Detection of basal acetylcholine in rat brain microdialysate

Muscarinic acetylcholine receptor 3 localized to primary endothelial cilia regulates blood pressure and cognition

We previously demonstrated that the inability of primary endothelial cilia to sense fluid shear stress can lead to nitric oxide (NO) deficiency and cause hypertension (HTN). Decreased biosynthesis of NO contributes to cerebral amyloid angiopathy in Alzheimer's disease (AD) patients through increased deposition of amyloid beta (Aβ). However, the molecular mechanisms underlying the pathogenesis of HTN and AD are incompletely understood. The objective of this study was to examine the pathophysiological roles of vascular primary cilia and muscarinic acetylcholine receptor 3 (CHRM3) in HTN and AD. We discovered, for the first time, that CHRM3 was localized to primary cilia of endothelial and cerebrovascular cells, and that CHRM3 expression was downregulated in cilialess cells. Moreover, CHRM3 activation enhanced cilia length and sensory function in terms of eNOS activation. To further examine the role of vascular CHRM3 in vivo, we showed that endothelial CHRM3 knockout was associated with increased BP and attenuated acetylcholine-mediated vascular relaxation. In addition, endothelial CHRM3 knockout resulted in altered fear behavior. This demonstrates the physiological significance of endothelial CHRM3 signaling and primary cilia-derived NO production as an important mechanism in the control of BP and cognition.

Detection of Acetylcholine at Nanoscale NPOE/Water Liquid/Liquid Interface Electrodes

The interface between two immiscible electrolyte solutions (ITIES) has become a very powerful analytical platform for sensing a diverse range of chemicals (e.g., metal ions and neurotransmitters) with the advantage of being able to detect non-redox electroactive species. The ITIES is formed between organic and aqueous phases. Organic solvent identity is crucial to the detection characteristics of the ITIES [half-wave transfer potential (E_1/2), potential window range, limit of detection, transfer coefficient (α), standard heterogeneous ion-transfer rate constant (k⁰), etc.]. Here, we demonstrated, for the first time at the nanoscale, the detection characteristics of the NPOE/water ITIES. Linear detection of the diffusion-limited current at different concentrations of acetylcholine (ACh) was demonstrated with cyclic voltammetry (CV) and i-t amperometry. The E_1/2 of ACh transfer at the NPOE/water nanoITIES was -0.342 ± 0.009 V versus the E_1/2 of tetrabutylammonium (TBA⁺). The limit of detection of ACh at the NPOE/water nanoITIES was 37.1 ± 1.5 μM for an electrode with a radius of ∼127 nm. We also determined the ion-transfer kinetics parameters, α and k⁰, of TBA⁺ at the NPOE/water nanoITIES by fitting theoretical cyclic voltammograms to experimental voltammograms. This work lays the basis for future cellular studies using ACh detection at the nanoscale and for studies to detect other analytes. The NPOE/water ITIES offers a potential window distinct from that of the 1,2-dichloroethane (DCE)/water ITIES. This unique potential window would offer the ability to detect analytes that are not easily detected at the DCE/water ITIES.

In Vivo Detection of Redox-Inactive Neurochemicals in the Rat Brain with an Ion Transfer Microsensor

Electrochemical tracking of redox-inactive neurochemicals remain a challenge due to chemical inertness, almost no Faraday electron transfer for these species, and the complex brain atmosphere. In this work, we demonstrate a low-cost, simple-making liquid/liquid interface microsensor (LLIM) to monitor redox-inactive neurochemicals in the rat brain. Taking choline (Ch) as an example, based on the difference in solvation energies of Ch in cerebrospinal fluid (aqueous phase) and 1,2-dichloroethane (1,2-DCE; organic phase), Ch is recognized in the specific ion-transfer potential and distinctive ion-transfer current signals. The LLIM has an excellent response to Ch with good linearity and selectivity, and the detection limit is 0.37 μM. The LLIM can monitor the dynamics of Ch in the cortex of the rat brain by both local microinfusion and intraperitoneal injection of Ch. This work first demonstrates that the LLIM can be successfully applied in the brain and obtain electrochemical signals in such a sophisticated system, allowing one new perspective of sensing at the liquid/liquid interface for nonelectrically active substances in vivo to understand the physiological function of the brain.

In Vivo Chemical Monitoring at High Spatiotemporal Resolution Using Microfabricated Sampling Probes and Droplet-Based Microfluidics Coupled to Mass Spectrometry

An essential approach for in vivo chemical monitoring is to use sampling probes coupled with analytical methods; however, this method traditionally has limited spatial and temporal resolution. To address this problem, we developed an analytical system that combines microfabricated push-pull sampling probes with droplet-based microfluidics. The microfabricated probe provides spatial resolution approximately 1000-fold better than that of common microdialysis probes. Microfabrication also facilitated integration of an extra channel into the probe for microinjection. We created microfluidic devices and interfaces that allowed manipulation of nanoliter droplet samples collected from the microfabricated probe at intervals of a few seconds. Use of droplet-based microfluidics prevented broadening of collected zones, yielding 6 s temporal resolution at 100 nL/min perfusion rates. Resulting droplets were analyzed by direct infusion nanoelectrospray ionization (nESI) mass spectrometry for simultaneous determination of glutamine, glutamate, γ-aminobutyric acid, and acetylcholine. Use of low infusion rates that enabled nESI (50 nL/min) was critical to allowing detection in the complex samples. Addition of ¹³C-labeled internal standards to the droplet samples was used for improved quantification. Utility of the overall system was demonstrated by monitoring dynamic chemical changes evoked by microinjection of high potassium concentrations into the brain of live rats. The results showed stimulated neurochemical release with rise times of 15 s. This work demonstrates the potential of coupling microfabricated sampling probes to droplet-based mass spectrometric assays for studying chemical dynamics in a complex microenvironment at high spatiotemporal resolution.

A multi-enzyme microreactor-based online electrochemical system for selective and continuous monitoring of acetylcholine

This study demonstrates an online electrochemical system (OECS) for selective and continuous measurements of acetylcholine (ACh) through efficiently integrating in vivo microdialysis, a multi-enzyme microreactor and an electrochemical detector.

