Effects of tDCS-like electrical stimulation on retinal ganglion cells

Abstract

PurposeTranscranial direct current stimulation (tDCS) has been studied in humans for its effects on enhancement of learning, amelioration of psychiatric disorders, and modification of other behaviors for over 50 years. Typical treatments involve injecting 2 mA current through scalp electrodes for 20 minutes, sometimes repeated weekly for two to five sessions. Little is known about the direct effects of tDCS at the neural circuit or the cellular level. This study assessed the effects of tDCS-like currents on the central nervous system by recording effects on retinal ganglion cell responsiveness using the rabbit retina eyecup preparation.Materials and methodsWe examined changes in firing to On and Off light stimuli during and after brief applications of a range of currents and polarity and in different classes of ganglion cells.ResultsThe responses of Sustained cells were consistently suppressed during the first round of current application, but responses could be enhanced after subsequent rounds of stimulation. The observed first round suppression was independent of current polarity, amplitude, or number of trials. However, the light responses of Transient cells were more likely to be enhanced by negative currents and unaffected or suppressed by first round positive currents. Short-duration currents, that is, minutes, as low as 2.5 µA produced a remarkable persistency of firing changes, for up to 1.5 hours, after cessation of current.ConclusionThe results are consistent with postulated tDCS alteration of central nervous system function, which outlast the tDCS session and provide evidence for the isolated retina as a useful model to understand tDCS actions at the neuronal level.

Effects of tACS-Like Electrical Stimulation on On-Center Retinal Ganglion Cells: Part I

“…All methods were similar to those in a previous tDCS study 28 except for the use of microelectrode arrays and alternating, rather than direct stimulating current.…”

“…The recording and electrical stimulus configurations were similar to those we used for the tDCS study. 28 One current electrode was a stainless steel wire shallowly immersed in the superfusion bath over the retina near the recording electrode(s). The reference electrode was a silver–silver chloride ring at the chamber bottom that served as both the ground/reference electrode for recording and as the other electrode for tACS current application.…”

“…The analog recordings of ganglion cell responses, digitized at 4 kHz with 12 bit resolution, were processed in MATLAB as described previously 28 to extract spikes. The stimulus spot (called a case in figures) with the largest unequivocal light-evoked response(s) prior to electrical stimulation is generally shown in most detail in this report.…”

“…In a previous study, we examined the effects of tDCS in retina, 28 showing that currents of a few microamperes affected most ganglion cell classes during current application, and up to hours afterward. In this study, we have made a similar examination of tACS effects on On-center ganglion cells with currents of 1–2 microamperes, at a frequency of 10 Hz.…”

Implantable Direct Current Neural Modulation: Theory, Feasibility, and Efficacy

“…The other reason that DC neuromodulation has gained interest in recent years is that the field of neuromodulation has become refined sufficiently to be faced with new challenges that are more difficult to address using pulsatile waveforms. Because DC directly controls membrane potential, it can increase or decrease firing rate, altogether block neural activity, control AP propagation velocity, and modulate synaptic connectivity (Goldberg et al, 1984; Bikson et al, 2004; Vrabec et al, 2017; Strang et al, 2018; Yang et al, 2018). DC also appears to maintain the stochastic properties of AP inter-pulse intervals on each neuron (Goldberg et al, 1984), in contrast to conventional pulsatile stimulation for which an evoked AP in phase with the stimulation pulse is the intended effect.…”