Interference of polychlorinated biphenyls in hepatic and brain thyroid hormone metabolism in fetal and neonatal rats

The effects of prenatal oral administration of 0.2, 0.6, and 1.8 mg/kg body wt of 3,3',4,4'5,5'-hexachlorobiphenyl (HCB) on Day 1 of gestation and a combination of 1 mg/kg 3,3',4,4'-tetrachlorobiphenyl (TCB) from Day 2 to Day 18 with 0.6 mg HCB/kg body wt on Day 1 of gestation on thyroid hormone status and peripheral thyroid metabolism were studied in pregnant Wistar rats and their fetuses and offspring. Plasma total thyroxine and free thyroxine levels were reduced by HCB in a dose-dependent fashion in pregnant rats (Days 12 and 20 of gestation) and neonates (Day 21 postpartum), while only a combined dose of HCB and TCB was effective in decreasing fetal thyroid hormone levels by 65% on Day 20 of gestation. The activity of type II thyroxine 5'-deiodinase (5'D-II), the enzyme responsible for the deiodination of thyroxine (T4) to biologically active triiodothyronine in the brain, was examined in whole brain homogenates in fetuses and neonates. Decreases in plasma thyroid hormones were accompanied by significant increases, up to 100%, in 5'D-II activity in brain homogenates from fetuses (Day 20 of gestation) and neonates (Days 7 and 21 postpartum). The glucuronidation of 125I-T4 by hepatic microsomes was increased by at least 100% relative to control levels by all treatments in fetuses (Day 20 of gestation) and increased at least 40% in neonates (Days 7 and 21 postpartum) by a dose of 0.6 and 1.8 mg HCB/kg and the combined dose. These data indicate that prenatal HCB and/or TCB administration result in increased peripheral T4 metabolism. The increase in 5'D-II activity suggests that local hypothyroidism occurs in the brains of fetal and neonatal rats exposed to HCB and/or TCB. Since these effects occur during a period in which thyroid hormones play an important role in brain maturation, they may help explain the mechanism of developmental neurotoxicity induced by polychlorinated biphenyls.