Melatonin regulates the mRNA expression of the mt(1) melatonin receptor in the rat Pars tuberalis

Antidepressant agomelatine attenuates behavioral deficits and concomitant pathology observed in streptozotocin-induced model of Alzheimer's disease in male rats

Experimental findings suggest that the melatonin system has a beneficial role in models of Alzheimer's disease (ADs). The aim of the present study was to explore whether the atypical antidepressant agomelatine (Ago), which is a melatonin MT₁ and MT₂ agonist and 5-HT_2C antagonist, is effective against behavioral, biochemical and histological impairments in streptozotocin (STZ)-induced model of ADs in male rats. Male Sprague Dawley rats were treated intraperitoneally (i.p.) with Ago (40 mg/kg) for 30 days starting three months following the intracerebroventricular (icv) injection of STZ. Chronic Ago treatment reduced anxiety-like behavior of STZ-treated rats in the elevated plus maze, increased the preference to saccharine and corrected the spatial memory impairment in the eight-arm radial arm maze test. This melatonin analogue restored STZ-induced biochemical changes, including an increase of beta amyloid (Aβ) protein, and signal markers of inflammation (TNF-alpha and IL-1 beta). Ago exerted partial neuroprotection, specifically in the temporal CA3b subfield of the dorsal hippocampus and temporal piriform cortex. The ability of Ago to alleviate behavioral symptoms and concomitant neuropathological events observed in a model of sporadic ADs suggests that this melatonin alternative can be considered a promising adjuvant in this disease.

Endotoxin-Induced Inflammation Suppresses the Effect of Melatonin on the Release of LH from the Ovine Pars Tuberalis Explants-Ex Vivo Study

The secretion of the hormone melatonin reliably reflects environmental light conditions. Among numerous actions, in seasonal breeders, melatonin may regulate the secretion of the gonadotropins acting via its corresponding receptors occurring in the Pars Tuberalis (PT). However, it was previously found that the secretory activity of the pituitary may be dependent on the immune status of the animal. Therefore, this study was designed to determine the role of melatonin in the modulation of luteinizing hormone (LH) secretion from the PT explants collected from saline- and endotoxin-treated ewes in the follicular phase of the oestrous cycle. Twelve Blackhead ewes were sacrificed 3 h after injection with lipopolysaccharide (LPS; 400 ng/kg) or saline, and the PTs were collected. Each PT was cut into 4 explants, which were then divided into 4 groups: I, incubated with 'pure' medium 199; II, treated with gonadotropin-releasing hormone (GnRH) (100 pg/mL); III, treated with melatonin (10 nmol/mL); and IV, incubated with GnRH and melatonin. Melatonin reduced (p < 0.05) GnRH-induced secretion of LH only in the PT from saline-treated ewes. Explants collected from LPS-treated ewes were characterized by lower (p < 0.05) GnRH-dependent response in LH release. It was also found that inflammation reduced the gene expression of the GnRH receptor and the MT1 melatonin receptors in the PT. Therefore, it was shown that inflammation affects the melatonin action on LH secretion from the PT, which may be one of the mechanisms via which immune/inflammatory challenges disturb reproduction processes in animals.

Comparison of arylalkylamine N-acetyltransferase and melatonin receptor type 1B immunoreactivity between young adult and aged canine spinal cord

Melatonin affects diverse physiological functions through its receptor and plays an important role in the central nervous system. In the present study, we compared immunoreactivity patterns of arylalkylamine N-acetyltransferase (AANAT), an enzyme essential for melatonin synthesis, and melatonin receptor type 1B (MT2) in the spinal cord of young adult (2~3 years) and aged (10~12 years) beagle dogs using immunohistochemistry and Western blotting. AANAT-specific immunoreactivity was observed in the nuclei of spinal neurons, and was significantly increased in aged dog spinal neurons compared to young adult spinal neurons. MT2-specific immunoreactivity was found in the cytoplasm of spinal neurons, and was predominantly increased in the margin of the neuron cytoplasm in aged spinal cord compared to that in the young adult dogs. These increased levels of AANAT and MT2 immunoreactivity in aged spinal cord might be a feature of normal aging and associated with a feedback mechanism that compensates for decreased production of melatonin during aging.

Molecular mechanisms of melatonin's inhibitory actions on breast cancers

Melatonin is involved in many physiological functions and it plays an important role in many pathological processes as well. Melatonin has been shown to reduce the incidence of experimentally induced cancers and can significantly inhibit the growth of some human tumors, namely hormone-dependent cancers. The anticancer effects of melatonin have been observed in breast cancer, both in in vivo with models of chemically induced rat mammary tumors, and in vitro studies on human breast cancer cell lines. Melatonin acts at different physiological levels and its antitumoral properties are supported by a set of complex, different mechanisms of action, involving apoptosis activation, inhibition of proliferation, and cell differentiation.

Alterations of melatonin receptors MT1 and MT2 in the hypothalamic suprachiasmatic nucleus during depression

BACKGROUND

The pineal hormone melatonin regulates circadian rhythms, largely by feedback on the central biological clock of the brain, the hypothalamic suprachiasmatic nucleus (SCN). This feedback is mediated by the melatonin receptors, melatonin receptor 1 (MT1) and melatonin receptor 2 (MT2). The circadian system may play a role in the pathophysiology of mood disorders, and indeed, melatonin-receptor agonists are considered a potential therapy for depression.

METHOD

In order to investigate melatonin receptors in the SCN during depression, and their relationship to the major neuropeptides in the SCN, vasopressin (AVP) and vasoactive intestinal peptide (VIP), we studied the SCN in 14 depressed patients (five major depression and nine bipolar disorder) and 14 matched controls by immunocytochemistry.

