Determination of 5-methoxyindoles in pineal gland and plasma samples by high-performance liquid chromatography with electrochemical detection

Diurnal profiles of melatonin synthesis-related indoles, catecholamines and their metabolites in the duck pineal organ

This study characterizes the diurnal profiles of ten melatonin synthesis-related indoles, the quantitative relations between these compounds, and daily variations in the contents of catecholamines and their metabolites in the domestic duck pineal organ. Fourteen-week-old birds, which were reared under a 12L:12D cycle, were killed at two-hour intervals. The indole contents were measured using HPLC with fluorescence detection, whereas the levels of catecholamines and their metabolites were measured using HPLC with electrochemical detection. All indole contents, except for tryptophan, showed significant diurnal variations. The 5-hydroxytryptophan level was approximately two-fold higher during the scotophase than during the photophase. The serotonin content increased during the first half of the photophase, remained elevated for approximately 10 h and then rapidly decreased in the middle of the scotophase. N-acetylserotonin showed the most prominent changes, with a more than 15-fold increase at night. The melatonin cycle demonstrated only an approximately 5-fold difference between the peak and nadir. The 5-methoxytryptamine content was markedly elevated during the scotophase. The 5-hydroxyindole acetic acid, 5-hydroxytryptophol, 5-methoxyindole acetic acid and 5-methoxytryptophol profiles were analogous to the serotonin rhythm. The norepinephrine and dopamine contents showed no significant changes. The DOPA, DOPAC and homovanillic acid levels were higher during the scotophase than during the photophase. Vanillylmandelic acid showed the opposite rhythm, with an elevated level during the daytime.

The endogenous substrates of brain CYP2D

CYP2D6, one of the major cytochrome P450 isoforms present in the human brain, is associated with the incidence and prevalence of central nervous system (CNS) diseases. Human CYP2D6 and rat CYP2D are involved in the metabolism of various neurotransmitters and neurosteroids. Brain CYP2D can be regulated by endogenous steroids, including sex hormones. The alteration of CYP2D-mediated metabolism induced by endogenous steroids may cause changes in sensitivity to environmental and industrial toxins and carcinogens as well as physiological and pathophysiological processes controlled by biologically active compounds. This review summarizes the current knowledge regarding the distribution, endogenous substrates, and regulation of brain CYP2D.

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.

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.

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.

Polymorphic cytochrome P450 2D6: humanized mouse model and endogenous substrates

Cytochrome P450 2D6 (CYP2D6) is the first well-characterized polymorphic phase I drug-metabolizing enzyme, and more than 80 allelic variants have been identified for the CYP2D6 gene, located on human chromosome 22q13.1. Human debrisoquine and sparteine metabolism is subdivided into two principal phenotypes--extensive metabolizer and poor metabolizer--that arise from variant CYP2D6 genotypes. It has been estimated that CYP2D6 is involved in the metabolism and disposition of more than 20% of prescribed drugs, and most of them act in the central nervous system or on the heart. These drug substrates are characterized as organic bases containing one nitrogen atom with a distance about 5, 7, or 10 A from the oxidation site. Aspartic acid 301 and glutamic acid 216 were determined as the key acidic residues for substrate-enzyme binding through electrostatic interactions. CYP2D6 transgenic mice, generated using a lambda phage clone containing the complete wild-type CYP2D6 gene, exhibits enhanced metabolism and disposition of debrisoquine. This transgenic mouse line and its wild-type control are models for human extensive metabolizers and poor metabolizers, respectively, and would have broad application in the study of CYP2D6 polymorphism in drug discovery and development, and in clinical practice toward individualized drug therapy. Endogenous 5-methoxyindole- thylamines derived from 5-hydroxytryptamine were identified as high-affinity substrates of CYP2D6 that catalyzes their O-demethylations with high enzymatic capacity and specificity. Thus, polymorphic CYP2D6 may play an important role in the interconversions of these psychoactive tryptamines, including a crucial step in a serotonin-melatonin cycle.

