Examining reelin expression and neural plasticity in animal models of depression

Stress is an important risk factor for the development of depression, but little is known about the neurobiological mechanisms by which stress might promote depressive symptomatology. The hippocampus and amygdala are susceptible to the detrimental effects of prolonged elevated stress hormone levels and neuroplastic changes within these brain regions have been linked to the onset of depression. Some of the neurobiological changes associated with prolonged elevated glucocorticoids include decreased neurogenesis, synaptic plasticity, dendritic morphology, and spine density within the hippocampus and increased dendritic morphology and spine density in the amygdala. Interestingly, recent evidence has described a regulatory role for the extracellular matrix protein reelin in synaptic plasticity, hippocampal neurogenesis, dendritic arborization, and spine density. Moreover, reelin has been shown to be decreased in neuropsychiatric disorders, such as schizophrenia, bipolar disorder, and depression. Combined, these results suggest that reelin may be an interesting protein to examine in regard to the pathogenesis of depression and to further elucidate potential therapeutic targets for the treatment of this disorder. The goal of the current research was to provide a comprehensive examination into the role of repeated stress on reelin and neural plasticity in the pathogenesis of depression through multiple preclinical studies. Given the association between reelin and hippocampal plasticity, in chapter 2 the effects of repeated exposure to corticosterone (CORT) or physical restraint on reelin expression in specific hippocampal regions were examined. Results revealed that there was a significant decrease in the number of reelin-positive cells in the CA1 stratum lacunosum and the subgranular zone of the dentate gyrus in rats that received CORT, but not in rats that received restraint. Interestingly, these results parallel our laboratory’s previous observation that CORT increases depression-like behavior but physical restraint does not. As reelin was decreased in the subgranular zone, it suggests that this protein is in a prime location to influence neurogenesis. Accordingly, chapter 3 focused on assessing the effects of different durations of CORT on behavior, hippocampal reelin expression, and neurogenesis, by subjecting rats to 7, 14 or 21 days of repeated CORT injections (40 mg/kg, s.c.) or vehicle injections. Results revealed that both the 14-day and 21-day CORT-treated rats showed increased depressive-like behavior in the forced swim test, significantly fewer reelin-positive cells and decreased neurogenesis compared to the control rats. In chapter 4, mice with a genetic deficit in reelin expression were used to examine their vulnerability to the depressogenic effects of CORT. We hypothesized that heterozygous reeler mice (HRM), with approximately 50% normal levels of reelin, would be more sensitive to the depressogenic effects of CORT than wild-type mice (WTM). Mice received injections of either vehicle, 5 mg/kg, 10 mg/kg, or 20 mg/kg of CORT, and then were assessed for changes in depression-like behavior, reelin expression, and neurogenesis. The effects of CORT on behavior, the number of reelin-positive cells, and hippocampal neurogenesis were more pronounced in the HRM than in the WTM, providing support for the idea that mice with impaired reelin signaling are more vulnerable to the deleterious effects of glucocorticoids. As reelin is expressed in GABAergic interneurons and our previous studies consistently revealed decreases in reelin number following CORT exposure, in chapter 5 the effects of repeated CORT and restraint stress on GABAergic and glutamatergic markers in the hippocampus and amygdala were examined. Western blotting analyses revealed that CORT significantly decreased the GABAergic markers, GAD65 and the α2 receptor subunit, and increased the vesicular glutamate transporter VGLUT2 within the hippocampus. We also found that corticosterone decreased the GABAergic markers, GAD67 and the α2 receptor subunit, in the amygdala. Restraint stress had no significant effect in either of these areas. These findings suggest that the depressogenic effects of CORT may be related to alterations in GABAergic and glutamatergic neurotransmission within these structures. Together these results support a relationship between glucocorticoid-induced depressive-like behavior and decreases in reelin, neurogenesis and GABAergic signaling and provide support for investigating reelin as a novel therapeutic target for the treatment of depression