Fig. 1

Intracranial GES for 5xFAD mouse and the FEM simulation analysis

A-C. Electrode implantation. Two stainless steel screws were implanted as the paired electrodes to deliver the intracranial sinusoidal alternating current stimulation (A), as shown in the skull at: Anterior-Posterior (AP) = -2 mm, and Medial-Lateral (ML) = 4 mm (left and right) to the bregma (B-C). The electrodes were screwed and immobilized in the skull with the distal end reaching the dura. The positioning of the electrodes was determined following computer simulation that indicates maximal electric fields/currents to the targeted regions – cortex and hippocampus [15]

D. A mouse with electrodes implanted. Following the implantation surgery, the animals were checked twice daily to ensure no infection, no changes in health and behaviors, and normal activities until the end of the experiment

E-F. Three-dimensional (3D) simulation of the distribution of electric fields in a mouse brain, based on C57BL/6 mouse brain atlas with MRI and Nissl histology, with 39 regions represented (in different colors). The simulation helped to determine the positioning of the electrodes (in red and blue) to achieve desired field distribution as in G

G. The Finite element method (FEM) simulation suggests electric field (EF) distribution in the brain, which would effectively stimulate the cortex (with EF intensity at ~ 100–150 mV/mm) and hippocampus (with EF intensity at ~ 10–80 mV/mm), two main regions affected by Aβ overload in AD.

H. Behavior tests (blue bars) and Gamma Electric Stimulation (GES, red bars). The 5xFAD mice (3-month-old, male) were randomly assigned into sham, Low-current, and High-current groups. Before the GES, the Morris Water Maze assessment consisted of training and probe trials performed in Week 0 (blue bar). The electrodes were implanted 24 h before the first stimulation. The stimulation was delivered (red bars) for 1 h each day on Monday, Wednesday, Friday, and Sunday of Week 1; for 1 h on Monday of Week 3 and 4, and then for 1 h on Sunday of Week 4, followed by immediate euthanization and brain tissue collection. The GES details were: 40 Hz at 25 µA (n = 8), 50 µA (n = 8), 100 µA (n = 6), and 200 µA (n = 7) (amplitudes produced and monitored by the Neuroelectrics® Starstim®). The sham mice (n = 6) underwent every procedure, except that GES was not switched on. The Morris Water Maze test was performed in Week 4 (blue bar) as the learning and memory behavior assessment after the 4-week GES respectively