Determination of endogenous melatonin in the individual pineal glands of inbred mice using precolumn oxidation reversed-phase micro-high-performance liquid chromatography

The amount of endogenous melatonin in the individual pineal glands of inbred mice has been determined using reversed-phase micro-high-performance liquid chromatography after precolumn oxidation of melatonin to a compound having strong fluorescence. The fluorescent compound was identified as N-[(6-methoxy-4-oxo-1,4-dihydroquinolin-3-yl)methyl]acetamide. The excitation and emission wavelengths of this compound are 245 and 380 nm, respectively, and the fluorescence intensity is 6.8 times greater than that of melatonin. Molar absorptivity and fluorescence quantum yield of this compound are 46,300[L mol(-1)cm(-1)] and 0.31 (245 nm), respectively. The lower quantification limit of melatonin in biological samples using this precolumn oxidation method is 200 amol, and the calibration curve of spiked melatonin is linear from 200 amol to 50 fmol (r>0.999). The sensitivity of the present method is almost 10 times higher than that of the previous method. The values of endogenous melatonin obtained for ICR, C57BL, BALB/c, and AKR mice are 4.7, 6.1, 7.4, and 18.8 fmol/pineal gland, respectively. The amounts of endogenous pineal melatonin of these strains had not been clearly reported due to the poor enzymatic activities for melatonin biosynthesis; this is the first report that clearly demonstrates the existence of endogenous melatonin in these inbred mice.

Voltammetric and flow amperometric methods for the determination of melatonin in pharmaceuticals

Melatonin can be sensitively detected in pharmaceuticals by two different and simple electrochemical methods: cyclic voltammetry (CV) and amperometric detection in a flow injection analysis system (FIA-ED). An adequate pre-treatment of the carbon paste electrode in the first case and the employ of a high flow rate in the second one were the key for obtaining a very good reproducibility (R.S.D. values of 1.5 (n=10) and 1.3% (n=20), respectively). Low limits of detection were achieved and with the coupling of a flow system a linear dynamic range of three orders of magnitude (from 10(-8) to 10(-5) M) was obtained. Both methods were applied to the determination of melatonin in pharmaceuticals. In order to best validate these methodologies a fluorescent procedure was developed to contrast the results. As no interferences from the matrix were found the employ of a separation technique is not necessary. In this way the procedure is fastened and simplified. Moreover, the low price, ease of handling, possibility of automation and high sample throughput are important advantages that convert the flow methodology in an attractive alternative for quality control of pharmaceuticals.

Determination of free and total (free plus protein-bound) melatonin in plasma and cerebrospinal fluid by high-performance liquid chromatography with fluorescence detection

A simple, sensitive and accurate method for the estimation of free and total (free plus protein-bound) melatonin (MLT) in human plasma and cerebrospinal fluid (CSF) is described. Via Chem-Elut cartridges, free and total MLT (the latter obtained after a deproteinization step) were quantified in dichloromethane-extracted samples and analyzed in one chromatographic run by high-performance liquid chromatography (HPLC) with fluorimetric detection. The column used was an Extrasil ODS-2 (3 microm, 150 x 4.6 mm I.D.), while the mobile phase consisted of 75 mM sodium acetate-acetonitrile (72:28, v/v) (pH 5.0). Repeatability and reproducibility of the method were 3.24 and 9.4%, respectively. The recovery of melatonin from plasma and CSF was 99.9+/-4.0% for non-deproteinized samples and 93.2+/-4.8% for deproteinized samples. The detection limit of the assay was 0.5 pg/ml. In human plasma, the mean+/-SD concentrations in the darkness period were 23.18+/-7.44 pg/ml for free melatonin and 82.5+/-36.48 pg/ml for total melatonin, while the lowest concentrations detected during daytime were 2.23+/-2.22 and 7.40+/-5.68 pg/ml, respectively. Detection of MLT in CSF was 5.01+/-2.31 and 28.55+/-6.95 pg/ml for the free and total fraction, respectively.

