Detection of basal acetylcholine in rat brain microdialysate

Liquid chromatographic determination of residual hydrogen peroxide in pharmaceutical excipients using platinum and wired enzyme electrodes

Hydrogen peroxide (H(2)O(2)) is a chemically reactive reagent that can oxidize and degrade many pharmaceutical compounds under normal conditions. Unfortunately, H(2)O(2) is often introduced into pharmaceutical excipients during manufacturing and it may significantly affect the chemical stability of drugs in formulations. Thus, a sensitive analytical method for determination of residual H(2)O(2) in excipients is of importance in formulation development and product quality control. A liquid chromatographic system with a dual channel electrochemical detector (LCEC) was equipped with either a platinum electrode or a wired peroxidase electrode for determination of H(2)O(2). The excipient (0.1 g) was dissolved in 10 ml of mobile phase and 5 microl of the dissolved solution was directly injected. The chromatographic run time for each sample was 1 min with a detection limit of 10 ng/ml (S/N=5) using the platinum electrode and 1 ng/ml (S/N=5) using the wired enzyme coated electrode, respectively. The peak purity was assured by comparing the peak ratios at different potentials for both the standard and the samples. The H(2)O(2) levels in different batches of PVP, PEG, and other surfactants from different manufacturers were determined and the values ranged from 0 to 244 ppm. The LCEC method is exceptionally fast, accurate and convenient for quantitation of low levels of residual H(2)O(2) in pharmaceutical formulation excipients.

Simultaneous Determination of Choline and Acetylcholine Based on a Trienzyme Chemiluminometric Biosensor in a Single Line Flow Injection System

A detector for the simultaneous determination of choline (Ch) and acetylcholine (ACh) based on a sensitive trienzyme chemiluminometric biosensor in a single line flow injection (FI) system is described. Immobilized choline oxidase (ChOx), immobilized peroxidase (POx), immobilized acetylcholinesterase, and coimmobilized ChOx/POx were packed, in turn, in a transparent ETFE tube (1 mm i.d., 75 cm) and the tube was placed in front of a photomultipier tube as a flow cell. Two-peak response was obtained by one injection of the sample solution. The first and second peaks were dependent on the concentrations of Ch and ACh, respectively. The influence of some experimental parameters such as flow rate, amounts of immobilized enzymes on the behavior of the sensor was studied in order to optimize the sensitivity, sample throughput and resolution. Calibration curves were linear at 1 - 1000 nM for Ch and 3 - 3000 nM for ACh. The sample throughput was 25/h without carryover. The FI system was applied to the simultaneous determination of Ch and ACh in rabbit brain tissue homogenates.

Differential cortical acetylcholine release in rats performing a sustained attention task versus behavioral control tasks that do not explicitly tax attention

The present study used microdialysis techniques to compare acetylcholine release in the frontoparietal cortex of rats performing in a task requiring sustained attention with that of rats performing in two control procedures. The two control procedures were a fixed-interval 9-s schedule of reinforcement assessing primarily the effects of operant responding and comparable reward rates, and an operant procedure designed to test the effects of lever extension to prompt responding. These two control procedures involved comparable sensory-motor and motivational variables to those of the sustained attention task, but did not explicitly tax attentional processes. Performance of the sustained attention task was associated with a significant increase in cortical acetylcholine efflux, reaching a maximum of nearly 140%. Performance of the two control procedures was associated with significantly smaller (approximately 50%) increases in cortical acetylcholine release. This robust dissociation between attentional and control performance-associated increases in cortical acetylcholine release resulted, in part, from the elimination of the pre-task transfer of the animals into the operant chambers and the associated increases in acetylcholine release observed in previous studies. The present results support the hypothesis that demands on attentional performance, as opposed to the frequency of lever pressing, reward delivery and other task-related variables, selectively activate the basal forebrain corticopetal cholinergic system.

