Differences in in vitro responses of the hypothalamo-pituitary-gonadal hormonal axis between low and high egg producing turkey hens

Low egg producing hens (LEPH) ovulate less frequently than high egg producing hens (HEPH) and exhibit differences in mRNA levels for components of the hypothalamo-pituitary-gonadal (HPG) axis, suggesting differential responsiveness to trophic stimulation. Ovulation frequency is governed by the production and feedback of pituitary gonadotropins and ovarian follicle steroid hormones, which are regulated by HPG axis stimulation and inhibition at the hypothalamic level. Pituitary and follicle cells from LEPH and HEPH were subjected to in vitro hormonal treatments to stimulate or inhibit the HPG axis, followed by expression analysis of mRNA levels for HPG axis genes and radioimmunoassays for steroid hormone production. Statistical analysis was performed using the mixed models procedure of SAS. Pituitary cells from HEPH showed up-regulation of genes associated with ovulation stimulation, whereas LEPH cells showed up-regulation of genes associated with inhibition of ovulation. HEPH follicle cells displayed a higher sensitivity and responsiveness to gonadotropin treatment. Level of egg production impacted ovulation-related gene expression in pituitary cells as well as steroid hormone production in follicle cells, with HEPH displaying a greater positive response to stimulation. These findings indicate that differences in egg production among turkey hens likely involve differential responsiveness of the cells within the HPG axis.