Some studies show that testosterone may
Some studies show that testosterone may be involved in regulating cell proliferation and its stimulating effect has been proved in castrated rats (Wainwright et al., 2011), but not in mice and meadow voles (Ormerod and Galea, 2003; Benice and Raber, 2010). However, the seasonal difference in cell proliferation of gerbils emerged in subadults and adults, whereas the seasonal variation in testosterone didn't appear until adulthood. It seems that seasonal changes in Itraconazole cell proliferation are not likely attributable to plasma testosterone. It is well-known that locomotion (exercise) and exploration increase cell proliferation and neurogenesis in hippocampus in mice and wild rodents (van Praag et al., 1999; Lieberwirth et al., 2016; Ramírez-Rodríguez et al., 2018). In addition, a larger social network can lead to better memory function and reduced neuroinflammation in aged mice (Smith et al., 2018). Therefore, it seems reasonable to speculate on the possible causal connection between the seasonal increases in locomotion and exploration and the seasonal increases in brain cell proliferation. The present study also showed that the cell proliferation in some specific brain regions, including SVZ, DG, Arc, and Amy, revealed sex differences, where adult females generated more new cells than males. Similar sexual dimorphism is also observed in adult rats and meadow voles (Galea and Mcewen, 1999; Spritzer et al., 2017), but not in grey squirrels (Sciurus carolinensis) (Lavenex et al., 2000). The sex hormones may be involved in this sexual dimorphism (Galea et al., 2006), and the stimulating effect of estradiol on hippocampal cell proliferation has been confirmed in female rodents (Tanapat et al., 1999; MeFrick and Kim, 2018). The sexual dimorphism in cell proliferation in specific brain regions may have some implications for sex differences in reproductive behavior or other behavior. Short-day length is a reliable cue for predicting winter and its suppressing effects on reproduction have been confirmed in majority of small mammals, including white-footed mice (Peromyscus leucopus) (Pyter et al., 2005), Syrian hamsters (Revel et al., 2006) and Siberian hamsters (Greives et al., 2008). Several previous studies examining the photoperiodic response of reproduction in Mongolian gerbils demonstrated that 10-week SD acclimation (<10 hour light) induced testicular regression in adult males in comparison with LD control (Petterborg et al., 1984; Karakaş and Gündüz, 2002). However, gerbils treated with 10-week SD acclimation did not shrink sexual organs, decrease sexual hormones, or change exploratory behavior, anxiety-like behavior, or short-term memory in the present study. Moreover, the transformation of the photoperiod still had no effects on the recovery of photoperiodic sensitivity. Recently, the view of “cyclical histogenesis” has received more and more attention (Hazlerigg and Lincoln, 2011), and the annual cycle from anestrus to reproduction may be a recapitulation of early brain developmental mechanisms (Murphy and Ebling, 2011). Hence, we examined cell proliferation and survival in several brain regions. No photoperiodic effects were found in the two main regions (SVZ and DG). However, gerbils treated with LD had more cell proliferation in VMH and Arc than SD gerbils. Less is known about the role of these new cells and why this photoperiodic difference became undetectable in cell survival. Previous studies indicate that Mongolian gerbils can maintain stable energy balance in different light cycles (Karakaş and Gündüz, 2002; Li et al., 2003; Li and Wang, 2005; Yao et al., 2018). In consistence with our results, SD acclimation for 10 weeks in 14L-born prepubertal gerbils did not induce testicular regression except under 0 L, 2 L and 24 L (Karakaş and Gündüz, 2002). Snell strain house mice (Mus musculus) acclimated under SD also had unchanged reproductive organ weights compared to LD control (Petterborg et al., 1984). Different photoperiod experiences in early life may affect testis development and different species may have diverse responses to photoperiod. Food restriction is critical to reduce body weight and inhibit reproduction in Mongolian gerbils (Karakaş et al., 2005) and Abert's Towhees Melozone aberti (Davies et al., 2015). These data indicate that the interaction of photoperiod and food shortage or low temperature, but not photoperiod alone, induced seasonal breeding. This photoperiodic refractoriness in gerbils may explain the adaptive mechanism for their wide distribution from west to east (>1500 km) stretching over desert, desert steppe and typical steppe of northern China (Zhou et al., 2001), and also contribute to stepping over the barrier for winter reproduction in the context of enough food availability.