[Studies on the behavioral development of rats treated neonatally with l-thyroxine]

The effect of l-thyroxine administration from 2 to 10 days of age (Neo-T4) on the behavioral development was studied. Serum T4 levels in Neo-T4 rats showed a marked and dose-related decrease at 15 days of age and a less marked decrease at 62 days of age. The locomotor activity of the Neo-T4 rats was higher at 13 approximately 19 days of age and tended to be lower at 62 days of age than in the controls. At 17 days of age, apomorphine (0.1 mg/kg, s.c.) induced a marked increase in the frequency of sniffing down in the control males, whereas a decrease was noted in the Neo-T4 males. Age-matched females in both groups responded to apomorphine with a slight decrease in sniffing down behavior. An increase in the frequency of sniffing down by apomorphine (0.5 mg/kg) in adult Neo-T4 rats was less marked. TRH administration failed to induce hyperthermia in adult Neo-T4 males. The results indicate that neonatal administration of T4 in the rat induces alterations in the functional development of the brain in varying degrees dependent upon the functional integration of the neurons. An involvement of sex hormones is suggested.