Brain complexity in stroke recovery after bihemispheric transcranial direct current stimulation in mice

Abstract

Stroke is one of the leading causes of disability worldwide. There are many different rehabilitation approaches aimed at improving clinical outcomes for stroke survivors. One of the latest therapeutic techniques is the non-invasive brain stimulation. Among non-invasive brain stimulation, transcranial direct current stimulation has shown promising results in enhancing motor and cognitive recovery both in animal models of stroke and stroke survivors. In this framework, one of the most innovative methods is the bihemispheric transcranial direct current stimulation that simultaneously increases excitability in one hemisphere and decreases excitability in the contralateral one. As bihemispheric transcranial direct current stimulation can create a more balanced modulation of brain activity, this approach may be particularly useful in counteracting imbalanced brain activity, such as in stroke. Given these premises, the aim of the current study has been to explore the recovery after stroke in mice that underwent a bihemispheric transcranial direct current stimulation treatment, by recording their electric brain activity with local field potential and by measuring behavioural outcomes of Grip Strength test. An innovative parameter that explores the complexity of signals, namely the Entropy, recently adopted to describe brain activity in physiopathological states, was evaluated to analyse local field potential data. Results showed that stroke mice had higher values of Entropy compared to healthy mice, indicating an increase in brain complexity and signal disorder due to the stroke. Additionally, the bihemispheric transcranial direct current stimulation reduced Entropy in both healthy and stroke mice compared to sham stimulated mice, with a greater effect in stroke mice. Moreover, correlation analysis showed a negative correlation between Entropy and Grip Strength values, indicating that higher Entropy values resulted in lower Grip Strength engagement. Concluding, the current evidence suggests that the Entropy index of brain complexity characterizes stroke pathology and recovery. Together with this, bihemispheric transcranial direct current stimulation can modulate brain rhythms in animal models of stroke, providing potentially new avenues for rehabilitation in humans.

Keywords: entropy; local field potential; stroke; tDCS.

Graphical Abstract

Figure 1

Timeline of experimental protocols. Timeline shows experimental design with respect to stroke induction (days from stroke) and to the end of tDCS or sham treatment (hours or week post tDCS). All mice enrolled in the study were subjected to LFP recordings; a subgroup of mice (n = 5 Stroke-tDCS; n = 5 Stroke-Sham) were also tested in the forelimb grip strength test. Figure was created with BioRender.com.

Figure 2

Total ApEn results. The ANOVA for the evaluation of Total ApEn between the experimental Groups and Time Points showed a statistically significant interaction (F(3,38) = 4.2419, P = 0.01112) for the factor Groups. Total Approximate Entropy (Total ApEn) values in Healthy-Sham (n = 12 mice), Stroke-Sham (n = 10 mice), Healthy-tDCS (n = 11 mice) and Stroke-tDCS (n = 9) groups are reported in the figure. Vertical bars represent the standard errors. *represents the significant results P < 0.05; ANOVA followed by Duncan post hoc.

Figure 3

Hem ApEn results. The ANOVA for the evaluation of Total ApEn between hemispheres (Left Hem ApEn, Right Hem ApEn), Groups (Healthy-Sham, Stroke-Sham, Healthy-tDCS, Stroke-tDCS) and time points (1, 2, 3 and 4 weeks after stimulation) showed a significant interaction (F(3, 38) = 2.8486, P = 0.050) between Hemispheres and Groups. Total ApEn values computed in the left (Left Hem ApEn, red line) and in the right hemisphere (Right Hem ApEn, blue line) in Healthy-Sham (n = 12 mice), Stroke-Sham (n = 10 mice), Healthy-tDCS (n = 11 mice) and Stroke-tDCS (n = 9 mice) groups are reported in the figure. Vertical bars represent the standard errors. *represents the significant results P < 0.05; ANOVA followed by Duncan post hoc.

Figure 4

Time course of post-stroke recovery in the grip strength test in mice subjected tDCS and sham stimulation. Stroke-tDCS mice (n = 5) displayed higher forelimb strength values than Stroke-Sham mice (n = 5), starting from day 6 after stroke (24 h post stim) and throughout the entire follow-up period. Data are expressed as mean ± SEM. *P < 0.05; **P < 0.001; two-way RM ANOVA, followed by Bonferroni post hoc. b.w. indicates body weight.

Figure 5

Correlation analysis between total ApEn and grip strength. Scatterplots of Pearson’s correlation analysis between Total ApEn and Grip Strength values (r = −0.3554, P = 0.0285) in all mice tested (n = 5 Stroke-tDCS mice; n = 5 Stroke-Sham mice) and time points (at 1, 2, 3 and 4 weeks after stimulation).

Figure 6

Correlation analysis between hem ApEn and grip strength. Scatterplots of Pearson’s correlation analysis between the right Hem ApEn and the Grip Strength values (r = −0.3258, P = 0.0402) in all mice tested (n = 5 Stroke-tDCS mice; n = 5 Stroke-Sham mice) and time points (at 1, 2, 3 and 4 weeks after stimulation).