Retinotectal Map Properties During Epiretinal Electrical Stimulation in Rats

Abstract

Purpose: : To compare the spatial distribution of responses in the superior colliculus of normal and retinal degenerate (RD) rats during light stimulation and epiretinal electrical stimulation.

Methods: : Epiretinal electrical stimulation was performed in normal pigmented rats (Long Evans) and S334ter–line–3 RD rats (397–432 day old). A platinum–iridium stimulating electrode (75µm) was advanced through the vitreous of the eye through an incision made in the sclera until the electrode tip was positioned over the epiretinal surface. A constant current source was used for electrical stimulation (range, 25–200 µA, 1 ms biphasic pulses) and a full–field strobe flash (1500 cd/m²) was used for light stimulation. Multiunit signals were recorded electrophysiologically from the surface of the SC at 150–250µm apart.

Results: : Light evoked responses in normal pigmented rats could be recorded from the entire surface of the SC. No light evoked visual responses could be recorded in RD rats. Electrically evoked responses could be recorded in normal pigmented rats and RD rats from an area of the SC corresponding to the placement of the stimulating electrode inside the eye. In RD rats, the threshold level for electrically evoked SC response was significantly higher (120.00 ± 33.67 µA, p<0.05) when compared to the normal control rats (45.83 ± 13.57 µA) and in RD rats, electrically evoked responses were recordable at fewer SC sites than in normal rats.

Conclusions: : RD rats had a significantly higher stimulus threshold than normal rats. In normal and RD rats, epiretinal electrical stimulation produces a spatially localized response in the SC that is consistent with the known retinotopic projection.

Keywords: superior colliculus/optic tectum • retina • electrophysiology: non-clinical