A highly sensitive assay of melatonin at the femotogram level in human plasma by gas chromatography/negative ion chemical ionization mass spectrometry

Copper(II) phthalocyanine as an electrocatalytic electrode for cathodic detection of urinary tryptophan

Herein, we introduce a novel method for tryptophan detection via a reduction reaction facilitated by its interaction with a copper(II) phthalocyanine (CuPc) electrocatalytic electrode. This method addresses challenges associated with the susceptibility of the oxidation response to interference from various species when measuring tryptophan in bodily fluids. The reduction currents exhibit a linear increase with tryptophan concentrations in two ranges: 0.0013-0.10 mM and 0.10-1.20 mM, with the sensitivities of 14.7 ± 0.5 μA mM-1 and 3.5 ± 0.1 μA mM-1, respectively. The limit of detection (LOD, 3SB/m) is determined to be 0.39 μM. The sensor exhibits excellent reproducibility, with the relative standard deviation of <5%. Application of the sensor to authentic urine samples yields a % recovery of 101 ± 4%.

Recent Advances in Nanomaterials Based Optical Sensors for the Detection of Melatonin and Serotonin

In this review paper we discussed the detection of melatonin and serotonin by using various optical methods. Melatonin and serotonin are very necessary body hormones these are also called neuroregulatory hormones secreted by pineal gland in brain by pinealocytes and shape of pineal gland is cone like. Sensitive detection of melatonin and serotonin in pharmacological samples and human serum is crucial for human beings, lots of research publications available in literature for melatonin and serotonin and we overviewed these papers. We have deeply reviewed many research papers where sensitively sensing of melatonin and serotonin occurs, by using of various interfering agents and nanomaterials. This review aims presenting colorimetry, fluorometry and spectrophotometric detection of melatonin (MEL) and serotonin (SER) by using different metal oxides, carbon nanomaterials (nanosheets, nanorods, nanofibers) and many other agents. Nanomaterials typically possess favourable optical, electrical and mechanical characteristics, they provide up new avenues for enhancing the efficacy of sensors. It is crucial to provide an optical sensors platform that is dependable, sensitive and low price. The development of sensors and biosensors to use nanomaterials for neurotransmitters has advanced significantly in recent years. There are currently many developing biomarkers in biological fluids, and bionanomaterial-based biosensor systems, as well as clinical and pharmacological settings, have garnered significant interest. Biomarkers have been found using optical devices in a quick, selective and sensitive manner. Our aim is to compile all the data that already published on MEL, SER sensing and comparison of each method, we mainly focused on principle, observations, sensitivity, selectivity, limit of detection, mechanism behind the reaction, effect of temperature, pH and concentration. In the last of this paper, we discuss some challenges of these methods and future projects.

A Cross-Sectional Study of Myopia and Morning Melatonin Status in Northern Irish Adolescent Children

Purpose

Previous studies have demonstrated an association between melatonin status and both refractive error and axial length in young adult myopes. This study aimed to determine if this relationship extends to a younger adolescent cohort.

Methods

Healthy children aged 12-15 years provided morning saliva samples before attending Ulster University (55°N) for cycloplegic autorefraction and axial length measures. Participants completed questionnaires describing recent sleep habits and physical activity. Salivary melatonin was quantified using high-performance liquid chromatography-tandem mass spectrometry. Data collection for all participants occurred over a 1-week period (April 2021).

Results

Seventy participants aged 14.3 (95% CI: 14.2-14.5) years were categorised by spherical equivalent refraction [SER] (range: -5.38DS to +1.88DS) into two groups; myopic SER ≤ -0.50DS (n = 22) or nonmyopic -0.50DS < SER ≤ +2.00DS (n = 48). Median morning salivary melatonin levels were 4.52 pg/ml (95% CI: 2.60-6.02) and 4.89 pg/ml (95% CI: 3.18-5.66) for myopic and nonmyopic subjects, respectively, and did not differ significantly between refractive groups (P = 0.91). Melatonin levels were not significantly correlated with SER, axial length, sleep, or activity scores (Spearman's rank, all P > 0.39). Higher levels of physical activity were associated with higher sleep quality (Spearman's rank, ρ = -0.28, P = 0.02).

