Melatonin Distribution Reveals Clues to Its Biological Significance in Basal Metazoans

Although nearly ubiquitous in nature, the precise biological significance of endogenous melatonin is poorly understood in phylogenetically basal taxa. In the present work, we describe insights into the functional role of melatonin at the most “basal” level of metazoan evolution. Hitherto unknown morphological determinants of melatonin distribution were evaluated in Nematostella vectensis by detecting melatonin immunoreactivity and examining the spatial gene expression patterns of putative melatonin biosynthetic and receptor elements that are located at opposing ends of the melatonin signaling pathway. Immuno-melatonin profiling indicated an elaborate interaction with reproductive tissues, reinforcing previous conjectures of a melatonin-responsive component in anthozoan reproduction. In situ hybridization (ISH) to putative melatonin receptor elements highlighted the possibility that the bioregulatory effects of melatonin in anthozoan reproduction may be mediated by interactions with membrane receptors, as in higher vertebrates. Another intriguing finding of the present study pertains to the prevalence of melatonin in centralized nervous structures. This pattern may be of great significance given that it 1) identifies an ancestral association between melatonin and key neuronal components and 2) potentially implies that certain effects of melatonin in basal species may be spread widely by regionalized nerve centers.     

Melatonin synthesis by rabbit platelets

Platelets of reserpinized rabbits, incubated in buffer containing tritiated 5-hydroxytryptamine (³H-5HT), have the ability to convert ³H-5HT into labelled compound (s) extractable with alkaline-chloroform. The bulk of the chloroform-extractable radioactivity showed a Rf value similar to authentic melatonin on silicagel thin-layer chromatography.The labelled product eluted with ethanol from the silicagel area where melatonin was suspected to reside was indentified as melatonin by mass spectrometry and radioimmunoassay.Our in vitro experiments demonstrate that rabbit platelets are capable of converting 5-HT into melatonin apparently because they possess the enzymatic equipment necessary for this biosynthesis i.e. serotonin N-acetyltransferase and hydroxyindole-O-methyltransferase.     

Nyctohemeral Rhythm in the Levels of S-Adenosylmethionine in the Rat Pineal Gland and Its Relationship to Melatonin Biosynthesis

Abstract: Liquid chromatographic techniques that permit the simultaneous analysis of S -adenosylmethionine, melatonin, and its intermediary metabolites N -acetyl-5-hydroxytryptamine and 5-hydroxytryptamine within individual pineal glands have been developed. S -Adenosylmethionine has been shown to undergo a marked nyctohemeral rhythm in the pineal gland of the rat, with maximal levels occurring during the light period and minimal levels during the dark period. Detailed studies of the temporal relationships between the levels of S -adenosylmethionine and those of melatonin and its intermediary metabolites suggest that an association exists between the levels of S -adenosylmethionine and the status of the biosynthesis of melatonin. Exposure of animals to continuous light and the administration of the β-adrenoreceptor antagonist propranolol were both found to inhibit the induction of melatonin synthesis and prevent the reduction in the levels of S -adenosylmethionine during the dark period. As a corollary the induction of melatonin biosynthesis following the administration of the β-adrenoreceptor agonist isoproterenol during the light period was accompanied by a marked decrease in the levels of S -adenosylmethionine in the pineal gland. The significance of the link between the nyctohemeral rhythms in the levels of S -adenosylmethionine and the biosynthesis of melatonin in the pineal gland is discussed in the context of the therapeutic efficacy of S -adenosylmethionine as an antidepressant.     

Age-related characteristics of the effects of epithalamin on serotonin metabolism in the pineal gland of rats]

The biosynthesis of serotonin into melatonin was decreased in old (18-20-month) in comparison to young (4-5-month) male Wistar rats. 5-day morning injections to young and old rats with polypeptide pineal preparation (epithalamin) in a dose of 2.5 mg/kg of body weight induced the increase in the night peak of serotonin, N-acetylserotonin and melatonin in young and melatonin alone in old rats and did not influence 5-methoxytryptamine, 5-oxy- and 5-methoxyindoleacetic acids level. These data support suggestion of ultrashort loop between pineal peptides and indoles and that epithalamin increases the metabolism of serotonin into melatonin.     

