Melatonin MT2 receptor is expressed and potentiates contraction in human airway smooth muscle

Nocturnal asthma is characterized by heightened bronchial reactivity at night, and plasma melatonin concentrations are higher in patients with nocturnal asthma symptoms. Numerous physiological effects of melatonin are mediated via its specific G protein-coupled receptors (GPCRs) named the MT1 receptor, which couples to both Gq and Gi proteins, and the MT2 receptor, which couples to Gi. We investigated whether melatonin receptors are expressed on airway smooth muscle; whether they regulate intracellular cyclic AMP (cAMP) and calcium concentrations ([Ca2+]i), which modulate airway smooth muscle tone; and whether they promote airway smooth muscle cell proliferation. We detected the mRNA and protein expression of the melatonin MT2 but not the MT1 receptor in native human and guinea pig airway smooth muscle and cultured human airway smooth muscle (HASM) cells by RT-PCR, immunoblotting, and immunohistochemistry. Activation of melatonin MT2 receptors with either pharmacological concentrations of melatonin (10-100 µM) or the nonselective MT1/MT2 agonist ramelteon (10 µM) significantly inhibited forskolin-stimulated cAMP accumulation in HASM cells, which was reversed by the Gαi protein inhibitor pertussis toxin or knockdown of the MT2 receptor by its specific siRNA. Although melatonin by itself did not induce an initial [Ca2+]i increase and airway contraction, melatonin significantly potentiated acetylcholine-stimulated [Ca2+]i increases, stress fiber formation through the MT2 receptor in HASM cells, and attenuated the relaxant effect of isoproterenol in guinea pig trachea. These findings suggest that the melatonin MT2 receptor is expressed in ASM, and modulates airway smooth muscle tone via reduced cAMP production and increased [Ca2+]i.

Melatonin receptor ligands: A pharmaco-chemical perspective

Melatonin MT₁ and MT₂ receptor ligands have been vigorously explored for the last 4 decades. Inspection of approximately 80 publications in the field revealed that most melatonergic ligands were structural analogues of melatonin combining three essential features of the parent compound: an aromatic ring bearing a methoxy group and an amide side chain in a relative arrangement similar to that present in melatonin. While several series of MT₂ -selective agents-agonists, antagonists, or partial agonists-were reported, the field was lacking MT₁ -selective agents. Herein, we describe various approaches toward the development of melatonergic ligands, keeping in mind that most of the molecules/pharmacophores obtained were essentially melatonin copies, even though diverse tri- or tetra-cyclic compounds were explored. In addition to lack of structural diversity, only few studies examined the activity of the reported melatonergic ligands in vivo. Moreover, an extensive pharmacological characterization including biopharmaceutical stability, pharmacokinetic properties, specificity toward other major receptors to name a few remained scarce. For example, many of the antagonists described were not stable in vivo, were not selective for the melatonin receptor subtype of interest, and were not fully characterized from a pharmacological standpoint. Indeed, virtual screening of large compound libraries has led to the recent discovery of potent and selective melatonin receptor agonists and partial agonists of new chemotypes. Having said this, the melatonergic field is still lacking subtype-selective melatonin receptor antagonists "active" in vivo, which are critical to our understanding of melatonin and melatonin receptors' role in basic physiology and disease.

Characterization of the various functional pathways elicited by synthetic agonists or antagonists at the melatonin MT1 and MT2 receptors

Melatonin is a neurohormone that translates the circadian rhythm to the peripheral organs through a series of binding sites identified as G protein-coupled receptors MT1 and MT2. Due to minute amounts of receptor proteins in target organs, the main tool of studies of the melatoninergic system is recombinant expression of the receptors in cellular hosts. Although a number of studies exist on these receptors, studies of several signaling pathways using a large number of melatoninergic compounds are rather limited. We chose to fill this gap to better describe a panel of compounds that have been only partially characterized in terms of functionality. First, we characterized HEK cells expressing MT1 or MT2, and several signaling routes with melatonin itself to validate the approach: GTPγS, cAMP production, internalization, β-arrestin recruitment, and cell morphology changes (CellKey ® ). Second, we chose 21 compounds from our large melatoninergic chemical library and characterized them using this panel of signaling pathways. Notably, antagonists were infrequent, and their functionality depended largely on the pathway studied. This will permit redefining the availability of molecular tools that can be used to better understand the in situ activity and roles of these receptors.

Sleep well. Untangling the role of melatonin MT1 and MT2 receptors in sleep

The pharmacological potential of targeting selectively melatonin MT1 or MT2 receptors has not yet been exploited in medicine. Research using selective MT1/MT2 receptor ligands and MT1/MT2 receptor knockout mice has indicated that the activation of MT2 receptors selectively increases non-rapid eye movement (NREM) sleep whereas MT1 receptors seem mostly implicated in the regulation of REM sleep. Moreover, MT1 knockout mice show an increase in NREM sleep, while MT2 knockout a decrease, suggesting an opposite role of these two receptors. A recent paper in mice by Sharma et al (J Pineal Res, 2018, e12498) found that MT1 but not MT2 receptors are expressed on orexin neurons in the perifornical lateral hypothalamus (PFH). Moreover, after injecting melatonin or luzindole into the mouse PFH, the authors suggest that melatonin promotes NREM sleep because activates PFH MT1 receptors, which in turn inhibit orexin neurons that are important in promoting arousal and maintaining wakefulness. In this commentary, we have critically commented on some of these findings on the bases of previous literature. In addition, we highlighted the fact that no conclusions could be drawn on the melatonin receptor subtype mediating the effects of melatonin on sleep because the authors used the non-selective MT1/MT2 receptors antagonist luzindole. More solid research should further characterize the pharmacological function of these two melatonin receptors in sleep.

New MT₂ Melatonin Receptor-Selective Ligands: Agonists and Partial Agonists

The search for melatonin receptor agonists and antagonists specific towards one of the receptor subtypes will extend our understanding of the role of this system in relaying circadian information to the body. A series of compounds derived from a hit compound discovered in a screening process led to powerful agonists specific for one of the isoform of the melatonin receptor namely, MT₂. The compounds are based on a poorly explored skeleton in the molecular pharmacology of melatonin. By changing the steric hindrance of one substituent (i.e., from a hydrogen atom to a tributylstannyl group), we identified a possible partial agonist that could lead to antagonist analogues. The functionalities of these compounds were measured with a series of assays, including the binding of GTPγS, the inhibition of the cyclic AMP production, the β-arrestin recruitment, and the cell shape changes as determined by cellular dielectric spectroscopy (CellKey^®). The variations between the compounds are discussed.

