Highly sensitive and specific assay of plasma melatonin using high-performance liquid chromatography with fluorescence detection preceded by solid-phase extraction

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

Effect of melatonin and analogues on corneal wound healing: involvement of Mt2 melatonin receptor

PURPOSE

We have investigated the effect of melatonin and its analogues on rabbit corneal epithelial wound healing.

METHODS

New Zealand rabbits were anaesthetised and wounds were made by placing Whatman paper discs soaked in n-heptanol on the cornea. Melatonin and analogues (all 10 nmol) were instilled. Wound diameter was measured every 2 hours by means of fluorescein application with a Topcon SL-8Z slit lamp. Melatonin antagonists (all 10 nmol) were applied 2 hours before the application of the n-heptanol-soaked disc and then every 6 hours together with melatonin. To confirm the presence of MT2 receptors in corneal epithelial cells immunohistochemistry, Western blot and RT-PCR assays in native tissue and in rabbit corneal epithelial cells were performed. The tear components were extracted then processed by HPLC to quantify melatonin in tears.

RESULTS

Migration assays revealed that melatonin and particularly the treatment with the MT2 agonist IIK7, accelerated the rate of healing (p < 0.001). The application of the non-selective melatonin receptor antagonist luzindole and the MT2 antagonist DH97 (but not prazosin), prevented the effect of melatonin on wound healing (both p < 0.001). Immunohistochemistry, Western blot and RT-PCR assays showed the presence of MT2 melatonin receptor in corneal epithelial cells. In addition, we have identified melatonin in tears and determined its daily variations.

CONCLUSIONS

These data suggest that MT2 receptors are implicated in the effect of melatonin on corneal wound healing regulating migration rate. This suggests the potential use of melatonin and its analogues to enhance epithelial wound healing in ocular surface disease.

Immune modulatory and antioxidant effects of melatonin in experimental periodontitis in rats

Melatonin is an important antioxidant, and through its anti-inflammatory effects it can control immune responses, oxidative stress, and defense cell infiltration. Periodontitis is a disease of the oral cavity and the generation of free radicals is an important consideration in this disease. Therefore, we examined the immune-modulatory and antioxidant roles of melatonin in the treatment of periodontitis. In all, 30 male Wistar rats were randomly divided into three groups: the control group, the periodontitis-induced (PED) group, and the periodontitis+melatonin treatment (MEL+PED) group. The control group received no treatment, whereas periodontitis was induced in both the PED and the MEL+PED groups, with the MEL+PED group being treated with systemic melatonin. For the periodontitis-induced groups, the rats' mandibular first molar teeth were ligatured (3-0 cotton) in a submarginal position for 4 weeks, and then the ligature was removed. After removal of the ligature, melatonin was administered only to the MEL+PED group (an ip dose of 10mg/kg body wt for 15 days at 11:00 PM each day). In the histological examination, the MEL+PED group, which received the melatonin, showed reduced inflammatory cytokines (IL-1β, from 97.47 to 84.24pg/ml; TNF-α, from 0.22530 to 0.22519pg/ml), regulated oxidative stress parameters (MDA, from 41,458 to 30,708nmol/g; GSH, from 18,166 to 25,858nmol/mg), and less periodontal tissue destruction (CEJ-PL, lingual, from 244.54 to 140.57μm; buccal, from 235.6 to 158.93μm; and CEJ-BC, lingual, from 383.65 to 287.76μm; buccal, from 391.92 to 296.12μm). From these findings we conclude that, even when periodontitis was induced, melatonin reduced the oxidative damage in the rats' periodontal tissue by inhibiting the inflammatory effects and by restoring the antioxidants.

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.

