Variation of melatonin and serotonin content in rat pineal gland with sex and oestrous phase difference determined by high-performance liquid chromatography with fluorimetric detection

Up-to-date studies regarding the determination of melatonin by chromatographic methods

Melatonin is an indolic compound that has been reported in the literature to exist in human-based samples, vertebrates, vegetables, fruits, and pharmaceutical products. Melatonin is considered a dietary supplement and can regulate circadian rhythms, although it has not been classified as a drug by the US Food and Drug Administration. Several analytical methods have been used for its detection. This study aimed to summarize the recent outcomes of the chromatographic methods such as electrophoretic methods, gas chromatography, and liquid chromatography, which have been used for the determination of melatonin in the last three decades, with the focus on gas chromatography and high-performance liquid chromatography methods.

Melatonin signaling regulates locomotion behavior and homeostatic states through distinct receptor pathways in Caenorhabditis elegans

Melatonin is a hormone that controls circadian rhythms and seasonal behavioral changes in vertebrates. Recent studies indicate that melatonin participates in diverse physiological functions including the modulation of neural activities. Melatonin is also detected in many other organisms that do not exhibit obvious circadian rhythms, but their precise functions are not known. To understand the role of melatonin and its genetic pathway in vivo, we examined the effects of melatonin and its receptor antagonists on various behaviors in Caenorhabditis elegans. Exogenously applied melatonin specifically decreased locomotion rates in 15-min treatments, suggesting that melatonin directly regulates neural activities for locomotion. This melatonin signaling functions through MT1-like melatonin receptors, because the MT1/2 receptor antagonist luzindole effectively blocked the effect of melatonin on locomotion, while MT2-specific antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT) and MT3-selective antagonist prazosin had no effect. Alternatively, long-term treatment with prazosin specifically altered homeostatic states of the worm, suggesting another melatonin-signaling pathway through MT3-like receptors. We also found that two G-protein alpha subunit mutants and newly isolated five mutants exhibited defects in response to melatonin. Our findings imply that melatonin acts as a neuromodulator by regulating locomotion behavior and as a ligand for homeostatic control through distinct receptor pathways in C. elegans.

Human lymphocyte-synthesized melatonin is involved in the regulation of the interleukin-2/interleukin-2 receptor system

Since melatonin was first isolated in 1958 up to the last few years, this substance was considered a hormone exclusive to the pineal gland. Although melatonin has lately been identified in a large number of extrapineal sites, its potential biological actions have not yet been studied. This paper shows that human lymphocyte-synthesized melatonin plays a crucial role modulating IL-2/IL-2 receptor system because when blocking melatonin biosynthesis by the tryptophan hydroxylase inhibitor, parachlorophenylalanine, both IL-2 and IL-2 receptor levels fell, restoring them by adding exogenous melatonin. Moreover, we demonstrated that this endogenous melatonin interfered with the exogenous melatonin effect on IL-2 production. Melatonin exerted these effects by a receptor-mediated action mechanism because both IL-2 and IL-2 receptor expressions significantly decreased when lymphocytes were incubated in the presence of the specific membrane and/or nuclear melatonin receptor antagonists, luzindole, and/or CGP 55644, respectively. Finally, we made the real significance of the membrane melatonin receptors in this process clear, so prostaglandin E(2)-induced inhibition on IL-2 production increased when we blocked the membrane receptors using luzindole. In conclusion, these data show that endogenous melatonin is an essential part for an accurate response of human lymphocytes through the modulation of IL-2/IL-2 receptor system.

A reverse-phase HPLC and fluorescence detection method for measurement of 5-hydroxytryptamine (serotonin) in Planaria

INTRODUCTION

Planaria have proven to be a good model system in which to investigate mammalian behaviors and responses to drugs. We have recently studied the response of planarians to dopaminergic ligands and to the effects of cocaine and opioids. To correlate behavior (specifically, drug withdrawal) with neurotransmitter levels, we developed a method to quantify 5-hydroxytryptamine (5-HT; serotonin) in planarians.

METHODS

Following the homogenization of planarians in aqueous solvent (perchloric acid, L-cystine, and Na(2)EDTA) and centrifugation of supernatant (14,000 x g at 4 degrees C for 20 min), 5-HT was measured using HPLC (aqueous citric acid buffer mobile phase; 5-microm C(18) column with fluorescence detection, 280/340 nm). N(omega)-methyl-5-HT was used as an internal standard (IS).

RESULTS

5-HT was rapidly extracted and conveniently measured from the planarians. The detection limit of the procedure (0.35 ng) was below the amount of 5-HT in one animal.

