Pharmacological characterization of the human melatonin Mel1a receptor following stable transfection into NIH3T3 cells

2-Arylmelatonin analogues: Probing the 2-phenyl binding pocket of melatonin MT1 and MT2 receptors

In crystal structures of melatonin MT₁ and MT₂ receptors, a lipophilic subpocket has been characterized which accommodates the phenyl ring of the potent agonist 2-phenylmelatonin. This subpocket appears a key structural element to achieve high binding affinity and selectivity for the MT₂ receptor. A series of 2-arylindole ligands was synthesized to probe the requirements for the optimal occupation and interaction with the 2-phenyl binding pocket. Thermodynamic integration simulations applied to MT₁ and MT₂ receptors in complex with the α-naphthyl derivative provided a rationale for the MT₂-selectivity and investigation on the binding mode of a couple of atropisomers allowed to define the available space and arrangement of substituents inside the subpocket. Interestingly, more hydrophilic 2-aza-substituted compounds displayed high binding affinity and molecular dynamics simulations highlighted polar interaction with residues from the subpocket that could be responsible for their potency.

Pharmacological Actions of Carbamate Insecticides at Mammalian Melatonin Receptors

Integrated in silico chemical clustering and melatonin receptor molecular modeling combined with in vitro 2-[125I]-iodomelatonin competition binding were used to identify carbamate insecticides with affinity for human melatonin receptor 1 (hMT1) and human melatonin receptor 2 (hMT2). Saturation and kinetic binding studies with 2-[125I]-iodomelatonin revealed lead carbamates (carbaryl, fenobucarb, bendiocarb, carbofuran) to be orthosteric ligands with antagonist apparent efficacy at hMT1 and agonist apparent efficacy at hMT2 Furthermore, using quantitative receptor autoradiography in coronal brain slices from C3H/HeN mice, carbaryl, fenobucarb, and bendiocarb competed for 2-[125I]-iodomelatonin binding in the suprachiasmatic nucleus (SCN), paraventricular nucleus of the thalamus (PVT), and pars tuberalis (PT) with affinities similar to those determined for the hMT1 receptor. Carbaryl (10 mg/kg i.p.) administered in vivo also competed ex vivo for 2-[125I]-iodomelatonin binding to the SCN, PVT, and PT, demonstrating the ability to reach brain melatonin receptors in C3H/HeN mice. Furthermore, the same dose of carbaryl given to C3H/HeN mice in constant dark for three consecutive days at subjective dusk (circadian time 10) phase-advanced circadian activity rhythms (mean = 0.91 hours) similar to melatonin (mean = 1.12 hours) when compared with vehicle (mean = 0.04 hours). Carbaryl-mediated phase shift of overt circadian activity rhythm onset is likely mediated via interactions with SCN melatonin receptors. Based on the pharmacological actions of carbaryl and other carbamate insecticides at melatonin receptors, exposure may modulate time-of-day information conveyed to the master biologic clock relevant to adverse health outcomes. SIGNIFICANCE STATEMENT: In silico chemical clustering and molecular modeling in conjunction with in vitro bioassays identified several carbamate insecticides (i.e., carbaryl, carbofuran, fenobucarb, bendiocarb) as pharmacologically active orthosteric melatonin receptor 1 and 2 ligands. This work further demonstrated that carbaryl competes for melatonin receptor binding in the master biological clock (suprachiasmatic nucleus) and phase-advances overt circadian activity rhythms in C3H/HeN mice, supporting the relevance of circadian effects when interpreting toxicological findings related to carbamate insecticide exposure.

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.

Chiral Recognition of Flexible Melatonin Receptor Ligands Induced by Conformational Equilibria

N-anilinoethylamides are a class of melatoninergic agents with the aniline portion mimicking the indole ring of the natural ligand and the ethylamide chain reproducing that of melatonin. The simplest compound in this class, N-{2-[(3-methoxyphenyl)methylamino]ethyl}acetamide (UCM793), has nanomolar binding affinity for MT₁ and MT₂ membrane receptors. To explore the effect of chain conformation on receptor binding, a methyl group was inserted on the methylene alpha or beta to the amide nitrogen and conformational equilibria were investigated by NMR spectroscopy and molecular dynamics simulations. Receptor affinity was conserved only for the beta-methyl derivative, which also showed significant stereoselectivity, with the (S) enantiomer being the eutomer. Molecular dynamics simulations, validated by NMR spectroscopy, showed that the beta-methyl group affects the conformational preferences of the ethylamide chain. Docking into the receptor crystal structure provides a rationale for the observed chiral recognition, suggesting that the (S)-beta-methyl group favors the conformation that better fits the receptor binding site.

