 nM respectively) did differ significantly, suggesting differences in dopamine and 5-HT activity in these brain regions. Maintenance of these gradients depends upon a variety of mechanisms. Such gradients likely underlie highly localized effects of drugs and control of behavior that have been found using other techniques.

A multifunctional pipette

Microfluidics has emerged as a powerful laboratory toolbox for biologists, allowing manipulation and analysis of processes at a cellular and sub-cellular level, through utilization of microfabricated features at size-scales relevant to that of a single cell. In the majority of microfluidic devices, sample processing and analysis occur within closed microchannels, imposing restrictions on sample preparation and use. We present an optimized non-contact open-volume microfluidic tool to overcome these and other restrictions, through the use of a hydrodynamically confined microflow pipette, serving as a multifunctional solution handling and dispensing tool. The geometries of the tool have been optimised for use in optical microscopy, with integrated solution reservoirs to reduce reagent use, contamination risks and cleaning requirements. Device performance was characterised using both epifluorescence and total internal reflection fluorescence (TIRF) microscopy, resulting in ~200 ms and ~130 ms exchange times at ~100 nm and ~30 μm distances to the surface respectively.

Development and characterization of a microfluidic chamber incorporating fluid ports with active suction for localized chemical stimulation of brain slices

We report a novel microfluidic chamber incorporating fluid ports with active suction to achieve localized chemical stimulation of brain slices. A two-level soft-lithography process is used to fabricate fluid ports with integrated injection and suction holes that are connected to underlying microchannels. Fluorescence imaging, particle tracking velocimetry, and cell staining are used to characterize flows around a fluid port with or without active suction to validate effective localization of injected chemicals. To demonstrate biological applicability of the chamber, we show an induction of cortical spreading depression (CSD) waves in mouse brain slices through controlled focal delivery of potassium chloride solution.

Push-pull perfusion sampling with segmented flow for high temporal and spatial resolution in vivo chemical monitoring

Low-flow push-pull perfusion is a sampling method that yields better spatial resolution than competitive methods like microdialysis. Because of the low flow rates used (50 nL/min), it is challenging to use this technique at high temporal resolution which requires methods of collecting, manipulating, and analyzing nanoliter samples. High temporal resolution also requires control of Taylor dispersion during sampling. To meet these challenges, push-pull perfusion was coupled with segmented flow to achieve in vivo sampling at 7 s temporal resolution at 50 nL/min flow rates. By further miniaturizing the probe inlet, sampling with 200 ms resolution at 30 nL/min (pull only) was demonstrated in vitro. Using this method, L-glutamate was monitored in the striatum of anesthetized rats. Up to 500 samples of 6 nL each were collected at 7 s intervals, segmented by an immiscible oil and stored in a capillary tube. The samples were assayed offline for L-glutamate at a rate of 15 samples/min by pumping them into a reagent addition tee fabricated from Teflon where reagents were added for a fluorescent enzyme assay. Fluorescence of the resulting plugs was monitored downstream. Microinjection of 70 mM potassium in physiological buffered saline evoked l-glutamate concentration transients that had an average maxima of 4.5 ± 1.1 μM (n = 6 animals, 3-4 injections each) and rise times of 22 ± 2 s. These results demonstrate that low-flow push-pull perfusion with segmented flow can be used for high temporal resolution chemical monitoring and in complex biological environments.

Minimizing tissue damage in electroosmotic sampling

Electroosmotic sampling is a potentially powerful method for pulling extracellular fluid into a fused-silica capillary in contact with the surface of tissue. An electric field is created in tissue by passing current through an electrolyte-filled capillary and then through the tissue. The resulting field acts on the counterions to the surface charges in the extracellular space to create electroosmotic fluid flow within the extracellular space of a tissue. Part of the development of this approach is to define conditions under which electroosmotic sampling minimizes damage to the tissue, in this case organotypic hippocampal slice cultures (OHSCs). We have assessed tissue damage by measuring fluorescence resulting from exposing sampled tissue to propidium iodide solution 16-24 h after sampling. Sampling has been carried out with a variety of capillary diameters, capillary tip-tissue distances, and applied voltages. Tissue damage is negligible when the power (current x potential drop) created in the tissue is less than 120 microW. In practical terms, smaller capillary i.d.s, lower voltages, and greater tissue to capillary distances lead to lower power.

