Simultaneous oxytocin and arg-vasopressin measurements in microdialysates using capillary liquid chromatography-mass spectrometry

Systematic review: pain, cognition, and cardioprotection-unpacking oxytocin's contributions in a sport context

Introduction

This systematic review investigates the interplay between oxytocin and exercise; in terms of analgesic, anti-inflammatory, pro-regenerative, and cardioprotective effects. Furthermore, by analyzing measurement methods, we aim to improve measurement validity and reliability.

Methods

Utilizing PRISMA, GRADE, and MECIR protocols, we examined five databases with a modified SPIDER search. Including studies on healthy participants, published within the last 20 years, based on keywords "oxytocin," "exercise" and "measurement," 690 studies were retrieved initially (455 unique records). After excluding studies of clinically identifiable diseases, and unpublished and reproduction-focused studies, 175 studies qualified for the narrative cross-thematic and structural analysis.

Results

The analysis resulted in five categories showing the reciprocal impact of oxytocin and exercise: Exercise (50), Physiology (63), Environment (27), Social Context (65), and Stress (49). Exercise-induced oxytocin could promote tissue regeneration, with 32 studies showing its analgesic and anti-inflammatory effects, while 14 studies discussed memory and cognition. Furthermore, empathy-associated OXTR rs53576 polymorphism might influence team sports performance. Since dietary habits and substance abuse can impact oxytocin secretion too, combining self-report tests and repeated salivary measurements may help achieve precision.

Discussion

Oxytocin's effect on fear extinction and social cognition might generate strategies for mental training, and technical, and tactical development in sports. Exercise-induced oxytocin can affect the amount of stress experienced by athletes, and their response to it. However, oxytocin levels could depend on the type of sport in means of contact level, exercise intensity, and duration. The influence of oxytocin on athletes' performance and recovery could have been exploited due to its short half-life. Examining oxytocin's complex interactions with exercise paves the way for future research and application in sports science, psychology, and medical disciplines.

Systematic Review Registration

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=512184, identifier CRD42024512184.

Recent advances in mass spectrometry analysis of neuropeptides

Due to their involvement in numerous biochemical pathways, neuropeptides have been the focus of many recent research studies. Unfortunately, classic analytical methods, such as western blots and enzyme-linked immunosorbent assays, are extremely limited in terms of global investigations, leading researchers to search for more advanced techniques capable of probing the entire neuropeptidome of an organism. With recent technological advances, mass spectrometry (MS) has provided methodology to gain global knowledge of a neuropeptidome on a spatial, temporal, and quantitative level. This review will cover key considerations for the analysis of neuropeptides by MS, including sample preparation strategies, instrumental advances for identification, structural characterization, and imaging; insightful functional studies; and newly developed absolute and relative quantitation strategies. While many discoveries have been made with MS, the methodology is still in its infancy. Many of the current challenges and areas that need development will also be highlighted in this review.

Optimized Sawhorse Waveform for the Measurement of Oxytocin Release in Zebrafish

Oxytocin is a nonapeptide hormone involved in numerous physiological functions. Real-time electrochemical measurements of oxytocin in living tissue are challenging due to electrode fouling and the large potentials needed to oxidize the tyrosine residue. Here, we used fast-scan cyclic voltammetry at carbon-fiber microelectrodes and flow injection analysis to optimize a waveform for the measurement of oxytocin. This optimized waveform employed an accumulation potential of -0.6 V, multiple scan rates, and a 3 ms holding potential at a positive, oxidizing potential of +1.4 V before linearly scanning the potential back to -0.6 V (versus Ag/AgCl). We obtained a limit of quantitation of 0.34 ± 0.02 μM, and our electrodes did not foul upon multiple injections. Moreover, to demonstrate the utility of our method, we measured the release of oxytocin, evoked by light application and mechanical perturbation, in whole brains from genetically engineered adult zebrafish that express channelrhodopsin-2 selectively on oxytocinergic neurons. Collectively, this work expands the toolkit for the measurement of peptides in living tissue preparations.

Oxytocin testing and reproductive health: Status and clinical applications

Oxytocin (OT) is a nonapeptide hormone mainly synthesized in the magnocellular neurons of the paraventricular and supraoptic nucleus of the hypothalamus. In the extra-hypothalamic brain areas, OT acts like neurotransmitters and modulators. The physiological functions of OT are multiple. OT participates to the coordination and control of gonadal development and reproduction. OT appears also as an important regulator of social behaviors such as affiliative, parental, and romantic behaviors. Recent evidence suggests other roles for OT such as potent effects on cardiometabolic functions or involvement in stress-related disorders. The growing interest around the clinical role of OT raised the question of the measurement of OT levels and performances of assays.

