Demonstration of a System to Study Multisite Response to Focal Glutamate Stimulation in a Rat Retina

This study was undertaken to investigate how a mammalian retina responds to focalized stimulation with a neurotransmitter as part of a larger research project seeking to establish the feasibility of a chemical-based approach to a retinal prosthesis. The specific goals of this thesis are: i) to focally stimulate a rat retina ex vivo with glutamate and record its response at multiple sites using a Multielectrode Array (MEA) system, and ii) to analyze the recorded signals from multiple neurons for a physiological response. For this study, retinas from dark-adapted Long Evans rats were used, and the tissue samples were prepared by carefully extracting the retina from the excised eye of the euthanized animal. The retinal tissue was placed on the MEA chip with the ganglion cell layer (GCL) contacting the electrodes while being continuously superperfused with a physiological medium. The basal level of the spontaneous activity of the neurons was recorded using the MC Rack software associated with the MEA system. The GCL was stimulated by light from a custom built LED light source placed underneath the MEA system and the response of the tissue was recorded from the 60 electrodes of the MEA chip. Following confirmation of a physiological response of the tissue to the light stimulation, 0.2 µl of glutamate was ejected on the photoreceptor side of the retina through a capillary connected to a custom syringe pump. The response of the retina to this controlled ejection was recorded from 60 different sites on the tissue. The above set of stimulation experiments were repeated for three concentrations of glutamate, viz. 2 mM,1 mM and 0.5 mM on genetically identical animals. The responses obtained from the spontaneous activity, the light-stimulated activity and the chemically-stimulated activity were analyzed using the tools available within the MC Rack software. The analyses indicate that the retina responds electrophysiologically to glutamate stimulation and the response activity level increases with an increase in the concentration of the glutamate. The findings of this study preliminarily demonstrate the feasibility of chemically stimulating the retina in a physiological manner, which could potentially lead to a chemical based — as opposed to electrical based — retinal implant for correcting the loss of vision due to photoreceptor degenerative diseases.