Stress is a universally experienced phenomenon capable of inducing rapid psychological and physiological changes. While long-term exposure to stress has been associated with a myriad of diseases including the development of anxiety and depression, acute stress causes a number of beneficial and detrimental physiological adaptations that can be used to probe the origins of pathological states. An acute stress response is mediated by the tripartite activation of the sympatho-adrenal-medullary, hypothalamic-spinal-adrenal, and hypothalamic-pituitary-adrenal axes. These three systems generate a coordinated response releasing catecholamines and glucocorticoids (cortisol in humans and corticosterone in rodents), ultimately liberating energy stores. A by-product of increased energy production is the formation of reactive oxygen species (ROS) through metabolic oxidative phosphorylation. These have important physiological roles for several enzymes and transcription factors in addition to regulating cell cycle progression and stem cell differentiation. Typically, ROS are quickly reduced to water through a series of reactions via enzymatic and non-enzymatic antioxidants such as glutathione peroxidase (GPx) and the glutathione redox couple respectively. This antioxidative chain, including ROS intermediates, constitutes the reduction/oxidation (redox) system. Although the stress and redox systems are intrinsically linked in pathological states, little is known about the acute physiology between these systems. Therefore, the primary aim of this thesis was to establish temporal profiles of redox parameters in central and peripheral tissues following an acute stress challenge and further demonstrate the alleviation of these indicators using a novel aromatherapeutic approach.
Male Wistar rats aged 6-8 weeks postnatal were maintained under control conditions or subjected to a single two hour acute restraint stress using wire mesh restrainers. Blood samples were collected at time-points immediately before, during, and following stress treatment for the determination of stress hormones, glucose, and redox parameters including oxidative status, glutathione (GSH) and glutathione disulphide (GSSG), and lipid peroxidation. Post mortem neural tissue was collected and specific brain regions were isolated for the determination of oxidative status, GSH and GSSG, and lipid peroxidation in addition to expression of stress and redox-related genes. Following establishment of these measures, a separate group of animals was used to determine the effectiveness of plant-derived odour exposure on these stress-induced markers.
Exposure to acute restraint stress was extremely effective at transiently elevating circulating levels of prolactin and robustly increasing corticosterone and glucose. Peripheral oxidative status in erythrocytes and plasma demonstrated delayed and continuous increases for two hours following the removal of stress exposure. These results were supported with increases in plasma lipid peroxidation and GSSG, and decreases in GSH and the ratio of GSH/GSSG, indicating the onset of oxidative stress. In addition, exposure to plant-derived odours effectively reduced the stress-induced changes in corticosterone, glucose, and all peripheral indicators of the redox system. In the brain, the striatum demonstrated a robust induction of oxidative stress with increases in oxidative status and lipid peroxidation, and decreases in GSH and GSH/GSSG ratios persisting up to four hours following stress application. This was accompanied by an increase in expression of the antioxidant and zinc-binding gene, metallothionein 1a (Mt1a) while there was no change in the major GSH-utilising antioxidant enzyme, GPx1. Conversely, the hippocampus demonstrated an initial oxidative insult in all of the above redox parameters following exposure to restraint stress. However, this recovered quickly back to control levels indicating a recovery in this region. While Mt1a was also upregulated, the increase in expression was double that observed in the striatum while GPx1 remained unchanged.
In conclusion, this thesis is the first to establish an acute profile of stress-induced oxidative stress using a novel fluorometric assay for general oxidative state that is both simple and sensitive. This assay was validated alongside established redox assays of GSH, GSSG, and lipid peroxidation in central tissue and peripheral blood. These results support the hypothesis that exposure to an acute stressor produces significant changes in redox physiology within central and peripheral tissues. Furthermore, the amelioration of these stress-induced changes via exposure to plant-derived odours offers a novel therapeutic mechanism for the reduction of stress.