On-line amperometric assay of glucose,L-glutamate, and acetylcholine using microdialysis probes and immobilized enzyme reactors

A novel method for simultaneous glutamate and extracellular activity measurement in brain slices with high temporal resolution

Measurement of neurotransmitters during normal or altered function in cerebral slices could be an important tool to better understand the relationship between biochemical changes and electrophysiological activity. Some attempts of this analysis have been made; however, the current techniques do not have the appropriate time resolution to establish this relationship. The use of electrochemical biosensors has allowed for good time resolution, but problems related to the reduction of signal noise and biofouling of the electrode surface could be an important issue. In this work, we propose a new alternative to simultaneously measure glutamate and electrical activity with a high temporal resolution in brain slices. This approach is based on the use of enzymatic reactors that generate a fluorescent derivative from glutamate that can be measured at high temporal resolution. The results presented here show a reliable measurement of this neurotransmitter in brain slices obtained from intact animals under the effect of a glutamate transporter blocker DL-threo-beta-benzyloxyaspartate as well as the potassium channel blocker 4-aminopyridine. Differences in the levels of glutamate and high frequency and amplitude discharges as an effect of drug administration were found in brain slices obtained from epileptic rats (p<0.05). In conclusion, this method could be used to measure neurotransmitter concentration online at a near physiological temporal resolution, which can then be correlated to the electrical activity that is simultaneously recorded.

RECENT DEVELOPMENTS IN ELECTROCHEMICAL SENSORS FOR THE DETECTION OF NEUROTRANSMITTERS FOR APPLICATIONS IN BIOMEDICINE

Neurotransmitters are important biological molecules that are essential to many neurophysiological processes including memory, cognition, and behavioral states. The development of analytical methodologies to accurately detect neurotransmitters is of great importance in neurological and biological research. Specifically designed microelectrodes or microbiosensors have demonstrated potential for rapid, real-time measurements with high spatial resolution. Such devices can facilitate study of the role and mechanism of action of neurotransmitters and can find potential uses in biomedicine. This paper reviews the current status and recent advances in the development and application of electrochemical sensors for the detection of small-molecule neurotransmitters. Measurement challenges and opportunities of electroanalytical methods to advance study and understanding of neurotransmitters in various biological models and disease conditions are discussed.

Prolonged exposure to ammonia increases extracellular glutamate in cultured rat astrocytes

Abnormal alteration of brain function is a characteristic complication of hepatic encephalopathy in both acute and chronic liver failure. Previous studies suggest that the pathogenesis of hepatic encephalopathy involves chronic glial edema with subsequent alteration of glioneuronal communication, N-methyl-d-aspartate (NMDA) receptor activation, and oxidative/nitrosative stress. In the present study, we investigated extracellular glutamate levels in cultured astrocytes under prolonged exposure to ammonia. Using an enzyme-linked high-performance liquid chromatography assay to detect glutamate, prolonged (48 h) exposure of cultured astrocytes to ammonia resulted in a concentration- and time-dependent increase in extracellular glutamate. Similar increases were observed when ammonia-containing medium (pH 7.8) was adjusted to the pH of control medium (pH 7.4), indicating that the effect is not due to pH. Treatment of astrocytes with an antioxidant (l-ascorbic acid), an NADPH oxidase inhibitor (apocynin), a Ca2+ chelator (BAPTA-AM), an NMDA receptor antagonist (NK801), or a mitochondrial permeability transition inhibitor (cyclosporine A) suppressed the increase of extracellular glutamate in response to prolonged ammonia exposure. Prolonged exposure to ammonia increased extracellular glutamate through the NMDA receptor, increased intracellular Ca2+ levels, and upregulation of excitatory amino acids. The addition of ATP further increased extracellular glutamate levels in astrocytes subjected to prolonged ammonia treatment (5mM, 48 h) in a dose-dependent manner. These results indicate that the deregulation of glutamate release from astrocytes may contribute to the dysfunction of glutamatergic neurons in patients with acute liver failure (ALF).

High-sensitive glutamate biosensor based on NADH at Lauth's violet/multiwalled carbon nanotubes composite film on gold substrates

A highly sensitive amperometric L-glutamate biosensor based on the electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide has been developed on Lauth's Violet (known as thionine)/multiwalled carbon nanotubes (Th-MWCNTs) composite film, which is used as a mediator and an enzyme immobilization matrix. The glutamate biosensor, which is fabricated by immobilizing glutamate dehydrogenase (GLDH) on the surface of Th-MWCNTs, displayed a precipitous response (ca. 3 s), a low detection limit (15.9 nM), a wide linear dynamic range (0.1 to 500 microM), and high sensitivity of 281.6 microAmM(-1) cm(-2), higher biological affinity, as well as good stability and repeatability. Interferences from other biological compounds were also studied for the fabricated sensor. The Th-MWCNTs system exemplifies a simple and efficient approach to the assimilation of GLDH and electrodes, which can provide analytical access to a large group of enzymes for wide range of bioelectrochemical applications in health care fields.

