Abstract
Seasonal control of reproduction is critical for the perpetuation of species living in temperate zones that display major changes in climatic environment and availability of food resources. In mammals, seasonal cues are mainly provided by the annual change in the 24-h light/dark ratio (i.e., photoperiod), which is translated into the nocturnal production of the pineal hormone melatonin. The annual rhythm in this melatonin signal acts as a synchronizer ensuring that breeding occurs when environmental conditions favor survival of the offspring. Although specific mechanisms might vary among seasonal species, the hypothalamic RF (Arg–Phe) amide-related peptides (RFRP-1 and -3) are believed to play a critical role in the central control of seasonal reproduction and in all seasonal species investigated, the RFRP system is persistently inhibited in short photoperiod. Central chronic administration of RFRP-3 in short day-adapted male Syrian hamsters fully reactivates the reproductive axis despite photoinhibitory conditions, which highlights the importance of the seasonal changes in RFRP expression for proper regulation of the reproductive axis. The acute effects of RFRP peptides, however, depend on species and photoperiod, and recent studies point toward a different role of RFRP in regulating female reproductive activity. In this review, we summarize the recent advances made to understand the role and underlying mechanisms of RFRP in the seasonal control of reproduction, primarily focusing on mammalian species.
Highlights
Animals living in temperate and boreal latitudes experience marked seasonal changes in their environment
We found that the number of related peptideRFRP in Seasonal Reproduction (RFRP) neurons and the intensity of the immunoreactive labeling were markedly higher in females than in males adapted to long photoperiod (LP) conditions
RFRPs were thought to act with a similar inhibitory effect as the avian ortholog gonadotropin-inhibitory hormone (GnIH) in all species and is still widely referred to as GnIH, despite its well-documented stimulatory effect in Siberian and Syrian hamster species [6, 7]
Summary
Animals living in temperate and boreal latitudes experience marked seasonal changes in their environment. To overcome these environmental changes, increasing their chances of survival, they show seasonal changes in several aspects of their physiology, i.e., reproduction, metabolism, and behavior. Annual variations in day length are translated into an endocrine message, namely, the nocturnal secretion of the pineal hormone melatonin, which acts as a potent seasonal synchronizer of biological functions, especially reproductive activity [1,2,3,4]. The RFamide peptide kisspeptin (kp), a very potent stimulator of gonadotropinreleasing hormone (GnRH) release, shows photoperiodic variations and was thought to be a likely candidate for the photoperiodic control of reproduction in seasonal breeders, but increasing evidence points toward another RFamide peptide, the RF (Arg-Phe) amide-related peptide. We review our current understanding of the RFRP system in seasonal breeders, reporting the mechanisms through which melatonin impacts on RFRP synthesis and the effects of RFRPs in the seasonal control of reproduction including speciesdependant and sex-specific variations in the RFRP system
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