The Melatonin Signaling Pathway in a Long-Term Memory In Vitro Study

The activation of cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB) via phosphorylation in the hippocampus is an important signaling mechanism for enhancing memory processing. Although melatonin is known to increase CREB expression in various animal models, the signaling mechanism between melatonin and CREB has been unknown in vitro. Thus, we confirmed the signaling pathway between the melatonin receptor 1 (MT1) and CREB using melatonin in HT-22 cells. Melatonin increased MT1 and gradually induced signals associated with long-term memory processing through phosphorylation of Raf, ERK, p90RSK, CREB, and BDNF expression. We also confirmed that the calcium, JNK, and AKT pathways were not involved in this signaling pathway by melatonin in HT-22 cells. Furthermore, we investigated whether melatonin regulated the expressions of CREB-BDNF associated with long-term memory processing in aged HT-22 cells. In conclusion, melatonin mediated the MT1-ERK-p90RSK-CREB-BDNF signaling pathway in the in vitro long-term memory processing model and increased the levels of p-CREB and BDNF expression in melatonin-treated cells compared to untreated HT-22 cells in the cellular aged state. Therefore, this paper suggests that melatonin induces CREB signaling pathways associated with long-term memory processing in vitro.

Antiproliferative and pro-apoptotic activity of melatonin analogues on melanoma and breast cancer cells

Melatonin plays different physiological functions ranging from the regulation of circadian rhythms to tumor inhibition, owing to its antioxidant, immunomodulatory and anti-aging properties. Due to its pleiotropic functions, melatonin has been shown to elicit cytoprotective processes in normal cells and trigger pro-apoptotic signals in cancer cells. The therapeutic potential of melatonin analogues prompted us to investigate the in vitro and in vivo antitumor activity of new melatonin derivatives and explore the underlying molecular mechanisms. The experiments revealed that the new melatonin analogues inhibited the growth of melanoma and breast cancer cells in a dose- and time-dependent manner. In addition, our results indicated that melatonin derivative UCM 1037 could induce apoptosis in melanoma and breast cancer cells, as well as cell necrosis, in MCF-7. Together, apoptosis and necrosis could be two possible mechanisms to explain the cytotoxic effect of the melatonin analogue against cancer cells. The suppression of tumor growth by the melatonin analogues was further demonstrated in vivo in a xenograft mice model. A decrease in the activation of MAPK pathway was observed in all cancer cells following UCM 1037 treatment. Overall, this study describes a promising antitumor compound showing antiproliferative and cytotoxic activity in melanoma and breast cancer cells.

New evidence of melatonin receptor contribution to ram sperm functionality

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

Anatomical and cellular localization of melatonin MT1 and MT2 receptors in the adult rat brain

The involvement of melatonin in mammalian brain pathophysiology has received growing interest, but information about the anatomical distribution of its two G-protein-coupled receptors, MT1 and MT2 , remains elusive. In this study, using specific antibodies, we examined the precise distribution of both melatonin receptors immunoreactivity across the adult rat brain using light, confocal, and electron microscopy. Our results demonstrate a selective MT1 and MT2 localization on neuronal cell bodies and dendrites in numerous regions of the rat telencephalon, diencephalon, and mesencephalon. Confocal and ultrastructural examination confirmed the somatodendritic nature of MT1 and MT2 receptors, both being localized on neuronal membranes. Overall, striking differences were observed in the anatomical distribution pattern of MT1 and MT2 proteins, and the labeling often appeared complementary in regions displaying both receptors. Somadendrites labeled for MT1 were observed for instance in the retrosplenial cortex, the dentate gyrus of the hippocampus, the islands of Calleja, the medial habenula, the suprachiasmatic nucleus, the superior colliculus, the substantia nigra pars compacta, the dorsal raphe nucleus, and the pars tuberalis of the pituitary gland. Somadendrites endowed with MT2 receptors were mostly observed in the CA3 field of the hippocampus, the reticular thalamic nucleus, the supraoptic nucleus, the inferior colliculus, the substantia nigra pars reticulata, and the ventrolateral periaqueductal gray. Together, these data provide the first detailed neurocytological mapping of melatonin receptors in the adult rat brain, an essential prerequisite for a better understanding of melatonin distinct receptor function and neurophysiology.

