Neonatal thyroxine administration, behavioral maturation, and brain growth in mice of different brain weight

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Abstract
L‐thyroxine in amounts of .5, 1, and 2 μg was injected daily from age 5 through 14 days in 3 lines of mice selected for high (H), medium (M), or low (L) brain weight. Controls (injected with vehicle only) of these lines differed in ages of eye‐opening, first auditory startle, and perfection of a surface‐righting response. Thyroxine at .5 μg/day accelerated the maturation of eye‐opening, auditory startle, and aerial‐righting response, but had no effect on a grasping response or open‐field activity. Brain weights were depressed significantly in all thyroxine treated groups except in the .5‐μg dosage in H mice.