High-performance liquid chromatography/tandem mass spectrometry assay for the rapid high sensitivity measurement of basal acetylcholine from microdialysates

A high-throughput liquid chromatography tandem mass spectrometry (LC/MS/MS) method has been developed for the analysis of acetylcholine (ACh) in brain dialysates. This separation of ACh is based on cation exchange chromatography with elution buffer consisting of a mixture of ammonium acetate, ammonium formate and acetonitrile. Using isocratic separation conditions, ACh was resolved within a minute and detected using tandem mass spectrometry in the positive ion electrospray mode. The limit of detection for ACh was found to be 1 fmol on column with a S/N ratio of 3:1. The assay has been used routinely for the measurement of ACh in brain dialysates from awake freely moving rats. Furthermore, separation conditions were modified to allow simultaneous measurement of ACh and the acetylcholine esterase inhibitor, neostigmine.

Stimulation of cortical acetylcholine release following blockade of ionotropic glutamate receptors in nucleus accumbens

In vivo microdialysis techniques were used to determine the ability of glutamate receptors within the nucleus accumbens to trans-synaptically modulate the basal forebrain cortical cholinergic system. Rats were implanted with a dialysis probe in the medial prefrontal cortex to measure changes in cortical acetylcholine efflux and in the ipsilateral nucleus accumbens to locally manipulate glutamate receptor activity. Intra-accumbens perfusion of the broad spectrum ionotropic glutamate receptor antagonist kynurentate (1.0, 5.0 mm) led to a dose-dependent increase (maximum of 200%) in cortical acetylcholine efflux. This stimulated efflux was reproduced with the intra-accumbens perfusion of the AMPA/kainate antagonist DNQX (0.1, 0.25, 2.5 mm; maximum increase of 200%) or the NMDA antagonist D-CPP (10.0, 100.0, 200 micro M; maximum increase of 400%). These results reveal a significant glutamatergic tone within the accumbens of awake rats and support the hypothesis that accumbens efferents to basal forebrain modulate the excitability of the basal forebrain cortical cholinergic system.

Development of an integrated capillary electrophoresis/sensor for L-ascorbic acid detection

A CE/biosensor for measuring ascorbic acid was developed by coupling a polyaniline optical sensor and capillary electrophoresis (CE). The capillary column was partially modified with a thin film of polyaniline redox sensitive material. Ascorbic acid was detected by monitoring the changes in optical absorbance occurring to the polyaniline film upon the reduction reaction. The sensor response (change in optical absorbance at 650 nm) is proportional to the concentration of ascorbic acid over a range of 2.5-250 mg/L and the response range has shown a clear dependence on the characteristics of the polymerized film. High specificity and sensitivity of the present method, low sample consumption, short times of response (ca. 2 min) and the reproducibility of the results demonstrate that the CE/polyaniline-sensor could be further employed in the study of the relation between the content of L-ascorbic acid in body fluids and clinical parameters, e.g., cell ageing.

The effects of manipulations of attentional demand on cortical acetylcholine release

In vivo microdialysis was used to measure acetylcholine (ACh) efflux in the frontoparietal cortex while rats performed in one of two operant tasks. One task was designed and validated to generate measures of sustained attention, while the other task was designed to minimize explicit demands on sustained attentional resources (low-demand task). Transferring animals from the baseline environment into the operant chambers robustly increased cortical ACh efflux regardless of subsequent task demands. Performance in the sustained attention task further increased frontoparietal ACh efflux, and these increases were not observed when animals were simply exposed to the operant chamber without task performance. Manipulations of the task parameters within a session, to either increase or decrease explicit demands on sustained attention, were not associated with fluctuations in ACh efflux. Unexpectedly, performance in the low-demand task was also associated with significant increases in ACh efflux that were similar to those observed during the sustained attention task. However, widespread depletions of cortical cholinergic inputs produced by intra-basalis infusions of 192 IgG-saporin failed to impair performance in the low-demand task, suggesting that cholinergic transmission is not necessary for performance in this task. The present results indicate that although a wider range of instrumental processes than previously hypothesized are associated with increases in cortical ACh release, the dependence of performance on the integrity of cortical cholinergic inputs may be limited to tasks with explicit attentional demands.

