Distribution Of Melatonin Receptors Research Articles

The roles of angiotensin-converting enzyme 2 inhibitor, melatonin and its agonist on angiotensin II reactivity in intact and denuded rat aortic rings

Background The pineal product melatonin (MEL) modulates blood vessels through G protein-coupled receptors (GPCRs) called melatonin type 1 receptor (MT1R) and melatonin type 2 receptor (MT2R), in that order. The renin-angiotensin system (RAS), which breaks down angiotensin II (Ang II) to create Ang 1–7, is thought to be mostly controlled by angiotensin-converting enzyme-2 (ACE2). Aim The current work examines the involvement of ACE2 inhibitor, MEL, and ramelteon (RAM) in the vascular response to Ang II activities in the endothelial denuded (E-) and intact (E+) rat isolated thoracic aortic rings. Method The isometric tension was measured to evaluate the vascular Ang II contractility using dose response curve (DRC). Results MEL and RAM caused a rightward shift of Ang II in endothelium E + and endothelium E- aorta. Conclusion According to the current study, the distribution of MEL receptors and the endothelium’s condition are related to the vasomodulatory effect of MEL and ACE2 on Ang II attenuation. These physiological interactions can control vascular tone and increase Ang II reactivity denude endothelial layaer.

The effect of melatonin on sheep endometrial epithelial cell apoptosis through the receptor and non-receptor pathways

Melatonin potentially regulates the female animal reproductive function, but its regulatory mechanism in the apoptosis of sheep endometrial epithelial cells (SEECs) remains to be elucidated. In the present study, immunofluorescence staining, western blotting, and quantitative real-time polymerase chain reaction were performed to detect the distribution of melatonin receptors (MT1 and MT2) in the uterus of sheep and the effect of melatonin via the receptor and non-receptor pathways on the apoptosis of SEECs in vitro. The results showed that melatonin inhibits the apoptosis of SEECs to varying degrees to regulate the expression of estrogen receptors (ERs) and progesterone receptors (PGR) via its interaction with MT1 and MT2. In addition, the ER antagonist partially relieved the inhibitory effect of melatonin on the apoptosis of SEECs, while the PGR antagonist did not. Thus, melatonin mediates endometrial epithelial apoptosis through the MT receptors and also by regulating estrogen function. This study provides evidence of the regulatory mechanism of melatonin on the physiological function of the sheep uterus.

Melatonin membrane receptors MT1 and MT2 are expressed in ram spermatozoa from non-seasonal breeds.

In mammals, many melatonin biological functions are mediated through its interaction with the membrane receptors MT1 and MT2. We have previously reported their presence in ram spermatozoa from males located in temperate climates, but there is no information on their presence in spermatozoa from rams in areas with an equatorial photoperiod (12L:12D). Thus, we have investigated the existence and cellular distribution of melatonin receptors in spermatozoa from three sheep breeds in Colombia (Colombian Creole, Hampshire, and Romney Marsh) during dry and rainy seasons, using indirect immunofluorescence and western blot. Our results indicated the presence of melatonin receptors in spermatozoa from these rams, and that their distribution differs from that previously found in spermatozoa from rams in temperate climates. Moreover, two new immunotypes of MT2 were identified: type N, with staining only in the neck, and type E with a band of immunofluorescence in the upper part of the post-acrosome and the apical edge. Likewise, differences between breeds and climate seasons were detected for both receptors. However, densitometry analysis of western blot bands only revealed differences between seasons in the Creole rams for MT1 and the Romney Marsh rams for MT2, whereas differences between breeds were only detected for MT2. It could be inferred that melatonin receptors in rams subjected to an equatorial photoperiod might be more closely related to sperm quality than seasonal control. Therefore, the presence of these receptors suggests that melatonin could be a useful tool to increase the fertility of rams located in tropical or equatorial climates.

Melatonin receptors: distribution in mammalian brain and their respective putative functions.

