Postnatal Development of Mouse Plasma and Brain Corticosterone Levels: New Findings Contingent Upon the Use of a Competitive Protein-Binding Assay

A competitive protein-binding (CPB) assay, suitable for measuring corticosterone levels in 20 mul of mouse plasma or 100 mg of brain, is described. The postnatal development of adrenocortical function was determined in C57BL/10 and DBA/1 mice by CPB assay of basal and stressinduced levels of plasma corticosterone and resting levels of brain corticosterone. Marked increases in both basal and stressed levels of plasma corticosterone were found beginning at day 12 after birth: mean basal levels rose from about 1 mug/u99 ml on day 12 to peak values of about 10-15 mug/100 ml on days 18-20, and then declined by day 30 to the 13-day level of 2.6 mug/100 ml. This pattern differs significantly from results obtained with standard fluorometric assays for corticosterone; it was determined that a major part of this discrepancy is due to the lack of specificity of the fluorometric assay. The developmental change in brain corticosterone was similar to the pattern found in plasma. Only the stress-induced levels of plasma corticosterone showed significant genetic variation, and this did not appear until about one week after the end of the relative stress-nonresponsive period. These findings should be useful in evaluating hypotheses concerning the developmental regulation of adrenocortical function and the action of glucocorticoids in regulating the biochemical differentiation of other tissues.