Cyclical regulation of GnRH gene expression in GT1-7 GnRH-secreting neurons by melatonin

Experimental Data on the Role of Melatonin in the Pathogenesis of Nonalcoholic Fatty Liver Disease

Despite the increasing prevalence of nonalcoholic fatty liver disease (NAFLD) worldwide, its complex pathogenesis remains incompletely understood. The currently stated hypotheses cannot fully clarify the interrelationships between individual pathogenetic mechanisms of the disease. No appropriate health strategies have been developed for treating NAFLD. NAFLD is characterized by an accumulation of triglycerides in hepatic cells (steatosis), with the advanced form known as nonalcoholic steatohepatitis. In the latter, superimposed inflammation can lead to fibrosis. There are scientific data on NAFLD's association with components of metabolic syndrome. Hormonal factors are thought to play a role in the development of metabolic syndrome. Endogenous melatonin, an indoleamine hormone synthesized by the pineal gland mainly at night, is a powerful chronobiotic that probably regulates metabolic processes and has antioxidant, anti-inflammatory, and genomic effects. Extrapineal melatonin has been found in various tissues and organs, including the liver, pancreas, and gastrointestinal tract, where it likely maintains cellular homeostasis. Melatonin exerts its effects on NAFLD at the cellular, subcellular, and molecular levels, affecting numerous signaling pathways. In this review article, we discuss the experimental scientific data accumulated on the involvement of melatonin in the intimate processes of the pathogenesis of NAFLD.

Heterogeneity in GnRH and kisspeptin neurons and their significance in vertebrate reproductive biology

Vertebrate reproduction is essentially controlled by the hypothalamus-pituitary-gonadal (HPG) axis, which is a central dogma of reproductive biology. Two major hypothalamic neuroendocrine cell groups containing gonadotropin-releasing hormone (GnRH) and kisspeptin are crucial for control of the HPG axis in vertebrates. GnRH and kisspeptin neurons exhibit high levels of heterogeneity including their cellular morphology, biochemistry, neurophysiology and functions. However, the molecular foundation underlying heterogeneities in GnRH and kisspeptin neurons remains unknown. More importantly, the biological and physiological significance of their heterogeneity in reproductive biology is poorly understood. In this review, we first describe the recent advances in the neuroendocrine functions of kisspeptin-GnRH pathways. We then view the recent emerging progress in the heterogeneity of GnRH and kisspeptin neurons using morphological and single-cell transcriptomic analyses. Finally, we discuss our views on the significance of functional heterogeneity of reproductive endocrine cells and their potential relevance to reproductive health.

MiR-375 potentially enhances GnRH expression by targeting Sp1 in GT1-7 cells

Gonadotropin-releasing hormone (GnRH) is the initial central regulator of the animal reproduction system, which is crucial for puberty onset and fertility. However, the mechanisms regulating GnRH production and release remain unclear. In addition, few studies reported that miR-375 expressed in mouse hypothalamus, but up to now there are limited functional studies of miR-375 in regulating GnRH secretion. According to our recent findings that miR-375 was involved in regulating the synthesis and secretion of pituitary hormones, thus, we aimed to identify the role of miR-375 in regulating GnRH production in GT1-7 cells. Immunofluorescence results demonstrated that miR-375 was expressed in all of the GT1-7 cells. The functional studies showed that miR-375 overexpression enhanced GnRH mRNA expression level, but decreased the mRNA expressions of Sp1, Cebpb, Msx1, and Tle4. Transcriptomics analysis demonstrated Sp1 and Tle4 acted as the targeting genes of miR-375, and Sp1 negatively regulated Gnrh mRNA expression by binding to the Gnrh promoter. Thus, we conclude that miR-375 potentially enhances GnRH expression by targeting Sp1 and Tle4 in GT1-7 cells. Our results highlight a critical role of miR-375 in regulating GnRH production, which may provide a novel potential therapeutic approach to neuroendocrine-disorder-related dysfunctions.

Melatonin Suppresses the Kainate Receptor-Mediated Excitation on Gonadotropin-Releasing Hormone Neurons in Female and Male Prepubertal Mice

Melatonin, a pineal gland secretion, is an amphiphilic neurohormone involved in the biological and physiologic regulation of bodily functions. Numerous studies have shown the effects of melatonin on the release of gonadotropins and their actions at one or several levels of the hypothalamic–pituitary–gonadal axis. However, direct melatonin action on gonadotropin-releasing hormone (GnRH) neurons and its mechanism of action remain unclear. Here, plasma melatonin levels were measured and the effect of melatonin on GnRH neurons was assessed using brain slice patch clamp techniques. The plasma melatonin levels in prepubertal mice were higher than those in the adults. Melatonin itself did not change the firing activity of GnRH neurons. Interestingly, the kainate receptor-mediated responses but not the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)- and N-methyl-D-aspartic acid (NMDA)-induced responses were suppressed by melatonin in both the voltage clamp and current clamp modes. The inhibitory effects of the kainate-induced response by melatonin tended to increase with higher melatonin concentrations and persisted in the presence of tetrodotoxin, a voltage-sensitive Na⁺ channel blocker, or luzindole, a non-selective melatonin receptor antagonist. However, the response was completely abolished by pretreatment with pertussis toxin. These results suggest that melatonin can regulate GnRH neuronal activities in prepubertal mice by partially suppressing the excitatory signaling mediated by kainate receptors through pertussis toxin-sensitive G-protein-coupled receptors.

