Table 1 Included studies on noninvasive electrical stimulation effects in animals with retinal disorder

Takeshi Morimoto, 2007 (Japan) [39]

RP

RCS rats (rdy/rdy) (sex not specified, 3 weeks old)

RCS rats

TES: n = 6, sham: n = 6 per group

TES: contact lens electrode; biphasic rectangular current pulses; 20 Hz; 0 (sham), 50, and 100 μA; 1 ms/phase; 1 h; left eye

Once a week; 3–9 weeks of age

Two, 4, 6 weeks after start of treatment

Eye ball

ERG, toluidine blue stain

The mean thickness of the ONL at 5, 7, and 9 weeks of age was thicker in eyes treated with TES of 100 μA, and the retinal function up to 7 weeks of age was preserved in RP rats

Hanif, 2016 (USA) [40]

RP

P23H-1 rats (sex not specified, neonatal)

P23H-1 mutant rats

WES: n = 10, sham: n = 15

Whole-eye ES: silver (Ag/AgCl) pellet electrode; sine wave current (4 μA peak to peak at 5 Hz); 30 min; single eye

Twice a week, 4–24 weeks of age

Four, 8, 12, 17, and 20 weeks after start of treatment

Eye ball

OKT, ERG, toluidine blue stain, RT-PCR

(1) WES increased the spatial frequency thresholds, improved the amplitude of OPs waves in ERG, and preserved the number of ganglion cells in RP rats; (2) One hour post-WES, BDNF, caspase 3, FGF-2, and GS increased at gene levels

Feng Liu, 2022 (China) [41]

RP

Rd10 mice (both sexes, neonatal)

Rd10 mutant mice

TsES: n = 59 or n = 92 per group, sham: n = 165

Transscleral ES: gold pads electrode; bi-phasic electric pulse (square wave, 2.5 ms pulse width,1 ms inter-pulse-interval); 20 Hz; 0 (sham), 50, 100 μA; 1 ms; 30 min; left eye

Daily or every other day, P20—P25

Five days after start of treatment

Eye ball

MEA, black-and-white transition box, IHC

(1) TsES modified the retinal light responses and reduced the high spontaneous firing of retinal ganglion cells; (2) 100 µA of TsES increased the light sensitivities of ganglion cells as well as their signal-to-noise ratios, and improved the survival of photoreceptors in RP mice

Ying-qin Ni, 2009 (China) [42]

RD

SD rats (male, adult, 225–250 g)

Exposure to bright blue light with 2500 lx for 24 h

TES: n = 6 or n = 2 per group, control: n = 6

TES: contact lens electrode; biphasic rectangular current pulses; 20–100 Hz; 100–500 μA; 3 ms; 1.5 h or 20 Hz; 200, 300 μA; 3 ms; 1 h; both eyes

Once before exposure or every 3 days after exposure, up to the 14th day

Seven, 14 days after light exposure

Eye ball

ERG, HE stain, IHC, qRT‐PCR, WB

(1) Both pre- and post-TES ameliorated the progressive photoreceptor degeneration, with the latter showing a relatively better and longer-term protective effect; (2) An upregulation of Bcl-2, CNTF, and BDNF and a downregulation of Bax in the retinas after TES, Bcl-2 and CNTF were selectively upregulated in Müller cells

Takeshi Morimoto, 2012 (Japan) [43]

RP

Transgenic rabbits (sex not specified, 6 weeks old)

Rhodopsin P347L transgenic rabbits

TES and sham: n = 5

TES: contact lens electrode; biphasic rectangular current pulses; 20 Hz; 700 μA; 1 h; left eye

Once a week, 6–12 weeks of age

Six weeks after start of treatment

Eye ball

ERG, IHC

(1) TES improved the a- and b-wave amplitudes of the photopic ERG and the b-wave amplitudes of the scotopic ERG; (2) TES preserved the thickness of ONL and promoted photoreceptor survival in the RP retinas

Andreas Schatz, 2012 (Germany, Switzerland, Brazil) [44]

RD

SD rats (sex and age not specified, 210.19 ± 14.63 g)

Exposure to bright light with 16,000 lx

TES and sham: n = 15, control: n = 3

TES: DTL electrodes; biphasic rectangular current pulses; 20 Hz; 200 μA; 10 ms/phase; 1 h; right eye

Once, 2 h before light exposure

Thirty-six hours, 7, 14, 21 days after light exposure

Eye ball

ERG, HE stain, IHC, TUNEL assay

(1) One week after light exposure, TES increased the Vmax of the retinas, and lowed the b-wave implicit time for the rod response after 2 weeks of light damage; (2) TES preserved a complete outer nuclear layer thickness, reduced photoreceptor cell death, and preserved outer segment length

