Cholinesterase inhibitors improve both memory and complex learning in aged beagle dogs

Abstract

Similar to patients with Alzheimer's disease (AD), dogs exhibit age-dependent cognitive decline, amyloid-β (Aβ) pathology, and evidence of cholinergic hypofunction. The present study sought to further investigate the role of cholinergic hypofunction in the canine model by examining the effect of the cholinesterase inhibitors phenserine and donepezil on performance of two tasks, a delayed non-matching-to-position task (DNMP) designed to assess working memory, and an oddity discrimination learning task designed to assess complex learning, in aged dogs. Phenserine (0.5 mg/kg; PO) significantly improved performance on the DNMP at the longest delay compared to wash-out and partially attenuated scopolamine-induced deficits (15 μg/kg; SC). Phenserine also improved learning on a difficult version of an oddity discrimination task compared to placebo, but had no effect on an easier version. We also examined the effects of three doses of donepezil (0.75, 1.5, and 6 mg/kg; PO) on performance of the DNMP. Similar to the results with phenserine, 1.5 mg/kg of donepezil improved performance at the longest delay compared to baseline and wash-out, indicative of memory enhancement. These results further extend the findings of cholinergic hypofunction in aged dogs and provide pharmacological validation of the canine model with a cholinesterase inhibitor approved for use in AD. Collectively, these studies support utilizing the aged dog in future screening of therapeutics for AD, as well as for investigating the links among cholinergic function, Aβ pathology, and cognitive decline.

Fig. 1

Mean performance accuracy on the DNMP at the 20 s delay in (A) placebo treated dogs and (B) phenserine treated dogs. A three-way ANOVA revealed a significant interaction between treatment and challenge condition, which was further examined using Fisher’s post-hoc test. Post-hoc Fisher’s indicated that scopolamine significantly impaired performance of the placebo treated dogs compared to saline performance. By contrast, scopolamine impairment was not evident in the phenserine treated dogs. Significant differences determined using post-hoc Fisher’s test are depicted by *. Error bars represent S.E.M.

Fig. 2

Mean performance accuracy on the DNMP at the 80 s delay in (A) placebo treated dogs and (B) phenserine treated dogs. A three-way ANOVA revealed a significant interaction among treatment, time-point and challenge condition, which was further examined using Fisher’s post-hoc test. In placebo treated animals, a significant scopolamine-induced impairment was found compared to saline performance during the wash-out phase. By contrast, post-hoc Fisher’s test revealed that scopolamine challenge significantly impaired performance at 80 s compared to saline while the dogs were treated with phenserine. Moreover, post-hoc Fisher’s also revealed that performance under saline was significantly better under phenserine compared to after phenserine was discontinued (i.e., wash-out) for at least 10 days. No scopolamine effect was evident during wash-out, partially due to lower levels of performance at the 80 s delay after phenserine was discontinued. Significant differences determined using post-hoc Fisher’s test are depicted by *. Error bars represent S.E.M.

Fig. 3

Mean performance accuracy of placebo and phenserine treated dogs on the DNMP at the 80 s delay during the treatment phase. Post-hoc Fisher’s indicated that phenserine marginally (p < 0.08) enhanced performance during stabilization (i.e., 3 control test days) and saline challenge compared to saline challenge under the placebo. Marginally significant differences determined using post-hoc Fisher’s test are depicted by *. Error bars represent S.E.M.

Fig. 4

Mean errors to criterion on both ODD1 and ODD2. A two-way repeated measures ANOVA revealed a significant interaction between drug treatment and treatment order, which was further analyzed using post-hoc Fisher’s test. No differences were found between phenserine and placebo on the more simple oddity problem (ODD1); however, phenserine significantly reduced learning errors on the more difficult problem (ODD2) compared to placebo. The placebo groups committed significantly more errors on ODD2 than either treatment group on ODD1, which confirmed ODD2 was more difficult than ODD1. Significant differences determined using post-hoc Fisher’s tests are depicted by *. Error bars represent SEM.

Fig. 5

Mean performance accuracy on the DNMP at the (A) 20 and (B) 90 s delays by donepezil dose. For each delay, performance under each dose across time-points (i.e., baseline, 1 h, 3 h, 5 h, and following a 5 day wash-out) was analyzed using a 2-way repeated measures ANOVA. No significant effects of dose or time-point were found at 20 s. At the 90 s delay, a significant interaction between dose and time-point was found. Post-hoc Fisher’s revealed a significant improvement in performance compared to baseline at 3 and 5 h following the 1.5 mg/kg donepezil dose. By contrast, the 6 mg/kg dose significantly impaired performance at the 90 s delay compared to baseline three hours following dosing. No differences from baseline were found at any other dose and wash-out performance did not differ from baseline in any dose group. Significant differences from baseline are depicted by * as determined using post-hoc Fisher’s test. Error bars represent S.E.M.