RESULTS

We show here that hypothalamic MT2 receptor immunoreactivity was limited to SCN, the supraoptic nucleus and paraventricular nucleus. We found that numbers of MT1-immunoreactive (MT1-ir) cells and AVP and/or VIP-ir cells were increased in the central SCN in depression, but numbers of MT2-ir cells were not altered. Moreover, the number of MT1-ir cells, but not MT2-ir cells was negatively correlated with age at onset of depression, while positively correlated with disease duration. CONCLUSION AND LIMITATIONS: Although every post-mortem study has limitations, MT1 receptors appeared specifically increased in the SCN of depressed patients, and may increase during the course of the disease. These changes may be involved in the circadian disorders and contribute to the efficacy of MT agonists or melatonin in depression. Moreover, we suggest that melatonin receptor agonists for depression should be targeted towards the MT1 receptor selectively.

Analysis of the daily changes of melatonin receptors in the rat liver

Melatonin membrane (MT1 and MT2) and nuclear (RORα) receptors have been identified in several mammalian tissues, including the liver. The mechanisms regulating hepatic melatonin receptors are yet unknown. This study investigated whether these receptors exhibit daily changes and the effects of melatonin on their levels. Our results show that mRNAs for MT1/MT2 receptors exhibit circadian rhythms that were followed by rhythms in their respective protein levels; the acrophases for the two rhythms were reached at 04:00 and 05:00 hr, respectively. Pinealectomy blunted the rhythms in both mRNAs and protein levels. In contrast, mRNA and protein levels of nuclear receptor RORα increased significantly after pinealectomy. The cycles of the latter receptor also exhibited circadian rhythms which peaked at 03:00 and 03:45 hr, respectively. Melatonin administration (10-200 mg/kg) increased in a dose-dependent manner the protein content of MT1/MT2 receptors, with no effects on RORα. Lunzindole treatment, however, did not affect melatonin receptor expression or content of either the membrane or nuclear receptors. Together with previously published findings which demonstrated the intracellular distribution of melatonin in rat liver, the current results support the conclusion that the circadian rhythms of MT1/MT2 and RORα receptors are under the control of the serum and intracellular melatonin levels. Moreover, the induction of MT1/MT2 receptors after the administration of high doses of melatonin further suggests that the therapeutic value of melatonin may not be restricted to only low doses of the indoleamine.

Circadian clock gene Per2 is not necessary for the photoperiodic response in mice

In mammals, light information received by the eyes is transmitted to the pineal gland via the circadian pacemaker, i.e., the suprachiasmatic nucleus (SCN). Melatonin secreted by the pineal gland at night decodes night length and regulates seasonal physiology and behavior. Melatonin regulates the expression of the β-subunit of thyroid-stimulating hormone (TSH; Tshb) in the pars tuberalis (PT) of the pituitary gland. Long day-induced PT TSH acts on ependymal cells in the mediobasal hypothalamus to induce the expression of type 2 deiodinase (Dio2) and reduce type 3 deiodinase (Dio3) that are thyroid hormone-activating and hormone-inactivating enzymes, respectively. The long day-activated thyroid hormone T₃ regulates seasonal gonadotropin-releasing hormone secretion. It is well established that the circadian clock is involved in the regulation of photoperiodism. However, the involvement of the circadian clock gene in photoperiodism regulation remains unclear. Although mice are generally considered non-seasonal animals, it was recently demonstrated that mice are a good model for the study of photoperiodism. In the present study, therefore, we examined the effect of changing day length in Per2 deletion mutant mice that show shorter wheel-running rhythms under constant darkness followed by arhythmicity. Although the amplitude of clock gene (Per1, Cry1) expression was greatly attenuated in the SCN, the expression profile of arylalkylamine N-acetyltransferase, a rate-limiting melatonin synthesis enzyme, was unaffected in the pineal gland, and robust photoperiodic responses of the Tshb, Dio2, and Dio3 genes were observed. These results suggested that the Per2 clock gene is not necessary for the photoperiodic response in mice.

A circadian clock entrained by melatonin is ticking in the rat fetal adrenal

The adrenal gland in the adult is a peripheral circadian clock involved in the coordination of energy intake and expenditure, required for adaptation to the external environment. During fetal life, a peripheral circadian clock is present in the nonhuman primate adrenal gland. Whether this extends to the fetal adrenal gland like the rat is unknown. Here we explored in vivo and in vitro whether the rat fetal adrenal is a peripheral circadian clock entrained by melatonin. We measured the 24-h changes in adrenal content of corticosterone and in the expression of clock genes Per-2 and Bmal-1 and of steroidogenic acute regulatory protein (StAR), Mt1 melatonin receptor, and early growth response protein 1 (Egr-1) expression. In culture, we explored whether oscillatory expression of these genes persisted during 48 h and the effect of a 4-h melatonin pulse on their expression. In vivo, the rat fetal adrenal gland showed circadian expression of Bmal-1 and Per-2 in antiphase (acrophases at 2200 and 1300 h, respectively) as well as of Mt1 and Egr-1. This was accompanied by circadian rhythms of corticosterone content and of StAR expression both peaking at 0600 h. The 24-h oscillatory expression of Bmal-1, Per-2, StAR, Mt1, and Egr-1 persisted during 48 h in culture; however, the antiphase between Per-2 and Bmal-1 was lost. The pulse of melatonin shifted the acrophases of all the genes studied and restored the antiphase between Per-2 and Bmal-1. Thus, in the rat, the fetal adrenal is a strong peripheral clock potentially amenable to regulation by maternal melatonin.