Appetite regulation: the central role of melatonin in Danio rerio

Melatonin is the hormonal mediator of photoperiodic information to the central nervous system in vertebrates and allows the regulation of energy homeostasis through the establishment of a proper balance between energy intake and energy expenditure. The aim of this study was to evaluate the role of melatonin in appetite central control analyzing the involvement of this hormone in the regulation of feeding behavior in the zebrafish Danio rerio. For this purpose, the effect of two different melatonin doses (100nM and 1μM) administered for 10 days, via water, to zebrafish adults was evaluated at both physiological and molecular level and the effect of melatonin was considered in relation to the most prominent systems involved in appetite regulation. For the first time, in fact, melatonin control of food intake by the modulation of leptin, MC4R, ghrelin, NPY and CB1 gene expression was evaluated. The results obtained indicate that melatonin significantly reduces food intake and the reduction is in agreement with the changes observed at molecular level. A significant increase in genes codifying for molecules involved in feeding inhibition, such as leptin and MC4R, and a significant reduction in the major orexigenic signals including ghrelin, NPY and CB1 are showed here. Taken together these results support the idea that melatonin falls fully into the complex network of signals that regulate food intake thus playing a key role in central appetite regulation.

Development and validation of new methods for the determination of melatonin and its oxidative metabolites by high performance liquid chromatography and capillary electrophoresis, using multivariate optimization

Melatonin (N-acetyl-5-metoxytriptamine, MEL) has focused a lot of attention as consequence of its multiple functions. MEL is a potent endogenous antioxidant and a free radical scavenger that reacts with several sort of radicals generating various metabolites. Two of them are N1-acetyl-N2-formyl-5-methoxykynurenine (AFMK) and N1-acetyl-5-methoxykynurenine (AMK). These compounds are important because they have also antioxidant actions as well as other important biological properties. In the present work, we develop two methods to detect and quantify these compounds (MEL, AFMK and AMK) in the same sample. For this purpose we used an experimental design, and utilized high performance liquid chromatography (HPLC-DAD) and micellar electrokinetic chromatography (MEKC) techniques with diode array detector in both of them. The limit of detection/quantification for MEL, AFMK and AMK were respectively 44/94, 18/38 and 23/51 ng mL(-1) by using HPLC and 13/44, 37/124 and 47/156 ng mL(-1) by using MEKC. This is the first time that these compounds have been separated in the same chromatogram or electroferogram. The time of analysis was faster using MEKC. Furthermore, this technique showed better resolution but HPLC offered better limit of detection and quantification for metabolites. Both methods were validated and correlation coefficients were higher than 0.999 and the range of recovery of those methods were 99.6-103.7%. Precision was evaluated as repeatability and intermediate precision with relative standard derivation <5%. When a 5 microg mL(-1) solution of these compounds were analyzed with both methods we do not observed any statistically significance differences. Moreover, we analyzed 3COHM (cyclic-3-hydroximelatonin), another known metabolite of melatonin, by using the same methods. The employment of these methods will offer a useful tool to contribute to answer the role of MEL, AFMK and AMK in biological system and both methods can be used in routine analysis for these compounds.

Melatonin concentrations during larval and postlarval development of gilthead sea bream Sparus auratus: more than a time-keeping molecule?

In this study, melatonin (MEL) and thyroxine (T(4)) concentrations were measured during larval and postlarval development of gilthead sea bream Sparus auratus Hormones were measured in whole bodies of larvae or the head and trunk of postlarvae after 67 days of exposure to constant light, 24L:0D, constant darkness, 0L:24D or 12L:12D and in the plasma of 6 month juveniles kept under the 12L:12D, 0L:24D and 24L:0D regimes. High MEL concentrations in larvae suggested a distinct role of MEL in early organogenesis and development of S. auratus. In larvae, the gastro-intestinal tract seemed to be an important extrapineal and extraretinal source of MEL. No endogenous rhythm of MEL synthesis was demonstrated in 67 day larvae; however, in 6 month juveniles, it was evident. At early ontogenesis of S. auratus, the role of MEL is probably related mostly to the control of development and protection against free radicals, whereas its action as a time-keeping molecule develops later. The increase in T(4) concentration during the S. auratus larva-juvenile transition, i.e. between 50 and 70 days post-hatch, which was observed concurrently with the decrease of MEL concentration, may suggest an inverse relationship between T(4) and MEL.

Melatonin synthesis under calcium constraint in gilthead sea bream (Sparus auratus L.)