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

Dendritic Release of Neurotransmitters

Release of neuroactive substances by exocytosis from dendrites is surprisingly widespread and is not confined to a particular class of transmitters: it occurs in multiple brain regions, and includes a range of neuropeptides, classical neurotransmitters, and signaling molecules, such as nitric oxide, carbon monoxide, ATP, and arachidonic acid. This review is focused on hypothalamic neuroendocrine cells that release vasopressin and oxytocin and midbrain neurons that release dopamine. For these two model systems, the stimuli, mechanisms, and physiological functions of dendritic release have been explored in greater detail than is yet available for other neurons and neuroactive substances. © 2017 American Physiological Society. Compr Physiol 7:235-252, 2017.

Development of a High-Sensitivity Quantitation Method for Arginine Vasopressin by High-Performance Liquid Chromatography Tandem Mass Spectrometry, and Comparison with Quantitative Values by Radioimmunoassay

Human plasma arginine vasopressin (AVP) levels serve as a clinically relevant marker of diabetes and related syndromes. We developed a highly sensitive method for measuring human plasma AVP using high-performance liquid chromatography tandem mass spectrometry. AVP was extracted from human plasma using a weak-cation solid-phase extraction plate, and separated on a wide-bore octadecyl reverse-phase column. AVP was quantified in ion-transition experiments utilizing a product ion (m/z 328.3) derived from its parent ion (m/z 542.8). The sensitivity was enhanced using 0.02% dichloromethane as a mobile-phase additive. The lower limit of quantitation was 0.200 pmol/L. The extraction recovery ranged from 70.2 ± 7.2 to 73.3 ± 6.2% (mean ± SD), and the matrix effect ranged from 1.1 - 1.9%. Quality-testing samples revealed interday/intraday accuracy and precision ranging over 0.9 - 3% and -0.3 - 2%, respectively, which included the endogenous baseline. Our results correlated well with radioimmunoassay results using 22 human volunteer plasma samples.

The frontal cortex as a network hub controlling mood and cognition: Probing its neurochemical substrates for improved therapy of psychiatric and neurological disorders

The highly-interconnected and neurochemically-rich frontal cortex plays a crucial role in the regulation of mood and cognition, domains disrupted in depression and other central nervous system disorders, and it is an important site of action for their therapeutic control. For improving our understanding of the function and dysfunction of the frontal cortex, and for identifying improved treatments, quantification of extracellular pools of neuromodulators by microdialysis in freely-moving rodents has proven indispensable. This approach has revealed a complex mesh of autoreceptor and heteroceptor interactions amongst monoaminergic pathways, and led from selective 5-HT reuptake inhibitors to novel classes of multi-target drugs for treating depression like the mixed α₂-adrenoceptor/5-HT reuptake inhibitor, S35966, and the clinically-launched vortioxetine and vilazodone. Moreover, integration of non-monoaminergic actions resulted in the discovery and development of the innovative melatonin receptor agonist/5-HT_2C receptor antagonist, Agomelatine. Melatonin levels, like those of corticosterone and the "social hormone", oxytocin, can now be quantified by microdialysis over the full 24 h daily cycle. Further, the introduction of procedures for measuring extracellular histamine and acetylcholine has provided insights into strategies for improving cognition by, for example, blockade of 5-HT₆ and/or dopamine D₃ receptors. The challenge of concurrently determining extracellular levels of GABA, glutamate, d-serine, glycine, kynurenate and other amino acids, and of clarifying their interactions with monoamines, has also been resolved. This has proven important for characterizing the actions of glycine reuptake inhibitors that indirectly augment transmission at N-methyl-d-aspartate receptors, and of "glutamatergic antidepressants" like ketamine, mGluR5 antagonists and positive modulators of AMPA receptors (including S47445). Most recently, quantification of the neurotoxic proteins Aβ42 and Tau has extended microdialysis studies to the pathogenesis of neurodegenerative disorders, and another frontier currently being broached is microRNAs. The present article discusses the above themes, focusses on recent advances, highlights opportunities for clinical "translation", and suggests avenues for further progress.

