In vivo neurochemical monitoring using benzoyl chloride derivatization and liquid chromatography-mass spectrometry

Analysis of cerebrospinal fluid γ-aminobutyric acid by capillary electrophoresis with laser-induced fluorescence detection

The measurement of γ-aminobutyric acid (GABA) is suitable for investigating various neurological disorders. In this study, a sensitive and selective method for free GABA quantification in cerebrospinal fluid (CSF) has been standardised. This method is based on CE with LIF detection using 4-fluoro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-F) as a derivatisating agent. The reaction conditions (NBD-F concentration, pH, temperature and reaction time) and the electrophoretic parameters (run buffer composition and pH and separation voltage) were optimised to obtain the maximum derivatisation efficiency and electrophoretic resolution. The best resolution was obtained using 200 mM sodium borate, 10 mM SDS, 8.5 mM β-CD, pH 10 and 20 kV voltage. The method was linear in the concentration range of 2.5-1000 nM with good inter- and intra-assay precision values. The effects of CSF handling on free GABA concentrations were also evaluated. Our results show that the time delay between CSF collection and freezing strongly increases the CSF GABA values. Age-related reference values were established in 55 paediatric controls. The influence of antiepileptic therapy on free CSF GABA was studied in 38 neuropaediatric patients. Significantly, higher GABA values were obtained in patients taking valproic acid or vigabatrin therapy, which are antiepileptic drugs that modulate GABA metabolism.

Amphetamine stimulates movement through thalamocortical glutamate release

The ventrolateral thalamus (VL) is a primary relay point between the basal ganglia and the primary motor cortex (M1). Using dual probe microdialysis and locomotor behavior monitoring, we investigated the contribution of VL input into M1 during amphetamine (AMPH)-stimulated monoamine release and hyperlocomotion in rats. Tetrodotoxin (10 μM) perfusion into the VL significantly lowered hyperactivity induced by AMPH (1 mg/kg i.p.). This behavioral response corresponded to reduced cortical glutamate and monoamine release. To determine which glutamate receptors the thalamocortical projections acted upon, we perfused either the α-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA)/kainate receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX) (10 μM) or the N-methyl-D-aspartic acid (NMDA) receptor antagonist (MK-801) intracortically followed by systemic AMPH. The results show that AMPA/kainate, and to a lesser extent NMDA receptors, mediated the observed effects. As glutamate-monoamine interactions could possibly occur through local or circuit-based mechanisms, we isolated and perfused M1 tissue ex vivo to determine the extent of local glutamate-dopamine interactions. Taken together, these results demonstrate that AMPH generates hyperlocomotive states via thalamocortical signaling and that cortical AMPA receptors are an important mediator of these effects. This study utilizes dual probe microdialysis sampling and comprehensive LC-MS analysis to determine the effects of amphetamine (1 mg/kg i.p.) on thalamocortical neurotransmission. Using pharmacological tools such as local thalamic tetrodotoxin (TTX) perfusion and glutamate antagonist at the cortical level, we demonstrate that thalamocortical glutamate (acting primarily through cortical AMPA receptors) is an essential component in amphetamine-induced hyperlocomotion.

Comprehensive analysis of neurotransmitters from regenerating planarian extract using an ultrahigh-performance liquid chromatography/mass spectrometry/selected reaction monitoring method

RATIONALE

Absolute quantification of neurotransmitters (NTs) from biological systems is imperative to track how changes in concentration of active neurochemicals may affect biological behavior. A sensitive method for the absolute quantification of multiple NTs in a single method is highly needed.

METHODS

A stable-isotope dilution ultrahigh-performance liquid chromatography/mass spectrometry/selected reaction monitoring (UHPLC/MS/SRM) assay has been developed for a sensitive and quantitative assessment of NTs in planaria. We used this method for the simultaneous quantification of 16 NTs. All analytes showed a linear relationship between concentrations (0.78-50 ng/mL), regression coefficients higher than 0.97, accuracy (91-109%) and low coefficients of variation (CVs). The inter-day CVs for the lowest quality controls (1.56 ng/mL) were in the range between 2-11%.

RESULTS

The levels of most of the NTs were similar in both sexual and asexual planarians except for glutamic acid, which was about two-fold higher in asexual compared to sexual planarians. We identified high levels of serotonin and failed to detect tryptamine suggesting that the pathway essential for the conversion of tryptophan into tryptamine is absent in planarians. Interestingly, we also found high levels of dopamine and L-DOPA in regenerating planarians suggesting their possible role in regeneration.

