Brain corticosterone in lactating rats: possible relation to the attenuation of the pituitary-adrenocortical response to stress

In an attempt to elucidate the mechanism of suppression of the plasma corticosterone stress response in the lactating female rats, nonstress levels of corticosterone in plasma and hypothalamus, cerebral cortex, hippocampus and pituitaries were determined in the lactating and nonlactating Sprague-Dawley rats at 1600-1700 hr by the protein binding assay. The brain corticosterone levels were significantly higher (64-92%) in the lactating than in nonlactating rats, while the plasma corticosterone levels were identical. We conclude that the suppression of the stress response in the lactating female rats is directly or indirectly related to the high basal levels of corticosterone in the brain, which result from a chronic hyperactivation of the endocrine axis.