|BRADY, KRISTEN - University Of Maryland|
|PORTER, TOM - University Of Maryland|
|LIU, HSIAO-CHING - North Carolina State University|
Submitted to: Research Journal of Poultry Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2019
Publication Date: 12/17/2019
Citation: Brady, K., Porter, T.E., Liu, H., Long, J.A. 2019. Characterization of gene expression in the hypothalamo-pituitary-gonadal axis during the preovulatory surge in the turkey hen. Research Journal of Poultry Sciences. 98(12):7041–7049. https://doi.org/10.3382/ps/pez437.
Interpretive Summary: Ovulation in the female bird is stimulated by sudden increase in luteinizing hormone (produced by the pituitary gland), known as the preovulatory surge, which signals the ovary to produce the hormone progesterone. The amount of time between ovulation of a follicle and production of a hard-shelled egg in the turkey hen is 24 to 26 hours. The timing of this process is controlled primarily by hormones from three organs (hypothalamus, pituitary gland and ovary) that are part of a loop known as the hypothalamo-pituitary-gonadal (HPG) axis; however, egg production also can be manipulated by the hypothalamo-pituitary-thyroidal (HPT) axis, which involves the thyroid gland rather than the ovary. The hormone profiles and expression of genes related to the HPG and HPT axes were characterized in turkey hens, either during or after the preovulatory surge. It was found that, during the preovulatory surge, the hormone estradiol was not affected, while the hormones progesterone and thyroxine were significantly higher, compared to samples taken outside of the preovulatory surge. Expression of genes within the HPG axis related to ovulation stimulation and progesterone production was reduced during the preovulatory surge; while HPT axis gene expression was decreased during the preovulatory surge. Both the HPG and HPT axis plasma hormone and gene expression levels were significantly impacted by the preovulatory surge, suggesting that the individual roles of each axis on ovulation regulation should be further investigated.
Technical Abstract: A preovulatory surge (PS) of luteinizing hormone (LH) and progesterone triggers the first step of egg production, follicle ovulation. The PS, ovulation, and, ultimately, egg production is controlled by the hypothalamo-pituitary-gonadal (HPG) axis but can be manipulated by the hypothalamo-pituitary-thyroidal (HPT) axis. In the HPG axis, gonadotropin releasing hormone (GNRH) and gonadotropin inhibitory hormone (GNIH) release from the hypothalamus impacts follicle stimulating hormone (FSH) and luteinizing hormone (LH) production from the pituitary, which, in turn, impacts ovarian production of estradiol and progesterone. At the ovarian level, less mature follicles respond to FSH to produce estradiol while more mature follicle respond to LH to produce progesterone. In the HPT axis, thyroid releasing hormone production in the hypothalamus causes pituitary production of thyroid stimulating hormone, causing production of thyroid hormones in the thyroid. Aside from crosstalk in the hypothalamus and pituitary, thyroid hormone receptors are present in the ovary. The hormone profiles and gene expression related to the HPG and HPT axes were characterized in turkey hens, sampled outside and inside of the PS (n=3 per group). Results were analyzed by a one-way ANOVA using the mixed models procedure of SAS. Plasma estradiol was not affected by the PS while plasma progesterone was significantly increased during the PS. Plasma triiodothyronine was decreased during the PS while plasma thyroxine was increased. HPG axis gene expression related to ovulation stimulation and progesterone production was reduced during the PS while expression related to follicle development and estradiol production was increased during the PS. HPT axis gene expression was decreased during the PS in the hypothalamus and pituitary, while expression differences were not seen in the follicle cells types outside and inside of the PS. HPG and HPT axis plasma hormone levels and gene expression levels were significantly impacted by the PS, requiring further investigation to refine the individual roles of each axis on ovulation regulation as well as the crosstalk between the two axes.