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

Abstract

Dysregulation of the preovulatory surge (PS) leads to lowered egg production. The hypothalamo-pituitary-thyroid (HPT) axis has been shown to influence plasma progesterone levels and follicle ovulation. The presence of thyroid hormone receptors (THR) in the reproductive axis suggests possible effects of thyroid hormone. To further understand the potential role of thyroid hormone on the PS, HPT axis plasma hormone concentrations and gene expression were characterized surrounding the PS 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. Average egg producing hens 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. Low egg producing hens displayed higher expression of mRNA for hypothalamic TRH as well as pituitary TSHB and DIO2 compared with HEPH. Average egg producing hens expression of THR mRNAs was upregulated during the PS in the hypothalamus but downregulated in the pituitary. High egg producing hens showed decreased expression of THR mRNAs in both the hypothalamus and pituitary when compared with LEPH. In ovarian follicles, THR mRNAs were more prevalent in the thecal layer of the follicle wall compared with 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 occurring between LEPH and HEPH. Generally, downregulation of the HPT axis was seen during the PS in AEPH and HEPH, whereas upregulation of the HPT axis was seen in LEPH. 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.

Keywords: HPG axis; HPT axis; egg production; preovulatory surge; turkey.

Figure 1

Plasma triiodothyronine (T3) and thyroxine (T4) hormone profiles in average. egg producing hens (AEPH) (Figures 1A and 1C) as well as low egg producing hens (LEPH) and high egg producing hens (HEPH) (Figures 1B and 1D). All 3 groups of hens were sampled outside (basal) and inside (surge) of the preovulatory surge (PS). Significant plasma thyroid hormone concentration differences between LEPH and HEPH for a given condition are denoted with an asterisk, whereas significant differences between basal and surge plasma thyroid hormone concentrations for a given egg production group are denoted with a dagger.

Figure 2

Triiodothyronine (T3) to thyroxine (T4) ratio in average egg producing hens. (AEPH) (A) as well as low egg producing hens (LEPH) and high egg producing hens (HEPH) (B). All 3 groups of hens were sampled outside (basal) and inside (surge) of the preovulatory surge (PS). Combined T3 and T4 plasma concentrations of AEPH under basal and surge conditions as well as of LEPH and HEPH, taking the PS into account, are shown in Figures 2C and 2D, respectively. Significant thyroid hormone ratio or combined plasma concentration differences between LEPH and HEPH for a given condition are denoted with an asterisk, whereas significant differences between basal and surge plasma thyroid hormone concentrations for a given egg production group are denoted with a dagger.

Figure 3

Hypothalamic gene expression of hypothalamo–pituitary–thyroid axis releasing factors, thyroid hormone receptors, and thyroid hormone metabolism enzymes in average egg producing hens (AEPH), low egg producing hens (LEPH), and high egg producing hens (HEPH) sampled outside (basal) and inside (surge) of the preovulatory surge (PS). The AEPH expression under basal and surge conditions is presented in Figure 3A, whereas LEPH and HEPH expression, taking the PS into account, is presented in Figure 3B. Normalized data are presented relative to basal expression for each gene. Significant expression differences between LEPH and HEPH for a given condition are denoted with an asterisk, whereas significant differences between basal and surge expression for a given egg production group are denoted with a dagger.

Figure 4

Pituitary gene expression for hypothalamo–pituitary–thyroid axis. releasing factor receptors, thyrotropin subunits, thyroid hormone receptors, and thyroid hormone metabolism enzymes in average egg producing hens (AEPH), low egg producing hens (LEPH), and high egg producing hens (HEPH) sampled outside (basal) and inside (surge) of the preovulatory surge (PS). The AEPH expression under basal and surge conditions is presented in Figure 4A, whereas LEPH and HEPH expression, taking the PS into account, is presented in Figure 4B. Normalized data are presented relative to basal expression for each gene. Significant expression differences between LEPH and HEPH for a given condition are denoted with an asterisk, whereas significant differences between basal and surge expression for a given egg production group are denoted with a dagger.

Figure 5

F1 follicle granulosa layer gene expression of hypothalamo–pituitary–thyroid. axis thyroid hormone receptors in average egg producing hens (AEPH), low egg producing hens (LEPH), and high egg producing hens (HEPH) sampled outside (basal) and inside (surge) of the preovulatory surge (PS). The AEPH expression under basal and surge conditions is presented in Figure 5A, whereas LEPH and HEPH expression, taking the PS into account, is presented in Figure 5B. Normalized data are presented relative to basal expression for each gene. Significant expression differences between LEPH and HEPH for a given condition are denoted with an asterisk, whereas significant differences between basal and surge expression for a given egg production group are denoted with a dagger.

Figure 6

F5 follicle theca externa layer gene expression of hypothalamo–pituitary–thyroid axis thyroid hormone receptors in average egg producing hens (AEPH), low egg producing hens (LEPH), and high egg producing hens (HEPH) sampled outside (basal) and inside (surge) of the preovulatory surge (PS). The AEPH expression under basal and surge conditions is presented in Figure 6A, whereas LEPH and HEPH expression, taking the PS into account, is presented in Figure 6B. Normalized data are presented relative to basal expression for each gene. Significant expression differences between LEPH and HEPH for a given condition are denoted with an asterisk, whereas significant differences between basal and surge expression for a given egg production group are denoted with a dagger.