An update of the classical and novel methods used for measuring fast neurotransmitters during normal and brain altered function

To understand better the cerebral functions, several methods have been developed to study the brain activity, they could be related with morphological, electrophysiological, molecular and neurochemical techniques. Monitoring neurotransmitter concentration is a key role to know better how the brain works during normal or pathological conditions, as well as for studying the changes in neurotransmitter concentration with the use of several drugs that could affect or reestablish the normal brain activity. Immediate response of the brain to environmental conditions is related with the release of the fast acting neurotransmission by glutamate (Glu), γ-aminobutyric acid (GABA) and acetylcholine (ACh) through the opening of ligand-operated ion channels. Neurotransmitter release is mainly determined by the classical microdialysis technique, this is generally coupled to high performance liquid chromatography (HPLC). Detection of neurotransmitters can be done by fluorescence, optical density, electrochemistry or other detection systems more sophisticated. Although the microdialysis method is the golden technique to monitor the brain neurotransmitters, it has a poor temporal resolution. Recently, with the use of biosensor the drawback of temporal resolution has been improved considerably, however other inconveniences have merged, such as stability, reproducibility and the lack of reliable biosensors mainly for GABA. The aim of this review is to show the important advances in the different ways to measure neurotransmitter concentrations; both with the use of classic techniques as well as with the novel methods and alternant approaches to improve the temporal resolution.

Mass spectrometry "sensor" for in vivo acetylcholine monitoring

Developing sensors for in vivo chemical monitoring is a daunting challenge. An alternative approach is to couple sampling methods with online analytical techniques; however, such approaches are generally hampered by lower temporal resolution and slow analysis. In this work, microdialysis sampling was coupled with segmented flow electrospray ionization mass spectrometry (ESI-MS) to perform in vivo chemical monitoring. The use of segmented flow to prevent Taylor dispersion of collected zones and rapid analysis with direct ESI-MS allowed 5 s temporal resolution to be achieved. The MS "sensor" was applied to monitor acetylcholine in the brain of live rats. The detection limit of 5 nM was sufficient to monitor basal acetylcholine as well as dynamic changes elicited by microinjection of neostigmine, an inhibitor of acetylcholinesterase, that evoked rapid increases in acetycholine and tetrodotoxin, a blocker of Na(+) channels, that lowered the acetylcholine concentration. The versatility of the sensor was demonstrated by simultaneously monitoring metabolites and infused drugs.

Aging-related deficits in orexin/hypocretin modulation of the septohippocampal cholinergic system

The medial septum (MS) of the basal forebrain contains cholinergic neurons that project to the hippocampus, support cognitive function, and are implicated in age-related cognitive decline. Hypothalamic orexin/hypocretin neurons innervate and modulate basal forebrain cholinergic neurons and provide direct inputs to the hippocampus. However, the precise role of orexin in modulating hippocampal cholinergic transmission--and how these interactions are altered in aging--is unknown. Here, orexin A was administered to CA1 and the MS of young (3-4 months) and aged (27-29 months) Fisher 344/Brown Norway rats, and hippocampal acetylcholine efflux was analyzed by in vivo microdialysis. At both infusion sites, orexin A dose-dependently increased hippocampal acetylcholine in young, but not aged rats. Moreover, immunohistochemical characterization of the MS revealed no change in cholinergic cell bodies in aged animals, but a significant decrease in orexin fiber innervation to cholinergic cells. These findings indicate that: (1) Orexin A modulates hippocampal cholinergic neurotransmission directly and transsynaptically in young animals, (2) Aged animals are unresponsive to orexin A, and (3) Aged animals undergo an intrinsic reduction in orexin innervation to cholinergic cells within the MS. Alterations in orexin regulation of septohippocampal cholinergic activity may contribute to age-related dysfunctions in arousal, learning, and memory.

High throughput enzyme inhibitor screening by functionalized magnetic carbonaceous microspheres and graphene oxide-based MALDI-TOF-MS

In this work, a high throughput methodology for screening enzyme inhibitors has been demonstrated by combining enzyme immobilized magnetic carbonaceous microspheres and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) with grapheme oxide as matrix. First, model enzyme acetylcholinesterase (AChE) was immobilized onto the 3-glycidoxypropyltrimethoxysilane (GLYMO)-modified magnetic carbonaceous (MC) microspheres, displaying a high enzyme activity and stability, and also facilitating the separation of enzyme from substrate and product. The efficiency of immobilized AChE was monitored by biochemical assay, which was carried out by mixing enzyme-immobilized MC microspheres with model substrate acetylcholine (ACh), and subsequent quantitative determination of substrate ACh and product choline using graphene oxide-based MALDI-TOF-MS with no background inference. The limit of detection (LOD) for ACh was 0.25 fmol/μL, and excellent linearity (R(2)=0.9998) was maintained over the range of 0.5 and 250 fmol/μL. Choline was quantified over the range of 0.05 and 15 pmol/μL, also with excellent linearity (R(2)=0.9994) and low LOD (0.15 fmol/μL). Good accuracy and precision were obtained for all concentrations within the range of the standard curves. All together, eight compounds (four known AChE inhibitors and four control chemical compounds with no AChE inhibit effect) were tested with our promoted methodology, and the obtained results demonstrated that our high throughput screening methodology could be a great help to the routine enzyme inhibitor screening.

Measuring multiple neurochemicals and related metabolites in blood and brain of the rhesus monkey by using dual microdialysis sampling and capillary hydrophilic interaction chromatography-mass spectrometry

In vivo measurement of multiple functionally related neurochemicals and metabolites (NMs) is highly interesting but remains challenging in the field of basic neuroscience and clinical research. We present here an analytical method for determining five functionally and metabolically related polar substances, including acetylcholine (quaternary ammonium), lactate and pyruvate (organic acids), as well as glutamine and glutamate (amino acids). These NMs are acquired from samples of the brain and the blood of non-human primates in parallel by dual microdialysis, and subsequently analyzed by a direct capillary hydrophilic interaction chromatography (HILIC)-mass spectrometry (MS) based method. To obtain high sensitivity in electrospray ionization (ESI)-MS, lactate and pyruvate were detected in negative ionization mode whereas the other NMs were detected in positive ionization mode during each HILIC-MS run. The method was validated for linearity, the limits of detection and quantification, precision, accuracy, stability and matrix effect. The detection limit of acetylcholine, lactate, pyruvate, glutamine, and glutamate was 150 pM, 3 μM, 2 μM, 5 nM, and 50 nM, respectively. This allowed us to quantitatively and simultaneously measure the concentrations of all the substances from the acquired dialysates. The concentration ratios of both lactate/pyruvate and glutamine/glutamate were found to be higher in the brain compared to blood (p < 0.05). The reliable and simultaneous quantification of these five NMs from brain and blood samples allows us to investigate their relative distribution in the brain and blood, and most importantly paves the way for future non-invasive studies of the functional and metabolic relation of these substances to each other.

