Mitochondria play a major role in the maintaining the cellular homeostasis, it is therefore to be expected that if this homeostasis is challenged, the mitochondria would be among the first to respond. Since homeostasis is essential to survival, the stress response achieves what is termed as "ALLOSTASIS" which can be defined as the process of maintaining homeostasis by active means, through the release of stress response mediators. Depression has been described as a representation of the activation of primary stress mediators. This thesis explores the effect of stress on neuronal energy metabolism. To achieve this, various experimental studies were designed to encompass a variety of stimuli; e.g. age, anxiety, inflammation and physiological stress i.e. elevated serum corticosterone. Mitochondria were prepared using differential centrifugation through 18% (v/v) percoll density gradient. To establish a sound base on which the study was to be based the mitochondrial preparation was validated using respiratory control index (RCI), marker enzymes and electron micrographs, and compared to various other studies using the same technique for preparation. The preparation used in this study was demonstrably superior in purity and function. Initial studies dealt with age and anxiety as sources of stress. It was demonstrated that older rats (24months+) have lower (20-25%) baseline mitochondrial function as compared to young animals (2-3 months). When exposed to anxiety, young animals demonstrated a transient decline in mitochondrial function seen as a fall in RCI, but older animals showed an inhibitory effect on Complex 11 of the mitochondrial respiratory chain which was seen as a significant (P<0.01) decrease in reactive oxygen species (ROS) production in the presence of 5 mM succinate but not 5 mM glutamate plus 5 mM malate (G+M). This could be attributable to the phenomenon of partial uncoupling, a protective mechanism that stems the production of large amounts of ROS in stressed mitochondria by lowering the membrane potential, and the consequent loss of reverse electron transfer that is responsible for the large amounts of ROS being produced at complex I in the presence of a complex 11 substrate. lnterleukin-tp (IL-1P) was found to block the improvement of brain mitochondrial function resulting from exposure to brain derived neurotropic factor (BDNF; 333 ng rnl"). This effect was found to be concentration-dependent. IL-1 p was seen to have no effect on the mitochondrial function in the absence of BDNF. An earlier study by this laboratory had shown the involvement of RAS/MAPK pathway in the mitochondrial propagation of the signal initiated by BDNF. IL-1 P was seen to interfere with this pathway at the site of scaffolding proteins that are essential for MAPK docking and therefore for signal propagation. Using an in vivo model of chronic stress involving oral corticosterone (0.5%) in drinking water for 14 days was developed. This model showed successfully that chronic exposure significantly reduced the RCI of treated animals from 8.70 ± 0.30 to 7.17 ± 0.17 for G+M and from 7.56 ± 0.60 to 5.50 ± 0.20 for succinate (P<0.05; n=5). To further validate the model, daily water consumption and weight gain charts were maintained and they showed that there was no significant variation between the treated and the untreated group. This model was then used to simulate chronic stress induced depression in subsequent studies. The prototypes of the three generations of antidepressants: imipramine for 1st generation, fluoxetine for 2nd generation and tianeptine for the novel agents or 3'd generation, were used in subsequent in vivo experiments designed to study the effect of stress and antidepressant therapy on mitochondrial function expressed in terms of RCI, ROS production and the effect on serum corticosterone through a quantitative enzyme immunoassay. The main findings of the in vivo studies were that imipramine acts primarily through a corticosterone- dependent pathway because it significantly lowered serum corticosterone concentration in corticosterone treated animals. However its effects on the mitochondrial function were not significant. Fluoxetine was found to have no effect on mitochondrial function, either in vitro or in vivo. Tianeptine was found to have an effect on the mitochondrial function in vitro but not in vivo. This study has identified a potential pivotal role for mitochondria in relation to the manifestation of stress responses, regardless of the origin of the stressor.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:555649 |
| Date | January 2011 |
| Creators | Bains, Rasneer |
| Publisher | University of Sunderland |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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