Tao Ye, 2016 (China) [45]

RP

C57/BL mice (both sexes, 8–9 weeks old)

MNU (60 mg/kg body weight single, ip)

TES, sham and control: n = 50 per group

TES: contact lens electrode; biphasic rectangular current pulses; 20 Hz; 0(sham), 100, 200 μA; 1 h; both eyes

Days 1,3,6 after MNU injection

Eight days after start of treatment

Eye ball and whole retina

ERG, MEA, HE stain, IHC, qRT‐PCR

(1) TES increased the amplitudes of ERG b-wave. There were different rescuing kinetics existed among regional photoreceptors, the central region is more easily preserved than other regions; (2) The signal-to-noise ratio of TES-treated mice increased, indicating that the RGCs could transmit visual signals much more reliably; (3) Apoptosis factors Bax, Bcl-2, Calpain-2, neurotrophin BDNF, CNTF were involved in the protective effect of TES

Honghua Yu, 2020 (USA, Norway, China, The Netherlands) [46]

RP

Rho^−/− mice; (sex not specified, 6 weeks of age)

Rhodopsin knockout mice

TpES: n = 6 or n = 12 per group, control: n = 6, sham: n = 6

TpES: portable electrode probe; monophasic rectangular pulse; 2—200 PPS; 100 μA; random left or right eyes

Seven consecutive days (week 1) or 7 consecutive days every other week (weeks 1 and 3)

Before and 1, 2, 3, 4 weeks after start of treatment

Eye ball

ERG, IHC, TUNEL assay, qPCR

(1) TpES in Rho^−/− mice improved photoreceptor survival and electroretinography function; (2) TpES triggered residential retinal progenitor-like cells such as Müller cells to reenter the cell cycle, possibly producing new photoreceptors; (3) TpES directly stimulated cell proliferation and the expression of progenitor cell markers in Müller cells cultures, at least partially through bFGF signaling

Agadagba, 2022 (China, Egypt) [47]

RD

Rd10 mice (both sexes, P60-P90)

Rd10 mutant mice

TES: n = 6 per group, sham: n = 10

TES: silver wire electrode; biphasic square-wave pulses; 10 Hz; 400, 500, 600 μA; 2 ms/phase; 30 min; right eye

Once a day, repeated for 7 days

One day, 1, 2 weeks after start of treatment

/

ECoG

In retinal degeneration mice, the application of electrical stimulation to the retina clearly neuromodulates brain coherence and connection of visual and non-visual cortices, and the observed modifications are largely preserved

Xin Wang, 2011 (China) [48]

H-IOP

SD rats (famale, adult, 220–250 g)

Left eye, retinal ischemia by elevated IOP (IOP 120 mm Hg, maintained 60 min)

TES, control and sham: n = 3–7 per group

TES: contact lens electrode; biphasic rectangular current pulses; 20 Hz; 0 (sham), 300 μA; 3 ms/phase; 1 h; left eye

Once every 2 days until day 14, after ischemia

Six and 24 h, 3, 7, and 14 days after start of treatment

Eye ball and whole retina

FG retrograde labeling, ERG, HE stain, IHC, WB

(1) TES increased the average density of RGCs in retinas and better preserved the mean thickness of separate retinal layers; (2) TES preserved the ERG b-wave amplitude on day 7 after ischemia and recovered robustly on day 14; (3) The neuroprotective effect of TES is associated with an increase in GS levels in Müller cells

Lin Fu, 2018 (China) [49]

H-IOP

Mongolian gerbils (sex not specified, 3–5 months old)

Right eye, acute ocular hypertensive (IOP maintained 60 min)

TES, control and sham: n = 3–10 per group

TES: contact lens electrode; bipolar rectangular current; 20 Hz; 100 μA; 1 ms/phase; 1 h; right eye

Twice (day 1, day 4) every other week after IOP elevation, for 1 month

One, 4 weeks after start of treatment

Whole retina

ERG, IHC, WB, RT-PCR

(1) TES improved RGC survival and preserved the b-wave and PhNR amplitudes of ERG; (2) TES-treated eyes showed an increase in IL-10 expression, with a corresponding decrease in IL-6 and COX-2 expression as well as a decrease in NF-κB phosphorylation, which was related to suppression in microglial cell activation

Assraa Hassan Jassim, 2021 (USA) [50]