Melatonin, through its different receptors, has pleiotropic functions in mammalian brain. Melatonin is secreted mainly by the pineal gland and exerts its effects via receptor-mediated and non-receptor-mediated actions. With recent advancement in neuroanatomical mapping, we may now understand better the localizations of the two G protein-coupled melatonin receptors MT1 and MT2. The abundance of these melatonin receptors in respective brain regions suggests that receptor-mediated actions of melatonin might play crucial roles in the functions of central nervous system. Hence, this review aims to summarize the distribution of melatonin receptors in the brain and to discuss the putative functions of melatonin in the retina, cerebral cortex, reticular thalamic nucleus, habenula, hypothalamus, pituitary gland, periaqueductal gray, dorsal raphe nucleus, midbrain and cerebellum. Studies on melatonin receptors in the brain are important because cumulative evidence has pointed out that melatonin receptors not only play important physiological roles in sleep, anxiety, pain and circadian rhythm, but might also be involved in the pathogenesis of a number of neurodegenerative diseases including Alzheimer's disease, Parkinson's disease and Huntington's disease.

Melatonin in sperm biology: breaking paradigms.

Melatonin is a ubiquitous molecule, present in a wide range of organisms, and involved in multiple functions. Melatonin relays the information about the photoperiod to the tissues that express melatonin-binding sites in both central and peripheral nervous systems. This hormone has a complex mechanism of action. It can cross the cell plasma membrane and exert its actions in all cells of the body. Certain melatonin actions are mediated by receptors that belong to the superfamily of G-protein-coupled receptors (GPCRs), the MT1 and MT2 membrane. Melatonin can also bind to calmodulin as well as to nuclear receptors of the retinoic acid receptor family, RORα1, RORα2 and RZRβ. The purpose of this review is to report on recent developments in the physiological role of melatonin and its receptors. Specific issues concerning the biological function of melatonin in mammalian seasonal reproduction and spermatozoa are considered. The significance of the continuous presence of melatonin in seminal plasma with a fairly constant concentration is also discussed.

Anatomical and histological profile of bronchus-associated lymphoid tissue and localization of melatonin receptor types (Mel1a and Mel1b) in the lung-associated immune system of a tropical bird, Perdicula asiatica

The histological distribution of the lung-associated immune system (LAIS) and the expressional pattern of melatonin receptors are still unknown in birds. The aim of the present study was to determine the localization of the bronchus-associated lymphoid tissue (BALT nodule) in a tropical bird, the Indian jungle bush quail, Perdicula asiatica. We also demonstrate the expression of melatonin receptor types (Mel1a and Mel1b) in order to propose an immunomodulatory role of melatonin in LAIS. Localization of melatonin receptors in the lung of the Indian jungle bush quail, P. asiatica was supported immunohistochemically and by Western blot analysis using specific antibodies for those receptors. Immunolocalization for Mel1b receptor was noted in the bronchial region of the lungs, in finger-like projections of mucosal foldings, in lymphocytes in the BALT nodule as well as in free form. In contrast, immunolocalization for Mel1a receptor was noted in various areas of the lung instead of in the bronchial region. Western blot analysis showed a single band at 37 and 39kDa for Mel1a and Mel1b receptors, respectively, with the latter showing higher expression. The results demonstrate a well-developed LAIS and region-specific distribution of melatonin receptors in the lung and provide evidence for a possible functional role for melatonin in the LAIS of birds.

Melatonin receptor density in Area X of European starlings is correlated with reproductive state and is unaffected by plasma melatonin concentration

Several of the song control nuclei of songbirds, including HVc (higher vocal center) and Area X, contain melatonin receptor (MelR). In laboratory-housed male starlings, the densities of MelR in Area X change markedly according to reproductive state. MelR are down-regulated when starlings are photostimulated (in full breeding condition) and are subsequently up-regulated when starlings become photorefractory (reproductively quiescent). However, seasonal regulation of MelR densities in Area X has only been investigated during the light phase of the light:dark cycle. Variation in MelR densities are physiologically relevant only if they also occur during the dark phase, when melatonin is present in the circulation. Brains from male starlings that were in different reproductive states but exposed to the same 18L:6D photoperiod were collected during either the mid-point of the light phase or the dark phase. Melatonin receptor distribution was assessed in vitro by 125Iodomelatonin (IMEL) receptor autoradiography. All photostimulated birds exhibited down-regulation of MelR in Area X, and all photorefractory birds exhibited high MelR density in Area X, regardless of time of sampling or plasma melatonin concentration. Thus, within each reproductive state, MelR density in Area X did not differ over the course of a circadian cycle. The functional significance of seasonal regulation of MelR in this song control nucleus remains unclear, but it is likely to involve a release of cellular inhibition by melatonin during photostimulation, with possible consequences for song learning, memory consolidation or regulation of the context of song production.