褪黑素对谷氨酸钠致痫大鼠海马5-羟色胺水平的影响

目的观察褪黑素(Melatonin,MT)对谷氨酸钠(Glutamate,Glu)致痫大鼠海马5-羟色胺(5-hydroxytryptamine,5-HT)水平的影响,研究其抑制癫痫的作用机制。方法40只健康雄性SD大鼠随机分为4组(每组10只),分别为生理盐水对照组(NS组);谷氨酸钠致痫组(Glu组);褪黑素 谷氨酸钠组(MT Glu组);Luzidole 褪黑素 谷氨酸钠组(Luz MT Glu组)。观察并记录大鼠行为学及脑电图改变,用免疫组织化学方法检测大鼠海马内5-HT含量变化。结果行为学观察和EEG显示,NS组无痫样发作和痫样放电,Glu组和Luz MT Glu组痫样发作重(Ⅲ-Ⅴ级),脑电图显示频发高幅的痫样波,TM Glu组无或仅有轻微发作(0-Ⅱ级),脑电图上无或偶见散在单个微小痫样波;免疫组织化学分析结果显示,Glu组和Luz MT Glu组大鼠海马内5-HT含量与对照组比较均减少,差异性明显(P<0.05),MT Glu组较Glu组和Luz MT Glu组5-HT含量升高,差异性明显(P<0.05)。结论MT对谷氨酸钠致痫大鼠痫样发作程度、痫样放电有抑制作用,其机制之一是经由其特异性的膜受体,通过某种机制增强5-HT作用,进而发挥抑痫效应。     

褪黑素对谷氨酸钠致痫大鼠海马5-色胺水平的影响

目的观察褪黑素（Melatonin,MT）对谷氨酸钠（Glutamate,Glu）致痫大鼠海马5-羟色胺（5-hydroxytryptamine,5-HT）水平的影响,研究其抑制癫痫的作用机制。方法40只健康雄性SD大鼠随机分为4组（每组10只）,分别为生理盐水对照组（NS组）;谷氨酸钠致痫组（Glu组）;褪黑素＋谷氨酸钠组（MT＋Glu组）;Luzidole＋褪黑素＋谷氨酸钠组（Luz＋MT＋Glu组）。观察并记录大鼠行为学及脑电图改变,用免疫组织化学方法检测大鼠海马内5-HT含量变化。结果行为学观察和EEG显示,NS组无痫样发作和痫样放电,Glu组和Luz＋MT＋Glu组痫样发作重（Ⅲ—Ⅴ级）,脑电图显示频发高幅的痫样波,TM＋Glu组无或仅有轻微发作（0-Ⅱ级）,脑电图上无或偶见散在单个微小痫样波;免疫组织化学分析结果显示,Glu组和Luz＋MT＋Glu组大鼠海马内5-HT含量与对照组比较均减少,差异性明显（P〈0．05）,MT＋Glu组较Glu组和Luz＋MT＋Glu组5-HT含量升高,差异性明显（P〈0．05）。结论MT对谷氨酸钠致痫大鼠痫样发作程度、痫样放电有抑制作用,其机制之一是经由其特异性的膜受体,通过某种机制增强5-HT作用,进而发挥抑痫效应。     