Analysis of acetylcholine from extracellular fluid in brain by in vivo microdialysis and LC-ESI-MS/MS with the stable isotope-labeled internal standard

Acetylcholine (ACh) associated with Alzheimer's and Parkinson's disease is the major neurotransmitter in vertebrates. In support of clinical studies on the mechanism of the illnesses and development of medicines for these diseases, the LC-ESI-MS/MS method was developed and validated for the direct quantification of ACh in dialysate samples with acetylcholine-D(9) bromide (IS) as the isotope-labeled internal standard. The analytes were separated on the Waters Hilics C(18) Column (2.1 mm×100 mm, 3 μm) on LC with mobile phase ultrapure water-200 mM ammonium formate (pH 3.04)-acetonitrile (30:5:65, vol/vol/vol) at a flow rate of 300 μL/min, and monitored with a fragment ion of m/z 87 formed from a molecular ion of m/z 146 for ACh and that of m/z 87 from m/z 155 for IS during multiple reaction monitoring (MRM) positive ion mode. The lower limit of quantitation (LLOQ) of ACh was lower than 0.1 nmol/L in dialysate samples, equivalent to 0.2fmol injected on-column. The developed method could be utilized in the analysis of ACh in dialysate samples and these results were in good agreement with the gradient elution study.

The parafascicular thalamic nucleus concomitantly influences behavioral flexibility and dorsomedial striatal acetylcholine output in rats

Recent evidence suggests that a circuit involving the centromedian-parafascicular (Pf) thalamus and basal ganglia is critical for a shift away from biased actions. In particular, excitatory input from the Pf onto striatal cholinergic neurons may facilitate behavioral flexibility. Accumulating evidence indicates that an endogenous increase in dorsomedial striatal acetylcholine (ACh) output enhances behavioral flexibility. The present experiments investigated whether the rat (Rattus norvegicus) Pf supports flexibility during reversal learning, in part, by modifying dorsomedial striatal ACh output. This was determined first by examining the effects of Pf inactivation, through infusion of the GABA agonists baclofen and muscimol, on place acquisition and reversal learning. Additional experiments examined Pf inactivation on dorsomedial striatal ACh output during reversal learning and a resting condition. Behavioral testing was performed in a cross-maze. In vivo microdialysis combined with HPLC/electrochemical detection was used to sample ACh from the dorsomedial striatum. Pf inactivation selectively impaired reversal learning in a dose-dependent manner. A subsequent study showed that an increase in dorsomedial striatal ACh efflux (∼30% above basal levels) during reversal learning was blocked by Pf inactivation, which concomitantly impaired reversal learning. In the resting condition, a dose of baclofen and muscimol that blocked a behaviorally induced increase in dorsomedial striatal ACh output did not reduce basal ACh efflux. Together, the present findings indicate that the Pf is an intralaminar thalamic nucleus critical for behavioral flexibility, in part, by directly affecting striatal ACh output under conditions that require a shift in choice patterns.

Quantitative analysis of acetylcholine in rat brain microdialysates by liquid chromatography coupled with electrospray ionization tandem mass spectrometry

A liquid chromatography tandem mass spectrometry (LC/MS/MS) method has been developed for the quantitative analysis of acetylcholine in rat brain dialysates. The separation of acetylcholine (ACh), choline (Ch), acetyl-β-methylcholine (IS) from endogenous compounds and Ringer's salts was achieved with cation exchange chromatography. Optimization of chromatographic and mass spectrometry parameters were perfomed in order to improve sensitivity of the method. The limit of detection were 0.05 and 3.75 fmol on column with S/N ratio of 3:1 for ACh and Ch, respectively. The limit of quantitation (LOQ) for ACh and Ch measured in Ringer's solution were 0.05 nM (0.25 fmol) and 3.75 nM (18.75 fmol), respectively at S/N ratio of 10:1. Linearity of the method has been evaluated in the concentrations range between 0.05 and 5.00 nM and 3.75 and 200 nM for ACh and Ch respectively. The correlation coefficients were 0.999 and 0.995 for ACh and Ch respectively, indicating very good linearity. The LC/MS/MS method developed has been applied to evaluate the effect of oral administration of 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine 1,1-dioxide (IDRA21), a positive modulators of AMPA receptor, on the release of ACh in the rat prefrontal cortex by microdialysis.

Quantification of acetylcholine, an essential neurotransmitter, in brain microdialysis samples by liquid chromatography mass spectrometry

Chemical neurotransmission has been the subject of intensive investigations in recent years. Acetylcholine is an essential neurotransmitter in the central nervous system as it has an effect on alertness, memory and learning. Enzymatic hydrolysis of acetylcholine in the synaptic cleft is fast and quickly metabolizes to choline and acetate by acetylcholinesterase. Hence the concentration in the extracellular fluid of the brain is low (0.1-6 nm). Techniques such as microdialysis are routinely employed to measure acetylcholine levels in living brain systems and the microdialysis sample volumes are usually less than 50 microL. In order to develop medicine for the diseases associated with cognitive dysfunction like mild cognitive impairment, Alzheimer's disease, schizophrenia and Parkinson's disease, or to study the mechanism of the illness, it is important to measure the concentration of acetylcholine in the extracellular fluid of the brain. Recently considerable attention has been focused on the development of chromatographic-mass spectrometric techniques to provide more sensitive and accurate quantification of acetylcholine collected from in-vivo brain microdialysis experiments. This review will provide a brief overview of acetylcholine biosynthesis, microdialysis technique and liquid chromatography mass spectrometry, which is being used to quantitate extracellular levels of acetylcholine.

Change of cholinergic transmission and memory deficiency induced by injection of beta-amyloid protein into NBM of rats

The change of cholinergic transmission of beta-amyloid protein (beta-AP) treated rats was studied by intracerebral microdialysis sampling combined with HPLC analysis. beta-AP(1-40) was injected into nucleus basalis magnocellularis (NBM). Passive avoidance response test (step-down test) and delayed alternation task were used for memory testing. The impairment of memory after injection of beta-AP(1-40) into NBM exhibited mainly the deficiency of short-term working memory. One week after injection of beta-AP(1-40) the release of acetylcholine (ACh) from frontal cortex of freely-moving rats decreased significantly, and the response of cholinergic nerve ending to the action of high [K(+)] solution was rather weak. In control animals the percentage of increase of ACh-release during behavioral performance was 57%, while in beta-AP(1-40)-treated rats it was 34%. The temporary increase of the ACh-release of the rat put into a new place was also significantly diminished in beta-AP(1-40) -treated rats. The results show that the injection of beta-AP(1-40) into NBM impairs the cholinergic transmission in frontal cortex, and the impairment of cholinergic transmission may be the main cause of the deficit of working memory.