[Luzindol accelerates the aging of estrous function of female rats]

In this paper, we investigated the dynamics of aging of the estrous function of female rats kept in the conditions of standard vivarium lighting and receiving luzindol - the blocker of melatonin receptors. Every three months, daily, for two weeks, vaginal smears were taken from the animals and cytological examination of the vaginal contents was conducted. Despite different mechanisms of the development of melatonin system failure (a decreased production of melatonin and a blockade of melatonin receptors), the effects of the influence on the ovulatory function are similar. In case of the blockade of melatonin receptors, the appearance of premature signs of aging of the reproductive function in rats was observed. It was manifested by an increased duration of ovulatory cycle; a decrease in the number of regular cycles; the emergence of irregular cycles; a decrease in the number of short estrous cycles and an increase of long cycles; the early development of persistent estrus.

Melatonin receptor signaling contributes to neuroprotection upon arousal from torpor in thirteen-lined ground squirrels

The brain of mammalian hibernators is naturally protected. Hibernating ground squirrels undergo rapid and extreme changes in body temperature and brain perfusion as they cycle between lengthy torpor bouts and brief periods of euthermia called interbout arousals (IBAs). Arousal from torpor to IBA occurs rapidly, but there is no evidence of brain injury accompanying this extreme physiological transition. Production of the hormone melatonin accompanies arousal, suggesting that it plays a protective role at this time. Here, we investigated mechanisms of melatonin receptor-mediated protection in the brain of the hibernating ground squirrel. We administered the competitive melatonin receptor antagonist luzindole (30 mg/kg ip) to ground squirrels at the predicted end of a torpor bout, triggering an arousal. We found that luzindole-treated animals exhibited caspase-3 activity two times higher than vehicle-treated animals in the hypothalamus at midarousal (P = 0.01), suggesting that melatonin receptor signaling is important for protection in this brain region. We also found a 30% decline in succinate-fueled mitochondrial respiration in luzindole-treated animals compared with vehicle-treated animals (P = 0.019), suggesting that melatonin receptor signaling is important for optimal mitochondrial function during arousal from torpor. The mitochondrial effects of luzindole treatment were seen only during the hibernation season, indicating that this effect is specifically important for arousal from torpor. These data provide evidence for the protective role of melatonin receptor signaling during the extreme physiological transition that occurs when a hibernating mammal arouses from torpor and provide further evidence for regional and seasonal changes in the hibernator brain.

Synthesis and functional characterization of substituted isoquinolinones as MT2-selective melatoninergic ligands

A series of substituted isoquinolinones were synthesized and their binding affinities and functional activities towards human melatonin MT₁ and MT₂ receptors were evaluated. Structure-activity relationship analysis revealed that substituted isoquinolinones bearing a 3-methoxybenzyloxyl group at C5, C6 or C7 position respectively (C5>C6>C7 in terms of their potency) conferred effective binding and selectivity toward the MT₂ receptor, with 15b as the most potent compound. Most of the tested compounds were MT₂-selective agonists as revealed in receptor-mediated cAMP inhibition, intracellular Ca²⁺ mobilization and phosphorylation of extracellular signal-regulated protein kinases. Intriguingly, compounds 7e and 7f bearing a 4-methoxybenzyloxyl group or 4-methylbenzyloxyl at C6 behaved as weak MT₂-selective antagonists. These results suggest that substituted isoquinolinones represent a novel family of MT₂-selective melatonin ligands. The position of the substituted benzyloxyl group, and the substituents on the benzyl ring appeared to dictate the functional characteristics of these compounds.

The influence od melatonin receptors antagonists, luzindole and 4-phenyl-2-propionamidotetralin (4-P-PDOT), on melatonin-dependent vasopressin and adrenocorticotropic hormone (ACTH) release from the rat hypothalamo-hypophysial system. In vitro and in vivo studies

Melatonin exerts its biological role acting via G protein-coupled membrane receptors - MT1 and MT2, as well as through cytoplasmic and/or nuclear receptors. Melatonin has previously been shown to change vasopressin (AVP) and adrenocorticotropic hormone (ACTH) secretion dependently on its concentration. To determine whether the response of vasopressinergic neurones to different concentrations of melatonin is mediated through the membrane MT1 and/or MT2 receptors, the influence of luzindole - an antagonist of both MT1 and MT2 receptors, and 4-phenyl-2-propionamidotetralin (4-P-PDOT) - a selective MT2 receptor antagonist, on melatonin-dependent AVP release from the rat hypothalamo-neurohypophysial (H-NH) system was studied in vitro (melatonin at the concentrations of 10(-9), 10(-7) and 10(-3) M) and in vivo (melatonin at the concentrations of 10(-9) and 10(-7) M). Moreover, the second goal of this study was to find out whether melatonin receptors MT1 and/or MT2 are involved in the regulation of ACTH and corticosterone secretion into the blood. We have demonstrated that melatonin, at the concentrations of 10(-9) and 10(-7) M, significantly inhibited AVP secretion from isolated rat H-NH explants when antagonists solvent (i.e. 0.1% DMSO) was present in the medium. Neither luzindole, nor 4-P-PDOT, applied without melatonin, did influence AVP release in vitro. Luzindole applied together with melatonin (10(-7) M and 10(-9) M) significantly suppressed melatonin-dependent effect, while 4-PPDOT did not eliminate the inhibitory influence of 10(-7) M and 10(-9) M melatonin on AVP secretion from isolated rat H-NH explants. Melatonin at a concentration of 10(-3) M significantly increased AVP release when the H-NH explants were incubated in the medium containing luzindole or 4-P-PDOT. Under present experimental in vivo conditions, infused intracerebroventricularly (i.c.v.) melatonin, at a concentration close to its physiological level in the blood, significantly diminished AVP secretion into the blood, however, at higher concentration (10(-7) M) it remained inactive in this process. Moreover, melatonin at both concentrations of 10(-9) M and 10(-7) M, was able to inhibit AVP secretion into the blood (and increase its neurohypophysial content) when animals were previously i.c.v. injected with 4-P-PDOT, but not with luzindole. Blood plasma concentration of ACTH was diminished significantly by 10(-7) M melatonin in DMSO-infused, but not in luzindole- or 4-P-PDOT-injected rats, however, it remained inactive in modifying the corticosterone blood plasma concentrations in any of the studied subgroups. The present study demonstrates that subtype MT1 membrane receptor may contribute to the inhibitory effect of physiological concentration of melatonin on functional regulation of vasopressinergic neurones in the rat. However, for the stimulatory effect of pharmacological dose of the hormone on AVP secretion in vitro, mechanisms different from membrane MT1/MT2 receptors are involved. The present experiment do not determines whether MT1 and/or MT2 receptors affect the function of the rat pituitary-adrenal cortex axis.