Conclusion

The present study found no significant relationship between morning salivary melatonin levels and refractive error or axial length in young adolescents. This contrasts with outcomes from a previous study of adults with comparable methodology, season of data collection, and geographical location. Prospective studies are needed to understand the discrepancies between adult and childhood findings and evaluate whether melatonin levels in childhood are indicative of an increased risk for future onset of myopia and/or faster axial growth trajectories and myopia progression in established myopes. Future work should opt for a comprehensive dim-light melatonin onset protocol to determine circadian phase.

Development and Validation of an Ultrasensitive LC-MS/MS Method for the Quantification of Melatonin in Human Saliva

A growing body of literature describes the potential effects of circadian disruption on human health. Indeed, psychiatric diseases, metabolic syndrome, and cancers may be linked to disturbance of the circadian rhythm. Currently, the best practice to assess circadian rhythm is the measurement of melatonin levels. Our goal was thus to develop and validate a highly sensitive LC-MS/MS method to follow salivary melatonin levels throughout the day and night. Our method reached a lower limit of the measuring interval (LLMI) of 0.8 pg/mL. To our knowledge, it is the most sensitive method allowing quantitation of melatonin in saliva. Saliva, obtained from passive drooling or salivette, was extracted by an efficient and quick liquid-liquid extraction with no further cleanup needed. The method was validated according to the European Medicines Agency (EMA) guidelines and provided excellent results regarding accuracy, precision, linearity, selectivity, and specificity. Comparison between radioimmunoassay and our method was performed and showed differences at low levels, most likely due to cross-reactivity with other indols. To assess daytime melatonin levels in humans, salivary melatonin levels of ten volunteers were monitored throughout the day and showed lower daytime levels than reported in previous studies.

Quantification of serotonin and eight of its metabolites in plasma of healthy volunteers by mass spectrometry

Serotonin is transformed into melatonin under the control of the light/dark cycle, representing a cornerstone of circadian rhythmicity. Serotonin also undergoes extensive metabolism to produce 5-hydroxyindoleacetic acid (5-HIAA), a biomarker for the diagnosis and monitoring of serotonin secreting neuroendocrine tumors (NETs). While serotonin, melatonin and their metabolites are part of an integrated comprehensive system, human observations about their respective plasma concentrations are still limited. We report here for the first time a multiplex UHPLC-MS/MS assay for the quantification of serotonin, 5-HIAA, 5-hydroxytryptophol (5-HTPL), N-acetyl-serotonin (NAS), Mel, 6-OH-Mel, 5-methoxytryptamine (5-MT), 5-methoxytryptophol (5-MTPL), and 5-methoxyindoleacetic acid (5-MIAA) in human plasma. Analytes were extracted by protein precipitation and solid phase extraction. Plasma concentrations for these analytes were determined in 102 healthy volunteers. The LLOQ of the assay ranges from 2.2 nM for serotonin to 1.0 pM for 6-OH-Mel. This sensitivity enables the quantification of circulating serotonin, 5-HIAA, NAS, Mel, and 5-MIAA, even at their lowest diurnal concentrations. This assay will enable specific, precise and accurate measurement of serotonin, Mel and their metabolites to draw a detailed picture of this complex pineal metabolism, allowing a dynamic understanding of these pathways and providing promising biomarkers and a metabolic signature for serotonin-secreting NETs.

What do we really know about the safety and efficacy of melatonin for sleep disorders?

Melatonin is a hormonal product of the pineal gland, a fact that is often forgotten. Instead it is promoted as a dietary supplement that will overcome insomnia, as an antioxidant and as a prescription only drug in most countries outside the United States of America and Canada. The aim of this review is to step back and highlight what we know about melatonin following its discovery 60 years ago. What is the role of endogenous melatonin; what does melatonin do to sleep, body temperature, circadian rhythms, the cardiovascular system, reproductive system, endocrine system and metabolism when administered to healthy subjects? When used as a drug/dietary supplement, what safety studies have been conducted? Can we really say melatonin is safe when it has not been systematically studied and many studies show interactions with a wide range of physiological processes? Finally the results of studies investigating the efficacy of melatonin as a drug to alleviate insomnia are critically evaluated. In summary, melatonin is an endogenous pineal gland hormone with specific physiological functions in animals and humans, with its primary role in humans to maintain synchrony of sleep with the day/night cycle. When administered as a drug it affects a wide range of physiological systems and has clinically important drug interactions. With respect to efficacy for treating sleep disorders, melatonin can advance the time of sleep onset but the effect is modest and variable. In children with neurodevelopmental disabilities melatonin appears to have the greatest impact on sleep onset but little effect on sleep efficiency.