Effects of Melatonin on Anterior Pituitary Plasticity: A Comparison Between Mammals and Teleosts

Melatonin is a key hormone involved in the photoperiodic signaling pathway. In both teleosts and mammals, melatonin produced in the pineal gland at night is released into the blood and cerebrospinal fluid, providing rhythmic information to the whole organism. Melatonin acts via specific receptors, allowing the synchronization of daily and annual physiological rhythms to environmental conditions. The pituitary gland, which produces several hormones involved in a variety of physiological processes such as growth, metabolism, stress and reproduction, is an important target of melatonin. Melatonin modulates pituitary cellular activities, adjusting the synthesis and release of the different pituitary hormones to the functional demands, which changes during the day, seasons and life stages. It is, however, not always clear whether melatonin acts directly or indirectly on the pituitary. Indeed, melatonin also acts both upstream, on brain centers that control the pituitary hormone production and release, as well as downstream, on the tissues targeted by the pituitary hormones, which provide positive and negative feedback to the pituitary gland. In this review, we describe the known pathways through which melatonin modulates anterior pituitary hormonal production, distinguishing indirect effects mediated by brain centers from direct effects on the anterior pituitary. We also highlight similarities and differences between teleosts and mammals, drawing attention to knowledge gaps, and suggesting aims for future research.

The usefulness of melatonin in the field of obstetrics and gynecology

Disorders of the female reproductive system, including those associated with hormone regulation, fertility rate and fetal health, are issues of great concern worldwide. More recently, melatonin supplementation has been suggested as a therapeutic approach in gynecological practice. In both animal models and in women, melatonin supplementation suggests a therapeutic and preventative potential, effects attributed mainly to its antioxidant properties and action as hormone modulator. The aim of this literature review is to further investigate the evidence available on the effects of melatonin supplementation in animal and human studies, focusing on its potential application to gynecology. Melatonin-containing supplements are easily found in online and high street retailers, and despite its supplementation deemed to be relatively safe, no consensus has been reached on effective dosage and supplementation period. Short term supplementation studies, of up to six months, suggest that a daily posology of 2-18 mg of melatonin may have the potential to improve fertility rate, oocyte quality, maturation and number of embryos. However, the evidence available so far on the effects of melatonin supplementation covering gestational age and gestational outcomes is very scarce. Clinical trials and longer-term supplementation studies are required to assess any clinical outcome associated with melatonin supplementation in the field of gynecology.

Gonadoliberin – Synthesis, Secretion, Molecular Mechanisms and Targets of Action

Decapeptide gonadoliberin (GnRH) is the most important regulator of the hypothalamic-pituitary-gonadal (HPG) axis that controls the synthesis and secretion of the luteinizing and follicle-stimulating hormones by gonadotrophs in the adenohypophysis. GnRH is produced by the specialized hypothalamic neurons using the site-specific proteolysis of the precursor protein and is secreted into the portal pituitary system, where it binds to the specific receptors. These receptors belong to the family of G protein-coupled receptors, and they are located on the surface of gonadotrophs and mediate the regulatory effects of GnRH on the gonadotropins production. The result of GnRH binding to them is the activation of phospholipase C and the calcium-dependent pathways, the stimulation of different forms of mitogen-activated protein kinases, as well as the activation of the enzyme adenylyl cyclase and the triggering of cAMP-dependent signaling pathways in the gonadotrophs. The gonadotropins, kisspeptin, sex steroid hormones, insulin, melatonin and a number of transcription factors have an important role in the regulation of GnRH1 gene expression, which encodes the GnRH precursor, as well as the synthesis and secretion of GnRH. The functional activity of GnRH-producing neurons depends on their migration to the hypothalamic region at the early stages of ontogenesis, which is controlled by anosmin, ephrins, and lactosamine-rich surface glycoconjugate. Dysregulation of the migration of GnRH-producing neurons and the impaired production and secretion of GnRH, lead to hypogonadotropic hypogonadism and other dysfunctions of the reproductive system. This review is devoted to the current state of the problem of regulating the synthesis and secretion of GnRH, the mechanisms of migration of hypothalamic GnRH-producing neurons at the early stages of brain development, the functional activity of the GnRH-producing neurons in the adult hypothalamus and the molecular mechanisms of GnRH action on the pituitary gonadotrophs. New experimental data are analyzed, which significantly change the current understanding of the functioning of GnRH-producing neurons and the secretion of GnRH, which is very important for the development of effective approaches for correcting the functions of the HPG axis.

The hypothalamus-pituitary-gonad axis: Tales of mice and men

Reproduction is controlled by the hypothalamic-pituitary-gonadal (HPG) axis. Gonadotropin-releasing hormone (GnRH) neurons play a central role in this axis through production of GnRH, which binds to a membrane receptor on pituitary gonadotrophs and stimulates the biosynthesis and secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Multiple factors affect GnRH neuron migration, GnRH gene expression, GnRH pulse generator, GnRH secretion, GnRH receptor expression, and gonadotropin synthesis and release. Among them anosmin is involved in the guidance of the GnRH neuron migration, and a loss-of-function mutation in its gene leads to a failure of their migration from the olfactory placode to the hypothalamus, with consequent anosmic hypogonadotropic hypogonadism (Kallmann syndrome). There are also cases of hypogonadotropic hypogonadim with normal sense of smell, due to mutations of other genes. Another protein, kisspeptin plays a crucial role in the regulation of GnRH pulse generator and the pubertal development. GnRH is the main hypothalamic regulator of the release of gonadotropins. Finally, FSH and LH are the essential hormonal regulators of testicular functions, acting through their receptors in Sertoli and Leydig cells, respectively. The main features of the male HPG axis will be described in this review.

Membrane receptor-independent inhibitory effect of melatonin on androgen production in porcine theca cells

Excessive secretion of androgens including androstenedione and testosterone in theca cells frequently causes female infertility in mammals. Melatonin is a potent inhibitor of androgen production in gonadal cells of several species in a membrane receptor-dependent manner. However, the function of melatonin in steroidogenesis of porcine theca cells remains unclear. Here we report that melatonin inhibits androgen biosynthesis independently of its membrane receptors in pigs. Using flow cytometry, immunofluorescence and RT-PCR we showed that the vast majority of cells isolated from the theca layer of antral follicles are indeed theca cells. Furthermore, we demonstrated that of the two of melatonin membrane receptors encoded in the porcine genome, theca cells exclusively express melatonin receptor 1B. Cell counting analysis indicated that different concentrations of melatonin did not alter the normal viability and proliferation of theca cells. Additionally, hormone radioimmunoassay and qPCR respectively showed that a high concentration of melatonin significantly repressed both androgen production and expression of steroidogenic genes involving StAR, CYP11A1, HSD3β and SET (P < 0.05), but did not impair progesterone production. Interestingly, these effects were not reversed by N-acetyl-2-benzyltryptamin, a melatonin membrane receptor antagonist. Overall, these results demonstrate that melatonin inhibits androgen production in porcine theca cells independently of its membrane receptor.

