Hamster Melatonin Receptors: Cloning and Binding Characterization of MT₁ and Attempt to Clone MT₂

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.

Alternative Ligands at Melatonin Receptors

Melatonin (N-acetyl-5-methoxytryptamine) is a neurohormone that possesses a wide range of biological effects. Most of the main recognized effects of this hormone in mammals are due to its interaction with two G protein-coupled receptors, MT1 and MT2. Ligand-binding studies have been based on the use of its radioligand analog, 2[125I]-iodomelatonin, a super agonist discovered in the early 1990s. This compound has been used in most of the binding studies reported in the literature. Nevertheless, more recently other possibilities arose. This chapter is a brief summary of those alternative radioligands and of their benefits one can find in using them.

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.

A putative new melatonin binding site in sheep brain, MTx: preliminary observations and characteristics

In mammals, MT1 and MT2 melatonin receptors are high affinity G protein-coupled receptors and are thought to be involved in the integration of the melatonin signaling throughout the brain and periphery. In the present study, we describe a new melatonin binding site, named MTx, with a peculiar pharmacological profile. This site had a low affinity for 2-[125I]-melatonin in saturation assays in hypothalamus and retina (pKD = 9.13 {plus minus} 0.05, Bmax = 1.12 {plus minus} 0.11 fmol/mg protein and pKD = 8.81 {plus minus} 0.50, Bmax = 7.65 {plus minus} 2.64 fmol/mg protein, respectively) and a very high affinity, in competition assays, for melatonin (pKi = 13.08 {plus minus} 0.18), and other endogenous compounds. Using autoradiography, we showed a preferential localization of the MTx in periventricular areas of the sheep brain, with a density 3 to 8 times higher than those observed for ovine MT1 In addition, using a set of well-characterized ligands, we showed that this site did not correspond to any of the following receptors: MT1, MT2, MT3 , D1, D2, noradrenergic, nor 5-HT2 Based on its affinity for melatonin, MTx did not seem to be implicated in the integration of cerebral melatonin concentration variations since they were saturating for MTx. Nevertheless, it remained of prime importance because of its periventricular distribution, in close contact with the CSF, and its peculiar pharmacological profile responding to both melatoninergic and serotoninergic compounds. Significance Statement Herein a putative new melatonin binding site is described in sheep brain parts in close contact with the 3rd ventricle. The characteristics of the pharmacological profile of this site is different from anything previously reported in the literature. The present work forms the basis of future full pharmacological characterization.