Recent Advances in Electrochemical and Optical Sensors for Detecting Tryptophan and Melatonin

Tryptophan and melatonin are pleiotropic molecules, each capable of influencing several cellular, biochemical, and physiological responses. Therefore, sensitive detection of tryptophan and melatonin in pharmaceutical and human samples is crucial for human well-being. Mass spectrometry, high-performance liquid chromatography, and capillary electrophoresis are common methods for both tryptophan and melatonin analysis; however, these methods require copious amounts of time, money, and manpower. Novel electrochemical and optical detection tools have been subjects of intensive research due to their ability to offer a better signal-to-noise ratio, high specificity, ultra-sensitivity, and wide dynamic range. Recently, researchers have designed sensitive and selective electrochemical and optical platforms by using new surface modifications, microfabrication techniques, and the decoration of diverse nanomaterials with unique properties for the detection of tryptophan and melatonin. However, there is a scarcity of review articles addressing the recent developments in the electrochemical and optical detection of tryptophan and melatonin. Here, we provide a critical and objective review of high-sensitivity tryptophan and melatonin sensors that have been developed over the past six years (2015 onwards). We review the principles, performance, and limitations of these sensors. We also address critical aspects of sensitivity and selectivity, limit and range of detection, fabrication process and time, durability, and biocompatibility. Finally, we discuss challenges related to tryptophan and melatonin detection and present future outlooks.

Melatonin controversies, an update

Melatonin was discovered more than 60 years ago. Since then, several seminal discoveries have allowed us to define its function as a neuroendocrine hormone and its molecular targets in mammals and many other species. However, many fundamental issues have not yet been solved such as the subcellular localization of melatonin synthesis and the full spectrum of its molecular targets. In addition, a considerable number of controversies persist in the field, mainly concerning how many functions melatonin has. Altogether, this illustrates how "immature" the field still is. The intention of this opinion article is to note the controversies and limitations in the field, to initiate a discussion and to make proposals/guidelines to overcome them and move the field forward.

Melatonin, an ubiquitous metabolic regulator: functions, mechanisms and effects on circadian disruption and degenerative diseases

The last four decades, we assist to an increasing scientific interest on melatonin, a circadian hormone, a metabolic regulator which influences not only plants' metabolism and their defense against pathogens but mostly the animals and humans' metabolic pathways, their response to circadian disruption, stress and burnout syndrome. In humans, as a hormonal regulator, produced in the pineal grand as well in mitochondria, melatonin is involved in different, complex intracellular signaling pathways, with antioxidant and immune stimulating effects, proving to act as a circadian synchronizer, as a preventive and therapeutic agent in many degenerative diseases, and especially in hormone-dependent cancers. Preclinical or clinical studies showed recently the mechanisms involved in regulating the cellular activity, its role in aging and circadian disturbances and impact on degenerative diseases. Melatonin proved to have an anti-inflammatory, antiapoptotic and powerful antioxidant effect by subtle mechanisms in mitochondrial metabolic pathways. This overview includes recent and relevant literature data related to the impact of endogenous and exogeneous melatonin on the prevention of cancer progression and treatment of various degenerative diseases. Metabolomics, an emerging new omics' technology, based on high performance liquid chromatography coupled with mass spectrometry is presented as an encouraging technique to fingerprint and realize a precise evaluation and monitoring of the turnover of melatonin and its metabolites in different pathological circumstances.

Melatonin rich foods in our diet: food for thought or wishful thinking?

Melatonin continues to generate interest in the scientific community and the general public. In recent years, there has been growing interest in the possibility that melatonin present in human foods may have physiological effects. This has led to the promotion of "melatonin-rich" foods and "phyto-melatonin". The night time secretion of endogenous melatonin from the pineal gland provides a daily circadian signal which is detected by receptors in various tissues. In animals the changing circadian pattern of melatonin secretion across seasons is important to them to program their reproductive behaviours to ensure optimal reproductive success, while in humans it probably plays a prominent role in anchoring sleep to the night period. When melatonin is administered in non-physiological, milligram amounts to humans, the onset of sleep can be manipulated and in larger doses anti-oxidant properties may emerge. Melatonin-rich foods are considered in this context too, but the question remains whether the amounts of melatonin in the food can be expected to be high enough to realistically change sleep or have antioxidant properties. In this review, papers reporting the effects of ingestion of melatonin-rich food on plasma or saliva melatonin or its urinary metabolite are critically evaluated. Unfortunately many of the papers are compromised by poor experimental design and assay methodologies and uncritical evaluation of results. The conclusion drawn from this review is that it is wishful thinking to expect that the amount of melatonin in "melatonin-rich" foods will impact on sleep or have any other physiological impact.