Seasonal Reproduction in Vertebrates: Melatonin Synthesis, Binding, and Functionality Using Tinbergen's Four Questions

One of the many functions of melatonin in vertebrates is seasonal reproductive timing. Longer nights in winter correspond to an extended duration of melatonin secretion. The purpose of this review is to discuss melatonin synthesis, receptor subtypes, and function in the context of seasonality across vertebrates. We conclude with Tinbergen's Four Questions to create a comparative framework for future melatonin research in the context of seasonal reproduction.

PHYSIOLOGY AND ENDOCRINOLOGY SYMPOSIUM: Alterations in uteroplacental hemodynamics during melatonin supplementation in sheep and cattle

Compromised placental function can result in fetal growth restriction which is associated with greater risk of neonatal morbidity and mortality. Large increases in transplacental nutrient and waste exchange, which support the exponential increase in fetal growth during the last half of gestation, are dependent primarily on the rapid growth and vascularization of the uteroplacenta. The amplitude of melatonin secretion has been associated with improved oxidative status and altered cardiovascular function in several mammalian species; however, melatonin mediated alterations of uteroplacental capacity in sheep and cattle are lacking. Therefore, our laboratories are examining uteroplacental blood flow and fetal development during maternal melatonin supplementation. Using a mid- to late-gestation ovine model of intrauterine growth restriction, we examined uteroplacental blood flow and fetal growth during supplementation with 5 mg/d of dietary melatonin. Maternal nutrient restriction decreased uterine arterial blood flow, while melatonin supplementation increased umbilical arterial blood flow compared with non-supplemented controls. Although melatonin treatment did not rescue fetal weight in nutrient restricted ewes; we observed disproportionate fetal size and fetal organ development. Elevated fetal concentrations of melatonin may result in altered blood flow distribution during important time points of development. These melatonin specific responses on umbilical arterial hemodynamics and fetal development may be partially mediated through vascular melatonin receptors. Recently, we examined the effects of supplementing Holstein heifers with 20 mg/d of dietary melatonin during the last third of gestation. Uterine arterial blood flow was increased by 25% and total serum antioxidant capacity was increased by 43% in melatonin supplemented heifers vs. non-supplemented controls. In addition, peripheral concentrations of progesterone were decreased in melatonin supplemented heifers vs. non-supplemented controls. Using an in vitro model, melatonin treatment increased the activity of cytochrome P450 2C, a progesterone inactivating enzyme, which was blocked by treatment with the melatonin receptor antagonist, luzindole. Elucidating the consequences of specific hormonal supplements on the continual plasticity of placental function will allow us to determine important endogenous mediators of offspring growth and development.

Melatonin as an Anti-Inflammatory Agent Modulating Inflammasome Activation

Inflammation may be defined as the innate response to harmful stimuli such as pathogens, injury, and metabolic stress; its ultimate function is to restore the physiological homeostatic state. The exact aetiology leading to the development of inflammation is not known, but a combination of genetic, epigenetic, and environmental factors seems to play an important role in the pathogenesis of many inflammation-related clinical conditions. Recent studies suggest that the pathogenesis of different inflammatory diseases also involves the inflammasomes, intracellular multiprotein complexes that mediate activation of inflammatory caspases thereby inducing the secretion of proinflammatory cytokines. Melatonin, an endogenous indoleamine, is considered an important multitasking molecule with fundamental clinical applications. It is involved in mood modulation, sexual behavior, vasomotor control, and immunomodulation and influences energy metabolism; moreover, it acts as an oncostatic and antiaging molecule. Melatonin is an important antioxidant and also a widespread anti-inflammatory molecule, modulating both pro- and anti-inflammatory cytokines in different pathophysiological conditions. This review, first, gives an overview concerning the growing importance of melatonin in the inflammatory-mediated pathological conditions and, then, focuses on its roles and its protective effects against the activation of the inflammasomes and, in particular, of the NLRP3 inflammasome.

New evidence of melatonin receptor contribution to ram sperm functionality

The present study analysed the involvement of melatonin, acting via its receptors (MT1 and MT2), in ram sperm functionality. Indirect immunofluorescence assays revealed no changes in the distribution or intensity of MT1 receptors, whereas different subpopulations were established for MT2 receptors in control, in vitro capacitated and acrosome-reacted ram spermatozoa. Chlortetracycline staining revealed the following correlations between the pattern of staining for MT2 receptors in: (1) non-capacitated (NC) sperm rate and the proportion of spermatozoa with equal immunostaining intensity in the acrosome and post-acrosome (r = 0.59, P < 0.001); (2) in capacitated (C) sperm rate and the proportion of spermatozoa with stronger reactivity in the acrosome (r = 0.60, P < 0.001); and (3) in acrosome-reacted (AR) sperm rate and the proportion of spermatozoa with more intense staining on the post-acrosome (r = 0.67, P < 0.001). Incubation of swim-up-selected samples with either 1 μM melatonin or MT1 and MT2 receptor agonists (2-phenylmelatonin 1 µM and 8-Methoxy-2-propionamidotetralin (8M-PDOT) 1 µM and 10 nM) at 39°C and 5% CO2 for 3 h resulted in a higher proportion of the NC pattern compared with the control group (P < 0.05), whereas treatment with MT1 and MT2 receptor antagonists (luzindole 1 µM and 4-phenyl-2-propionamidotetralin (4P-PDOT) 1 µM and 10 nM) decreased the proportion of spermatozoa exhibiting the NC pattern (P < 0.001) concomitant with an increase in those exhibiting the C pattern (P < 0.01). In conclusion, melatonin exerts a modulating effect on ram sperm functionality, primarily via activation of the MT2 receptor.