Melatonin binding proteins identified in the rat brain by affinity labeling

N-Bromoacetyl-2-iodo-5-methoxytryptamine (BIM), a novel derivative of the biologically active melatonin analog, 2-iodomelatonin, was prepared and used to identify melatonin binding proteins in rat brain synaptosomes. Incubation of the synaptosomes with BIM resulted in a time and concentration dependent, irreversible inhibition of 2-[125I]iodomelatonin binding. In parallel, the radioactive form of BIM, N-bromoacetyl-2-[125I]iodo-5-methoxytryptamine ([125I]BIM) became incorporated into the synaptosomes. The incorporation of [125I]BIM was inhibited by BIM, 2-iodomelatonin and melatonin but not by 5-methoxytryptamine or N-acetyl serotonin. [125I]BIM became covalently attached to three polypeptides with apparent molecular weight values of 92, 55 and 45 kDa; the labeling of all three proteins was markedly inhibited by melatonin. These results indicate that the 92, 55 and 45 kDa polypeptides are melatonin binding proteins.     

Affinity labeling of melatonin binding sites in the hamster brain.

N-Bromoacetyl-2-iodo-5-methoxytryptamine (BIM), a novel derivative of the biologically active melatonin analog, 2-iodomelatonin, was used to identify melatonin binding proteins in synaptosomes from Syrian hamster brain. Incubation of the synaptosomes with BIM resulted in a concentration dependent, irreversible inhibition of 2-125I-iodomelatonin binding. The radioactive form of BIM, N-Bromoacetyl-2-125I-iodo-5-methoxytryptamine (125I-BIM), became covalently attached to three proteins in the synaptosomes, in a concentration dependent manner. These proteins had apparent molecular weight values of 92, 55 and 45 kilodaltons. The incorporation of 125I-BIM into all three proteins was inhibited by BIM greater than 2-iodomelatonin greater than melatonin whereas the melatonin antagonist N-(1,4 dinitrophenyl)- 5-methoxytryptamine (ML-23) selectively inhibited the labeling of the 45 kDa protein. These results indicate that the 92, 55 and 45 KDa polypeptides are melatonin binding proteins.     

Melatonin-induced stimulation of rat corpus epididymal epithelial cell proliferation.

Stimulation of rat epididymal epithelial cell proliferation by melatonin was demonstrated by thymidine incorporation and flow cytometric analyses. The stimulatory effect of melatonin was dependent on the hormone concentration and the duration of cell exposure to the hormone. Maximal stimulation of [3H]thymidine incorporation into epididymal epithelial cells by melatonin was observed at 1 x 10(-9) M 5alpha-dihydrotestosterone in medium, while lower or higher concentrations of androgen attenuated the stimulatory effect of melatonin. Interestingly, a nuclear melatonin receptor agonist (1-[3-allyl-4-oxothiazolidine-2-ylidene]-4-methyl-thiosemi-carb azone, CGP 52608) induced opposite effect on epithelial cell proliferation to that produced by melatonin. Our data suggest that melatonin-induced stimulation of rat epididymal epithelial cell proliferation is not likely to be mediated by nuclear receptor. Furthermore, sequential changes of cell cycle distribution with melatonin treatment also supports a stimulatory action of melatonin on epididymal epithelial cell proliferation.     

Uptake and effects of melatonin on the synthesis of proteins by the rat cerebral cortex

Slices of rat parietal cerebral cortex took up and retained [³H] melatonin up to a tissue concentration about 4-fold to that present in the incubation medium. This phenomenon was time-dependent, maxima being observed after 180 min-incubations Eighty to 93% of the radioactivity present in the cerebral cortex slices was chromatographically identified as melatonin. Even at the highest melatonin concentration that could be dissolved in the incubation media, a constant proportion of [³H] melatonin was bound to cortical slices, indicating that within this concentration range, melatonin binding is independent of its concentration. Melatonin effects on protein synthesis in the rat cerebral cortex were investigated by studying the incorporation of [³H] L-leucine into proteins in cerebral cortex of rats injected s.c. with 10 or 100 μg/day of melatonin for 5 to 10 days. Both treatments caused leucine incorporation into proteins to increase significantly by about 50 to 60%.     