Continuous and selective measurement of oxytocin and vasopressin using boron-doped diamond electrodes

The electrochemical detection of oxytocin using boron-doped diamond (BDD) electrodes was studied. Cyclic voltammetry of oxytocin in a phosphate buffer solution exhibits an oxidation peak at +0.7 V (vs. Ag/AgCl), which is attributable to oxidation of the phenolic group in the tyrosyl moiety. Furthermore, the linearity of the current peaks obtained in flow injection analysis (FIA) using BDD microelectrodes over the oxytocin concentration range from 0.1 to 10.0 μM with a detection limit of 50 nM (S/N = 3) was high (R(2) = 0.995). Although the voltammograms of oxytocin and vasopressin observed with an as-deposited BDD electrode, as well as with a cathodically-reduced BDD electrode, were similar, a clear distinction was observed with anodically-oxidized BDD electrodes due to the attractive interaction between vasopressin and the oxidized BDD surface. By means of this distinction, selective measurements using chronoamperometry combined with flow injection analysis at an optimized potential were demonstrated, indicating the possibility of making selective in situ or in vivo measurements of oxytocin.

Challenges for the in vivo quantification of brain neuropeptides using microdialysis sampling and LC-MS

In recent years, neuropeptides and their receptors have received an increased interest in neuropharmacological research. Although these molecules are considered relatively small compared with proteins, their in vivo quantification using microdialysis is more challenging than for small molecules. Low microdialysis recoveries, aspecific adsorption and the presence of various multiply charged precursor ions during ESI-MS/MS detection hampers the in vivo quantification of these low abundant biomolecules. Every step in the workflow, from sampling until analysis, has to be optimized to enable the sensitive analysis of these compounds in microdialysates.

Proteomics tools reveal startlingly high amounts of oxytocin in plasma and serum

The neuropeptide oxytocin (OT) is associated with a plethora of social behaviors, and is a key topic at the intersection of psychology and biology. However, tools for measuring OT are still not fully developed. We describe a robust nano liquid chromatography-mass spectrometry (nanoLC-MS) platform for measuring the total amount of OT in human plasma/serum. OT binds strongly to plasma proteins, but a reduction/alkylation (R/A) procedure breaks this bond, enabling ample detection of total OT. The method (R/A + robust nanoLC-MS) was used to determine total OT plasma/serum levels to startlingly high concentrations (high pg/mL-ng/mL). Similar results were obtained when combining R/A and ELISA. Compared to measuring free OT, measuring total OT can have advantages in e.g. biomarker studies.

Improving the Sensitivity, Resolution, and Peak Capacity of Gradient Elution in Capillary Liquid Chromatography with Large-Volume Injections by Using Temperature-Assisted On-Column Solute Focusing

Capillary HPLC (cLC) with gradient elution is the separation method of choice for the fields of proteomics and metabolomics. This is due to the complementary nature of cLC flow rates and electrospray or nanospray ionization mass spectrometry (ESI-MS). The small column diameters result in good mass sensitivity. Good concentration sensitivity is also possible by injection of relatively large volumes of solution and relying on solvent-based solute focusing. However, if the injection volume is too large or solutes are poorly retained during injection, volume overload occurs which leads to altered peak shapes, decreased sensitivity, and lower peak capacity. Solutes that elute early even with the use of a solvent gradient are especially vulnerable to this problem. In this paper, we describe a simple, automated instrumental method, temperature-assisted on-column solute focusing (TASF), that is capable of focusing large volume injections of small molecules and peptides under gradient conditions. By injecting a large sample volume while cooling a short segment of the column inlet at subambient temperatures, solutes are concentrated into narrow bands at the head of the column. Rapidly raising the temperature of this segment of the column leads to separations with less peak broadening in comparison to solvent focusing alone. For large volume injections of both mixtures of small molecules and a bovine serum albumin tryptic digest, TASF improved the peak shape and resolution in chromatograms. TASF showed the most dramatic improvements with shallow gradients, which is particularly useful for biological applications. Results demonstrate the ability of TASF with gradient elution to improve the sensitivity, resolution, and peak capacity of volume overloaded samples beyond gradient compression alone. Additionally, we have developed and validated a double extrapolation method for predicting retention factors at extremes of temperature and mobile phase composition. Using this method, the effects of TASF can be predicted, allowing determination of the usefulness of this technique for a particular application.