CONCLUSIONS

For the first time, we developed novel methodology based on UHPLC/MS/SRM and quantified 16 NTs with high sensitivity and specificity from sexual and asexual strains of planarian Schmidtea mediterranea. This method will also have great application in quantifying various NTs with great precision in different model systems.

Rapid preconcentration for liquid chromatography-mass spectrometry assay of trace level neuropeptides

Measurement of neuropeptides in the brain through in vivo microdialysis sampling provides direct correlation between neuropeptide concentration and brain function. Capillary liquid chromatography-multistage mass spectrometry (CLC-MS(n)) has proven to be effective at measuring endogenous neuropeptides in microdialysis samples. In the method, microliter samples are concentrated onto nanoliter volume packed beds before ionization and mass spectrometry analysis. The long times required for extensive preconcentration present a barrier to routine use because of the many samples that must be analyzed and instability of neuropeptides. In this study, we evaluated the capacity of 75 μm inner diameter (i.d.) capillary column packed with 10 μm reversed phase particles for increasing the throughput in CLC-MS(n) based neuropeptide measurement. Coupling a high injection flow rate for fast sample loading/desalting with a low elution flow rate to maintain detection sensitivity, this column has reduced analysis time from ∼30 min to 3.8 min for 5 μL sample, with 3 pM limit of detection (LOD) for enkephalins and 10 pM LOD for dynorphin A1-8 in 5 μL sample. The use of isotope-labeled internal standard lowered peptide signal variation to less than 5 %. This method was validated for in vivo detection of Leu and Met enkephalin with microdialysate collected from rat globus pallidus. The improvement in speed and stability makes CLC-MS(n) measurement of neuropeptides in vivo more practical.

A novel liquid chromatography/tandem mass spectrometry method for the quantification of glycine as biomarker in brain microdialysis and cerebrospinal fluid samples within 5min

Glycine is an important amino acid neurotransmitter in the central nervous system (CNS) and a useful biomarker to indicate biological activity of drugs such as glycine reuptake inhibitors (GRI) in the brain. Here, we report how a liquid chromatography/tandem mass spectrometry (LC-MS/MS) method for the fast and reliable analysis of glycine in brain microdialysates and cerebrospinal fluid (CSF) samples has been established. Additionally, we compare this method with the conventional approach of high performance liquid chromatography (HPLC) coupled to fluorescence detection (FD). The present LC-MS/MS method did not require any derivatisation step. Fifteen microliters of sample were injected for analysis. Glycine was detected by a triple quadrupole mass spectrometer in the positive electrospray ionisation (ESI) mode. The total running time was 5min. The limit of quantitation (LOQ) was determined as 100nM, while linearity was given in the range from 100nM to 100μM. In order to demonstrate the feasibility of the LC-MS/MS method, we measured glycine levels in striatal in vivo microdialysates and CSF of rats after administration of the commercially available glycine transporter 1 (GlyT1) inhibitor LY 2365109 (10mg/kg, p.o.). LY 2365109 produced 2-fold and 3-fold elevated glycine concentrations from 1.52μM to 3.6μM in striatal microdialysates and from 10.38μM to 36μM in CSF, respectively. In conclusion, we established a fast and reliable LC-MS/MS method, which can be used for the quantification of glycine in brain microdialysis and CSF samples in biomarker studies.

Synergistic activity between the delta-opioid agonist SNC80 and amphetamine occurs via a glutamatergic NMDA-receptor dependent mechanism

Glutamate is known to cause the release of dopamine through a Ca(2+)-sensitive mechanism that involves activation of NMDA ionotropic glutamate receptors. In the current study, we tested the hypothesis that the delta opioid agonist SNC80 acts indirectly, via the glutamatergic system, to enhance both amphetamine-stimulated dopamine efflux from striatal preparations and amphetamine-stimulated locomotor activity. SNC80 increased extracellular glutamate content, which was accompanied by a concurrent decrease in GABA levels. Inhibition of NMDA signaling with the selective antagonist MK801 blocked the enhancement of both amphetamine-induced dopamine efflux and hyperlocomotion observed with SNC80 pretreatment. Addition of exogenous glutamate also potentiated amphetamine-stimulated dopamine efflux in a Mg(2+)- and MK801-sensitive manner. After removal of Mg(2+) to relieve the ion conductance inhibition of NMDA receptors, SNC80 both elicited dopamine release alone and produced a greater enhancement of amphetamine-evoked dopamine efflux. The action of SNC80 to enhance amphetamine-evoked dopamine efflux was mimicked by the GABA(B) antagonist 2-hydroxysaclofen. These cumulative findings suggest SNC80 modulates amphetamine-stimulated dopamine efflux through an intra-striatal mechanism involving inhibition of GABA transmission leading to the local release of glutamate followed by subsequent activation of NMDA receptors.

