Invest Ophthalmol Vis Sci 47(6): 2606-2612

Inner Retinal Mechanisms Engaged by Retinal Electrical Stimulation

^{Departments of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, Nebraska}

^{Departments of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center, Omaha, Nebraska}

Corresponding author: Eyal Margalit, Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, 985540 Nebraska Medical Center, Omaha NE 68198-5540; ude.cmnu@tilagrame

^{Both authors contributed equally to the work presented here and should therefore be regarded as equivalent authors.}

Abstract

Purpose

Retinal prosthetic devices are being developed to bypass degenerated retinal photoreceptors by directly activating retinal neurons with electrical stimulation. However, little is known about retinal activity during such stimulation.

Methods

Whole cell patch-clamp recordings were obtained from ganglion and bipolar cells in the salamander retinal slice preparation. A stimulating electrode was positioned at the vitreal surface of the slice.

Results

Brief pulses of cathodic current evoked transient inward currents in ganglion cells arising from action potentials. Longer pulses (>5 milliseconds) also evoked sustained inward currents in ganglion cells that appeared synaptic in origin because, unlike transient currents, sustained currents were blocked by inhibiting synaptic transmission with Cd^{. These synaptic currents reversed around E}_Cl and were blocked by picrotoxin, strychnine, or both, suggesting they were mediated by GABA_a/c and glycine receptors. Synaptic currents were also blocked by the NMDA antagonist MK801 and the KA/AMPA antagonist NBQX, suggesting that epiretinal stimulation evoked glutamate release from bipolar cells, which in turn stimulated the release of GABA and glycine from amacrine cells. Sustained currents were also evoked by epiretinal stimulation in bipolar cells. These currents reversed near E_Cl and were blocked by picrotoxin, suggesting they arose from GABA_a/c receptors.

Conclusions

Pulse duration is an important parameter for effective activation of the inner retina by epiretinal stimulation. Brief pulses evoke action potentials only in ganglion cells. However, longer pulses also evoke sustained synaptic currents by stimulating glutamate release from bipolar cell terminals, which, in turn, evokes the release of GABA and glycine from amacrine cells.

Abstract

One approach to treat blindness associated with outer retinal degenerative disorders, such as retinitis pigmentosa, Usher syndrome, and age-related macular degeneration, is to develop a multielectrode prosthetic device that will bypass the missing outer retinal elements (e.g., photoreceptors) and will electrically stimulate the remaining inner retinal elements (e.g., bipolar and ganglion cells) in a patterned manner. Indeed, such devices have been developed and implanted in human subjects.¹2 Subjects report that retinal stimulation evokes the perception of phosphenes. However, studies examining the electrophysiological mechanisms engaged by retinal electrical stimulation have been limited in extent. Several in vitro experiments with extracellular electrophysiological recordings in isolated retina preparations have shown that neuronal activity can be evoked by electrical stimulation. Early experiments show that secondary slow-wave retinal potentials can be induced by transretinal electrical stimulation of an amphibian eyecup preparation.³4 In bullfrog eyecups, the charge-density threshold for evoking responses by epiretinal stimulation was found to be 2.98 μC/cm^{Charge-density threshold values were much higher—between 30 and 917}μC/cm^{—in isolated rabbit retinas, and similar findings were reported in isolated human retinas.}⁶ Similar threshold values of 500 μC/cm^{using subretinal electrical stimulation have been found in chicken and retinal degenerate Royal College of Surgeons rats (Stett A, et al.}IOVS 1999;40:ARVO Abstract 3892).⁷

However, the main goal of these studies was not to determine underlying mechanisms but to delineate electrode shapes and pulse parameters for safe and effective electrical stimulation in controlled settings. The objective of the present study was to analyze retinal mechanisms during electrical stimulation. To do so, we obtained whole cell recordings from retinal ganglion and bipolar cells in a salamander retina slice preparation. Salamander retina is a widely used model system because its relatively large cells facilitate whole cell recording. In these studies, we tested whether various electrical stimulation parameters induce differential ganglion or bipolar retinal cell responses, and we pharmacologically dissected the retinal circuitry engaged by such stimulation.

Footnotes

Disclosure: E. Margalit, None; W.B. Thoreson, None

Footnotes

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