Haemodynamic and renal sympathetic responses to V1b vasopressin receptor activation within the paraventricular nucleus

NEW FINDINGS

What is the central question of this study? Does antagonism of V1b receptors prevent the haemodynamic and renal sympathetic nerve responses that occur with application of exogenous vasopressin into the paraventricular nucleus (PVN) of conscious, chronically instrumented rats? What is the main finding and its importance? Microinjection of vasopressin into the PVN increased mean arterial pressure, heart rate and renal sympathetic nerve activity, all of which were inhibited by pre-injection of the PVN with the V1b antagonist, nelivaptan. The administered vasopressin did not enter the peripheral circulation or increase plasma vasopressin. Ganglionic blockade prevented each of the responses, consistent with mediation by enhanced sympathetic output rather than an increase in circulating vasopressin. Vasopressin (VP) participates in regulation of haemodynamics and volume. Besides more classical actions as a circulating hormone, VP may act via release from axons and dendrites within the CNS. The paraventricular nucleus (PVN) possesses vasopressinergic neurons and a dense complement of VP receptors, including the V1b receptor, which has been implicated in several types of stress responses. We tested the hypothesis that antagonism of V1b receptors will prevent VP-induced increases in mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA). Studies were performed in conscious male Sprague-Dawley rats chronically instrumented with vascular catheters, renal nerve electrodes and a cannula stereotaxically directed into the PVN. Unilateral microinjection of VP into the PVN significantly increased MAP, HR and RSNA, peaking at 10 min. Pre-injection of the PVN with the selective V1b receptor antagonist, nelivaptan, did not alter baseline values but blocked the responses to VP. Ganglionic blockade with chlorisondamine decreased MAP and HR and abolished their increase in response to subsequent PVN application of VP. Injection of VP into the PVN did not alter plasma VP levels. Paraventricular nucleus injection with radiolabelled VP resulted in negligible radiolabelled VP in peripheral blood. These findings support the concept that, in basal conditions, PVN V1b receptor activation (rather than VP release into the periphery) may be implicated in the increases in MAP, HR and RSNA due to increased sympathetic outflow. While the role of V1a and oxytocin receptors cannot be excluded, these data suggest that further studies of the role of V1b receptor activation by endogenous VP during stress to effect neuroexcitation are warranted.

An ultrasensitive nano UHPLC-ESI-MS/MS method for the quantification of three neuromedin-like peptides in microdialysates

AIM

An ultrasensitive nano UHPLC-ESI-MS/MS method is developed to simultaneously monitor three low-concentration neuromedin-like peptides in microdialysates.

RESULTS

Peptide preconcentration and sample desalting is performed online on a trap column. A shallow gradient slope at 300 nl/min on the analytical column maintained at 35°C, followed by two saw-tooth column wash cycles, results in the highest sensitivity and the lowest carryover. The validated method allows the accurate and precise quantification of 0.5 pM neurotensin and neuromedin N (2.5 amol on column), and of 3.0 pM neuromedin B (15.0 amol on column) in in vivo microdialysates without the use of internal standards.

CONCLUSION

The assay is an important tool for elucidating the role of these neuromedin-like peptides in the pathophysiology of neurological disorders.

Reducing adsorption to improve recovery and in vivo detection of neuropeptides by microdialysis with LC-MS

Neuropeptides are an important class of neurochemicals; however, measuring their concentration in vivo by using microdialysis sampling is challenging due to their low concentration and the small samples generated. Capillary liquid chromatography with mass spectrometry (cLC-MS) can yield attomole limits of detection (LOD); however, low recovery and loss of sample to adsorptive surfaces can still hinder detection of neuropeptides. We have evaluated recovery during sampling and transfer to the cLC column for a selection of 10 neuropeptides. Adding acetonitrile to sample eliminated carryover and improved LOD by 1.4- to 60-fold. The amount of acetonitrile required was found to have an optimal value that correlated with peptide molecular weight and retention time on a reversed phase LC column. Treating AN69 dialysis membrane, which bears negative charge due to incorporated sulfonate groups, with polyethylenimine (PEI) improved recovery by 1.2- to 80-fold. The effect appeared to be due to reducing electrostatic interaction between peptides and the microdialysis probe because modification increased recovery only for peptides that carried net positive charge. The combined effects improved LOD of the entire method by 1.3- to 800-fold for the different peptides. We conclude that peptides with both charged and hydrophobic regions require combined strategies to prevent adsorption and yield the best possible detection. The method was demonstrated by determining orexin A, orexin B, and a novel isoform of rat β-endorphin in the arcuate nucleus. Dialysate concentrations were below 10 pM for these peptides. A standard addition study on dialysates revealed that while some peptides can be accurately quantified, some are affected by the matrix.