Synthesis and characterization of new bivalent agents as melatonin- and histamine H3-ligands

Melatonin is an endogenous molecule involved in many pathophysiological processes. In addition to the control of circadian rhythms, its antioxidant and neuroprotective properties have been widely described. Thus far, different bivalent compounds composed by a melatonin molecule linked to another neuroprotective agent were synthesized and tested for their ability to block neurodegenerative processes in vitro and in vivo. To identify a novel class of potential neuroprotective compounds, we prepared a series of bivalent ligands, in which a prototypic melatonergic ligand is connected to an imidazole-based H₃ receptor antagonist through a flexible linker. Four imidazolyl-alkyloxy-anilinoethylamide derivatives, characterized by linkers of different length, were synthesized and their binding affinity for human MT₁, MT₂ and H₃ receptor subtypes was evaluated. Among the tested compounds, 14c and 14d, bearing a pentyl and a hexyl linker, respectively, were able to bind to all receptor subtypes at micromolar concentrations and represent the first bivalent melatonergic/histaminergic ligands reported so far. These preliminary results, based on binding affinity evaluation, pave the way for the future development of new dual-acting compounds targeting both melatonin and histamine receptors, which could represent promising therapeutic agents for the treatment of neurodegenerative pathologies.

Melatonin MT₁ and MT₂ receptors display different molecular pharmacologies only in the G-protein coupled state

BACKGROUND AND PURPOSE

Melatonin receptors have been extensively characterized regarding their affinity and pharmacology, mostly using 2-[(125)I]-melatonin as a radioligand. Although [(3)H]-melatonin has the advantage of corresponding to the endogenous ligand of the receptor, its binding has not been well described.

EXPERIMENTAL APPROACH

We characterized [(3)H]-melatonin binding to the hMT₁ and hMT₂ receptors expressed in a range of cell lines and obtained new insights into the molecular pharmacology of melatonin receptors.

KEY RESULTS

The binding of [(3)H]-melatonin to the hMT₁ and hMT₂ receptors displayed two sites on the saturation curves. These two binding sites were observed on cell membranes expressing recombinant receptors from various species as well as on whole cells. Furthermore, our GTPγS/NaCl results suggest that these sites on the saturation curves correspond to the G-protein coupled and uncoupled states of the receptors, whose pharmacology was extensively characterized.

CONCLUSIONS AND IMPLICATIONS

hMT₁ and hMT₂ receptors spontaneously exist in two states when expressed in cell lines; these states can be probed by [(3)H]-melatonin binding. Overall, our results suggest that physiological regulation of the melatonin receptors may result from complex and subtle mechanisms, a small difference in affinity between the active and inactive states of the receptor, and spontaneous coupling to G-proteins.

A molecular and chemical perspective in defining melatonin receptor subtype selectivity

Melatonin is primarily synthesized and secreted by the pineal gland during darkness in a normal diurnal cycle. In addition to its intrinsic antioxidant property, the neurohormone has renowned regulatory roles in the control of circadian rhythm and exerts its physiological actions primarily by interacting with the G protein-coupled MT₁ and MT₂ transmembrane receptors. The two melatonin receptor subtypes display identical ligand binding characteristics and mediate a myriad of signaling pathways, including adenylyl cyclase inhibition, phospholipase C stimulation and the regulation of other effector molecules. Both MT₁ and MT₂ receptors are widely expressed in the central nervous system as well as many peripheral tissues, but each receptor subtype can be linked to specific functional responses at the target tissue. Given the broad therapeutic implications of melatonin receptors in chronobiology, immunomodulation, endocrine regulation, reproductive functions and cancer development, drug discovery and development programs have been directed at identifying chemical molecules that bind to the two melatonin receptor subtypes. However, all of the melatoninergics in the market act on both subtypes of melatonin receptors without significant selectivity. To facilitate the design and development of novel therapeutic agents, it is necessary to understand the intrinsic differences between MT₁ and MT₂ that determine ligand binding, functional efficacy, and signaling specificity. This review summarizes our current knowledge in differentiating MT₁ and MT₂ receptors and their signaling capacities. The use of homology modeling in the mapping of the ligand-binding pocket will be described. Identification of conserved and distinct residues will be tremendously useful in the design of highly selective ligands.

