Influence of triiodothyronine (T 3) on secretion of steroids and thyroid hormone receptor expression in chicken ovarian follicles

Abstract

The present study was designed to (1) assess the role of triiodothyronine (T 3) with regard to in vitro steroid hormone secretion by chicken ovarian follicles; (2) determine whether T 3 influences the in vivo function of the pituitary-ovarian axis in the hen; and (3) detect expression of thyroid hormone receptor (TR) mRNA in chicken ovarian follicles. In the first experiment, laying hens were decapitated 22.5 h before ovulation. White prehierarchical follicles (1–8 mm) and fragments of theca and granulosa layers of the 3 largest yellow preovulatory follicles F3-F1 (22–35 mm) were incubated in a medium supplemented with T 3 (0, 0.1, 1, 10, 100, or 1000 ng/mL) or ovine luteinizing hormone (LH) (10 ng/mL) in combination with doses of T 3 (1, 10, and 100 ng/mL). Triiodothyronine decreased basal and LH-stimulated estradiol secretion by white follicles and the theca layer of all preovulatory follicles. On the other hand, it increased progesterone secretion by F2 and F1 follicles. In the second experiment, hens were injected 1 h after ovulation with saline (control) or T 3 (10 μg/100 g body weight, intraperitoneally). Results indicated that exogenous T 3 decreased plasma concentrations of LH and estradiol and increased plasma concentrations of progesterone. In the third experiment, using reverse transcription polymerase chain reaction (RT-PCR) analysis, expression of thyroid hormone receptor (TRα and TRβ0), mRNA was detected in all of the ovarian compartments. The expression of TRα mRNA was relatively greater in comparison with TRβ0. There were no differences between white ovarian follicles in the expression of TRα and TRβ0 mRNA. A considerably higher TRα and lower TRβ0 expression was detected in the granulosa layer of preovulatory follicles in comparison with the theca layer. In conclusion, the data indicate that thyroid hormones acting via nuclear receptors are involved in regulation of the pituitary-ovarian axis and processes associated with follicle growth and maturation.