Defining the Neuropeptidome of the Spiny Lobster Panulirus interruptus Brain Using a Multidimensional Mass Spectrometry-Based Platform

Decapod crustaceans are important animal models for neurobiologists due to their relatively simple nervous systems with well-defined neural circuits and extensive neuromodulation by a diverse set of signaling peptides. However, biochemical characterization of these endogenous neuropeptides is often challenging due to limited sequence information about these neuropeptide genes and the encoded preprohormones. By taking advantage of sequence homology in neuropeptides observed in related species using a home-built crustacean neuropeptide database, we developed a semi-automated sequencing strategy to characterize the neuropeptidome of Panulirus interruptus, an important aquaculture species, with few known neuropeptide preprohormone sequences. Our streamlined process searched the high mass accuracy and high-resolution data acquired on a LTQ-Orbitrap with a flexible algorithm in ProSight that allows for sequence discrepancy from reported sequences in our database, resulting in the detection of 32 neuropeptides, including 19 novel ones. We further improved the overall coverage to 51 neuropeptides with our multidimensional platform that employed multiple analytical techniques including dimethylation-assisted fragmentation, de novo sequencing using nanoliquid chromatography-electrospray ionization-quadrupole-time-of-flight (nanoLC-ESI-Q-TOF), direct tissue analysis, and mass spectrometry imaging on matrix-assisted laser desorption/ionization (MALDI)-TOF/TOF. The high discovery rate from this unsequenced model organism demonstrated the utility of our neuropeptide discovery pipeline and highlighted the advantage of utilizing multiple sequencing strategies. Collectively, our study expands the catalog of crustacean neuropeptides and more importantly presents an approach that can be adapted to exploring neuropeptidome from species that possess limited sequence information.

Advances in Mass Spectrometric Tools for Probing Neuropeptides

Neuropeptides are important mediators in the functionality of the brain and other neurological organs. Because neuropeptides exist in a wide range of concentrations, appropriate characterization methods are needed to provide dynamic, chemical, and spatial information. Mass spectrometry and compatible tools have been a popular choice in analyzing neuropeptides. There have been several advances and challenges, both of which are the focus of this review. Discussions range from sample collection to bioinformatic tools, although avenues such as quantitation and imaging are included. Further development of the presented methods for neuropeptidomic mass spectrometric analysis is inevitable, which will lead to a further understanding of the complex interplay of neuropeptides and other signaling molecules in the nervous system.

Ventral tegmental area neurotensin signaling links the lateral hypothalamus to locomotor activity and striatal dopamine efflux in male mice

Projections from the lateral hypothalamic area (LHA) innervate components of the mesolimbic dopamine (MLDA) system, including the ventral tegmental area (VTA) and nucleus accumbens (NAc), to modulate motivation appropriately for physiologic state. Neurotensin (NT)-containing LHA neurons respond to multiple homeostatic challenges and project to the VTA, suggesting that these neurons could link such signals to MLDA function. Indeed, we found that pharmacogenetic activation of LHA NT neurons promoted prolonged DA-dependent locomotor activity and NAc DA efflux, suggesting the importance of VTA neurotransmitter release by LHA NT neurons for the control of MLDA function. Using a microdialysis-mass spectrometry technique that we developed to detect endogenous NT in extracellular fluid in the mouse brain, we found that activation of LHA NT cells acutely increased the extracellular concentration of NT (a known activator of VTA DA cells) in the VTA. In contrast to the prolonged elevation of extracellular NAc DA, however, VTA NT concentrations rapidly returned to baseline. Intra-VTA infusion of NT receptor antagonist abrogated the ability of LHA NT cells to increase extracellular DA in the NAc, demonstrating that VTA NT promotes NAc DA release. Thus, transient LHA-derived NT release in the VTA couples LHA signaling to prolonged changes in DA efflux and MLDA function.

Hydration and beyond: neuropeptides as mediators of hydromineral balance, anxiety and stress-responsiveness

Challenges to body fluid homeostasis can have a profound impact on hypothalamic regulation of stress responsiveness. Deficiencies in blood volume or sodium concentration leads to the generation of neural and humoral signals relayed through the hindbrain and circumventricular organs that apprise the paraventricular nucleus of the hypothalamus (PVH) of hydromineral imbalance. Collectively, these neural and humoral signals converge onto PVH neurons, including those that express corticotrophin-releasing factor (CRF), oxytocin (OT), and vasopressin, to influence their activity and initiate compensatory responses that alleviate hydromineral imbalance. Interestingly, following exposure to perceived threats to homeostasis, select limbic brain regions mediate behavioral and physiological responses to psychogenic stressors, in part, by influencing activation of the same PVH neurons that are known to maintain body fluid homeostasis. Here, we review past and present research examining interactions between hypothalamic circuits regulating body fluid homeostasis and those mediating behavioral and physiological responses to psychogenic stress.

