Effects of the destruction of starburst-cholinergic amacrine cells by the toxin AF64A on rabbit retinal directional selectivity

Abstract

The effects of intraocular injections of ethylcholine mustard aziridinium ion (AF64A), an irreversible inhibitor of choline uptake, on the rabbit retina were assessed electrophysiologically, pharmacologically, anatomically, and behaviorally. Survival times from 1 day to 30 days were investigated. After 24 h, the shortest time tested, the directional selectivity of On–Off responding ganglion cells having the characteristic morphology of On–Off directionally selective directionally selective (DS) ganglion cells, as revealed by intracellular dye injection, was significantly reduced, both by an apparent decrease of preferred direction responses and an increase in responses to null-direction movement. No toxin-mediated changes in the dendritic trees of these cells were noted. Cells in AF64A-affected retinas having the DS morphology did not respond significantly to GABAergic or cholinergic agents such as picrotoxin and eserine, but did respond to nicotine. Recordings from a small random sample of other ganglion cell classes in the same retinas yielded no obvious changes in response properties. The direct effects on starburst (cholinergic) amacrine cells, which were identified by intraocular injection of the fluorescent dye DAPI with the AF64A, were investigated by intracellular injections of Lucifer yellow, and by immunohistochemical staining with antibodies to choline acetyltransferase (ChAT). Although starburst amacrine cell somas survived the AF64A treatment for at least several days, the dendrites could not be visualized by fluorescent dye injection in affected retinas due to dye leakage of the injected fluorescent dye from either the soma or proximal dendritic region. ChAT staining revealed a sequence in which ChAT-positive cells were undetectable first in the inner nuclear layer, and then in the ganglion cell layer. Cholinergic amacrine cells in the central retina were also affected before those in the periphery. The electrophysiological changes observed typically preceded the loss of ChAT activity. Behavioral tests for optokinetic nystagmus responses also revealed a lack of such responses in the affected eyes.