Therapeutic perspectives for melatonin agonists and antagonists.

Melatonin is a neurohormone synthesized in the pineal gland during the dark period in all species, including humans. The diversity and differences in melatonin receptor distribution in the brain and extracerebral organs suggest multiple functional roles for melatonin. Administration of melatonin agonists reduces neophobia and treatment with a melatonin antagonist during the dark period reverses the anxiolytic-like effect of endogenous melatonin. Chronic treatment with agonists prevents various perturbations induced by chronic mild stress. Melatonin in vivo directly constricts cerebral arterioles in rats and decreases the lower limit of cerebral blood flow autoregulation, suggesting that melatonin may diminish the risk of hypoperfusion-induced cerebral ischemia. At the extracerebral level, melatonin regulates intestinal motility in rats. The intestinal postprandial motor response is shorter in the dark phase than in the light phase and this reduction is reversed in animals pretreated with a melatonin antagonist. Moreover, melatonin reduces the duration of cholecystokinin excitomotor effect. Endogenous melatonin may modulate intestinal motility to coordinate intestinal functions such as digestion and transit and control the metabolism of the animal. An adipocyte melatonin binding site may also participate in this control. Melatonin is involved in a wide range of physiological functions. The question remains as to whether evolution, adaptation and diurnal life have modified the physiological role of melatonin in humans. Moreover, the functional role of each of the receptor subtypes has to be characterized to design selective ligands to treat specific diseases.

Differential distribution of melatonin receptors in the pituitary gland of Xenopus laevis.

A major target site for melatonin action is thought to be the pituitary gland. We have detected differential expression and co-localization of the Mel(1a) and Mel(1c) receptors in cells of the Xenopus laevis pituitary gland. Sections of Xenopus pituitary glands were labeled with Mel(1a) and/or Mel(1c) antibodies, in combination with antibodies to arginine vasotocin (AVT), alpha-melanocyte stimulating hormone (alpha-MSH), prolactin (PRL), and luteinizing hormone (LH). Mel(1a) immunoreactivity was localized to cells of the pars intermedia and to elements within the pars nervosa. Mel(1c) immunoreactivity was also localized to the pars nervosa, and significant labeling was also observed in discrete clusters of cells in the pars distalis. Mel(1a) was absent from the pars distalis, while Mel(1c) was absent from the pars intermedia. Mel(1a) and Mel(1c) were co-localized in the pars nervosa. AVT was present in the pars nervosa, and appeared to be localized to the cell clusters of the pars distalis in which the Mel(1c) receptor was localized. alpha-MSH co-localized with the Mel(1a) receptor in the pars intermedia. LH appeared to localize to many of the cells in the pars distalis, with the notable exception of the Mel(1c) receptor-positive clusters of cells. PRL did not appear to co-localize with either melatonin receptor. The pattern of differential expression of the Mel(1a) and Mel(1c) receptors suggests that the receptors specifically mediate the cellular response to melatonin binding in the specific cell populations.

Sexual differences and effect of photoperiod on melatonin receptor in avian brain.

Several data suggest that melatonin may influence avian reproduction by acting at the level of the hypothalamic-hypophisial-gonadal axis, and/or on neural circuits controlling reproductive behaviours. The action of melatonin is exerted through specific receptors whose distribution and pharmacological properties have been extensively investigated. This review will focus on the distribution, sexual dimorphism, and dependence upon the photoperiod of melatonin binding sites in avian species with a special emphasis on Japanese quail. Melatonin receptors are widely distributed in avian brain. They are mostly present in the visual pathways of all the investigated species and in the song controlling nuclei of oscine birds. Sexual dimorphism of melatonin binding sites (higher density in males than in females) was detected in some telencephalic nuclei of songbirds, in the visual pathways, and in the preoptic area of quail. The last region plays a key role in the activation of male quail copulatory behaviour and it hosts a large population of gonadotropin-releasing hormone-containing neurons. Sexual dimorphism of melatonin-binding sites in the above-mentioned regions suggests a differential role for this hormone in the modulation of visual perception, gonadotropin production, and seasonally activated behaviours in male and female quail. Further studies are necessary to understand interrelationships among photic cues, gonadal steroids, density, and sexually dimorphic distribution of melatonin receptors.

