Spared piriform cortical single-unit odor processing and odor discrimination in the Tg2576 mouse model of Alzheimer's disease

Abstract

Alzheimer's disease is a neurodegenerative disorder that is the most common cause of dementia in the elderly today. One of the earliest reported signs of Alzheimer's disease is olfactory dysfunction, which may manifest in a variety of ways. The present study sought to address this issue by investigating odor coding in the anterior piriform cortex, the primary cortical region involved in higher order olfactory function, and how it relates to performance on olfactory behavioral tasks. An olfactory habituation task was performed on cohorts of transgenic and age-matched wild-type mice at 3, 6 and 12 months of age. These animals were then anesthetized and acute, single-unit electrophysiology was performed in the anterior piriform cortex. In addition, in a separate group of animals, a longitudinal odor discrimination task was conducted from 3-12 months of age. Results showed that while odor habituation was impaired at all ages, Tg2576 performed comparably to age-matched wild-type mice on the olfactory discrimination task. The behavioral data mirrored intact anterior piriform cortex single-unit odor responses and receptive fields in Tg2576, which were comparable to wild-type at all age groups. The present results suggest that odor processing in the olfactory cortex and basic odor discrimination is especially robust in the face of amyloid β precursor protein (AβPP) over-expression and advancing amyloid β (Aβ) pathology. Odor identification deficits known to emerge early in Alzheimer's disease progression, therefore, may reflect impairments in linking the odor percept to associated labels in cortical regions upstream of the primary olfactory pathway, rather than in the basic odor processing itself.

Figure 1. Single unit activity in Tg2576 versus age-matched WT mice at 3, 6 and 12 MO.

Tg2576 showed a trend towards higher baseline activity (A) versus age-matched WT (p = .06) but no difference in highest odor-evoked response (B). Unit entrainment to respiration (C) was diminished in Tg2576, though this did not emerge until 12 MO (* = p<.05). Data presented as mean ± SEM.

Figure 2. Single unit receptive fields in aPCX.

(A) Representative single-unit response to stimulus set. Rasters represent unit activity from a single cell tested with multiple odors and histogram indicates tally of rasters for each odor. Shaded area indicates 2 seconds starting at the onset of stimulus delivery. (B) Odor receptive fields in Tg2576 versus age-matched WT at 3, 6 and 12 MO. X-axis is odor stimuli organized by response strength. Y-axis is odor-evoked spikes per second normalized to the highest response of the six odors. Tg2576 single-units showed no difference in receptive field specificity compared to age-matched WT mice.

Figure 3. Short-term odor habituation was impaired in Tg2576 mice compared to WT controls.

For example, as shown here, 12 MO Tg2576 mice (n = 12) showed less habituation over the course of four repeated odor stimuli than age-matched WT controls (n = 12).

Figure 4. Behavioral discrimination of overlapping mixtures in WT mice.

The pseudo-color panels (color corresponds to proportion correct) show performance for individual mice in each task. Mean performance for each task (top right). Different mice were used for each task. As previously shown in rats, the 10c vs. 10cR1 discrimination was significantly easier to acquire than 10c vs 10c-1. However, so few individual animals acquired the 10c vs. 10c-1 task it was not feasible to use it to test the effects of APP over-expression. There was no significant difference between WT and Tg2576 mice in performance on an odor mixture discrimination task (10c vs. 10cR1) across age (bottom right). Of the initial 7 WT and 6 Tg2576 mice, 1 WT and 2 Tg2576 animals died prior to 12 months and are not included here. Animals were trained in the two-alternative forced choice task prior to 5 months of age and then tested bimonthly until 12 months. Initial acquisition of the discrimination was not affected by genotype. Furthermore, there was no significant effect of genotype on performance of this well learned odor discrimination task through 12 months of age.

Figure 5. Histological examples of thioflavin S staining at 12 MO in Tg2576 mice for olfactory bulb (OB), anterior piriform cortex (aPCX), hippocampus (HPX) and lateral entorhinal cortex (LEC).

Scale bar is 500 µm.

Figure 6. Summary of data obtained for 20+ month old Tg2576.

(A) Mice over 20 months of age showed abundant thioflavin S positive staining in (from left to right) OB, anterior piriform cortex (aPCX), hippocampus (HPX) and lateral entorhinal cortex (LEC). Scale bar is 500 µm. (B) Thioflavin S area fractions for 20MO animals in OB, aPCX, HPX and LEC. (C) 20 MO Tg2576 showed no difference in baseline or maximal odor-evoked unit activity and (D) no difference in unit entrainment respiration. (E) Finally, no difference was observed in single-unit receptive field specificity compared to age-matched WT.