Adrenocortical hormones and brain growth: reversibility and differential sensitivity during development.

The brain growth that follows adrenalectomy can be arrested by corticosterone replacement. Administered daily in combination with deoxycorticosterone, the brain and body conditions of intact rats are closely matched. Using this combination dose (corticosterone, 7.0 mg/kg; deoxycorticosterone, 1.0 mg/kg), hormonal replacement was systematically administered and withheld in a balanced design in order to assess the relative sensitivity of brain tissue to corticosteroids across time, and to determine if brain growth suppressed during one phase could recover upon hormone withdrawal during a second phase. Female forebrains were less sensitive to the hormones during an early phase which spanned ages 27 to 46 days. In contrast, 70 to 80% of potential growth was suppressed by replacement during a later period (ages 46 to 65 days). Brain size differences included weight and forebrain length, width, and depth. Hind brains were more sensitive to hormone replacement during phase 1. However, this suppressed growth was completely regained after hormone withdrawal during phase 2. On the other hand, growth that had occurred in the absence of adrenal steroids was not reversed, only arrested, by subsequent administration. We conclude that the rat brain became increasingly sensitive to the suppressive influence of adrenal steroids as females matured from juveniles to adults. Coupled with this sensitivity was the ability--apparently complete--to recover from steroid-induced brain growth suppression.