A Rat Model of Alzheimer's Disease Based on Abeta42 and Pro-oxidative Substances Exhibits Cognitive Deficit and Alterations in Glutamatergic and Cholinergic Neurotransmitter Systems

Abstract

Alzheimer's disease (AD) is one of the most serious human, medical, and socioeconomic burdens. Here we tested the hypothesis that a rat model of AD (Samaritan; Taconic Pharmaceuticals, USA) based on the application of amyloid beta42 (Abeta42) and the pro-oxidative substances ferrous sulfate heptahydrate and L-buthionine-(S, R)-sulfoximine, will exhibit cognitive deficits and disruption of the glutamatergic and cholinergic systems in the brain. Behavioral methods included the Morris water maze (MWM; long-term memory version) and the active allothetic place avoidance (AAPA) task (acquisition and reversal), testing spatial memory and different aspects of hippocampal function. Neurochemical methods included testing of the NR1/NR2A/NR2B subunits of NMDA receptors in the frontal cortex and CHT1 transporters in the hippocampus, in both cases in the right and left hemisphere separately. Our results show that Samaritan rats(™) exhibit marked impairment in both the MWM and active place avoidance tasks, suggesting a deficit of spatial learning and memory. Moreover, Samaritan rats exhibited significant changes in NR2A expression and CHT1 activity compared to controls rats, mimicking the situation in patients with early stage AD. Taken together, our results corroborate the hypothesis that Samaritan rats are a promising model of AD in its early stages.

Keywords: Alzheimer’s disease; animal model; cognition; hippocampus; learning and memory; neurochemistry of the acetylcholine system; sporadic AD.

Figure 1

Results from the Morris water maze (MWM). (A) Total distance to reach the platform. In four daily acquisition sessions with a hidden platform in a stable position (d01–d04), Samaritan rats were significantly impaired and their performance did not improve during subsequent sessions, suggesting severe behavioral impairments. The visible platform test showed that both groups had both the motivation and physical ability to reach the platform (the apparent trend towards poorer performance in the Samaritan group was not statistically significant). (B) The probe trial (a single swim without a platform) was performed on the fifth session, after hidden platform training but before visible platform testing. Samaritan rats spent significantly less time in the quadrant that had previously contained the platform, which suggests an impairment of memory or spatial navigation. Their performance was even below the expected random value (15 s), as indicated by the dashed line. Columns in the charts show group means, and SEM is indicated by the error bars. Significant differences between groups or sessions at p < 0.05 as evaluated by a t-test on session-averaged data are indicated by an asterisk (*), n.s. indicates a non-significant result.

Figure 2

Results from the Carousel maze. The Carousel maze testing consisted of two habituation sessions (d01–02), acquisition (d03–07) and reversal training (d08–12). (A) Total distance walked is a measure of locomotor activity. In the habituation sessions, the difference between groups was not significant, but locomotion increased in the second session (with a rotating arena) relative to the first (with a stable arena). In the acquisition phase, the effect of group was also not significant; however, the control group gradually increased its locomotion, whereas the locomotion of the Samaritan rats was stable. In the reversal stage, no difference was found. (B) Maximum avoidance time indicates the longest period of successful avoidance during a session. Again, the effect of groups was not significant, but there was a significant interaction suggesting slower learning in the Samaritan group in the acquisition phase. In the reversal phase, neither the group effect nor interaction was significant, despite the apparent trend visible in the graph. (C) Number of errors (entrances into the sector) is another measure of avoidance behavior. The difference between groups was again not significant, but a significant interaction indicated slower learning and poorer final performance in Samaritans during the acquisition phase. In reversal, the apparent tendency toward increased number of errors in the Samaritan group remained non-significant. (D) Time to first error is a measure of between-session (long-term) memory. There was only a trend for group difference, but a significant interaction term again indicated poorer final performance in Samaritans during acquisition. There was no significant difference in the reversal phase, which may have been caused by the large variance in this parameter. Bars or data points in the charts show group means, and SEM is indicated by the error bars. Significant differences between groups at p < 0.05 as evaluated by the t-test on session-averaged data are indicated by an asterisk (*), n.s. indicates a non-significant result.

Figure 3

Results of western blotting. Representative images of samples from the L hemisphere were used (all data are presented in Table 2).