Distinct changes in cortical acetylcholine and noradrenaline efflux during contingent and noncontingent performance of a visual attentional task

Optimization of cognitive processing may depend on specific and distinct functions of the cortical cholinergic and noradrenergic systems. This investigation dissociates functions of cortical acetylcholine (ACh) and noradrenaline (NA) in arousal and visual attention by simultaneously measuring ACh and NA efflux in the rat prefrontal cortex during sustained attentional performance. The five-choice serial reaction time task was used to provide a continuous assessment of visuospatial attention. Previous studies using this task have established a critical role for the cortical cholinergic system in the detection of visual targets. However, selective lesions of the locus coeruleus noradrenergic system impair performance only when additional attentional demands are placed on the subject by distractors or temporally unpredictable targets. To test the hypothesis that the cortical noradrenergic system is particularly sensitive to novel task contingencies, we also assessed NA and ACh efflux in rats that been trained previously on the task but for whom the instrumental contingency coupling responding with stimulus detection and reward was abolished. Cortical ACh efflux showed a robust and task-related increase during established contingent performance. This response was significantly attenuated in noncontingent subjects, although it still exceeded pretask values. In contrast, NA efflux only increased transiently in contingent subjects after task onset but showed sustained elevations in noncontingent subjects on the first day when contingencies were changed. These data also implicate cortical ACh in aspects of attentional functioning but highlight a specific involvement of the cortical noradrenergic system in detecting shifts in the predictive relationship between instrumental action and reinforcement.

Dissociations between the effects of intra-accumbens administration of amphetamine and exposure to a novel environment on accumbens dopamine and cortical acetylcholine release

Previous research has demonstrated an interaction between the effects of amphetamine and exposure to a novel environment on the activity of neurons in the nucleus accumbens. Given a model in which these accumbens efferents gate the excitability of basal forebrain cholinergic corticopetal neurons, the administration of intra-accumbens amphetamine was hypothesized to potentiate the increase in cortical acetylcholine produced by introduction to a novel environment. Dual probe microdialysis revealed no synergistic interactions between exposure to a novel environment and amphetamine on nucleus accumbens dopamine or cortical acetylcholine efflux. This finding indicates that exposure to a novel environment failed to recruit the telencephalic activation of the nucleus accumbens presumably necessary to reveal modulatory effects of accumbens dopaminergic transmission on cortical acetylcholine release.

Amphetamine-stimulated cortical acetylcholine release: role of the basal forebrain

Systemic administration of amphetamine results in increases in the release of acetylcholine in the cortex. Basal forebrain mediation of this effect was examined in three experiments using microdialysis in freely-moving rats. Experiment 1 examined whether dopamine receptor activity within the basal forebrain was necessary for amphetamine-induced increase in cortical acetylcholine by examining whether intra-basalis perfusion of dopamine antagonists attenuates this increase. Systemic administration of 2.0 mg/kg amphetamine increased dopamine efflux within the basal forebrain nearly 700% above basal levels. However, the increase in cortical acetylcholine efflux following amphetamine administration was unaffected by intra-basalis perfusions of high concentrations of D1- (100 microM SCH 23390) or D2-like (100 microM sulpiride) dopamine receptor antagonists. Experiments 2 and 3 determined whether glutamatergic or GABAergic local modulation of the excitability of the basal forebrain cholinergic neurons influences the ability of systemic amphetamine to increase cortical acetylcholine efflux. In Experiment 2, perfusion of kynurenate (1.0 mM), a non-selective glutamate receptor antagonist, into the basal forebrain attenuated the increase in cortical acetylcholine produced by amphetamine. Experiment 3 revealed that positive modulation of GABAergic transmission by bilateral intra-basalis infusion of the benzodiazepine receptor agonist chlordiazepoxide (40 microg/hemisphere) also attenuated the amphetamine-stimulated increase in cortical acetylcholine efflux. These data suggest that amphetamine increases cortical acetylcholine release via a complex neuronal network rather than simply increasing basal forebrain D1 or D2 receptor activity.

