117.6 Saturday, Jan. 7 Gonadotropin suppression alone does not induce the short-day ovarian phenotype in Siberian hamsters ZYSLING, DA*; PARK, S-U; MCMILLAN, EL; PLACE, NJ; Cornell Univ, Ithaca NY email@example.com
Many seasonal breeders time their reproductive effort to the spring and summer months to insure adequate resource availability for the production and care of young. Females born before the summer solstice (long days, LD) reach maturity quickly and often breed that same year. In contrast, females born after the summer solstice (short days, SD) may delay reproductive development to the following spring when conditions are favorable. In Siberian hamsters, development in SD is associated with significant structural and functional differences in the ovary as compared to females held in LD. These SD-induced differences include a greater number of primordial follicles and an abundance of hypertrophied granulosa cells (HGCs), which are immunoreactive for anti-Müllerian hormone (AMH). Ovarian developmental differences in SD are thought to result from suppression of gonadotropins, but this hypothesis has not been adequately tested. Therefore, the goal of this study was to determine whether gonadotropin suppression explains the SD ovarian phenotype. Specifically, we treated LD females with the GnRH antagonist acyline every other day, beginning at weaning (postnatal day 18) and continuing until 10 wk of age. Control LD and SD females were similarly treated with vehicle. We found that acyline induced SD-like decreases in ovarian and uterine horn mass and serum FSH and AMH. Conversely, primordial follicle numbers and ovarian Amh mRNA levels were not significantly different than in controls, and HGCs were not observed. Our data suggest that gonadotropin suppression alone does not completely account for the SD ovarian phenotype and other factors may be involved, including SD-induced changes in other hormones, such as melatonin and prolactin. This possibility will be the subject of future studies.