Addressing the need for biomarker liquid chromatography/mass spectrometry assays: a protocol for effective method development for the bioanalysis of endogenous compounds in cerebrospinal fluid

RATIONALE

Research on disorders of the central nervous system (CNS) has shown that an imbalance in the levels of specific endogenous neurotransmitters may underlie certain CNS diseases. These alterations in neurotransmitter levels may provide insight into pathophysiology, but can also serve as disease and pharmacodynamic biomarkers. To measure these potential biomarkers in vivo, the relevant sample matrix is cerebrospinal fluid (CSF), which is in equilibrium with the brain's interstitial fluid and circulates through the ventricular system of the brain and spinal cord. Accurate analysis of these potential biomarkers can be challenging due to low CSF sample volume, low analyte levels, and potential interferences from other endogenous compounds.

METHODS

A protocol has been established for effective method development of bioanalytical assays for endogenous compounds in CSF. Database searches and standard-addition experiments are employed to qualify sample preparation and specificity of the detection thus evaluating accuracy and precision.

RESULTS

This protocol was applied to the study of the histaminergic neurotransmitter system and the analysis of histamine and its metabolite 1-methylhistamine in rat CSF.

CONCLUSIONS

The protocol resulted in a specific and sensitive novel method utilizing pre-column derivatization ultra high performance liquid chromatography/tandem mass spectrometry (UHPLC/MS/MS), which is also capable of separating an endogenous interfering compound, identified as taurine, from the analytes of interest.

Emerging trends in in vivo neurochemical monitoring by microdialysis

Mapping chemical dynamics in the brain of live subjects is a challenging but highly rewarding goal because it allows neurotransmitter fluctuations to be related to behavior, drug effects, and disease states. A popular method for such measurements is microdialysis sampling coupled to analytical measurements. This method has become well-established for monitoring low molecular weight neurotransmitters, metabolites, and drugs, especially in pharmacological and pharmacokinetic studies. Recent technological developments which improve the temporal and spatial resolution of the methods will enable it to be used for studying behavior and small brain nuclei. Better assays allow monitoring more neurotransmitters simultaneously. Extension to analysis of aggregating proteins like amyloid β is proving extremely useful for uncovering the roles of these molecules and how they contribute to neurodegenerative diseases.

Recent Advances in Mass Spectrometry for the Identification of Neuro-chemicals and their Metabolites in Biofluids

Recently, mass spectrometric related techniques have been widely applied for the identification and quantification of neurochemicals and their metabolites in biofluids. This article presents an overview of mass spectrometric techniques applied in the detection of neurological substances and their metabolites from biological samples. In addition, the advances of chromatographic methods (LC, GC and CE) coupled with mass spectrometric techniques for analysis of neurochemicals in pharmaceutical and biological samples are also discussed.

Determination of Serotonin and Dopamine Metabolites in Human Brain Microdialysis and Cerebrospinal Fluid Samples by UPLC-MS/MS: Discovery of Intact Glucuronide and Sulfate Conjugates

An UPLC-MS/MS method was developed for the determination of serotonin (5-HT), dopamine (DA), their phase I metabolites 5-HIAA, DOPAC and HVA, and their sulfate and glucuronide conjugates in human brain microdialysis samples obtained from two patients with acute brain injuries, ventricular cerebrospinal fluid (CSF) samples obtained from four patients with obstructive hydrocephalus, and a lumbar CSF sample pooled mainly from patients undergoing spinal anesthesia in preparation for orthopedic surgery. The method was validated by determining the limits of detection and quantification, linearity, repeatability and specificity. The direct method enabled the analysis of the intact phase II metabolites of 5-HT and DA, without hydrolysis of the conjugates. The method also enabled the analysis of the regioisomers of the conjugates, and several intact glucuronide and sulfate conjugates were identified and quantified for the first time in the human brain microdialysis and CSF samples. We were able to show the presence of 5-HIAA sulfate, and that dopamine-3-O-sulfate predominates over dopamine-4-O-sulfate in the human brain. The quantitative results suggest that sulfonation is a more important phase II metabolism pathway than glucuronidation in the human brain.

