Enhancing Continuous Online Microdialysis Using Dexamethasone: Measurement of Dynamic Neurometabolic Changes during Spreading Depolarization

Computational Modeling as a Tool to Drive the Development of a Novel, Chemical Device for Monitoring the Injured Brain and Body

Real-time measurement of dynamic changes, occurring in the brain and other parts of the body, is useful for the detection and tracked progression of disease and injury. Chemical monitoring of such phenomena exists but is not commonplace, due to the penetrative nature of devices, the lack of continuous measurement, and the inflammatory responses that require pharmacological treatment to alleviate. Soft, flexible devices that more closely match the moduli and shape of monitored tissue and allow for surface microdialysis provide a viable alternative. Here, we show that computational modeling can be used to aid the development of such devices and highlight the considerations when developing a chemical monitoring probe in this way. These models pave the way for the development of a new class of chemical monitoring devices for monitoring neurotrauma, organs, and skin.

Dexamethasone-Enhanced Continuous Online Microdialysis for Neuromonitoring of O2 after Brain Injury

Traumatic brain injury (TBI) is a major public health crisis in many regions of the world. Severe TBI may cause a primary brain lesion with a surrounding penumbra of tissue that is vulnerable to secondary injury. Secondary injury presents as progressive expansion of the lesion, possibly leading to severe disability, a persistent vegetive state, or death. Real time neuromonitoring to detect and monitor secondary injury is urgently needed. Dexamethasone-enhanced continuous online microdialysis (Dex-enhanced coMD) is an emerging paradigm for chronic neuromonitoring after brain injury. The present study employed Dex-enhanced coMD to monitor brain K⁺ and O₂ during manually induced spreading depolarization in the cortex of anesthetized rats and after controlled cortical impact, a widely used rodent model of TBI, in behaving rats. Consistent with prior reports on glucose, O₂ exhibited a variety of responses to spreading depolarization and a prolonged, essentially permanent decline in the days after controlled cortical impact. These findings confirm that Dex-enhanced coMD delivers valuable information regarding the impact of spreading depolarization and controlled cortical impact on O₂ levels in the rat cortex.

Addressable microfluidics technology for non-sacrificial analysis of biomaterial implants in vivo

Tissue regeneration-promoting and drug-eluting biomaterials are commonly implanted into animals as a part of late-stage testing before committing to human trials required by the government. Because the trials are very expensive (e.g., they can cost over a billion U.S. dollars), it is critical for companies to have the best possible characterization of the materials' safety and efficacy before it goes into a human. However, the conventional approaches to biomaterial evaluation necessitate sacrificial analysis (i.e., euthanizing a different animal for measuring each time point and retrieving the implant for histological analysis), due to the inability to monitor how the host tissues respond to the presence of the material in situ. This is expensive, inaccurate, discontinuous, and unethical. In contrast, our manuscript presents a novel microfluidic platform potentially capable of performing non-disruptive fluid manipulations within the spatial constraints of an 8 mm diameter critical calvarial defect-a "gold standard" model for testing engineered bone tissue scaffolds in living animals. In particular, here, addressable microfluidic plumbing is specifically adapted for the in vivo implantation into a simulated rat's skull, and is integrated with a combinatorial multiplexer for a better scaling of many time points and/or biological signal measurements. The collected samples (modeled as food dyes for proof of concept) are then transported, stored, and analyzed ex vivo, which adds previously-unavailable ease and flexibility. Furthermore, care is taken to maintain a fluid equilibrium in the simulated animal's head during the sampling to avoid damage to the host and to the implant. Ultimately, future implantation protocols and technology improvements are envisioned toward the end of the manuscript. Although the bone tissue engineering application was chosen as a proof of concept, with further work, the technology is potentially versatile enough for other in vivo sampling applications. Hence, the successful outcomes of its advancement should benefit companies developing, testing, and producing vaccines and drugs by accelerating the translation of advanced cell culturing tech to the clinical market. Moreover, the nondestructive monitoring of the in vivo environment can lower animal experiment costs and provide data-gathering continuity superior to the conventional destructive analysis. Lastly, the reduction of sacrifices stemming from the use of this technology would make future animal experiments more ethical.

Validation of Dexamethasone-Enhanced Continuous-Online Microdialysis for Monitoring Glucose for 10 Days after Brain Injury

Traumatic brain injury (TBI) induces a pathophysiologic state that can be worsened by secondary injury. Monitoring brain metabolism with intracranial microdialysis can provide clinical insights to limit secondary injury in the days following TBI. Recent enhancements to microdialysis include the implementation of continuously operating electrochemical biosensors for monitoring the dialysate sample stream in real time and dexamethasone retrodialysis to mitigate the tissue response to probe insertion. Dexamethasone-enhanced continuous-online microdialysis (Dex-enhanced coMD) records long-lasting declines of glucose after controlled cortical impact in rats and TBI in patients. The present study employed retrodialysis and fluorescence microscopy to investigate the mechanism responsible for the decline of dialysate glucose after injury of the rat cortex. Findings confirm the long-term functionality of Dex-enhanced coMD for monitoring brain glucose after injury, demonstrate that intracranial glucose microdialysis is coupled to glucose utilization in the tissues surrounding the probes, and validate the conclusion that aberrant glucose utilization drives the postinjury glucose decline.

