Study On Characteristic Alterations Of HPA Axis And Its Feedback Regulation In Sert Knockout Mice

Genetic factors play an important role in the etiology of affective disorders. Recently, a polymorphism in the promoter region of the serotonin transporter gene (5-HT transporter linked promoter region, 5-HTTLPR) was found to be involved in personality and affective disorders. The cellular and molecular mechanisms mediating the increasing sensitivity to the stress in individuals with short variants of 5-HTTLPR has not been unknown as yet. Understanding these mechanisms will assist us to develop better approaches to prevent and treat the disorders from the stresses. However, it is difficult to pursue these studies in humans because of ethics. Genetic deletion of the serotonin transporter gene in mice generated a model in which the SERT function in heterozygous (SERT +/-) mice was similar to that in humans with short alleles of 5-HTTLPR. For example, like short alleles of 5-HTTLPR, SERT +/- mice only express a half of the SERT relative to their SERT normal littermates (SERT +/+). Behaviorally, SERT knockout mice (SERT+/- and -/- mice) are more anxious and sensitive than SERT+/+ after they undergo a stress. Therefore, SERT knockout mice can be used as a tool to study the mechanisms that mediate the increasing sensitivity to stress in individuals with short alleles of 5-HTTLPR.The aim of the present study was to test the functional, cellular and molecular alterations in the components of hypothalamic-pituitary-adrenal cortex (HPA) axis and its feedback regulation was characterized by determining the expressions and functions of MR, GR, and CRF1 receptor, the CRF expression, as well as the secretions of corticosterone and ACTH under the conditions of either base or stress in SERT knockout mice, using autoradiographic binding assay, in situ hybridization, radioimmunoassay, and immunoblot technique, respectively.. The results showed that 1) the basal expression of CRF mRNA in the paraventricular nucleus (PVN) of the hypothalamus was lower in both SERT+/- and -/- mice than that in SERT+/+ mice. Stress from exposure to the elevated plus maze (EPM) for 5 min significantly increased CRF mRNA in the PVN of SERT+/- and -/- mice, but not in SERT+/+ mice; 2) an increasing response to CRF challenge was found in SERT-/- mice, suggesting that the function of CRF type 1 receptors (CRF R1) in the pituitary was increased. Consistently, 125I-sauvagine (a CRF receptor antagonist) binding assay revealed an increasing density of CRF R1 in the pituitary of SERT -/- under basal condition. These data suggest that CRF R1 in the pituitary of SERT-/- mice is up-regulated. However, function of CRF R1 was not changed and the density of CRF R1 was reduced in the pituitary of SERT+/- mice compared with SERT+/+ mice; 3) the expression of glucocorticoid receptors (GRs) was significantly increased, but mineralocorticoid receptors (MRs) was reduced in the hippocampus of SERT+/- and -/- mice. In contrast, the expression of GR in the hypothalamus, pituitary and adrenal cortex was significantly reduced in the SERT+/- and -/- mice compared with SERT+/+ mice under basal condition. Consistently, the secretion of corticosterone in the response of SERT-/- mice to dexamethasone was blunted in comparison of SERT+/+ and +/- mice. Furthermore, stress induced a rapid increase of the GR expression in the hypothalamus of SERT +/- and -/- mice. All above indicate that the HPA axis and its feedback regulation are altered in SERT knockout mice, which could account for the increasing sensitivity to stress in these mice.