Abstract:
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.