Fiber-Based Electrochemical Biosensors for Monitoring pH and Transient Neurometabolic Lactate

Developing tools that are able to monitor transient neurochemical dynamics is important to decipher brain chemistry and function. Multifunctional polymer-based fibers have been recently applied to monitor and modulate neural activity. Here, we explore the potential of polymer fibers comprising six graphite-doped electrodes and two microfluidic channels within a flexible polycarbonate body as a platform for sensing pH and neurometabolic lactate. Electrodes were made into potentiometric sensors (responsive to pH) or amperometric sensors (lactate biosensors). The growth of an iridium oxide layer made the fiber electrodes responsive to pH in a physiologically relevant range. Lactate biosensors were fabricated via platinum black growth on the fiber electrode, followed by an enzyme layer, making them responsive to lactate concentration. Lactate fiber biosensors detected transient neurometabolic lactate changes in an in vivo mouse model. Lactate concentration changes were associated with spreading depolarizations, known to be detrimental to the injured brain. Induced waves were identified by a signature lactate concentration change profile and measured as having a speed of ∼2.7 mm/min (n = 4 waves). Our work highlights the potential applications of fiber-based biosensors for direct monitoring of brain metabolites in the context of injury.

Recent Advances in In Vivo Neurochemical Monitoring

The brain is a complex network that accounts for only 5% of human mass but consumes 20% of our energy. Uncovering the mysteries of the brain's functions in motion, memory, learning, behavior, and mental health remains a hot but challenging topic. Neurochemicals in the brain, such as neurotransmitters, neuromodulators, gliotransmitters, hormones, and metabolism substrates and products, play vital roles in mediating and modulating normal brain function, and their abnormal release or imbalanced concentrations can cause various diseases, such as epilepsy, Alzheimer's disease, and Parkinson's disease. A wide range of techniques have been used to probe the concentrations of neurochemicals under normal, stimulated, diseased, and drug-induced conditions in order to understand the neurochemistry of drug mechanisms and develop diagnostic tools or therapies. Recent advancements in detection methods, device fabrication, and new materials have resulted in the development of neurochemical sensors with improved performance. However, direct in vivo measurements require a robust sensor that is highly sensitive and selective with minimal fouling and reduced inflammatory foreign body responses. Here, we review recent advances in neurochemical sensor development for in vivo studies, with a focus on electrochemical and optical probes. Other alternative methods are also compared. We discuss in detail the in vivo challenges for these methods and provide an outlook for future directions.

Dexamethasone-Enhanced Microdialysis and Penetration Injury

Microdialysis probes, electrochemical microsensors, and neural prosthetics are often used for in vivo monitoring, but these are invasive devices that are implanted directly into brain tissue. Although the selectivity, sensitivity, and temporal resolution of these devices have been characterized in detail, less attention has been paid to the impact of the trauma they inflict on the tissue or the effect of any such trauma on the outcome of the measurements they are used to perform. Factors affecting brain tissue reaction to the implanted devices include: the mechanical trauma during insertion, the foreign body response, implantation method, and physical properties of the device (size, shape, and surface characteristics. Modulation of the immune response is an important step toward making these devices with reliable long-term performance. Local release of anti-inflammatory agents such as dexamethasone (DEX) are often used to mitigate the foreign body response. In this article microdialysis is used to locally deliver DEX to the surrounding brain tissue. This work discusses the immune response resulting from microdialysis probe implantation. We briefly review the principles of microdialysis and the applications of DEX with microdialysis in (i) neuronal devices, (ii) dopamine and fast scan cyclic voltammetry, (iii) the attenuation of microglial cells, (iv) macrophage polarization states, and (v) spreading depolarizations. The difficulties and complexities in these applications are herein discussed.

Real-time in vivo detection techniques for neurotransmitters: a review

Functional synapses in the central nervous system depend on a chemical signal exchange process that involves neurotransmitter delivery between neurons and receptor cells in the neuro system.

