MT1 Melatonin Receptors Mediate Somatic, Behavioral, and Reproductive Neuroendocrine Responses to Photoperiod and Melatonin in Siberian Hamsters (Phodopus sungorus)

• Published in: Endocrinology (Philadelphia) Vol. 151; no. 2; pp. 714 - 721
• Main Author: Prendergast, Brian J
• Format: Journal Article
• Language: English
• Published: Chevy Chase, MD Endocrine Society 01.02.2010; The Endocrine Society
• Subjects: 5-Methoxytryptamine - analogs & derivatives; 5-Methoxytryptamine - pharmacology; Animals; Biological and medical sciences; Circadian Rhythm - drug effects; Circadian Rhythm - physiology; Cricetinae; Fundamental and applied biological sciences. Psychology; Indenes - pharmacology; Male; Melatonin - pharmacology; Motor Activity - drug effects; Phodopus; Photoperiod; Receptor, Melatonin, MT1 - physiology; Receptor, Melatonin, MT2 - physiology; Seasons; Signal Transduction - drug effects; Signal Transduction - physiology; Vertebrates: endocrinology; Vertebrata; Melatonin; Mammalia; Animal; Rodentia; Photoperiod; Behavior; Phodopus sungorus; MT1 melatonin receptor; Hamster; Pineal hormone
• ISSN: 0013-7227; 1945-7170
• DOI: 10.1210/en.2009-0710

Environmental day length drives nocturnal pineal melatonin secretion, which in turn generates or entrains seasonal cycles of physiology, reproduction, and behavior. In mammals, melatonin (MEL) binds to a number of receptor subtypes including high-affinity (MT1 and MT2) and low-affinity (MT3, nuclear orphan receptors) binding sites, which are distributed throughout the central nervous system and periphery. The MEL receptors that mediate photoperiodic reproductive and behavioral responses to MEL have not been identified in a reproductively photoperiodic species. Here I tested the hypothesis that MT1 receptors are necessary and sufficient to engage photoperiodic responses by challenging male Siberian hamsters (Phodopus sungorus), a species that does not express functional MT2 receptors, with ramelteon (RAM), a specific MT1/MT2 receptor agonist. In hamsters housed in a long-day photoperiod, late-afternoon RAM treatment inhibited gonadotropin secretion, induced gonadal regression, and suppressed food intake and body mass, mimicking effects of MEL. In addition, chronic (24 h/d) RAM infusions were sufficient to obscure endogenous MEL signaling, and these treatments attenuated gonadal regression in short days. Together, the outcomes indicate that signaling at the MT1 receptor is sufficient and necessary to mediate the effects of photoperiod-driven changes in MEL on behavior and reproductive function in a reproductively photoperiodic mammal. In seasonally breeding Siberian hamsters, melatonin signaling at the MT1 receptor is necessary and sufficient for changes in day length to trigger seasonal adaptations in body mass, food intake, gonadotrophin secretion, and reproductive physiology.