Beneficial effects of melatonin on in vitro bovine embryonic development are mediated by melatonin receptor 1

In the current study, a fundamental question, that is, the mechanisms related to the beneficial effects of melatonin on mammalian embryonic development, was addressed. To examine the potential beneficial effects of melatonin on bovine embryonic development, different concentrations of melatonin (10(-11), 10(-9), 10(-7), 10(-5), 10(-3) M) were incubated with fertilized embryos. Melatonin in the range of 10(-11) to 10(-5) M significantly promoted embryonic development both in early culture medium (CR1aa +3 mg/mL BSA) and in later culture medium (CR1aa + 6%FBS). The most effective concentrations applied in the current studies were 10(-9) and 10(-7) M. Using quantitative real-time PCR with immunofluorescence and Western blot assays, the expression of melatonin receptor MT1 and MT2 genes was identified in bovine embryos. Further studies indicate that the beneficial effects of melatonin on bovine embryo development were mediated by the MT1 receptor. This is based on the facts that luzindole, a nonselective MT1 and MT2 antagonist, blocked the effect on melatonin-induced embryo development, while 4-P-PDOT, a selective MT2 antagonist, had little effect. Mechanistic explorations uncovered that melatonin application during bovine embryonic development significantly up-regulated the expression of antioxidative (Gpx4, SOD1, bcl-2) and developmentally important genes (SLC2A1, DNMT1A, and DSC2) while down-regulating expression of pro-apoptotic genes (P53, BAX, and Caspase-3). The results obtained from the current studies provide new information regarding the mechanisms by which melatonin promotes bovine embryonic development under both in vitro and in vivo conditions.

Labeled [³H]--thymidine incorporation in the DNA of 3T3-L1 preadipocytes due to MT₂- and not MT₃- melatonin receptor

The discovery of MT₁, MT₂, and MT₃ melatonin receptors on adipose tissue cells gives grounds for considering the possibility of melatonin as a factor which influences energy storage through modulation of metabolism and adipocyte proliferation. To date only a few contradictory studies have been published on the influence of melatonin on preadipocytes. The aim of the present study is to evaluate the influence of melatonin at physiological and supraphysiological concentrations on the proliferation of 3T3-L1 murine preadipocytes after 3 and 24 hours of the experiment and to determine the participation of membrane melatonin MT₂ receptors, and for the first time--MT₃, in its melatonin action during a 24-hour experiment. The 3T3-L1 murine preadipocyte cell line were cultured with or without melatonin at 10⁻³ and 10⁻⁹ mol/L, with or without melatonin antagonists luzindole (10⁻⁴ mol/L) and prazosin (10⁻⁵ mol/L). Cell proliferation was determined by means of labeled [³H]-thymidine incorporation in the DNA of the cell. Melatonin at both physiological and supraphysiological concentrations has a stimulating effect on the number of 3T3-L1 preadipocytes. The application of luzindole inhibits the above effect of melatonin both at 10⁻³ mol/L and 10⁻⁹ mol/L concentrations (P<0.05). The presence of prazosin does not have a statistically significant influence on the effects of melatonin action. Summarizing, it has been proven that melatonin exerts a proproliferative effect on 3T3-L1 preadipocytes at physiological and supraphysiological concentrations, partially by MT₂, and not by MT₃ receptors.

[Investigation into the vestibular protective properties of melatoninergic agents]

Experiments with rats showed that melatonin (2.5 mg/kg) produces a distinct vestibular protective effect excelling promethazine (50 mg/kg) as a reference agent, and also antidepressant agomelatine (5 mg/kg) as another melatoninergic agent. Lusindol, a blocker of MT1/MT2-receptors (2.5 mg/kg), and bicuculline (1.5 mg/kg), a specific GABA-receptors antagonist, weakened the melatonin effect significantly. The results testify mediation of the melatonin action by these receptors. Whole-cell patch clamp in an experiment with convoluted oblongata sections from white nonlinear infant male rats (14-d old) disclosed that melatonin (2 mM) inhibited drastically (29 +/- 3%) the excitatory postsynaptic current caused by depolarization step in neurons of the medial vestibular nucleus. Lusindol (0.1 mM) inhibited the effect of melatonin (2 mM) significantly (71 +/- 6%) which suggests involvement of melatonin MT1/MT2-receptors.

Characterization of signaling pathways coupled to melatonin receptors in gastrointestinal smooth muscle

Melatonin, a close derivative of serotonin, is involved in physiological regulation of circadian rhythms. In the gastrointestinal (GI) system, melatonin exhibits endocrine, paracrine and autocrine actions and is implicated in the regulation of GI motility. However, it is not known whether melatonin can also act directly on GI smooth muscle cells. The aim of the present study was to determine the expression of melatonin receptors in smooth muscle and identify their signaling pathways. MT1, but not MT2 receptors are expressed in freshly dispersed and cultured gastric smooth muscle cells. Melatonin selectively activated Gq and stimulated phosphoinositide (PI) hydrolysis in freshly dispersed and cultured muscle cells. PI hydrolysis was blocked by the expression of Gq, but not Gi minigene in cultured muscle cells. Melatonin also caused rapid increase in cytosolic Ca(2+) as determined by epifluorescence microscopy in fura-2 loaded single smooth muscle cells, and induced rapid contraction. Melatonin-induced PI hydrolysis and contraction were blocked by a non-selective MT1/MT2 antagonist luzindole (1 μM), but not by a selective MT2 antagonist 4P-PDOT (100 nM), and by the PLC inhibitor U73122. MT2 selective agonist IIK7 (100 nM) had no effect on PI hydrolysis and contraction. We conclude that rabbit gastric smooth muscle cells express melatonin MT1 receptors coupled to Gq. Activation of these receptors causes stimulation of PI hydrolysis and increase in cytosolic Ca(2+), and elicits muscle contraction.

Electrophysiological effects of melatonin on mouse Per1 and non-Per1 suprachiasmatic nuclei neurones in vitro

The master circadian pacemaker in the suprachiasmatic nuclei (SCN) regulates the nocturnal secretion of the pineal hormone melatonin. Melatonin, in turn, has feedback effects on SCN neuronal activity rhythms via high affinity G protein-coupled receptors (MT(1) and MT(2) ). However, the precise effects of melatonin on the electrical properties of individual SCN neurones are unclear. In the present study, we investigated the acute effects of exogenous melatonin on SCN neurones using whole-cell patch-clamp recordings in brain slices prepared from Per1::d2EGFP-expressing transgenic mice. In current-clamp mode, bath applied melatonin, at near-physiological concentrations (1 nM), hyperpolarised the majority (63.7%) of SCN neurones tested at all times of the projected light/dark cycle. In addition, melatonin depolarised a small proportion of cells (11.0%). No differences were observed for the effects of melatonin between Per1::GFP or non-Per1::GFP SCN neurones. Melatonin-induced effects were blocked by the MT(1)/MT(2) antagonist, luzindole (1 μM) and the proportion of SCN neurones responsive to melatonin was greatly reduced in the presence of either tetrodotoxin (200 or 500 nM) or gabazine (20 μM). In voltage-clamp recordings, 1 nM melatonin increased the frequency of GABA-mediated currents. These findings indicate, for the first time, that exogenous melatonin can alter neuronal excitability in the majority of SCN neurones, regardless of whether or not they overtly express the core clock gene Per1. The results also suggest that melatonin acts mainly by modulating inhibitory GABAergic transmission within the SCN. This may explain why exogenous application of melatonin has heterogenous effects on individual SCN neurones.

