IN OVO 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN EXPOSURE IN 3 AVIAN SPECIES .1. EFFECTS ON THYROID-HORMONES AND GROWTH DURING THE PERINATAL-PERIOD

Thyroid hormones are important in the perinatal growth and development of avian species, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds have been shown to cause alterations in these hormon es in laboratory animals. Since the decreased reproductive success in certain fish-eating bird populations exposed to TCDD and related compo unds is characterized by high embryo and hatchling mortality, we exami ned the effects of in ovo TCDD exposure on plasma thyroid hormone conc entrations (total T-3, total T-4) and body and skeletal growth during the perinatal period in the domestic chicken (Gallus gallus), domestic pigeon (Columba livia), and great blue heron (Ardea herodias), Hepati c ethoxyresorufin O-deethylase (EROD) activity was also determined as an enzymatic marker of cytochrome P450IA induction by TCDD. [H-3]TCDD was injected into the air cell of chicken eggs (21-day incubation peri od) on Embryonic Day 4.5 (0.1 mu g/kg egg), pigeon eggs (18-day incuba tion period) on Embryonic Day 3.5 (1 mu g/kg egg) and Embryonic Day 14 (3 mu g/kg egg), and heron eggs (28-day incubation period) at approxi mately the midpoint of incubation (2 mu g/kg egg). Chickens were eutha nized on Embryonic Days 17 and 19, day of hatch (Embryonic Day 21), an d Days 2 and 4 after hatch, Pigeons and herons were euthanized either at hatch (Embryonic Days 18 and 28, respectively), or fed an uncontami nated diet for 7 days prior to sacrifice, Although hepatic EROD activi ty was induced 13- to 43-fold above controls in chickens, there was no effect of TCDD exposure on hatchability, body growth, subcutaneous ed ema, or plasma thyroid hormone levels. In pigeons exposed to TCDD on E mbryonic Day 3.5, EROD was induced 6- to 15-fold, hatchability was dec reased, liver to body weight ratio was elevated, and body and skeletal growth were decreased (p < 0.01); however, there was no effect of TCD D exposure on plasma thyroid hormone levels, Similarly, in pigeons exp osed to TCDD on Embryonic Day 14, EROD was induced 10- to 14-fold, liv er to body weight ratio was elevated, and body and skeletal growth wer e decreased (p < 0.01), but there was no effect of TCDD treatment on p lasma thyroid hormone levels, In herons, hepatic EROD activity was ind uced 2- to 3-fold above control birds, similar to EROD activities meas ured in heron hatchlings exposed to environmental levels of TCDD and r elated chemicals in the Strait of Georgia, British Columbia. However, this level of TCDD exposure had no effect on plasma thyroid hormone le vels or body growth in herons, Collectively, these results suggest tha t perinatal plasma thyroid hormone levels cannot be used as relatively noninvasive biomarkers of TCDD exposure during embryonic development in chickens, pigeons, and great blue herons. (C) 1996 Academic Press, Inc.