Excitation of the Pre-frontal and Primary Visual Cortex in Response to Transcorneal Electrical Stimulation in Retinal Degeneration Mice

Abstract

Retinal degeneration (rd) is one of the leading causes of blindness in the modern world today. Various strategies including electrical stimulation are being researched for the restoration of partial or complete vision. Previous studies have demonstrated that the effectiveness of electrical stimulation in somatosensory, frontal and visual cortices is dependent on stimulation parameters including stimulation frequency and brain states. The aim of the study is to investigate the effect of applying a prolonged electrical stimulation on the eye of rd mice with various stimulation frequencies, in awake and anesthetized brain states. We recorded spontaneous electrocorticogram (ECoG) neural activity in prefrontal cortex and primary visual cortex in a mouse model of retinitis pigmentosa (RP) after prolonged (5-day) transcorneal electrical stimulation (pTES) at various frequencies (2, 10, and 20 Hz). We evaluated the absolute power and coherence of spontaneous ECoG neural activities in contralateral primary visual cortex (contra Vcx) and contralateral pre-frontal cortex (contra PFx). Under the awake state, the absolute power of theta, alpha and beta oscillations in contra Vcx, at 10 Hz stimulation, was higher than in the sham group. Under the anesthetized state, the absolute power of medium-, high-, and ultra-high gamma oscillations in the contra PFx, at 2 Hz stimulation, was higher than in the sham group. We also observed that the ultra-high gamma band coherence in contra Vcx-contra PFx was higher than in the sham group, with both 10 and 20 Hz stimulation frequencies. Our results showed that pTES modulates rd brain oscillations in a frequency and brain state-dependent manner. These findings suggest that non-invasive electrical stimulation of retina changes patterns of neural oscillations in the brain circuitry. This also provides a starting point for investigating the sustained effect of electrical stimulation of the retina to brain activities.

Keywords: ECoG activity; pre-frontal cortex; primary visual cortex; retina; transcorneal electrical stimulation.

FIGURE 1

(A) Pictorial representation of mouse skull suture lines showing recording sites. PFx, pre-frontal cortex; Vcx, primary visual cortex. Ref: Position of the reference electrode in the cerebellum. ML; Mid-line. (B) Representative electrocorticogram (ECoG) raw data trace during awake and anesthetized states after surgery. Both ECoG traces were captured over a period of 1 s.

FIGURE 2

Electrical evoked potential of one rd mouse (rd10) showing activation of retinal-cortical pathway by transcorneal electrical stimulation (TES). The stimulating electrode (silver wire) was placed on the right eye corneal surface and the mouse was stimulated with 400 μA current, 10 Hz frequency and charge balanced biphasic pulse (2 ms/phase). The charge injected per phase was 0.8 μC. Electrical evoked potential was recorded from two locations of the primary visual cortex namely ipsilateral primary visual cortex (Vcx) (A) and contralateral Vcx (B). Contralateral Vcx showed more robust response evoked by TES compared to ipsilateral Vcx with lower response.

FIGURE 3

Schematic representation of experimental design for recording of spontaneous ECoG. Each step was done sequentially from breeding of rd10 mice to the end of the experiments in which all animals were sacrificed.

FIGURE 4

Example data of post-stimulation electroencephalographic (EEG) raw trace before (top) and after pre-processing (bottom). Both traces were captured over a period of 5 s.

FIGURE 5

Impedance monitoring. (A) Impedance measurements of silver wire electrode in 0.1 M phosphate buffer saline (PBS). Impedance magnitude was measured across a wide range of frequencies (1 Hz–1,000 kHz). Frequency is represented on a log scale (log₁₀). (B) Representative plot of impedance at the interface between silver wire electrode and rd10 mice cornea (n = 6) measured at 1 kHz. No significant difference (p > 0.05) was observed between the pre-stimulation and post-stimulation impedance monitored in vivo at 1 kHz. Inset figure: Voltage waveform recordings of a silver wire electrode to 400 μA biphasic stimulus pulses recorded at the rd10 mice cornea (measurement was captured from an oscilloscope).

FIGURE 6

Box-plots showing post-stimulation changes in spontaneous absolute power in the primary visual cortex of rd10 mice during awake (A) and anesthetized (B) brain states. Absolute power is represented in a log scale. All results were compared to the sham group. All p-values were adjusted by Holm–Bonferroni method. * denotes significant change in spontaneous absolute power compared to sham group. Short horizontal lines embedded in each boxplot represent the median while the small square symbols represent the mean spontaneous absolute power of each group. Sham (n = 8), 2 Hz (n = 8), 10 Hz (n = 8), 20 Hz (n = 9).

FIGURE 7

Box-plots showing post-stimulation changes in spontaneous absolute power in the pre-frontal cortex of rd10 mice during awake (A) and anesthetized (B) brain states. Absolute power is represented in a log scale. All results were compared to the sham group. All p-values were adjusted by Holm–Bonferroni method. * denotes significant change in spontaneous absolute power compared to sham group. Short horizontal lines embedded in each boxplot represent the median while the small square symbols represent the mean spontaneous absolute power of each group. Sham (n = 8), 2 Hz (n = 8), 10 Hz (n = 8), 20 Hz (n = 9).

FIGURE 8

Bar-plots showing post-stimulation changes in contralateral primary visual cortex (contra Vcx)- contralateral pre-frontal cortex (contra PFx) spontaneous coherence of rd10 mice during awake (A) and anesthetized (B) brain states. Spontaneous coherence is represented as mean ± standard error of the mean (SEM). All results were compared to the sham group. All p-values were adjusted by Holm–Bonferroni method. * denotes significant change in spontaneous coherence compared to sham group. Sham (n = 8), 2 Hz (n = 8), 10 Hz (n = 8), 20 Hz (n = 9).