Mechanisms of adaptation of the hypothalamic-pituitary-adrenal axis in male mice under chronic social defeat stress

The hypothalamic-pituitary-adrenal axis (HPA) plays an important role in the mechanisms of adaptation to chronic stress. A model of chronic social defeat stress (CSDS), based on the experience of defeat in daily agonistic interactions, causes the development of a depressive-like state in mice, which is often accompanied by an increase in blood corticosterone levels. In this work, we assessed what changes occur in the central (hypothalamus) and peripheral (adrenal glands) parts of the HPA axis under the influence of chronic social stress, which can affect the regulation of corticosterone synthesis and its level in the blood. The experience of chronic social stress causes an increase in the relative weight of the adrenal glands, an increase in the expression level of Crh gene in the hypothalamus and the expression of the genes for the corticosterone synthesis enzymes Star, Cyp11a1, Cyp11b1 in the adrenal glands. At the same time, in the hypothalamus the expression of Fkbp5 and Nr3c1 decreases and the expression of Crhbp increases, and in the adrenal glands the expression of the Mc2r and Hsd11b1 genes decreases, which is ultimately aimed at reducing the amount of corticosterone secreted by the adrenal glands, and thus limiting the glucocorticoid response. Thus, chronic stress leads to an imbalance of the activating and stabilizing mechanisms of HPA axis regulation and a possible inadequate response to additional stress stimuli.