The five dimensions of receptor pharmacology exemplified by melatonin receptors: An opinion

Industrial and academic approaches to the search for alternative melatonin receptor ligands: An historical survey

The search for melatonin receptor agonists formed the main part of melatonin medicinal chemistry programs for the last three decades. In this short review, we summarize the two main aspects of these programs: the development of all the necessary tools to characterize the newly synthesized ligands at the two melatonin receptors MT1 and MT2, and the medicinal chemist's approaches to find chemically diverse ligands at these receptors. Both strategies are described. It turns out that the main source of tools were industrial laboratories, while the medicinal chemistry was mainly carried out in academia. Such complete accounts are interesting, as they delineate the spirits in which the teams were working demonstrating their strength and innovative character. Most of the programs were focused on nonselective agonists and few of them reached the market. In contrast, discovery of MT1-selective agonists and melatonergic antagonists with proven in vivo activity and MT1 or MT2-selectivity is still in its infancy, despite the considerable interest that subtype selective compounds may bring in the domain, as the physiological respective roles of the two subtypes of melatonin receptors, is still poorly understood. Poly-pharmacology applications and multitarget ligands have also been considered.

The location, physiology, pathology of hippocampus Melatonin MT2 receptor and MT2-selective modulators

Melatonin, a neurohormone secreted by the pineal gland and regulated by the suprachiasmatic nucleus (SCN) of the hypothalamus, is synthesized and directly released into the cerebrospinal fluid (CSF) of the third ventricle (3rdv), where it undergoes rapid absorption by surrounding tissues to exert its physiological function. The hippocampus, a vital structure in the limbic system adjacent to the ventricles, plays a pivotal role in emotional response and memory formation. Melatonin MT1 and MT2 receptors are G protein-coupled receptors (GPCRs) that primarily mediate melatonin's receptor-dependent effects. In comparison to the MT1 receptor, the widely expressed MT2 receptor is crucial for mediating melatonin's biological functions within the hippocampus. Specifically, MT2 receptor is implicated in hippocampal synaptic plasticity and memory processes, as well as neurogenesis and axogenesis. Numerous studies have demonstrated the involvement of MT2 receptors in the pathophysiology and pharmacology of Alzheimer's disease, depression, and epilepsy. This review focuses on the anatomical localization of MT2 receptor in the hippocampus, their physiological function in this region, and their signal transduction and pharmacological roles in neurological disorders. Additionally, we conducted a comprehensive review of MT2 receptor ligands used in psychopharmacology and other MT2-selective ligands over recent years. Ultimately, we provide an outlook on future research for selective MT2 receptor drug candidates.

Evaluation of the Anxiolytic Effect of Ramelteon in Various Rat Models of Anxiety

Background Melatonin was found to have anxiolytic properties in several animal and human studies. The melatonin receptor agonist ramelteon might also have a similar anxiolytic action. Objectives The objective of the study was to evaluate the effect of ramelteon in various rat models of anxiety and to explore the possible mechanism of action. Methods The anxiolytic effect was compared between the control group, diazepam group (1 mg/kg and 0.5 mg/kg), and ramelteon group (0.25 mg/kg, 0.5 mg/kg, and 1 mg/kg) by an elevated plus maze, light-dark box, hole board apparatus, and open field test in Sprague Dawley rats. Antagonists flumazenil, picrotoxin, and Luzindole were used to explore the possible mechanism of action of ramelteon if it showed an anxiolytic property. Results Ramelteon as a standalone drug did not show an anxiolytic effect. However, a combination of ramelteon (1 mg/kg) and diazepam (0.5 mg/kg) showed an anxiolytic effect. Conclusion Further studies can evaluate the use of a fixed-dose combination of ramelteon and already-approved anxiolytic drugs to reduce the dose of the latter.