Integrated microfluidic probe station

The microfluidic probe (MFP) consists of a flat, blunt tip with two apertures for the injection and reaspiration of a microjet into a solution--thus hydrodynamically confining the microjet--and is operated atop an inverted microscope that enables live imaging. By scanning across a surface, the microjet can be used for surface processing with the capability of both depositing and removing material; as it operates under immersed conditions, sensitive biological materials and living cells can be processed. During scanning, the MFP is kept immobile and centered over the objective of the inverted microscope, a few micrometers above a substrate that is displaced by moving the microscope stage and that is flushed continuously with the microjet. For consistent and reproducible surface processing, the gap between the MFP and the substrate, the MFP's alignment, the scanning speed, the injection and aspiration flow rates, and the image capture need all to be controlled and synchronized. Here, we present an automated MFP station that integrates all of these functionalities and automates the key operational parameters. A custom software program is used to control an independent motorized Z stage for adjusting the gap, a motorized microscope stage for scanning the substrate, up to 16 syringe pumps for injecting and aspirating fluids, and an inverted fluorescence microscope equipped with a charge-coupled device camera. The parallelism between the MFP and the substrate is adjusted using manual goniometer at the beginning of the experiment. The alignment of the injection and aspiration apertures along the scanning axis is performed using a newly designed MFP screw holder. We illustrate the integrated MFP station by the programmed, automated patterning of fluorescently labeled biotin on a streptavidin-coated surface.

In vivo monitoring of the transfer kinetics of trace elements in animal brains with hyphenated inductively coupled plasma mass spectrometry techniques

The roles of metal ions to sustain normal function and to cause dysfunction of neurological systems have been confirmed by various studies. However, because of the lack of adequate analytical method to monitor the transfer kinetics of metal ions in the brain of a living animal, research on the physiopathological roles of metal ions in the CNS remains in its early stages and more analytical efforts are still needed. To explicitly model the possible links between metal ions and physiopathological alterations, it is essential to develop in vivo monitoring techniques that can bridge the gap between metalloneurochemistry and neurophysiopathology. Although inductively coupled plasma mass spectrometry (ICP-MS) is a very powerful technique for multiple trace element analyses, when dealing with chemically complex microdialysis samples, the detection capability is largely limited by instrumental sensitivity, selectivity, and contamination that arise from the experimental procedure. As a result, in recent years several high efficient and clean on-line sample pretreatment systems have been developed and combined with microdialysis and ICP-MS for the continuous and in vivo determination of the concentration-time profiles of metal ions in the extracellular space of rat brain. This article reviews the research relevant to the development of analytical techniques for the in vivo determination of dynamic variation in the concentration levels of metal ions in a living animal.

Microfluidic chip for high efficiency electrophoretic analysis of segmented flow from a microdialysis probe and in vivo chemical monitoring

An effective method for in vivo chemical monitoring is to couple sampling probes, such as microdialysis, to online analytical methods. A limitation of this approach is that in vivo chemical dynamics may be distorted by flow and diffusion broadening during transfer from sampling probe to analytical system. Converting a homogeneous sample stream to segmented flow can prevent such broadening. We have developed a system for coupling segmented microdialysis flow with chip-based electrophoresis. In this system, the dialysis probe is integrated with a PDMS chip that merges dialysate with fluorogenic reagent and segments the flow into 8-10 nL plugs at 0.3-0.5 Hz separated by perfluorodecalin. The plugs flow to a glass chip where they are extracted to an aqueous stream and analyzed by electrophoresis with fluorescence detection. The novel extraction system connects the segmented flow to an electrophoresis sampling channel by a shallow and hydrophilic extraction bridge that removes the entire aqueous droplet from the oil stream. With this approach, temporal resolution was 35 s and independent of distance between sampling and analysis. Electrophoretic analysis produced separation with 223,000 +/- 21,000 theoretical plates, 4.4% RSD in peak height, and detection limits of 90-180 nM for six amino acids. This performance was made possible by three key elements: (1) reliable transfer of plug flow to a glass chip; (2) efficient extraction of aqueous plugs from segmented flow; (3) electrophoretic injection suitable for high efficiency separation with minimal dilution of sample. The system was used to detect rapid concentration changes evoked by infusing glutamate uptake inhibitor into the striatum of anesthetized rats. These results demonstrate the potential of incorporating segmented flow into separations-based sensing schemes for studying chemical dynamics in vivo with improved temporal resolution.