Abnormal proliferation and differentiation of osteoblasts from girls with adolescent idiopathic scoliosis to melatonin

Melatonin deficiency has been postulated as an etiologic factors in adolescent idiopathic scoliosis (AIS). In previous studies, melatonin was shown to regulate skeletal growth and bone formation in both humans and rats. Although it remains controversial whether there are differences in serum melatonin level between AIS and control subjects, melatonin signaling pathway dysfunction in osteoblasts has been reported in patients with AIS. Recently, our group found that melatonin receptor 1B (MT2) gene polymorphism was associated with the occurrence of AIS. Hence, the present study investigated the effect of melatonin on AIS osteoblasts. In vitro assays were performed with osteoblasts isolated from 17 severe AIS girls and nine control subjects. The osteoblasts were exposed to different concentrations of melatonin for 3 days. The effects of melatonin on cell proliferation (as evidenced by MTT assay) and differentiation (demonstrated by alkaline phosphatase activity) were determined. In the control group, melatonin significantly stimulated osteoblasts to proliferate and differentiate. However, in the AIS group, the stimulatory effects of melatonin were not discernible. Importantly, this finding demonstrated that there is a significant difference between AIS and control osteoblasts in functional response toward melatonin. Melatonin-stimulated proliferation of control osteoblasts was inhibited by the MT2 antagonist, 4-phenyl-2-propionamidotetraline, as well as by luzindole, a nonselective melatonin receptor antagonist, suggesting that MT2 is associated with the proliferative action of melatonin. The lack of response in AIS osteoblasts might be because of dysfunction of the melatonin signaling pathway, which may contribute to the low bone mineral density and abnormal skeletal growth observed in patients with AIS.

Melatonin potentiates glycine currents through a PLC/PKC signalling pathway in rat retinal ganglion cells

In vertebrate retina, melatonin regulates various physiological functions. In this work we investigated the mechanisms underlying melatonin-induced potentiation of glycine currents in rat retinal ganglion cells (RGCs). Immunofluorescence double labelling showed that rat RGCs were solely immunoreactive to melatonin MT(2) receptors. Melatonin potentiated glycine currents of RGCs, which was reversed by the MT(2) receptor antagonist 4-P-PDOT. The melatonin effect was blocked by intracellular dialysis of GDP-beta-S. Either preincubation with pertussis toxin or application of the phosphatidylcholine (PC)-specific phospholipase C (PLC) inhibitor D609, but not the phosphatidylinositol (PI)-PLC inhibitor U73122, blocked the melatonin effect. The protein kinase C (PKC) activator PMA potentiated the glycine currents and in the presence of PMA melatonin failed to cause further potentiation of the currents, whereas application of the PKC inhibitor bisindolylmaleimide IV abolished the melatonin-induced potentiation. The melatonin effect persisted when [Ca(2+)](i) was chelated by BAPTA, and melatonin induced no increase in [Ca(2+)](i). Neither cAMP-PKA nor cGMP-PKG signalling pathways seemed to be involved because 8-Br-cAMP or 8-Br-cGMP failed to cause potentiation of the glycine currents and both the PKA inhibitor H-89 and the PKG inhibitor KT5823 did not block the melatonin-induced potentiation. In consequence, a distinct PC-PLC/PKC signalling pathway, following the activation of G(i/o)-coupled MT(2) receptors, is most likely responsible for the melatonin-induced potentiation of glycine currents of rat RGCs. Furthermore, in rat retinal slices melatonin potentiated light-evoked glycine receptor-mediated inhibitory postsynaptic currents in RGCs. These results suggest that melatonin, being at higher levels at night, may help animals to detect positive or negative contrast in night vision by modulating inhibitory signals largely mediated by glycinergic amacrine cells in the inner retina.

Melatonin protects against experimental reflux esophagitis

Reflux esophagitis (RE), a major gastrointestinal disorder results from excess exposure of the esophageal mucosa to acidic gastric juice or bile-containing duodenal contents refluxed via an incompetent lower esophageal sphincter. Recent studies implicated oxygen derived free radicals in RE induced esophageal mucosal damage resulting in mucosal inflammation. Thus, control over free radical generation and modulation of inflammatory responses might offer better therapeutic effects to counteract the severity of RE. In this context we investigated the effect of melatonin against experimental RE in rats. Melatonin pretreatment significantly reduced the haemorrhagic lesions and decreased esophageal lipid peroxidation aggravated by RE. Moreover, the depleted levels of superoxide dismutase and glutathione observed in RE were replenished by melatonin signifying its free radical scavenging properties and antioxidant effects resulting in the improvement of esophageal defense mechanism. Further melatonin repressed the upregulated levels of expression of proinflammatory cytokines like, TNF-alpha, IL-1beta and IL-6 in RE. However, increased levels of the anti-inflammatory cytokine IL-10 remained unaltered after melatonin administration signifying its immunomodulatory effect through suppression of Th1-mediated immune responses. The involvement of receptor dependent actions of melatonin against RE were also investigated with MT2 receptor antagonist, luzindole (LUZ). LUZ failed to antagonize melatonin's protective effects against RE indicating that melatonin mediated these beneficial effects in a receptor-independent fashion. Thus, esophageal mucosal protection elicited by melatonin against experimental RE is not only dependent on its free radical scavenging activity but also mediated in part through its effect on the associated inflammatory events in a receptor-independent manner.

High-throughput screening assay for new ligands at human melatonin receptors

AIM

Melatonin (MT) is a neurohormone produced and secreted primarily by the pineal gland in a circadian manner, and mainly acts through 2 receptor subtypes: MT1 and MT2 in humans. The diversity in their tissue distribution is in favor of different functions for each receptor subtype. Selective modulators are therefore required to determine the physiological roles of these melatonin receptor subtypes and their implications in pathological processes.

METHODS

A homogenous MT1/MT2 receptor binding assay was established for high-throughput screening of new ligands at the hMT1 and/or hMT2 receptors. The functional properties (agonists or antagonists) were assessed by a conventional guanosine-5'[gamma-(35)S] triphosphate (GTP-gammaS) assay.

RESULTS

Three hMT1 receptor-selective small molecule antagonists and 1 hMT2 receptor-selective small molecule antagonist with novel structural features were identified following a high-throughput screening campaign of 48,240 synthetic and natural compounds.

CONCLUSION

The findings may assist in the expansion of chemical probes to these 2 receptor subtypes.