Measuring melatonin by immunoassay

The pineal gland hormone melatonin continues to be of considerable interest to biomedical researchers. Of particular interest is the pattern of secretion of melatonin in relation to sleep timing as well as its potential role in certain diseases. Measuring melatonin in biological fluids such as blood and saliva presents particular methodological challenges since the production and secretion of the hormone are known to be extremely low during the light phase in almost all situations. Active secretion only occurs around the time of lights out in a wide range of species. The challenge then is to develop practical high-throughput assays that are sufficiently sensitive and accurate enough to detect levels of melatonin less than 1 pg/mL in biological fluids. Mass spectrometry assays have been developed that achieve the required sensitivity, but are really not practical or even widely available to most researchers. Melatonin radioimmunoassays and ELISA have been developed and are commercially available. But the quality of the results that are being published is very variable, partly not only because of poor experimental designs, but also because of poor assays. In this review, I discuss issues around the design of studies involving melatonin measurement. I then provide a critical assessment of 21 immunoassay kits marketed by 11 different companies with respect to validation, specificity and sensitivity. Technical managers of the companies were contacted in an attempt to obtain information not available online or in kit inserts. A search of the literature was also conducted to uncover papers that have reported the use of these assays, and where possible, both daytime and night-time plasma or saliva melatonin concentrations were extracted and tabulated. The results of the evaluations are disturbing, with many kits lacking any validation studies or using inadequate validation methods. Few assays have been properly assessed for specificity, while others report cross-reaction profiles that can be expected to result in over estimation of the melatonin levels. Some assays are not fit for purpose because they are not sensitive enough to determine plasma or saliva DLMO of 10 and 3 pg/mL, respectively. Finally, some assays produce unrealistically high daytime melatonin levels in humans and laboratory animals in the order of hundreds of pg/mL. In summary, this review provides a comprehensive and unique assessment of the current commercial melatonin immunoassays and their use in publications. It provides researchers new to the field with the information they need to design valid melatonin studies from both the perspective of experimental/clinical trial design and the best assay methodologies. It will also hopefully help journal editors and reviewers who may not be fully aware of the pitfalls of melatonin measurement make better informed decisions on publication acceptability.

A critical review of melatonin assays: Past and present

There has been increased interest in the measurement of melatonin in plasma and saliva recently either as a marker of circadian phase or to understand the physiological role of melatonin. For both situations, there is a need for a specific assay for melatonin that is sensitive enough to detect low concentrations (<2 pg/mL). Since the mid-1970s, there have been many assays developed to measure melatonin in blood and saliva. Radioimmunoassays and ELISA have predominated because of their relative simplicity and high throughput. In this review, I show that the early radioimmunoassays while providing valuable information about nocturnal melatonin levels in humans, generally produced inaccurate basal (daytime) levels. Mass spectrometry assays, however, have provided us with the target values that immunoassays need to achieve, that is, daytime plasma melatonin levels <1 pg/mL. There are now many contemporary commercial assays available utilising both RIA and ELISA technologies, but not all achieve the standards set by the mass spectrometry assays. The performance of these assays is reviewed. I conclude with recommendations on issues researchers need to consider when conducting melatonin studies, including the importance of time of day of collection, validation of assays, the potential causes of poor assay specificity at low levels, the advantages/disadvantages of using saliva vs plasma and extraction assays vs direct assays, kit manufacturers responsibilities and the reporting requirements when publishing melatonin studies.