Review of recent advances in analytical techniques for the determination of neurotransmitters

Methods and advances for monitoring neurotransmitters in vivo or for tissue analysis of neurotransmitters over the last five years are reviewed. The review is organized primarily by neurotransmitter type. Transmitter and related compounds may be monitored by either in vivo sampling coupled to analytical methods or implanted sensors. Sampling is primarily performed using microdialysis, but low-flow push-pull perfusion may offer advantages of spatial resolution while minimizing the tissue disruption associated with higher flow rates. Analytical techniques coupled to these sampling methods include liquid chromatography, capillary electrophoresis, enzyme assays, sensors, and mass spectrometry. Methods for the detection of amino acid, monoamine, neuropeptide, acetylcholine, nucleoside, and soluble gas neurotransmitters have been developed and improved upon. Advances in the speed and sensitivity of these methods have enabled improvements in temporal resolution and increased the number of compounds detectable. Similar advances have enabled improved detection at tissue samples, with a substantial emphasis on single cell and other small samples. Sensors provide excellent temporal and spatial resolution for in vivo monitoring. Advances in application to catecholamines, indoleamines, and amino acids have been prominent. Improvements in stability, sensitivity, and selectivity of the sensors have been of paramount interest.

Recent trends in microdialysis sampling integrated with conventional and microanalytical systems for monitoring biological events: a review

Microdialysis (MD) is a sampling technique that can be employed to monitor biological events both in vivo and in vitro. When it is coupled to an analytical system, microdialysis can provide near real-time information on the time-dependent concentration changes of analytes in the extracellular space or other aqueous environments. Online systems for the analysis of microdialysis samples enable fast, selective and sensitive analysis while preserving the temporal information. Analytical methods employed for online analysis include liquid chromatography (LC), capillary (CE) and microchip electrophoresis and flow-through biosensor devices. This review article provides an overview of microdialysis sampling and online analysis systems with emphasis on in vivo analysis. Factors that affect the frequency of analysis and, hence, the temporal resolution of these systems are also discussed.

Mass spectrometry-based neurochemical analysis: perspectives for primate research

The analysis of neurochemicals from the brain represents a challenge for current analytical techniques due to a variety of factors, such as compositional complexity, limited amounts of sample and endogenous inferences. Advances in mass spectrometry (MS) provide great opportunities for the sensitive measurement of neurochemicals, offering benefits including simple sample preparation, broad capability for analysis of diverse compounds and rich structural information of analytes. Until recently, however, limited numbers of studies have reported on the analysis of small molecular neurochemicals, such as classical neurotransmitters, in part due to the difficulties in separation of polar molecules by using current chromatography techniques with MS-compatible conditions. By contrast, MS has become an indispensable tool for neuropeptide analysis , offering tremendous potential in the discovery of novel signaling peptides and biomarkers. This review covers recent advances in MS-based neurochemical analysis , including a comparison with related detection techniques, chromatographic separation and neuropeptide discovery. Issues relating to in vivo sample collection and sample preparation are discussed. To provide a wider view of the capability of MS in basic neuroscience and clinical research, we discuss MS-based neurochemical analysis conducted in different animal models and humans. We specifically highlight perspectives for the use of MS for brain functional studies and drug discovery in nonhuman primates.

High temporal resolution coupling of low-flow push-pull perfusion to capillary electrophoresis for ascorbate analysis at the rat vitreoretinal interface

A system is presented demonstrating the high-temporal resolution coupling of low-flow push-pull perfusion sampling (LFPS) to capillary electrophoresis for the absorbance measurement of ascorbate at the rat vitreoretinal interface. This system holds all separation components at a low pressure as the means for withdrawing sample during LFPS. The system uses a flow-gated interface to directly couple the withdrawal capillary from the LFPS probe to a separation capillary and eliminates the need for any offline sample handling. The temporal resolution of the system was limited by injection time and is less than 16 s. This high temporal resolution was applied to the monitoring of in vivo ascorbate levels at the rat vitreoretinal interface. Baseline concentrations of ascorbate were found to be 86 microM +/- 18 microM at the vitreoretinal interface. Baseline concentrations matched well with those obtained for the postmortem bulk vitreous analysis. Upon stimulation with 145 mM K(+), a maximum increase in baseline values between 32-107% for n = 3 was observed. This system demonstrates the first in vivo temporal study of ascorbate at the rat vitreoretinal interface.