9.6  Summary

A neurotransmitter-based retina prosthesis is a feasible option for restoration of visual function in humans with retinal degeneration. The diverse and differential actions of glutamate, GABA, glycine and acetylcholine on surviving retinal cells allow for both excitatory and inhibitory stimulation of the retina. These neurotransmitters differentially influence a number cell types that underlie feature detection processed in the retina. Although, pathophysiological changes in retinal degeneration may reduce the effectiveness of stimulation, neurotransmitter based prostheses offer the ability to activate retinal circuits, or suppress hyperactive ones. Technical considerations such as diffusion and EAATs ensure that exogenous local application of transmitters will affect a small restricted area of retina with responses that are temporally dampened. In addition, since passing axons are not stimulated, a neurotransmitter-based retinal prosthesis can maintain a high spatial specificity, even at suprathreshold stimulation. Technological advances in electrical prosthesis will aid in the development of a neurotransmitter based prosthesis, since existing circuits may be used to control electro-osmotic ejection of transmitters. In addition, there are new advances in other drug delivery technologies, such as caging and tethering molecules, which may be adapted for a neurotransmitter based retinal prosthesis. In conclusion, neurotransmitters offer a promising new approach to stimulation for retinal prosthesis, which may also supplement and piggy-back upon existing technology.

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