Reproductive hormones, hepatic deiodinase messenger ribonucleic acid, and vasoactive intestinal polypeptide-immunoreactive cells in hypothalamus in the heat stress-induced or chemically induced hypothyroid laying hen

Abstract

Heat stress (HS) effects on reproductive and thyroid hormones have been well documented; however, mechanisms of action are not well understood. Two studies were conducted to determine whether HS-induced and hypothyroid-induced effects are similar in the laying hen, with regard to reproductive hormones and vasoactive intestinal polypeptide (VIP)-immunoreactive cells in the hypothalamus. In study 1, thirty 32-wk-old Hy-Line W-36 laying hens, housed at 22 degrees C, were cannulated. On d 0 and then on d 1 to 5 of HS (35 degrees C, 50% RH), a daily blood sample was obtained and assayed for triiodothyronine (T(3)), thyroxine (T(4)), 17beta-estradiol (E(2)), progesterone (P(4)), prolactin (PRL), and VIP, and T(3):T(4)was calculated. On d 0, 1, 3, and 5, livers were obtained for hepatic type I deiodinase mRNA (cDI-1) determination. In study 2, eighty 32-wk-old hens were randomly assigned to 4 treatments of 20 birds each: 1) HS (36.5 degrees C, 50% RH), 2) thiouracil-induced hypothyroidism (HY), 3) HY + T(4) administration, and 4) control (22 degrees C). Beginning on d 1 of the 5-d study, daily blood samples (3.0 mL) were removed and assayed as in study 1. On d 5, brains were removed from 3 hens/treatment and immunoreactivity of VIP cells was determined. In study 1, HS reduced E(2), P(4), T(3) (P = 0.0001), T(3):T(4) ratio (P = 0.0078), and hepatic type I deiodinase mRNA (P = 0.0204) and increased T(4) (P = 0.0013); there was no effect on VIP or PRL. In study 2, HS and HY reduced T(3), T(3):T(4) ratio, and E(2) (P = 0.0001) and increased PRL (P = 0.0045); HS alone decreased P(4) (P = 0.0001). In HY + T(4), plasma E(2) and PRL were similar to control. Vasoactive intestinal polypeptide increased in plasma of HY birds, but there was no effect of HS or HY + T(4). Immunoreactive VIP cells increased (P = 0.0036) in nucleus inferior hypothalami of HS and HY brains. In HY + T(4), VIP immunoreactive cell numbers were similar to control. It appears that HY induced chemically or by HS exerts similar effects on reproductive hormones in the hen; the results suggest involvement of the VIP-PRL pathway even though peripheral blood concentrations were not consistent between studies.