Continuous monitoring of extracellular glucose concentrations in the striatum of freely moving rats with an implanted glucose biosensor

We have used a glucose oxidase-based sensor implanted in the striatum of freely moving rats to determine the concentration of extracellular glucose in two distinct ways. With a modification of the zero net flux method, in which different concentrations of glucose are infused through a dialysis probe glued to the biosensor, we calculated the concentration at which there was no change in glucose current by regression analysis; this gave a concentration of 0.351 ± 0.016 mM. Calculating the concentration from the basal current and the in vitro calibration of the biosensor was not significantly different from this. The basal extracellular glucose concentration determined by either method remained constant over a period of several days. Infusion of 50 µM veratridine through the adjacent dialysis probe caused a steep decrease in glucose current as soon as the drug reached the brain in contrast to the delayed fall (7.5 min) seen with microdialysis in previous experiments from this laboratory. These results demonstrate that this biosensor provides a direct, real-time measure of the extracellular concentration of glucose

Real-time detection of carboplatin using a microfluidic system

A microfluidic sensor system based on a carbon nanotube-epoxy composite electrode was fabricated to allow detection of the presence of the anti-cancer drug carboplatin in healthy tissue in real time during chemotherapy. Detection of carboplatin was carried out by observing the effects of the drug on the differential pulse voltammetry of free purine bases using a novel carbon nanotube-epoxy composite electrode. In free solution these electrodes performed better than glassy carbon electrodes for oxidation of the free purine bases AMP and GMP, and than DNA-modified carbon nanotube-epoxy composite sensors for detection of carboplatin. On-line carboplatin detection was performed using a computer-controlled microfluidic platform. The methodology for on-line carboplatin detection was optimised in terms of the analysis time and of to allow repeated carboplatin measurement using the same electrode. Microdialysis sampling and our microfluidic platform were combined to give a proof of concept system for real-time carboplatin detection with a limit of detection of 0.014 mM carboplatin in the sampled media. This paper is dedicated to Craig Lunte’s pioneering work in analysis and microdialysis

Measurement of brain tissue oxygen at a carbon paste electrode can serve as an index of increases in regional cerebral blood flow

Simultaneous monitoring of tissue O2 and regional cerebral blood flow (rCBF) was performed in the striatum of freely-moving rats. Differential pulse amperometry and constant potential amperometry were used to monitor O2 levels at a carbon paste electrode (CPE), while rCBF values were obtained using the H2 clearance technique. Two forms of behavioural activation were studied and the resultant changes in tissue O2 and blood flow compared. Both tail pinch and induced grooming produced immediate and parallel increases in O2 and blood flow which returned to baseline on cessation of activity. These findings indicate that under conditions of physiological stimulation the direct voltammetric measurement of O2 in brain tissue with a CPE can be used as a reliable index of increases in rCBF, resulting in an improvement in time resolution from 5 min (H2 clearance) to <1 s (amperometry). Because tissue O2 is a balance between supply by the blood stream and utilisation by the cells, increases in O2 current are an index of increased blood flow only when supply significantly exceeds utilisation

The role of astrocytes and noradrenaline in neuronal glucose metabolism

In the classical model the energy requirements during neuronal activation are provided by the delivery of additional glucose directly into the extracellular compartment that results from the increase in local cerebral blood flow (rCBF). The present review proposes that astrocytes play a key role in the response to neuronal activation. Arginine for the synthesis of NO, which has a major role in the increase in rCBF, is released from astrocytes in response to stimulation of astrocytic glutamate receptors. The increased delivery of glucose by the blood stream enters astrocytes, where some of it is converted to glycogen. During neuronal activation there is a decrease in extracellular glucose owing to increased utilization followed by a delayed increase; this results from stimulation of astrocytic β-adrenergic receptors, which leads to a breakdown of glycogen and the export of glucose

An amperometric glucose-oxidase/poly(o-phenylenediamine) biosensor for monitoring brain extracellular glucose: in vivo characterisation in the striatum of freely-moving rats