Profiling of Tryptophan-related Plasma Indoles in Patients with Carcinoid Tumors by Automated, On-Line, Solid-Phase Extraction and HPLC with Fluorescence Detection

Background: Profiling of the plasma indoles tryptophan, 5-hydroxytryptophan (5-HTP), serotonin, and 5-hydroxyindoleacetic acid (5-HIAA) is useful in the diagnosis and follow-up of patients with carcinoid tumors. We describe an automated method for the profiling of these indoles in protein-containing matrices as well as the plasma indole concentrations in healthy controls and patients with carcinoid tumors.

Methods: Plasma, cerebrospinal fluid, and tissue homogenates were prepurified by automated on-line solid-phase extraction (SPE) in Hysphere Resin SH SPE cartridges containing strong hydrophobic polystyrene resin. Analytes were eluted from the SPE cartridge by column switching. Subsequent separation and detection were performed by reversed-phase HPLC combined with fluorometric detection in a total cycle time of 20 min. We obtained samples from 14 healthy controls and 17 patients with metastasized midgut carcinoid tumors for plasma indole analysis. In the patient group, urinary excretion of 5-HIAA and serotonin was compared with concentrations of plasma indoles.

Results: Within- and between-series CVs for indoles in platelet-rich plasma were 0.6–6.2% and 3.7–12%, respectively. Results for platelet-rich plasma serotonin compared favorably with those obtained by single-component analysis. Plasma 5-HIAA, but not 5-HTP was detectable in 8 of 17 patients with carcinoid tumors. In the patient group, platelet-rich plasma total tryptophan correlated negatively with platelet-rich plasma serotonin (P = 0.021; r = −0.56), urinary 5-HIAA (P = 0.003; r = −0.68), and urinary serotonin (P &lt;0.0001; r = −0.80).

Conclusions: The present chromatographic approach reduces analytical variation and time needed for analysis and gives more detailed information about metabolic deviations in indole metabolism than do manual, single-component analyses.

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 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.

Evidence for melatonin synthesis in rodent Harderian gland: a dynamic in vitro study

Melatonin content and release from Harderian glands (HGs) has been measured by an in vitro perifusion technique in three rodent species: Wistar rat, Syrian hamster, and Siberian hamster. Melatonin immunoreactive concentrations in HGs of animals killed at 10.00 hr were 0.31 +/- 0.031 pg/mg gland in male Wistar rat, 0.54 +/- 0.026 pg/mg gland in male Siberian hamster, 0.17 +/- 0.070 and 0.20 +/- 0.059 pg/mg gland in male and female Syrian hamster, respectively. In all species examined, isolated HGs perifused for 9-15 hr released melatonin but did not stabilize their melatonin release rate. No sex-related difference could be noted in the HG melatonin release rate. The total amount of melatonin released over a 15 hr long perifusion was about 0.075 +/- 0.004 ng/15 h/mg gland and 0.063 +/- 0.010 ng/15 hr/mg gland in male and female Wistar rat, respectively; 0.155 +/- 0.019 ng/15 hr/mg gland and 0.141 +/- 0.006 ng/15 hr/mg gland in male and female Siberian hamster, respectively; 0.035 +/- 0.003 ng/15 hr/mg gland and 0.045 +/- 0.004 ng/15 hr/mg gland in male and female Syrian hamster, respectively. This amount, which is higher than the tissue levels, demonstrates the de novo melatonin synthesis. This is confirmed by the fact that infusion of the indoleamine precursor, tryptophan (TRP), stimulated melatonin secretion from HGs. The melatonin release is increased by 2.5-fold in male and female Wistar rat, 1.5-fold in male and female Siberian hamster, and 2.0- and 3.0-fold in male and female Syrian hamster, respectively. Treatment with a TRP hydroxylase inhibitor, para-chlorophenylalanine, reduced basal melatonin release and inhibited the TRP-induced melatonin stimulation. Kinetics and amounts of melatonin released were not affected by pinealectomy, ruling out a possible plasmatic origin of the HG melatonin. Isoproterenol, a beta-adrenergic agonist, and dibutyryl cyclic AMP, a cyclic AMP analogue, failed to stimulate HG melatonin secretion. In conclusion, these results confirm the presence of melatonin in the HGs and demonstrate that melatonin is synthesized in and released from isolated rodent HGs.