Continuous light and melatonin: daily and seasonal variations of brain binding sites and plasma concentration during the first reproductive cycle of sea bass

The present study reports on the daily and seasonal variations in plasma melatonin concentration, and also in optic tectum and hypothalamus melatonin binding sites, in male European sea bass maintained under natural photoperiod (NP) or continuous light (LL) from early stages of development. Samples were collected on a 24-h cycle, at four physiological phases of their first annual reproductive cycle, i.e., pre-spermatogenesis, spermatogenesis, spermiation and post-spermiation. Under NP, (1) plasma melatonin levels were higher at night than during the day regardless of the year period, and the duration of the signal matched the duration of the dark phase; (2) daily variations in Kd and Bmax were found in the optic tectum, but only during spermiation, with the acrophase being 180° out of phase with the plasma melatonin variations; and (3) significant seasonal Kd and Bmax changes were seen in the hypothalamus. Under LL, (1) plasma melatonin showed no elevation during the subjective night; and (2) Kd and Bmax exhibited seasonal variations in the hypothalamus. These results led to the conclusion that long-term exposure to LL affected both plasma melatonin and receptor oscillations; particularly, LL disrupted the receptor density circadian oscillation found in the optic tectum during spermiation under NP. This oscillation appears to be important for sea bass to pursue gametogenesis until full spermiation. The persistence of both daily and seasonal variation of receptor affinity and density in the hypothalamus under LL indicates that these variations are controlled by internal circadian and circannual clocks that do not involve melatonin.

Melatonin receptors, heterodimerization, signal transduction and binding sites: what's new?

Melatonin is a neurohormone that has been claimed to be involved in a wide range of physiological functions. Nevertheless, for most of its effects, the mechanism of action is not really known. In mammals, two melatonin receptors, MT1 and MT2, have been cloned. They belong to the G-protein-coupled receptor (GPCR) superfamily. They share some specific short amino-acid sequences, which suggest that they represent a specific subfamily. Another receptor from the same subfamily, the melatonin-related receptor has been cloned in different species including humans. This orphan receptor also named GPR50 does not bind melatonin and its endogenous ligand is still unknown. Nevertheless, this receptor has been shown to behave as an antagonist of the MT1 receptor, which opens new pharmacological perspectives for GPR50 despite the lack of endogenous or synthetic ligands. Moreover, MT1 and MT2 interact together through the formation of heterodimers at least in cells transfected with the cDNA of these two receptors. Lastly, signalling complexes associated with MT1 and MT2 receptors are starting to be deciphered. A third melatonin-binding site has been purified and characterized as the enzyme quinone reductase 2 (QR2). Inhibition of QR2 by melatonin may explain melatonin's protective effect that has been reported in different animal models and that is generally associated with its well-documented antioxidant properties.

Inverse agonist exposure enhances ligand binding and G protein activation of the human MT1 melatonin receptor, but leads to receptor down-regulation

Melatonin binds and activates G protein-coupled melatonin receptors. The density and affinity of the endogenous melatonin receptors change throughout the 24-hr day, and the exposure of recombinant melatonin receptors to melatonin often results in desensitization of the receptors. Receptor density, G protein activation and expression level were analyzed in CHO cell lines stably expressing the human MT1 receptors after 1 or 72 hr of exposure to melatonin (agonist, 10 nm) and luzindole (antagonist/inverse agonist, 10 microm). The 72-hr exposure to luzindole significantly increased the apparent receptor density in cell lines with both high and low MT1 receptor expression levels (MT1(high) and MT1(low) cells, respectively). In the constitutively active MT1(high) cells, luzindole pretreatment also stimulated the functional response to melatonin in [(35)S]GTPgammaS binding assays, whereas melatonin pretreatment attenuated the functional response at both time points. Receptor ELISA was used to analyze the cell membrane and total expression level of the MT1 receptor in intact and permeabilized cells, respectively. Luzindole pretreatment decreased the total cellular level of MT1 receptor in the MT1(high) cells at both time points but increased the cell surface expression of MT1 receptor at 72 hr. Melatonin significantly decreased MT1 receptor cell surface expression only in MT1(high) cells after a 1-hr treatment. These results indicate that melatonin treatment desensitizes MT1 receptors, whereas luzindole increases ligand binding and G-protein activation. Luzindole also stimulates downregulation of the MT1 receptor protein, interfering with the synthesis and/or degradation of the receptor.

Does a melatonin-dependent circadian oscillator in the pars tuberalis drive prolactin seasonal rhythmicity?

The pars tuberalis (PT) of the adenohypophysis expresses a high density of melatonin receptors and is thought to be a crucial relay for the actions of melatonin on seasonal rhythmicity of prolactin secretion by the pars distalis (PD). In common with the suprachiasmatic nucleus of the hypothalamus and most other peripheral tissues, the PT rhythmically expresses a range of 'clock genes'. Interestingly, this expression is highly dependent upon melatonin/photoperiod, with several aspects unique to the PT. These observations led to the establishment of a conceptual framework for the encoding of seasonal timing in this tissue. This review summarises current knowledge of the morphological, functional and molecular aspects of the PT and considers its role in seasonal timing. The strengths and weaknesses of current hypotheses that link melatonin action in the PT to its seasonal effect on lactotrophs of the PD are discussed and alternative working hypotheses are suggested.

Expression of the melatonin receptor Mel1c 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.