Disruption of mesolimbic regulation of prefrontal cholinergic transmission in an animal model of schizophrenia and normalization by chronic clozapine treatment

Abnormal mesolimbic control of cortical cholinergic activity has been hypothesized to contribute to the cognitive symptoms of schizophrenia. Stimulation of NMDA receptors in nucleus accumbens (NAC) increases acetylcholine (ACh) release in prefrontal cortex (PFC), an activation thought to contribute to attentional processing. Thus, the effects of intra-NAC perfusion of NMDA (250-400 microM) on ACh release in PFC were determined in rats receiving lesions of the ventral hippocampus (VH) as neonates (nVHLX), a neurodevelopmental model of schizophrenia, or as adults (aVHLX). NMDA elevated ACh release (100-150% above baseline) in adults sham-lesioned as neonates or in aVHLX rats. Adult nVHLX were unresponsive to NAC NMDA receptor stimulation. The inability of nVHLX to respond to NMDA emerged over development as a separate experiment demonstrated that evoked ACh release was normal before puberty (100-150% increase) yet, in these same nVHLX animals, absent after puberty. Amphetamine-evoked ACh release was assessed in nVHLX animals to exclude potential limitations in release capacity. Amphetamine produced greater increases in ACh release than in shams, indicating that nVHLX does not impair the capacity of cholinergic neurons to release ACh. Finally, the ability of 13 days of pretreatment with clozapine (1.25 mg/kg/day) to reinstate NMDA-evoked cortical ACh efflux was determined. Clozapine treatment normalized NMDA-evoked ACh release in nVHLX animals. These experiments show that mesolimbic regulation of cortical ACh release is disrupted in postpubertal nVHLX rats and normalized by low-dose treatment of clozapine; supporting the usefulness of nVHLX animals for research on the neuronal mechanisms underlying the cognitive symptoms of schizophrenia.

Measurement of Acetylcholine in Rat Brain Microdialysates by LC - Isotope Dilution Tandem MS

An LC-MS/MS method was developed for measuring acetylcholine (ACh) in an aqueous medium using reversed-phase ion-pair chromatography, electrospray ionization on a quadrupole ion trap instrument and a tetradeuterated analogue (ACh-1,1,2,2-d(4)) as an internal standard. A rapid separation was achieved on a 5-cm long octadecylsilica column (2.1 mm i.d.) by employing heptafluorobutyric acid (0.1% v/v) as an ion-pairing agent and requiring 10% v/v acetonitrile in 20 mM ammonium formate buffer under isocratic elution at 200 μl/min flow rate. The instrument's response was calibrated with samples containing known mole ratios of ACh and ACh-1,1,2,2-d(4) in an artificial cerebrospinal fluid, which afforded the conclusion that analyte concentrations could be determined by multiplying the measured analyte to internal standard ion-current ratio with the known molar concentration of the ACh-1,1,2,2-d(4) added. The rapid and simple assay was tested by measuring the basal neurotransmitter concentration in rat brain microdialysates without the use of a cholinesterase inhibitor upon sample collection.

Critical Evaluation of Acetylcholine Determination in Rat Brain Microdialysates using Ion-Pair Liquid Chromatography with Amperometric Detection

Liquid chromatography with amperometric detection remains the most widely used method for acetylcholine quantification in microdialysis samples. Separation of acetylcholine from choline and other matrix components on a microbore chromatographic column (1 mm internal diameter), conversion of acetylcholine in an immobilized enzyme reactor and detection of the produced hydrogen peroxide on a horseradish peroxidase redox polymer coated glassy carbon electrode, achieves sufficient sensitivity for acetylcholine quantification in rat brain microdialysates. However, a thourough validation within the concentration range required for this application has not been carried out before. Furthermore, a rapid degradation of the chromatographic columns and enzyme systems have been reported. In the present study an ion-pair liquid chromatography assay with amperometric detection was validated and its long-term stability evaluated. Working at pH 6.5 dramatically increased chromatographic stability without a loss in sensitivity compared to higher pH values. The lower limit of quantification of the method was 0.3 nM. At this concentration the repeatability was 15.7%, the inter-day precision 8.7% and the accuracy 103.6%. The chromatographic column was stable over 4 months, the immobilized enzyme reactor up to 2-3 months and the enzyme coating of the amperometric detector up to 1-2 months. The concentration of acetylcholine in 30 μl microdialysates obtained under basal conditions from the hippocampus of freely moving rats was 0.40 ± 0.12 nM (mean ± SD, n = 30). The present method is therefore suitable for acetylcholine determination in rat brain microdialysates.

Analysis of acetylcholine, choline and butyrobetaine in human liver tissues by hydrophilic interaction liquid chromatography-tandem mass spectrometry

The strong polar quaternary ammoniums, acetylcholine (ACh), choline (Ch) and butyrobetaine (BB, (3-carboxypropyl)trimethylammonium), are believed playing important roles in liver metabolism. These metabolites are at low levels and are weakly retained on reversed-phase liquid chromatographic (RP-LC) columns. Several hydrophilic interaction liquid chromatography-tandem mass spectrometry (HILIC-MS/MS) methods have been reported to analyze these compounds from different samples. However, no application to human liver tissues has been published. In this study, HILIC-MS/MS method was developed to simultaneously determine these three metabolites in human liver tissues. They were simply extracted from tissue, separated on a HILIC column, and detected by tandem MS in the mode of multiple reaction monitoring (MRM). Further studies on the recovery and repeatability based on real samples indicated the method was accurate and reliable. This method was successfully applied to measure the levels of ACh, Ch and BB in 61 human liver tissue samples including normal, hepatocellular carcinoma (HCC) and matched non-cancerous liver tissues. By comparison of Ch and ACh contents in 29 HCC with their matched non-cancerous liver tissues, it was found that ACh content increased in 11/29 HCC cases and decreased in 13/29 cases. Furthermore, the ACh/Ch ratio increased in 16/29 HCC cases, while it decreased in 8/29 cases. These results strongly indicated that there exist different patterns of ACh content in cancer tissues among HCC patients, thus highlighting the understanding of ACh and its relevant signal pathways in hepatic carcinogenesis and HCC progression.

Quantification of acetylcholine in microdialysate of subcutaneous tissue by hydrophilic interaction chromatography/tandem mass spectrometry

It has recently been suggested that acetylcholine plays an important role in the modulation of tissue inflammation. In order to further understand the newly discovered cholinergic anti-inflammatory pathway, tracking the concentration changes of acetylcholine in tissue is required. This paper describes the development of a method coupling hydrophilic interaction chromatography with electrospray ionization tandem mass spectrometry (HILIC/ESI-MS/MS) for the separation and quantification of acetylcholine in microdialysis samples of normal rats and of rats with local inflammation. The separation of acetylcholine from interferential endogenous compounds and inorganic cations was achieved with a zwitterionic stationary phase column using isocratic elution. Low-energy collision-induced dissociation tandem mass spectrometric (CID-MS/MS) analysis was carried out in the positive ion mode using multiple reaction monitoring (MRM) of the following mass transitions: m/z 146 --> 87 for acetylcholine and m/z 155 --> 87 for the internal standard acetylcholine-D9. The limit of detection for acetylcholine was found to be 0.075 fmol on-column with a signal-to-noise ratio of 3:1. The lower limit of quantification was 0.25 fmol on-column. The calibration curves obtained for acetylcholine in blank microdialysates were linear in the ranges of 0.025-50 nM and 0.025-0.5 nM, with correlation coefficients equal to or greater than 0.9994 and 0.9969, respectively. The recoveries of acetylcholine for high (2 nM) and low (0.5 nM) concentrations were in the ranges of 90-96% and 95-109%, respectively. The coefficients of variation for intra-day and inter-day reproducibility were equal to or less than 7.3% and 10.4%, respectively. The method has been successfully applied in the measurement of acetylcholine in microdialysates from normal and inflamed rat tissue.