Determination of melatonin in rat pineal, plasma and retina by high-performance liquid chromatography with electrochemical detection

A sensitive method for the routine measurement of endogenous melatonin (MEL) in pineal, retina and plasma rat tissues has been developed using reversed-phase high-performance liquid chromatography with electrochemical detection. Quantification limit for MEL was 0.2 ng/mg protein in pineal, 15 pg/ml in plasma and 2.0 pg/mg protein in retina. To improve both MEL quantification and the reproducibility of the assay, an internal standard was used when an extraction in organic solvent was required, in contrast with other available chromatographic methods. MEL values and the circadian profile obtained in this study from both rat pineal and plasma agree with those reported previously. This method allows MEL detection in mammal retina, particularly in rat, where MEL levels are very low.

Identification and diurnal studies of pineal and serum 5-methoxytryptamine in the rat and quail

Picomole to femtomole per gland/ml levels of 5-methoxytryptamine (5MT) in the pineal gland and serum of the rat and quail were determined, in the absence of a monoamine oxidase inhibitor, by using the ultra-sensitive technique of capillary column gas chromatography/electron-capture negative ion chemical ionization mass spectrometry. Diurnal rhythms of pineal 5MT with high levels at mid-light and low levels at mid-dark were found in both species, while serum 5MT levels showed less or no diurnal variations. The definitive presence of 5MT in the pineal gland and blood circulation provides further evidence that it is potentially a neurohormone. Whether 5MT is implicated in the photoperiodic responses and/or other physiological functions of animals, however, remains to be investigated.

Analysis of 5-methoxytryptamine at the femtomole level in the rat and quail brain by gas chromatography-electron-capture negative-ion chemical ionization mass spectrometry

A sensitive method for the measurement of endogenous 5-methoxytryptamine in brain tissue has been developed using capillary column gas chromatography-electron-capture negative-ion chemical ionization mass spectrometry. 5-Methoxytryptamine was first converted to N-[2H3]acetyl-5-methoxytryptamine by reaction with hexa-deuterated acetic anhydride, followed by reaction with pentafluoropropionic anhydride to yield the highly electron-capturing 3,3'-spirocyclic pentafluoro-propionyl indolenine derivative. Quantitative analysis was carried out by selected-ion monitoring of the [M-HF].- and [M-HF-DF].- ion intensity of the 3.3'-spirocyclic pentafluoropropionyl indolenine derivative, using 5-methoxy-[alpha, alpha, beta, beta-2H4]tryptamine as the internal standard. The presence of 5-methoxytryptamine in the brain tissue was demonstrated. In the absence of a monoamine oxidase inhibitor, the mean +/- S.D. levels of 5-methoxytryptamine in the rat and quail whole brain were found to be 30 +/- 6 and 347 +/- 52 pg/g, respectively. The possible physiological functions of 5-methoxytryptamine as a neuromodulator and/or neurotransmitter have to be considered.

HPLC validation of a circadian melatonin rhythm in the pineal gland of inbred mice

Production of melatonin in the pineal gland of inbred mice such as C57B1/6J, BALB/c and AKR strains is still a matter of debate. In previous studies, we and other authors showed that these strains of inbred mice have a clear-cut circadian rhythm of serum melatonin and urinary 6-hydroxy-melatonin-sulphate. In contrast, other groups claimed these mice are unable to synthesize melatonin. These studies were based on RIA measurements and/or estimates of N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) activities. In the present study we validate the presence of melatonin in the pineal gland of C57B1/6, BALB/c, and AKR mice by HPLC determinations. We found a short-term melatonin peak in the middle of the dark period with a pattern which mirrors that found previously in the serum. The possibility remains, although it seems unlikely, that the pineal melatonin rhythm measured here represents melatonin produced elsewhere which then was subsequently taken up by the pineal gland.

