Development of home orientation in hypothyroid and hyperthyroid rat pups.

In order to assess the effects of perinatal hypothyroidism or hyperthyroidism on the development of an integrated behavioral response, we tested hypothyroid, hyperthyroid, and control pups, as well as pups receiving thyroxine replacement therapy, for the development of the home orientation response. Hypothyroidism was induced in the pups by feeding the pregnant or lactating female a diet of .2% propylthiouracil from Day 15 of gestation to Day 22 postpartum. Pups receiving replacement therapy and pups made hyperthyroid were injected daily with thyroxine, starting at birth. The ability of the pups to initiate and maintain locomotion toward the nest was assessed between Days 4 and 22. Hyperthyroid, control, and replacement therapy pups behaved very similarly on the task, showing a peak in the percentage of pups homing between Days 12 and 16. Hypothyroid pups showed a delay in the peak percentage until Day 20, although the percentage of pups was similar to that found in other treatments. An integrated behavioral response can be delayed by hypothyroidism and still emerge apparently intact at a later age.