Photoperiod-dependent and -independent regulation of melatonin receptors in the forebrain of songbirds.

Melatonin was recently identified as playing a role in fine-tuning the effects of gonadal steroids in the regulation of seasonal neuroplasticity within the telencephalic song control system of European starlings. The present study investigated possible seasonal regulation of melatonin receptors (MelR) within the starling song control system, in the presence or absence of gonadal steroids. Brains were sampled from photosensitive starlings exposed to short days, photostimulated starlings exposed to long days and photorefractory starlings also exposed to long days. Each condition contained a group of gonad-intact birds and a group of castrated birds. Melatonin receptor distribution was assessed in vitro by 125-iodomelatonin (IMEL) receptor autoradiography. In general, MelR distribution was similar to that described in other songbird species. However, there was a striking downregulation of MelR in the song control nucleus Area X of intact and castrated photostimulated birds on long days compared to their photorefractory counterparts on the same long days and to the short-day groups. Downregulation of MelR occurred independently of gonadal steroids. Nevertheless, superimposed on this general pattern of MelR downregulation during photostimulation, IMEL binding was observed in a medial subdivision of Area X when gonadal steroids were present. Downregulation of MelR in Area X during the short breeding season has implications for seasonal regulation of the song control system. Subsequent upregulation of MelR as birds become photorefractory, in the absence of any change in photoperiod, gonadal steroids or melatonin signal is the first description of photoperiod-independent regulation of MelR in adults of any vertebrate class.

Expression of clock gene in the brain of rainbow trout: comparison with the distribution of melatonin receptors.

To identify brain structures potentially acting as biological clocks in rainbow trout (Oncorhynchus mykiss), the expression sites of a trout homolog of the mouse clock gene were studied and compared with that of melatonin receptors (Mel-R). For this purpose, a partial sequence of the trout clock gene, including a PAS domain, was obtained by reverse transcription-polymerase chain reaction and used to perform in situ hybridization. The highest density of clock transcripts was observed in the periventricular layer (SPV) of the optic tectum, but a weaker expression was detected in some pretectal nuclei, such as the posterior pretectal nucleus (PO) and the periventricular regions of the diencephalon. Comparison of the hybridization signal in fish sacrificed at 08:00 and 17:00 did not indicate major changes in clock expression levels. Comparison of adjacent sections alternatively treated with clock and Mel-R probes suggests that both messengers are probably expressed in the same cells in the SPV and PO. In addition, in situ hybridization with a glutamate decarboxylase 65 probe, demonstrates that cells expressing clock and Mel-R in the optic tectum are gamma-aminobutyric acid neurons. The tight overlapping between the expression of Mel-R and clock transcripts in cells of the PO and SPV suggests a functional link between these two factors. These results indicate that the optic tectum and the pretectal area of the rainbow trout are major sites of integration of the melatonin signal, express the clock gene, and may act as biological clocks to influence behavioral and endocrine responses in trout.

Melatonin and its physiological and therapeutic properties.

Melatonin is a hormone produced mainly by the pineal gland in most vertebrate species, including humans. Recent metabolic, receptor and functional studies created a picture of the melatoninergic system(s) in living organisms, its organization, physiology and a role in some pathologic conditions. The melatonin-generating system is characterized by three basic features: (1) photosensitivity, (2) diurnal (or circadian) rhythmicity (with highest levels of melatonin production occurring at night in darkness), and (3) age-related decline in its activity. Cyclic nocturnal increases of melatonin levels are proportional to the length of nights (or dark periods of an imposed light-dark cycle); the hormone thus conveys a photoperiodic message, and functions in an organism as an internal biochemical clock and calendar. Biological actions of melatonin are mediated via specific melatonin receptors, whose distribution in the body is uneven, yet with decisively highest density in the suprachiasmatic nuclei of the hypothalamus, pars tuberalis of the pituitary, and the retina (particularly in birds and lower vertebrates). Such a distribution of melatonin receptors suggests that the principal physiological role of the hormone is related to both chronobiology and modulation of the body hormonal milieu. This review surveys recent developments in the melatonin field, and summarizes current knowledge on the melatoninergic mechanisms, including the therapeutic aspect related to the hormone.