Evidence for melatonin synthesis in the rat brain during development

Melatonin production is not restricted to the pineal gland. Several extrapineal sources of this indole such as retina, Harderian gland, and immune system are well documented. Melatonin of pineal origin is not present in the rat at early stages of development. To assess the potential capacity of local melatonin synthesis by the immature brain and to gain insight into the relationship between melatonin production by the brain (without the pineal gland) and pineal gland during rat development, the melatonin content as well as the expression and activity of the melatonin-synthesizing enzymes, N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), were studied at fetal and postnatal stages. Moreover, melatonin-membrane receptor (MT(1)) expression was also analyzed. Both, the expression and activity of NAT and HIOMT were found in the brain with significant day/night differences in enzymes activities. Additionally, melatonin content was detected in all stages showing day/night differences depending on the stage of development. The brain nocturnal melatonin content was higher than diurnal content on postnatal day 16 and in adult rats which is in accordance with the pineal melatonin synthesis. To investigate the origin of this brain melatonin, pinealectomized rats were used and we found that the developing brain produced its own melatonin. Also, MT(1) expression was detected in brain during development. These results demonstrate that, when the pineal is not yet producing melatonin, there is melatonin synthesis by the brain that could be used as protection from free radical damage and/or could exert some actions through MT(1) receptors.

Regulation of the ovine MT1 melatonin receptor promoter: interaction between multiple pituitary transcription factors at different phases of development

Pineal secretion of melatonin provides a neuroendocrine representation of the light-dark cycle, which is used to synchronise daily and annual rhythms of physiology and behaviour. In mammals, melatonin primarily acts through MT(1) melatonin receptors that exhibit a highly restricted tissue distribution. Expression of MT(1) receptors is subject to developmental and circadian control, which likely modulates the physiological actions of melatonin. To investigate the mechanisms controlling MT(1) expression we cloned the proximal 1.5kb region of the ovine MT(1) promoter. Sequence analysis revealed putative cis-elements for transcription factors involved in pituitary development, namely Pitx-1 and Egr-1, and multiple putative E-boxes, which are involved in both circadian and developmental gene regulation. Nuclear protein from ovine pars tuberalis (PT) cells, a site of high endogenous MT(1) expression, stimulated gene expression from a MT(1) expression construct, indicating the presence of a functional promoter. Pitx-1 was strongly expressed in the ovine PT and stimulated MT(1) promoter activity in transfection assays. Co-transfection with Egr-1 induced promoter-specific effects: Pitx-1-stimulated MT(1) activity was inhibited, whereas betaLH promoter activity was enhanced. In addition to Pitx-1 the circadian clock genes Clock and Bmal1 were also expressed in the PT. However, despite multiple putative E-boxes in the MT(1) promoter, transfected Clock and Bmal1 were unable to regulate either basal or Pitx-1-stimulated MT(1) promoter activity. The current data, in conjunction with our previous study of the rat MT(1) promoter, suggests a general model in which melatonin receptor expression in the mammalian pituitary is determined by the developmentally changing balance between stimulatory and inhibitory transcription factors. Furthermore, our data suggest that circadian variation in MT(1) gene expression does not depend upon the direct action of circadian clock genes on E-box cis-elements.

Diurnal and circadian regulation of a melatonin receptor, MT1, in the golden rabbitfish, Siganus guttatus

The golden rabbitfish Siganus guttatus is a reef fish with a restricted lunar-synchronized spawning rhythmicity and releases gametes simultaneously around the first quarter moon period during the spawning season. In order to understand the molecular aspects of the "circa" rhythms in this species, the full-length melatonin receptor (MT1) cDNA was cloned, and its diurnal/circadian regulation was examined. The full-length MT1 cDNA (1257 bp) contained an open reading frame that encodes a protein of 350 amino acids; this protein is highly homologous to MT1 of nonmammalian species. A high expression of MT1 mRNA with a day-night difference was observed in the whole brain, retina, liver, and kidney. When diurnal variations in MT1 mRNA expression in the retina and whole brain were examined using real-time quantitative RT-PCR, an increase in the mRNA expression was observed during nighttime in both tissues under conditions of light/dark, constant darkness, and constant light. This suggests that MT1 mRNA expression is under circadian regulation. The expression of MT1 mRNA in the cultured pineal gland also showed diurnal variations with high expression levels during nighttime; this suggests that the increased expression level observed in the whole brain is partially of pineal origin. Alternation of light conditions in the pineal gland cultures resulted in the changes in melatonin release into the culture medium as well as MT1 mRNA expression in the pineal gland. The present results suggest that melatonin and its receptors play an important role in the exertion of daily and circadian variations in the neural tissues.

Melatonin induces gene-specific effects on rhythmic mRNA expression in the pars tuberalis of the Siberian hamster (Phodopus sungorus)

In mammals, circadian and photoperiodic information is encoded in the pineal melatonin signal. The pars tuberalis (PT) of the pituitary is a melatonin target tissue, which transduces photoperiodic changes and drives seasonal changes in prolactin secretion from distal lactotroph cells. Measurement of photoperiodic time in the PT is believed to occur through melatonin dependent changes in clock gene expression, although it is unclear whether the PT should be considered a melatonin sensitive peripheral oscillator. We tested this hypothesis in the Siberian hamster (Phodopus sungorus) firstly by investigating the effects of melatonin injection, and secondly by determining whether temporal variation in gene expression within the PT persists in the absence of a rhythmic melatonin signal. Hamsters preconditioned to long days were treated with melatonin during the late light phase, to advance the timing of the nocturnal melatonin peak, or placed in constant light for one 24 h cycle, thereby suppressing endogenous melatonin secretion. Gene expression in the PT was measured by in situ hybridization. We show that melatonin rapidly induces cry1 mRNA expression without the need for a prolonged melatonin-free interval, acutely inhibits mt1 expression, advances the timing of peak rev-erb alpha expression and modulates per1 expression. With the exception of cry1, these genes continue to show temporal changes in expression over a first cycle in the absence of a melatonin signal. Our data are consistent with the hypothesis that the hamster PT contains a damped endogenous circadian oscillator, which requires a rhythmic melatonin signal for long-term synchronization.