H-IOP

DBA/2J (D2) mice (both sexes, 3–5 or 10 months old)

Glaucoma model of DBA/2J (D2) mouse strain

TES: n = 18, control: n = 8, non-TES: n = 25

TES: contact lens electrodes; symmetric biphasic square wave; 20 Hz; 100 μA; 1 ms/phase; 10 min; both eyes

Once every 3 days for 8 weeks

Eight weeks after start of treatment

Whole retina

FG retrograde labeling, IHC, WB

TES resulted in RGC axon protection, a reduction in inflammatory cells and their activation (by inhibiting microglia activation and T cell infiltration), improved energy homeostasis (by reducing the pAMPK/AMPK ratio), and a reduction of the cell death-associated p75^NTR

Ken-Ichiro Miyake, 2007 (Japan) [51]

ONC

Long-Evans rats (sex not specified, P100)

Left optic nerve crush (0.02 N, 5 s)

TES and sham: n = 4 or 5 per group

TES: contact lens electrode; biphasic square wave pulses; 20 Hz; 500 μA; 50 μs; 6 h; left eye

Once, after the post-crush VEP recording

Before, immediately after, 6 h, and 1 week after ONC

Eye ball

VEP, fluorescent anterograde tracer labeling

(1) TES immediately increased VEP amplitude impaired by ONC, and this augmentation was often preserved after 1 week; (2) After TES, a larger amount of retinal axons projected centrally beyond the crushed region, indicating that it protected retinal axons from the ensuing degeneration

Yuichi Tagami, 2009 (Japan) [52]

ONC

Wistar rats (male, adult, 250–300 g)

Left optic nerve crush (10 s)

TES: n = 5–10 per group, sham: n = 10, non-TES: n = 13

TES: contact electrode; biphasic rectangular current pulses; 20 Hz; 100 μA; 1 ms; 1 h; left eye

Once immediately after ONC (day 0); twice (days 0, 7); four times (days 0, 4, 7, 10); daily (days 0–12)

Three, 7, 12 days after ONC

Eye ball

Anterograde labeling, FG retrograde labeling, IHC

In retinas treated daily with TES, the mean number of regenerating axons significantly increased at 250 μm distal from the lesion, increased IGF-1 immunoreactivity was observed, and the survival of RGCs was enhanced

Petra Henrich-Noack, 2013 (Germany) [53]

ONC

Hooded rats (male, 7 weeks of age)

Bilateral optic nerve crush (0.1 mm jaw gap, 30 s)

TES: n = 12–13 per group, sham: n = 10–12 per group, non-TES: n = 8

TES: 3 mm diameter gold ring electrode; 100 μA; 1 ms; 30 s; different frequencies in order: 10–12-9–11-8–10- 9–12 Hz; both eyes

Immediately after and on days 3, 7, 11, 15, 19, 23 after ONC, frequencies in order with 5 s breaks, repeated once after a 2 min pause, 8 min of total stimulation time for one eye

During 42 days before and 43 days after ONC

–

VIST, ICON, EEG

TES induced long-term neuronal protection from delayed retrograde cell death, but in this case of severe axonal damage TES did not influence functional restoration and EEG signals recorded over the visual cortex

Petra Henrich-Noack, 2013 (Germany) [54]

ONC

Hooded rats (male, 6–7 weeks of age)

Bilateral optic nerve crush (0.1 mm jaw gap, 30 s)

TES and sham: n = 8 per group

TES: 3 mm diameter gold ring electrode; biphasic square-wave pulses; 20 Hz; 100 μA; 1 ms; 1 h; single eye

Twice (immediately after and day 11 after ONC)

Four days before and 3, 7, 11, 15 days after ONC

–

ICON

(1) TES delayed cell death after ONC, and RGC survival rate decreased over time; (2) TES reduced ONC-associated neuronal swelling and shrinkage, maintained cell morphology, especially in RGCs which survived long-term

Petra Henrich-Noack, 2017 (Germany,China, USA) [55]

ONC

(1) Hooded rats (male, 6–7 weeks of age); (2) B6.Cg-Tg (Thy1-YFP) HJrs/J transgenic mice (sex and age not specified)

(1) Rats: bilateral optic nerve crush (0.1 mm jaw gap, 30 s); (2) Mice: bilateral optic nerve crush (5 s)

(1) Rats: ES: n = 11, sham: n = 8, and control: n = 9; (2) Mice: ES and ES-control: n = 9, sham: n = 9