DISCUSSION

The ability to measure neurotransmitter levels provides a methodological opportunity to correlate behavior with biochemical changes in planarians and to extend behavioral observations to intracellular transmitter and second messenger transduction pathways.

Evidence of melatonin synthesis by human lymphocytes and its physiological significance: possible role as intracrine, autocrine, and/or paracrine substance

It has been historically assumed that the pineal gland is the major source of melatonin (N-acetyl-5-methoxytryptamine) in vertebrates. Melatonin plays a central role in fine-tuning circadian rhythms in vertebrate physiology. In addition, melatonin shows a remarkable functional versatility exhibiting antioxidant, oncostatic, antiaging, and immunomodulatory properties. Melatonin has been identified in a wide range of organisms from bacteria to human beings. Its biosynthesis from tryptophan involves four well-defined intracellular steps catalyzed by tryptophan hydroxylase, aromatic amino acid decarboxylase, serotonin-N-acetyltransferase, and hydroxyindole-O-methyltransferase. Here, for the first time, we document that both resting and phytohemagglutinin-stimulated human lymphocytes synthesize and release large amounts of melatonin, with the melatonin concentration in the medium increasing up to five times the nocturnal physiological levels in human serum. Moreover, we show that the necessary machinery to synthesize melatonin is present in human lymphocytes. Furthermore, melatonin released to the culture medium is synthesized in the cells, because blocking the enzymes required for its biosynthesis or inhibiting protein synthesis in general produced a significant reduction in melatonin release. Moreover, this inhibition caused a decrease in IL-2 production, which was restored by adding exogenous melatonin. These findings indicate that in addition to pineal gland, human lymphoid cells are an important physiological source of melatonin and that this melatonin could be involved in the regulation of the human immune system, possibly by acting as an intracrine, autocrine, and/or paracrine substance.

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.

Determination of pineal melatonin by precolumn derivatization reversed-phase high-performance liquid chromatography and its application to the study of circadian rhythm in rats and mice

Determination of minute amounts of endogenous melatonin in rat and mouse pineal gland was performed using an RP-HPLC system. Melatonin was separated following precolumn derivatization and determined with a fluorescence detector at the emission wavelength of 380 nm with the excitation at 245 nm. The calibration curve of melatonin constructed by adding known amounts of melatonin to the homogenates of mouse pineal gland was linear over the range of 1-500 fmol (injection amount/20 microl). The detection limit of added melatonin was 1 fmol (S/N = 5). Repeatability and day-to-day precision for the melatonin spiked sample of mouse pineal gland was 4.0 and 3.8% (RSD), respectively. Using the present method, circadian changes of melatonin content in rat (Wistar) and mouse (C3H) pineal gland were determined. In addition, a minute amount of melatonin in ddY mouse pineal gland was determined, because pineal melatonin of many inbred mouse strains has been reported to be lower than the detection limit.

Sensitive determination of melatonin by precolumn derivatization and reversed-phase high-performance liquid chromatography

A sensitive determination method for melatonin was developed. Melatonin was derivatized under alkaline conditions in the presence of hydrogen peroxide. The resultant fluorophore was excited at 247 nm and the emission wavelength was 384 nm. The Stokes shift was 137 nm, which was longer than that of melatonin itself (lambda ex 280 nm, lambda em 330 nm). The melatonin derivative was separated by reversed-phase HPLC in about 15 min and the calibration curve was linear from 500 amol to 5 pmol (r > 0.999) with the detection limit of 500 amol (S/N = 5). The sensitivity of this method was about ten times higher than that of previous methods. Both the day-to-day precision and within-day precision were about 5%, and the derivative of melatonin in the aqueous solution was stable for more than 10 days. This method was successfully applied to the determination of melatonin in rat pineal gland.

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.

Pineal and plasma melatonin as determined by high-performance liquid chromatography with electrochemical detection

A rapid and sensitive method for the routine quantitative determination of melatonin in pineal and plasma is described. The assay used reversed-phase high-performance liquid chromatography (RP-HPLC) separation combined with either amperometric (system A) or coulometric (system B) detection. The method gave satisfactory reproducibility and accuracy, and detection limits for melatonin were as low as 8.5 pg (system A) and 1 pg (system B). This high sensitivity, together with the short analysis time (less than 10 min), and the simplicity of sample procedure make the present RP-HPLC method suitable for a wide range of studies concerning melatonin measurements. Melatonin values obtained in this study from both rat pineal and human plasma agree with those reported previously, and clearly determined a circadian pattern.