Simultaneous determination of L-glutamate, acetylcholine and dopamine in rat brain by a flow-injection biosensor system with microdialysis sampling

A flow-injection biosensor system with an on-line microdialysis sampling system is proposed for the simultaneous detection of neurotransmitters (L-glutamate, acetylcholine and dopamine) released from rat brain cells. The dialysate collected in the sample loop from the microdialysis probe was automatically injected into the flow-injection line with a triple electrode arranged perpendicular to the flow direction. The triple electrode was constructed by hybridizing a poly(1,2-diaminobenzene) film to two enzyme sensing-parts which respond to L-glutamate and acetylcholine, and by coating a Nafion film on a remaining sensing part which responds to dopamine, respectively, without any cross-reactivity. The three sensing parts of the triple electrode responded linearly to the concentrations of L-glutamate and acetylcholine in the range of 0.002-5 mM and to that of dopamine in the range of 0.002-20 mM, respectively, without any interference from oxidizable species present in the dialysate. The proposed flow-injection analytical method could be applied to an in vivo assay of these neurotransmitters released from rat-brain cells by the continuous KCl stimulation.

Low potential detection of glutamate based on the electrocatalytic oxidation of NADH at thionine/single-walled carbon nanotubes composite modified electrode

A glutamate biosensor based on the electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH), which was generated by the enzymatic reaction, was developed via employing a single-walled carbon nanotubes/thionine (Th-SWNTs) nanocomposite as a mediator and an enzyme immobilization matrix. The biosensor, which was fabricated by immobilizing glutamate dehydrogenase (GlDH) on the surface of Th-SWNTs, exhibited a rapid response (ca. 5s), a low detection limit (0.1 microM), a wide and useful linear range (0.5-400 microM), high sensitivity (137.3+/-15.7) microA mM(-1)cm(-2), higher biological affinity, as well as good stability and repeatability. In addition, the common interfering species, such as ascorbic acid, uric acid, and 4-acetamidophenol, did not cause any interference due to the use of a low operating potential (190 mV vs. NHE). The biosensor can be used to quantify the concentration of glutamate in the physiological level. The Th-SWNTs system represents a simple and effective approach to the integration of dehydrogenase and electrodes, which can provide analytical access to a large group of enzymes for wide range of bioelectrochemical applications including biosensors and biofuel cells.

Ocular blood flow changes after dynamic exercise in humans

PURPOSE

To investigate control mechanisms for ocular blood flow changes after dynamic exercise using two different methods.

METHODS

Changes over time in the tissue blood flow in the retina and choroid-retina of healthy volunteers were determined after dynamic exercise (Master's double two-step test), using scanning laser Doppler flowmetry (SLDF) and laser speckle flowgraphy (LSFG). Changes in intraocular pressure (IOP), blood pressure, plasma CO(2) gas concentration (pCO(2)), and levels of nitric oxide (NO) metabolites were examined.

RESULTS

Retinal blood flow measured by SLDF increased significantly only at 15 min after exercise. In contrast, normalized blur (NB) values in the choroid-retina, obtained by LSFG, increased significantly up to 60 min after exercise. Ocular perfusion pressure (OPP), calculated from IOP and blood pressure, increased significantly immediately and 15 min after exercise. The plasma NO metabolite levels increased significantly, although pCO(2) levels were unchanged.

CONCLUSIONS

Dynamic exercise changes OPP and produces increased tissue blood flow in the retina in the immediate postexercise period, while blood flow increases more persistently in the choroid-retina. Difference in control of blood flow in these two regions may be related to stronger autoregulatory mechanism of blood flow in the retina. Nitric oxide may play a role in the regulation of blood flow.

Carbon fibre-based microbiosensors for in vivo measurements of acetylcholine and choline