Gene structures, biochemical characterization and distribution of rat melatonin receptors

G-protein coupled receptors for the pineal hormone melatonin have been partially cloned from rats. However, insufficient information about their cDNA sequences has hindered studies of their distribution and physiological responses to melatonin using rats as an animal model. We have cloned cDNAs of two rat membrane melatonin receptor subtypes, melatonin receptor 1a (MT1) and melatonin receptor 1b (MT2), using a rapid amplification of cDNA end (RACE) method. The rat MT1 and MT2 cDNAs encode proteins of 353 and 364 amino acids, respectively, and show 78-93% identities with the human and mouse counterparts. Stable expression of either rat MT1 or MT2 in NIH3T3 cells resulted in high affinity 2-[(125)I]-iodomelatonin ((125)I-Mel) binding (K (d) = 73.2 +/- 9.0 and 73.7 +/- 2.9 pM, respectively), and exhibited a similar rank order of inhibition of specific (125)I-Mel binding by five ligands (2-iodomelatonin > melatonin > 6-hydroxymelatonin > luzindole > N-acetyl-5-hydroxytryptamine). RT-PCR analysis showed that MT1 is highly expressed in the hypothalamus, lung, kidney, adrenal gland, stomach, and ovary, while MT2 is highly expressed in the hippocampus, kidney, and ovary. We also performed multi-cell RT-PCR to examine the expression of mRNAs encoding MT1 and MT2 in adult GnRH neurons. MT1 was weakly expressed in male GnRH neurons, and was less expressed in the female neurons. MT2 expression was undetectable in GnRH neurons from either sex. This study delineates the gene structures, fundamental properties, and distribution of both rat melatonin receptor subtypes, and may offer opportunities to assess the physiological significance of melatonin in rats.

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.

Synthesis and biological activity of new melatonin dimeric derivatives

A new series of melatonin (MLT) dimers were obtained by linking together two melatonin units with a linear alkyl chain through the MLT acetamido group or through a C-2 carboxyalkyl function. The binding properties of these ligands were evaluated in in vivo experiments on cloned human MT(1) and MT(2) receptors expressed in NIH3T3 rat fibroblast cells. The class of 2-carboxyalkyl dimers was the most interesting one with compounds having good MT(1)/MT(2) nanomolar affinity. The data obtained suggest that the spacer length is crucial for optimal interaction at both receptor subtypes as well as to determine functional activity of the resulting dimers.

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.

Reassessing the melatonin pharmacophore—Enantiomeric resolution, pharmacological activity, structure analysis, and molecular modeling of a constrained chiral melatonin analogue

3-(Acetylaminomethyl)-2-(ethoxycarbonyl)-6-methoxy-1,3,4,5-tetrahydrobenzo[cd]indole (2) is a rigid melatonin analogue that as a racemate displays about the same affinity and intrinsic activity of melatonin (1) in in vitro experiments. We report here the resolution of the racemate by preparative medium pressure liquid chromatography (MPLC) and the X-ray determination of the R absolute configuration of the (-)-enantiomer. The two enantiomers were separately tested as MT1 and MT2 ligands, and the (+)-(S)-2 showed a potency comparable to that of melatonin and about three orders of magnitude greater than that of its enantiomer. The information obtained by crystallographic analysis and NMR studies about the conformational preference for 2 and by the pharmacological characterization of (R)-2 and (S)-2 was employed in a molecular modeling study, aimed at reassessing the melatonin receptor pharmacophore model for agonist compounds. Chiral enantioselective agonists reported in the literature were also included in the study.

