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USING ZEBRAFISH TO INVESTIGATE COGNITIVE DEFICITS AND VASCULAR COMPLICATIONS ASSOCIATED WITH TYPE II DIABETES
The prevalence of obesity and associated diabetes mellitus is at an unprecedented level globally and is an epidemic in the United States today. All forms of diabetes are characterized by high blood sugar levels (or hyperglycemia), which, if not corrected, carries the risk of long-term complications that typically develop after 10-20 years due to the damage of blood vessels. Two such complications are diabetic retinopathy and dementia, with recent reports suggesting there may be a correlation between changes observed in retina with those occurring in the brain. The studies described in this dissertation aimed to use the zebrafish model of chronic hyperglycemia developed in our lab to (1) assess cognitive (brain-based behavior) changes, (2) correlate those cognitive changes with changes in visually-guided behaviors, and (3) relate anatomical and neurochemical changes in retina and brain to the observed behaviors. Our analyses were performed after 4- and 8-weeks of hyperglycemia, with the expectation that more deleterious effects would be seen at the later time point. To do this, we improved and implemented the three-chamber choice associative learning task typically used in rodents with zebrafish using a live shoal as a reinforcer (Chapter 2). We then employed this behavioral task (Chapter 3) to assess hyperglycemia-induced changes in cognition and vision. At the 4-week time point, glucose-treated fish displayed an impaired ability to learn and maintain memory evidenced by an increase in the number of force-rewarded trials, a decrease in the number of high performing fish by ~20% on the first day of reversal, and a decrease in discrimination ratio. At 8-weeks, behavioral analyses revealed dampened effects of glucose with responses that were either comparable to controls or due to osmotic effects. However, examining the 8-week behavioral data based on fish performance (high performing vs. low performing), revealed a glucose-specific effect on discrimination ratio in the high performing group. The optomotor response, a vision-based behavior, was greater in glucose-treated fish at both time points. After 4 weeks of treatment (Chapter 4) tight junction markers were reduced, while inflammatory markers (NF-kB, IKK) were increased in brain and retina, consistent with observed behavioral deficits. At 8-weeks, inflammatory markers in brain were still elevated, as were levels of two enzymes, tyrosine hydroxylase and glutamic acid decarboxylase, important for neurotransmitter synthesis. In retina, though, only tyrosine hydroxylase and Nf-KB were upregulated. No changes in tight junction markers were evident at 8-weeks. Taken together, these findings suggest differential sensitivity to glucose by individual fish, hyperglycemia-induced changes in brain and retina after 4-weeks of treatment, but tissue-specific effects after 8-weeks of treatment.