Basal forebrain glutamatergic modulation of cortical acetylcholine release

The mediation of cortical ACh release by basal forebrain glutamate receptors was studied in awake rats fitted with microdialysis probes in medial prefrontal cortex and ipsilateral basal forebrain. Repeated presentation of a stimulus consisting of exposure to darkness with the opportunity to consume a sweetened cereal resulted in a transient increase in cortical ACh efflux. This stimulated release was dependent on basal forebrain glutamate receptor activity as intrabasalis perfusion with the ionotropic glutamate receptor antagonist kynurenate (1.0 mM) markedly attenuated darkness/cereal-induced ACh release. Activation of AMPA/kainate receptors by intrabasalis perfusion of kainate (100 microM) was sufficient to increase cortical ACh efflux even under basal (nonstimulated) conditions. This effect of kainate was blocked by coperfusion with the antagonist DNQX (0.1-5.0 mM). Stimulation of NMDA receptors with intrabasalis perfusion of NMDA (50 or 200 microM) did not increase basal cortical ACh efflux. However, perfusion of NMDA in rats following exposure to the darkness/cereal stimulus resulted in a potentiation of both the magnitude and duration of stimulated cortical ACh efflux. Moreover, intrabasalis perfusion of the higher dose of NMDA resulted in a rapid increase in cortical ACh efflux even before presentation of the darkness/cereal stimulus, suggesting an anticipatory change in the excitability of basal forebrain cholinergic neurons. These data demonstrate that basal forebrain glutamate receptors contribute to the stimulation of cortical ACh efflux in response to behavioral stimuli. The specific roles of basal forebrain glutamate receptor subtypes in mediating cortical ACh release differ and depend on the level of activity of basal forebrain cholinergic neurons.

Application of in vivo microdialysis to the study of cholinergic systems

The application of in vivo microdialysis to the study of acetylcholine (ACh) release has contributed greatly to our understanding of cholinergic brain systems. This article reviews standard experimental procedures for dialysis probe selection and implantation, perfusion parameters, neurochemical detection, and data analysis as they relate to microdialysis assessments of cholinergic function. Particular attention is focused on the unique methodological considerations that arise when in vivo microdialysis is dedicated expressly to the recovery and measurement of ACh as opposed to other neurotransmitters. Limitations of the microdialysis technique are discussed, as well as methodological adaptations that may prove useful in overcoming these limitations. This is followed by an overview of recent studies in which the application of in vivo microdialysis has been used to characterize the basic pharmacology and physiology of cholinergic neurons. Finally, the usefulness of the microdialysis approach for testing hypotheses regarding the cholinergic systems' involvement in cognitive processes is examined. It can be concluded that, in addition to being a versatile and practical method for studying the neurochemistry of cholinergic brain systems, in vivo microdialysis represents a valuable tool in our efforts to better comprehend ACh's underlying role in a variety of behavioral processes.

Separation methods used in the determination of choline and acetylcholine

Cholinergic neurotransmission has been the subject of intensive investigations in recent years due to increasing recognition of the importance of its roles in physiology, pathology and pharmacology. The fact that the disposition of a neurotransmitter may reflect its functional status has made the measurement of acetylcholine and/or its precursors and metabolites in biological fluids an integral part of cholinergic research. With evolving complexity in experimental approaches and designs, and correspondingly increasing demand on sensitivity, specificity and accuracy matching advancements in sophistication in analytical methods have been made. The present review attempts to survey the array of analytical techniques that have been adopted for the measurement of acetylcholine or its main precursor/metabolite choline ranging from simple bioassays, radioenzymatic assays, gas chromatography (GC) with flame ionization detection, GC with mass spectrometry (GC-MS) detection, high-performance liquid chromatography (HPLC) with electrochemical detection (ED), HPLC with MS (HPLC-MS) to the sophisticated combination of micro-immobilized enzymatic reactor, microbore HPLC and modified electrode technology for the detection of ultra-low levels with particular emphasis on the state of the art techniques.

