Effects of neonatal thyroid hormone stimulation and differential preweaning rearing on open-field behavior in adult rats

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Abstract
In 2 experiments, rats were given a mixture containing high doses of 1‐tri‐iodothyronine and 1‐thyroxine‐sodium on Day 4 after birth (Group T)Controls (C) received equivalent amounts of saline. During the preweaning period, half of the litters in the T and C groups were raised in an impoverished (I) condition, and half in an enriched (E) condition. Open‐field ambulation, rearing, defecation, and, in Experiment 2, grooming behavior were recorded during a 4‐day period at 210 days (Experiment 1) and at 90 days of age (Experiment 2). A consistent augmenting action of neonatal hormone stimulation on ambulation and rearing was found for both males and females. Hormone‐treated animals did not habituate their rate of general motor activity with repeated testings in the open‐field. An over‐all pattern of a larger effect of hormone‐treatment in the I condition than in the E condition was noted, particularly among the females. Thus, the experience of preweaning enrichment partly “normalized” the open‐field behavior of the T groups. The results are discussed in relation to an hypothesis concerning the interaction of rate of development of the central nervous system and environmental stimulation in determining later behavior.