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G protein-coupled receptors (GPCRs) are classically characterized as cell-surface receptors transmitting extracellular signals into cells. Here we show that central components of a GPCR signaling system comprised of the melatonin type 1 receptor (MT 1 ), its associated G protein, and β-arrestins are on and within neuronal mitochondria. We discovered that the ligand melatonin is exclusively synthesized in the mitochondrial matrix and released by the organelle activating the mitochondrial MT 1 signal-transduction pathway inhibiting stress-mediated cytochrome c release and caspase activation. These findings coupled with our observation that mitochondrial MT 1 overexpression reduces ischemic brain injury in mice delineate a mitochondrial GPCR mechanism contributing to the neuroprotective action of melatonin. We propose a new term, "automitocrine," analogous to "autocrine" when a similar phenomenon occurs at the cellular level, to describe this unexpected intracellular organelle ligand-receptor pathway that opens a new research avenue investigating mitochondrial GPCR biology., Competing Interests: The authors declare no conflict of interest.

C3-Substituted-4-azaindoles were synthesized from pyridylacetonitriles in a two-step sequence allowing the easy introduction of a range of substituents. This strategy permits the rapid synthesis of 4-azamelatonin and a protected 4-azatryptophan.

The synthesis of benzoxathiins bearing a retroamide function is described from 8-hydroxythiochroman, the key step involving the synthesis of the benzoxathiin ring through a sulfonium salt. These new melatonin analogues were evaluated on human receptors MT1 and MT2 and have a similar affinity to that of melatonin itself.

Background and Purpose: The paradigm that GPCRs are able to prolong or initiate cellular signalling through intracellular receptors recently emerged. Melatonin binds to G protein-coupled MT1 and MT2 receptors. In contrast to most other hormones targeting GPCRs, melatonin and its synthetic analogues are amphiphilic molecules easily penetrating into cells, but the existence of intracellular receptors is still unclear mainly due to a lack of appropriate tools.Experimental Approach: We therefore designed and synthesized a series of hydrophilic melatonin receptor ligands coupled to the Cy3 cyanin fluorophore to reliably monitor its inability to penetrate cells. Two compounds, one lipophilic and one hydrophilic, were then functionally characterized in terms of their affinity for human and murine melatonin receptors expressed in HEK293 cells and their signalling efficacy.Key Results: Among the different ligands, ICOA-13 showed the desired properties as it was cell-impermeant and bound to human and mouse MT1 and MT2 receptors. ICOA-13 showed differential activities on melatonin receptors ranging from partial to full agonistic properties for the Gi /cAMP and ERK pathway and β-arrestin 2 recruitment. Notably, ICOA-13 enabled us to discriminate between Gi /cAMP signalling of the MT1 receptor initiated at the cell surface and neuronal mitochondria.Conclusions and Implications: We report here the first cell-impermeant melatonin receptor agonist, ICOA-13, which allows us to discriminate between signalling events initiated at the cell surface and intracellular compartments. Detection of mitochondrial MT1 receptors may have an important impact on the development of novel melatonin receptor ligands relevant for neurodegenerative diseases, such as Huntington disease. [ABSTRACT FROM AUTHOR]

Melatonin receptors have been studied for several decades. The low expression of the receptors in tissues led the scientific community to find a substitute for the natural hormone melatonin, the agonist 2-[125I]-iodomelatonin. Using the agonist, several hundreds of studies were conducted, including the discovery of agonists and antagonists for the receptors and minute details about their molecular behavior. Recently, we attempted to expand the panel of radioligands available for studying the melatonin receptors by using the newly discovered compounds SD6, DIV880, and S70254. These compounds were characterized for their affinities to the hMT1 and hMT2 recombinant receptors and their functionality in the classical GTP S system. SD6 is a full agonist, equilibrated between the receptor isoforms, whereas S70254 and DIV880 are only partial MT2 agonists, with Ki in the low nanomolar range while they have no affinity to MT1 receptors. These new tools will hopefully allow for additions to the current body of information on the native localization of the receptor isoforms in tissues. [ABSTRACT FROM AUTHOR]

Abstract: A series of 7-azaindolic ligands bearing a methoxy group and a N-acetyl chain as melatoninergic pharmacophores were synthesized and their binding affinities towards MT1 and MT2 receptors were evaluated. Compounds 7a–c and 12 (cyclohexyl ring connected at C-2 and C-3 position) appears as important melatonin MT2 and MT1 receptors agonists. On the other hand, the presence of basic groups (amines) at position C-3 was detrimental to the melatoninergic affinities. [Copyright &y& Elsevier]

We report herein an efficient synthesis of 2-substituted furo[3,2- b ]pyridines and their biological evaluation as melatonin receptors ligands. The proposed eight-step sequence ending with a Suzuki coupling allowed a rapid access to various analogues. The steric hindrance and the conformation of the aryl group in C 2-position were evaluated regarding the selectivity of the molecule for one of the two high affinity melatonin receptors as well as the activity profile of the compound. Introduction of 1-naphthyl substituent gave the best result in terms of selectivity with a MT 1 /MT 2 ratio of about 150 (MT 1 K i = 198 nM, MT 2 K i = 1.3 nM). [ABSTRACT FROM AUTHOR]