Synthesis and conformational study of 3,4-carbocyclic bridged indole melatonin and serotonin analogues

Tetrahydrobenz[cd]indole, has been usually assumed to be a rigid scaffold of arylethylamines of pharmaceutical interest, such as melatonin and serotonin. A series of molecules containing this scaffold has been synthesized and their conformation in solution has been determined by 1H NMR. The values of the coupling constants show that the carbocycle fused with the indole ring is a mixture of the two conformers with substituent in equatorial or axial orientation. The molar fraction of the conformers appears to be sensibly affected by the bulkiness of the C-2 indole substituent. A pseudo-axial orientation of the C-3 alkylamido side chain is important for melatonin ligands to access the binding site and exhibit potent in vitro affinity, as illustrated for melatonin ligand 1 (pK(i)=9.32).

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.

Recent Advances in Melatonin Receptor Ligands

Melatonin is a hormone exerting its multiple actions mainly through two G-protein-coupled receptors MT(1) and MT(2). Exploring the physiological role of each of these subtypes requires subtype selective MT(1) and MT(2) ligands. While several MT(2)-selective ligands were developed in the 1990s, no selective agonists and antagonists for the MT(1) subtype were described. The present article reviews mela toninergic ligands developed in the current millennium focusing on subtype selective agents and on drug candidates. Notable compounds are the MT(1)-selective agonists 35 and 134, MT(1)-selective antagonists 117 and 131, MT(2)-selective agonists 58, 70, 79, 97 and 125, MT(2)-selective antagonists 27, 73 and 119, and the highly potent non-selective agonist 120. The non-selective agonists agomelatine 2, and ramelteon 87 are drug candidates as antidepressive agent and for the treatment of insomnia and circadian rhythm disfunction, respectively.

Binding characteristics and daily rhythms of melatonin receptors are distinct in the retina and the brain areas of the European sea bass retina (Dicentrarchus labrax)

Melatonin is synthesized, with a circadian rhythm, in the pineal organ of vertebrates, high levels being produced during the scotophase and low levels during the photophase. The retina also produces melatonin, although in the case of the European sea bass, its secretion pattern appears to be inverted. In the study described here, radioreceptor assay techniques were used to characterize the melatonin binding sites, their regional distribution and their daily variations. Brain and retina membrane preparations were used in all the binding assays and 2-[125I]iodomelatonin ([125I]Mel) as radioligand at 25 degrees C. The specific binding of [125I]Mel was seen to be saturable, reversible, specific and of high affinity. In all the tissues assayed, the power of the ligands to inhibit [125I]Mel binding decreased in the following order: melatonin>>4-P-PDOT>luzindole> or =N-acetylserotonin, which points to the presence of Mel1-like receptors. The inhibition curves of 4-P-PDOT suggested the presence of two different binding sites in the brain areas, but only one type of site of low affinity in the neural retina. No daily variations in [125I]Mel binding capacity (Bmax) or affinity (Kd) were detected in the brain areas, while a clear rhythm in Kd melatonin receptor affinity and Bmax binding capacity was observed in the retina. Kd and Bmax retinal rhythms were out of phase with the lowest Kd and the highest Bmax occurring at scotophase. This result suggests that retinal melatonin is a paracrine factor able to control receptor desensitization during photophase when ocular melatonin is higher in this species.

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.

Conformation by NMR of two tetralin-based receptor ligands

The conformation in solution of 1-phenyl-3-propionamido-1,2,3,4-tetrahydronaphthalene and 1-phenyl-3-(N,N-dimethylamino)-1,2,3,4-tetrahydronaphthalene has been determined by a combination of nuclear magnetic resonance measurements and molecular mechanics calculations. The results indicate that in the cis isomers the cyclohexene ring is in a locked conformation and the trans isomers correspond to a mixture of the two inverted half chairs. Moreover, the data allowed the identification of the two purposely-synthesized geometrical isomers of 1-phenyl-3-propionamidotetralin. Binding studies on melatonin receptor subtypes showed that the (+/-)-cis-1-phenyl-3-propionamido-1,2,3,4-tetrahydronaphthalene has higher affinity and selectivity ratio toward the MT(2) subtype than the (+/-)-trans-isomer.