Sympathetic nervous system modulates the ocular hypotensive action of MT2-melatonin receptors in normotensive rabbits

The aim of this study was to investigate the hypotensive effect of the melatonin analogue, N-butanoyl-2-(2-methoxy-6H-isoindolo[2,1-a]indol-11-yl)ethanamine (IIK7), through MT(2)-melatonin receptors and the involvement of the sympathetic nervous system in this action in New Zealand rabbit eyes. The topical application of melatonin or IIK7 produced a reduction in intraocular pressure of 20.2 +/- 5.3% and 38.5 +/- 3.2% respectively. This effect was concentration-dependent; it was blocked by selective MT(2) receptor antagonists and was severely diminished after chemical sympathectomy. Immunohistochemistry and western blot analysis showed the ciliary processes as the site of this action and no co-localization of MT(2)-melatonin receptor with the sympathetic nervous system was observed. The beta-adrenergic agonists, terbutaline and salbutamol, potentiated the hypotensive effect of IIK7 reducing intraocular pressure (IOP) 41.75 +/- 4.26% and 44.7 +/- 5.6% respectively. Also, IIK7 in presence of the nonspecific protein phosphatase inhibitor okadaic acid, lowered IOP 32.2 +/- 4.5% and in presence of forskolin plus 3-isobutyl-1-methylxanthine decreased IOP in 32.2 +/- 5.47%. These data suggest that the melatonin agonist IIK7 reduces intraocular pressure by acting through MT(2)-melatonin receptors presumably decreasing aqueous humour formation. Also, in the presence of beta-adrenoceptor agonists MT(2)-melatonin receptors activity increase their ability to reduce IOP.

Involvement of the cGMP pathway in mediating the insulin-inhibitory effect of melatonin in pancreatic beta-cells

Recent investigations have demonstrated an influence of melatonin on insulin secretion in pancreatic beta-cells. The effects are receptor-mediated via two parallel signaling pathways. The aim of this study was to examine the relevance of a second melatonin receptor (MT2) as well as the involvement of a third signaling cascade in mediating melatonin effects, i.e. the cyclic guanosine monophosphate (cGMP) pathway. Our results demonstrate that the insulin-inhibiting effect of melatonin could be partly reversed by preincubation with the unspecific melatonin receptor antagonist luzindole as well as by the MT2-receptor-specific antagonist 4P-PDOT (4-phenyl-2-propionamidotetraline). As melatonin is known to modulate cGMP concentration via the MT2 receptor, these data indicate transmission of the melatonin effects via the cGMP transduction cascade. Molecular investigations established the presence of different types of guanylate cyclases, cGMP-specific phosphodiesterases and cyclic nucleotide-gated channels in rat insulinoma beta-cells (INS1). Moreover, variations in mRNA expression were found when comparing day and night values as well as different states of glucose metabolism. Incubation experiments provided evidence that 3-isobutyl-1-methylxanthine (IBMX)-stimulated cGMP concentrations were significantly decreased in INS1 cells exposed to melatonin for 1 hr in a dose- and time-dependent manner. This effect could also be reversed by application of luzindole and 4P-PDOT. Stimulation with 8-Br-cGMP resulted in significantly increased insulin production. In conclusion, it could be demonstrated that the melatonin receptor subtype MT2 as well as the cGMP signaling pathway are involved in mediating the insulin-inhibiting effect of melatonin.

Melatonin antagonizes the intrinsic pathway of apoptosis via mitochondrial targeting of Bcl-2

We have recently shown that melatonin antagonizes damage-induced apoptosis by interaction with the MT-1/MT-2 plasma membrane receptors. Here, we show that melatonin interferes with the intrinsic pathway of apoptosis at the mitochondrial level. In response to an apoptogenic stimulus, melatonin allows mitochondrial translocation of the pro-apoptotic protein Bax, but it impairs its activation/dimerization The downstream apoptotic events, i.e. cytochrome c release, caspase 9 and 3 activation and nuclear vesiculation are equally impaired, indicating that melatonin interferes with Bax activation within mitochondria. Interestingly, we found that melatonin induces a strong re-localization of Bcl-2, the main Bax antagonist to mitochondria, suggesting that Bax activation may in fact be antagonized by Bcl-2 at the mitochondrial level. Indeed, we inhibit the melatonin anti-apoptotic effect (i) by silencing Bcl-2 with small interfering RNAs, or with small-molecular inhibitors targeted at the BH3 binding pocket in Bcl-2 (i.e. the one interacting with Bax); and (ii) by inhibiting melatonin-induced Bcl-2 mitochondrial re-localization with the MT1/MT2 receptor antagonist luzindole. This evidence provides a mechanism that may explain how melatonin through interaction with the MT1/MT2 receptors, elicits a pathway that interferes with the Bcl-2 family, thus modulating the cell life/death balance.

Synthesis, Enantiomeric Resolution, and Structure–Activity Relationship Study of a Series of 10,11-Dihydro-5H-Dibenzo[a,d]cycloheptene MT2 Receptor Antagonists

Racemic N-(8-methoxy-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-10-ylmethyl)acetamide (compound 5) was previously identified as a novel selective MT(2) antagonist fulfilling the requirements of pharmacophore and 3D QSAR models. In this study the enantiomers of 5 were separated by medium-pressure liquid chromatography and behaved as the racemate. Compound 5 was modified at the acylaminomethyl side chain and at position C8. The resulting analogues generally behaved as melatonin receptor antagonists (GTPgammaS test) with a modest degree of selectivity (up to 10-fold) for the MT(2) receptor. Changes at the amide side chain led to a decrease in binding affinity, whereas 8-acetyl and 8-methyl derivatives 12 and 11, respectively, were as potent as the 8-methoxy parent compound 5. Docking experiments with an MT(2) receptor model suggested binding modes consistent with the observed SARs and with the lack of selectivity of the enantiomers of 5.

Design and synthesis of new N1 and C3-substituted 4-fluoroindolic melatoninergics

A series of new C-3 and N1-substituted 4-fluorotryptamides have been prepared and tested for their ability to activate pigment granule aggregation in Xenopus laevis melanophores and bind to the recombinant human MT(1) and MT(2) melatonin receptor subtypes expressed in NIH 3T3 cells. Planar sp(2) geometry at C-3-betaC seems to decrease the population of the preferred conformation as it renders 4-fluoroindoles 4b-d weaker antagonists than their C-3-betaC-unsubstituted congeners 3a-e. This effect is not preclusively linked with the C-3 region, as the same geometry around N1 (compounds 5a-c) similarly leads to weak antagonistic action. Last, the new C-3 substituted 4-fluorotryptamides presented herein are substantially more potent than their respective N-OMe functionalized congeners, previously reported.

