Abstract
Melatonin receptor agonists such as melatonin and ramelteon [(S)-N-[2-(1,6,7,8-tetrahydro-2H-indeno-[5,4-b]furan-8-yl)ethyl]-propionamide; TAK-375] have sleep-promoting effects in humans. In preclinical models, these effects are more similar to those observed in monkeys than in other species. However, in contrast to the human melatonin receptors, the pharmacological characteristics of the monkey melatonin receptors have yet to be elucidated. In this study, we cloned the cynomolgus monkey MT1 and MT2 melatonin receptors based on rhesus monkey genome sequences and then characterized the monkey melatonin receptors and compared their pharmacological properties with those of the human homologs. The overall amino acid sequences of the monkey MT1 and MT2 melatonin receptors showed high homology to the human MT1 (95%) and MT2 (96%) receptors, respectively. Saturation binding experiments with 2-[125I]iodomelatonin revealed that the dissociation constants (Kd) for the monkey MT1 and MT2 melatonin receptors were 19.9 and 70.4 pM, respectively. In ligand competition assays using 2-[125I]iodomelatonin, ramelteon displayed approximately 3- to 7-fold higher affinities than melatonin for the recombinant monkey MT1 and MT2 melatonin receptors and monkey suprachiasmatic nucleus membranes. This higher affinity of ramelteon compared with melatonin has also been observed in human melatonin receptors. Furthermore, ramelteon inhibited pituitary adenylate cyclase-activating polypeptide-27-stimulated cAMP production with higher potency than melatonin. In conclusion, this information will help us to understand the pharmacological effects of melatonin receptor agonists in monkeys.
Footnotes
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.109.155283.
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ABBREVIATIONS: SCN, suprachiasmatic nucleus; PCR, polymerase chain reaction; CHO, Chinese hamster ovary; MG-132, N-benzoyloxycarbonyl(Z)-Leu-Leu-leucinal; PACAP, pituitary adenylate cyclase-activating polypeptide; TAK-375, (S)-N-[2-(1,6,7,8-tetrahydro-2H-indeno-[5,4-b]furan-8-yl)ethyl] propionamide.
- Received May 5, 2009.
- Accepted June 23, 2009.
- The American Society for Pharmacology and Experimental Therapeutics
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