Quantitative determination of melatonin in milk by LC-MS/MS

A rapid, reliable and sensitive LC-MS/MS method for the determination of melatonin in milk was developed and validated. Sample was extracted with dichloromethane and cleaned by passing through Chem Elut solid phase extraction cartridge. The solvent was evaporated to dryness, reconstituted with methanol and analysed by LC-MS/MS on Agilent zorbax Eclipse XDB C-18 rapid resolution analytical column. The analytical procedure was found to be accurate, precise and linear. The method accuracy was 92.2 % (range 90.06-94.58) and the mean precision was 1.55 % and the calibration was linear for 1 to 150 pg mL(-1) (R(2) > 0.99), the lowest limit of quantification (LLOQ) was 1 pg mL(-1). 7-D Melatonin (7-DM) was used as an internal standard. This method was proved to be a promising method for the determination of melatonin for market milk and human milk samples.

Comparative ANNs with different input layers and GA-PLS study for simultaneous spectrofluorimetric determination of melatonin and pyridoxine HCl in the presence of melatonin’s main impurity

Melatonin (MLT) has many health implications, therefore it is important to develop specific analytical methods for the determination of MLT in the presence of its main impurity, N-{2-[1-({3-[2-(acetylamino)ethyl]-5-methoxy-1H-indol-2-yl}methyl)-5-methoxy-1H-indol-3-yl]ethyl}acetaamide (DMLT) and pyridoxine HCl (PNH) as a co-formulated drug. This work describes simple, sensitive, and reliable four multivariate calibration methods, namely artificial neural network preceded by genetic algorithm (GA-ANN), principal component analysis (PCA-ANN) and wavelet transform procedures (WT-ANN) as well as partial least squares preceded by genetic algorithm (GA-PLS) for the spectrofluorimetric determination of MLT and PNH in the presence of DMLT. Analytical performance of the proposed methods was statistically validated with respect to linearity, accuracy, precision and specificity. The proposed methods were successfully applied for the assay of MLT in laboratory prepared mixtures containing up to 15% of DMLT and in commercial MLT tablets with recoveries of no less than 99.00%. No interference was observed from common pharmaceutical additives and the results compared favorably with those obtained by a reference method.

An ultra-high sensitive bioanalytical method for plasma melatonin by liquid chromatography-tandem mass spectrometry using water as calibration matrix

For the endogenous substances with an ultra-low level in biological fluids, such as melatonin, the blank biological matrix is obviously not "blank". This problem leads to a serious issue of the bioanalytical methods development and validation by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). This work developed and validated an ultra-high sensitive bioanalytical method for plasma melatonin by LC-MS/MS using water as calibration matrix. The lower limit of quantitation of the method was verified to be 1.0 pg/mL and the method exhibited a linear range of 1-5000 pg/mL. Potential matrix effects, accuracy and precision were fully monitored and validated by two complementary quality control approaches respectively using water and the pooled plasma as matrix. The intra-run and inter-run precisions were less than 11.5% and 12.2%, respectively, and the relative error was below ± 13.8% for all of 5 quality control levels. The method was successfully applied to investigate the daytime (8:00 AM-8:00 PM) baseline level of endogenous plasma melatonin, as well as the pharmacokinetic profiles of exogenous melatonin after oral administration in beagle dogs.

Development and validation of a reversed-phase ion-pair liquid chromatography method for the determination of magnesium ascorbyl phosphate and melatonin in cosmetic creams

A reversed-phase ion-pair high-performance liquid chromatographic method was developed and validated for the determination of magnesium ascorbyl phosphate and melatonin in cosmetic creams. The determination was performed on a BDS C 18 analytical column (250 x 4.6 mm i.d., 5 microm particle size); the mobile phase consisted of 0.020 M tetrabutylammonium hydroxide and 0.025 M potassium dihydrogen phosphate (pH 6.8) mixed with acetonitrile in a ratio (77:23, v/v) and pumped at a flow rate 1.00 ml min(-1). The UV detector was operated at 260 nm. The retention times of the magnesium ascorbyl phosphate, melatonin and chlorthalidone that was used as internal standard, were 6.55, 9.18 and 11.07 min, respectively. Calibration graphs are linear (r better than 0.9990, n=6), in concentration range 1.00-10.00 microg ml(-1) for magnesium ascorbyl phosphate and 0.63-6.25 microg ml(-1) for melatonin. The intra- and inter-day R.S.D. values were less than 6.0%, while the relative percentage error E(r) was less than 3.5% (n=5). The quantitation limits were 0.69 and 0.47 microg ml(-1), for magnesium ascorbyl phosphate and melatonin, respectively. The method was applied to the analysis of a cosmetic cream and proved to be suitable for rapid and reliable quality control.