Design, synthesis and melatoninergic activity of new unsubstituted and β,β′-difunctionalised 2,3-dihydro-1H-pyrrolo[3,2,1-ij]quinolin-6-alkanamides

A series of new 2,3-dihydro-1H-pyrrolo[3,2,1-ij]quinolin-6-alkanamides, with and without alkyl and cycloalkyl moieties in the beta-position of the alkanamido side chain, have been prepared and tested for their ability to activate pigment granule aggregation in Xenopus laevis melanophores and bind to the recombinant human MT(1) and MT(2) melatonin receptor subtypes expressed in NIH 3T3 cells. An increase of the spacer's length in the side chain by a methylene unit (from 17d to 21d) leads to a six-fold decrease in antagonistic activity. On the other hand, the introduction of two methyl groups in the beta-position of the side chain of 17a induces agonist potency (compound 24), implying thus that the two beta-methyl groups are not only tolerated by the receptor, but constitute functional probes in its dynamic agonist-antagonist conformational equilibrium. The presence of more bulky beta-substituents, regardless of the size of the R group, compounds 24a,b, seems to lead to antagonism and to a noteworthy MT(2) subtype selectivity. Last, the new N1-C7 annulated derivatives presented herein are substantially more potent than their respective N1-C2 annulated counterparts, previously reported.

Delayed rectifier outward K+ current mediates the migration of rat cerebellar granule cells stimulated by melatonin

Melatonin (MT) may work as a neuromodulator through the associated MT receptors in the central nervous system. Previously, our studies have shown that MT increased the I(K) current via a G protein-related pathway. In the present study, patch-clamp whole-cell recording, transwell migration assays and organotypic cerebellar slice cultures were used to examine the effect of MT on granule cell migration. MT increased the I(K) current amplitude and migration of granule cells. Meanwhile, TEA, the I(K) channel blocker, decreased the I(K) current and slowed the migration of granule cells. Furthermore, the effects of MT on the I(K) current and cell migration were not abolished by pre-incubation with P7791, a specific antagonist of MT(3)R, but were eliminated by the application of the MT(2)R antagonists K185 and 4-P-PDOT. I(K) current and cell migration were decreased by the application of dibutyryl cyclic AMP (dbcAMP), which was in contrast to the MT effect on the I(K) current and cell migration. Incubation with dbcAMP essentially blocked the MT-induced increasing effect. Moreover, incubation of isolated cell cultures in the MT-containing medium also decreased the cAMP immunoreactivity in the granule cells. It is concluded, therefore, that I(K) current, downstream of a cAMP transduction pathway, mediates the migration of rat cerebellar granule cells stimulated by MT.

Design, synthesis and melatoninergic potency of new N-acyl 8,9-dihydro-4-methoxy-7H-2-benzo[de]quinolinalkanamines

A series of new N-acyl 8,9-dihydro-4-methoxy-7H-2-benzo[de]quinolinalkanamines have been prepared and tested for their ability to activate pigment granule aggregation in Xenopus laevis melanophores and bind to the recombinant human MT(1) and MT(2) melatonin receptor subtypes expressed in NIH 3T3 cells. Compounds with a single methylene spacer in the side chain (7) have no agonist activity, but are weak antagonists in the Xenopus melanophore assay, irrespectively of the size or shape of the R substituent (R=CH(3) to c-C(4)H(7)). In contrast, compounds with two (8) or three (9) methylene spacers show partial agonist activity, though this does vary with the nature of the R substituent. Interestingly, the cyclopropane and cyclobutane R substituents, which are usually linked with antagonism, render the cyclopropanecarboxamido analog 9d and its cyclobutanecarboxamido congener 9e weak agonists. It seems, therefore, that in these compounds the R substituent constitutes a functional probe in the dynamic agonist-antagonist conformational equilibrium. One of the new molecules, antagonist 8c, exhibits a noteworthy MT(2) subtype selectivity (13-fold), whereas the acetamido analog 9a (with a three methylene units spacer) also acts as an antagonist and is the only analog exhibiting MT(1) selectivity (>10-fold). In contrast to the analogous N1-C7 annulated indole derivatives, recently reported, the new C1-C8 condensed isoquinolines are not all pure antagonists. Despite their modest receptor affinity at the binding site these compounds demonstrate that the nature of the response (agonist or antagonist activity) is dependent, in this case, on both the side chain spacer's length and the size and shape of the R group.

Design and synthesis of N-(3,3-diphenylpropenyl)alkanamides as a novel class of high-affinity MT2-selective melatonin receptor ligands

A novel series of melatonin receptor ligands was discovered by opening the cyclic scaffolds of known classes of high affinity melatonin receptor antagonists, while retaining the pharmacophore elements postulated by previously described 3D-QSAR and receptor models. Compounds belonging to the classes of 2,3- and [3,3-diphenylprop(en)yl]alkanamides and of o- or [(m-benzyl)phenyl]ethyl-alkanamides were synthesized and tested on MT(1) and MT(2) receptors. The class of 3,3-diphenyl-propenyl-alkanamides was the most interesting one, with compounds having MT(2) receptor affinity similar to that of MLT, remarkable MT(2) selectivity, and partial agonist or antagonist behavior. In particular, the (E)-m-methoxy cyclobutanecarboxamido derivative 18f and the di-(m-methoxy) acetamido one, 18g, have sub-nM affinity for the MT(2) subtype, with more than 100-fold selectivity over MT(1), 18f being an antagonist and 18g a partial agonist on GTPgammaS test. Docking of 18g into a previously developed MT(2) receptor model showed a binding scheme consistent with that of other antagonists. The MT(2) expected binding affinities of the new compounds were calculated by a previously developed 3D-QSAR CoMFA model, giving satisfactory predictions.

Towards the Development of Mixed MT1-Agonist/MT2-Antagonist Melatonin Receptor Ligands

Herein we report attempts to optimize the pharmacological properties of 5-(2-hydroxyethoxy)-N-acetyltryptamine (5-HEAT), a melatonin receptor ligand previously described by us. Several 5-substituted and 2,5-disubstituted N-acyltryptamines were synthesized and evaluated in vitro for the human cloned MT(1) and MT(2) receptors. From this series of N-acyltryptamines the 2-bromo derivative (5 c) retains the interesting efficacy profile of 5-HEAT and shows increased melatonin receptor affinities; it represents one of the first examples of a high-affinity MT(1) agonist/MT(2) antagonist. Some other full agonists for both melatonin receptors which exhibit similar or increased affinity relative to that of melatonin were obtained.

The orphan GPR50 receptor specifically inhibits MT1 melatonin receptor function through heterodimerization

One-third of the approximately 400 nonodorant G protein-coupled receptors (GPCRs) are still orphans. Although a considerable number of these receptors are likely to transduce cellular signals in response to ligands that remain to be identified, they may also have ligand-independent functions. Several members of the GPCR family have been shown to modulate the function of other receptors through heterodimerization. We show that GPR50, an orphan GPCR, heterodimerizes constitutively and specifically with MT(1) and MT(2) melatonin receptors, using biochemical and biophysical approaches in intact cells. Whereas the association between GPR50 and MT(2) did not modify MT(2) function, GPR50 abolished high-affinity agonist binding and G protein coupling to the MT(1) protomer engaged in the heterodimer. Deletion of the large C-terminal tail of GPR50 suppressed the inhibitory effect of GPR50 on MT(1) without affecting heterodimerization, indicating that this domain regulates the interaction of regulatory proteins to MT(1). Pairing orphan GPCRs to potential heterodimerization partners might be of clinical importance and may become a general strategy to better understand the function of orphan GPCRs.

