Forty minutes of experience increase the weight and RNA content of cerebral cortex in periadolescent rats.

The goal of this work was to determine the shortest period of exposure to environmental complexity (EC) that would produce measurable changes in brain weight or RNA content. This period varied with the age of subjects. In young adult rats, 3 months old, 4 days of exposure to EC increased the RNA concentration in the occipital cortex above the level of littermates kept in impoverished condition (IC). Cortical weight was not affected. Seven or one daily hrs of exposure to EC did not produce significant differences between rats in EC and IC. In periadolescent rats, 30- to 40-day-old, 1 daily hr of enrichment for 4 days increased both the weight of occipital cortex and its RNA content. Exposures of 7 or 24 hrs/day produced similar results. In another experiment with periadolescent rats 1 hr or even 10 min of exposure to EC per day, for 4 days, increased total cortical weight and RNA content. Thus, periadolescent rat brain display remarkably high plasticity. These very short exposure periods are comparable to the duration of learning sessions in simple formal trainings.