Fear Conditioning as a Measuring Tool for Cognitive Deficits Related to Amyloid Burden Coupled with Iron, Zinc, and Copper in the Transgenic Tg2576 Mouse Model for Alzheimer’s Disease




Burns, Andrew J.

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This study utilized the transgenic Tg2576 mouse model of Alzheimer’s Disease (AD) that expresses the human amyloid precursor protein (APP). Iron, zinc, and copper in the brain are thought to interact with amyloid proteins, specifically Aβ, to facilitate the cognitive decline associated with AD. Transgenic (Tg) and Wildtype (Wt) control mice were given different doses of metal in their water supply beginning at approximately 3 months of age and were tested for memory impairment with a fear conditioning (FC) test at 14 months of age. The FC test was 6 minutes in duration, with the shock administered on the last 3 minutes of the Training Day (Day 1). The water groups were lab (no added metal), iron [Fe(NO3)2 at 10ppm], zinc [Zn(CO3) at 10ppm], and zinc + copper [Zn(CO3) at 10ppm and CuCl2 at .25ppm respectively]. There was an unexpected difference between the genotypes on the Training Day. The Tg mice displayed abnormally low freezing behaviors when compared to the Wt mice and it is unclear whether this was caused by plaque associated brain damage, or a differential response to the fear stimulus due to hyperactivity. Due to this difference between the genotypes an individual growth curve (IGC) analysis was conducted and indicated that there was a significant difference between genotypes, but not between water groups. In the contextual environment, the Tg mice exhibited significantly lower freezing behavior than the Wt mice. Conversely, in the cued environment the Tg mice exhibited significantly more freezing behavior than the Wt mice. This result in the cued environment was unexpected, but when the contextual freezing behavior of the Tg mice was subtracted from their cued freezing behavior, there was no significant difference between the two genotypes. This suggests that there was impaired contextual conditioning in the Tg mice and the inability to distinguish between the contextual and cued environments resulted in an additive effect on the freezing behavior of the Tg group in the cued environment. The mice were sacrificed at 18 months of age and the plaque load of the Tg lab and iron water groups was analyzed. Linear regression analyses were conducted on these water groups and a significant negative correlation was found between plaque burden and freezing behavior for minutes 456 of the Training Day (Day 1) and minutes 456 of Day 2 (the first trial in the contextual environment). Higher plaque burden was associated with a lower freezing behavior. Also the lab water group had a higher plaque burden than the iron water group in the basal ganglia and exhibited significantly less freezing on minutes 456 of Day 2 in the contextual environment. This supports the theory that a higher plaque burden is disrupting the hippocampus and possibly the basal ganglia, which results in a change in motor behavior (hyperactivity and reduced freezing), and that the intake of metals (such as iron) could affect the deposition of plaques in specific brain areas. The overall conclusion is that when looking at the effects of water type on freezing behavior, the strong differences found between the genotypes could have overshadowed weaker differences between water types that may have been present.



Amyloid, Tg2576, Fear, Metal, Plaque, Context