The regional and cellular expression profile of the melatonin receptor MT1 in the central dopaminergic system

The physiological effects of pineal melatonin are primarily mediated by melatonin receptors located in the brain and periphery. Even though there are a number of studies demonstrating the regulatory role of melatonin in the development of dopaminergic behaviors, such as psychostimulant-induced diurnal locomotor sensitization or drug seeking, little is known about the contribution of melatonin receptors (i.e., MT1) to this role. Therefore, as a first step in understanding the functional role of melatonin receptors in dopaminergic behaviors, we focused on determining the expression pattern of MT1 receptors in the dopaminergic system of the human and rodent brain. Regional (e.g., nucleus accumbens shell) and cellular (e.g., tyrosine hydroxylase immunopositive cells) expression of MT1 mRNA was characterized by applying the immuno-laser capture microdissection (immuno-LCM) technique coupled with nested RT-PCR. Moreover, employing quantitative Western immunoblotting and RT-PCR, we found that the mouse MT1 receptor expression presents diurnal variations (i.e., low mRNA and high protein levels at night, ZT21). The dopaminergic system-based presence of MT1 receptor proteins was not limited to rodents; we found these receptors in postmortem human brain as well. Further research is needed to understand the regional/cellular functional role of melatonin receptors in the regulation of dopaminergic behaviors, using models such as melatonin receptor knockout mice.

Stimulation of Melatonin Receptors Decreases Calcium Levels in Xenopus Tectal Cells by Activating GABAC Receptors

To investigate the physiological effects of melatonin receptors in the Xenopus tectum, we have used the fluorescent indicator Fluo-4 AM to monitor calcium dynamics of cells in tectal slices. Bath application of KCl elicited fluorescence increases that were reduced by melatonin. This effect was stronger at the end of the light period than at the end of the dark period. Melatonin increased γ-aminobutyric acid-C (GABAC)–receptor activity, as demonstrated by the ability of the GABAC-receptor antagonists, picrotoxin and TPMPA, to abolish the effects of melatonin. In contrast, neither the GABAA-receptor antagonist bicuculline nor the GABAB-receptor antagonist CGP 35348 diminished the effects of melatonin. RT-PCR analyses revealed expression of the 3 known melatonin receptors, MT1 (Mel1a), MT2 (Mel1b), and Mel1c. Because the effect of melatonin on tectal calcium increases was antagonized by an MT2-selective antagonist, 4-P-PDOT, we performed Western blot analyses with an antibody to the MT2 receptor; the data indicate that the MT2 receptor is expressed primarily as a dimeric complex and is glycosylated. The receptor is present in higher amounts at the end of the light period than at the end of the dark period, in a pattern complementary to the changes in melatonin levels, which are higher during the night than during the day. These results imply that melatonin, acting by MT2 receptors, modulates GABAC receptor activity in the optic tectum and that this effect is influenced by the light–dark cycle.

Transition from embryonic to adult transcription pattern of serotonin N-acetyltransferase gene in avian pineal gland

The study reports the change of transcription pattern of serotonin N-acetyltransferase gene and melatonin receptor genes during ontogenesis of the avian pineal gland. The RT-PCR technique was used to investigate the expression of the arylalkylamine N-acetyltransferase (AA-NAT) and melatonin receptor genes during development of the pineal glands isolated from Japanese quail (Coturnix coturnix japonica) embryos incubated from 3 days on until hatching (17 days), and in some organs (pineal, brain hemisphere, eye, leg, heart) of the 3-day-old quail embryo. It was shown that two phases of AA-NAT expression are observed during pineal gland development. The first, embryonic-type phase, lasts from the beginning until 7-10 days of incubation, and is marked by the presence of two RT-PCR products for AA-NAT: the shorter mature form without intron (238 bp), and the longer form (323 bp) containing an unprocessed intron of 85 bp. The second, adult-type phase is characterized by the presence of a single mature transcript, containing no intron; it starts from 7 to 10 days of incubation and lasts until hatching and in the adult pineal. The duration of this transition time from the embryonic to the adult transcription pattern in the quail pineal gland from 7 to 10 days of incubation we attribute to asynchronic embryo development, because quail chicks usually hatch between the 16th and 19th day of incubation. Analysis of the AA-NAT protein sequences for chick and quail (GeneBank accession no. U 46 502 and AF 007 068, respectively) revealed their perfect homology with the part of protein read from the sequence present in the adult-type phase of the pineal gland (the RT-PCR product of 238 bp). The presence of the intron (in the 323 bp RT-PCR product, accession no. AY 197 460) in the embryonic-phase of the pineal gland changes the reading frame of the mRNA sequence and the hypothetical resulting protein loses its homology with the chick and quail AA-NAT enzyme starting with 105th amino acid of the complete chick AA-NAT protein comprising 205 amino acids (accession no. U 46 502). In the whole embryos at stages 1-8 (according to the Hamburger-Hamilton classification) both RT-PCR products with and without intron were consistently found, and individual tissues from 3-day-old embryos also produced two AA-NAT products, i.e., the expression was of the embryonic-type. At the time of transition from the embryonic to the adult AA-NAT transcription pattern, in 7-11-day-old embryos, all three melatonin receptor transcripts (mel-1a, mel-1b, and mel-1c) were observed in the pineals, without consistent modifications of the band intensity. In the adult pineal, a single mature AA-NAT transcript was present as well as all three melatonin receptor transcripts, usually with preferential expression of the mel-1a band. The transition time from the embryonic to adult AA-NAT expression pattern coincides well with the acquisition of functional activity and the appearance of melatonin synthesis in the embryonic pineal reported for chicken, as related to quail. We suggest that the change in transcription pattern of the AA-NAT gene may reflect another, still unknown mechanism of regulating AA-NAT activity during ontogenesis, at the level of mRNA processing, whose specificity (or not) for embryonic development we wish to establish in the future.

