Impact of Transcutaneous Auricular Vagus Nerve Stimulation on Spatial Learning and Memory in Acrolein-Induced Alzheimer's Disease-Like Hippocampal Neuronal Damage in Wistar Rats

Abstract

Background: Data about the utility of vagus nerve stimulation (VNS) as a potential therapy for neurodegenerative disorders are still inconclusive. We used a rat model of acrolein-induced hippocampal neurodegeneration to investigate the effect of VNS on spatial learning and memory.

Methods: A total of 24 Wistar rats were randomly allocated to one of the four groups: no acrolein exposure (n = 6), control (n = 6), sham (n = 6), and experimental (n = 6). The control, sham, and experimental groups were exposed to acrolein 2.5 mg/kg/day by gastric gavage for eight weeks. After acrolein exposure, the experimental and sham groups received transcutaneous auricular VNS and greater auricular nerve stimulation, respectively, under 2% isoflurane anesthesia for four weeks. Then, all animal groups were assessed for spatial learning and memory in a Morris water maze before being euthanized for hippocampus histological examination.

Results: The mean time to find the hidden platform varied significantly between the no acrolein exposure group and each of the acrolein-exposed groups. The results of one-way ANOVA indicated a significant difference in the average swimming time between the four study groups (F = 14.64, p < 0.001). Results from the post-hoc analysis indicated that the mean difference was statistically significant between the "no acrolein exposure" and "control" groups (p < 0.001), the "no acrolein exposure" and "experimental" groups (p = 0.001), and between the "control" and "sham" groups (p< 0.001). There was no statistically significant difference in swimming time to find the hidden escape platform between the sham and experimental groups (p = 0.060).

Conclusion: Transcutaneous auricular VNS has no significant effect on spatial learning or memory in Wistar rats with acrolein-induced hippocampus neuronal damage, indicating the need to review the long-standing notion that hippocampal neuronal loss causes spatial navigation deficits.

Keywords: acrolein; alzheimer’s disease; brain stimulation; dementia; morris water maze; neurodegeneration; rats; spatial learning; spatial memory; vagus nerve stimulation.

Figure 1. H&E staining of the hippocampus cornu ammonis regions CA1, CA2, CA3, and CA4, molecular layer (M), and the dentate gyrus (DG), providing histologic evidence of acrolein-induced hippocampal neuronal damage.

(A) Low-power section (300 μm) of the hippocampus of a rat that was not exposed to acrolein showed normal neurons in regions CA3, CA2, and CA1, with some scatter astrocytes in M. (B) High-power image (200 μm) of the hippocampus of a normal rat showing neuronal cell bodies in CA3 and CA2, with vesicular chromatin pattern. Both slides A and B from non-acrolein exposed animals had no evidence of apoptosis on H&E staining (x20). (C) Low-power images (500 μm) of the hippocampus of a rat exposed to acrolein showed extensive apoptosis in regions CA3 extending through CA2, CA1, DG, and M. (D) High-power image (200 μm) of the hippocampus of a rat exposed to acrolein showing apoptosis of neurons in region CA1. The neurons had condensed chromatin and were shrunken with a retraction artifact around the neurons. Slides C and D demonstrated that acrolein exposure was associated with hippocampal neurocellular damage. CA: cornu ammonis regions (CA1, CA2, CA3) of the hippocampus; DG: dentate gyrus; M: hippocampal molecular layer; μm: micrometer.

Figure 2. A box plot of average swimming time taken to find the escape platform in a Morris water maze (MWM) for the different study groups.

The lowest median swimming time for the control group was 4.6 seconds. The median swimming time to find the escape platform was significantly higher in the no acrolein exposure group (18 seconds) than for each of the other study groups. Data in both sham and experimental groups were skewed to the right but the median swimming time to find the escape platform was shorter in the experiment group (about nine seconds) compared to the sham group (about 15 seconds).

Figure 3. Line graph showing trends of average swimming time for the different study groups across the test trials.

The trend lines showed a divergent tendency between the control and experimental groups with the latter spending less time to locate the hidden platform, and a convergent tendency between the no acrolein exposure and experimental groups.