Physiological and metabolic functions of melatonin

Melatonin is a lipophilic hormone, mainly produced and secreted at night by the pineal gland. Melatonin synthesis is under the control of postganglionic sympathetic fibers that innervates the pineal gland. Melatonin acts via high affinity G protein-coupled membrane receptors. To date, three different receptor subtypes have been identified in mammals: MT1 (Mel 1a) and MT2 (Mel 1b) and a putative binding site called MT3. The chronobiotic properties of the hormone for resynchronization of sleep and circadian rhythms disturbances has been demonstrated both in animal models or in clinical trials. Several other physiological effects of melatonin in different peripheral tissues have been described in the past years. In this way, it has been demonstrated that the hormone is involved in the regulation of seasonal reproduction, body weight and energy balance. This contribution has been focused to review some of the physiological functions of melatonin as well as the role of the hormone in the regulation of energy balance and its possible involvement in the development of obesity.     

Diurnal expressions of four subtypes of melatonin receptor genes in the optic tectum and retina of goldfish

Four subtypes of melatonin receptor genes (Mel(1a) 1.4, Mel(1a) 1.7, Mel(1b), and Mel(1c)) are considered to be expressed to mediate various physiological functions of melatonin in goldfish (Carassius auratus). To examine their tissue distribution and diurnal changes in expression levels, we cloned partial gene fragments for these melatonin receptor subtypes, and established specific RT-PCR and quantitative real-time PCR systems. Mel(1a) 1.4 and Mel(1b) were predominantly expressed in various neuronal and peripheral tissues, while Mel(1a) 1.7 and Mel(1c) were expressed in the restricted tissues. All subtype genes were expressed in the optic tectum, diencephalon, mesencephalon, vagal lobe, retina and spleen. The real-time PCR analyses showed that significant differences among time were observed for Mel(1a) 1.4 in the optic tectum and for Mel(1a) 1.7 and Mel(1b) in the retina. In the retina, the levels of Mel(1a) 1.7 and Mel(1b) mRNAs showed diurnal changes with one peak at ZT24. The present results show differential distribution of four subtypes of melatonin receptor mRNAs in the neuronal and peripheral tissues. However, the expressions of all subtype genes in the retinorecipient brain regions and retina reinforce the role of the melatonin receptor in processing visual information. Furthermore, the present study demonstrates diurnal expressions of the major subtype genes, i.e. Mel(1a) 1.4 in the optic tectum and Mel(1a) 1.7 in the retina.     

Expression of the melatonin receptor Mel(1c) in neural tissues of the reef fish Siganus guttatus

The golden rabbitfish, Siganus guttatus, is a reef fish exhibiting a restricted lunar-related rhythm in behavior and reproduction. Here, to understand the circadian rhythm of this lunar-synchronized spawner, a melatonin receptor subtype-Mel(1c)-was cloned. The full-length Mel(1c) melatonin receptor cDNA comprised 1747 bp with a single open reading frame (1062 bp) that encodes a 353-amino acid protein, which included 7 presumed transmembrane domains. Real-time PCR revealed high Mel(1c) mRNA expression in the retina and brain but not in the peripheral tissues. When the fish were reared under light/dark (LD 12:12) conditions, Mel(1c) mRNA in the retina and brain was expressed with daily variations and increased during nighttime. Similar variations were noted under constant conditions, suggesting that Mel(1c) mRNA expression is regulated by the circadian clock system. Daily variations of Mel(1c) mRNA expression with a peak at zeitgeber time (ZT) 12 were observed in the cultured pineal gland under LD 12:12. Exposure of the cultured pineal gland to light at ZT17 resulted in a decrease in Mel(1c) mRNA expression. When light was obstructed at ZT5, the opposite effect was obtained. These results suggest that light exerts certain effects on Mel(1c) mRNA expression directly or indirectly through melatonin actions.     

Pineal and cortical melatonin receptors MT1 and MT2 are decreased in Alzheimer's disease

The pineal hormone melatonin is involved in physiological transduction of temporal information from the light dark cycle to circadian and seasonal behavioural rhythms, as well as possessing neuroprotective properties. Melatonin and its receptors MT1 and MT2, which belong to the family of G protein-coupled receptors, are impaired in Alzheimer's disease (AD) with severe consequences to neuropathology and clinical symptoms. The present data provides the first immunohistochemical evidence for the cellular localization of the both melatonin receptors in the human pineal gland and occipital cortex, and demonstrates their alterations in AD. We localized MT1 and MT2 in the pineal gland and occipital cortex of 7 elderly controls and 11 AD patients using immunohistochemistry with peroxidase-staining. In the pineal gland both MT1 and MT2 were localized to pinealocytes, whereas in the cortex both receptors were expressed in some pyramidal and non-pyramidal cells. In patients with AD, parallel to degenerative tissue changes, there was an overall decrease in the intensity of receptors in both brain regions. In line with our previous findings, melatonin receptor expression in AD is impaired in two additional brain areas, and may contribute to disease pathology.     