Caloxin-derived peptides for the inhibition of plasma membrane calcium ATPases

Plasma membrane calcium ATPases (PMCAs) are a family of transmembrane proteins responsible for the extrusion of cytosolic Ca²⁺ to the extracellular milieu. They are important players of the calcium homeostasis possibly implicated in some important diseases. The reference inhibitors of PMCA extruding activity are on one hand ortho-vanadate (IC₅₀ in the 30 mM range), and on the other a series of 12- to 20-mer peptides named caloxins (IC₅₀ in the 100 µM scale). As for all integral membrane proteins, biochemistry and pharmacology are difficult to study on isolated and/or purified proteins. Using a series of reference blockers, we assessed a pharmacological window with which we could study the functionality of PMCAs in living cells. Using this system, we screened for alternative versions of caloxins, aiming at shortening the peptide backbone, introducing non-natural amino acids, and overall trying to get a glimpse at the structure-activity relationship between those new peptides and the protein in a cellular context. We describe a short series of equipotent 5-residue long analogues with IC₅₀ in the low µM range.

Functionality of Melatonin Receptors: Internalization

The main step of classical desensitization of a receptor, by mean of its disappearance from the plasma membrane, is its internalization. This is a key factor in the regulation of agonist-mediated signaling pathways, as it most of the time stops the activation of the receptor. Internalization is thus important to evaluate, as a complementary information for a natural ligand or an alternative synthetic agonist. Enzyme fragment complementation is an elegant but delicate way to measure this phenomenon, by fusing two complementary parts of an enzyme to two partners, and to measure the activity of the reconstituted enzyme upon complexation of the partners. In the present chapter, using two parts of β-galactosidase, one fused to the C-terminus of the MT1 receptor, the other to an endosomal protein, one can measure the formation of the complex; thus, the transfer of the receptor to the endosome from which MT1 will be recirculated.

MT1 Receptor Signaling Pathways by Impedance Measurement

Melatonin exerts its classical effects of relay of the circadian rhythm through two G protein-coupled receptors, MT1 and MT2. The functions attributed to melatonin are so numerous that the action of this neurohormone should be through several protein targets or through new coupled biochemistry routes at its receptors. In order to better explore and understand these melatonin-dependent activities, we enlarged the functional pathways linked to the activation of the receptors in living system. Impedance has been shown to rely on the shape-shifting capacity of receptor-associated mechanisms. Those changes elicited by an agonist lead to changes in the actual shape of the cells, and thus to their electric conductivity. The impact of those changes onto the physiology of the cells is not completely understood from a mechanistic point of view, but the measure of these changes associated with various ligands at the melatonin receptor(s) might bring new information on melatonin-dependent cell reactivity. The following chapter is a detailed account of the way impedance can be measured in MT1-experssing cells.

Why Are We Still Cloning Melatonin Receptors? A Commentary

Cloning may seem to be a view from the past. The time before software, computers and AI were invented. It seems to us worth discussing these points in view of our favorite target: the melatoninergic system. In a few stances, it might be important to point out that even in the new era of dry science, there is still a need to experiment and to prove at the bench that our in silico assertions are right. Most of the living animals express to some extend the melatonin receptors. Some of these animal genomes were completely or partially sequenced, and it is tempting to extract from this huge information the sequence(s) of our favorite genes (MLT receptors). Then, why bother cloning, as opposed to simply built the gene and express it in a host cell? Because the genetic boundaries of the expressed sequence(s) are not 100% sure. Because the melatonin receptor gene(s) comprise a first exon 25,000 base pair far from the second one and the limits between this Ex1 and In1-as between In1 and Ex2-are subject to changes that might have a huge impact on the biochemical properties of the receptor, once expressed. Because a receptor is a biochemical entity with characteristics that are important for the functioning of this particular pathway, and more generally, for the functioning of life.

N-(Anilinoethyl)amide Melatonergic Ligands with Improved Water Solubility and Metabolic Stability

The MT2 -selective melatonin receptor ligand UCM765 (N-(2-((3-methoxyphenyl)(phenyl)amino)ethyl)acetamide), showed interesting sleep inducing, analgesic and anxiolytic properties in rodents, but suffers from low water solubility and modest metabolic stability. To overcome these limitations, different strategies were investigated, including modification of metabolically liable sites, introduction of hydrophilic substituents and design of more basic derivatives. Thermodynamic solubility, microsomal stability and lipophilicity of new compounds were experimentally evaluated, together with their MT1 and MT2 binding affinities. Introduction of a m-hydroxymethyl substituent on the phenyl ring of UCM765 and replacement of the replacement of the N,N-diphenyl-amino scaffold with a N-methyl-N-phenyl-amino one led to highly soluble compounds with good microsomal stability and receptor binding affinity. Docking studies into the receptor crystal structure provided a rationale for their binding affinity. Pharmacokinetic characterization in rats highlighted higher plasma concentrations for the N-methyl-N-phenyl-amino derivative, consistent with its improved microsomal stability and makes this compound worthy of consideration for further pharmacological investigation.