Melatonin binding sites in sciurid and hystricomorph rodents: studies on ground squirrels and guinea pigs

Little is known about the distribution of binding sites for the pineal hormone melatonin in non-myomorph rodents. We used 2-[ 125I]iodomelatonin (IMEL) to analyze the distribution, affinity, and specificity of binding sites in the golden-mantled ground squirrel, a sciurid rodent that reportedly lacks IMEL binding sites in the brain. Specific binding was found not only in the pars tuberalis, but also in several telencephalic areas including the hypothalamic suprachiasmatic region. The affinity and specificity of IMEL binding are comparable to those reported in other rodents. IMEL binding studies in a hystricomorph rodent, the guinea pig, revealed high concentrations of receptor in the nucleus accumbens and dorsolateral thalamus. Central melatonin binding sites have now been demonstrated in species of all three rodent families. The heterogeneous distribution of melatonin receptors appears similar in the species studied, and no evidence is found to link IMEL binding sites at any particular locus to photoperiodic, circannual, or non-seasonal breeding patterns.

The density of melatonin receptors is dependent upon the prevailing photoperiod in the Japanese quail ( Coturnix japonica)

Two groups of Japanese quail ( Coturnix japonica) were exposed to two different photoperiods (short and long-days: LD 8:16 and LD 16:8, respectively), and their brains examined for the presence and distribution of melatonin receptors by means of quantitative in vitro autoradiography. Animals belonging to the LD 8:16 group expressed a significantly higher melatonin receptor density in the optic tectum and nucleus triangularis, while the LD 16:8 animals had a higher density of receptors in the hyperstriatum and nucleus preopticus dorsalis. These data demonstrate an apparent influence of the photoperiod on the density of melatonin receptors, especially in nuclei of the tectofugal pathway, related to the control of visual pattern and intensity discrimination, localization and orientation.

The melatonin rhythm: both a clock and a calendar.

The paper briefly reviews the data which shows that the circadian production and secretion of melatonin by the pineal gland can impart both daily, i.e., clock, and seasonal, i.e., calendar, information to the organism. The paper summarizes the 3 patterns of nocturnal melatonin production that have been described. Clearly, regardless of the pattern of nocturnal melatonin production a particular species normally displays, the duration of nightime elevated melatonin is proportional to the duration of the night length. Since daylength under natural conditions changes daily the melatonin rhythm, which adjusts to the photoperiod sends time of year information to the organism. The melatonin receptors which subserve the clock message sent by the pineal gland in the form of a melatonin cycle may reside in the biological clock itself, namely, the suprachiasmatic nuclei (SCN). The melatonin receptors that mediate seasonal changes in reproductive physiology are presumably those that are located on the pars tuberalis cells of the anterior pituitary gland. Besides these receptors which likely mediate clock and calendar information, melatonin receptors have been described in other organs. Interestingly, the distribution of melatonin receptors is highly species-specific. Whereas the clock and calendar information that the melatonin cycle imparts to the organism relies on cell membrane receptors, a fact that is of some interest considering the high lipophilicity of melatonin, recent studies indicate that other functions of melatonin may require no receptor whatsoever.