Melatonin

Melatonin, originally discovered as a hormone of the pineal gland, is produced by bacteria, protozoa, plants, fungi, invertebrates, and various extrapineal sites of vertebrates, including gut, skin, Harderian gland, and leukocytes. Biosynthetic pathways seem to be identical. Actions are pleiotropic, mediated by membrane and nuclear receptors, other binding sites or chemical interactions. Melatonin regulates the sleep/wake cycle, other circadian and seasonal rhythms, and acts as an immunostimulator and cytoprotective agent. Circulating melatonin is mostly 6-hydroxylated by hepatic P450 monooxygenases and excreted as 6-sulfatoxymelatonin. Pyrrole-ring cleavage is of higher importance in other tissues, especially the brain. The product, N1-acetyl-N2-formyl-5-methoxykynuramine, is formed by enzymatic, pseudoenzymatic, photocatalytic, and numerous free-radical reactions. Additional metabolites result from hydroxylation and nitrosation. The secondary metabolite, N1-acetyl-5-methoxykynuramine, supports mitochondrial function and downregulates cyclooxygenase 2. Antioxidative protection, safeguarding of mitochondrial electron flux, and in particular, neuroprotection, have been demonstrated in many experimental systems. Findings are encouraging to use melatonin as a sleep promoter and in preventing progression of neurodegenerative diseases.     

Melatonin and biological rhythms

Circadian and seasonal rhythms are a fundamental feature of all living organisms. The functional mechanism involved is built around internal biological clock(s) and the hormone melatonin (Mel) is one of its critical components. Although numerous other sources have been identified (retina, harderian gland, gut), in vertebrates Mel is primarily produced by the pineal gland during the dark period of the light-dark cycle. This rhythmic Mel is generated directly by circadian clock(s). The Mel rhythm is thus an important efferent hormonal signal from the clock. The periodic secretion of Mel might thus be used as a circadian mediator of a system that can 'read' the message.The duration of the nocturnal Mel production is directly proportional to the length of the dark period. It is through these changes in duration that the brain integrates the photoperiodic information. In essence, the Mel rhythm appears to be an endocrine code of the environmental light-dark cycle conveying photic information that is used by organisms for both circadian and seasonal temporal organization. The major question arising from this effect of Mel concerns it precise mechanism of action. From the data reported in the present minireview, it appears that the photoneuroendocrine mechanism is not fundamentally different in vertebrates at least as far as the role of Mel is concerned.     

Reproductive phase dependent daily variation in melatonin receptors (Mel(1a) and Mel(1b)), androgen receptor (AR) and lung associated immunity of Perdicula asiatica

Our knowledge about the involvement of melatonin in the regulation of lung associated immune system (LAIS) is still poor though the melatonin receptor types (Mel(1a) and Mel(1b)) have been localized in lungs of some wild birds. We thought to explore the correlation between daily variation (within a 24h time scale) in peripheral melatonin and testosterone along with expression of melatonin receptors (Mel(1a) and Mel(1b)) and androgen receptor (AR) in lungs during reproductively active and inactive phases. Receptor expression of Mel(1b) was more prominent than Mel(1a) at all the time points during both the reproductive phases. The expression of AR was inversely related to both the melatonin and its receptor expression at the 24h time scale during both the reproductive phases. Results also reflected a parallel relationship of melatonin, melatonin receptors and all the immune parameters (total leukocyte count, lymphocyte count, % stimulation ratio) suggesting that peripheral melatonin might be responsible for daily periodicity of LAIS. The presence of androgen receptors in lung led us to propose that gonadal steroid does influence the LAIS. Therefore melatonin along with testosterone might be acting as a temporal synchronizer for daily rhythms in lung associated immunity in Perdicula asiatica during different reproductive phases.     