Melatonin replacement restores the circadian behavior in adult rat Leydig cells after pinealectomy

Melatonin actions on oscillators in reproductive organs are poorly understood. Here we analyzed melatonin effects on rhythmic expression of clock and steroidogenesis-related genes in adult rat Leydig cells (LCs). The effect of melatonin was tested both in vivo using pinealectomized and melatonin-substituted rats and in vitro on isolated LCs. Data revealed 24-h-rhythmic expression of clock genes (Bmal1, Per1,2,3, Rev-erba,b, Rorb), steroidogenic genes (Star, Cyp11a1, Cyp17a1), and genes of steroidogenic regulators (positive-Nur77, negative-Arr19). Pinealectomy increased 24-h-oscillations of serum testosterone and LC's cAMP levels, expression of Insl3, Per1, Star/StAR, Hsd3b1/2, Nur77, decreased Arr19 and canceled Per2 oscillatory expression pattern. At hypothalamic-pituitary level, pinealectomy increased mesor of Gnrh, Lhb and rhythm robustness of Mntr1a expression. All parameters disturbed were restored by melatonin-replacement. In vitro studies did not confirm direct melatonin effects on neither clock nor steroidogenic genes. Accordingly, melatonin influence 24-h-rhythmic LC-function likely through hypothalamic-pituitary axis and consequently cAMP-signaling in LCs.

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.

Co-expression of two subtypes of melatonin receptor on rat M1-type intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are involved in circadian and other non-image forming visual responses. An open question is whether the activity of these neurons may also be under the regulation mediated by the neurohormone melatonin. In the present work, by double-staining immunohistochemical technique, we studied the expression of MT₁ and MT₂, two known subtypes of mammalian melatonin receptors, in rat ipRGCs. A single subset of retinal ganglion cells labeled by the specific antibody against melanopsin exhibited the morphology typical of M1-type ipRGCs. Immunoreactivity for both MT₁ and MT₂ receptors was clearly seen in the cytoplasm of all labeled ipRGCs, indicating that these two receptors were co-expressed in each of these neurons. Furthermore, labeling for both the receptors were found in neonatal M1 cells as early as the day of birth. It is therefore highly plausible that retinal melatonin may directly modulate the activity of ipRGCs, thus regulating non-image forming visual functions.

Orofacial inflammatory pain affects the expression of MT1 and NADPH-d in rat caudal spinal trigeminal nucleus and trigeminal ganglion

Very little is known about the role of melatonin in the trigeminal system, including the function of melatonin receptor 1. In the present study, adult rats were injected with formaldehyde into the right vibrissae pad to establish a model of orofacial inflammatory pain. The distribution of melatonin receptor 1 and nicotinamide adenine dinucleotide phosphate diaphorase in the caudal spinal trigeminal nucleus and trigeminal ganglion was determined with immunohistochemistry and histochemistry. The results show that there are significant differences in melatonin receptor 1 expression and nicotinamide adenine dinucleotide phosphate diaphorase expression in the trigeminal ganglia and caudal spinal nucleus during the early stage of orofacial inflammatory pain. Our findings suggest that when melatonin receptor 1 expression in the caudal spinal nucleus is significantly reduced, melatonin's regulatory effect on pain is attenuated.

Melatonin: functions and ligands

Melatonin is a chronobiotic substance that acts as synchronizer by stabilizing bodily rhythms. Its synthesis occurs in various locations throughout the body, including the pineal gland, skin, lymphocytes and gastrointestinal tract (GIT). Its synthesis and secretion is controlled by light and dark conditions, whereby light decreases and darkness increases its production. Thus, melatonin is also known as the 'hormone of darkness'. Melatonin and analogs that bind to the melatonin receptors are important because of their role in the management of depression, insomnia, epilepsy, Alzheimer's disease (AD), diabetes, obesity, alopecia, migraine, cancer, and immune and cardiac disorders. In this review, we discuss the mechanism of action of melatonin in these disorders, which could aid in the design of novel melatonin receptor ligands.

A comparative study of reproductive and metabolic responses to administration of exogenous melatonin and aldosterone in xeric and mesic spiny mice populations

A comparative study of reproduction revealed differences between desert-adapted Acomys russatus and Mediterranean Acomys cahirinus populations with respect to the environmental cues used for reproductive activity. Long day (LD) conditions were noted as initial reproductive cue for both populations. This research is a follow-up affects comparative endocrine and metabolic study in regards to reproduction where LD-acclimated mice were treated with, exogenous aldosterone (ALDO) and melatonin (MLT). Only the reproductive system of A. russatus females was significantly affected by both hormones. In A. cahirinus females, MLT decreased leptin levels, while in A. russatus, a treatment with both hormones increased leptin levels. In A. russatus males, MLT affects both reproductive and metabolic functions. However, in A. cahirinus males, ALDO and MLT treatments caused an increase in leptin levels, and a decrease in free fatty acid (FFA) levels, respectively. Correlations between leptin and FFA in general were affected by both MLT and ALDO treatments in A. russatus males and A. cahirinus females. Our results support the general idea, that although the reproductive system of A. russatus responded to an osmotic stress, in our case expressed by ALDO treatment, which can be considered as an ultimate signal, where, photoperiod changes are an initial signal.

Potency of melatonin in living beings

Living beings are surrounded by various changes exhibiting periodical rhythms in environment. The environmental changes are imprinted in organisms in various pattern. The phenomena are believed to match the external signal with organisms in order to increase their survival rate. The signals are categorized into circadian, seasonal, and annual cycles. Among the cycles, the circadian rhythm is regarded as the most important factor because its periodicity is in harmony with the levels of melatonin secreted from pineal gland. Melatonin is produced by the absence of light and its presence displays darkness. Melatonin plays various roles in creatures. Therefore, this review is to introduce the diverse potential ability of melatonin in manifold aspects in living organism.