Brain or blood plasma melatonin was analysed as a measure for pineal melatonin production in sea bream. Access to calcium was limited by diluting the seawater to 2.5 per thousand and removing calcium from the diet or by prolonged feeding of vitamin D-deficient diet. Interactions/relations between melatonin and calcium balance and the hypercalcemic endocrines PTHrP and calcitriol were assessed. Restricting calcium availability in both water and diet had no effect on plasma melatonin, but when calcium was low in the water or absent from food, increased and decreased plasma melatonin was observed, respectively. Fish on a vitamin D-deficient diet (D- fish) showed decreased plasma calcitriol levels and remained normocalcemic. Decreased brain melatonin was found at all sampling times (10-22 weeks) in the D- fish compared to the controls. A positive correlation between plasma Ca2+ and plasma melatonin was found (R(2)=0.19; N=41; P <0.01) and brain melatonin was negatively correlated with plasma PTHrP (R(2)=0.78; N=4; P <0.05). The positive correlation between plasma levels of melatonin and Ca2+ provides evidence that melatonin synthesis is influenced by plasma Ca2+. The decreased melatonin production in the D- fish points to direct or indirect involvement of calcitriol in melatonin synthesis by the pineal organ in teleosts. The hypercalcemic factors PTHrP and calcitriol appeared to be negatively correlated with melatonin and this substantiates an involvement of melatonin in modulating the endocrine response to cope with hypocalcemia. It further points to the importance of Ca2+ in melatonin physiology.

Melatonin and thyroxine response to pollution in white stork nestlings (Ciconia ciconia): aspects of rhythmicity and age

There is growing evidence that ubiquitous environmental contaminants may interfere with vertebrate endocrine systems. The selected endocrine biomarkers are used to indicate the condition of free-ranging populations of wildlife, including avian species. The aim of this study was to determine the impact of environment quality on serum thyroxine (T4) and melatonin (Mel) in white stork nestlings (Ciconia ciconia) living in different locations: small villages in natural areas surrounded by forests and crop fields, near the city and near the copper smelter. We extended our analyses to examine the hormones' day-night changes in conjunction with chicks' age. Total serum T4 and Mel was measured by RIA. T4 level, as a decisive measure of thyroid hormone productivity, was significantly lower in the nestlings exposed to pollutants from the copper smelter. Mel, as a well-known scavenger of free radicals, was elevated in the nestlings in the area near the copper smelter. This study indicates that alteration in T4 and Mel levels could be a useful marker of exposure of nestling wild storks to different toxic substances in field studies. Mel is postulated to be a susceptible defensive molecule as a protective mechanism for organisms.

Arginine vasotocin, isotocin and melatonin responses following acclimation of gilthead sea bream (Sparus aurata) to different environmental salinities

Gilthead sea bream (Sparus aurata) is a euryhaline species with a capacity to cope with demands in a wide range of salinities and thus is a perfect model-fish to study osmoregulatory responses to salinity-adaptive processes and their hormonal control. Immature sea bream acclimated to different salinities, i.e. SW (38 per thousand), LSW (5 per thousand) and HSW (55 per thousand), were kept at 18 degrees C under natural photoperiod. Arginine vasotocin (AVT) and isotocin (IT) in plasma and pituitary were determined by HPLC. Plasma melatonin (Mel) was assayed by RIA. Plasma osmolality, ion concentrations (Na(+), K(+), Ca(2+), Cl(-)) and Na(+),K(+)-ATPase activity in gill were measured. A steady increase in plasma AVT, along with increasing water salinity was observed. Pituitary IT concentration in HSW-acclimated fish was significantly higher than that in LSW group. AVT/IT secretory system of sea bream does appear to be involved in the mechanism of long-term acclimation to different salinities. The distinct roles and control mechanisms of both nonapeptides are suggested. Plasma Mel was significantly higher in LSW compared with both HSW and SW groups. Data indicate that the changes in Mel level are linked to osmoregulation. Further studies are required to elucidate a complex role of AVT, IT and Mel in sea bream osmoregulation.

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.