Liquid chromatography-mass spectrometry platform for both small neurotransmitters and neuropeptides in blood, with automatic and robust solid phase extraction

Neurons communicate via chemical signals called neurotransmitters (NTs). The numerous identified NTs can have very different physiochemical properties (solubility, charge, size etc.), so quantification of the various NT classes traditionally requires several analytical platforms/methodologies. We here report that a diverse range of NTs, e.g. peptides oxytocin and vasopressin, monoamines adrenaline and serotonin, and amino acid GABA, can be simultaneously identified/measured in small samples, using an analytical platform based on liquid chromatography and high-resolution mass spectrometry (LC-MS). The automated platform is cost-efficient as manual sample preparation steps and one-time-use equipment are kept to a minimum. Zwitter-ionic HILIC stationary phases were used for both on-line solid phase extraction (SPE) and liquid chromatography (capillary format, cLC). This approach enabled compounds from all NT classes to elute in small volumes producing sharp and symmetric signals, and allowing precise quantifications of small samples, demonstrated with whole blood (100 microliters per sample). An additional robustness-enhancing feature is automatic filtration/filter back-flushing (AFFL), allowing hundreds of samples to be analyzed without any parts needing replacement. The platform can be installed by simple modification of a conventional LC-MS system.

Structural characterization of arginine-vasopressin and lysine-vasopressin by Fourier- transform ion cyclotron resonance mass spectrometry and infrared multiphoton dissociation

Arginine-vasopressin (AVP) and lysine-vasopressin (LVP) were analyzed by reversed-phase liquid chromatography/mass spectrometry (LC-MS) using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) electrospray ionization (ESI) in the positive ion mode. LVP and AVP exhibited the protonated adduct [M+H](+) as the predominant ion at m/z 1056.43965 and at m/z 1084.44561, respectively. Infrared multiphoton dissociation (IRMPD), using a CO(2) laser source at a wavelength of 10.6 μm, was applied to protonated vasopressin molecules. The IRMPD mass spectra presented abundant mass fragments essential for a complete structural information. Several fragment ions, shared between two target molecules, are discussed in detail. Some previously unpublished fragments were identified unambiguously utilizing the high resolution and accurate mass information provided by the FT-ICR mass spectrometer. The opening of the disulfide loop and the cleavage of the peptide bonds within the ring were observed even under low-energy fragmentation conditions. Coupling the high-performance FT-ICR mass spectrometer with IRMPD as a contemporary fragmentation technique proved to be very promising for the structural characterization of vasopressin.

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.

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.

In vivo dialysis setup with a loop injection valve facilitates retrodialysis studies

INTRODUCTION

Retrodialysis, as used in neuropharmacological research, is a technique for in vivo delivery of neuroactive agents with concurrent monitoring of their effects on cellular activity with a separation between certain degree of spatial and temporal resolution. Typically, this is accomplished either by the use of a liquid-switch requiring multiple pumps, or by exchange of flow tubing requiring stopping and restarting dialysis. In the present study, we describe the use of a medium pressure injection valve for retrodialysis that overcomes these problems.

METHODS

The valve was configured with a loop to deliver 20μL of solution, and artificial CSF flow from the pump to the probe was established via this device. The application of this setup was evaluated in urethane anesthetized adult male C57BL/6J mice prepared with a CMA 11 probe implanted in the ventral hippocampus. By switching between the load and inject positions, the loop was filled with escitalopram solution (0.3μM) and delivered at a rate of 1μL/min at the probe for retrodialysis. Escitalopram (2mg/kg BW) was administered subcutaneously for microdialysis studies. During these treatments, dialysate fractions were collected for the determination of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA).

RESULTS

Irrespective of route of escitalopram administration, the pattern of dialysate 5-HT, and 5-HIAA response was comparable to that reported by other investigators. Accordingly, the in-line valve assembly did not compromise retrodialysis or microdialysis sampling. The manipulations to carry out retrodialysis using the valve setup are easy and simple.

DISCUSSION

An in-line injection valve is a promising adaptation for retrodialysis studies and can be incorporated as a standard part of in vivo dialysis instrumentation.