No abstract

Transcription factor CREM appears to play a key physiological and developmental role within the hypothalamic-pituitary-gonadal axis. This axis is modulated by the pineal hormone melatonin, whose production is in turn driven by the endogenous clock. There is striking circadian fluctuation of a novel CREM isoform, ICER, which is expressed at high levels during the night. ICER is generated from an alternative, intronic promoter and functions as a powerful repressor of cyclic AMP-induced transcription. Rhythmic adrenergic signals originated by the clock direct ICER expression by stimulation of the cAMP signal transduction pathway.

The KiSS-1 gene encodes kisspeptin, the endogenous ligand of the G-protein-coupled receptor GPR54. Recent data indicate that the KiSS-1/GPR54 system is critical for the regulation of reproduction and is required for puberty onset. In seasonal breeders, reproduction is tightly controlled by photoperiod (i.e., day length). The Syrian hamster is a seasonal model in which reproductive activity is promoted by long summer days (LD) and inhibited by short winter days (SD). Using in situ hybridization and immunohistochemistry, we show that KiSS-1 is expressed in the arcuate nucleus of LD hamsters. Importantly, the KiSS-1 mRNA level was lower in SD animals but not in SD-refractory animals, which spontaneously reactivated their sexual activity after several months in SD. These changes of expression are not secondary to the photoperiodic variations of gonadal steroids. In contrast, melatonin appears to be necessary for these seasonal changes because pineal-gland ablation prevented the SD-induced downregulation of KiSS-1 expression. Remarkably, a chronic administration of kisspeptin-10 restored the testicular activity of SD hamsters despite persisting photoinhibitory conditions. Overall, these findings are consistent with a role of KiSS-1/GPR54 in the seasonal control of reproduction. We propose that photoperiod, via melatonin, modulates KiSS-1 signaling to drive the reproductive axis.

In seasonal species, various physiological processes including reproduction are organized by photoperiod via melatonin, but the mechanisms of melatonin action are still unknown. In birds, the peptide gonadotropin-inhibiting hormone (GnIH) has been shown to have inhibitory effects on reproductive activity and displays seasonal changes of expression. Here we present evidence in mammals that the gene orthologous to GnIH, the RFamide-related peptide (RFRP) gene, expressed in the mediobasal hypothalamus, is strongly regulated by the length of the photoperiod, via melatonin. The level of RFRP mRNA and the number of RFRP-immunoreactive cell bodies were reduced in sexually quiescent Syrian and Siberian hamsters acclimated to short-day photoperiod (SD) compared with sexually active animals maintained under long-day photoperiod (LD). This was contrasted in the laboratory Wistar rat, a non-photoperiodic breeder, in which no evidence for RFRP photoperiodic modulation was seen. In Syrian hamsters, the reduction of RFRP expression in SD was independent from secondary changes in gonadal steroids. By contrast, the photoperiodic variation of RFRP expression was abolished in pinealectomized hamsters, and injections of LD hamsters with melatonin for 60 d provoked inhibition of RFRP expression down to SD levels, indicating that the regulation is dependent on melatonin. Altogether, these results demonstrate that in these hamster species, the RFRP neurons are photoperiodically modulated via a melatonin-dependent process. These observations raise questions on the role of RFRP as a general inhibitor of reproduction and evoke new perspectives for understanding how melatonin controls seasonal processes via hypothalamic targets.

In seasonal mammals, a distinct photoneuroendocrine circuit that involves the pineal hormone melatonin tightly synchronizes reproduction with seasons. In the Syrian hamster, a seasonal model in which sexual activity is inhibited by short days, we have previously shown that the potent GnRH stimulator, kisspeptin, is crucial to convey melatonin's message; however, the precise mechanisms through which melatonin affects kisspeptin remain unclear. Interestingly, rfrp gene expression in the neurons of the dorsomedial hypothalamic nucleus, a brain region in which melatonin receptors are present in the Syrian hamster, is strongly down-regulated by melatonin in short days. Because a large body of evidence now indicates that RFamide-related peptide (RFRP)-3, the product of the rfrp gene, is an inhibitor of gonadotropin secretion in various mammalian species, we sought to investigate its effect on the gonadotrophic axis in the Syrian hamster. We show that acute central injection of RFRP-3 induces c-Fos expression in GnRH neurons and increases LH, FSH, and testosterone secretion. Moreover, chronic central administration of RFRP-3 restores testicular activity and Kiss1 levels in the arcuate nucleus of hamsters despite persisting photoinhibitory conditions. By contrast RFRP-3 does not have a hypophysiotrophic effect. Overall, these findings demonstrate that, in the male Syrian hamster, RFRP-3 exerts a stimulatory effect on the reproductive axis, most likely via hypothalamic targets. This places RFRP-3 in a decisive position between the melatonergic message and Kiss1 seasonal regulation. Additionally, our data suggest for the first time that the function of this peptide depends on the species and the physiological status of the animal model.