Correlations of serotonin and its metabolites in individual rat pineal glands over light:dark cycles and after acute light exposure

Profiles of pineal indolealkylamines were estimated by high performance liquid chromatography and were correlated in individual glands of male rats sacrificed over several light:dark cycles and after acute exposure to light at night. A significant and positive correlation of 5HIAA vs 5HT in individual glands over both normal and experimental lighting conditions suggested that oxidative deamination is not a major factor in photic regulation of pineal 5HT levels and that the formation of 5HIAA is dependent on substrate availability. Regression analysis of other indole constituents revealed that there was a positive and significant correlation between 5HT vs N-acetylserotonin, but not between 5HT vs melatonin and N-acetylserotonin vs melatonin in individual glands during the dark phase of a light:dark cycle. We propose that this effect may be related to a pulsatile release of melatonin into the blood stream and is the result of sampling glands at different stages in the storage/release of melatonin.     

褪黑素对绒山羊皮肤FGF5等绒毛生长相关基因的影响

目的:旨在分析埋植褪黑素后对绒山羊皮肤绒毛生长相关基因表达的影响,进而探讨褪黑素促绒毛生长的机理。方法:以内蒙古白绒山羊为研究对象,选取体况相近的4月龄母羔羊10只,按试验要求随机分为2组,处理组按照2mg/kg BW(体重)的剂量每两个月耳后皮下埋植褪黑素,对照组不埋植,试验时间为1年。实验开始后每月采集绒山羊体侧皮肤和毛样,利用荧光定量PCR技术检测9个绒毛生长相关基因的表达变化。结果:(1)埋植褪黑素后各个基因的平均表达水平都有所上调,其中显著上调FGF5(P<0.0001),而对β-catenin、MSX-2、Wnt10b、BMP2、BMPRIB、NTRK3、Delta1、hairless等无显著影响(P>0.05);(2)埋植褪黑素明显提高了FGF5与hairless的相关性,并且使FGF5与β-catenin、NTRK3的相关性由负相关变成正相关,与Wnt10b的相关性降低,FGF5与其余4个基因的相关性变化不大。结论:结果提示,褪黑素上调绒山羊皮肤FGF的表达,影响FGF5与hairless、β-catenin、NTRK3之间的相关性;山羊中FGF5基因对被毛表型的作用与其他动物不同;褪黑素对复杂组织和单个细胞的作用不同;MAPK信号通路是受褪黑素影响最大的一个信号途径。推测褪黑素可能通过调节绒山羊皮肤中RORα来发挥作用,影响FGF5与hairless、β-catenin、NTRK3和Wnt10b等的相关性,同时抑制MSX2和BMP2的功能,进而促进绒毛的生长。     

Melatonin metabolism,signaling and possible roles in plants

Melatonin is a multifunctional biomolecule found in both animals and plants. In this review, the biosynthesis, levels, signaling, and possible roles of melatonin and its metabolites in plants is summarized. Tryptamine 5-hydroxylase (T5H), which catalyzes the conversion of tryptamine into serotonin, has been proposed as a target to create a melatonin knockout mutant presenting a lesion-mimic phenotype in rice. With a reduced anabolic capacity for melatonin biosynthesis and an increased catabolic capacity for melatonin metabolism, all plants generally maintain low melatonin levels. Some plants, including Arabidopsis and Nicotiana tabacum (tobacco), do not possess tryptophan decarboxylase (TDC), the first committed step enzyme required for melatonin biosynthesis. Major melatonin metabolites include cyclic 3-hydroxymelatonin (3-OHM) and 2-hydroxymelatonin (2-OHM). Other melatonin metabolites such as N¹-acetyl-N²-formyl-5-methoxykynuramine (AFMK), N-acetyl-5-methoxykynuramine (AMK) and 5-methoxytryptamine (5-MT) are also produced when melatonin is applied to Oryza sativa (rice). The signaling pathways of melatonin and its metabolites act via the mitogen-activated protein kinase (MAPK) cascade, possibly with Cand2 acting as a melatonin receptor, although the integrity of Cand2 remains controversial. Melatonin mediates many important functions in growth stimulation and stress tolerance through its potent antioxidant activity and function in activating the MAPK cascade. The concentration distribution of melatonin metabolites appears to be species specific because corresponding enzymes such as M2H, M3H, catalases, indoleamine 2,3-dioxygenase (IDO) and N-acetylserotonin deacetylase (ASDAC) are differentially expressed among plant species and even among different tissues within species. Differential levels of melatonin and its metabolites can lead to differential physiological effects among plants when melatonin is either applied exogenously or overproduced through ectopic overexpression.     

