Characterization of hypothalamo-pituitary-thyroid axis gene expression in the hypothalamus, pituitary gland, and ovarian follicles of turkey hens during the preovulatory surge and in hens with low and high egg production

Authors: Brady, Kristen; Long, Julie; LIU, HSIAO-CHING - University Of North Carolina; PORTER, TOM - University Of Maryland

Submitted to: Poultry Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2020
Publication Date: 12/26/2020
Citation: Brady, K.M., Long, J.A., Liu, H., Porter, T. 2020. Characterization of hypothalamo-pituitary-thyroid axis gene expression in the hypothalamus, pituitary gland, and ovarian follicles of turkey hens during the preovulatory surge and in hens with low and high egg production. Poultry Science. https://doi.org/10.1016/j.psj.2020.12.026.
DOI: https://doi.org/10.1016/j.psj.2020.12.026

Interpretive Summary: The influence of the thyroid axis on the initiation and cessation of egg production is well established, though the role of the thyroid axis during egg production, and more specifically during the preovulatory surge that triggers follicle ovulation, is not well established. This study sought to characterize thyroid hormone plasma concentrations and gene expression related to the thyroid axis outside and during the preovulatory surge. Additionally, the same parameters were examined in low egg producing and high egg producing hens to see how the thyroid axis is perturbed in hens with differing egg production levels. This study found that the thyroid axis is generally down-regulated during the preovulatory surge; however, low egg producing hens showed up-regulation of the thyroid axis during the preovulatory surge. The up-regulation of the thyroid axis in low egg producing hens may play a role in the reduced egg production levels seen in these hens.

Technical Abstract: The hypothalamo-pituitary-thyroid (HPT) axis has been shown to influence plasma progesterone levels of the preovulatory surge (PS) that triggers follicle ovulation. Dysregulation of the PS leads to lowered egg production. The presence of thyroid hormone receptors (THRs) in the reproductive axis indicates possible effects of thyroid hormone. Characterization of HPT axis plasma hormone concentrations and gene expression surrounding the PS was performed in average egg producing hens (AEPH), low egg producing hens (LEPH), and high egg producing hens (HEPH) (n=3 hens/group). Data were analyzed using the mixed models procedure of SAS, with significance indicated at P<0.05. AEPH and HEPH displayed lower levels of triiodothyronine (T3) and higher levels of thyroxine (T4) inside of the PS, whereas LEPH showed inverse T3 and T4 levels relative to the PS. Expression of mRNA for hypothalamic thyrotropin-releasing hormone (TRH), pituitary thyrotropin (TSHB), and the main thyroid hormone metabolism enzyme (DIO2) were downregulated during the PS in AEPH and HEPH. LEPH displayed higher expression of mRNA for hypothalamic TRH as well as pituitary TSHB and DIO2 compared to HEPH. AEPH expression of thyroid hormone receptor (THR) mRNAs was upregulated during the PS in the hypothalamus but downregulated in the pituitary. HEPH showed decreased expression of THR mRNAs in both the hypothalamus and pituitary when compared to LEPH. In ovarian follicles, THR mRNAs were more prevalent in the thecal layer of the follicle wall compared to the granulosa layer, and expression tended to decrease with follicle maturity. Minimal differences in follicular THR expression were seen between LEPH and HEPH, indicating that THR expression is unlikely to be responsible for steroid hormone production differences seen in LEPH and HEPH. Generally, down-regulation of the HPT axis was seen during the PS, and up-regulation of the HPT axis was seen in LEPH compared to HEPH. Further studies will be required to clarify the role of the HPT axis in the regulation of ovulation and egg production rates in turkey hens.