Intraovarian expression of GnRH-1 and gonadotropin mRNA and protein levels in Siberian hamsters during the estrus cycle and photoperiod induced regression/recrudescence

The hypothalamic-pituitary-gonadal (HPG) axis is the key reproductive regulator in vertebrates. While gonadotropin releasing hormone (GnRH), follicle stimulating (FSH), and luteinizing (LH) hormones are primarily produced in the hypothalamus and pituitary, they can be synthesized in the gonads, suggesting an intraovarian GnRH-gonadotropin axis. Because these hormones are critical for follicle maturation and steroidogenesis, we hypothesized that this intraovarian axis may be important in photoperiod-induced ovarian regression/recrudescence in seasonal breeders. Thus, we investigated GnRH-1 and gonadotropin mRNA and protein expression in Siberian hamster ovaries during (1) the estrous cycle; where ovaries from cycling long day hamsters (LD;16L:8D) were collected at proestrus, estrus, diestrus I, and diestrus II and (2) during photoperiod induced regression/recrudescence; where ovaries were collected from hamsters exposed to 14 weeks of LD, short days (SD;8L:16D), or 8 weeks post-transfer to LD after 14 weeks SD (PT). GnRH-1, LHβ, FSHβ, and common α subunit mRNA expression was observed in cycling ovaries. GnRH-1 expression peaked at diestrus I compared to other stages (p < 0.05). FSHβ and LHβ mRNA levels peaked at proestrus and diestrus I (p < 0.05), with no change in the α subunit across the cycle (p > 0.05). SD exposure decreased ovarian mass and plasma estradiol concentrations (p < 0.05) and increased GnRH-1, LHβ, FSHβ, and α subunit mRNA expression as compared to LD and, except for LH, compared to PT (p < 0.05). GnRH and gonadotropin protein was also dynamically expressed across the estrous cycle and photoperiod exposure. The presence of cycling intraovarian GnRH-1 and gonadotropin mRNA suggests that these hormones may be locally involved in ovarian maintenance during SD regression and/or could potentially serve to prime ovaries for rapid recrudescence.

Research highlights
► Intraovarian GnRH mRNA peaks at diestrus I in the hamster estrus cycle.
► Ovarian FSH and LH mRNA peak at diestrus I and proestrus, no change in subunit.
► Inhibitory short days increase ovarian GnRH-1, LH, FSH, and subunit mRNA.
► GnRH, LH, and FSH protein also change across the estrous and photoperiod cycles.