Simultaneous determination of serotonin, N-acetylserotonin and melatonin in the pineal gland of the juvenile golden hamster by high-performance liquid chromatography with electrochemical detection

A simple and simultaneous determination of melatonin and its precursors, serotonin (5-HT) and N-acetylserotonin, was achieved by reversed-phase high-performance liquid chromatography with electrochemical detection. The addition of an ion-pairing agent, sodium 1-octanesulfonate, to the chromatographic mobile phase caused an increase of the retention time of 5-HT, and resulted in the successful simultaneous resolution of these three indoleamines. This method was used to quantitate these indoleamines in the pineal gland of juvenile golden hamsters.

Detection and quantification of melatonin in a dinoflagellate, Gonyaulax polyedra: solutions to the problem of methoxyindole destruction in non-vertebrate material

Preservative procedures are described for the extraction and quantification of melatonin in cell material from the dinoflagellate Gonyaulax polyedra, an organism in which this indoleamine is rapidly degraded due to interaction with free oxygen radicals and photooxidation. Cells were shock-frozen in liquid nitrogen and pulverized. Various extraction methods were applied to the powder, and rates of recovery were compared. For the determination of melatonin by high performance liquid chromatography (HPLC), extractions with acetone or perchloric acid were more suitable than the use of other solvents. For purposes of radioimmunoassay (RIA), extraction with acetone gave the best results. Several other inorganic solvents, which are often applied in melatonin research, such as chloroform, dichloromethane, and diethyl ether, led to considerable losses of the indoleamine. The procedures developed for HPLC with either electrochemical or fluorescence detection also allow the quantification of other indolic compounds, in particular, tryptophan and 5-methoxytryptamine. The methods described may be of value in the further search for melatonin and related indoleamines in non-vertebrate material, especially, from unicells, multicellular plants, and invertebrates.

High performance liquid chromatographic profiling of tryptophan and related indoles in body fluids and tissues of carcinoid patients

A high performance liquid chromatographic method with quaternary gradient elution and fluorometric detection was developed for profiling of tryptophan (TRP), 5-hydroxytryptophan, serotonin (5-HT) and 5-hydroxyindole-3-acetic acid (5-HIAA) in urine, platelet-rich plasma and (tumour) tissue of patients with carcinoid tumours. Prior to injection, urine samples were diluted and filtered. Platelet-rich plasma and tissue homogenates were prepurified by C18 solid phase extraction. Detection limits were approx. 2 pmol. Results of urinary 5-HT and 5-HIAA compared favourably with those of single component analyses. No consistent diurnal variations were found for TRP, 5-HT and 5-HIAA in 12-h urine samples from 15 healthy adults. Abstinence of 5-HT-rich foods reduced urinary levels of 5-HT and 5-HIAA. C18 extraction of indoles from protein-containing matrices was studied in platelet-rich plasma. Although time-consuming and complicated for daily routine use, the present approach offers particular advantages over single component analyses in the study of TRP metabolism in patients with carcinoid tumours.

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.

Low ambient temperature does not affect the pineal concentrations of either 5-methoxytryptamine or melatonin in golden hamsters kept under short photoperiod

This work investigates the rhythmic synthesis of pineal 5-methoxytryptamine and related indoles in golden hamsters as a function of ambient temperature. 5-methoxytryptamine, 5-methoxytryptophol, 5-methoxyindole acetic acid, and melatonin were measured by high pressure liquid chromatography (HPLC) in the pineal gland of golden hamsters after inhibition of monoamine oxidase. In our experimental conditions, the pineal 5-methoxyindole concentrations of hamsters kept at 5 degrees C were similar to those observed at 20 degrees C. These results suggest that low temperature, which is known to accelerate the short photoperiod-induced gonadal regression, acts at a level different to that of the pineal gland or affects another compound in the pineal gland.