Capillary electrophoresis coupled to biosensor detection

The present review highlights some modern aspects of biosensor revelation, a detection method which has already found a large number of applications in healthcare, food industry and environmental analysis. First, the concept of bio-recognition, which is at the heart of biosensor technology, is discussed, with emphasis on host-guest-like recognition mechanisms. This detection device has been successfully coupled, in its first applications, to chromatographic columns, which allow a high resolution of complex mixtures of analytes prior to interaction with the biosensing unit. The properties of the transducing elements, which should generate a signal (e.g., electrochemical, thermal, acoustic, optical) of proper intensity and of relative fast rise, are additionally evaluated and discussed. The review then focuses on potential applications of biosensing units in capillary electrophoresis (CE) devices. CE appears to be an excellent separation methodology to be coupled to biosensor detection, since it is based on miniaturized electrophoretic chambers, fast analysis times, complete automation in sample handling and data treatment and requires extremely small sample volumes. Although only a few applications of CE-based biosensors have been described up to the present, it is anticipated that this hyphenated technique could have a considerable expansion in the coming years.

Increased acetylcholine release in the rat medial prefrontal cortex during performance of a visual attentional task

Recent studies have suggested a functional link between cortical cholinergic output and attentional task demands, whereby acetylcholine (ACh) release is regulated according to the outcome of ongoing behaviour. To explore this hypothesis we measured ACh efflux in the rat medial prefrontal cortex (mPFC) during between-session manipulations of the cognitive demands of an attentional task. Rats were trained to detect visual stimuli in a five-choice serial reaction time task (5-CSRTT) which involves sustained and divided attention. Following habituation to tethering and implantation with a microdialysis probe in the mPFC, rats were tested in the 5-CSRTT for three consecutive days, with different lengths of stimulus duration. During performance of the 5-CSRTT we measured robust, reproducible, task-related increases in ACh release in the mPFC across all sessions. Variations of the stimulus duration from the standard 0.5 s resulted in the predicted behavioural effects (reductions and increases in choice accuracy with 0.25 s and 5 s, respectively), but there was no evidence of either greater changes in ACh release in the more demanding condition or smaller changes in the less demanding condition. By contrast, in the session with 5-s stimulus duration there was a positive correlation between prefrontal cortical ACh efflux and the total number of trials completed. In summary, the present study shows that ACh efflux in the rat mPFC is increased during performance of a 5-CSRTT, but has found no evidence to support a specific relationship between cholinergic cortical output and attentional performance.

In vivo microdialysis and reverse phase ion pair liquid chromatography/tandem mass spectrometry for the determination and identification of acetylcholine and related compounds in rat brain

A method using liquid chromatography/tandem mass spectrometry (LC/MS/MS) has been developed for the determination of basal acetylcholine (ACh) in microdialysate from the striatum of freely moving rats. A microdialysis probe was surgically implanted into the striatum of the rats and Ringer's solution was used as the perfusion medium at a flow rate of 2 microL per minute. The samples were then analyzed off-line by LC/MS/MS experiments. The separation of ACh and choline (Ch) was carried out using reverse phase ion pair liquid chromatography with heptafluorobutyric acid as a volatile ion pairing reagent. Analytes were detected by electrospray ionization tandem mass spectrometry in the positive ion mode. The detection limit for ACh was 1.4 fmol on column, which is at least three times lower than previously reported. Three quaternary ammonium compounds in the rat brain microdialysate were also identified by tandem mass spectrometry experiments in which the unknown mass spectra were compared with standard reference compounds. These compounds were identified as carnitine, acetylcarnitine and (3-carboxypropyl)trimethylammonium. This is the first known report of the compound (3-carboxypropyl)trimethylammonium being found in rat brain.

