Thyroid hormone receptors are required for the melatonin-dependent control of Rfrp gene expression in mice.

Mammals adapt to seasons using a neuroendocrine calendar defined by the photoperiodic change in the nighttime melatonin production. Under short photoperiod, melatonin inhibits the pars tuberalis production of TSHß, which, in turn, acts on tanycytes to regulate the deiodinase 2/3 balance resulting in a finely tuned seasonal control of the intra-hypothalamic thyroid hormone T3. Despite the pivotal role of this T3 signaling for synchronizing reproduction with the seasons, T3 cellular targets remain unknown. One candidate is a population of hypothalamic neurons expressing Rfrp, the gene encoding the RFRP-3 peptide, thought to be integral for modulating rodent's seasonal reproduction. Here we show that nighttime melatonin supplementation in the drinking water of melatonin-deficient C57BL/6J mice mimics photoperiodic variations in the expression of the genes Tshb, Dio2, Dio3, and Rfrp, as observed in melatonin-proficient mammals. Notably, we report that this melatonin regulation of Rfrp expression is no longer observed in mice carrying a global mutation of the T3 receptor, TRα, but is conserved in mice with a selective neuronal mutation of TRα. In line with this observation, we find that TRα is widely expressed in the tanycytes. Altogether, our data demonstrate that the melatonin-driven T3 signal regulates RFRP-3 neurons through non-neuronal, possibly tanycytic, TRα.

Sex differences in the photoperiodic regulation of RF-Amide related peptide (RFRP) and its receptor GPR147 in the syrian hamster.

RF-(Arg-Phe) related peptides (RFRP-1 and -3) are considered to play a role in the seasonal regulation of reproduction; however, the effect of the peptides depends on species and gender. This study aimed at comparing the RFRP system in male and female Syrian hamsters over long and short photoperiods to investigate the neuroanatomical basis of these differential effects. The neuroanatomical distribution of RFRP neurons and fibers, revealed using an antiserum recognizing RFRP-1 and -3, as well as GPR147 mRNA, are similar in male and female Syrian hamsters. RFRP neurons are mainly found in the medial hypothalamus, whereas RFRP projections and GPR147 mRNA are observed in the preoptic area, anteroventral-periventricular nucleus, suprachiasmatic nucleus, paraventricular nucleus, bed nucleus of the stria terminalis, ventromedial hypothalamus, habenular nucleus, and arcuate nucleus. The number of RFRP neurons is higher in females than in males, and in both sexes, the number of RFRP neurons is reduced in short photoperiods. GPR147 mRNA levels are higher in females than in males and are downregulated in short photoperiods, particularly in females. Interestingly, the number of RFRP-positive fibers in the anteroventral-periventricular nucleus is higher only in females adjusted to a short photoperiod. Our results suggest that the RFRP system, which is strongly regulated by photoperiod in both male and female Syrian hamsters, is particularly important in females, with a distinct role in the anteroventral-periventricular nucleus, possibly in the regulation of the preovulatory luteinizing hormone surge via kisspeptin neurons.

MT1 melatonin receptor mRNA tissular localization by PCR amplification.

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.

Pineal arylalkylamine N-acetyltransferase gene expression is highly stimulated at night in the diurnal rodent, Arvicanthis ansorgei.

The different mechanisms underlying the control of diurnal vs. nocturnal activity are still unknown. Regarding the nocturnal synthesis of the pineal hormone, melatonin, experiments performed on diurnal sheep or bovine and on nocturnal rat or hamster revealed important differences in the regulation of the melatonin rate-limiting enzyme, arylalkylamine N-acetyltransferase (AA-NAT). These observations raised the hypothesis that melatonin synthesis may be different in nocturnal vs. diurnal animals. In this study, we cloned the cDNA coding for Aa-nat and analysed the mechanisms of AA-NAT enzyme activation in the pineal gland of the diurnal grass rat, Arvicanthis ansorgei, and compared them to those of the nocturnal Wistar rat, Rattus norvegicus. Aa-nat gene sequences of both species are 86.6% identical. In Arvicanthis, Aa-nat gene expression is markedly increased at the beginning of the night and is followed by a large increase in AA-NAT activity and melatonin content. In contrast, at the end of the night, the decrease in AA-NAT activity and melatonin content precedes that of Aa-nat mRNA. A beta-adrenergic agonist given at daytime reproduces the nocturnal activation of melatonin synthesis, whereas, a beta-adrenergic antagonist given at night-time inhibits AA-NAT activity and melatonin synthesis independently of Aa-nat mRNA. The day-night regulation of melatonin synthesis in the pineal of the diurnal Arvicanthis, involving a transcriptional activation in early night and a post-translational inhibition at late night, is very similar to that of the nocturnal Wistar rat. In conclusion, the fundamental differences underlying melatonin synthesis among species rely upon phylogenetic rather than behavioural differences.