RESUMO
In mammals, interacting transcriptional/post-translational feedback loops involving 'clock genes' and their protein products control circadian organisation. These genes are not only expressed in the master circadian clock of the suprachiasmatic nuclei (SCN) but also in many peripheral tissues where they exhibit similar but not identical dynamic to that seen in the SCN. Among these peripheral tissues, the pars tuberalis (PT) of the pituitary expresses clock genes. We show here that the PT of the rat, like that of other rodents, rhythmically expresses Per1. We also report rhythmic expression of another clock gene, Cry1. The peak of Cry1 mRNA expression occurs during the night concomitantly with rising blood plasma melatonin concentrations. Using an acute injection paradigm, we demonstrate that Cry1 expression is directly induced by melatonin in the PT. Melatonin injection at the end of the subjective day also affects Per1 expression, leading to diminished mRNA levels. These data support the existence of a time-measurement model in the PT based on direct opposite actions of melatonin on Per1 and Cry1 expression.
Assuntos
Relógios Biológicos/genética , Ritmo Circadiano/fisiologia , Proteínas de Drosophila , Proteínas do Olho , Flavoproteínas/genética , Melatonina/sangue , Células Fotorreceptoras de Invertebrados , Hipófise/metabolismo , Ratos Wistar/metabolismo , Animais , Relógios Biológicos/efeitos dos fármacos , Proteínas de Ciclo Celular , Ritmo Circadiano/efeitos dos fármacos , Criptocromos , Regulação da Expressão Gênica/efeitos dos fármacos , Regulação da Expressão Gênica/genética , Masculino , Melatonina/farmacologia , Proteínas Nucleares/genética , Proteínas Circadianas Period , Hipófise/citologia , Hipófise/efeitos dos fármacos , RNA Mensageiro/efeitos dos fármacos , RNA Mensageiro/metabolismo , Ratos , Ratos Wistar/anatomia & histologia , Receptores Acoplados a Proteínas GRESUMO
OBJECTIVES: The pineal gland transduces photoperiodic informations to the neuroendocrine axis through the nocturnally melatonin secretion. This hormonal message plays a major role in the biological rhythm regulation. By autoradiography, more than 130 melatonin putative targets have been reported in the central nervous system (CNS) and in peripheral tissues. However, cross-species consensus concern only a few of them like the suprachiasmatic nuclei (SCN), the master circadian clock, and the pars tuberalis of the pituitary. Recently, MT1 melatonin receptor cDNA have been cloned in several mammals providing us with new tools to investigate its tissular location at the gene level. In the present study, we report a screening for MT1 mRNA by RT-PCR amplification of numerous tissue mRNA. METHOD: mRNA were extracted from a large variety of rat tissues. To semi-quantify the melatonin receptor mRNA expression level, each cDNA was amplified concomitantly with both beta-actin and MT1 specific primers. RESULTS: In central and peripheral tissues previously reported to bind melatonin, strong PCR signals were logically observed. More surprisingly, a vast majority of studied tissues express MT1 mRNA and then might be responsive to melatonin. CONCLUSION: Numerous biological functions express diurnal rhythmicity and internal-synchronization. As, most of them apparently do not receive any out-coming neuronal message from the SCN, endocrine communication was proposed to support biological rhythm synchronization. Our present data strengthen the idea that the nocturnally restricted melatonin secretion could be one internal zeitgeber that putatively distributes the endogenous circadian rhythmicity to all tissues expressing melatonin receptors.