CHANGES IN SEROTONIN LEVEL AND TURNOVER IN DISCRETE HYPOTHALAMIC NUCLEI AFTER PINEALECTOMY AND MELATONIN ADMINISTRATION TO RATS

The influence of the pineal gland on the hypothalamic serotonergic function was examined by studying the effects of long-term pinealectomy (1 month) and melatonin replacement (500 μg/kg; 10 days) on serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) content as well as on the in vivo 5-HT synthesis rate in discrete hypothalamic nuclei. Pinealectomy was followed by a significant decrease of 5-HT content in the anterior hypothalamic nuclei (AHN) and the ventromedial hypothalamic nuclei (VMHN), and also in 5-HIAA content in lateral (LPON) and medial preoptic nuclei (MPON). The 5-HT synthesis rate, estimated from the accumulation of 5-hydroxytryptophan after blockade of the 1-amino acid decarboxylase activity, were also decreased in the AHN and the paraventricular hypothalamic nuclei (PVHN) of pinealectomized rats. In contrast, an enhanced 5-HT synthesis rate and basal 5-HIAA content were found in the suprachiasmatic nuclei (SCN) after pinealectomy. Daily treatment with melatonin for 10 days reversed most of the effects induced by pinealectomy. Thus, melatonin increased the levels of 5-HT in the AHN and VMHN, and slightly increased the 5-HIAA content in preoptic nuclei. In addition, melatonin increased the 5-HT synthesis rate in the AHN and VMHN, but also in the MPON, VMHN and dorsomedial hypothalamic nuclei (DMHN) where pinealectomy had no effect. By contrast, melatonin treatment did not affect SCN 5-HT synthesis rate, although it decreased 5-HIAA levels. The results demonstrate that melatonin is able to stimulate 5-HT metabolism in most of the hypothalamic areas, but inhibits SCN 5-HT function. Some of the effects of melatonin seems to be exerted by modulating the synthesis of the amine, although melatonin likely also interacts with other regulatory processes of 5-HT function (i.e. release/uptake). The well defined presence of melatonin receptors in the rat SCN, and its absence in other hypothalamic structures, suggest that this may be the mechanism mediating the differential response to endogenous melatonin. Moreover, the larger effect of exogenous melatonin in relation to pinealectomy suggests the presence of melatonin unespecific effects possibly owing to supraphysiological doses. The present findings may be relevant for the mode of action of melatonin and its implication in several endocrine and behavioral functions mediated by serotonergic neurons. Copyright © 1996 Elsevier Science Ltd     

