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.
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.
Elnagar
S A
SA
Poultry Production Department, Faculty of Agriculture, Alexandria University, Egypt.
Scheideler
S E
SE
Beck
M M
MM
D000818:Animals; D002645:Chickens / Q000502:physiology*; D004967:Estrogens / Q000097:blood; D005260:Female; D006358:Hot Temperature; D007031:Hypothalamus / Q000166:cytology*; D007037:Hypothyroidism / Q000139:chemically induced / Q000662:veterinary*; D007453:Iodide Peroxidase / Q000235:genetics / Q000378:metabolism*; D010058:Oviposition; D011374:Progesterone / Q000097:blood; D011388:Prolactin / Q000097:blood; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012098:Reproduction / Q000502:physiology; D013312:Stress, Physiological; D013963:Thyroid Hormones / Q000097:blood; D014660:Vasoactive Intestinal Peptide / Q000097:blood / Q000378:metabolism*
D004967:Estrogens; D012333:RNA, Messenger; D013963:Thyroid Hormones; D014660:Vasoactive Intestinal Peptide; D011374:Progesterone; D011388:Prolactin; D007453:Iodide Peroxidase