Melatonin receptor activation suppresses adrenocorticotropin production via BMP-4 action by pituitary AtT20 cells

The role of melatonin, a regulator of circadian rhythm, in adrenocorticotropin (ACTH) production by corticotrope cells has not been elucidated. In this study, we investigated the effect of melatonin on ACTH production in relation to the biological activity of bone morphogenetic protein (BMP)-4 using mouse corticotrope AtT20 cells that express melatonin type-1 (MT1R) but not type-2 (MT2R) receptors. We previously reported that BMP-4 inhibits corticotropin-releasing hormone (CRH)-induced ACTH production and proopiomelanocortin (POMC) transcription by inhibiting MAPK signaling. Both melatonin and an MT1R/MT2R agonist, ramelteon, suppressed CRH-induced ACTH production, POMC transcription and cAMP synthesis. The inhibitory effects of ramelteon on basal and CRH-induced POMC mRNA and ACTH levels were more potent than those of melatonin. Treatment with melatonin or ramelteon in combination with BMP-4 additively suppressed CRH-induced ACTH production. Of note, the level of MT1R expression was upregulated by BMP-4 stimulation. The suppressive effects of melatonin and ramelteon on POMC transcription and cAMP synthesis induced by CRH were not affected by an MT2R antagonist, luzindole. On the other hand, BMP-4-induced Smad1/5/8 phosphorylation and the expression of a BMP target gene, Id-1, were augmented in the presence of melatonin and ramelteon. Considering that the expression levels of BMP receptors, ALK-3/BMPRII, were increased by ramelteon, MT1R action may play an enhancing role in BMP-receptor signaling. Among the MT1R signaling pathways including AKT, ERK and JNK pathways, inhibition of AKT signaling functionally reversed the MT1R effects on both CRH-induced POMC transcription and BMP-4-induced Id-1 transcription. Collectively, MT1R signaling and BMP-4 actions were mutually augmented, leading to fine-tuning of ACTH production by corticotrope cells.

The melatonergic system in mood and anxiety disorders and the role of agomelatine: implications for clinical practice

Melatonin exerts its actions through membrane MT1/MT2 melatonin receptors, which belong to the super family of G-protein-coupled receptors consisting of the typical seven transmembrane domains. MT1 and MT2 receptors are expressed in various tissues of the body either as single ones or together. A growing literature suggests that the melatonergic system may be involved in the pathophysiology of mood and anxiety disorders. In fact, some core symptoms of depression show disturbance of the circadian rhythm in their clinical expression, such as diurnal mood and other symptomatic variation, or are closely linked to circadian system functioning, such as sleep-wake cycle alterations. In addition, alterations have been described in the circadian rhythms of several biological markers in depressed patients. Therefore, there is interest in developing antidepressants that have a chronobiotic effect (i.e., treatment of circadian rhythm disorders). As melatonin produces chronobiotic effects, efforts have been aimed at developing agomelatine, an antidepressant with melatonin agonist activity. The present paper reviews the role of the melatonergic system in the pathophysiology of mood and anxiety disorders and the clinical characteristics of agomelatine. Implications of agomelatine in "real world" clinical practice will be also discussed.

Melatonin inhibits GnRH-1, GnRH-3 and GnRH receptor expression in the brain of the European Sea Bass, Dicentrarchus labrax

Several evidences supported the existence of melatonin effects on reproductive system in fish. In order to investigate whether melatonin is involved in the modulation of GnRH systems in the European sea bass, we have injected melatonin (0.5 μg/g body mass) in male specimens. The brain mRNA transcript levels of the three GnRH forms and the five GnRH receptors present in this species were determined by real time quantitative PCR. Our findings revealed day–night variations in the brain expression of GnRH-1, GnRH-3 and several GnRH receptors (dlGnRHR-II-1c, -2a), which exhibited higher transcript levels at mid-light compared to mid-dark phase of the photocycle. Moreover, an inhibitory effect of melatonin on the nocturnal expression of GnRH-1, GnRH-3, and GnRH receptors subtypes 1c, 2a and 2b was also demonstrated. Interestingly, the inhibitory effect of melatonin affected the expression of hypophysiotrophic GnRH forms and GnRH receptors that exhibit day–night fluctuations, suggesting that exogenous melatonin reinforce physiological mechanisms already established. These interactions between melatoninergic and GnRH systems could be mediating photoperiod effects on reproductive and other rhythmic physiological events in the European sea bass.

Thyroid transcription factor 1, a homeodomain containing transcription factor, contributes to regulating periodic oscillations in GnRH gene expression

Thyroid transcription factor 1 (TTF1), a member of the Nkx family of transcription factors required for basal forebrain morphogenesis, functions in the postnatal hypothalamus as a transcriptional regulator of genes encoding neuromodulators and hypophysiotrophic peptides. One of these peptides is gonadotrophin-releasing hormone (GnRH). In the present study, we show that Ttf1 mRNA abundance varies in a diurnal and melatonin-dependent fashion in the preoptic area of the rat, with maximal Ttf1 expression attained during the dark phase of the light/dark cycle, preceding the nocturnal peak in GnRH mRNA content. GnRH promoter activity oscillates in a circadian manner in GT1-7 cells, and this pattern is enhanced by TTF1 and blunted by small interfering RNA-mediated Ttf1 gene silencing. TTF1 transactivates GnRH transcription by binding to two sites in the GnRH promoter. Rat GnRH neurones in situ contain key proteins components of the positive (BMAL1, CLOCK) and negative (PER1) limbs of the circadian oscillator, and these proteins repress Ttf1 promoter activity in vitro. By contrast, Ttf1 transcription is activated by CRY1, a clock component required for circadian rhythmicity. In turn, TTF1 represses transcription of Rev-erbα, a heme receptor that controls circadian transcription within the positive limb of the circadian oscillator. These findings suggest that TTF1 is a component of the molecular machinery controlling circadian oscillations in GnRH gene transcription.

