人工視覚システム実現に向けた網膜電気刺激に対する神経節細胞応答の検証

抄録

To develop retinal prosthesis for blind patients using implanted electrodes, it is important to study the properties of the retinal ganglion cell responses induced by local current stimuli. Intracellular calcium concentration ([Ca²⁺]_i) is a good indicator of excitation as well as the bio-chemical condition of the neuron. Therefore, we conducted a [Ca²⁺]_i imaging study using the frog retina to investigate the response properties to local current stimuli with a cooled-CCD imaging system, and relative changes in [Ca²⁺]_i were quantified as changes in the fluorescence of Ca²⁺-sensitive fluorophore. Bipolar current pulses were applied using a sharp tungsten electrode to mimic epi-retinal stimulation. To mimic sub-retinal stimulation, current pulses were applied from the photoreceptor side with a planar electrode. The elevation of [Ca²⁺]_i was observed when the current was increased above 50 μA for both epi-retinal and subretinal stimuli. The stimulus applied to the epi-retinal side elevated the [Ca²⁺]_i of the ganglion cell somata around the stimulus electrode. This stimulus also elevated the [Ca²⁺]_i of the ganglion cell axons around the electrode. This observation indicates that the current stimulus applied from the epi-retinal side excites the ganglion cell axons. When the stimulus was applied from the photoreceptor side, [Ca²⁺]_i elevation was observed only in the ganglion cell somata around the stimulus electrode. The diameter of the response area to the tetanic stimuli (10 times at 100 Hz, 50 μA) was 145±39 μm measured parallel to the axon and 132±28 μm perpendicular to the axon. The response area was broadened by increasing the stimulus current or by increasing the number of pulses in the tetanic stimulation. We concluded that sub-retinal stimulation is more suitable than epi-retinal stimulation for localizing the excited cells.