Study of the heavy atom-induced room temperature phosphorescence properties of melatonin and its analytical application

Liquid phase room temperature phosphorescence (RTP) properties of melatonin were studied using heavy atom induced-room temperature phosphorescence (HAI-RTP) technique. 1.2 M potassium iodide was used as a heavy atom reagent together with 0.002 M sodium sulphite as deoxygenating agent to produce the RTP signal. The maximum phosphorescence emission and excitation wavelengths of melatonin were 290 and 457 nm, respectively. The effect of potassium iodide concentration on the RTP lifetime of melatonin was also investigated and based on the results, the rate constants for phosphorescence decay (k(p)) and radiationless deactivation through reaction with heavy atom (k(h)) were determined. Based on the obtained results, a simple and sensitive room temperature phosphorimetric method was developed for the determination of melatonin. The method allowed the determination of 10.0-200 ng ml(-1) melatonin in aqueous solution with the limits of detection and quantification of 3.6 and 12 ng ml(-1), respectively. The proposed method was satisfactorily applied to the determination of melatonin in commercial pharmaceutical formulations.

Quantification of melatonin in human saliva by liquid chromatography-tandem mass spectrometry using stable isotope dilution

A method for the determination of melatonin in human saliva has been developed using high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS-MS). Saliva was collected in plastic tubes. 7-D-Melatonin was added as internal standard and the samples were cleaned and concentrated by solid-phase extraction. The limit of detection was 1.05 pg x ml(-1) and the limit of quantification was 3.0 pg x ml(-1). The accuracy of the method was +/-14% at 5.60 pg x ml(-1) and +/-9% at 19.6 pg x ml(-1). The precision was +/-13% at 6.18 pg x ml(-1) and +/-11% at 31.2 pg x ml(-1), respectively. Our HPLC-MS-MS method shows a high sensitivity and specificity for melatonin and more reliable results compared with a radioimmunoassay. The chromatographic method has been used to determine the circadian rhythm of melatonin among three nurses working the night shift and a patient suffering from an inability to fall asleep at night.

Role of melatonin in the induction and maintenance of sleep

Pharmacokinetic studies of melatonin in young and elderly human volunteers, and the measurement of hypnotic effects in chicks under alternate light-dark or permanent light conditions, show that melatonin is a bioprecursor of hypnotic acetyl metabolites produced by the enzymatic acetylation of both melatonin and 2-oxomelatonin under the control of serotonin N-acetyltransferases (NATs), which are present in the pineal gland. The acetyl metabolite of melatonin, which we call carbo2, is an N-acetyl-β-carboline. The electroencephalographs (EEG) architecture of the sleep produced by this compound is similar to thai of physiological sleep, and is characterized by the significant proportion of slow-wave deep sleep and rapid eye movement sleep. This is in sharp contrast to the EEG sleep architecture observed with GABAergic (GABA, γ-aminobutyric acid) compounds. Since insomnia and sleep disorders are believed to be due to a lack of NAT enzymes in the pineal gland, a new therapeutic approach of sleep disorders by administration of such hypnotic acetyl metabolites of melatonin, or synthetic analogs thereof, can be en visaged.

Rapid determination of serum melatonin by ESI-MS-MS with direct sample injection

This paper describes a rapid, simple and sensitive analytical method for the quantitative determination of melatonin in human serum by ESI-MS-MS with direct serum sample injection and on-line extraction. The method uses N-acetyltryptamine as the internal standard. It has high specificity and sensitivity for serum melatonin analysis. The internal calibration curve shows a wide linear range from 0.500 to 200 ng/ml with a correlation coefficient, R(2) > 0.999. The limit of quantitation is 0.500 ng/ml and the limit of detection is 0.100 ng/ml with 10-microl sample injection. The recoveries of serum melatonin at three levels are approximately 70%. The intra-assay precision (n = 5) is between 0.8 and 2.0% and the inter-assay precision (n = 3) is between 1.5 and 5.9% over the calibration range. This method has a total analysis time of less than 9 min. It can be used for the measurement of melatonin in human blood.