Amperometric glucose biosensors based on the immobilization of glucose oxidase (GOx) on Pt electrodes with electropolymerized o-phenylenediamine (PPD) were implanted in the right striatum of freely-moving rats. Carbon paste electrodes for the simultaneous monitoring of ascorbic acid (AA) and/or tissue O2 were implanted in the left striatum. A detailed in vivo characterization of the Pt/PPD/GOx signal was carried out using various pharmacological manipulations. Confirmation that the biosensor responded to changing glucose levels in brain extracellular fluid (ECF) was obtained by intraperitoneal (i.p.) injection of insulin that caused a decrease in the Pt/PPD/GOx current, and local administion of glucose (1 mM) via an adjacent microdialysis probe that resulted in an increase in the biosensor current. An insulin induced increase in tissue O2 in the brain was also observed. Interference studies involved administering AA and subanaesthetic doses of ketamine i.p. Both resulted in increased extracellular AA levels with ketamine also causing an increase in O2. No significant change in the Pt/PPD/GOx current was observed in either case indicating that changes in O2 and AA, the principal endogenous interferents, have minimal effect on the response of these first generation biosensors. Stability tests over a successive 5-day period revealed no significant change in sensitivity. These in vivo results suggest reliable glucose monitoring in brain ECF

Association of seizures with cortical spreading depression and peri-infarct depolarisations in the acutely injured human brain

OBJECTIVE: To test the co-occurrence and interrelation of ictal activity and cortical spreading depressions (CSDs) - including the related periinfarct depolarisations in acute brain injury caused by trauma, and spontaneous subarachnoid and/or intracerebral haemorrhage. METHODS: 63 patients underwent craniotomy and electrocorticographic (ECoG) recordings were taken near foci of damaged cortical tissue for up to 10 days. RESULTS: 32 of 63 patients exhibited CSDs (5 to 75 episodes), and 11 had ECoGraphic seizure activity (1-81 episodes). Occurrence of seizures was significantly associated with CSD, as 10 of 11 patients with seizures also had CSD (p=0.007, 2-tailed Fishers exact test). Clinically overt seizures were only observed in one patient. Each patient with CSD and seizures displayed one of four different patterns of interaction between CSD and seizures. In four patients CSD was immediately preceded by prolonged seizure activity. In three patients the two phenomena were separated in time: multiple CSDs were replaced by ictal activity. In one patient seizures appeared to trigger repeated CSDs at the adjacent electrode. In two patients ongoing repeated seizures were interrupted each time CSD occurred. CONCLUSIONS: Seizure activity occurs in association with CSD in the injured human brain. SIGNIFICANCE: ECoG recordings in brain injury patients provide insight into pathophysiological mechanisms that is not accessible by scalp EEG recordings

Monitoring of metabolite gradients in tissue-engineered constructs

At present, the assessment of developing tissue-engineered constructs is almost always carried out destructively using biochemical or histological methods to determine cell number, viability and tissue growth throughout the construct. Since many of these experiments are long, taking weeks or even months to complete, simple and readily applicable non-destructive methods of monitoring changes in cell metabolism, viability and tissue deposition within the construct would be invaluable; such methods could point out adverse responses during the early stages of culture. Here, we describe the use of microdialysis for detecting local changes in cellular metabolism within a tissue-engineered construct. Three-dimensional constructs consisting of bovine articular chondrocytes entrapped in an alginate gel were cultured in a bioreactor for two weeks. Glucose and lactate were monitored by microdialysis, as the major nutrient and metabolite, respectively. Concentration gradients within the construct were evident, with the highest lactate concentrations in the construct centre. The local lactate concentration was a measure of cellular metabolic activity, decreasing as cellular activity fell and increasing as cellular activity was stimulated. Nutrient starvation and cell death in the construct centre could be readily detected in constructs deliberately cultured under adverse conditions. The results show that probe measurements can give an early warning of inappropriate local metabolic changes. Such information during the growth of tissue-engineered constructs would allow either corrective action or else an early end to an unsuccessful test