Brain Physiology and Pathophysiology in Mental Stress

Exposure to various forms of stress is a common daily occurrence in the lives of most individuals, with both positive and negative effects on brain function. The impact of stress is strongly influenced by the type and duration of the stressor. In its acute form, stress may be a necessary adaptive mechanism for survival and with only transient changes within the brain. However, severe and/or prolonged stress causes overactivation and dysregulation of the hypothalamic pituitary adrenal (HPA) axis thus inflicting detrimental changes in the brain structure and function. Therefore, chronic stress is often considered a negative modulator of the cognitive functions including the learning and memory processes. Exposure to long-lasting stress diminishes health and increases vulnerability to mental disorders. In addition, stress exacerbates functional changes associated with various brain disorders including Alzheimer’s disease and Parkinson’s disease. The primary purpose of this paper is to provide an overview for neuroscientists who are seeking a concise account of the effects of stress on learning and memory and associated signal transduction mechanisms. This review discusses chronic mental stress and its detrimental effects on various aspects of brain functions including learning and memory, synaptic plasticity, and cognition-related signaling enabled via key signal transduction molecules. 1. Introduction Although stress is a necessary mechanism for survival, severe and/or long-term stress disrupts normal brain structure and function [1–4]. Mental stress, which may range in intensity from mild to severe posttraumatic stress disorder (PTSD), has been reported to impair memory [5–15] possibly by elevating excitatory amino acid and glucocorticoid levels, which in turn induce excitotoxicity and hippocampal atrophy [16]. The endocrinologist, Selye [17], defined “stress” as “the non-specific response of the body to any demand placed on it.” The body’s principal physiological responses to stressful stimuli are mediated by the sympathoadrenal system and the hypothalamic pituitary adrenocortical (HPA) axis, which are, in turn, mediated by the hippocampus [18–21]. Stress stimulates the release of corticotropin-releasing factor (CRF), from the hypothalamic paraventricular nucleus (PVN), into the hypophysial-portal circulation, where it induces the release of adrenocorticotropin hormone (ACTH) from the anterior pituitary and glucocorticoids (cortisol in humans; corticosterone in rodents) from the adrenal glands [22]. The magnitude of the HPA stress response elicited by