Medical Institute of Environmental Hygiene, Heinrich Heine University of Düsseldorf, Düsseldorf, Germany
Polychlorinated biphenyls (PCBs) are wide-spread environmental contaminants which accumulate in the food chain and cause neurobehavioral alterations in animals and humans after prenatal exposure. Previous experiments in our laboratory demonstrated behavioral deficits by prenatal exposure of rats to a coplanar PCB congener but not to an orthochlorinated PCB congener. It is reported that coplanar congeners reduce thyroid hormone levels. Thyroid hormones play an important role during neuronal development. Therefore thyroid hormone disturbances produced by PCBs may mediate changes during neuronal development and alter behavior in the offspring.
We compared a coplanar PCB congener with the thyreostatic compound PTU. Dams were exposed to 3,4,3',4'-TCB from gestational day 7 to 18 at a dose of 1 mg/kg body weight per day. The positive control got propylthiouracil (PTU) in the drinking water from gestational day 7 to postnatal day 21 (PND 21). Measurement of thyroid hormone levels in the serum and the thyroid indicated reductions in T4 levels in the PCB and PTU group. Male offspring were examined for spatial learning in a Morris water maze starting at PND 117 for 18 days. The animals had to find an unvisible platform 1.5 cm above the water surface for 12 days and 5 reversal learning days. The PTU exposed offspring swam a longer distance during the first 2 days and had a higher velocity during these test days than the control animals and the PCB exposed animals. Analysis of zones around the platform resulted in changes during one experimental day only. The results demonstrate that changes in thyroid hormone levels do not produce changes in spatial learning in the Morris water maze. Either the hypothyreosis was too weak pronounced to affect the hippocampus and spatial learning behavior or the learning task was not sensitive to detect subtle alterations in neurobehavior.