This report describes technical improvements to the manufacture of a carbon fibre electrode for the stable and sensitive detection of H2O2 (detection limit at 0.5 microM). This electrode was also modified through the co-immobilisation of acetylcholinesterase (AChE) and/or choline oxidase (ChOx) in a bovine serum albumin (BSA) membrane for the development of a sensor for in vivo measurements of acetylcholine and choline. Amperometric measurements were performed using a conventional three-electrode system forming part of a flow-injection set-up at an applied potential of 800-1100 mV relative to an Ag/AgCl reference electrode. The optimised biosensor obtained was reproducible and stable, and exhibited a detection limit of 1 microM for both acetylcholine and choline. However, due to the high operating potential used, the biosensor was prone to substantial interference from other electroactive compounds, such as ascorbic acid. Therefore, in a further step, a mediated electron transfer approach was used that incorporated horseradish peroxidase into an osmium-based redox hydrogel layered onto the active surface of the electrode. Afterwards, a Nafion layer and a coating containing AChE and/or ChOx co-immobilised in a BSA membrane were successively deposited. This procedure further increased the selectivity of the biosensor, when operated in the same flow-injection system but at an applied potential of -50 mV relative to an Ag/AgCl reference electrode. The sensor exhibited good selectivity and a high sensitivity over a concentration range (0.3-100 microM) suitable for the measurement of choline and acetylcholine in vivo.

Simultaneous determination of glucose and L-lactate in rat brain by an electrochemical in vivo flow-injection system with an on-line microdialysis sampling

An electrochemical in vivo flow-injection system with an on-line microdialysis sampling is proposed for the simultaneous monitoring of L-lactate and glucose in rat brain. In the first stage of the operation, the dialysate from the microdialysis probe is delivered to a sample loop of the six-way autoinjector by perfusing Ringer's solution for 80 s at 5 microl min(-1). In the second stage, the dialysate collected in the sample loop is automatically injected for 10 s into the flow-injection line. Injected dialysate is split into two streams and two portions pass through two channels with two different immobilized enzyme reactors (glucose oxidase and lactate oxidase immobilized reactors) to produce hydrogen peroxide from glucose and L-lactate in the dialysate. After a subsequent confluence of the streams, produced hydrogen peroxide can be detected amperometrically at a downstream poly(1,2-diaminobenzene) film-coated platinum electrode, without any interference from oxidizable species and proteins present in the dialysate. Because each channel has a different residence time, two peaks are obtained. The first peak corresponds to L-lactate and the second peak to glucose. The peak current is linearly related to the concentrations of L-lactate between 0.2 and 10 mM and glucose between 0.1 and 20 mM. The present method can be successfully applied to the simultaneous in vivo monitoring of L-lactate and glucose in rat brain. The analytical speed is 45 dialysates h(-1).

Flow-through chemiluminescence sensor using immobilized oxidases for the selective determination of L-glutamate in a flow-injection system

A selective and sensitive chemiluminometric flow sensor for the determination of L-glutamate in serum, based on immobilized oxidases such as glutamate oxidase (GOD), uricase (UC) and peroxidase (POD), is described herein. The principle for the selective chemiluminometric detection for L-glutamate is based on coupled reactions of four sequentially aligned immobilized oxidases, UC/POD/GOD/POD in a flow cell. The immobilized UC was employed to decompose urate, which is one of the major interfering components in serum for a luminol-H2O2 chemiluminescence reaction. The H2O2 produced from the UC reaction readily reacted with reducing components, such as ascorbate and glutathione, and then the excess H2O2 was decomposed by the immobilized POD. L-Glutamate in the sample plug was enzymatically converted to H2O2 with immobilized GOD. Subsequently, the peroxide reacts with luminol on the immobilized POD to produce chemiluminescence, proportional to glutamate concentration. The enzymes were immobilized on tresylated poly(vinyl alcohol beads). The immobilized enzymes were packed into TPFE tube (1.0 mm i.d. x 60 cm), in turn, and used as a flow cell. The sampling rate was 30 h-1. The calibration graph for L-glutamate is linear for 20 nM-5 microM; the detection limit (signal-to-noise = 3) is 10 nM.

Micro-flow in vivo analysis of L-glutamate with an on-line enzyme amplifier based on substrate recycling

A micro-flow enzyme system with a microdialysis probe is proposed for the amperometric detection of trace amounts of neurotransmitter L-glutamate released from rat brain cells. The L-glutamate oxidase (EC 1.4.3.11)/glutamate dehydrogenase (EC 1.4.1.4) coimmobilized reactor was used to enhance the sensitivity of L-glutamate as an on-line amplifier based on substrate recycling. A poly(1,2-diaminobenzene) film-coated platinum electrode was also used to selectively detect only the hydrogen peroxide generated into a upstream enzyme reactor, without interference from oxidizable species, such as L-ascorbate, the adsorption of low molecular-weight proteins in a dialysate, and NADPH added to the carrier solution to initiate substrate recycling. By the present in vivo system, L-glutamate was selectively assayed with about a 600-fold increase in sensitivity compared with the unamplified responses. The detection limit was 0.08 mumol dm-3. This method was applied to an in vivo assay of L-glutamate in the extracellular space of rat brain; also, monitoring of the L-glutamate level changed after a continuous stimulation of KCl to demonstrate the reliability of the system.