Aβ induces astrocytic glutamate release, extrasynaptic NMDA receptor activation, and synaptic loss

Synaptic loss is the cardinal feature linking neuropathology to cognitive decline in Alzheimer's disease (AD). However, the mechanism of synaptic damage remains incompletely understood. Here, using FRET-based glutamate sensor imaging, we show that amyloid-β peptide (Aβ) engages α7 nicotinic acetylcholine receptors to induce release of astrocytic glutamate, which in turn activates extrasynaptic NMDA receptors (eNMDARs) on neurons. In hippocampal autapses, this eNMDAR activity is followed by reduction in evoked and miniature excitatory postsynaptic currents (mEPSCs). Decreased mEPSC frequency may reflect early synaptic injury because of concurrent eNMDAR-mediated NO production, tau phosphorylation, and caspase-3 activation, each of which is implicated in spine loss. In hippocampal slices, oligomeric Aβ induces eNMDAR-mediated synaptic depression. In AD-transgenic mice compared with wild type, whole-cell recordings revealed excessive tonic eNMDAR activity accompanied by eNMDAR-sensitive loss of mEPSCs. Importantly, the improved NMDAR antagonist NitroMemantine, which selectively inhibits extrasynaptic over physiological synaptic NMDAR activity, protects synapses from Aβ-induced damage both in vitro and in vivo.

In vivo calibration of microdialysis using infusion of stable-isotope labeled neurotransmitters

In vivo calibration of microdialysis probes is required for interpreting measured concentrations. The most popular method of in vivo calibration is no-net-flux (NNF), which requires infusing several concentrations of neurotransmitters to determine in vivo recoveries (extraction fraction or Ed) and extracellular concentrations. A new method for in vivo calibration of microdialysis of neurotransmitters using glutamate (GLU) and dopamine (DA) as model analytes is reported. (13)C6-DA and (13)C5-GLU were perfused through microdialysis probes as internal calibrators. Using liquid chromatography with mass spectrometry, it was possible to distinguish the (13)C-forms from the endogenous forms of each neurotransmitter. Ed was directly calculated by measuring the loss of the (13)C-forms during infusion. The measured endogenous (12)C forms of the neurotransmitters could be corrected for Ed to give calibrated extracellular concentrations in vivo. Retrodialysis of stable-isotope-labeled (SIL) neurotransmitters gave Ed and extracellular concentrations of (13)C5-GLU and (13)C6-DA that matched no-net-flux measurements; however, the values were obtained in a fraction of time because no added measurements were required to obtain the calibration. Ed was reduced during uptake inhibition for GLU and DA when measured by SIL retrodialysis. Because Ed is directly measured at each microdialysis fraction, it was possible to monitor changes in Ed under transient conditions created by systemic injection of uptake inhibitors. The results show that DA and GLU concentrations are underestimated by as much as 50% if not corrected for Ed during uptake inhibition. SIL retrodialysis provides equivalent information to NNF at much reduced time and animal use.

Microfabricated sampling probes for in vivo monitoring of neurotransmitters

Microfabricated fluidic systems have emerged as a powerful approach for chemical analysis. Relatively unexplored is the use of microfabrication to create sampling probes. We have developed a sampling probe microfabricated in Si by bulk micromachining and lithography. The probe is 70 μm wide by 85 μm thick by 11 mm long and incorporates two buried channels that are 20 μm in diameter. The tip of the probe has two 20 μm holes where fluid is ejected or collected for sampling. Utility of the probe was demonstrated by sampling from the brain of live rats. For sampling, artificial cerebral spinal fluid was infused in through one channel at 50 nL/min while sample was withdrawn at the same flow rate from the other channel. Analysis of resulting fractions collected every 20 min from the striatum of rats by liquid chromatography with mass spectrometry demonstrated reliable detection of 17 neurotransmitters and metabolites. The small probe dimensions suggest it is less perturbing to tissue and can be used to sample smaller brain nuclei than larger sampling devices, such as microdialysis probes. This sampling probe may have other applications such as sampling from cells in culture. The use of microfabrication may also enable incorporation of electrodes for electrochemical or electrophysiological recording and other channels that enable more complex sample preparation on the device.