Melatonin controversies, an update

Melatonin was discovered more than 60 years ago. Since then, several seminal discoveries have allowed us to define its function as a neuroendocrine hormone and its molecular targets in mammals and many other species. However, many fundamental issues have not yet been solved such as the subcellular localization of melatonin synthesis and the full spectrum of its molecular targets. In addition, a considerable number of controversies persist in the field, mainly concerning how many functions melatonin has. Altogether, this illustrates how "immature" the field still is. The intention of this opinion article is to note the controversies and limitations in the field, to initiate a discussion and to make proposals/guidelines to overcome them and move the field forward.

MCH-R1 Antagonist GPS18169, a Pseudopeptide, Is a Peripheral Anti-Obesity Agent in Mice

Melanin-concentrating hormone (MCH) is a 19 amino acid long peptide found in the brain of animals, including fishes, batrachians, and mammals. MCH is implicated in appetite and/or energy homeostasis. Antagonists at its receptor (MCH-R1) could be major tools (or ultimately drugs) to understand the mechanism of MCH action and to fight the obesity syndrome that is a worldwide societal health problem. Ever since the deorphanisation of the MCH receptor, we cloned, expressed, and characterized the receptor MCH-R1 and started a vast medicinal chemistry program aiming at the discovery of such usable compounds. In the present final work, we describe GPS18169, a pseudopeptide antagonist at the MCH-R1 receptor with an affinity in the nanomolar range and a Ki for its antagonistic effect in the 20 picomolar range. Its metabolic stability is rather ameliorated compared to its initial parent compound, the antagonist S38151. We tested it in an in vivo experiment using high diet mice. GPS18169 was found to be active in limiting the accumulation of adipose tissues and, correlatively, we observed a normalization of the insulin level in the treated animals, while no change in food or water consumption was observed.

Melatonin receptors, brain functions, and therapies

In mammals, including humans, the neurohormone melatonin is mainly secreted from the pineal gland at night and acts on two high-affinity G protein-coupled receptors, the melatonin MT₁ and MT₂ receptors. Major functions of melatonin receptors in the brain are the regulation of circadian rhythms and sleep. Correspondingly, the main indications of the currently available drugs for these receptors indicate this as targets. Yet these drugs may not only improve circadian rhythm- and sleep-related disorders but may also be beneficial for complex diseases like major depression, Alzheimer's disease, autism, and attention-deficit/hyperactivity disorders. Here, we will focus on the hypothalamic functions of melatonin receptors by updating our knowledge on their hypothalamic expression pattern at normal, aged, and disease states, by discussing their capacity to regulate circadian rhythms and sleep and by presenting the clinical applications of the melatonin receptor-targeting drugs ramelteon, tasimelteon, and agomelatine or of prolonged-release melatonin formulations. Finally, we speculate about future trends in the field of melatonin receptor drugs.

Biased Ligands Differentially Shape the Conformation of the Extracellular Loop Region in 5-HT2B Receptors

G protein-coupled receptors are linked to various intracellular transducers, each pathway associated with different physiological effects. Biased ligands, capable of activating one pathway over another, are gaining attention for their therapeutic potential, as they could selectively activate beneficial pathways whilst avoiding those responsible for adverse effects. We performed molecular dynamics simulations with known β-arrestin-biased ligands like lysergic acid diethylamide and ergotamine in complex with the 5-HT_2B receptor and discovered that the extent of ligand bias is directly connected with the degree of closure of the extracellular loop region. Given a loose allosteric coupling of extracellular and intracellular receptor regions, we delineate a concept for biased signaling at serotonin receptors, by which conformational interference with binding pocket closure restricts the signaling repertoire of the receptor. Molecular docking studies of biased ligands gathered from the BiasDB demonstrate that larger ligands only show plausible docking poses in the ergotamine-bound structure, highlighting the conformational constraints associated with bias. This emphasizes the importance of selecting the appropriate receptor conformation on which to base virtual screening workflows in structure-based drug design of biased ligands. As this mechanism of ligand bias has also been observed for muscarinic receptors, our studies provide a general mechanism of signaling bias transferable between aminergic receptors.

