Behavioral effects of neonatal thyroid hormones and differential postweaning rearing in rats.

Male and female rats (T rats) given an injection of 75 microgram tri-iodothyronine (T3) and 25 microgram thyroxine (T4) on Day 3 after birth were raised under enriched (E) and impoverished (I) postweaning conditions. Observations of their open-field behavior and Days 63 and 112 revealed a higher rate of activity in T groups as compared to controls (C) as well as faster running times and more entries/min in Hebb-Williams maze testing on Days 74-86, indicating behavioral hyperactivity in T rats. Active-avoidance testing at age 91 days revealed a higher number of intertrial crossings and significantly faster unconditioned response and conditioned response latencies in T as compared to C groups. Enriched postweaning rearing altered the behavioral effects in T rats comparatively little, least in the open-field and more so for escape- and avoidance-latencies and for the number of errors in the Hebb-Williams maze.