Effects of chronic psychosocial stress on HPA axis functionality in male C57BL/6 mice and the impact of trait anxiety on the individual stress vulnerability
Year: 2015
Type: dissertation
Abstract: Chronic stress, in humans especially mediated by psychosocial stressors, constitutes a risk factor for the development of affective and somatic disorders. Interestingly, the effects of chronic stress also vary between individuals, with some being highly susceptible, while others being resilient to its negative consequences. These differences in stress vulnerability are supposed to depend on the interplay between genetic predispositions and environmental factors like e.g. adverse events during early life. One approach to study the mechanisms underlying the development of stress-induced disorders in general and the individual vulnerability in particular, is the use of appropriate animal models. Therefore, in the present study the chronic subordinate colony housing (CSC) model, an established and preclinically validated model for chronic psychosocial stress in male mice was used. Four experimental C57BL/6 mice are housed together with a larger and more aggressive male mouse (resident) for 19 days. The resident is supposed to occupy the dominant while the experimental mice obtain the subordinate position. In a number of publications it could be demonstrated that CSC causes affective as well as somatic disorders. CSC mice show an increased anxiety on the elevated plus-maze compared with single housed control (SHC) mice. CSC also results in immunological alterations, like decreased thymus weight and development of a spontaneous colitis. The underlying mechanisms are still poorly investigated, however CSC mice showed alterations in the hypothalamic-pituitary-adrenal (HPA) axis functionality, f.e. at the level of the adrenal glands.\n\nThe first part of the thesis, therefore, focused on alterations of different parameters of the HPA axis, mainly at the level of the pituitary and the paraventricular nucleus (PVN) following CSC exposure. Adrenocorticotropic hormone (ACTH) concentrations under basal conditions in the morning as well as following exposure to an acute heterotypic stressor (6 min forced swim) were increased in CSC compared with SHC mice. The increased pituitary activity and reactivity in CSC mice, thereby, seems to be mediated by an increased pituitary weight and an increased number of corticotroph, i.e. ACTH producing cells, in the pituitary. The protein expression of the corticotropin-releasing hormone (CRH) receptor 1 (CRH-R1) was decreased in CSC mice while the protein expression of the arginine vasopressin (AVP) receptor 1b (AVPR-1b) was unchanged. In addition, the number of parvo- and magnocellular AVP positive neurons in the PVN was similar in SHC and CSC mice. Taken together, in the first part of the thesis I could demonstrate that CSC results in an increased pituitary activity and reactivity. An increased number of corticotroph cells together with an unchanged AVPR-1b and a decreased CRH-R1 expression indicated that the newly built corticotroph cells are more sensitive to AVP than CRH, mediating the increased ACTH release following CSC exposure.\n\nThe second part of the thesis focused on the negative feedback inhibition of the HPA axis, mainly mediated by the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) in the pituitary, hippocampus, in the prefrontal cortex and the PVN.\nIn the hippocampus, the protein expression of the GR was decreased, while the MR expression was unchanged following CSC exposure. However, analysis of the functionality of the GR in isolated hippocampal cells in vitro revealed an even increased functionality. The corticosterone (CORT)-induced hnRNA expression of the period 1 gene in isolated hippocampal cells as well as the hippocampal cell viability following 24 h CORT-stimulation in vitro was higher in CSC compared with SHC mice. In the prefrontal cortex, protein expression of the GR and MR was increased following CSC exposure while in the PVN no differences alterations of both receptors were found. In the pituitary, a decreased GR protein expression indicated a reduced negative feedback inhibition. However, a dexamethasone suppression test revealed an unchanged or if at all an even increased feedback inhibition at the level of the pituitary in CSC compared with SHC mice. Taken together, the expression of MR and GR was found to be differentially regulated in the pituitary and in the different brain regions. Moreover, I demonstrated that in the pituitary as well as in the hippocampus the reduced GR expression did not result in a decreased receptor functionality.\n\nThe vulnerability to the negative consequences of chronic stress varies between individuals, whereby negative life events together with genetic factors are supposed to play an important role. The third part of the thesis, therefore, focused on the influence of the genetic predisposition, in this case the innate anxiety, on the consequences of chronic psychosocial stress. CD1 mice, selectively bred for high (mHAB) and low anxiety-related behaviour (mLAB) and normal non-selected CD1 (mNAB) mice were used for this study. Given the fact that chronic stressor exposure is supposed to increase anxiety-related behaviour, the hypothesis was that the low anxiety of the mLAB mice might be stress-protective while mHAB mice are more vulnerable to the consequences of CSC compared with mNAB mice. In accordance with the known consequences of CSC in C57BL/6 mice, mHAB as well as mNAB CSC mice showed an increased adrenal weight, a decreased sensitivity of the adrenal glands to ACTH in vitro, a lower plasma CORT:ACTH ratio and an increased release of IFN-µ in isolated mesenteric lymph node cells in response to anti-CD3 stimulation compared with respective SHC mice. The most interesting result was that mLAB mice were resistant to the consequences of CSC, given the fact that all parameters analysed were unchanged following CSC exposure. Moreover, I demonstrated that mHAB mice were not more vulnerable compared with mNAB mice to the CSC-induced effects which might be due to the stressor intensity. To sum it up, the innate anxiety influenced the vulnerability to the consequences of chronic psychosocial stress, with a low anxiety phenotype protecting against the affective and somatic consequences of CSC. The combination of mice with low innate anxiety and the CSC paradigm constitutes an important tool in order to analyse the molecular mechanisms underlying the gene and environment interactions in general as well as the mechanisms of stress resilience in particular.
Author: Andrea M. Füchsl
Language: English
Subfield: Behavioral Neuroscience
Field: Neuroscience
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2206602489