Melatonin secretion occurs at a constant rate in both young and older men and women

The magnitude and duration of melatonin (MLT) secretion were measured over a period of 25 h with pharmacokinetic studies employing administration of D(7) MLT at midday and at midnight in two separate studies and two groups of subjects, 12 young and 11 older men and women. Plasma levels of endogenous MLT and D(7) MLT were quantified separately by use of a specific and sensitive method (gas chromatography-mass spectrometry) previously developed in our laboratory, enabling us to measure endogenous and exogenous MLT levels down to 0.5 pg/ml in plasma. In the two groups of subjects, MLT secretion occurred only at night: onset time of secretion was from 1915 to 2205 (Greenwich mean time), and offset was from 0305 to 0545. No MLT peak was observed in individual nocturnal MLT profiles that were similar to curves obtained for a rate-constant infusion. Modelization demonstrated the superimposition of observed data and simulated curves. MLT concentrations decreasing from the offset of secretion might correspond to the elimination of MLT present in the body at the end of nocturnal secretion. By use of the MLT clearance given by pharmacokinetics, the amount of secreted MLT was found to be 35.7 and 21.6 microg for men and women, respectively, and the rate of secretion was 4.6 and 2.8 microg/h, respectively. No significant gender difference was observed for these two parameters when normalized to body weight. No significant gender difference was observed for onset times of secretion or duration of secretion (7.6-8.6 h) within the two groups, or between young and older subjects.

Photostability of drugs: photodegradation of melatonin and its determination in commercial formulations

The photostability of melatonin, a hormone used as supplementary drug in the alleviation of jet-lag and other sleep disorders, was studied. The drug photodegradation at different pH values was monitored by HPLC methods. The main photoproduct was isolated and characterised on the basis of the NMR, FTIR, and mass spectra. A HPLC method, in combination with a post-column on-line photochemical derivatisation was developed for the selective and reliable quality control of commercially available melatonin containing products.

Bioavailability of melatonin in humans after day-time administration of D(7) melatonin

Absolute bioavailability of the neurohormone melatonin (MLT) was studied in 12 young healthy volunteers (six males, six females) after administration at midday, on two separate occasions, of 23 microg by intravenous (i.v.) infusion and 250 microg by oral solution of D(7) MLT, a molecule in which seven deuterium atoms replace seven hydrogen atoms. Exogenous (D(7)) and endogenous (D(0)) MLT were quantified simultaneously but separately by a highly specific assay: gas chromatography/negative ion chemical ionization mass spectrometry, developed in our laboratory, which enabled us to go down to 0.5 pg/mL in plasma samples. After i.v. administration, the maximum plasma concentration (C(max)) and the area under the plasma concentration-time curve (AUC) values were significantly different in male and female subjects, but there was no significant gender difference in total body clearance normalized to body weight: 1.27+/-0.20 L/h/kg and 1.18+/-0.22 L/h/kg for males and females, respectively. The apparent terminal half-life (t(1/2(z))) values were 36+/-2 and 41+/-10 min, respectively. After oral administration, pharmacokinetic parameters used to quantify bioavailability were near three-fold greater in female subjects than in males, with large inter-individual variations. The maximum plasma MLT concentration C(max)+/-S.D. was found at 243.7+/-124.6 pg/mL and 623.6+/-575.1 pg/mL for male and female subjects respectively, while the mean values for AUCs were 236+/-107 pg.h/mL and 701+/-645 pg.h/mL. The absolute bioavailability of MLT was from 1 to 37%: mean=8.6+/-3.9% and 16.8+/-12.7% for male and female subjects, respectively.

Determination of melatonin in biological fluids in the presence of the melatonin agonist S 20098: comparison of immunological techniques and GC-MS methods

Immunoassays were investigated for the determination of melatonin in biological samples in the presence of a naphthalenic structural analogue S 20098, which is currently under development as a melatonin agonist. The lack of specificity of commercially available antibodies in the presence of closely related molecules led us to develop an LC-RIA procedure with a quantification limit set at 15 pg/ml(-1). Because this technique was not sensitive enough and difficult to use on a routine basis, a more sensitive GC-MS technique was developed. This method involved automated solid-phase extraction (plasma) or liquid-liquid extraction (saliva), derivatization of the indolic moiety and GC separation with an automated switching device before MS detection. The method was validated over the range 1-100 pg/ml(-1), with a quantification limit set at 1 pg/ml(-1) in human plasma and saliva. Intra-assay and inter-assay precision and accuracy were within 16% for all concentrations investigated and each biological matrix. The stability of melatonin in plasma and saliva under various storage conditions was also determined. The specificity of the assay for the analysis of melatonin in the presence of S 20098 and its metabolises was demonstrated. The method was subsequently applied for the determination of endogenous melatonin concentrations in plasma and saliva samples from clinical studies performed with S 20098 to provide pharmacodynamic data.

