Memory-related hippocampal brain-derived neurotrophic factor activation pathways from repetitive transcranial magnetic stimulation in the 3xTg-AD mouse line

Abstract

Alzheimer's disease is associated with a loss of plasticity and cognitive functioning. Previous research has shown that repetitive transcranial magnetic stimulation (rTMS) boosts cortical neurotrophic factors, potentially addressing this loss. The current study aimed to expand these findings by measuring brain-derived neurotrophic factor (BDNF), its downstream hippocampal signaling molecules, and behavioral effects of rTMS on the 3xTg-AD mouse line. 3xTg-AD (n = 24) and B6 wild-type controls (n = 26), aged 12 months, were given 14 days of consecutive rTMS at 10 Hz for 10 min. Following treatment, mice underwent a battery of behavioral tests and biochemical analysis of BDNF and its downstream cascades were evaluated via Western blot and ELISA. Results showed that brain stimulation did improve performance on the Object Place Task and increased hippocampal TrkB, ERK, and PLCγ in 3xTg-AD mice with minimal effects on wild-type mice. There was no significant difference in the levels of AKT and Truncated TrkB (TrkB.T1) between treatment and sham. Thus, rTMS has the potential to provide an efficacious non-invasive therapy for the treatment of Alzheimer's disease through activation of neurotrophic factor signaling.

Keywords: 3xTg-AD; Alzheimer’s disease; Brain-derived neurotrophic factor; Object place test; Repetitive transcranial magnetic stimulation; TrkB.

Fig. 1.

rTMS had no effect on ambulatory differences and working memory in 3xTg-AD mice. Differences in ambulation were measured in 3xTg-AD mice aged 12 months with the A) total distance traveled in the Open Field Test, and B) total arm entries in the Y-Maze. C) Deficits in working memory were apparent in the 3xTg-AD mice using the percent correct entries in sequence in the Y-Maze. rTMS stimulation had no effect on these measures. All figures show mean ± SEM. *p < −0.05, ***p < −0.001.

Fig. 2.

rTMS improved object place memory in 3xTg-AD mice. Discrimination index for A) Object Recognition Test and B) Object Place Test. Within group differences in exploration of the Novel vs Familiar objects for C) Object Recognition Test, and D) Object Place Test. rTMS only improved memory in 3xTg-AD mice in the Object Place Test, there was no effect in the Object Recognition Test. All figures show mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001.

Fig. 3.

rTMS affects BDNF signaling in 3xTg-AD Mice. Hippocampal synaptic homogenates were used to measure A) brain derived neurotrophic factor (BDNF) using an ELISA, B) Tyrosine Receptor Protein Kinase B (TrkB) normalized to βActin, and C) it’s truncated form (TrkB.T1) normalized to TrkB.FL by Western blot. BDNF was reduced in 3xTg-AD mice which received SHAM stimulation only, and the levels of TrkB and TrkB.T1 expression were increased in 3xTg-AD-TMS mice compared to WT-SHAM and WT-TMS respectively. All figures show mean ± SEM. * p < 0.05, ** p < 0.01.

Fig. 4.

rTMS affects specific downstream targets of BDNF in 3xTg-AD mice. Hippocampal cytosolic homogenates were used to measure downstream targets of BDNF-TrkB signaling. A) Extracellular Signaling-Related Kinase (ERK), B) Protein Kinase B (AKT) and C) Phosphatase Cγ (PLCγ) were all measured using Western blot normalized to Cofflin. rTMS increased the amount of ERK in the 3xTg-AD mice and there was an overall interaction between treatment and genotype in the levels of PLCγ. All figures show mean ± SEM. *p < 0.05.