br Conclusion The wild study

Conclusion The wild study reports seasonal variations in reproductive physiology, behavior and atp gamma s synthesis plasticity at different age stages in wild-caught Mongolian gerbils. The gerbils in breeding seasons have more mature sexual development, higher exploratory behavior, higher novelty preference, higher GnRH expression in hypothalamus and higher cell proliferation in hypothalamus, amygdala and hippocampus than those in non-breeding (hoarding) season (Table 4). The laboratory study shows that photoperiod alone could not alter reproductive traits, behavior, cell proliferation or cell survival in the detected brain regions. All these data demonstrate that structural plasticity in GnRH expression and brain cell proliferation is associated with seasonal reproduction and food hoarding in gerbils, but a single photoperiod cue may not be critical to induce seasonal life-history traits and brain structural plasticity. The refractoriness to photoperiod in reproduction and neurogenesis may partly support the phenomenon of winter reproduction in Mongolian gerbils. These findings provide the proximate physiological and neural basis for these seasonal life-history traits of breeding and food hoarding in small mammals.
Acknowledgements This work was supported by the National Natural Science Foundation of China (31470474 and 31472006) and Beijing Natural Science Foundation (5172024). The authors thank Meghan Donovan from Department of Psychology and Program in Neuroscience, Florida State University for her editing help, and thank all the members of the Animal Physiological Ecology Group for the discussion about this study. All authors declare no conflict of interests.
Introduction In vertebrates, reproduction is regulated by the hypothalamic-pituitary-gonadal axis, which senses different internal and external signals. The first neuropeptide identified in this pathway was the decapeptide gonadotropin-releasing hormone (GnRH), discovered in the 70s in mammals, where it was demonstrated its key role in the control of pituitary gonadotropins and reproduction (Burgus et al., 1971, Matsuo et al., 1971). In all vertebrates, GnRH is synthetized as a prepro-hormone containing the decapeptide itself and a GnRH-associated peptide named GAP (Seeburg and Adelman, 1984, Roch et al., 2014). Nowadays, a number of variants of GnRH have been identified in vertebrates, and multiple GnRH variants are found in the brain of a single species (Guilgur et al., 2006, Roch et al., 2014). These GnRH variants are currently classified into three different types, according to their amino-acid sequence, neuroanatomical localization, embryological origin, and synteny: GnRH1, GnRH2, and GnRH3 (Guilgur et al., 2006, Kim et al., 2011, Tostivint, 2011, Decatur et al., 2013, Plachetzki et al., 2016). In vertebrates, GnRH1 is the most variable GnRH type according to its amino-acid sequence and it is expressed in neurons originated from the olfactory placode during embryogenesis (González-Martínez et al., 2004, Kah et al., 2007). This variant plays the classical hypophysiotropic function in mammals, as well as in most other gnathostomes. GnRH2, originally discovered in chicken (Miyamoto et al., 1984), was then found in almost all gnathostomates and it is mainly expressed by midbrain neurons. It has been proposed that this variant plays a key role on reproductive behavior (revised in Parhar et al. (2016)). Finally, GnRH3, originally discovered in salmon (Sherwood et al., 1983), is expressed in ventral forebrain neurons. Although it was considered to be present only in teleost fish species, some reports suggested a more ancient origin (Decatur et al., 2013, Roch et al., 2014). This variant is important as a neuromodulator of olfactory and visual information related to reproduction (Kawai et al., 2010, Umatani et al., 2015) and also plays hypophysiotropic functions, particularly in those teleost species expressing only GnRH2 and GnRH3 as most Cypriniformes and Salmoniformes (Guilgur et al., 2006, Zohar et al., 2010).