Low-frequency transcranial magnetic stimulation is beneficial for enhancing synaptic plasticity in the aging brain

Abstract

In the aging brain, cognitive function gradually declines and causes a progressive reduction in the structural and functional plasticity of the hippocampus. Transcranial magnetic stimulation is an emerging and novel neurological and psychiatric tool used to investigate the neurobiology of cognitive function. Recent studies have demonstrated that low-frequency transcranial magnetic stimulation (≤1 Hz) ameliorates synaptic plasticity and spatial cognitive deficits in learning-impaired mice. However, the mechanisms by which this treatment improves these deficits during normal aging are still unknown. Therefore, the current study investigated the effects of transcranial magnetic stimulation on the brain-derived neurotrophic factor signal pathway, synaptic protein markers, and spatial memory behavior in the hippocampus of normal aged mice. The study also investigated the downstream regulator, Fyn kinase, and the downstream effectors, synaptophysin and growth-associated protein 43 (both synaptic markers), to determine the possible mechanisms by which transcranial magnetic stimulation regulates cognitive capacity. Transcranial magnetic stimulation with low intensity (110% average resting motor threshold intensity, 1 Hz) increased mRNA and protein levels of brain-derived neurotrophic factor, tropomyosin receptor kinase B, and Fyn in the hippocampus of aged mice. The treatment also upregulated the mRNA and protein expression of synaptophysin and growth-associated protein 43 in the hippocampus of these mice. In conclusion, brain-derived neurotrophic factor signaling may play an important role in sustaining and regulating structural synaptic plasticity induced by transcranial magnetic stimulation in the hippocampus of aging mice, and Fyn may be critical during this regulation. These responses may change the structural plasticity of the aging hippocampus, thereby improving cognitive function.

Keywords: aging; brain-derived neurotrophic factor; cognitive function; hippocampus; neural regeneration; neuroplasticity; neurotrophic factor; non-invasive brain stimulation; transcranial magnetic stimulation.

Figure 1

Schematic diagram of the transcranial magnetic stimulation (TMS) protocol in aged mice. Four sessions of low-frequency TMS were performed once daily for 14 consecutive days. The pattern of one session of TMS consisted of 5 burst trains, with each train consisting of 20 pulses and 10-second inter-train intervals, with inter-session intervals of 30 seconds; i.e., 400 pulses in total.

Figure 2

Immunohistochemical staining of SYN and GAP43 in the hippocampus of aged mice. (A) Hippocampal CA1 and CA3 regions (both boxed) in the control (no primary antibody). (B) Immunoreactivity for SYN (S1–3, CA1 area; S4–6, CA3 area) and GAP43 (G1–3, CA1 area; G4–6, CA3 area) is shown as brownish-yellow granules in pyramidal cells. Scale bars: 100 μm. SYN: Synaptophysin; GAP43: growth associated protein 43.

Figure 3

Protein and mRNA expression of SYN, GAP43, BDNF, TrkB and Fyn in the hippocampus of aging mice. Western immunoblots (A) and their respective quantitative analysis (B) of SYN, GAP43, BDNF, TrkB, and Fyn. mRNA bands via reverse transcription polymerase chain reaction (RT-PCR) (C) and their respective quantitative analysis via semi-RT-PCR of SYN, GAP43, BDNF, TrkB and Fyn. (B, D) The data are expressed as the mean ± SD (8 mice per group). The expression of protein and mRNA was normalized to GAPDH levels, which was then normalized to the control (defined as 100%.). * P < 0.05, ** P < 0.01, vs. control group. One-way analysis of variance followed by the Fisher's Least Significant Difference was used. Each experiment was repeated three times. SYN: Synaptophysin; GAP43: growth associated protein 43; BDNF: brain-derived neurotrophic factor; TrkB: tropomyosin receptor kinase B.

Figure 4

Changes in spatial cognition in aged mice using the Morris water maze test. (A) Escape latency on each training day. (B) The time spent in the target quadrant during probe trial. The column 1 represents the target quadrant. (C) Mean swimming speed on each training day. (D) Number of platform crossings in the target quadrant during probe test (over 60 seconds). The data are expressed as the mean ± SD (6 mice per group). * P < 0.05, ** P < 0.01, vs. the control group (one-way analysis of variance followed by the Fisher's Least Significant Difference test).