Mapping the Melatonin Receptor. 7. Subtype Selective Ligands Based on β-SubstitutedN-Acyl-5-methoxytryptamines and β-SubstitutedN-Acyl-5-methoxy-1-methyltryptamines

A series of beta-substituted and beta,beta-disubstituted N-acyl 5-methoxy-1-methyltryptamines and 5-methoxytryptamines have been prepared as melatonin analogues to investigate the nature of the binding site of the melatonin receptor. The affinity of analogues was determined in a radioligand binding assay using cloned human MT(1) and MT(2) receptor subtypes expressed in NIH 3T3 cells. Agonist and antagonist potency of all analogues was measured using the pigment aggregation response of a clonal line of Xenopus laevis melanophores. beta-Methylmelatonin (17a) and beta,beta-dimethylmelatonin (17b), though showing a slight decrease in binding at human receptors, show an increase in potency on Xenopus. N-Butanoyl 5-methoxy-1-methyl-beta,beta-trimethylenetryptamine (12c) is an antagonist at human MT(1) receptors but an agonist at MT(2), while N-butanoyl 5-methoxy-1-methyl-beta,beta-tetramethylenetryptamine (13c) is an antagonist at MT(1) but had no action at MT(2) and is one of the first examples of an MT(1) selective antagonist.

Analysis of structure-activity relationships for MT2 selective antagonists by melatonin MT1 and MT2 receptor models

Three-dimensional homology models of human MT(1) and MT(2) melatonin receptors were built with the aim to investigate the structure-activity relationships (SARs) of MT(2) selective antagonists. A common interaction pattern was proposed for a series of structurally different MT(2) selective antagonists, which were positioned within the binding site by docking and simulated annealing. The proposed antagonist binding mode to the MT(2) receptor is characterized by the accommodation of the out-of-plane substituents in a hydrophobic pocket, which resulted as being fundamental for the explanation of the antagonist behavior and the MT(2) receptor selectivity. Moreover, to assess the ability of the MT(2) receptor model to reproduce the SARs of MT(2) antagonists, three new derivatives of the MT(2) selective antagonist N-[1-(4-chloro-benzyl)-4-methoxy-1H-indol-2-ylmethyl]-propionamide (7) were synthesized and tested for their receptor affinity and intrinsic activity. These compounds were docked into the MT(2) receptor model and were submitted to molecular dynamics studies, providing results in qualitative agreement with the experimental data. These results confirm the importance of the out-of-plane group in receptor binding and selectivity and provide a partial validation of the proposed G protein-coupled receptor model.

Melatonin affects the temporal organization of the song of the zebra finch

In birds and mammals, including humans, melatonin-binding sites are abundant in brain areas that have no known clock function. Although the role of such binding sites is still unclear, it is assumed that these sites link neural functions to circadian or circannual demands of neuroendocrine homeostasis and reproduction. To investigate a possible direct role of melatonin in motor control, we studied the song and neural song system of the zebra finch. Neurons of two sensory-motor areas of the descending song control circuit that are crucial for the organization of the song pattern, the HVC and RA, express the melatonin-1B receptor (Mel1B), while the hypoglossal motor neurons of the song circuit express melatonin-1C receptors (Mel1C). Application of melatonin to brain slices decreases the firing-rate of RA-neurons. Systemic administration of a Mel1B antagonist at the beginning of the night shortens the song and motif length and affects the song syllable lengths produced the next day. The temporal pattern of the song, however, does not undergo daily changes. Thus, melatonin is likely to affect a non-circadian motor pattern by local modulation of song control neurons and in consequence alters a sexual signal, the song of the zebra finch.

Exposure to continuous darkness ameliorates gastric and colonic inflammation in the rat: both receptor and non-receptor-mediated processes

BACKGROUND AND AIM

Melatonin is a hormone involved in the transduction of photoperiodic information, and appears to modulate a variety of neural and endocrine functions. The present study was designed to determine the impact of continuous darkness (CD) on acute gastric and colonic inflammation and the involvement of melatonin receptors in the darkness-related alterations in oxidant gut injury.

METHODS

Rats were housed either in CD or in standardized light/dark (12/12 h) cycles for 15 days before the induction of colitis or gastric ulcer. Luzindole (MT(2) receptor antagonist) was given at a dose of 0.25 mg/kg intraperitoneally 30 min before and 6 and 18 h following the induction of colitis with acetic acid or gastric ulcer with ethanol. Rats were decapitated at 24 h, and the colons and stomachs were removed for macroscopic scoring, histologic assessment and for the determination of tissue malondialdehyde and glutathione levels.

RESULTS

All inflammation parameters were increased by acetic acid-induced colitis or ethanol-induced gastric ulcer compared with the control group. Our results indicate that the severity of both gastric and colonic injury is reduced by a 2-week exposure to CD prior to the induction of inflammatory event, while luzindole treatment reversed the protective effect of CD on the colonic and gastric injury. However, darkness-related alterations in malondialdehyde and glutathione levels were not altered by luzindole.

CONCLUSION

Although the CD-induced amelioration of gut injury involves melatonin receptors, the direct antioxidant effects on melatonin appear to be independent of receptor activity.

Tetrahydroisoquinoline derivatives as melatonin MT2 receptor antagonists

A series of tetrahydroisoquinolines has yielded potent MT(2) receptor antagonists, which are selective versus the MT(1) receptor.

Intracellular second messengers involved in melatonin signal transduction in chicken splenocytes in vitro

The pineal hormone melatonin exhibits immunomodulatory activity well documented in mammals and birds. The mechanism of melatonin action within the immune system is, however, poorly understood. In mammalian immune cells in vitro, melatonin acts mainly as an antiapoptotic, oncostatic and antiproliferative agent, and these effects are exerted via specific receptors or are related to its free radical scavenging activity. In previous studies we have found that in short-term chicken splenocyte cultures in vitro melatonin stimulated basil proliferation and inhibited that stimulated with phytohemagglutinin, a T-cell mitogen. This paper is devoted to the involvement of membrane receptors, previously characterised by us as MT2 (Mel(1b)) and Mel(1c) subtypes, in the above mentioned melatonin effects in chicken splenocyte cultures. For this purpose, in present study a nonselective melatonin receptor antagonist, luzindole, and the selective MT2 blocker, 4P-PDOT, were used. The effect of melatonin on second messengers, cyclic adenosine-3',5'-monophosphate (cAMP) and inositol-1,4,5-trisphosphate (IP(3)), involved in the regulation of proliferation, was examined. We have found that the stimulation of proliferation occurs via Mel(1c) receptor and is associated with the changes in intracellular second messengers concentration: a decrease in cAMP and an increase in IP(3). In contrast, in mitogen-activated splenocytes, melatonin-induced inhibition of proliferation is mediated by MT2 receptors and is related to cAMP accumulation, as well as a decrease in IP(3). In conclusion, we have demonstrated that the stimulatory and inhibitory effect of melatonin on chicken splenocytes in vitro, dependent on the magnitude of cell stimulation, resulted from two different subtypes of membrane receptors.