(1) Rats: transorbital ES; 3 mm diameter gold ring electrode; biphasic square-wave pulses (200 μA; 10 ms; different frequencies in order: 2–3-4–5-6–7-8- 6–5-4–3- 2 Hz; 23 min; both eyes); (2) Mice: transorbital ES; 2 mm diameter gold ring electrode; biphasic square-wave pulses; (100 μA; 1 ms; different frequencies in order: 10–12-9–11-8–10-9–12 Hz; 24 min; both eyes)

(1) Rats: days 0, 4 after ONC; (2) Mice: days 0, 3, 6, 9, 12 after ONC

(1) Rats: 21, 14, 10 days before and 4, 7, 18 days after ONC; (2) Mice: 8 days before and 3, 7, 14 days after ONC

(1) Rats: eye ball and whole retina; (2) Mice: /

(1) Rats:Oregon Green BAPTA retrograde labeling, ICON, VEP, luxol-fast-blue stain; (2) Mice: CSLO

(1) ES-induced dendritic pruning in surviving neurons in the initial post-ONC period; (2) Complete dendritic stripping following ES protects neurons from excitotoxic cell death by silencing them

Takeshi Morimoto, 2005 (Japan) [56]

ONT

Wistar rats (male, adult, 230–270 g)

Left optic nerve transection

TES: n = 6 per group, control: n = 12, non-ES: n = 8, sham: n = 6

TES: bipolar contact lens electrode; biphasic rectangular current pulses; 20 Hz; 100 μA; 0(sham)-3 ms/phase; 1 h; left eye

Once, commenced immediately after ONT

One hour to 14 days after start of treatment

Eye ball and whole retina

FG retrograde labeling, RT—PCR, Northern blot, WB, IHC

(1) TES rescued the axotomized RGCs by increasing the level of IGF-1 production by Müller cells; (2) IGF-1 immunoreactivity was originally localized in the Müller cell endfeet and then spread across the inner retina

Takeshi Morimoto, 2010 (Japan) [57]

ONT

Wistar rats (male, adult, 230–270 g)

Left optic nerve transection

TES: n = 6 per group, control: n = 12, non-ES: n = 8, sham: n = 6

TES: bipolar contact lens electrode; biphasic square pulses; (1) 0.5, 1, 2, 3, and 5 ms/phase, 100 μA, 20 Hz, 60 min; (2) 50, 100, 200, 300 and 500 μA, 1 ms/phase, 20 Hz, 60 min; (3) 0.5, 1, 5, 20, 50, and 100 Hz, 100 μA, 1 ms/phase, 60 min; (4) 15, 30, and 60 min, 100 μA, 1 ms/phase, 20 Hz; (5) 100 μA, 1 ms/phase, 20 Hz, 60 min; left eye

Once or four times (day 0, 4, 7, and 10) after ONT

Seven or 14 days after start of treatment

Whole retina

FG retrograde labeling

(1) Histologically, the optimal neuroprotective parameters for TES were pulse duration of 1 and 2 ms/phase, current intensity of 100 and 200 μA, stimulation frequency of 1, 5, and 20 Hz; (2) More than 30 min of TES was necessary to have a neuroprotective effect; (3) Symmetric pulses without an inter-pulse interval were most effective; (4) Repeated ES was more neuroprotective than a single ES

Houmin Yin, 2016 (China) [58]

ONT

SD rats (male, adult, 220–250 g)

Right optic nerve transection

TES, control and sham: n = 5 per group

TES: gold electrode; biphasic rectangular current pulses; 20 Hz; 0 (sham), 200 μA; 1 h; right eye

Days 0 and 4, or days 0, 4, 7 and 10 after ONT

Seven, 14 days after ONT

Whole retina

FG retrograde labeling, IF, WB

TES promoted RGC survival after ONT accompanied by reduced microglial activation and microglia-derived TNF-α production

Takako Osako, 2013 (Japan) [59]

NAION

SD rats (male, age not specified, 220–250 g)

RB-laser induction: RB (2.5 mM, 1 mL/kg, tail vein), laser (514 nm laser, 500 μm, 12 s, photoactivation of ON)

TES: n = 7 or12 per group, control: n = 7 or 8 per group

TES: monopolar contact lens electrode; biphasic square pulses; 20 Hz; 100 μA; 3 ms/phase; 1 h

Days 1, 4, 7, 14, and 28 after induction

Fourteen, 28 days after start of treatment

Whole retina

ERG (STR), FG retrograde labeling

TES preserved the decreasing STR amplitude and the decreasing RGC numbers in NAION. It was effective for preserving decreasing RGC numbers and function in the chronic stage of NAION