Simultaneous determination of acetylcholine, choline and physostigmine in microdialysis samples from rat hippocampus by microbore liquid chromatography/electrochemistry on peroxidase redox polymer coated electrodes

Microbore column liquid chromatography with post-column immobilized enzyme reactor (IMER) and electrochemical detection on redox polymer coated electrodes was used for detection of acetylcholine (ACh) and choline (Ch) in microdialysis samples. The sensitivity of the coated electrodes, decreased gradually by about 10%/day, with highest reduction of 30% within the first 16 h of use. A number of choline derivatives were tested as possible internal standards, of those acetylethylhomocholine (AEHCh) and butyrylcholine (BCh) were found the most suitable candidates since they both provided high enzymatic conversion in the IMER. Physostigmine produced a negative peak, possibly reflecting oxidation of eseroline--a decarbamoylated product of reversible reaction of physostigmine with immobilized acetylcholine esterase. The probes, implanted in the ventral hippocampi of awake rats were perfused at a flow-rate of 1.25 microl/min with Ringer solution containing 10 microM physostigmine or with artificial cerebrospinal fluid only. The concentrations of ACh in 10-microl samples at basal conditions were between 0.9-2.5 nM, whereas in the presence of physostigmine the ACh levels raised to 41-48 nM. Physostigmine concentration was reduced to 8.8 microM, indicating its in vivo delivery of about 12%. The coefficients of variation were reduced from 7.4% for external standard method after every sixth sample to 5.8% and 5.9% for internal standardization with AEHCh and BCh, respectively. The latter method shortened the total analysis time by about 15%, thus being especially suitable for continuous long-lasting off-line or on-line monitoring. Additionally, other endogenous cholines such as butyrylcholine or synthetic choline derivatives could be detected by the present method.

Disposable, enzymatically modified printed film carbon electrodes for use in the high-performance liquid chromatographic-electrochemical detection of glucose or hydrogen peroxide from immobilized enzyme reactors

Disposable screen-printed, film carbon electrodes (PFCE) were modified with cast-coated Osmium-polyvinylpyrridine-wired horse radish peroxidase gel polymer (Os-gel-HRP) to enable the detection of the reduction at 0 mV of hydrogen peroxide (H2O2) derived from a post-column immobilized enzyme reactor (IMER) containing acetylcholinesterase and choline oxidase. In another series of experiments PFCE were initially modified with cast-coated Os-gel-HRP and then treated with glucose oxidase in bovine serum albumin (BSA) and cross-linked with glutaraldehyde to form a bi-layer glucose-Os-gel-HRP PFCE. This bi-layer glucose-Os-gel-HRP PFCE generated a reduction current at 0 mV to H2O2 derived from the reaction of glucose oxidase and glucose in solution. These enzyme-modified PFCE were housed in a radial flow cell and coupled with cation-exchange liquid chromatographic methods to temporally separate substrates in solution for the determination of acetylcholine (ACh) and choline (Ch) in the first experimental series, or glucose in the second experimental series. These two disposable enzyme-modified PFCE exhibited linear current vs. substrate relations, were durable, being usable for approximately 40 determinations, and were sufficiently sensitive to be employed in biological sampling. Both assays utilized the same HPLC equipment. The limit of detection for ACh was 16 fmol/10 microl and that for glucose was 12 micromol/7.5 microl. ACh and Ch were measured from a microdialysate from the frontal cortex of a rat. Glucose in human urine was determined using the bi-layer glucose oxidase-Os-gel-HRP PFCE.

