Neonatal thyroxine stimulation accelerates the maturation of both locomotor and memory processes in mice

In two experiments mice were injected with thyroxine on Postnatal Days 1, 2, and 3, and the subsequent effects upon the development of the swimming reflex and the emergence of instrumental learning/memory processes were examined. In agreement with past studies, early thyroxine treatment accelerated the maturation of swimming capacities and general physical development compared with littermate controls receiving saline injections. In the second study, thyroxine- and saline-treated mice received 25 training trials on a shock-escape T-maze taks at 7, 9, 11, or 13 days of age with a retention test 24 hr later. The results indicated that while learning was equivalent within each of the ages between the treatment groups, onset of 24-hr retention capacity occurred approximately 2 days earlier in the thyroxine-treated mice than in controls. In addition, a performance deficit was observed in the thyroxine mice at the oldest age tested, in agreement with previous reports. The results of these experiments suggest that early hyperthyroidism results in earlier maturation of both locomotor and memory processes, followed by later performance deficits.