Three-dimensional quantitative structure-activity relationship studies on selected MT1 and MT2 melatonin receptor ligands: requirements for subtype selectivity and intrinsic activity modulation

The three-dimensional quantitative structure-activity relationship comparative molecular field analysis (3D-QSAR CoMFA) approach was applied to some classes of melatonin (MLT) membrane receptor ligands, with the principal aim of exploring the correlation between their steric features and MT(2)-selective antagonism. Binding data obtained from cloned MT(1) and MT(2) receptor subtypes were used to develop 3D-QSAR models for agonists and for antagonists at the two receptor subtypes, looking for the structural requirements for receptor subtype selectivity. In particular, we superposed the compounds showing antagonist activity, or very low intrinsic activity at the GTPgammaS test, following the hypothesis that the occupation of an additional pocket positioned out of the plane of MLT is one of the major determinants for MT(2) selectivity; the statistical models obtained confirmed this hypothesis. Structure-intrinsic activity relationship studies, applied to a set of compounds homogeneously tested, allowed the identification of the structural features whose modulation shifts the behavior from that of the agonist to that of the antagonist. The pocket out of the plane of MLT was identified as one of the key features for obtaining selective MT(2) antagonists. The reliability of our statistical models was further confirmed by the correct prediction of the pharmacological behavior of some N-substituted melatonin derivatives, which were prepared and tested on cloned receptor subtypes.

Synthesis, pharmacological characterization and QSAR studies on 2-substituted indole melatonin receptor ligands

A number of 6-methoxy-1-(2-propionylaminoethyl)indoles, carrying properly selected substituents at the C-2 indole position, were prepared and tested as melatonin receptor ligands. Affinities and intrinsic activities for the human cloned mt1 and MT2 receptors were examined and compared with those of some 2-substituted melatonin derivatives recently described by us. A quantitative structure activity relationship (QSAR) study of the sixteen 2-substituted indole compounds, 5a-k, 1, 8-11, using partial least squares (PLS) and multiple regression analysis (MRA) revealed the existence of an optimal range of lipophilicity for the C2 indole substituent. There are also indications that planar, electron-withdrawing substituents contribute to the affinity by establishing additional interactions with the binding pocket. No mt1/MT2 subtype selectivity was observed, with the relevant exception of the 2-phenethyl derivative 5e, which exhibited the highest selectivity for the h-MT2 receptor among all the compounds tested (MT2/mt1 ratio of ca. 50). Conformational analysis and superposition of 5e to other reported selective MT2 ligands revealed structural and conformational similarities that might account for the MT2/mt1 selectivity of 5e.

2-N-acylaminoalkylindoles: design and quantitative structure-activity relationship studies leading to MT2-selective melatonin antagonists

Several indole analogues of melatonin (MLT) were obtained by moving the MLT side chain from C(3) to C(2) of the indole ring. Binding and in vitro functional assays were performed on cloned human MT1 and MT2 receptors, stably transfected in NIH3T3 cells. Quantitative structure-activity relationship studies showed that 4-methoxy-2-(N-acylaminomethyl)indoles, with a benzyl group in position 1, were selective MT2 antagonists and, in particular, N-[(1-p-chlorobenzyl-4-methoxy-1H-indol-2-yl)methyl]propanamide (12) behaved as a pure antagonist at MT1 and MT2 receptors, with a 148-fold selectivity for MT2. We present a topographical model that suggests a lipophilic group, located out of the plane of the indole ring of MLT, as the key feature of the MT2 selective antagonists.

Design and synthesis of melatonin receptors agonists and antagonists

We review our work towards the design and synthesis of high-affinity melatonin (N-acetyl-5-methoxytryptamine) agonist and antagonist compounds. High affinity melatonergic agonists were obtained by shifting the melatonin side chain from C3 to N1 of the indole ring system. Conversely, by moving the side chain from C3 to C2 it was possible to obtain melatonin antagonist compounds, albeit of moderate affinity.