Effect of (R)-salsolinol and N-methyl-(R)-salsolinol on the balance impairment between dopamine and acetylcholine in rat brain: involvement in pathogenesis of Parkinson disease

BACKGROUND

Parkinson disease (PD), a progressive neurodegenerative disease, affects at least 1% of population above the age of 65. Although the specific etiology of PD remains unclear, recently the endogenous neurotoxins such as (R)-salsolinol [(R)-Sal] and N-methyl-(R)-salsolinol [(R)-NMSal] have been thought to play a major role in PD. Much interest is focused on the degeneration of dopamine neurons induced by these neurotoxins. However, little literature is available on the impact of endogenous neurotoxins on the balance between dopamine (DA) and acetylcholine (ACh).

METHODS

After injection of (R)-Sal or (R)-NMSal into the rat brain striatum, the concentrations of DA and its metabolites were detected by HPLC with electrochemical detection. We assessed the influence of neurotoxins on acetylcholinesterase (AChE) activity and developed a microdialysis-electrochemical device to measure ACh concentrations with enzyme-modified electrodes.

RESULTS

(R)-Sal and (R)-NMSal led to concentration-dependent decreases in the activity of AChE. ACh concentrations in striatum treated with (R)-Sal or (R)-NMSal were increased to 131.7% and 239.8% of control, respectively. As to the dopaminergic system, (R)-NMSal caused a significant decrease in DA concentrations and (R)-Sal reduced the concentrations of DA metabolites in the striatum.

CONCLUSIONS

(R)-Sal and (R)-NMSal exerted a considerable effect on the balance between DA and ACh by impairing the cholinergic system as well as the dopaminergic system. It is likely that the disruption of balance between DA and ACh plays a critical role in the pathogenesis of neurotoxin-induced PD.

Toward a neuro-cognitive animal model of the cognitive symptoms of schizophrenia: disruption of cortical cholinergic neurotransmission following repeated amphetamine exposure in attentional task-performing, but not non-performing, rats

Impairments in attentional functions and capacities represent core aspects of the cognitive symptoms of schizophrenia. Attentional performance has been demonstrated to depend on the integrity and activity of cortical cholinergic inputs. The neurobiological, behavioral, and cognitive effects of repeated exposure to psychostimulants model important aspects of schizophrenia. In the present experiment, prefrontal acetylcholine (ACh) release was measured in attentional task-performing and non-performing rats pretreated with an escalating dosing regimen of amphetamine (AMPH) and following challenges with AMPH. In non-performing rats, pretreatment with AMPH did not affect the increases in ACh release produced by AMPH-challenges. In contrast, attentional task performance-associated increases in ACh release were attenuated in AMPH-pretreated and AMPH-challenged rats. This effect of repeated AMPH exposure on ACh release was already present before task-onset, suggesting that the loss of cognitive control that characterized these animals' performance was a result of cholinergic dysregulation. The findings indicate that the demonstration of repeated AMPH-induced dysregulation of the prefrontal cholinergic input system depends on interactions between the effects of repeated AMPH exposure and cognitive performance-associated recruitment of this neuronal system. Repeated AMPH-induced disruption of prefrontal cholinergic activity and attentional performance represents a useful model to investigate the cholinergic mechanisms contributing to the cognitive impairments of schizophrenia.

D2-like receptors in nucleus accumbens negatively modulate acetylcholine release in prefrontal cortex

Glutamatergic and dopaminergic inputs converge on medium spiny neurons in nucleus accumbens and regulate the excitability of these projections to target areas including the cholinergic basal forebrain. NMDA receptors situated on these projections are locally modulated by D1- and D2-like receptors. We previously reported that the D1-like positive modulation of NMDA receptor activity is expressed trans-synaptically in the control of basal forebrain cholinergic projections to prefrontal cortex. The present experiments tested the hypothesis that D2-like receptors in accumbens negatively modulate cortical ACh release. Perfusion of NMDA (150 microM) into the shell region of the accumbens produced a sustained increase (150-200%) in ACh release in prefrontal cortex. This increase was completely blocked by co-perfusion with the D2-like agonist quinpirole (100 microM). Perfusion of quinpirole also reduced basal ACh release (approximately 50%) in prefrontal cortex. The contribution of D2 receptors to the quinpirole effect was assessed in two additional studies. The first study revealed that co-perfusion of the D2 antagonist haloperidol (100 microM) blocked the quinpirole-induced attenuation of NMDA mediated ACh release. The second experiment demonstrated that intra-accumbens perfusion of quinelorane (100 microM), a more selective D2 agonist than quinpirole, also attenuated the NMDA mediated ACh release. Collectively, these studies demonstrate that D2 receptors in accumbens negatively modulate basal and NMDA mediated increases in ACh release in prefrontal cortex. This negative modulation may contribute to the integration of normal attentional processing and goal directed behavior and to the therapeutic effects of antipsychotic medication on cognition in psychopathologies such as schizophrenia.

An investigation of acetylcholine released in skeletal muscle and protein unbound drug released in blood based on the pyridostigmine bromide (pretreatment drug) sustained-release pellets by microdialysis technique in the rabbit model

Pyridostigmine bromide (PB) is a reversible acetylcholinesterase inhibitor that has been used as a pretreatment drug for "Soman" nerve gas poisoning in combat to increase survival. The once-daily PB-sustained-release (SR) pellets were developed by extrusion-spheronization and fluid-bed methods in our laboratory, which was followed by zero-order release mechanism. The results showed that the released concentration of acetylcholine (ACh) in skeletal muscle and the released concentration of protein unbound drug in blood were determined by microdialysis technique to have significant differences (P<0.05) among the three dosage forms (IV injection, commercial IR tablets and the PB-SR pellet). The released concentrations of ACh and protein unbound drug for PB-SR pellets were slower than IV injection and commercial IR tablets; this phenomenon indicating that the retention period of drug efficacy in vivo for PB-SR pellet was longer than the others, that is to say, the PB-SR pellets provided with SR effect in vivo as well. We believe that once-daily administered PB-SR pellets would improve limitations of post-exposure antidotes, decrease the frequency of administration and enhance the retention period of drug efficacy in vivo for personnel exposed to contamination situations in wars or terrorist attacks in the future.