Development of a circadian melatonin rhythm in embryonic zebrafish

We investigated the time course of circadian system development in zebrafish and the role of environmental light cycles in this process, using a rhythm in melatonin content of embryos and larvae as a marker of circadian function. When zebrafish were raised in a cycle of 14 h light and 10 h dark at 28.5 degrees C, nocturnal increases in melatonin content were detectable beginning on the second night post-fertilization (PF). When embryos were transferred to constant darkness (DD) at the end of the second light period, a circadian rhythm of melatonin content persisted for at least three cycles. However, when embryos were transferred from light to DD at 14 h PF, no rhythm was detectable in the population. Phase-locked circadian melatonin rhythms were measurable after embryos were exposed to a transition from constant light (LL) to darkness at 26 or 32 h PF, but not at 20 h. These data indicate that a circadian oscillator that regulates melatonin synthesis becomes functional and responsive by light between 20 and 26 h PF. At this stage, pineal photoreceptors have begun to differentiate, but retinal photoreceptors have not, suggesting that the first circadian melatonin rhythms are of pineal origin. The absence of melatonin rhythms in populations of embryos exposed to DD beginning at earlier stages indicates that there is no timed developmental event that sets the circadian clock in the absence of environmental input. Exposure to DD starting at 14 or 20 h PF did not retard overall development as determined by gross morphological staging criteria, and did not prevent later synchronization of melatonin rhythms by light-dark (LD) cycles.

Determination of melatonin in pharmaceutical formulations and human plasma by gas chromatography-electron impact mass spectrometry

A simple and reliable method for gas chromatography-mass spectrometry (GC-MS) has been developed for the determination of melatonin in commercially available tablet formulations and human plasma. The dissolution of the tablets in ethyl acetate was simply obtained by sonication. The filtrate of the resultant solution, after concentration, was derivatized with pentafluoropropionic anhydride (PFPA) and analysed by GC-MS using splitless injection. The linear response range for melatonin was 5-250 ng on column. The same method can be used, with minor modifications, for the determination of melatonin in spiked human plasma in the range 50-1000 pg/mL plasma.

Capillary GC-MS determination of melatonin in several pharmaceutical tablet formulations

A simple capillary GC-MS method for the determination of melatonin from tablets is proposed. Melatonin (N-acetyl-5-methoxytryptamine) was determined by gas chromatography-mass spectrometry after extraction from ground tablets with ethyl acetate and derivatization with N-methyl-N-trimethylsilyl-heptafluorobutyramide (MTSHFBA). Splitless injection offers good reproducibility with a standard deviation of +/- 2%. The proposed method was applied to analyse the melatonin content from three different tablet formulations.

Simultaneous determination of melatonin-pyridoxine combination in tablets by zero-crossing derivative spectrophotometry and spectrofluorimetry

Two methods have been developed for the analysis of melatonin (M) and pyridoxine hydrochloride (PH) in combination. The first method depends on first- and second-derivative ultraviolet spectrophotometry, with the zero crossing technique of measurement. First-derivative amplitudes at 296 nm and second-derivative amplitudes at 294 and 322 nm are selected for the determination of M and PH, respectively. The second method is based on the native fluorescence of both M and PH, in methanol and 0.1 M hydrochloric acid, respectively, after a preliminary solvent extraction procedure. The relative standard deviation of both methods was less than 2.0%. The two methods have been successfully applied to the determination of both drugs in laboratory-prepared mixtures and in tablets.

Highly sensitive method for the determination of melatonin by normal-phase high-performance liquid chromatography with fluorometric detection

In the present study a new chromatographic method was developed to quantify melatonin in rat pineal that can be extended to other tissues. Melatonin was extracted from an acid homogenate with ethyl acetate to avoid amine interference. HPLC was performed with silica normal-phase column and fluorescence detection. This method is sensitive enough for detecting melatonin in a single pineal gland with a detection limit of 3 pg/mg tissue.