Distribution of melatonin receptors in the brain of the frog Rana pipiens as revealed by in vitro autoradiography

Melatonin binding sites were identified in the brain of the frog Rana pipiens using in vitro autoradiography. Coronal sections were incubated for 1 h in 100 pM 2- 125I-iodomelatonin. Specific binding was displaced with 1 μM nonradioactive melatonin. Autoradiographic labeling of 3H-Hyperfilm was observed in areas that receive primary, secondary, and tertiary visual input: the superficial layers of the optic tectum, anterior and posterior thalamic nuclei, striatum, medial pallium, and interpeduncular nucleus. Other areas that demonstrated binding included the medial and lateral septal nuclei, medial preoptic area, suprachiasmatic region, and anterodorsal tegmental nucleus. Binding was also apparent in the distribution of the lateral olfactory tract (lateral pallium), and in tracts associated with visual pathways: optic nerve, chiasm and tract, and lateral and medial forebrain bundles. A high degree of melatonin binding was observed in the left habenular nucleus, but not in the habenulum of the right side of the brain. Radioreceptor binding assays of frog whole-brain homogenate demonstrated specific saturable melatonin binding ( K d = 70 pM, B max = 0.80 fmol/mg protein). Melatonin and 6-chloromelatonin were potent displacers of 2- 125I-iodomelatonin, while 5-hydroxytryptamine, 5-methoxytryptamine, and N-acetylserotonin were much less potent. Melatonin inhibited the forskolin-stimulated increase in cAMP synthesis in optic tectum expiants. These results suggest that high-affinity melatonin receptor binding sites are widely distributed in the telencephalon, diencephalon, and mesencephalon and are very prominent in areas of the frog brain that are associated with visual processing.

Vascular melatonin receptors.

High-affinity melatonin receptors are expressed in the tail artery of the rat and arteries forming the circle of Willis of the rat and certain primates. The characteristics of the vascular melatonin receptors seem to be similar to the ones described in the central nervous system. The expression of vascular melatonin receptors in the rat is differentially regulated by factors such as strain and age, and in the female by reproductive hormones. Functional studies using the caudal artery of the rat suggest that melatonin regulates vascular tone. The highly restricted distribution of melatonin receptors in arteries involved in regulating blood flow to areas involved in heat dissipation suggests that vascular melatonin receptors may have a specific function in thermoregulatory homeostasis.

Asymmetric distribution of melatonin receptors in the brain of the lizard Anolis carolinensis

The pineal hormone melatonin may regulate seasonal reproduction and entrainment of circadian rhythms by binding to specific brain receptors. An analysis of melatonin receptor distribution in the lizard brain revealed an asymmetry of melatonin binding in the diencephalon. A high degree of melatonin binding was present in the left habenular nucleus, but no binding was observed in the habenulum of the right brain hemisphere. It is intriguing that the left habenular nucleus, in contrast to the right habenulum, both possesses a high density of melatonin receptors and receives primary photic input from the parietal eye. Similarly, the optic tectum, which receives primary visual input from the retina, is also rich in melatonin receptors. These observations suggest that the left habenulum is under dual control (neuronal and hormonal) of the parietal eye/pineal complex, and that melatonin may play a significant role in neural processing of visual information.

Melatonin receptors and signal transduction during development in Siberian hamsters ( Phodopus sungorus)

Maternal melatonin communicates daylength information to the fetus in Siberian hamsters. Fetal sensitivity to melatonin declines near birth. In this report, we describe melatonin receptor distribution and a second messenger response to melatonin in Siberian hamsters during the perinatal period. The sites of high-affinity 2-[ 125I]iodomelatonin ([ 125I]MEL) binding were generally similar throughout the perinatal period. The non-hydrolyzable GTP analog, guanosine-5′- O-(3-thiotriphosphate) (100 μM) inhibited [ 125I]MEL binding at each age, suggesting the melatonin receptors are associated with guanine nucleotide binding proteins (G proteins). Furthermore, melatonin (10 nM) inhibited forskolin-stimulated cAMP accumulation in median eminence/pars tuberalis (ME/PT) explants as early as 4 days before birth, when sensitivity to melatonin in vivo is high. The cAMP regulatory system appeared disrupted on the day of birth, in that forskolin (10 μM) stimulation of cAMP accumulation was reduced, and melatonin did not inhibit cAMP accumulation stimulated by forskolin. A higher forskolin dose (100 μM) elevated cAMP levels more clearly on the day of birth, and melatonin inhibited forskolin-stimulated cAMP accumulation. These results suggest that the decreased physiological responsiveness to melatonin at the end of gestation may be due to alterations in the cAMP regulatory system.