Bidirectional communication between the pineal gland and the immune system

The pineal gland is a vertebrate neuroendocrine organ converting environmental photoperiodic information into a biochemical message (melatonin) that subsequently regulates the activity of numerous target tissues after its release into the bloodstream. A phylogenetically conserved feature is increased melatonin synthesis during darkness, even though there are differences between mammals and birds in the regulation of rhythmic pinealocyte function. Membrane-bound melatonin receptors are found in many peripheral organs, including lymphoid glands and immune cells, from which melatonin receptor genes have been characterized and cloned. The expression of melatonin receptor genes within the immune system shows species and organ specificity. The pineal gland, via the rhythmical synthesis and release of melatonin, influences the development and function of the immune system, although the postreceptor signal transduction system is poorly understood. Circulating messages produced by activated immune cells are reciprocally perceived by the pineal gland and provide feedback for the regulation of pineal function. The pineal gland and the immune system are, therefore, reciprocally linked by bidirectional communication.     

Differential daily rhythms of melatonin in the pineal gland and gut of goldfish Carassius auratus in response to light

The objectives of this study were to test the effects of light on melatonin rhythms in the pineal gland and gut of goldfish Carassius auratus and to investigate whether melatonin function differed in these two tissues, which are photosensitive and non-photosensitive respectively. Rhythms were evaluated by measuring arylalkylamine N-acetyltransferase (AANAT2) and melatonin receptor 1 (MT-R1) mRNA expression and melatonin concentration in the pineal gland, gut (in vivo), and cell cultures of the two tissues (in vitro). Compared to control, pineal gland melatonin secretion was higher at night, whereas the 24-h dark and ophthalmectomy groups maintained higher AANAT2 and MT-R1 mRNA expression during the day. Melatonin levels and AANAT2 and MT-R1 mRNA expression in the gut were also the highest at night, but the 24-h light, dark, and ophthalmectomy groups did not significantly differ from control. Furthermore, we measured AANAT2 and MT-R1 mRNA expression in high temperature water (30 °C) to investigate differences in the antioxidant capacity of pineal gland vs. gut melatonin. Melatonin and H₂O₂ levels, as well as AANAT2 and MT-R1 mRNA expression, were all higher in the two tissues under thermal stress, compared with their levels at 22 °C. Taken together, our results suggest that light has no effect on melatonin patterns in the gut, which appears to exhibit its own circadian rhythm, but both gut and pineal gland melatonin exhibit similar antioxidant function.     

Melatonin and alcohol-related disorders

ABSTRACT

This review concerns the current knowledge of melatonin and alcohol-related disorders. Chronobiological effects of ethanol are related to melatonin suppression and in relation to inflammation, stress, free radical scavenging, autophagy and cancer risk. It is postulated that both alcohol- and inflammation-induced production of reactive oxygen species (ROS) alters cell membrane properties leading to tissue dysfunction and, subsequent further ROS production. Lysosomal enzymes are often used to assess the relationships between intensified inflammation states caused by alcohol abuse and oxidative stress as well as level of tissue damage estimated by the increased release of cellular enzymes into the extracellular space. Studies have established a link between alcoholism and desynchronosis (circadian disruption). Desynchronosis results from the disorganization of the body’s circadian time structure and is an aspect of the pathology of chronic alcohol intoxication. The inflammatory conditions and the activity of lysosomal enzymes in acute alcohol poisoning or chronic alcohol-dependent diseases are in most cases interrelated. Inflammation can increase the activity of lysosomal enzymes, which can be regarded as a marker of lysosomal dysfunction and abnormal cellular integrity. Studies show alcohol toxicity is modulated by the melatonin (Mel) circadian rhythm. This hormone, produced by the pineal gland, is the main regulator of 24 h (sleep-wake cycle) and seasonal biorhythms. Mel exhibits antioxidant properties and may be useful in the prevention of oxidative stress reactions known to be responsible for alcohol-related diseases. Naturally produced Mel and exogenous sources in food can act in free radical reactions and activate the endogenous defense system. Mel plays an important role in the normalization of the post-stress state by its influence on neurotransmitter systems and the synchronization of circadian rhythms. Acting simultaneously on the neuroendocrine and immune systems, Mel optimizes homeostasis and provides protection against stress.

