| Abstract: | Rats were handled daily from day 2 to 11 after birth, inclusive. These animals and unhandled controls were injected on day 11 with a radioactively-labeled precursor of deoxyribonucleic acid (DNA) and killed either 6 hr, 3 or 30 days later. In a subsequent experiment, uninjected rats were likewise permitted to survive until 11, 14, 41, or 101 days of age. Brain weight measurements were taken in all animals, and the brains from the rats of the first experiment prepared for histology and autoradiography, and evaluated quantitatively. The following differences were established: (1) The brains of the handled rats were consistently lighter than controls at 11 and 14 days; (2) These differences were not associated with differences in body weight; (3) Planimetric measurements of sampled regions showed that brain weight differences were correlated with areal size differences; (4) Autoradiographic cell counting indicated that cell proliferation and the formation of new microneurons were higher after injection in the handled than unhandled rats; (5) No differences attributable to the treatment were obtained between relative areas occupied in cortex by perikarya and neuropil. We concluded that handling leads to prolongation of brain maturation and hypothesized that it represents environmentally-induced “infantilization.” |