Simultaneous Liquid Chromatographic Measurement of Melatonin and Related Indoles through Post-column Electrochemical Demethylation and Fluorescence Derivatization

In this paper we describe a highly sensitive and selective liquid chromatographic method for the determination of 5-methoxyindoles (5-methoxyindole-3-acetic acid, 5-methoxytryptamine, 5-methoxytryptophol, and melatonin) using a post-column technique involving electrolytic demethylation followed by fluorescence derivatization with benzylamine. We separated these compounds within 30 min by reversed-phase liquid chromatography using acetate buffer (pH 6.5)-acetonitrile-methanol [8:1:1 (v/v); isocratic elution] and then demethylated them, using a commercial coulometric system, to give the corresponding 5-hydroxyindoles. Next, we converted the 5-hydroxyindole products into fluorescent derivatives by their reactions with benzylamine in the presence of potassium hexacyanoferrate(III). We detected the derivatives spectrofluorometrically at 480 nm upon excitation at 345 nm. The detection limits (signal-to-noise ratio = 3) of the 5-methoxyindoles were in the range from 12 to 93 fmol per 20-microL injection.

Day–night specific binding of 2-[125I]Iodomelatonin and melatonin content in gill, small intestine and kidney of three fish species

Some of melatonin's (Mel) well-established physiological effects are mediated via high-affinity cell-membrane receptors belonging to the superfamily of G-protein-coupled receptors. Specific binding of ligand 2-[(125)I]iodomelatonin, using membrane preparations from osmoregulatory tissues of flounder, rainbow trout and sea bream, together with Mel concentrations in the tissues and plasma were studied. The kidney, gill and small intestine samples were collected during the day and at night. The dissociation constants (K (d)) and maximal binding densities (B (max)) were calculated for each tissue at 11:00 and 23:00 h. The binding sites with K (d) values in the tissues in the picomolar range indicated the high affinity. K (d) and B (max) values were tissue- and species-dependent. The GTP analogue [Guanosine 5'-O-(3-thiotriphosphate)] treatment significantly reduced the B (max) value, indicating that the 2-[(125)I]iodomelatonin-binding sites are probably coupled to a G-protein. No daily variations in K (d) and B (max) values were observed. These are the first studies of the presence of 2-[(125)I]iodomelatonin-binding sites in the small intestine, kidney tubule and gill of fish. The data strongly suggest new potential targets for Mel action and the influence of Mel on water/ion balance in fish. The intestine seems to be a site of Mel synthesis and/or an active accumulation of the hormone.

Determination of melatonin and its precursors and metabolites using capillary electrophoresis with UV and fluorometric detection

A micellar electrokinetic chromatography method has been developed for simultaneous determination of melatonin and its precursors and metabolites. A 20 mM borate buffer pH 9.5 with 50 mM SDS served as the electrolyte. Tryptophan, 5-methoxyindoleacetic acid, 6-hydroxymelatonin, melatonin, serotonin, and 5-methoxytryptamine were baseline separated in less than 13 min. The limits of detection for UV detection and fluorometric detection based on native fluorescence of analytes were at the sub-ppm level. The proposed method with UV detection was applied to melatonin content control in pharmaceutical tablets with a precision expressed as RSD (n = 7) = 1.6%. For biological samples extraction with chloroform and ethyl acetate was examined. With ethyl acetate and chloroform recoveries of 87.2% and 82.1% melatonin, respectively, were obtained from plasma samples. The recovery of melatonin from spiked urine samples was 80.0% for ethyl acetate and 82.5% for chloroform. Fluorometric detection provides about two-fold improvement over UV in the detection of melatonin and minor improvements for three other analytes, but is much poorer than UV for tryptophan and 6-hydroxymelatonin in applied conditions.

Seasonal changes in brain melatonin concentration in the three-spined stickleback (Gasterosteus aculeatus): towards an endocrine calendar

Pineal organ and its hormone melatonin (N-acetyl-5-methoxytryptamine) is likely involved in timing and synchronisation of many internal processes, such as reproduction, with annual changes in environmental cues, i.e., photoperiod and water temperature. The seasonal changes in melatonin profile in stickleback brains related to the following reproductive phases were examined, and the link between melatonin concentrations and the stages of spawning cycle was analysed. Two wild populations of sticklebacks were exposed to annual environmental changes in their natural habitats. Brains, gonads, kidneys and livers were collected over 2 years. Melatonin was measured using RIA and the indices, gonadosomatic (GSI), nephrosomatic (NSI) and hepatosomatic (HSI), were calculated. The role of melatonin, as a component of internal calendar engaged in the control of seasonal breeding in this species, is discussed. The extremely high melatonin levels observed in early spring (March) and autumn (October) seem to mark out a time frame for spawning in sticklebacks. The seasonal pattern of melatonin production and identified development stages of gonads suggests the potential inhibitory effect of the hormone on stickleback reproduction in shortening photoperiod and stimulatory effect in lengthening photoperiod.