MTNR1A melatonin receptors in the ovine premammillary hypothalamus: day-night variation in the expression of the transcripts

Melatonin regulation of reproductive functions in sheep is mediated by action in the premammillary hypothalamus (PMH). The aim of this study was to identify the high-affinity melatonin-receptor subtypes expressed in this structure. To achieve this, we used reverse transcription-polymerase chain reaction (RT-PCR) and developed in situ hybridization techniques (ISH). By using RT-PCR, we detected a band corresponding to the MTNR1A melatonin-receptor cDNA in the PMH as well as in the pars tuberalis (PT). On the opposite, MTNR1B melatonin-receptor transcripts were not detected using degenerate primers in any of the structures considered, confirming the lack of expression of this receptor subtype in sheep. The expression of MTNR1A mRNA was further confirmed in the PMH by ISH with a 35S-labeled ovine MTNR1A riboprobe. We next investigated the variation in the expression of MTNR1A mRNA between the end of the day and the end of the night (absence and presence of melatonin, respectively). MTNR1A transcript expression was greater at the end of the night than at the end of the day in the PMH. In contrast, MTNR1A mRNA expression was lower at the end of the night than at the end of the day in the PT. No significant variation in the MTNR1A mRNA expression was observed in a more dorsal hypothalamic area. Overall, these results show that MTNR1A transcripts are expressed in the ovine PMH and that their expression follows a diurnal rhythm, which is different from the pattern of expression observed in the PT.

Repression of gonadotropin-releasing hormone (GnRH) gene expression by melatonin may involve transcription factors COUP-TFI and C/EBP beta binding at the GnRH enhancer

Melatonin activates membrane-bound G-protein-coupled receptors mt1 and MT2, but may also bind a family of orphan nuclear receptors, including RORalpha and RZRbeta, representing another potential molecular mechanism of melatonin action. Recently, we demonstrated that melatonin downregulates gonadotropin-releasing hormone (GnRH) gene expression in the GT1-7 cell line, specifically at the level of the neuron-specific GnRH gene enhancer. In this study, we have examined the region located at -1736/-1728 of the GnRH enhancer shown to be involved in the repression of GnRH by melatonin. This region includes hexameric consensus binding sites for orphan nuclear receptors, including ROR/RZR and COUP-TFI, as well as other defined consensus binding sites for AP-1 and C/EBP. Using electrophoretic mobility shift analysis (EMSA), we have demonstrated that GT1-7 nuclear proteins bind specifically to this region of the GnRH enhancer to form 4 complexes. EMSA antibody supershift analysis indicates that the transcription factors COUP-TFI and C/EBP beta bind two specific complexes, but RORalpha, Oct-1, Pbx-1, c-fos, or c-jun antibodies failed to produce any detectable supershifts. These results provide the first evidence that melatonin may mediate its direct neuroendocrine control of GnRH gene expression through transcription factor binding at specific regions of the GnRH enhancer.

Involvement of melatonin metabolites in the long-term inhibitory effect of the hormone on rat spinal nociceptive transmission

There is evidence that melatonin and its metabolites could bind to nuclear sites in neurones, suggesting that this hormone is able to exert long-term functional effects in the central nervous system via genomic mechanisms. This study was designed to investigate (i) whether systemically administered melatonin can exert long-term effects on spinal cord windup activity, and (ii) whether blockade of melatonin degradation with eserine could prevent this effect. Rats receiving melatonin (10 mg/kg ip), the same dose of melatonin plus eserine (0.5 mg/kg ip), or saline were studied. Seven days after administration of the drugs or saline, spinal windup of rats was assessed in a C-fiber reflex response paradigm. Results show that rats receiving melatonin exhibited a reduction in spinal windup activity. This was not observed in the animals receiving melatonin plus eserine or saline, suggesting a role for melatonin metabolites in long-term changes of nociceptive transmission in the rat spinal cord.

Melatonin receptor signaling in pregnant and nonpregnant rat uterine myocytes as probed by large conductance Ca2+-activated K+ channel activity

The mRNAs of MT1 and MT2 melatonin receptors are present in cells from nonpregnant (NPM) and pregnant (PM) rat myometrium. To investigate the coupling of melatonin receptors to Gq- and Gi-type of heterotrimeric G proteins, we analyzed the activity of large-conductance Ca2+-activated K+ (BKCa) channels, the expression of which in the uterus is confined to smooth muscle cells. The melatonin receptor agonist 2-iodomelatonin induced a pertussis toxin (PTX)-insensitive increase in channel open probability that was blocked by the nonselective antagonist luzindole. The 2-iodomelatonin effect on channel open probability was suppressed by overexpression of the Gqalpha-inactivating protein RGS16 and the phospholipase C inhibitor U-73122. The activity of BKCa channels is differentially regulated by protein kinase A (PKA) in NPM and PM cells. Thus, the beta-adrenoceptor agonist isoprenaline inhibited the BKCa channel conducted whole-cell outward current (Iout) in NPM cells and enhanced Iout in PM cells. Additional application of 2-iodomelatonin antagonized the isoprenaline effect on Iout in NPM cells but enhanced Iout in PM cells. All 2-iodomelatonin effects on Iout were sensitive to PTX treatment and the PKA inhibitor H-89. We therefore conclude that melatonin activates both the PTX-insensitive Gq/phospholipase C/Ca2+ and the PTX-sensitive Gi/cAMP/PKA signaling pathway in rat myometrium.