Evaluation of neurobiological and antioxidant effects of novel melatonin analogs in mice

Based on the pharmacophore model of melatonin (MT₁) receptor, we recently synthesized a series of indole derivatives that showed anticonvulsant activity with low neurotoxicity and hepatotoxicity in rodents. In the present study, the three most potent C3-modified derivatives with hydrazine structure 3c, 3e, and 3f, with 2-chlorophenyl, 2-furyl, and 2-thienyl fragments, respectively, were selected, and their neurobiological activity was explored in mice. In Experiment #1, the dose-dependent anxiolytic effect of a single i.p. administration of the novel compounds at doses of 10, 30, and 60 mg/kg were studied in the open field (OF) test. In Experiment#2, the analgesic effect of 3c, 3e, and 3f (30–100 mg/kg) was tested in the hot plate test and formalin test. Experiment#3 was designed to assess the antidepressant-like activity of 3c, 3e, and 3f (10–60 mg/kg). The forced swimming test (FST) and tail suspension test (TST)-induced effect on markers of oxidative stress in the frontal cortex (FC), and the hippocampus was evaluated. Melatonin was used in the same doses as melatonin analogs in all three experiments as a positive control. Desipramine (10 mg/kg) was also applied as a control in the FST. The three melatonin analogs bearing hydrazide/hydrazone substitution at 3C of the indol scaffold demonstrated improved antidepressant-like activity compared to the melatonin. The tested substances are devoided of anxiolytic effects. The antioxidant activity of the melatonin analogs and analgesic potential is comparable to that of melatonin. The 3C substitution with hydrazide/hydrazone moiety substantially contributes to the antidepressant and antioxidant activity of the melatonin analogs.

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.

Melatonin MT1 and MT2 receptor ERK signaling is differentially dependent on Gi/o and Gq/11 proteins

G protein-coupled receptors (GPCRs) transmit extracellular signals into cells by activating G protein- and β-arrestin-dependent pathways. Extracellular signal-regulated kinases (ERKs) play a central role in integrating these different linear inputs coming from a variety of GPCRs to regulate cellular functions. Here, we investigated human melatonin MT₁ and MT₂ receptors signaling through the ERK1/2 cascade by employing different biochemical techniques together with pharmacological inhibitors and siRNA molecules. We show that ERK1/2 activation by both receptors is exclusively G protein-dependent, without any participation of β-arrestin1/2 in HEK293 cells. ERK1/2 activation by MT₁ is only mediated though G_i/o proteins, while MT₂ is dependent on the cooperative activation of G_i/o and G_q/11 proteins. In the absence of G_q/11 proteins, however, MT₂ -induced ERK1/2 activation switches to a β-arrestin1/2-dependent mode. The signaling cascade downstream of G proteins is the same for both receptors and involves activation of the PI3K/PKCζ/c-Raf/MEK/ERK cascade. The differential G protein dependency of MT₁ - and MT₂ -mediated ERK activation was confirmed at the level of EGR1 and FOS gene expression, two ERK1/2 target genes. G_i/o /G_q/11 cooperativity was also observed in Neuroscreen-1 cells expressing endogenous MT₂ , whereas in the mouse retina, where MT₂ is engaged into MT₁ /MT₂ heterodimers, ERK1/2 signaling is exclusively G_i/o -dependent. Collectively, our data reveal differential signaling modes of MT₁ and MT₂ in terms of ERK1/2 activation, with an unexpected G_i/o /G_q/11 cooperativity exclusively for MT₂ . The plasticity of ERK activation by MT₂ is highlighted by the switch to a β-arrestin1/2-dependent mode in the absence of G_q/11 proteins and by the switch to a G_i/o mode when engaged into MT₁ /MT₂ heterodimers, revealing a new mechanism underlying tissue-specific responses to melatonin.