Evidence of a thyrotropin-releasing activity of ovine corticotropin-releasing factor in the domestic fowl (Gallus domesticus)

Ovine corticotropin-releasing factor (oCRF) administered to 19-day-old chicken embryos (E19) increased plasma concentration of pituitary glycoprotein alpha-subunit concentrations within 15 min for at least 4 hr. Follicle stimulating hormone levels were unchanged, while plasma luteinizing hormone concentrations only began to increase 1 hr after the oCRF treatment. Calculation of circulating thyrotropin (TSH) indicator values revealed a rapid elevation in TSH plasma levels following oCRF. Concentrations of thyroxine (T-4), 3,5,3'-triiodothyronine (T-3), 3,3',5'-triiodothyronine (rT(3)), and corticosterone increased from 1 hr postinjection. Hypothalamic outer ring deiodinating type II increased and hepatic inner ring deiodinating type III fell after 2 and 4 hr, explaining at least in part the plasma T-3 increase at the end of the experiment. In a second experiment, using E18 chicks, a comparison was made between the effects of a single injection of 2 mu g oCRF and 20 mIU bovine TSH. Both hormones increased T-4, T-3, and rT(3) plasma concentrations, supporting the hypothesis of a TSH-releasing activity for oCRF in the embryonic chicken. The proposed TSH-mediated effect of CRF on thyroid function was further confirmed in two in vitro experiments in which oCRF did not directly influence the thyroidal T-4 secretion but, when applied to pituitaries, clearly increased the alpha-subunit release. In chickens CRF is concluded to not only control the adrenal axis, but also to participate in the coordination of avian TSH release. (C) 1996 Academic Press, Inc.