Regulatory Effects Of Photoperiod On Follicular Development In Black-lined Hamsters

The striped hamster(Cricetulus barabensis)is a small rodent with strong reproductive ability and lives in the northern temperate zone.It distributes in a wide range of China's area,and is one of the main rodent pests in the farmlands of north China.Rodents,as key species in biomes,play an important role in maintaining the ecosystems' structure and function.Therefore,it is of great significance to understand the potential fluctuation mechanism of rodent population.The striped hamster possess a typical seasonal breeding character.Its population fluctuation has a typical seasonal rhythm,and the photoperiod is the most stable ecological signal to affect seasonal reproduction.By stimulating the pineal gland to secrete melatonin,the photoperiod converts light signals into physiological signals and stores them in animals' bodies.This mechanism synchronizes animal's reproduction state with the external environment to ensure that animals reproduce in the most favorable season and successfully give birth to offspring.Studies have shown that photoperiod has certain role to regulate the seasonal reproductive activities of animals by regulating the gonads' structures and functions.Gonad is the main organ to regulate animal reproduction.Therefore,the physiological and biochemical indicators of gonads are important parameters representing the breeding states and strength of animals.In this study,female striped hamsters were separately raised under short daylight(SD,8L: 16D),moderate daylight(MD,12L:12D),and long daylight(LD,16L: 8D)conditions to explore the effects of photoperiod on the structures and functions of the ovaries.Meanwhile,the potential mechanism of photoperiods regulating the ovaries' structure and function at molecular level was explored to lay the foundation for understanding the seasonal reproduction mechanism of the striped hamster.Firstly,the ELISA method was used to examine the serum concentration of follicle stimulating hormone(FSH)and luteinizing hormone(LH)and the estradiol(E2)content in feces of the striped hamsters under different photoperiods.The results showed that FSH concentration gradually increased with the lengthening of the light time,and the order of the FSH concentration under different photoperiods is LD > MD > SD.The serum LH concentration and the E2 content in feces did not vary significantly under different photoperiods.Secondly,the difference of the ovarian weight and the number of growing follicles among the striped hamsters under different photoperiods were explored.The results were as follows: the ratio of ovarian weight/body weight in the striped hamster under SD is significantly lower than that under MD and LD(P < 0.05),indicating that SD may cause ovarian atrophy.The number of small secondary follicles in the striped hamster in MD and LD groups was significantly higher than that in the SD group(P < 0.05).Compared to MD group,the total number of the growing follicles has an increasing tendency with the light time elongation and a decreasing tendency with the light time shortening.The number of growing follicles in LD group was higher than that in SD group,indicating that the striped hamsters in the LD group have more chances to ovulate.In addition,based on the consistency of follicle development cycle and estrus cycle,the estrus cycle of female striped hamsters under different photoperiods were continuously examined.The results showed that the estrus rate of the striped hamsters under SD,MD and LD conditions was 18.18%,70.00%and 72.27%,respectively.Under SD conditions,only a few striped hamsters can enter estrus normally,which means that ovulation is more difficult under SD conditions than that under MD and LD conditions.This result is in consistent with that the number of the growing follicles is lower under SD conditions than that under MD and LD conditions.Finally,the differential expression levels of the factors regulating the ovarian weight and the number of growing follicles in the striped hamsters were examined at the m RNA and the protein levels to understand the mechanisms that the photoperiod regulates the ovarian weight and the number of growing follicles in the striped hamsters.The results were as follows: the expression of Cyclin D2 at the protein levels,which controls the proliferation of granulosa cells,was positively regulated by the photoperiods,Cyclin D2 protein expression level in MD and LD groups was significantly higher than that in SD group(P < 0.05),While the expression levels of PCNA controlling the proliferation of granulosa cells at the m RNA and the protein levels were not significantly different(P > 0.05).In addition,photoperiod could stimulate the expression of CYP19A1,which is a rate-limiting enzyme for the estrogen synthesis,and the order of the expression level of CYP19A1 at the m RNA and protein levels is LD > MD > SD.Compared to the SD group,Photoperiod could also positively stimulate the expression of Inhibin-? in the LD group at the protein level(P < 0.05),which is a factor regulating follicular differentiation.The expression of FSHR at the protein level under SD conditions and MD conditions was significantly higher than that under LD conditions(P < 0.05).The expression of PACAP at protein level was positively regulated by the photoperiods,showing the highest expression level under LD conditions and the lowest under SD conditions.In conclusion,SD could lead to the ovarian atrophy,the ovarian mass loss,reducing the quantity of growing follicles,interrupting the estrus cycle and terminating the ovulation in the striped hamsters.In contrast,the striped hamster could keep the ovarian structure and function normal under MD and LD conditions.FSH may be the main gonadotropin that causes differences of the ovarian structure and function under different photoperiods by regulating the expression of follicular development factors,including granulosa cell proliferation genes,follicular differentiation genes,steroid-producing genes,FSHR and PACAP.Photoperiod may regulate the seasonal reproduction of the striped hamsters by regulating the structure and function of ovaries and the expression of follicle development factors.