Melatonin membrane receptors in peripheral tissues: distribution and functions

Many of melatonin's actions are mediated through interaction with the G-protein coupled membrane bound melatonin receptors type 1 and type 2 (MT1 and MT2, respectively) or, indirectly with nuclear orphan receptors from the RORα/RZR family. Melatonin also binds to the quinone reductase II enzyme, previously defined the MT3 receptor. Melatonin receptors are widely distributed in the body; herein we summarize their expression and actions in non-neural tissues. Several controversies still exist regarding, for example, whether melatonin binds the RORα/RZR family. Studies of the peripheral distribution of melatonin receptors are important since they are attractive targets for immunomodulation, regulation of endocrine, reproductive and cardiovascular functions, modulation of skin pigmentation, hair growth, cancerogenesis, and aging. Melatonin receptor agonists and antagonists have an exciting future since they could define multiple mechanisms by which melatonin modulates the complexity of such a wide variety of physiological and pathological processes.

Melatonin elicits protein kinase C-mediated calcium response in immortalized GT1-7 GnRH neurons

Melatonin is suggested to have effects on hypothalamic-pituitary-gonadal (HPG) axis. The pulsatile pattern of GnRH release, which results in the intermittent release of gonadotropic hormones from the pituitary, has a critical importance for reproductive function but the factors responsible from this release pattern are not known. Calcium is a second messenger involved in hormone release. Therefore, investigation of the effects of melatonin on intracellular free calcium levels ([Ca(2+)](i)) would provide critical information on hormone release in immortalized GnRH neurons. The pattern of melatonin-induced intracellular calcium signaling was investigated by fluorescence calcium imaging using the immortalized GnRH-secreting GT1-7 hypothalamic neurons. Melatonin caused a significant increase in [Ca(2+)](i,) which was greatly blocked by luzindole, a melatonin antagonist, or attenuated by pre-treatment with protein kinase C inhibitor. This study suggests that melatonin seems to have a direct effect on GnRH neurons.

Melatonin triggers p53Ser phosphorylation and prevents DNA damage accumulation

Several epidemiological studies have shown that high levels of melatonin, an indolic hormone secreted mainly by the pineal gland, reduce the risks of developing cancer, thus suggesting that melatonin triggers the activation of tumor-suppressor pathways that lead to the prevention of malignant transformation. This paper illustrates that melatonin induces phosphorylation of p53 at Ser-15 inhibiting cell proliferation and preventing DNA damage accumulation of both normal and transformed cells. This activity requires p53 and promyelocytic leukemia (PML) expression and efficient phosphorylation of p53 at Ser-15 residue. Melatonin-induced p53 phosphorylation at Ser-15 residue does not require ataxia telangiectasia-mutated activity, whereas it is severely impaired upon chemical inhibition of p38 mitogen-activated protein kinase activity. By and large, these findings imply that the activation of the p53 tumor-suppressor pathway is a critical mediator of melatonin and its anticancer effects. Therefore, it provides molecular insights into increasing observational evidence for the role that melatonin has in cancer prevention.

Anatomical and histological profile of bronchus-associated lymphoid tissue and localization of melatonin receptor types (Mel 1a and Mel 1b) 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 (Mel(1a) and Mel(1b)) 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 Mel(1b) 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 Mel(1a) 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 Mel(1a) and Mel(1b) 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.

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, classification, and pharmacology of G protein-coupled melatonin receptors

The hormone melatonin (5-methoxy-N-acetyltryptamine) is synthesized primarily in the pineal gland and retina, and in several peripheral tissues and organs. In the circulation, the concentration of melatonin follows a circadian rhythm, with high levels at night providing timing cues to target tissues endowed with melatonin receptors. Melatonin receptors receive and translate melatonin's message to influence daily and seasonal rhythms of physiology and behavior. The melatonin message is translated through activation of two G protein-coupled receptors, MT(1) and MT(2), that are potential therapeutic targets in disorders ranging from insomnia and circadian sleep disorders to depression, cardiovascular diseases, and cancer. This review summarizes the steps taken since melatonin's discovery by Aaron Lerner in 1958 to functionally characterize, clone, and localize receptors in mammalian tissues. The pharmacological and molecular properties of the receptors are described as well as current efforts to discover and develop ligands for treatment of a number of illnesses, including sleep disorders, depression, and cancer.

Hypothalamic cell lines to investigate neuroendocrine control mechanisms

The hypothalamus is the control center for most physiological processes; yet has been difficult to study due to the inherent heterogeneity of this brain region. For this reason, researchers have turned towards cell models. Primary hypothalamic cultures are difficult to maintain, are heterogeneous neuronal and glial cell populations and often contain a minimal number of viable peptide-secreting neurons. In contrast, immortalized, clonal cell lines represent an unlimited, homogeneous population of neurons that can be manipulated using a number of elegant molecular techniques. Cell line studies and in vivo experimentation are complementary and together provide a powerful tool to drive scientific discovery. This review focuses on three key neuroendocrine systems: energy homeostasis, reproduction, and circadian rhythms; and the use of hypothalamic cell lines to dissect the complex pathways utilized by individual neurons in these systems.

Melatonin and reproduction revisited

This brief review summarizes new findings related to the reported beneficial effects of melatonin on reproductive physiology beyond its now well-known role in determining the sexual status in both long-day and short-day seasonally breeding mammals. Of particular note are those reproductive processes that have been shown to benefit from the ability of melatonin to function in the reduction of oxidative stress. In the few species that have been tested, brightly colored secondary sexual characteristics that serve as a sexual attractant reportedly are enhanced by melatonin administration. This is of potential importance inasmuch as the brightness of ornamental pigmentation is also associated with animals that are of the highest genetic quality. Free radical damage is commonplace during pregnancy and has negative effects on the mother, placenta, and fetus. Because of its ability to readily pass through the placenta, melatonin easily protects the fetus from oxidative damage, as well as the maternal tissues and placenta. Examples of conditions in which oxidative and nitrosative stress can be extensive during pregnancy include preeclampsia and damage resulting from anoxia or hypoxia that is followed by reflow of oxygenated blood into the tissue. Given the uncommonly low toxicity of melatonin, clinical trials are warranted to document the protection by melatonin against pathophysiological states of the reproductive system in which free radical damage is known to occur. Finally, the beneficial effects of melatonin in improving the outcomes of in vitro fertilization and embryo transfer should be further tested and exploited. The information in this article has applicability to human and veterinary medicine.