Influence of exogenous thyroxine on plasma melatonin in juvenile Atlantic salmon (Salmo salar)

One of the most clearly defined endocrine changes during the parr-smolt transformation of anadromous salmonids is an increase in plasma levels of thyroid hormones. The role of pineal hormone melatonin in timing and synchronisation of smoltification is widely discussed. The effect of administration of exogenous thyroxine (T4) on plasma melatonin was investigated in juvenile Atlantic salmon (Salmo salar) at the early stages of parr-smolt transformation. Fish were kept in fresh water under simulated-natural photoperiod and exposed to exogenous T4. Fish were sampled at 12.00 and 24.00 h from treatment and control tanks, 2 and 14 days after treatment started. Plasma melatonin and L-thyroxine were measured using RIA and competitive enzyme immunoassay, respectively. After 2 days of T4 treatment, marked difference in plasma melatonin concentration measured at 12.00 and 24.00 h was still observed in both groups. However, 2-week exposure to T4 caused a reduction in night-time plasma melatonin level and thus, probably, inhibited melatonin related time-keeping system in juvenile salmon. Additional studies are needed to clarify the mechanism of the described phenomenon.

Determination of melatonin in human plasma with solid-phase extraction, high-performance liquid chromatography and fluorescence detection

A new bioanalytical method for the determination of melatonin in plasma with high-performance liquid chromatography (HPLC) and fluorescence detection preceded by solid-phase extraction has been developed and validated. Melatonin was extracted from 3 mL plasma using a Waters Oasis HLB solid-phase extraction cartridge and the elute was evaporated to dryness and dissolved in 200 microl mobile phase; acetonitrile-phosphate buffer, 0.01 M pH 7.2 (25:75, v/v). 125 microL was injected into the HPLC system and separation was carried out on a Waters SymmetryShield RP18 column 5 microm (250 x 4.6 mm). Excitation and emission wavelengths were set to 285 nm and 345 nm, respectively. The HPLC system was able to separate melatonin and internal standard (5-fluorotryptamine) from other endogenous indole compounds such as serotonin and tryptophan. Determination down to 0.10 nmol/L was possible, with an intra-assay precision of about 13%. Melatonin was stable in plasma for at least 30 days at about 23 degrees C.

Potassium Hexacyanoferrate(Iii)-Mediated Fluorometric Determination of Melatonin

This paper describes the development of a novel, simple and sensitive high-performance liquid chromatographic method for the determination of melatonin using precolumn fluorometric derivatization. A strong fluorescence was induced when alkaline solution of melatonin was heated in the presence of potassium hexacyanoferrate (III). This fluorescent reaction was used for the precolumn derivatization of melatonin. Solid phase extraction was used for concentrating melatonin below a level of detection limit.

Isolation of melatonin by immunoaffinity chromatography

A single-step, highly specific and easy-to-use method was developed for isolation and purification of melatonin from complex biological matrices. Polyclonal antibodies highly specific against melatonin (with cross-reactivities with related compounds below 0.02%, except for 6-hydroxymelatonin) were raised, characterised by enzyme-linked immunosorbent assay (ELISA) and used for preparation of immunoaffinity gel. Melatonin recovery by the immunoaffinity method was approximately 95%, allowing single-step processing of samples prior to electrospray HPLC-MS analysis (with detection limit 10 fmol). The method was successfully used for determining melatonin in human serum and turned out to be better than the non-specific solid-phase extraction published earlier.