Melatonin receptors and their regulation: biochemical and structural mechanisms

There is growing evidence demonstrating the complexity of melatonin's role in modulating a diverse number of physiological processes. This complexity could be attributed to the fact that melatonin receptors belong to two distinct classes of proteins, that is, the G-protein coupled receptor superfamily (MT(1), MT(2)) and the quinone reductase enzyme family (MT(3)) which makes them unique at the molecular level. Also, within the G-protein coupled receptor family of proteins, the MT(1) and MT(2) receptors can couple to multiple and distinct signal transduction cascades whose activation can lead to unique cellular responses. Also, throughout the 24-hour cycle, the receptors' sensitivity to specific cues fluctuates and this sensitivity can be modulated in a homologous fashion, that is, by melatonin itself, and in a heterologous manner, that is, by other cues including the photoperiod or estrogen. This sensitivity of response may reflect changes in melatonin receptor density that also occurs throughout the 24-hour light/dark cycle but out of phase with circulating melatonin levels. The mechanisms that underlie the changes in melatonin receptor density and function are still not well-understood, but data is beginning to show that transcriptional events and G-protein uncoupling may be involved. Even though this area of research is still in its infancy, great strides are being made everyday in elucidating the mechanisms that underlie melatonin receptor function and regulation. The focus of this review is to highlight some of these discoveries in an attempt to reveal the uniqueness of the melatonin receptor family while at the same time provide thought-provoking ideas to further advance this area of research. Thus, a brief overview of each of the mammalian melatonin receptor subtypes and the signal transduction cascades to which they couple will be discussed with a greater emphasis placed on the mechanisms underlying their regulation and the domains within the receptors essential for proper signaling.

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.

Overexpression of the MT1 melatonin receptor in MCF-7 human breast cancer cells inhibits mammary tumor formation in nude mice

Overexpression of the MT1 melatonin receptor in MCF-7 human breast cancer cells significantly enhances the response of these cells to the growth-inhibitory actions of melatonin. Athymic nude mice implanted with MT1-overexpressing MCF-7 cells developed significantly fewer palpable tumors (60% reduction) compared to mice receiving vector-transfected MCF-7 cells (vt-MCF-7). In response to exogenous melatonin, tumor incidence in the mice receiving the MT1-overexpressing MCF-7 cells was decreased by 80% compared to mice receiving vt-MCF-7 cells. Interestingly, daily melatonin administration did not decrease tumor incidence in mice receiving vt-MCF-7 cells, but rather stimulated overall tumor formation.

The mt1 melatonin receptor and RORbeta receptor are co-localized in specific TSH-immunoreactive cells in the pars tuberalis of the rat pituitary

The pars tuberalis (PT) of the pituitary represents an important target site for the time-pacing pineal hormone melatonin because it expresses a large number of mt1 receptors. Functional studies suggest that the PT mediates the seasonal effects of melatonin on prolactin (PRL) secretion. The aim of this study was the characterization of the phenotype of melatonin-responsive cells. Furthermore, we determined whether RORbeta, a retinoid orphan receptor present in the PT, was co-expressed in the same cells. We combined nonradioactive in situ hybridization (ISH) with hapten-labeled riboprobes for detection of the receptors and immunocytochemistry (ICC) for detection of alphaGSU (alpha-glycoprotein subunit), betaTSH, betaFSH, betaLH, GH, PRL, and ACTH. Expression of mt1 mRNA was found in small round cells, co-localized with alphaGSU and betaTSH. However, not all betaTSH-containing cells expressed mt1 mRNA. The distribution of mt1- and RORbeta-positive cells appeared to overlap, although more cells were labeled for RORbeta than for mt1. Gonadotrophs, as well as other pars distalis cell types, were never labeled for mt1 melatonin receptor. Therefore, this study identifies the "specific" cells of the PT as the mt1 melatonin receptor-expressing cells.

MT1 melatonin receptor mRNA expression exhibits a circadian variation in the rat suprachiasmatic nuclei

The aim of the present study was to investigate the daily regulation of both MT1 and MT2 melatonin receptor subtype mRNA expression in the rat SCN in order to clarify their role in the daily variation of SCN melatonin receptors. Existing MT1 and MT2 partial clones were extended by PCR to 982 and 522 bp, respectively. However, while the MT1 clone allowed us to set up a highly sensitive in situ hybridization (ISH) method, we could not detect MT2 expression within the SCN. Therefore, our results suggest that only MT1 mRNA can be correlated with 2-iodo-melatonin binding sites in the rat SCN. Investigation of MT1 mRNA expression throughout the 24 h light/dark cycle or in constant darkness clearly showed that in the two conditions, mRNA expression showed a robust rhythm with two peaks, one after the day/night and one after the night/day transitions in LD, and at the beginning of the subjective night and day in DD, respectively. Furthermore, these variations were not linked to the daily changes in melatonin receptor density. Thus, the transcriptional regulation of MT1 receptors does not appear to play a role in the daily regulation of melatonin binding sites availability.

