Impairments in olfactory function are found in 70–95% of PD patients, especially as a premorbid manifestation of the disease (Haehner et al. 2011). In the buried food test, both WT and KO adult female mice were able to detect and locate the hidden food although KO mice appeared slower to learn (Fig. 1a; RM ANOVA, effect of day F(2,34) = 8.72, P = 0.0009; split by genotype: WT, F(2,18) = 11.02, P = 0.0008; KO, F(2,16) = 1.58, n.s.). To monitor that mice showed similar interest and attention to the food stimulus, the latency to reach and eat a visible food pellet placed on top of the cage bedding was measured. KO mice showed a longer latency to reach the visible food than WT littermates but this difference was not statistically significant (Fig. 1a).
Figure 1. Olfactory tests. (a) Latency to grasp a buried or visible food pellet in adult female mice (*P < 0.05, **P < 0.005 vs. day 3 in WT group, paired t-test). (b) Time spent sniffing familiar blocks in trials 1–3 (average of all four blocks) or familiar and novel blocks in trial 4 in adult female mice, 6-days procedure (**P < 0.01 vs. blocks A–C, paired t-test). (c) Time spent sniffing familiar or novel blocks in trial 4 in aged female mice, 24-h procedure (*P < 0.05 familiar vs. novel block, Wilcoxon Signed Rank test). (d) Time spent sniffing a cartridge filled with habituated (trials 1–4) vs. novel (trial 5) scents in adult female mice (**P < 0.005 trial 4 vs. trial 1 and trial 5 in WT group, Fisher's PLSD test). (e) Time spent sniffing a cartridge filled with habituated (trials 1–4) vs. novel (trial 5) scents in aged female mice (*P < 0.05 trial 4 vs. trial 5 in KO group; #P < 0.05 WT vs. KO, unpaired t-test). Adult females WT, n = 9–10; KO, n = 9; aged females WT, n = 5; KO, n = 6.
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In the social odor discrimination task, no differences between genotypes were found in the time spent sniffing the familiar blocks during all trials. On trial 4, both genotypes spent significantly more time sniffing the novel block compared to all other blocks. Interestingly, KO mice seemed to spend more time sniffing the novel block compared to WT littermates although this difference was not statistically significant (Fig. 1b,c). Additionally, aged female KO mice, tested in the more challenging 24-h procedure, showed a shorter latency to contact the novel block also indicating a better odor detection (WT = 8.20 seconds, KO = 4.16 seconds; Mann-Withney U-test, P < 0.05). In the habituation/dishabituation test, the time spent sniffing the scented cartridge decreased significantly from trial 1 to trial 4 in adult WT female mice (habituation) and then significantly increased in trial 5 (novel scent). Conversely, adult KO littermates did not seem to habituate and to discriminate the novel scent (Fig. 1d; RM ANOVA split by genotype, effect of trial in WT, F(2,18) = 7.83, P = 0.0036; KO, F(2,16) = 2.33, n.s.). When a different group of female mice aged 12 months was exposed to the cartridge filled with similar odors, both genotypes failed to habituate to the peach odor (Fig. 1e). However, the time spent sniffing the cartridge was significantly higher in the KO mice (ANOVA, genotype F(1,9) = 7.09, P < 0.05, trial F(1,9) = 4.14, P < 0.05, genotype × trial F(2,18) = 1.40, n.s.). Although the interaction genotype × trial was not significant, only KO mice showed a significant increase in the time spent sniffing the novel scent (apricot) in trial 5 (RM ANOVA split by genotype, effect of trial in WT F(2,8) = 1.96, n.s; KO F(2,10) = 4.19, P < 0.05) possibly indicating a tendency to better discriminate between similar odors. No differences were observed in locomotor activity or motivation to explore because both genotypes showed a similar decrease in the number of transitions along the length of the cage across the five test trials (data not shown).