Melatonin and breast cancer: cellular mechanisms, clinical studies and future perspectives

Recent studies have suggested that the pineal hormone melatonin may protect against breast cancer, and the mechanisms underlying its actions are becoming clearer. Melatonin works through receptors and distinct second messenger pathways to reduce cellular proliferation and to induce cellular differentiation. In addition, independently of receptors melatonin can modulate oestrogen-dependent pathways and reduce free-radical formation, thus preventing mutation and cellular toxicity. The fact that melatonin works through a myriad of signalling cascades that are protective to cells makes this hormone a good candidate for use in the clinic for the prevention and/or treatment of cancer. This review summarises cellular mechanisms governing the action of melatonin and then considers the potential use of melatonin in breast cancer prevention and treatment, with an emphasis on improving clinical outcomes.

Daily changes in GT1-7 cell sensitivity to GnRH secretagogues that trigger ovulation

Circadian rhythms in behavior and physiology are orchestrated by a master biological clock located in the suprachiasmatic nucleus (SCN). Circadian oscillations are a cellular property, with 'clock' genes and their protein products forming transcription-translation feedback loops that maintain 24-hour rhythmicity. Although the expression of clock genes is thought to be ubiquitous, the function of local, extra-SCN timing mechanisms remains elusive. We hypothesized that extra-SCN clock genes control local temporal sensitivity to upstream modulatory signals, allowing system-specific processes to be carried out during individual, optimal times of day. To test this possibility, we examined changes in the sensitivity of immortalized GnRH neurons, GT1-7 cells, to timed stimulation by two key neuropeptides thought to trigger ovulation on the afternoon of proestrus, kisspeptin and vasoactive intestinal polypeptide (VIP). We noted a prominent daily rhythm of clock gene expression in this cell line. GT1-7 cells also exhibited daily changes in cellular peptide expression and GnRH secretion in response to kisspeptin and VIP stimulation. These responses occurred without changes in GnRH transcription. These findings are consistent with the notion that GnRH cells are capable of intrinsic circadian cycles that may be fundamental for coordinating daily changes in sensitivity to signals impacting the reproductive axis.

Hormonal regulation of clonal, immortalized hypothalamic neurons expressing neuropeptides involved in reproduction and feeding

The hypothalamus has been particularly difficult to study at the molecular level because of the inherent cellular heterogeneity and complexity of neuronal circuits within. We have generated a large number of immortalized, clonal cell lines through retroviral gene transfer of the oncogene SV40 T-Ag into primary murine hypothalamic neuronal cell cultures. A number of these neuronal cell lines express neuropeptides linked to the control of feeding behavior and reproduction, including neuropeptide Y (NPY) and neurotensin (NT). We review recent studies on the direct regulation of NPY gene expression by estrogen, and the leptin-mediated control of signal transduction pathways and NT transcription. These studies provide new insights into the direct control of neuropeptide synthesis by hormones and nutrients at a mechanistic level in the individual neuron, not yet possible in the whole brain. Using these novel cell models, we expect to contribute substantially to the understanding of how individual neuronal cell types control overall endocrine function, especially with regard to two of the most well-known roles of distinct peptidergic neurons; these being the control of reproduction and energy homeostasis.

Behavioral characterization and modulation of circadian rhythms by light and melatonin in C3H/HeN mice homozygous for the RORbeta knockout

This study reports for the first time the effects of retinoid-related orphan receptors [RORbeta; receptor gene deletion RORbeta(C3H)(-/-)] in C3H/HeN mice on behavioral and circadian phenotypes. Pineal melatonin levels showed a robust diurnal rhythm with high levels at night in wild-type (+/+), heterozygous (+/-), and knockout (-/-) mice. The RORbeta(C3H)(-/-) mice displayed motor ("duck gait," hind paw clasping reflex) and olfactory deficits, and reduced anxiety and learned helplessness-related behaviors. Circadian rhythms of wheel-running activity in all genotypes showed entrainment to the light-dark (LD) cycle, and free running in constant dark, with RORbeta(C3H)(-/-) mice showing a significant increase in circadian period (tau). Melatonin administration (90 microg/mouse sc for 3 days) at circadian time (CT) 10 induced phase advances, while exposure to a light pulse (300 lux) at CT 14 induced phase delays of circadian activity rhythms of the same magnitude in all genotypes. In RORbeta(C3H)(-/-) mice a light pulse at CT 22 elicited a larger phase advance in activity rhythms and a slower rate of reentrainment after a 6-h advance in the LD cycle compared with (+/+) mice. Yet, the rate of reentrainment was significantly advanced by melatonin administration at the new dark onset in both (+/+) and (-/-) mice. We conclude that the RORbeta nuclear receptor is not involved in either the rhythmic production of pineal melatonin or in mediating phase shifts of circadian rhythms by melatonin, but it may regulate clock responses to photic stimuli at certain time domains.

Physiological pineal effects on female reproductive function of laboratory rats: prenatal development of pups, litter size and estrous cycle in middle age

The present study investigates whether and how the pineal or its hormone melatonin influences female reproductive functions, namely the litter size, prenatal development of offsprings, and estrous cyclicity, especially its age-related cessation in a non-seasonal breeder, the laboratory rat. Wistar rats were maintained under a 24 h light-dark (12Lratio12D) cycle. Female rats were divided into 3 groups: non-operated (NO), sham-operated (SX), and pinealectomized (PX). Surgeries were performed in 35-40 day-old females. Starting at an age between 70 days and 7 months, female rats of all 3 groups were repeatedly mated with intact males. PX mothers more frequently delivered pups with malformations (e.g., taillessness, hydronephrosis, 7 out of 1263 pups) than control rats (0/1323; p<0.007). In the first delivery at 3 months of age, but not at later ages, PX mothers delivered more pups of lower body weight than control animals (p<0.001). Examination of vaginal smears showed that almost all female rats of the NO, SX, and PX groups had 4-day estrous cyclicity when they were young-between 60 days and 5 months of age. At an age of 17 to 18 months, most female rats of the NO and SX groups showed irregular, continuously diestrous or pseudopregnancy-like patterns, and 4-day estrous cyclicity was found in only 10% of the NO or SX animals. In contrast, about 50% of the PX rats showed 4-day estrous cyclicity at this older age (p< 0.001). Melatonin, when added to drinking water (0.4 mg/L) for 16 days during the dark phase increased the frequency of diestrous phase, except in continuously diestrous rats and very few others. This melatonin effect was strong in PX rats but relatively weak in SX rats. In conclusion, the pineal hormone appears to influence various reproductive functions and developmental processes, especially pregnancy and the timing of reproductive aging in rats. The effects of pinealectomy are more prominent at an age of 60 to 80 days (i.e., shortly after puberty) and at the beginning of the cessation of cycles in middle-aged females.