Melatonin receptor activation regulates GnRH gene expression and secretion in GT1-7 GnRH neurons. Signal transduction mechanisms

Melatonin plays a significant role in the control of the hypothalamic-pituitary-gonadal axis. Using the GT1-7 cell line, an in vitro model of GnRH-secreting neurons of the hypothalamus, we examined the potential signal transduction pathways activated by melatonin directly at the level of the GT1-7 neuron. We found that melatonin inhibits forskolin-stimulated adenosine 3'-, 5'-cyclic monophosphate accumulation in GT1-7 cells through an inhibitory G protein. Melatonin induced protein kinase C activity by 1.65-fold over basal levels, increased the phosphorylation of extracellular signal-regulated kinase 1 and 2 proteins, and activated c-fos and junB mRNA expression in GT1-7 cells. Using the protein kinase A inhibitor H-89, the protein kinase C inhibitor bisindolylmaleimide, and the mitogen-activated protein kinase kinase inhibitor PD98059, we found that the melatonin-mediated cyclical regulation of GnRH mRNA expression may involve the protein kinase C and the extracellular signal-regulated kinase 1 and 2 pathways, but not the protein kinase A pathway. We found that melatonin suppresses GnRH secretion by approximately 45% in the GT1-7 neurons. However, in the presence of the inhibitors H-89, bisindolylmaleimide, and PD98059 melatonin was unable to suppress GnRH secretion. These results provide insights into the potential signal transduction mechanisms involved in the control of GnRH gene expression and secretion by melatonin.

24h variation in the expression of the mt1 melatonin receptor subtype in coronary arteries derived from patients with coronary heart disease

Previous studies presented evidence for impaired nocturnal secretion and synthesis of melatonin in patients with coronary heart disease (CHD). This study aimed to investigate whether the melatonin receptor subtype mt1 is differentially expressed in coronary arteries derived from patients with CHD (n = 9) compared to patients with dilative cardiomyopathy (CMP; n = 10) who served as controls. Expression of the mt1 receptor was studied in sections of isolated coronary arteries by a reverse transcriptase-polymerase chain reaction (RT-PCR) and a Western immunoblot technique. In addition, the data from the Western blotting of 15 patients were interpolated against the exact time of aortic clamp to study the 24h expression of the mt1 receptor. The analyses of the results from both methods indicated the presence of the mt1 receptor in all of the individuals. No statistically significant difference was observed in the receptor expression between patients with CHD and those with CMP (in arbitrary units: 3.39 +/- 3.08 versus 3.91 +/- 2.78). Expression of the melatonin receptor in the coronary arteries of the whole patient group presented a 24h variation, with the lowest values detectable after 02:00 up to the late morning hours and a progressive increase beginning after 13:00 until 00:00 (mesor = 3.66, amplitude = 3.23, acrophase = 20.45, P = .0003). When studying the 24h variation in patients with CHD and CMP separately, a nearly similar circadian course was observed. In conclusion, we demonstrated for the first time a 24h variation of a melatonin receptor subtype in human vessels. Furthermore, in relation to our results, we suggest that the expression of the mt1 melatonin receptor in the coronary arteries is probably not impaired in patients with CHD.

Melatonin receptor RNA is expressed in photoreceptors and displays a diurnal rhythm in Xenopus retina

Melatonin is an output signal of an endogenous circadian clock of retinal photoreceptors, with highest levels occurring at night. Melatonin synthesized in the retina appears to act as a paracrine signal by binding to specific receptors in the eye. We have previously demonstrated that RNA encoding the Mel(1b) and Mel(1c) melatonin receptor subtypes is expressed in the Xenopus laevis retina. The goal of this study was to determine the distribution of the Mel(1b) and Mel(1c) receptor subtype RNA expression in the retina, and to determine if the level of expression of these receptors exhibits a diurnal rhythm. Sections of frog neural retina were analyzed by in situ hybridization with 35S-labeled Xenopus Mel(1c) and Mel(1b) riboprobes. Hybridization was present in cells of the inner nuclear layer and the ganglion cell layer. Moreover, there was hybridization in the photoreceptors, which has not been previously reported. To test the hypothesis that retinal melatonin receptor mRNA undergoes a diurnal rhythm of expression, total RNA was isolated from frog neural retinas obtained at 3-h intervals during a 24-h period. The total RNA was used in real-time PCR assays to quantify the differences in Mel(1b) and Mel(1c) receptor mRNA expression at various circadian times. Both the Mel(1b) and Mel(1c) receptor RNA demonstrated a diurnal rhythm of expression, with peak levels occurring late in the light period, and lowest levels late in the dark period. These results support the hypothesis that RNA encoding melatonin receptors undergo a diurnal rhythm of expression. To further investigate the possible expression of the Mel(1a) receptor subtype in Xenopus retina, we generated Mel(1a) PCR products in genomic DNA, and in reverse-transcribed neural retina and retinal pigment epithelium (RPE) RNA. The identity of the PCR product was confirmed by sequencing. Therefore, all three known Xenopus melatonin receptor subtypes appear to be expressed in the neural retina and RPE.

The circadian clock, light/dark cycle and melatonin are differentially involved in the expression of daily and photoperiodic variations in mt(1) melatonin receptors in the Siberian and Syrian hamsters

Mechanisms underlying the daily and photoperiodic variations in mt(1) melatonin receptors were investigated in the pars tuberalis (PT) and suprachiasmatic nuclei (SCN) of Siberian and Syrian hamsters. Whatever its daily profile, melatonin receptor density was strongly increased in both structures and species after constant light exposure or pinealectomy, and decreased after a single melatonin injection, indicating melatonin involvement in the daily regulation of the receptor protein. This was confirmed by a strong inverse correlation between melatonin binding capacity and plasma melatonin concentration. In contrast, regulation of mt(1) mRNA appeared more complex. The circadian clock, the light/dark cycle and melatonin are all implicated in mt(1) gene daily fluctuations, but the extent of their involvement depends upon the structure and the species studied. The photoperiodic decrease in melatonin receptor density observed in short photoperiod (PT of the two hamster species and Syrian hamster SCN) seems to be the consequence of a long-term mt(1) gene repression induced by the lengthening of the melatonin peak. Altogether, these results show that during daily variations, mt(1) melatonin receptor mRNA and protein are differentially regulated, while at the photoperiodic level, the mt(1) protein status depends on mRNA transcription.