Diffuse transmission by acetylcholine in the CNS

Recent immunoelectron microscopic studies have revealed a low frequency of synaptic membrane differentiations on ACh (ChAT-immunostained) axon terminals (boutons or varicosities) in adult rat cerebral cortex, hippocampus and neostriatum, suggesting that, besides synaptic transmission, diffuse transmission by ACh prevails in many regions of the CNS. Cytological analysis of the immediate micro-environment of these ACh terminals, as well as currently available immunocytochemical data on the cellular and subcellular distribution of ACh receptors, is congruent with this view. At least in brain regions densely innervated by ACh neurons, a further aspect of the diffuse transmission paradigm is envisaged: the existence of an ambient level of ACh in the extracellular space, to which all tissue elements would be permanently exposed. Recent experimental data on the various molecular forms of AChE and their presumptive role at the neuromuscular junction support this hypothesis. As in the peripheral nervous system, degradation of ACh by the prevalent G4 form of AChE in the CNS would primarily serve to keep the extrasynaptic, ambient level of ACh within physiological limits, rather than totally eliminate ACh from synaptic clefts. Long-lasting and widespread electrophysiological effects imputable to ACh in the CNS might be explained in this manner. The notions of diffuse transmission and of an ambient level of ACh in the CNS could also be of clinical relevance, in accounting for the production and nature of certain cholinergic deficits and the efficacy of substitution therapies.

Determination of erythromycin in urine and plasma using microbore liquid chromatography with tris(2,2'-bipyridyl)ruthenium(II) electrogenerated chemiluminescence detection

Erythromycin is determined in both urine and plasma samples using microbore reversed-phase liquid chromatography with tris(2,2'-bipyridyl)ruthenium(II) [Ru(bpy)3(2+)] electrogenerated chemiluminescence (ECL) detection. Ru(bpy)3(2+) is included in the mobile phase thus eliminating band broadening caused by post-column reagent addition. Extra column band broadening is an important concern in microbore liquid chromatography due to the small peak volumes. Erythromycin was studied in both water and biological samples. The detection limit for erythromycin in standards is 0.01 microM or 50 fmol injected with a S/N of 3 and a linear working range that extends four orders of magnitude. Human urine and blood plasma were also studied. Urine samples were diluted and filtered before injection. Ultrafiltration was used to remove protein from blood plasma samples prior to injection. Erythromycin was selectively detected in the body fluid samples without any further sample preparation. The detection limits obtained for erythromycin in urine and plasma are 0.05 and 0.1 microM, respectively, for 5 microl injected on a 150x1 mm I.D. C18 column.

Detection of basal acetylcholine release in the microdialysis of rat frontal cortex by high-performance liquid chromatography using a horseradish peroxidase-osmium redox polymer electrode with pre-enzyme reactor

To determine the basal acetylcholine level in the dialysate of rat frontal cortex, a horseradish peroxidase-osmium redox polymer-modified glassy carbon electrode (HRP-GCE) was employed instead of the conventional platinum electrode used in high-performance liquid chromatography-electrochemical detection (HPLC-ED). In initial experiments, an oxidizable unknown compound interfered with the detection of basal acetylcholine release on HPLC-HRP-GCE. An immobilized peroxidase-choline oxidase precolumn (pre-reactor) was included in the HPLC system, to eliminate the interference from the unknown compound. This combination could detect less than 10 fmol of standard acetylcholine and basal acetylcholine levels in the dialysate from a conventional concentric design microdialysis probe, without the use of cholinesterase inhibitor, and may facilitate physiological investigation of cholinergic neuronal activity in the central nervous system.

Electrochemical detection in bioanalysis

This review article presents an overview of current research on the use of amperometric electrochemical detectors in bioanalytical chemistry. Topics covered include microdialysis and ultrafiltration membranes for in-vivo sampling; microbore liquid chromatography; capillary electrophoresis; enzyme, photochemical, and chemical post-column reactions; electrochemiluminescence; and thin-film electrode materials. Selected references are cited.