Portable Microfluidic Biosensing System for Real-Time Analysis of Microdialysate in Transplant Kidneys

Currently, there is a severe shortage of donor kidneys that are fit for transplantation, due in part to a lack of adequate viability assessment tools for transplant organs. This work presents the integration of a novel wireless two-channel amperometric potentiostat with microneedle-based glucose and lactate biosensors housed in a 3D printed chip to create a microfluidic biosensing system that is genuinely portable. The wireless potentiostat transmits data via Bluetooth to an Android app running on a tablet. The whole miniaturized system is fully enclosed and can be integrated with microdialysis to allow continuous monitoring of tissue metabolite levels in real time. We have also developed a wireless portable automated calibration platform so that biosensors can be calibrated away from the laboratory and in transit. As a proof of concept, we have demonstrated the use of this portable analysis system to monitor porcine kidneys for the first time from organ retrieval, through warm ischemia, transportation on ice, right through to cold preservation and reperfusion. The portable system is robust and reliable in the challenging conditions of the abattoir and during kidney transportation and can detect clear physiological changes in the organ associated with clinical interventions.

Extended (10-Day) Real-Time Monitoring by Dexamethasone-Enhanced Microdialysis in the Injured Rat Cortex

Intracerebral microdialysis has proven useful for chemical monitoring in patients following traumatic brain injury. Recent studies in animals, however, have documented that insertion of microdialysis probes into brain tissues initiates a foreign-body response. Within a few days after probe insertion, the foreign body response impedes the use of microdialysis to monitor the K⁺ and glucose transients associated with spreading depolarization, a potential mechanism for secondary brain injury. Herein, we show that perfusing microdialysis probes with dexamethasone, a potent anti-inflammatory glucocorticoid, suppresses the foreign body response and facilitates the monitoring of spontaneous spreading depolarizations for at least 10 days following controlled cortical injury in the rat. In addition to spreading depolarizations, results of this study suggest that a progressive, apparently permanent, decline in pericontusional interstitial glucose may be an additional sequela of brain injury. This study establishes extended dexamethasone-enhanced microdialysis in the injured rodent cortex as a new paradigm for investigating trauma-induced metabolic crisis.

3D printed microfluidic device for online detection of neurochemical changes with high temporal resolution in human brain microdialysate

This paper presents the design, optimisation and fabrication of a mechanically robust 3D printed microfluidic device for the high time resolution online analysis of biomarkers in a microdialysate stream at microlitre per minute flow rates. The device consists of a microfluidic channel with secure low volume connections that easily integrates electrochemical biosensors for biomarkers such as glutamate, glucose and lactate. The optimisation process of the microfluidic channel fabrication, including for different types of 3D printer, is explained and the resulting improvement in sensor response time is quantified. The time resolution of the device is characterised by recording short lactate concentration pulses. The device is employed to record simultaneous glutamate, glucose and lactate concentration changes simulating the physiological response to spreading depolarisation events in cerebrospinal fluid dialysate. As a proof-of-concept study, the device is then used in the intensive care unit for online monitoring of a brain injury patient, demonstrating its capabilities for clinical monitoring.

Development of a non-human primate model to support CNS translational research: Demonstration with D-amphetamine exposure and dopamine response

BACKGROUND

Challenges specific to the discovery and development of candidate CNS drugs have led to implementation of various in silico, in vitro and in vivo approaches to improve the odds for commercialization of novel treatments.

NEW METHOD

Advances in analytical methodology and microdialysis probe design have enabled development of a non-human primate model capable of measuring concentrations of drugs or endogenous chemicals in brain extracellular fluid (ECF) and cerebrospinal fluid (CSF). Linking these to population modeling reduces animal numbers to support predictive translational sciences in primates. Application to measure D-amphetamine exposure and dopamine response in ECF and CSF demonstrate the approach.

RESULTS

Following a 0.1 mg/kg intravenous bolus dose of D-amphetamine, a population approach was used to build a plasma compartmental-based and brain physiologic-based pharmacokinetic (PK) model linking drug concentrations in plasma to brain ECF and CSF concentrations. Dopamine was also measured in brain ECF. The PK model was used to simulate the relationship between D-amphetamine exposure and dopamine response in ECF over a wide dose range.

COMPARISONS WITH EXISTING METHODS

Ability to co-sample and measure drug and endogenous substances in blood, brain ECF and/or CSF, coupled with population modeling, provides an in vivo approach to evaluate CNS drug penetration and effect in non-human primates.

CONCLUSIONS

A method to measure drug and endogenous neurochemicals in non-human primate brain fluids is demonstrated. Its basis in non-human primates merits improved confidence regarding predictions of drug exposure and target engagement in human CNS.