Determination of melatonin in galenic preparations by LC and voltammetry

The amount of melatonin in galenic tablets was determined by means of two feasible and accurate analytical methods. The high performance liquid chromatography (HPLC) with ultraviolet detection at 223 nm used a C(18) reversed-phase column; the linear scan voltammetric procedure (LSV) employs a pH 3.0 phosphate buffer as the supporting electrolyte for the oxidation process of melatonin, which has a current intensity maximum at +850 mV. The two methods gave satisfactory results in terms of precision and accuracy. In fact, the data of repeatability and intermediate precision expressed as percentage relative standard deviations (RSD%) were < or =2.8 and the accuracy values, resulting from recovery experiments, were between 99.0 and 101.3%. Both methods are suitable for quality control of melatonin in galenic preparations; the LSV procedure is more rapid, the HPLC method is more sensitive and more precise.

Day-night variations in plasma melatonin and arginine vasotocin concentrations in chronically cannulated flounder (Platichthys flesus)

Chronically catheterised, free swimming flounder (Platichthys flesus) have been used in experiments examining the day-night variations in circulating levels of melatonin (Mel) and arginine vasotocin (AVT). Under normal photoperiod (16 h light/8 h dark) serial blood samples taken from individual fish demonstrated a Mel rhythm with daytime levels at 09.00 and 15.00 h (238+/-14 and 179+/-12 fmol x ml(-1), respectively) lower than those at 23.00 h (1920+/-128 fmol x ml(-1)). Maintenance of fish in 24-h light abolished the light/dark Mel rhythm and circulating levels were comparable to those measured during the day in fish under normal photoperiod illumination. In fish maintained under 24 h dark, although a daily rhythm was still apparent, at the time when it would be normally dark, plasma Mel concentration was reduced and at times when it would be normally light, levels were higher than in fish maintained under normal light/dark illumination. Plasma AVT concentrations were higher in fish during the day (4.4+/-0.8 fmol x ml(-1)) than those at night (1.5+/-0.4 fmol x ml(-1)), the opposite to that seen with Mel. During acute study infusion of AVT resulted in reduced levels of plasma Mel, although this did not achieve statistical significance. Infusion of Mel did not alter circulating AVT concentration.

Determination of exogenous melatonin and its 6-hydroxy metabolite in human plasma by liquid chromatography-mass spectrometry

SUMMARY

Melatonin has recently garnered interest as a possible treatment for sleep disorders, and this has created a desire for appropriate pharmacokinetic studies. No method has yet been published that can measure the concentrations of both melatonin and its main metabolite, 6-hydroxymelatonin, in plasma or serum. Therefore, a liquid chromatography-mass spectrometry (LC/MS) method including enzymatic hydrolysis and one-step liquid-liquid extraction was developed for this purpose. The mean extraction recovery was 59% for melatonin and 42% for 6-hydroxymelatonin. The mean precision of the method as calculated from the interassay coefficient of variation of quality control samples at high and low concentrations was 18% for 6-hydroxymelatonin and 15% for melatonin. The inaccuracy was always less than 2%. The limit of detection was approximately 2 ng/mL for 6-hydroxymelatonin (signal/noise ratio = 4) and less than 2 ng/mL for melatonin (signal/noise ratio at 2 ng/mL = 10). The method was applied to the analysis of plasma from a healthy volunteer who had received a single oral dose of 25 mg melatonin.

Separation and assay methods for melatonin and its precursors

Melatonin is an indoleamine hormone that is synthesized from tryptophan via 5-hydroxytryptophan, serotonin and N-acetylserotonin in the vertebrate pineal gland. Many chromatographic and non-chromatographic techniques have been developed and improved for the determination and measurement of melatonin and its related indoleamines. At present, gas chromatography with mass spectrometry and reversed-phase high-performance liquid chromatography with fluorescence or electrochemical detection are widely used for indoleamine determinations in the pineal gland. This review will deal with methods for the separation and determination of the melatonin and its related indoleamines.

Advances in the analysis of tryptophan and its related compounds by chromatography

Advances in the analysis of tryptophan,--(both in its free form and bound, alone or together with its metabolites),--has been compiled on the basis of the relevant papers published in the last 4-5 years, including author's experiences associated with the preparation of derivatives and with any of those conditions arising from the analytical procedure itself. The special requirements of various, tryptophan containing matrices were also taken into consideration (biological tissues or fluids, food and feed stuffs, etc). For the sake of completeness in addition to the most common HPLC/UV/F1 techniques, HPLC/MS, GC/MS, CE/UV/F1 and spectrophotometry will be also discussed.