Abbreviations: ROS, reactive oxygen species; Mel, melatonin; SRV, resveratrol; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; ANT, arylalkylamine-N-acetyltransferase; EC cells, gastrointestinal enterochromaffin cells; MT1, melatonin high-affinity nanomolecular receptor site; MT2, melatonin low-affinity nanomolecular receptor site; ROR/RZR, orphan nuclear retinoid receptors; SOD, superoxide dismutase; CAT, catalase; GPx, glutathione peroxidase; GR, glutathione reductase; GSH, reduced form of glutathione; GSSG, oxidized form of glutathione; TAC, total antioxidant capacity; ONOO?^–, peroxynitrite radical; NCAM, neural cell adhesion molecules; LPO, lipid peroxidation; α-KG, α-ketoglutarate, HIF-1α, Hypoxia-inducible factor 1-α, IL-2, interleukin-2; HPA axis, hypothalamic-pituitary-adrenal axis; Tph1, tryptophan hydroxylase 1; AA-NAT, arylalkylamine-N-acetyltransferase; AS-MT, acetylserotonin O-methyltransferase; NAG, N-acetyl-beta-D-glucosaminidase; HBA1c glycated hemoglobin; LPS, lipopolysaccharide; AAP, alanyl-aminopeptidase; β-GR, β-glucuronidase; β-GD, β-galactosidase; LAP, leucine aminopeptidase.     

Distribution of 125I-labeled rat growth hormone in regional brain areas and peripheral tissue of the rat.

The uptake of intraperitoneally injected 125I-labeled rat growth hormone into brain and peripheral tissues was measured in normal and hypophysectomized adult rats. A significant level of radioactivity was observed in the seven brain regions examined -- the telencephalon, diencephalon, midbrain, pons-medulla, cerebellum, pineal and pituitary glands. The pineal and pituitary glands, which are outside the blood-brain barrier, contained three to four times the concentration of radioactivity of the other brain regions. Compared to brain, the level of radioactivity was much higher in peripheral tissues (the diaphragm, kidney, serum and liver). For example, the serum contained ten times the level of radioactivity of most brain regions. For a given tissue, however, the normal and hypophysectomized rats showed a comparable amount of 125I-growth hormone. Trichloroacetic acid precipitates from each tissue sample showed that peripheral tissues had a higher proportion of radioactivity (35-48% of total tissue radioactivity) than the brain samples (13-26%). The data support the view that growth hormone, or a metabolite can enter the central nervous system and may directly affect on-going metabolic processes.     

Extrapineal Melatonin: Location and Role within Diffuse Neuroendocrine System

During the last decade, much attention has centred on melatonin, one of the hormones of the diffuse neuroendocrine system. For many years it was considered to be only a hormone of the pineal gland. As soon as highly sensitive antibodies to indolealkylamines became available, melatonin was identified not only in pineal gland, but also in extrapineal tissues. These included the retina, Harderian gland, gut mucosa, cerebellum, airway epithelium, liver, kidney, adrenals, thymus, thyroid, pancreas, ovary, carotid body, placenta and endometrium. It has also been localized in non-neuroendocrine cells such as mast cells, natural killer cells, eosinophilic leukocytes, platelets and endothelial cells. This list of cells indicates that melatonin has a unique position among the hormones of the diffuse neuroendocrine system. It is found in practically all organ systems. Functionally, melatonin-producing cells are part and parcel of the diffuse neuroendocrine system as a universal system of response, control and organism protection. Taking into account the large number of such melatonin-producing cells in many organs, the wide spectrum of biological activities of melatonin and especially its main property as a universal regulator of biological rhythms, it is now possible to consider extrapineal melatonin as a key paracrine signal molecule for the local co-ordination of intercellular relationships.     

Melatonin Modulates Thyroid Hormones and Splenocytes Proliferation Through Mediation of Its MT1 and MT2 Receptors in Hyperthyroid Mice

Hyperthyroidism is characterized by an increased metabolic rate with the alteration of immune activity. The pineal hormone melatonin regulates various physiological activities through sensitization of MT1 and MT2 membrane receptors in mammals. In the present study we have evaluated the involvement of MT1 and MT2 receptors in melatonin mediated modulation of thyroid hormones and splenocyte proliferation in experimentally induced hyperthyroidic mice. The l-thyroxine treatment induced the hyperthyroidism in mice evidenced with hypersecretion of T3 and T4 hormones from thyroid gland. Hyperthyroidic state increased the TSH hormone level which might be inducing hyper activity in thyroid gland. Exogenous melatonin suppressed the thyroid hormones level as well as TSH level in circulation. The l-thyroxine treatment increased the splenocyte proliferation and showed synergic effects along with melatonin. l-thyroxine treated mice alone or along with melatonin treatment showed differential expression pattern of MT1 and MT2 receptors protein in thyroid and spleen tissues. It seems that melatonin regulates thyroid hormones and splenocyte proliferation through activation of MT1 and MT2 receptors.     