In vivo measurement of the dynamics of norepinephrine in an olfactory bulb following ischemia-induced olfactory dysfunction and its responses to dexamethasone treatment

Information on the dynamics of molecules following olfactory dysfunction remains essential for understanding the molecular events involved in the pathological process of olfactory dysfunction. This study for the first time demonstrates a method based on the combination of in vivo microdialysis with high performance liquid chromatography (HPLC) and electrochemical detection (ECD) for the measurement of the dynamics of norepinephrine (NE) in the olfactory bulbs of Sprague-Dawley rats following olfactory dysfunction induced by brain ischemia and its responses toward dexamethasone treatment. The method possesses a high spatial resolution and benefits from in vivo microdialysis and high selectivity and is thus capable of measuring NE in the olfactory bulb of rats. With this method, the basal level of NE in the olfactory bulb was evaluated to be ca. 235 ± 25 nM (n = 6). This level was found to increase by 260 ± 90% at a time point of 240 min after brain ischemia with bilateral ligation of both common carotid arteries. The increase was found to be suppressed upon the treatment of the animals with 0.2% dexamethasone in the olfactory bulb. These results suggest that NE is involved in the pathological process of ischemia-induced olfactory dysfunction and this information is useful to further understand the molecular events involved in olfactory dysfunction.

Electrokinetic infusions into hydrogels and brain tissue: Control of direction and magnitude of solute delivery

BACKGROUND

Delivering solutes to a particular region of the brain is currently achieved by iontophoresis for very small volumes and by diffusion from a microdialysis probe for larger volumes. There is a need to deliver solutes to particular areas with more control than is possible with existing methods.

NEW METHOD

Electrokinetic infusions of solutes were performed into hydrogels and organotypic hippocampal slice cultures. Application of an electrical current creates electroosmotic flow and electrophoresis of a dicationic fluorescent solute through organotypic hippocampal tissue cultures or larger hydrogels. Transport was recorded with fluorescence microscopy imaging in real-time.

RESULTS

Electrokinetic transport in brain tissue slice cultures and hydrogels occurs along an electrical current path and allows for anisotropic delivery over distances from several hundred micrometers to millimeters. Directional transport may be controlled by altering the current path. The applied electrical current linearly affects the measured solute fluorescence in our model system following infusions.

COMPARISON WITH EXISTING METHODS

Localized drug delivery involves iontophoresis, with diffusion primarily occurring beyond infusion capillaries under current protocols. Pressure-driven infusions for intraparenchymal targets have also been conducted. Superfusion across a tissue surface provides modest penetration, however is unable to impact deeper targets. In general, control over intraparenchymal drug delivery has been difficult to achieve. Electrokinetic transport provides an alternative to deliver solutes along an electrical current path in tissue.

CONCLUSIONS

Electrokinetic transport may be applied to living systems for molecular transport. It may be used to improve upon the control of solute delivery over that of pressure-driven transport.

Methods of Measuring Enzyme Activity Ex Vivo and In Vivo

Enzymes catalyze a variety of biochemical reactions in the body and, in conjunction with transporters and receptors, control virtually all physiological processes. There is great value in measuring enzyme activity ex vivo and in vivo. Spatial and temporal differences or changes in enzyme activity can be related to a variety of natural and pathological processes. Several analytical approaches have been developed to meet this need. They can be classified broadly as methods either based on artificial substrates, with the goal of creating images of diseased tissue, or based on natural substrates, with the goal of understanding natural processes. This review covers a selection of these methods, including optical, magnetic resonance, mass spectrometry, and physical sampling approaches, with a focus on creative chemistry and method development that make ex vivo and in vivo measurements of enzyme activity possible.

Measurement of Basal Neurotransmitter Levels Using Convolution-Based Nonfaradaic Current Removal

Fast-scan cyclic voltammetry permits robust subsecond measurements of in vivo neurotransmitter dynamics, resulting in its established use in elucidating these species’ roles in the actions of behaving animals. However, the technique’s limitations, namely the need for digital background subtraction for analytical signal resolution, have restricted the information obtainable largely to that about phasic neurotransmitter release on the second-to-minute time scale. The study of basal levels of neurotransmitters and their dynamics requires a means of isolating the portion of the background current arising from neurotransmitter redox reactions. Previously, we reported on the use of a convolution-based method for prediction of the resistive-capacitive portion of the carbon-fiber microelectrode background signal, to improve the information content of background-subtracted data. Here we evaluated this approach for direct analytical signal isolation. First, protocol modifications (i.e., applied waveform and carbon-fiber type) were optimized to permit simplification of the interfering background current to components that are convolution-predictable. It was found that the use of holding potentials of at least 0.0 V, as well as the use of pitch-based carbon fibers, improved the agreement between convolution predictions and the observed background. Subsequently, it was shown that measurements of basal dopamine concentrations are possible with careful control of the electrode state. Successful use of this approach for measurement of in vivo basal dopamine levels is demonstrated, suggesting the approach may serve as a useful tool in expanding the capabilities of fast-scan cyclic voltammetry.