Melatonin differentially affects vascular blood flow in humans

Melatonin is synthesized and released into the circulation by the pineal gland in a circadian rhythm. Melatonin has been demonstrated to differentially alter blood flow to assorted vascular beds by the activation of different melatonin receptors in animal models. The purpose of the present study was to determine the effect of melatonin on blood flow to various vascular beds in humans. Renal (Doppler ultrasound), forearm (venous occlusion plethysmography), and cerebral blood flow (transcranial Doppler), arterial blood pressure, and heart rate were measured in 10 healthy subjects (29±1 yr; 5 men and 5 women) in the supine position for 3 min. The protocol began 45 min after the ingestion of either melatonin (3 mg) or placebo (sucrose). Subjects returned at least 2 days later at the same time of day to repeat the trial after ingesting the other substance. Melatonin did not alter heart rate and mean arterial pressure. Renal blood flow velocity (RBFV) and renal vascular conductance (RVC) were lower during the melatonin trial compared with placebo (RBFV, 40.5±2.9 vs. 45.4±1.5 cm/s; and RVC, 0.47±0.02 vs. 0.54±0.01 cm·s(-1)·mmHg(-1), respectively). In contrast, forearm blood flow (FBF) and forearm vascular conductance (FVC) were greater with melatonin compared with placebo (FBF, 2.4±0.2 vs. 1.9±0.1 ml·100 ml(-1)·min(-1); and FVC, 0.029±0.003 vs. 0.023±0.002 arbitrary units, respectively). Melatonin did not alter cerebral blood flow measurements compared with placebo. Additionally, phentolamine (5-mg bolus) after melatonin reversed the decrease in RVC, suggesting that melatonin increases sympathetic outflow to the kidney to mediate renal vasoconstriction. In summary, exogenous melatonin differentially alters vascular blood flow in humans. These data suggest the complex nature of melatonin on the vasculature in humans.     

Melatonin Protects against Apoptosis-Inducing Factor (AIF)-Dependent Cell Death during Acetaminophen-Induced Acute Liver Failure

Acetaminophen (APAP) overdose is the most frequent cause of acute liver failure and is primarily caused by cytochrome P450 (CYP) 2E1-driven conversion of APAP into hepatotoxic metabolites. Several reports showed that melatonin attenuated APAP-induced acute liver failure. Nevertheless, the exact mechanism remains obscure. In the present study, we investigated the effects of melatonin on apoptosis-inducing factor (AIF)-dependent cell death in APAP-induced acute liver failure. Mice were intraperitoneally (i.p.) injected with different doses of melatonin (1.25, 5, 20 mg/kg) 30 min before APAP (300 mg/kg, i.p.). As expected, melatonin significantly alleviated APAP-induced cell death, as determined by TdT-mediated dUTP-biotin nick end labeling (TUNEL) assay. Further analysis showed that melatonin significantly attenuated APAP-induced activation of the serine/threonine kinase receptor interacting protein 1 (RIP1). In addition, melatonin inhibited APAP-induced hepatic c-Jun N-terminal kinase (JNK) phosphorylation and mitochondrial Bax translocation. Correspondingly, melatonin inhibited APAP-induced translocation of AIF from mitochondria to nuclei. Interestingly, no changes were induced by melatonin on hepatic CYP2E1 expression. In addition, melatonin had little effect on APAP-induced hepatic glutathione (GSH) depletion. In conclusion, melatonin protects against AIF-dependent cell death during APAP-induced acute liver failure through its direct inhibition of hepatic RIP1 and subsequent JNK phosphorylation and mitochondrial Bax translocation.     

Circadian Regulation of Melatonin in the Retina of Xenopus laevis: Limitation by Serotonin Availability

Treatments expected to increase retinal serotonin levels were found to stimulate melatonin production by cultured eyecups from Xenopus laevis. The monoamine oxidase inhibitor pargyline (100 microM) caused a sixfold increase in melatonin release, and the serotonin precursor 5-hydroxy-L-tryptophan (100 microM) caused a 70-fold increase. Both acted synergistically with eserine, an inhibitor of melatonin deacetylation in the retina. The effect of 5-hydroxytryptophan was dose dependent, with effects increasing from 1 to 100 microM. Increasing the tryptophan level in the culture medium had no effect on melatonin release. These results indicate that the rate-limiting step in retinal melatonin synthesis is 5-hydroxylation of tryptophan. Melatonin released from individual eyecups in superfusion culture in constant darkness with and without added 5-hydroxy-L-tryptophan was monitored over a 5-day period. Control eyecups released low levels of melatonin, with circadian rhythmicity persisting for 1-3 days. With 5-hydroxy-L-tryptophan added, melatonin levels were elevated 10-20-fold at all times, and rhythmicity was apparent for as long as five cycles. This provides a model system for studies of the circadian clock in the eye.     