Age-associated changes and sex differences in urinary 6-sulphatoxymelatonin circadian rhythm in the rat

The circadian rhythm of 6-sulphatoxymelatonin (aMT6s) excretion has been determined in male and female rats at 3 weeks and at 2, 8, 14 and 20 months of age. All animals have a pronounced circadian pattern of aMT6s excretion under a 12 hour dark: 12 hour light cycle. A significant increase in aMT6s excretion is observed from 3 weeks to 14 months followed by a decrease at 20 months. There is a highly significant correlation between aMT6s excretion and body weight (r = 0.73 for female rats and r = 0.74 for male rats; p values are all less than 0.001). Thus, a decrease in aMT6s excretion associated with increasing age occurs when body weight is taken into consideration. aMT6s excretion is higher in males at 3 weeks and at 2 and 8 months of ages. Urinary testosterone in male rats and estradiol in female rats increase from 3 weeks to 8 months and decrease at older ages. These data suggest that increase of body weight from 3 weeks to 14 months is an important factor responsible for the age-related alteration. The sex differences in aMT6s excretion in younger rats may be associated with their sex hormonal milieu.

Concurrent on-line sampling of melatonin in pineal microdialysates from conscious rat and its analysis by high-performance liquid chromatography with electrochemical detection

Dynamic changes of melatonin in microdialysates from the pineal gland of a freely moving rat were repeatedly determined by using on-line high-performance liquid chromatography with electrochemical detection. The detection limit for melatonin, ca. 5 pg, was well below that achieved with other systems. We observed a drastic increase of extracellular pineal melatonin during the transitional phase from the light period to the dark period. This application of microdialysis is a useful tool in the study of the physiological role of the mammalian pineal body.

Determination of six indolic compounds, including melatonin, in rat pineal using high-performance liquid chromatography with serial fluorimetric-electrochemical detection

A reversed-phase high-performance liquid chromatographic method has been developed for the simultaneous determination of 5-hydroxytryptophan, 5-hydroxyindoleacetic acid, N-acetylserotonin, tryptophan, 5-hydroxytryptamine (serotonin) and melatonin in rat pineal using a buffered aqueous eluent containing acetonitrile and methanol as organic modifiers and an ion-pairing agent to assist in controlling the retention of compounds containing an amine group. Serial fluorimetric-electrochemical detection provided additional assurance of compound identity. Analyte preparation simply involved the sonication of pineals in dilute perchloric acid containing an antioxidant and a chelating agent, followed by centrifugation to clarify. The method simplifies the determination of this range of indolic compounds, which normally would require at least two separate runs with different eluents. Detection limits for melatonin were 60 and 135 pg for fluorimetric and electrochemical detection, respectively. (This represented the "worst case" considering the levels and detection limits of all compounds present.) Using flow programming and a flow-rate varying between 1 and 1.5 ml/min, the analysis time was 27.5 min, which made the determination of ten samples in a working day possible.

Determination of pineal melatonin by high-performance liquid chromatography with electrochemical detection: application for rhythm studies and tissue explants

High-pressure liquid chromatography with electrochemical detection has been used for the determination of melatonin in rat pineal glands and tissue explants. Rat pineal melatonin content was measured at 2-hr intervals during a 24-hr period by direct injection of supernatants from centrifuged pineal homogenates into a reversed-phase C18 column. The lower limit of sensitivity was 32 pg per gland for a 2:1 signal-to-noise ratio, which allows melatonin measurements during daytime. Both absolute values and circadian rhythmicity obtained with the present method are in agreement with previously validated radioimmunoassays. In addition, isoproterenol-induced release of melatonin from individual rat pineal explants was measured after chloroform extraction of the incubation medium.

Kinetic study of melatonin release from rat pineal glands using a perifusion technique

In previous studies, noradrenaline was found to elicit a rise of melatonin secretion through activation of typical beta-adrenergic receptors. In the present study, a perifusion system was developed to characterize the kinetics of melatonin release from rat pineal glands. Isolated pineal glands from adult male rats were continuously perifused for 15 h in a Krebs-Ringer solution, and the concentration of melatonin in the effluent perifusate was monitored using a specific radioimmunoassay. The rate of release of melatonin declined during the first 3-4 h of perifusion and then remained fairly stable for at least 11 h. The spontaneous release of melatonin was around 20 pg per min and per gland. When pineal glands were stimulated with isoproterenol, melatonin release output linearly increased for at least 2 h after the stimulation. The increase in melatonin release depended on the isoproterenol concentration and on the duration of the stimulation. The analysis of the pattern of melatonin secretion by a single rat pineal gland showed that the secretion was irregular but did not present a clear feature of pulsatile or oscillatory release over a 11 h-long study. The perifusion system was found useful in order to follow the characteristics of melatonin release from pineal glands and should allow investigations of neuronal or hormonal control of pineal gland activities.