Identification of Stress-Inhibitory Mechanisms in the Brain

Stress may be defined as the adaptive response of the body to a challenge that exceeds or is perceived as likely to exceed the capacity of stimulus-specific homeostatic mechanisms. A major mediator of this response is the secretion of adrenocortical hormones (glucocorticoids) that always occurs during stress. This secretion is controlled by the hypothalamic-pituitary-adrenal (HPA) axis and excessive HPA axis activity has been linked to disorders such as immunosuppression, depression and hypertension. Recent research suggests that the brain may have intrinsic stress-inhibitory mechanisms that originate in the hippocampus and medial prefrontal cortex (mPFC) and that are capable of specifically inhibiting the HPA axis activity normally triggered by stress. The neural pathways by which such inhibitory effects are conveyed to the hypothalamic paraventricular nucleus (PVN), which contains the corticotropin-releasing factor (CRF)secreting cells that constitute the apex of the HPA axis, are currently unclear because there is no direct input to the PVN from hippocampal or mPFC neurons. Consequently, any inhibition of HPA function must involve a neuronal relay site. A prime candidate for this role is the dorsomedial hypothalamic nucleus (DMH), which provides a strong innervation to the medial PVN and receives direct projections from both the hippocampus and mPFC. Therefore, the aim of the proposed project will be to investigate the role of the DMH in the integration and regulation of the HPA axis activity under basal and stressful conditions. Moreover, we will examine two alternative models of how the HPA axis inhibition might be mediated via DMH projections to the PVN. Whereas model-A is proposed to involve a direct GABAergic neurononal projection from the DMH to PVN CRF cells, model-B is proposed to include GABAergic intemeurons within the DMH that serve to suppress a tonic excitatory input from the DMH to PVN CRF cells. Thus, the primary question to be addressed is: (1) Is there a difference in the HPA axis responses to stress in DMH lesioned animals compared to intact controls? (2) Are there GABAergic cells projecting from the DMH to the PVN activated during stress? (3) Does the release of GABA within the DMH increase in response to stress, and does the blockade of the GABAA receptors within the DMH induce a disinhibition of the stress induced HPA axis response?

Stress-related psychiatric disorders such as depression and anxiety disorders which elicit a high degree of individual suffering are becoming leading causes of morbidity in near future and represent one of the great preventive and therapeutic challenges for the 21st century. The World Health Organization predicts that depression will be the second most prevalent cause of illness-induced disability by 2020, trailing only ischaemic heart disease. Interestingly, most of these stress-related disorders are associated with a dysregulation of the neuroendocrine stress axis, the so called hypothalamic-pituitary-adrenal (HPA) axis. Consequently, it is of crucial importance to understand the neurobiological mechanisms that are involved in the regulation of such stress responses. The aim of the present project was to identify brain mechanisms that are capable of specifically inhibiting the HPA axis activity normally triggered by stress. Therefore, we investigated the role of the dorsomedial hypothalamic nucleus (DMH) in the integration and regulation of the neuroendocrine stress-axis. In various experiments we examined the influence of this area on the HPA axis activity in response to different types of stressors. Rats received bilateral lesions of the DMH and the concentration of the stress hormones corticotropin (ACTH) and corticosterone was measured in blood samples. Furthermore, animals were exposed either to a physical (immune challenge) or emotional stressor (elevated platform exposure) and blood samples were collected before (basal) and after stress exposure. Two hours after stress exposure animals were perfused, brains removed and post-fixed. In brain sections Fos (a marker for neuronal activity) was visualised and quantified by immunohistochemistry in a key area of the neuroendocrine stress reaction, the parvocellular division of the paraventricular nucleus (PVN). The results demonstrate, that animals with DMH lesions show higher neuroendocrine stress responses, indicated by higher stress-induced ACTH levels and increased neuronal activity in the PVN compared to sham-lesioned control rats. Interestingly, these differences were found only during emotional stress conditions, as under physical stress conditions no differences in the neuroendocrine stress response were found between DMH lesioned animals and sham-lesioned controls. Taken together, we could demonstrate that the DMH exerts a prominent inhibitory effect upon HPA axis activity, especially when elicited by an emotional stressor. Thus, these data provide some evidence about the neuronal mechanisms that might be activated to protect our body from the excessive secretion of adrenal hormones caused by emotional stress.