Regional concentrations of melatonin in the rat brain in the light and dark period

The levels of melatonin in five brain regions, whole brain, pineal and serum samples were studied in rats adapted under a photoperiod of 12h light and 12h dark. It was found that the melatonin levels for all the tissues obtained in the dark period were significantly higher than those obtained in the light period. Regional study of melatonin levels in the brain in the light and dark period demonstrated a high level in the hypothalamus, intermediate levels in the mid-brain, cerebellum and pons-medulla and low level in the telencephalon. Our findings indicate that melatonin in the brain is unevenly distributed and that there are diurnal rhythms of melatonin in all the five brain regions studied.     

Melatonin, melatonin receptors and melanophores: a moving story

Melatonin (5-methoxy N-acetyltryptamine) is a hormone synthesized and released from the pineal gland at night, which acts on specific high affinity G-protein coupled receptors to regulate various aspects of physiology and behaviour, including circadian and seasonal responses, and some retinal, cardiovascular and immunological functions. In amphibians, such as Xenopus laevis, another role of melatonin is in the control of skin coloration through an action on melanin-containing pigment granules (melanosomes) in melanophores. In these cells, very low concentrations of melatonin activate the Mel(1c) receptor subtype triggering movement of granules toward the cell centre thus lightening skin colour. Mel(1c) receptor activation reduces intracellular cAMP via a pertussis toxin-sensitive inhibitory G-protein (Gi), but how this and other intracellular signals regulate pigment movement is not yet fully understood. However, melanophores have proven an excellent model for the study of the molecular mechanisms which coordinate intracellular transport. Melanosome transport is reversible and involves both actin- (myosin V) and microtubule-dependent (kinesin II and dynein) motors. Melanosomes retain both kinesin and dynein during anterograde and retrograde transport, but the myosin V motor seems to be recruited to melanosomes during dispersion, where it assists kinesin II in dominating dynein thus driving net dispersion. Recent work suggests an important role for dynactin in coordinating the activity of the opposing microtubule motors. The melanophore pigment aggregation response has also played a vital role in the ongoing effort to devise specific melatonin receptor antagonists. Much of what has been learnt about the parts of the melatonin molecule required for receptor binding and activation has come from detailed structure-activity data using novel melatonin ligands. Work aiming to devise ligands specific for the distinct melatonin receptor subtypes stands poised to deliver selective agonists and antagonists which will be valuable tools in understanding the role of this enigmatic hormone in health and disease.     

Molecular determinants for the differential coupling of Galpha(16) to the melatonin MT1, MT2 and Xenopus Mel1c receptors

The pineal neurohormone melatonin modulates a variety of physiological processes through different receptors. It has recently been reported that the cloned melatonin receptors (MT1, MT2 and Mel1c) exhibit differential abilities to stimulate phospholipase C (PLC) via G(16). Here we examined the molecular basis of such differences in melatonin receptor signaling. Coexpression of MT1 or MT2 with the alpha subunit of G(16) (Galpha(16) ) allowed COS-7 cells to accumulate inositol phosphates in response to 2-iodomelatonin. In contrast, Mel1c did not activate Galpha(16) even though its expression was demonstrated by radioligand binding and agonist-induced inhibition of adenylyl cyclase. As Mel1c possesses an exceptionally large C-terminal tail, we further asked if this structural feature prevented productive coupling to Galpha(16). Eleven chimeric melatonin or mutant receptors were constructed by swapping all or part of the C-terminal tail between MT1, MT2 and Mel1c. All chimeras were fully capable of binding 2-[(125) I]iodomelatonin and inhibiting adenylyl cyclase. Chimeras containing the full-length Mel1c tail were incapable of activating Galpha(16), while those that contained the complete C-terminal region of either MT1 or MT2 stimulated PLC. Incorporation of the extra portion of the C-terminal tail of Mel1c to either MT1 or MT2 completely abolished the chimeras' ability to stimulate PLC via Galpha(16). In contrast, truncation of the C-terminal tail of Mel1c allowed interaction with Galpha(16). Our results suggest that Galpha(16) can discern structural differences amid the three melatonin receptors and provide evidence for functional distinction of Mel1c from MT1 and MT2 receptors.     

Correlations of Melatonin Content in Pineal Gland, Blood, and Brain of Some Birds and Mammals

Diurnal, rhythmic variation of melatonin content in the pinealgland, blood serum, and brain have been found in chickens, withgreater amounts present in all tissues during nighttime thanduring daytime. Similar daily rhythms appear to occur in thepineal gland and serum of rats and in the serum and urine ofhumans. It is proposed that these correlated fluctuations inmelatonin levels are causally related, elevated pineal contentresulting in increased melatonin content of the blood and increasedaccumulation by the brain. The brain of chickens, especiallythe hypothalamus, appears to accumulate melatonin and is probablya primary site of action of melatonin.