Development of a liquid chromatography/tandem mass spectrometry method for the quantitation of acetylcholine and related neurotransmitters in brain microdialysis samples

Monitoring concentrations of acetylcholine (ACh) in specific brain regions is important in understanding disease pathology, as well as in designing and evaluating novel disease-modifying treatments where cholinergic dysfunction is a hallmark feature. We have developed a sensitive and quantitative liquid chromatography/tandem mass spectrometry method to analyze the extracellular concentrations of ACh, choline (Ch) and (3-carboxylpropyl)-trimethylammonium (iso-ACh) in brain microdialysis samples of freely moving animals. One immediate advantage of this new method is the ability to monitor ACh in its free form without having to use a cholinesterase inhibitor in the perfusate. The separation of ACh, Ch, iso-ACh and related endogenous compounds was carried out based on cation exchange chromatography with a volatile elution buffer consisting of ammonium formate, ammonium acetate and acetonitrile. An unknown interference of ACh, which was observed in brain microdialysates from many studies, was well separated from ACh to ensure the accuracy of the measurement. Optimization of electrospray ionization conditions for these quaternary ammonium compounds achieved the limits of detection (S/N=3) of 0.2 fmol for ACh, 2 fmol for Ch and 0.6 fmol for iso-ACh using a benchtop tandem quadrupole mass spectrometer with moderate sensitivity. The limit of quantitation (S/N=10) was 1 fmol for ACh, 3 fmol for iso-ACh and 10 fmol for Ch. This method was selective, precise (<10% R.S.D.), and sensitive over a range of 0.05-10nM for ACh, 0.25-50 nM for iso-ACh and 15-3000 nM for Ch. To demonstrate that the developed method can be applied to monitoring changes in ACh concentrations in vivo, reference agents that have previously been shown to influence ACh levels were studied in rat dorsal hippocampus. This includes the 5-HT6 receptor antagonist, SB-271046, and the cholinesterase inhibitor, donepezil. Moreover, levels of ACh were demonstrated to be sensitive to infusion of tetrodotoxin (TTX) suggesting that the ACh being measured in vivo was of neuronal origin. Collectively, these biological data provided in vivo validation of this analytical method.

The effect of N-methyl-D-aspartate receptor blockade on acetylcholine efflux in the dorsomedial striatum during response reversal learning

Separate experiments found that activation of N-methyl-d-aspartate (NMDA) receptors or increased acetylcholine (ACh) efflux in the rat dorsomedial striatum is critical for learning when conditions require a shift in strategies. Increasing evidence indicates that NMDA receptor activity affects cholinergic efflux in the basal ganglia. The present studies determined whether NMDA receptor blockade in the dorsomedial striatum with dl-2-amino-5-phosphonopentanoic acid (AP-5) affects dorsomedial striatal ACh output in a resting condition, as well as during response reversal learning. Experiment 1 investigated the effects of AP-5 (12.5, 25 or 50 muM) infused into the dorsomedial striatum on ACh output in a resting condition. AP-5 infusion at 25 and 50 muM led to a 20% and 40% decrease in dorsomedial striatal ACh output, respectively. AP-5 (12.5 muM) infusion did not change dorsomedial striatal ACh output from basal levels. Experiment 2 determined whether dorsomedial striatal ACh efflux increases during response reversal learning and whether AP-5, at a dose that does not affect basal levels, modifies response reversal learning and ACh efflux. Following acquisition of a response discrimination, rats had microdialysis probes bilaterally inserted into the dorsomedial striatum prior to the reversal learning test. After baseline samples, rats received a response reversal learning test for 30 min. Control rats rapidly improved in the reversal learning session while simultaneously exhibiting an approximately 40% increase in ACh output compared with baseline levels. AP-5 (12.5 muM) treatment during testing significantly impaired response reversal learning while concomitantly blocking an increase in ACh output. These findings suggest that NMDA receptor activation in the dorsomedial striatum may facilitate a shift in response patterns, in part, by increasing ACh efflux.

Microdialysis coupled on-line to capillary liquid chromatography with tandem mass spectrometry for monitoring acetylcholine in vivo

Capillary liquid chromatography-mass spectrometry (cLC-MS) was coupled on-line to microdialysis sampling to monitor endogenous acetylcholine (ACh) from the rodent brain. In vivo microdialysate sampled at 0.6 microL/min from the striatum of ketamine or chloral hydrate anesthetized rats was loaded onto a sample loop and then injected onto a approximately 5 cm long strong cation exchange (SCX) capillary column. A step gradient was used to separate the analyte from ionization suppressing salts contained in dialysate in 2.4 min. Sampling coupled on-line with cLC-MS allowed for high temporal resolution (data points at 2.4 min intervals), good reproducibility (10-15% relative standard deviation, R.S.D.), and sensitive limits of detection (0.04 nM or 8 amol injected). The method successfully monitored basal and stimulated levels (induced by increased K+ concentrations) of ACh from the anesthetized rat without necessitating perfusion of an acetylcholinesterase (AChE) inhibitor. Absolute and percent basal levels of ACh from rats receiving different anesthetics were also compared.

Detection of γ-aminobutyric acid-induced glutamate release in acute mouse hippocampal slices with a patch sensor

gamma-Aminobutyric acid (GABA)-stimulated release of L-glutamate from various neuronal regions of acute mouse hippocampal slices was detected with a patch sensor that responds to L-glutamate at the sub-micromolar level. The response of the patch sensor to L-glutamate was evaluated in terms of an integrated current. The integrated current increased with the concentration of L-glutamate ranging from 0.50 to 5.0 microM. By using the patch sensor, GABA-induced L-glutamate release from acute mouse hippocampal slices was detected. The effect of antagonists for GABA(A) and GABA(B) receptors on the L-glutamate release was also investigated. The GABA (25 microM) stimulation induced the release of L-glutamate via GABA(A) receptor in the CA1 region, but GABA did not induce L-glutamate release in the CA3 region. However, in the presence of the GABA(B) receptor antagonist (3-aminopropyl)(diethoxymethyl)phosphinic acid (CGP-35348), release of L-glutamate in the CA3 region was evoked by GABA stimulation. The glutamate release was completely suppressed when both GABA(A) and GABA(B) receptor were inhibited. The current results show that the glutamate release in the CA3 region occurs via a GABA(A) pathway when GABA(B) receptors are inhibited.