Chemical gradients within brain extracellular space measured using low flow push-pull perfusion sampling in vivo

Although populations of neurons are known to vary on the micrometer scale, little is known about whether basal concentrations of neurotransmitters also vary on this scale. We used low-flow push-pull perfusion to test if such chemical gradients exist between several small brain nuclei. A miniaturized polyimide-encased push-pull probe was developed and used to measure basal neurotransmitter spatial gradients within brain of live animals with 0.004 mm(3) resolution. We simultaneously measured dopamine (DA), norepinephrine, serotonin (5-HT), glutamate, γ-aminobutyric acid (GABA), aspartate (Asp), glycine (Gly), acetylcholine (ACh), and several neurotransmitter metabolites. Significant differences in basal concentrations between midbrain regions as little as 200 μm apart were observed. For example, dopamine in the ventral tegmental area (VTA) was 4.8 ± 1.5 nM but in the red nucleus was 0.5 ± 0.2 nM. Regions of high glutamate concentration and variability were found within the VTA of some individuals, suggesting hot spots of glutamatergic activity. Measurements were also made within the nucleus accumbens core and shell. Differences were not observed in dopamine and 5-HT in the core and shell; but their metabolites homovanillic acid (460 ± 60 nM and 130 ± 60 nM respectively) and 5-hydroxyindoleacetic acid (720 ± 200 nM and 220 ± 50 nM respectively) did differ significantly, suggesting differences in dopamine and 5-HT activity in these brain regions. Maintenance of these gradients depends upon a variety of mechanisms. Such gradients likely underlie highly localized effects of drugs and control of behavior that have been found using other techniques.

In vivo brain microdialysis as a formulation-screening tool for a poorly soluble centrally acting drug

OBJECTIVE

Efficacy of a formulation of a poorly soluble centrally acting drug was evaluated by measuring dopamine responses using in vivo brain microdialysis.

METHODS

Co-crystals (1:1) of carbamazepine and nicotinamide (CBZ-NCT) were complexed with cyclodextrins (γ-CDs) using supercritical fluid processing. Phase solubility and intrinsic dissolution were studied. Pharmacodynamic studies were performed on rats divided into three groups getting either CBZ-NCT in CD (20 mg/kg CBZ), pure CBZ solution or vehicle. A guide cannula was implanted to attach the microdialysis probe. Dialysate samples were analyzed for dopamine levels, which were compared between groups.

RESULTS

The optimized CBZ formulation (5% w/w in γ-CD) with solubility - 10 mg/mL showed stepwise increase in dopamine response (maximum 250% of baseline) compared to neat CBZ or vehicle (p < 0.05). The pharmacokinetics of the drug required 30 min to elicit CNS response, which peaked at about 1.5-2 h.

CONCLUSION

Hence, brain microdialysis was successfully used to evaluate a dissolution rate enhancing formulation.

Quantitation of dopamine, serotonin and adenosine content in a tissue punch from a brain slice using capillary electrophoresis with fast-scan cyclic voltammetry detection

Methods to determine neurochemical concentrations in small samples of tissue are needed to map interactions among neurotransmitters. In particular, correlating physiological measurements of neurotransmitter release and the tissue content in a small region would be valuable. HPLC is the standard method for tissue content analysis but it requires microliter samples and the detector often varies by the class of compound being quantified; thus detecting molecules from different classes can be difficult. In this paper, we develop capillary electrophoresis with fast-scan cyclic voltammetry detection (CE-FSCV) for analysis of dopamine, serotonin, and adenosine content in tissue punches from rat brain slices. Using field-amplified sample stacking, the limit of detection was 5 nM for dopamine, 10 nM for serotonin, and 50 nM for adenosine. Neurotransmitters could be measured from a tissue punch as small as 7 µg (7 nL) of tissue, three orders of magnitude smaller than a typical HPLC sample. Tissue content analysis of punches in successive slices through the striatum revealed higher dopamine but lower adenosine content in the anterior striatum. Stimulated dopamine release was measured in a brain slice, then a tissue punch collected from the recording region. Dopamine content and release had a correlation coefficient of 0.71, which indicates much of the variance in stimulated release is due to variance in tissue content. CE-FSCV should facilitate measurements of tissue content in nanoliter samples, leading to a better understanding of how diseases or drugs affect dopamine, serotonin, and adenosine content.