In mammals, melatonin is the pivotal messenger synchronizing biological functions, notably reproductive activity, with annual daylength changes. Recently, two major findings clarified melatonin's mode of action. First, melatonin controls the production of thyroid stimulating hormone (TSH) by the pars tuberalis of the adenohypophysis. This TSH regulates local thyroid hormone availability in the mediobasal hypothalamus. Second, the RF-amides kisspeptin and RFRP-3, recently discovered regulators of the gonadotropic axis, are involved in the melatonin control of reproduction. This study aims to establish a mechanistic link between the melatonin-driven TSH and the RF-amide control of reproduction. We treated short-day-adapted male Djungarian and Syrian hamsters with a chronic central infusion of TSH. In both hamster species, the central administration of 5 mIU/d TSH for 4 to 6 wk restored the summer phenotype of both testicular activity and kisspeptin and RFRP expression. Vehicle treated hamsters remain sexually inactive. Furthermore, the TSH treatment increased the body weight of lean short-day-adapted Djungarian hamsters and reduced hypothalamic somatostatin expression to the summer phenotype. In summary, our study demonstrates the pivotal role of melatonin-driven TSH for the seasonal regulation of reproduction and body weight, and uncovers the neuropeptides relaying this signal within the hypothalamus.

Kisspeptins are a family of small peptides that play a key role in the neuroendocrine regulation of the reproductive function through neural pathways that have not yet been completely identified. The present study aimed to investigate the distribution of kisspeptin neurone fibres in the female rat brain by comparing precisely the immunoreactive pattern obtained with two antibodies: one specifically directed against kisspeptin-52 (Kp-52), the longest isoform, and the other directed against kisspeptin-10 (Kp-10), whose sequence is common to all putative mature isoforms. With both antibodies, immunoreactive cell bodies were exclusively observed in the arcuate nucleus, and immunoreactive fibres were confined to the septo-preoptico-hypothalamic continuum of the brain. Fibres were observed in the preoptic area, the diagonal band of Broca, the septohypothalamic area, the anteroventral periventricular, suprachiasmatic, supraoptic, paraventricular and periventricular nuclei, the dorsal border of the ventromedian nucleus, the dorsomedial and arcuate nuclei, and the median eminence. In the latter structure, varicose fibres were mainly distributed in the internal layer and were detected to a lesser extent throughout the external layer, including around the deeper part of the infundibular recess. Most regions of immunoreactive cells and fibres matched perfectly for the two antibodies. However, fibres in the dorsolateral septum, anterior fornix, accumbens nucleus and the lateral bed nucleus of the stria terminalis were only recognised by antibody anti-Kp-10, suggesting that anti-Kp-10 may recognise a wider range of kisspeptin isoforms than anti-Kp-52 or cross-react with molecules other than kisspeptin in rat tissue. Overall, these results illustrate the variety of projection sites of kisspeptin neurones in the rat and suggest that these peptides play a role in different functions.

The suprachiasmatic nucleus (SCN) controls the circadian rhythm of melatonin synthesis in the mammalian pineal gland by a multisynaptic pathway including, successively, preautonomic neurons of the paraventricular nucleus (PVN), sympathetic preganglionic neurons in the spinal cord and noradrenergic neurons of the superior cervical ganglion (SCG). In order to clarify the role of each of these structures in the generation of the melatonin synthesis rhythm, we first investigated the day- and night-time capacity of the rat pineal gland to produce melatonin after bilateral SCN lesions, PVN lesions or SCG removal, by measurements of arylalkylamine N-acetyltransferase (AA-NAT) gene expression and pineal melatonin content. In addition, we followed the endogenous 48 h-pattern of melatonin secretion in SCN-lesioned vs. intact rats, by microdialysis in the pineal gland. Corticosterone content was measured in the same dialysates to assess the SCN lesions effectiveness. All treatments completely eliminated the day/night difference in melatonin synthesis. In PVN-lesioned and ganglionectomised rats, AA-NAT levels and pineal melatonin content were low (i.e. 12% of night-time control levels) for both day- and night-time periods. In SCN-lesioned rats, AA-NAT levels were intermediate (i.e. 30% of night-time control levels) and the 48-h secretion of melatonin presented constant levels not exceeding 20% of night-time control levels. The present results show that ablation of the SCN not only removes an inhibitory input but also a stimulatory input to the melatonin rhythm generating system. Combination of inhibitory and stimulatory SCN outputs could be of a great interest for the mechanism of adaptation to day-length (i.e. adaptation to seasons).