Measurement of neurotransmitters from extracellular fluid in brain by in vivo microdialysis and chromatography–mass spectrometry

During the last three decades, a great deal of information has been discovered about chemical neurotransmission. However, the most important processes, namely the complex nature of neuronal circuitry, the "cross talk" between multiple neurotransmitter systems, and the varying effects neurochemicals have at different receptors, are still being explored. Techniques such as microdialysis are routinely employed to measure neurotransmitter levels in living tissue systems. Moreover, microdialysis studies have proven to be valuable in the investigation of neurodegenerative and psychiatric disease pathology, as well as in identifying novel drugs to treat such disorders. One particular challenge in performing these experiments is the requirement to couple microdialysis to sophisticated analytical equipment. Recently, considerable attention has been focused on the development of chromatographic-mass spectrometric techniques to provide more sensitive and accurate measurements of neurochemicals collected from in vivo microdialysis experiments. This review will provide a brief overview of the microdialysis technique, as well as how microdialysis and chromatography-mass spectrometry are being used to measure extracellular levels of neurotransmitters. The primary emphasis of this review will be on how these applications are used to measure levels of acetylcholine (ACh), dopamine, norepinephrine and gamma-aminobutyric acid (GABA).

NMDA and dopamine interactions in the nucleus accumbens modulate cortical acetylcholine release

The nucleus accumbens (NAC) plays a key role in directing appropriate motor output following the presentation of behaviorally relevant stimuli. As such, we postulate that accumbens efferents also participate in the modulation of neuronal circuits regulating attentional processes directed toward the identification and selection of these stimuli. In this study, N-methyl-d-aspartate (NMDA) and D1 ligands were perfused into the shell region of the NAC of awake rats. Cortical cholinergic transmission, a mediator of attentional processes, was measured via microdialysis probes inserted into the prefrontal cortex (PFC). NMDA perfusions (150 or 250 microm) into NAC resulted in significant increases in acetylcholine (ACh) efflux in PFC (150-200% above baseline levels). Co-administration of the D1 antagonist SCH-23390 (150 microm) markedly attenuated (by approx. 70%) ACh efflux following perfusions of 150 microm NMDA but not following 250 microm NMDA, suggesting that D1 receptor activity contributes to the ability of the lower but not the higher concentration of NMDA to increase cortical ACh release. Collectively, these data reveal a positive modulation of NMDA receptors by D1 receptors in NAC that is expressed trans-synaptically at the level of cortical transmission. This modulation may underlie the coordinated linking of attentional processes and motor output following exposure to salient and behaviorally relevant stimuli.

Enzyme Electrochemistry — Biocatalysis on an Electrode

Oxidoreductase enzymes catalyze single- or multi-electron reduction/oxidation reactions of small molecule inorganic or organic substrates, and they are integral to a wide variety of biological processes including respiration, energy production, biosynthesis, metabolism, and detoxification. All redox enzymes require a natural redox partner such as an electron-transfer protein (e.g. cytochrome, ferredoxin, flavoprotein) or a small molecule cosubstrate (e.g. NAD(P)H, dioxygen) to sustain catalysis, in effect to balance the substrate/product redox half-reaction. In principle, the natural electron-transfer partner may be replaced by an electrochemical working electrode. One of the great strengths of this approach is that the rate of catalysis (equivalent to the observed electrochemical current) may be probed as a function of applied potential through linear sweep and cyclic voltammetry, and insight to the overall catalytic mechanism may be gained by a systematic electrochemical study coupled with theoretical analysis. In this review, the various approaches to enzyme electrochemistry will be discussed, including direct and indirect (mediated) experiments, and a brief coverage of the theory relevant to these techniques will be presented. The importance of immobilizing enzymes on the electrode surface will be presented and the variety of ways that this may be done will be reviewed. The importance of chemical modification of the electrode surface in ensuring an environment conducive to a stable and active enzyme capable of functioning natively will be illustrated. Fundamental research into electrochemically driven enzyme catalysis has led to some remarkable practical applications. The glucose oxidase enzyme electrode is a spectacularly successful application of enzyme electrochemistry. Biosensors based on this technology are used worldwide by sufferers of diabetes to provide rapid and accurate analysis of blood glucose concentrations. Other applications of enzyme electrochemistry are in the sensing of macromolecular complexation events such as antigen–antibody binding and DNA hybridization. The review will include a selection of enzymes that have been successfully investigated by electrochemistry and, where appropriate, discuss their development towards practical biotechnological applications.

Prefrontal cortical modulation of acetylcholine release in posterior parietal cortex

Attentional processing is a crucial early stage in cognition and is subject to "top-down" regulation by prefrontal cortex (PFC). Top-down regulation involves modification of input processing in cortical and subcortical areas, including the posterior parietal cortex (PPC). Cortical cholinergic inputs, originating from the basal forebrain cholinergic system, have been demonstrated to mediate important aspects of attentional processing. The present study investigated the ability of cholinergic and glutamatergic transmission within PFC to regulate acetylcholine (ACh) release in PPC. The first set of experiments demonstrated increases in ACh efflux in PPC following AMPA administration into the PFC. These increases were antagonized by co-administration of the AMPA receptor antagonist DNQX into the PFC. The second set of experiments demonstrated that administration of carbachol, but not nicotine, into the PFC also increased ACh efflux in PPC. The effects of carbachol were attenuated by co-administration (into PFC) of a muscarinic antagonist (atropine) and partially attenuated by the nicotine antagonist mecamylamine and DNQX. Perfusion of carbachol, nicotine, or AMPA into the PPC did not affect PFC ACh efflux, suggesting that these cortical interactions are not bi-directional. These studies demonstrate the capacity of the PFC to regulate ACh release in the PPC via glutamatergic and cholinergic prefrontal mechanisms. Prefrontal regulation of ACh release elsewhere in the cortex is hypothesized to contribute to the cognitive optimization of input processing.

Rapid determination of trace amounts of minoxidil in hamster skin follicles with various formulations using narrow-bore LC/EC

A sensitive liquid chromatographic method with electrochemistry (LC/EC) was developed for the determination of trace of minoxidil in hamster skin follicles after topical administration of the ear using various formulations. The minoxidil in the sebaceous glands of the hamster ear was isolated from the skin and the follicles in different skin layers were treated with aqueous trichloroacetic acid followed by acetonitrile. The supernatant was directly injected into the LC/EC system and minoxidil was detected by oxidation at +800 mV versus Ag/AgCl using a glassy carbon electrode. The analytical recoveries were between 94.4 and 103.1% and the linearity was excellent up to 250 microg/ml with a regression coefficient (r(2)) of 0.9988. The LC/EC and the widely used radiolabeled scintillation methods agree well and both show high sensitivities. The LC/EC method is rapid and cost-effective with a detection limit of only 1 ng/ml.