Strategies leading to MT2 selective melatonin receptor antagonists

Studies of the physiological actions of melatonin have been hindered by the lack of specific, potent and subtype selective agonists and antagonists. This paper reviews our progress in developing subtype selective melatonin antagonists. Evidence is presented suggesting the structural features conferring MT2 selective antagonism.

Tricyclic alkylamides as melatonin receptor ligands with antagonist or inverse agonist activity

This work reports the design and synthesis of novel alkylamides, characterized by a dibenzo[a,d]cycloheptene nucleus, as melatonin (MLT) receptor ligands. The tricyclic scaffold was chosen on the basis of previous quantitative structure-activity studies on MT1 and MT2 antagonists, relating selective MT2 antagonism to the presence of an aromatic substituent out of the plane of the MLT indole ring. Some dibenzo seven-membered structures were thus selected because of the noncoplanar arrangement of their benzene rings, and an alkylamide chain was introduced to fit the requirements for MLT receptor binding, namely, dibenzocycloheptenes with an acylaminoalkyl side chain at position 10 and dibenzoazepines with this side chain originating from the nitrogen atom bridging the two phenyl rings. Binding affinity at human cloned MT1 and MT2 receptors was measured by 2-[125I]iodomelatonin displacement assay and intrinsic activity by the GTPgammaS test. The majority of the compounds were characterized by higher affinity at the MT2 than at the MT1 receptor and by very low intrinsic activity values, thus confirming the importance of the noncoplanar arrangement of the two aromatic rings for selective MT2 antagonism. Dibenzocycloheptenes generally displayed higher MT1 and MT 2affinity than dibenzoazepines. N-(8-Methoxy-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-10-ylmethyl)propionamide (4c) and -butyramide (4d) were the most selective MT2 receptor antagonists of the series, with MT2 receptor affinity comparable to that of melatonin and as such among the highest reported in the literature for MLT receptor antagonists. The acetamide derivative 4b produced a noticeable reduction of GTPgammaS binding at MT2 receptor, thus being among the few inverse agonists described.

Melatonin attenuates rat carotid chemoreceptor response to hypercapnic acidosis

Respiratory activity is under circadian modulation and the physiological mechanisms may involve the pineal secretory product, melatonin, and the carotid chemoreceptor. We hypothesized that melatonin modulates the carotid chemoreceptor response to hypercapnic acidosis. To determine whether the effect of melatonin on the chemoreceptor response to hypercapnic acidosis is mediated by melatonin receptors in the chemosensitive cells, cytosolic calcium ([Ca2+]i) was measured by spectrofluorometry in fura-2-loaded glomus cells dissociated from rat carotid bodies. Melatonin (0.01-10 nm) per se did not change the [Ca2+]i levels of the glomus cells but it concentration-dependently attenuated the peak [Ca2+]i response to hypercapnic acidosis in the glomus cells. In addition, the [Ca2+]i response was attenuated by 2-iodomelatonin, an agonist of melatonin receptors. The melatonin-induced attenuation of the [Ca2+]i response to hypercapnic acidosis was abolished by pretreatment with an non-selective mt1/MT2 antagonist, luzindole, and by MT2 antagonists, 4-phenyl-2-propionamidotetraline or DH97. In situ hybridization study with antisense mt1 and MT2 receptor mRNA oligonucleotide probes showed an expression of mt1 and MT2 receptors in the rat carotid body. Also, melatonin attenuated the carotid afferent response to hypercapnic acidosis in single- or pauci-fibers recorded from the sinus nerve in isolated carotid bodies superfused with bicarbonate-buffer saline. Results suggest that an activation of the melatonin receptors expressed in the glomus cells of the rat carotid body reduces the chemoreceptor response to hypercapnic acidosis. This modulation may play a physiological role in the influence of the circadian rhythms on the chemoreflex.

New selective ligands of human cloned melatonin MT1 and MT2 receptors

Melatonin has a key role in the circadian rhythm relay to periphery organs. Melatonin exerts its multiple roles mainly through two seven transmembrane domain, G-coupled receptors, namely MT1 or MT2 receptors. A pharmacological characterization of these human cloned melatonin hMT1 and hMT2 receptors stably expressed in HEK-293 or CHO cells is presented using a 2-[125I]-iodo-melatonin binding assay and a [35S]-GTPgammaS functional assay. Both reference compounds and new chemically diverse ligands were evaluated. Binding affinities at each receptor were found to be comparable on either HEK-293 or CHO cell membranes. Novel non-selective or selective hMT1 and hMT2 ligands are described. The [35S]-GTPgammaS functional assay was used to define the functional activity of these compounds which included partial, full agonist and/or antagonist activity. None of the compounds acted as an inverse agonist. We report new types of selective antagonists, such as S 25567 and S 26131 for MT1 and S 24601 for MT2. These studies brought other new molecular tools such as the selective MT1 agonist, S 24268, as well as the non-selective antagonist, S 22153. Finally, we also discovered S 25150, the most potent melatonin receptor agonist, so far reported in the literature.

Melatonin in the duodenal lumen is a potent stimulant of mucosal bicarbonate secretion

Melatonin, originating from intestinal enterochromaffin cells, mediates vagal and sympathetic neural stimulation of the HCO secretion by the duodenal mucosa. This alkaline secretion is considered the first line of mucosal defense against hydrochloric acid discharged from the stomach. We have studied whether luminally applied melatonin stimulates the protective secretion and whether a melatonin pathway is involved in acid-induced stimulation of the secretion. Rats were anaesthetized (Inactin) and a 12-mm segment of proximal duodenum with an intact blood supply was cannulated in situ. Mucosal HCO secretion (pH-stat) and the mean arterial blood pressure were continuously recorded. Luminal melatonin at a concentration of 1.0 micro m increased (P < 0.05) the secretion from 7.20 +/- 1.35 to 13.20 +/- 1.51 micro Eq/cm/hr. The MT2 selective antagonist luzindole (600 nmol/kg, i.v.) had no effect on basal HCO secretion, but inhibited (P < 0.05) secretion stimulated by luminal melatonin. Hexamethonium (10 mg/kg i.v. followed by continuous i.v. infusion at a rate of 10 mg/kg/hr), abolishes neurally mediated rises in secretion and also inhibited (P < 0.05) the stimulation by luminal melatonin. Exposure of the lumen to acid containing perfusate (pH 2.0) for 5 min increased (P < 0.05) the HCO secretion from 5.85 +/- 0.82 to 12.35 +/- 1.51 micro Eq/cm/hr, and luzindole significantly inhibited (P < 0.05) this rise in secretion. The study thus demonstrates that luminal melatonin is a potent stimulant of duodenal HCO secretion and, furthermore, strongly suggests melatonin as an important mediator of acid-induced secretion.