Tight association of the human Mel(1a)-melatonin receptor and G(i): precoupling and constitutive activity

If stably expressed in human embryonic kidney (HEK)293 cells, the human Mel(1a)-melatonin receptor activates G(i)-dependent, pertussis toxin-sensitive signaling pathways, i.e., inhibition of adenylyl cyclase and stimulation of phospholipase Cbeta; the latter on condition that G(q) is coactivated. The antagonist luzindole blocks the effects of melatonin and acts as an inverse agonist at the Mel(1a) receptor in both intact cells and isolated membranes. This suggests that the Mel(1a) receptor is endowed with constitutive activity, a finding confirmed on reconstitution of the Mel(1a) receptor with G(i). Because the receptor density is in the physiological range, constitutive activity is not an artifact arising from overexpression of the receptor. In addition, the following findings indicate that the Mel(1a) receptor forms a very tight complex with G(i) which can be observed both in the presence and absence of an agonist. 1) In intact cells and in membranes, high-affinity agonist binding is resistant to the destabilizing effect of guanine nucleotides. 2) The ability to bind an agonist with high affinity is preserved even after exposure of the cells to pertussis toxin, because a fraction of G(i) is inaccessible to the toxin in cells expressing Mel(1a) receptors (but not the A(1)-adenosine receptor, another G(i)-coupled receptor). 3) An antiserum directed against the Mel(1a) receptor coprecipitates G(i) even in the absence of an agonist. We therefore conclude that the Mel(1a) receptor is tightly precoupled and that its constitutive activity may play a role in pacing the biological clock, an action known to involve the melatonin receptors in the suprachiasmatic nucleus.

Ligand efficacy and potency at recombinant human MT2 melatonin receptors: evidence for agonist activity of some mt1-antagonists

NIH3T3 fibroblast cells transfected with the full-length coding region of the MT2 human melatonin receptor stably expressed the receptor that is coupled to a pertussis toxin-sensitive G protein and exhibits high affinity for melatonin (K(I) = 261 pM). The order of apparent affinity for selected compounds was: 4-phenyl-2-propionamidotetralin (4P-PDOT) > 2-phenylmelatonin > 2-iodomelatonin > 2-bromomelatonin > 6-chloromelatonin > or = melatonin > luzindole > N-acetyl-tryptamine > or = N-[(2-phenyl-1H-indol-3-yl)ethyl]cyclobutanecarboxamide (compound 6) > N-acetylserotonin. 4P-PDOT exhibited a very high selectivity (approximately 22,000 times) for the MT2 receptor with respect to the mt1 receptor subtype, as tested in comparative experiments with membrane preparations from NIH3T3 cells stably transfected with the human mt1 receptor. MT2 melatonin receptors mediated incorporation of [35S]-GTPgammaS into isolated membranes via receptor catalyzed exchange of [35S]-GTPgammaS for GDP. The relative intrinsic activity and potency of the compounds were subsequently studied by using [35S]-GTPgammaS incorporation. The order of potency was equal to the order of apparent affinity. Melatonin and full agonists increased [35S]-GTPgammaS binding by 250% over basal (taken as 100%). Luzindole did not increase basal [35S]-GTPgammaS binding but competitively inhibited melatonin-stimulated [35S]-GTPgammaS binding, thus exhibiting antagonist action. The other two mt1 antagonists used here, 4P-PDOT and N-[(2-phenyl-1H-indol-3-yl)ethyl]cyclobutanecarboxamide, behaved as partial agonists at the MT2 subtype, with relative intrinsic activities of 0.37 and 0.39, respectively. These findings show, for the first time, important differences in the intrinsic activity of analogues between the human mt1 and MT2 melatonin receptor subtypes.

2-[N-Acylamino(C1-C3)alkyl]indoles as MT1 melatonin receptor partial agonists, antagonists, and putative inverse agonists

The synthesis of several novel indole melatonin analogues substituted at the 2-position with acylaminomethyl (8-11), acylaminoethyl (5a-k), or acylaminopropyl (13) side chains is reported. On the basis of a novel in vitro functional assay (specific binding of [35S]GTPgammaS), which can discriminate agonist from partial agonist, antagonist, and inverse agonist ligands, 5a,g, h,j and 13 were shown to be partial agonists, 5d,e and 8-11 competitive antagonists, and 5b,c,k putative inverse agonists. Binding and functional assays were performed on cloned human MT1 receptor. Structure-activity relationship considerations indicate that N-[1-aryl-2-(4-methoxy-1H-indol-2-yl)(C1-C2)alkyl]alkanamides represent a lead structure for this type of ligands.