Exogenous melatonin delays gastric emptying rate in rats: role of CCK2 and 5-HT3 receptors.

Pineal hormone melatonin is proposed as a potential treatment for severe sleep disturbances, and various gastrointestinal disorders. It was shown that melatonin increases intestinal motility and influences the activity of myoelectric complexes of the gut. The aim of the study was to evaluate the mechanisms of the effect of exogenous melatonin on gastric emptying rate. Male Sprague-Dawley rats were fitted with gastric cannulas under anesthesia. The rate of gastric emptying of saline was determined after instillation into the gastric fistula, from the volume and phenol red concentrations recovered after 5 min. Melatonin injected intraperitoneally (ip; 0.001-100 mg/kg) delayed gastric emptying rate of saline at 3 and 10 mg/kg doses. When administered ip 15 min before melatonin (10 mg/kg) injections, CCK2 (L-365,260, 1 mg/kg) or 5-HT3 receptor (ramosetrone, 50 microg/kg) blockers abolished melatonin-induced delay in gastric emptying rate, while the blockade of sympathetic ganglia (bretylium tosylate, 15 mg/kg) significantly reduced the delay in gastric emptying rate. CCK1 receptor blocker (L-364,718, 1 mg/kg) had no significant effect on the delaying action of melatonin. Our results indicate that pharmacological doses of melatonin delay gastric emptying via mechanisms that involve CCK2 and 5-HT3 receptors. Moreover, it appears that exogenous melatonin inhibits gastric motility in part by activating sympathetic neurons.     

A novel melatonin antagonist, N-(2,4-dinitrophenyl)-5-methoxytryptamine neutralizes some effects of melatonin in the female Syrian hamster

In this present study we evaluated the ability of a recently synthesized melatonin antagonist, N-(2,4-dinitrophenyl)-5-methoxytryptamine (ML-23), to antagonize the effects of afternoon injections of melatonin on the reproductive and thyroid axes in the female Syrian hamster. Thirty-six animals were divided into four groups and treated daily for 13 weeks with an afternoon injection of melatonin (25 micrograms/injection) or saline diluent. ML-23 was given via the drinking water to both melatonin- and saline-treated groups. The experiment was continued until 78% of melatonin-treated animals exhibited acyclicity. The results show that ML-23 partially reversed the effects of melatonin on pituitary follicle-stimulating hormone concentrations but was without effect on the decreased pituitary and plasma prolactin concentrations induced by melatonin treatment. Furthermore, ML-23 antagonized the effects of melatonin on plasma thyroxine levels and significantly increased plasma triiodothyronine concentrations and the free triiodothyronine index when used in combination with melatonin. The decrease in ovarian weight and plasma estradiol, but not progesterone, obtained with melatonin treatment also was reversed by ML-23. Our data suggest that ML-23 prevents the effects of melatonin treatment on ovarian weight, pituitary follicle-stimulating hormone levels, plasma estradiol, and thyroxine concentrations in the female Syrian hamster. Since ML-23 did not prevent the effects of melatonin on pituitary weight, plasma luteinizing hormone and prolactin, and pituitary prolactin concentrations, the actions of ML-23 may involve only peripheral sites of action of melatonin. Alternatively, the dose of ML-23 may not have been optimal to prevent all of the central effects of the indoleamine.     

Melatonin in human cerebrospinal fluid in daytime; its origin and variation with age

Melatonin was measured in human daytime CSF samples originating in the lumbar sac and basal cisterns. No gradient was found for melatonin, although significantly more 5-hydroxyindoleacetic acid was found in the cisternal samples. This suggests that, in the daytime, melatonin is not released directly from the pineal into the third ventricle, as that would produce a CSF gradient. Presumably daytime CSF melatonin is derived from blood and the original sources may include the retina, intestine and pineal.Daytime human CSF melatonin showed a significant negative correlation with age over the age range 8 to 70 years.