Fiachra B. Bolger

An integrative dynamic model of brain energy metabolism using in vivo neurochemical measurements

An integrative, systems approach to the modelling of brain energy metabolism is presented. Mechanisms such as glutamate cycling between neurons and astrocytes and glycogen storage in astrocytes have been implemented. A unique feature of the model is its calibration using in vivo data of brain glucose and lactate from freely moving rats under various stimuli. The model has been used to perform simulated perturbation experiments that show that glycogen breakdown in astrocytes is significantly activated during sensory (tail pinch) stimulation. This mechanism provides an additional input of energy substrate during high consumption phases. By way of validation, data from the perfusion of 50 µM propranolol in the rat brain was compared with the model outputs. Propranolol affects the glucose dynamics during stimulation, and this was accurately reproduced in the model by a reduction in the glycogen breakdown in astrocytes. The model’s predictive capacity was verified by using data from a sensory stimulation (restraint) that was not used for model calibration. Finally, a sensitivity analysis was conducted on the model parameters, this showed that the control of energy metabolism and transport processes are critical in the metabolic behaviour of cerebral tissue.

Characterisation of carbon paste electrodes for real-time amperometric monitoring of brain tissue oxygen.

Tissue O₂ can be monitored using a variety of electrochemical techniques and electrodes. In vitro and in vivo characterisation studies for O₂ reduction at carbon paste electrodes (CPEs) using constant potential amperometry (CPA) are presented. Cyclic voltammetry indicated that an applied potential of -650 mV is required for O₂ reduction at CPEs. High sensitivity (-1.49 ± 0.01 nA/μM), low detection limit (ca. 0.1 μM) and good linear response characteristics (R² > 0.99) were observed in calibration experiments performed at this potential. There was also no effect of pH, temperature, and ion changes, and no dependence upon flow/fluid convection (stirring). Several compounds (e.g. dopamine and its metabolites) present in brain extracellular fluid were tested at physiological concentrations and shown not to interfere with the CPA O₂ signal. In vivo experiments confirmed a sub-second response time observed in vitro and demonstrated long-term stability extending over twelve weeks, with minimal O₂ consumption (ca. 1 nmol/h). These results indicate that CPEs operating amperometrically at a constant potential of -650 mV (vs. SCE) can be used reliably to continuously monitor brain extracellular tissue O₂.

An investigation of hypofrontality in an animal model of schizophrenia using real-time microelectrochemical sensors for glucose, oxygen, and nitric oxide.

Glucose, O2, and nitric oxide (NO) were monitored in real time in the prefrontal cortex of freely moving animals using microelectrochemical sensors following phencyclidine (PCP) administration. Injection of saline controls produced a decrease in glucose and increases in both O2 and NO. These changes were short-lived and typical of injection stress, lasting ca. 30 s for glucose and between 2 and 6 min for O2 and NO, respectively. Subchronic PCP (10 mg/kg) resulted in increased motor activity and increases in all three analytes lasting several hours: O2 and glucose were uncoupled with O2 increasing rapidly following injection reaching a maximum of 70% (ca. 62 μM) after ca. 15 min and then slowly returning to baseline over a period of ca. 3 h. The time course of changes in glucose and NO were similar; both signals increased gradually over the first hour post injection reaching maxima of 55% (ca. 982 μM) and 8% (ca. 31 nM), respectively, and remaining elevated to within 1 h of returning to baseline levels (after ca. 5 and 7 h, respectively). While supporting increased utilization of glucose and O2 and suggesting overcompensating supply mechanisms, this neurochemical data indicates a hyperfrontal effect following acute PCP administration which is potentially mediated by NO. It also confirms that long-term in vivo electrochemical sensors and data offer a real-time biochemical perspective of the underlying mechanisms.

In vitro development and in vivo application of a platinum-based electrochemical device for continuous measurements of peripheral tissue oxygen

Acute limb ischaemia is caused by compromised tissue perfusion and requires immediate attention to reduce the occurrence of secondary complications that could lead to amputation or death. To address this, we have developed a novel platinum (Pt)-based electrochemical oxygen (O2) device for future applications in clinical monitoring of peripheral tissue ischaemia. The effect of integrating a Pt pseudo-reference electrode into the O2 device was investigated in vitro with an optimum reduction potential of -0.80V. A non-significant (p=0.11) decrease in sensitivity was recorded when compared against an established Pt-based O2 sensor operating at -0.65V. Furthermore, a biocompatible clinical sensor (ClinOX) was designed, demonstrating excellent linearity (R2=0.99) and sensitivity (1.41±0.02nAμM-1) for O2 detection. Significant rapid decreases in the O2 current during in vivo ischaemic insults in rodent limbs were reported for Pt-Pt (p<0.001) and ClinOX (p<0.01) and for ClinOX (p<0.001) in porcine limbs. Ex vivo sensocompatibility investigations identified no significant difference (p=0.08) in sensitivity values over 14days of exposure to tissue homogenate. The Pt-Pt based O2 design demonstrated high sensitivity for tissue ischaemia detection and thus warrants future clinical investigation.

Real-Time In Vivo Sensing of Neurochemicals

The brain is one of the most complex biological structures known to science. How it works or, more specifically, how the physical brain gives rise to the properties of mind remains an unanswered question. However, it is clear that many drugs used empirically in the treatment of neurological disorders, such as Parkinson’s disease, work through their specific chemical actions on nerve cells in the brain. Thus, if we are to understand brain function and drug performance, there is a need to measure chemical signalling in the brain. Measurement technologies for neurochemical studies in the living brain include spectroscopy, such as NMR, sampling techniques, such as cerebral microdialysis, and the topic of this chapter - in situ electrochemical monitoring using long-term in vivo electrochemistry (LIVE). With LIVE, a microvoltammetric sensor is implanted in a specific brain region to monitor local changes in the concentration of specific substances in the extracellular fluid. It can do this with sub-second time resolution and with measurement periods extending over many hours, potentially days. Spatially, localised, high-temporal resolution, long-term sensing of this kind allows investigations of the functions of chemicals in neuronal signalling, drug actions and well-defined behaviours. In this chapter, we give an overview of the different electrochemical sensor types, the techniques used and the principal neurochemicals potentially associated with Parkinson’s disease that can be measured in vivo.