Functional MT1 and MT2 melatonin receptors in mammals

Melatonin, dubbed the hormone of darkness, is known to regulate a wide variety of physiological processes in mammals. This review describes well-defined functional responses mediated through activation of high-affinity MT1 and MT2 G protein-coupled receptors viewed as potential targets for drug discovery. MT1 melatonin receptors modulate neuronal firing, arterial vasocon-striction, cell proliferation in cancer cells, and reproductive and metabolic functions. Activation of MT2 melatonin receptors phase shift circadian rhythms of neuronal firing in the suprachiasmatic nucleus, inhibit dopamine release in retina, induce vasodilation and inhibition of leukocyte rolling in arterial beds, and enhance immune responses. The melatonin-mediated responses elicited by activation of MT1 and MT2 native melatonin receptors are dependent on circadian time, duration and mode of exposure to endogenous or exogenous melatonin, and functional receptor sensitivity. Together, these studies underscore the importance of carefully linking each melatonin receptor type to specific functional responses in target tissues to facilitate the design and development of novel therapeutic agent.

Molecular biology of gonadotropin-releasing hormone (GnRH)-I, GnRH-II, and their receptors in humans

In human beings, two forms of GnRH, termed GnRH-I and GnRH-II, encoded by separate genes have been identified. Although these hormones share comparable cDNA and genomic structures, their tissue distribution and regulation of gene expression are significantly dissimilar. The actions of GnRH are mediated by the GnRH receptor, which belongs to a member of the rhodopsin-like G protein-coupled receptor superfamily. However, to date, only one conventional GnRH receptor subtype (type I GnRH receptor) uniquely lacking a carboxyl-terminal tail has been found in the human body. Studies on the transcriptional regulation of the human GnRH receptor gene have indicated that tissue-specific gene expression is mediated by differential promoter usage in various cell types. Functionally, there is growing evidence showing that both GnRH-I and GnRH-II are potentially important autocrine and/or paracrine regulators in some extrapituitary compartments. Recent cloning of a second GnRH receptor subtype (type II GnRH receptor) in nonhuman primates revealed that it is structurally and functionally distinct from the mammalian type I receptor. However, the human type II receptor gene homolog carries a frameshift and a premature stop codon, suggesting that a full-length type II receptor does not exist in humans.

Diurnal changes of tonic nociceptive responses in mice: evidence for a proalgesic role of melatonin

Diurnal variations in tonic pain reactions have been described in mice tested in Spring, but the underlying mechanisms are still unknown. We tested the potential role of melatonin, a key hormone in the control of neuro-endocrine circadian rhythms. The experiments were performed in male CBA/J mice housed under controlled temperature, humidity, and light (12/12 dark/light cycle) conditions, during the Light (7-10a.m.) or Dark (7-10p.m.) phases of the diurnal cycle. In a first group of experiments, animals were either pretreated with i.p. saline (controls) or with the melatonin receptor antagonist, luzindole (30 mg/kg), before the s.c. injection of a dilute formalin solution into a hindpaw. In control animals, pain-related behavioral reactions (licking and flinching) were higher in the evening (Dark) than in the morning (Light), both during the first (0-10 min) and the second (11-55 min) phase of the response to s.c. formalin. In animals pre-treated with luzindole, no diurnal changes occurred, pain reactions in the Dark being similar to those of the Light Control group. In a second group of experiments, artificial pinealectomy, obtained by exposing animals to continuous light for 48 h, also reduced pain reactions in the evening to levels comparable to those in the morning. Receptor autoradiography showed lower binding availability at spinal cord level in mice sacrificed during the Dark, as expected from the circadian pattern of melatonin secretion. A further significant decrease of melatonin receptor binding was induced by noxious stimulation. These results suggest a proalgesic role of endogenous melatonin in tonic pain.

Immortalized cells from the rat suprachiasmatic nucleus express functional melatonin receptors

Immortalized SCN2.2 cells retain most biochemical and biophysical characteristics of the native rat SCN including the expression of clock genes and circadian regulatory proteins, and its distinctive pacemaker function. This study assessed the expression and signaling of MT(1) and MT(2) melatonin receptors in SCN2.2 cells. SCN2.2 cells express MT(1) and MT(2) receptors mRNA as detected by RT-PCR. In situ hybridization with digoxigenin-labeled probes demonstrated that mRNA for MT(1) and MT(2) melatonin receptors is expressed mostly in cells with neuronal-like morphology, representing 10.8+/-2.2% and 9.8+/-0.2%, respectively, of the SCN2.2 cell population. MT(1) and MT(2) melatonin receptor proteins are expressed in both rat SCN2.2 cells and rat SCN tissue as demonstrated by Western blot analysis with specific receptor antiserum. Melatonin (0.1-100 nM) inhibited forskolin (20 microM)-stimulated cAMP formation in a dose-dependent manner and this effect was blocked by the competitive melatonin receptor antagonist luzindole (100-1000 nM). Furthermore, melatonin (1 nM) stimulated protein kinase C (PKC) activity by approximately 2-fold. The selective MT(2) receptor antagonist 4P-PDOT (100 nM) blocked this effect, indicating that the melatonin-mediated increase in PKC activity occurs through activation of MT(2) melatonin receptors. We conclude that SCN2.2 cells express functional melatonin receptors, providing an in vitro model to unveil the melatonin signaling pathway(s) involved in the regulation of circadian rhythms.