Electrical Stimulation Parameters in Normal andDegenerate Rabbit Retina / 의학물리

ABSTRACT

Retinal prosthesis is regarded as the most feasible method for the blind caused by retinal diseases such as retinitis pigmentosa (RP) or age related macular degeneration (AMD). Recently Korean consortium launched for developing retinal prosthesis. One of the prerequisites for the success of retinal prosthesis is the optimization of the electrical stimuli applied through the prosthesis. Since electrical characteristics of degenerate retina are expected to differ from those of normal retina, we performed voltage stimulation experiment both in normal and degenerate retina to provide a guideline for the optimization of electrical stimulation for the upcoming prosthesis. After isolation of retina, retinal patch was attached with the ganglion cell side facing the surface of microelectrode arrays (MEA). 8x8 grid layout MEA (electrode diameter 30micrometer, electrode spacing 200micrometer, and impedance 50 k omega at 1 kHz) was used to record in-vitro retinal ganglion cell activity. Mono-polar electrical stimulation was applied through one of the 60 MEA channel, and the remaining channels were used for recording. The electrical stimulus was a constant voltage, charge-balanced biphasic, anodic-first square wave pulse without interphase delay, and 50 trains of pulse was applied with a period of 2 sec. Different electrical stimuli were applied. First, pulse amplitude was varied (voltage 0.5~3.0 V). Second, pulse duration was varied (100~1,200microns). Evoked responses were analyzed by PSTH from averaged data with 50 trials. Charge density was calculated with Ohm's and Coulomb's law. In normal retina, by varying the pulse amplitude from 0.5 to 3 V with fixed duration of 500 microns, the threshold level for reliable ganglion cell response was found at 1.5 V. The calculated threshold of charge density was 2.123 mC/cm2. By varying the pulse duration from 100 to 1,200microns with fixed amplitude of 2 V, the threshold level was found at 300microns. The calculated threhold of charge density was 1.698 mC/cm2. Even after the block of ON-pathway with L-(1)-2-amino-4-phosphonobutyric acid (APB), electrical stimulus evoked ganglion cell activities. In this APB-induced degenerate retina, by varying the pulse duration from 100 to 1200 microns with fixed voltage of 2 V, the threshold level was found at 300microns, which is the same with normal retina. More experiment with APB-induced degenerate retina is needed to make a clear comparison of threshold of charge density between normal and degenerate retina.