Application of immobilized enzyme reactor in on-line high performance liquid chromatography: A review

This review summarizes all the research efforts in the last decade (1994-2003) that have been spent to the various application of immobilized enzyme reactor (IMER) in on-line high performance liquid chromatography (HPLC). All immobilization procedures including supports, kind of assembly into chromatographic system and methods are described. The effect of immobilization on enzymatic properties and stability of biocatalysts is considered. A brief survey of the main applications of IMER both as pre-column, post-column or column in the chemical, pharmaceutical, clinical and commodities fields is also reported.

Stimulation of cortical acetylcholine release by orexin A

The basal forebrain cholinergic system is a critical component of the neurobiological substrates underlying attentional function. Orexin neurons are important for arousal and maintenance of wakefulness and are found in the area of the hypothalamus previously shown to project to the basal forebrain. We used dual-probe in vivo microdialysis in rats to test the hypothesis that orexin A (OxA) increases cortical acetylcholine (ACh) release. Intrabasalis administration of OxA (0, 0.1, 10.0 microM via reverse dialysis) dose-dependently increased ACh release within the prefrontal cortex (PFC). In a separate group of animals, local (intra-PFC) administration of OxA via reverse dialysis was found to have no significant effect on ACh release. In order to obtain anatomical corroboration of the basal forebrain as a site of orexin modulation of corticopetal cholinergic activity, we used immunohistochemistry to examine the relationship between orexin fibers and cholinergic neurons in the basal forebrain. We observed widespread distribution of orexin-immunoreactive fibers in cholinergic regions of the basal forebrain, particularly in more rostral areas where frequent instances of apparent appositional contact were observed between orexin fibers and choline acetyltransferase-positive cell bodies. Collectively, these data suggest that orexin projections to the basal forebrain form an important link between hypothalamic arousal and forebrain attentional systems.

Analysis of acetylcholine and choline in microdialysis samples by liquid chromatography/tandem mass spectrometry

A sensitive liquid chromatography/electrospray ionisation tandem mass spectrometric (LC/ESI-MS/MS) method was developed for the analysis of acetylcholine and choline in microdialysis samples. A Ringer's solution that contains high (150 mM) concentrations of inorganic salts was used to extract acetylcholine and choline from a rat or mouse brain. The separation of acetylcholine, choline, an internal standard acetyl-beta-methylcholine, endogenous compounds and inorganic cations was achieved with hydrophilic interaction chromatography using a diol column. The eluent consisted of 20 mM ammonium formate (pH 3.3) and acetonitrile (20:80) which is favourable for the ESI process. Limits of detection (signal-to-noise (S/N) ratio = 3) of 0.02 nM (0.2 fmol) for acetylcholine and 1 nM (10 fmol) for choline were observed using standards diluted in Ringer's solution. A good linearity was obtained from the limit of quantitation: 0.1 nM (S/N ratio = 10) to 50 nM (r = 0.999) for acetylcholine and within the concentration range of 100-3500 nM (r = 0.998) for choline. The between-day repeatability of the method was good; RSD was 3.1% at 1 nM level of acetylcholine and 3.5% at 1000 nM level of choline. The recoveries for addition of 1 or 2.5 nM acetylcholine and 0.2 or 1 microM choline in microdialysis balancing samples were between 93 and 101% indicating that no suppressing endogenous compounds were co-eluting with acetylcholine or choline. The developed method was applied to the analysis of microdialysis balancing samples collected from rat and mouse brains.

Dynamic changes in acetylcholine output in the medial striatum during place reversal learning

The present studies explored the role of the medial striatum in learning when task contingencies change. Experiment 1 examined whether the medial striatum is involved in place reversal learning. Testing occurred in a modified cross-maze across two consecutive sessions. Injections of the local anesthetic, bupivacaine, into the medial striatum, did not impair place acquisition, but impaired place reversal learning. The reversal-learning deficit was due to an inability to maintain the new choice pattern following the initial shift. Experiment 2 determined whether changes in acetylcholine (ACh) output occur during the acquisition or reversal learning of a place discrimination. Extracellular ACh output from the medial striatum was assessed in samples collected at 6-min intervals using in vivo microdialysis during behavioral testing. ACh output did not change from basal levels during place acquisition. During reversal learning, ACh output significantly increased as rats began to learn the new choice pattern, and returned to near basal levels as a rat reliably executed the new place strategy. The present results suggest that the medial striatum may be critical for flexible adaptations involving spatial information, and that ACh actions in this area enable the shifting of choice patterns when environmental conditions change.

Neuronal activity in the nucleus accumbens is necessary for performance-related increases in cortical acetylcholine release

In vivo microdialysis was used to determine the necessity of neuronal activity in the nucleus accumbens (NAC) for task-induced increases in cortical acetylcholine (ACh) efflux. Rats were trained in a behavioral task in which they were required to perform a defined number of licks of a citric acid solution in order to gain access to a palatable, cheese-flavored food. Upon reaching a consistent level of performance, rats were implanted with microdialysis cannula in the medial prefrontal cortex (mPFC) and either the ipsilateral shell of the NAC or in the dorsal striatum (STR; control site). Dialysis samples from the mPFC were analyzed for ACh concentrations and samples from the NAC were analyzed for dopamine (DA) concentrations. Performance in the task was associated with increases in both ACh efflux in the cortex (150-200%) and DA efflux in the NAC (50-75%). These increases were blocked by administration of tetrodotoxin (TTX; 1.0 microM) via reverse dialysis into the NAC. Administration of TTX into the dorsal STR control site was ineffective in blocking performance-associated increases in cortical ACh. The D2 antagonist sulpiride (10 or 100 microM) administered into the NAC via reverse dialysis was ineffective in blocking increases in cortical ACh efflux. The present data reveal that neuronal activity in the NAC is necessary for behaviorally induced increases in cortical ACh efflux and that this activation does not require increases in D2 receptor activity.

Improved method for the routine determination of acetylcholine and choline in brain microdialysate using a horseradish peroxidase column as the immobilized enzyme reactor

A modified microbore high-performance liquid chromatography-immobilized enzyme reactor-electrochemical detection system for acetylcholine (ACh) and choline (Ch) was developed. The system used the horseradish peroxidase and a solution mediator ferrocene to convert the analyte into an oxidized ferrocene species which was detected electrochemically by reduction at 0 mV. There was an excellent linear relationship between the concentration of ACh/Ch and the peak height over the range of 1-5000 nmol/l. The limit of detection for ACh was 2 fmol/5 microl (S/N=3:1). Compared with the common method recommended by Bioanalytical System Inc. (BAS), this method exhibits a 200-fold improvement in the detection limit. The ACh and Ch levels in rat brain microdialysate were examined.