Characterization of gene expression in the hypothalamo-pituitary-gonadal axis during the preovulatory surge in the turkey hen

Abstract

A preovulatory surge (PS) of luteinizing hormone (LH) and progesterone triggers follicle ovulation, which is the first step of egg production and is orchestrated by the hypothalamo-pituitary-gonadal (HPG) axis. In the HPG axis, hypothalamic peptides, gonadotropin releasing hormone, and gonadotropin inhibitory hormone, control the production of follicle stimulating hormone and LH by the pituitary, which subsequently regulate ovarian production of estradiol and progesterone, respectively. The goal of this study was to characterize the HPG axis function of average egg producing hens by assessing plasma hormone profiles and hypothalamic, pituitary, and follicle gene expression outside and during 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 (P > 0.05), but plasma progesterone levels increased 8-fold during the PS when compared to basal progesterone levels (P < 0.05). HPG axis gene expression related to ovulation stimulation (e.g., GNRH, GNRHR, and LHB) was down-regulated during the PS; whereas gene expression related to follicle development (e.g., FSHB) was up-regulated during the PS. Additionally, in the hypothalamus and pituitary, estradiol receptor expression was up-regulated during the PS, whereas progesterone receptor expression was down-regulated during the PS. In the follicle cells, gene expression pertaining to progesterone (e.g., STAR), androgen (e.g., HSD17B1), and estradiol (e.g., CYP19A1) production was up-regulated during the PS. Prior to this study, the HPG axis had yet to be characterized during the PS in the turkey hen. This study showed that the PS significantly impacted gene expression in the hypothalamus, pituitary, and ovarian follicles. These results provide a foundation for further research into the regulation of ovulation and egg production in turkey hens.

Keywords: egg production; gene expression; hypothalamo-pituitary-gonadal axis; ovulation; steroid hormone.

Figure 1.

Plasma progesterone and estradiol hormone profiles in hens sampled outside (basal) and inside (surge) of the preovulatory surge (PS). Significant steroid plasma concentration differences are denoted with an asterisk.

Figure 2.

Hypothalamic gene expression of hypothalamo-pituitary-gonadal (HPG) axis releasing factors and steroid hormone receptors in hens samples outside (basal) and inside (surge) of the preovulatory surge (PS). Normalized data are presented relative to basal expression for each gene. Significance is denoted with an asterisk.

Figure 3.

Pituitary gene expression of hypothalamo-pituitary-gonadal (HPG) axis releasing factor receptors, gonadotropin subunits, and steroid hormone receptors in hens samples outside (basal) and inside (surge) of the preovulatory surge (PS). Normalized data are presented relative to basal expression for each gene. Significance is denoted with an asterisk.

Figure 4.

Follicle granulosa layer gene expression related to gonadotropin action and progesterone production in the hypothalamo-pituitary-gonadal axis comparing the expression of hens samples outside (basal) and inside (surge) of the preovulatory surge (PS). Normalized data are presented relative to basal expression of the F1 follicle for each gene. Significance is denoted with an asterisk.

Figure 5.

Follicle theca interna layer gene expression related to gonadotropin action and androgen production in the hypothalamo-pituitary-gonadal (HPG) axis comparing the expression of hens samples outside (basal) and inside (surge) of the preovulatory surge (PS). Normalized data are presented relative to basal expression of the F1 follicle for each gene. Significance is denoted with an asterisk.

Figure 6.

Follicle theca externa layer gene expression related to gonadotropin action and estradiol production in the hypothalamo-pituitary-gonadal (HPG) axis comparing the expression of hens samples outside (basal) and inside (surge) of the preovulatory surge (PS). Normalized data are presented relative to basal expression of the F1 follicle for each gene. Significance is denoted with an asterisk.

Figure 7.

Summary of relative hypothalamo-pituitary-gonadal (HPG) axis gene expression changes during the preovulatory surge (PS). Red, white, and green represent increased expression, no change in expression, and decreased expression during the PS, respectively. The legend at the bottom of the figure displays how the color intensity relates to the fold change difference seen during the PS. Significant changes in gene expression are denoted by an asterisk.