Adolescent rats are resistant to adaptations in excitatory and inhibitory mechanisms that modulate mesolimbic dopamine during nicotine withdrawal

Adolescent smokers report enhanced positive responses to tobacco and fewer negative effects of withdrawal from this drug than adults, and this is believed to propel higher tobacco use during adolescence. Differential dopaminergic responses to nicotine are thought to underlie these age-related effects, as adolescent rats experience lower withdrawal-related deficits in nucleus accumbens (NAcc) dopamine versus adults. This study examined whether age differences in NAcc dopamine during withdrawal are mediated by excitatory or inhibitory transmission in the ventral tegmental area (VTA) dopamine cell body region. In vivo microdialysis was used to monitor extracellular levels of glutamate and gamma-aminobutyric acid (GABA) in the VTA of adolescent and adult rats experiencing nicotine withdrawal. In adults, nicotine withdrawal produced decreases in VTA glutamate levels (44% decrease) and increases in VTA GABA levels (38% increase). In contrast, adolescents did not exhibit changes in either of these measures. Naïve controls of both ages did not display changes in NAcc dopamine, VTA glutamate, or VTA GABA following mecamylamine. These results indicate that adolescents display resistance to withdrawal-related neurochemical processes that inhibit mesolimbic dopamine function in adults experiencing nicotine withdrawal. Our findings provide a potential mechanism involving VTA amino acid neurotransmission that modulates age differences during withdrawal.

A sensitive and selective quantification of catecholamine neurotransmitters in rat microdialysates by pre-column dansyl chloride derivatization using liquid chromatography-tandem mass spectrometry

A rapid and sensitive liquid chromatography tandem mass spectrometry method for simultaneous quantification of catecholamine neurotransmitters in microdialysates was developed. The catecholamine neurotransmitters dopamine (DA) and norepinephrine (NE) were pre-column derivatized with dansyl chloride and analyzed. A gradient elution method was used to separate the analytes from the interferences on an Agilent Poroshell 120 EC-C18 outer porous micro particulate column. The method was robust and sensitive to determine with the lower limit of quantification value of 0.068pmol/mL and 0.059pmol/mL for DA and NE, respectively. It has acceptable precision and accuracy for concentrations over the standard curve range. The method was successfully applied for simultaneous quantitation of DA and NE in the prefrontal cortex (PFC) dialysates of rats obtained from a microdialysis study dosed with vehicle and atomoxetine through intra peritoneal (i.p.) route at a dose of 3mg/kg to monitor the change in extracellular concentrations. Thus, accomplishment of this method would facilitate the neurochemical monitoring for discovery of new chemical entities targeted for the treatment of attention deficit hyperactivity disorder (ADHD).

Sodium dodecyl sulfate-modified electrochemical paper-based analytical device for determination of dopamine levels in biological samples

We report the development of an electrochemical paper-based analytical device (ePAD) for the selective determination of dopamine (DA) in model serum sample. The ePAD device consists of three layers. In the top layer, SU-8 photoresist defines a hydrophilic sample application spot on the filter paper. The middle layer was made from transparency film and contained two holes, one for sample preconcentration and the other for the surfactant to allow transfer to the third layer. A screen-printed carbon electrode formed the bottom layer and was used for electrochemical measurements. In the absence of the anionic surfactant, sodium dodecyl sulfate (SDS), the oxidation peaks of DA, ascorbic acid (AA) and uric acid (UA) overlapped. With the addition of SDS, the DA oxidation peak shifted to more negative values and was clearly distinguishable from AA and UA. The oxidation potential shift was presumably due to preferential electrostatic interactions between the cationic DA and the anionic SDS. Indeed, whilst the SDS-modified paper improved the DA current five-fold, the non-ionic Tween-20 and cationic tetradecyltrimethylammonium bromide surfactants had no effect or reduced the current, respectively. Furthermore, only the SDS-modified paper showed the selective shift in oxidation potential for DA. DA determination was carried out using square-wave voltammetry between -0.2 and 0.8 V vs. Ag/AgCl, and this ePAD was able to detect DA over a linear range of 1-100 μM with a detection limit (S/N=3) of 0.37 μM. The ePAD seems suitable as a low cost, easy-to-use, portable device for the selective quantitation of DA in human serum samples.

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.