α-Bungarotoxin receptors on neurons isolated from turtle retina: Molecular heterogeneity of bipolar cells

We studied localization of specific [¹²⁵I]-α-bungarotoxin (α-BTX) receptors on neurons dissociated from the adult turtle retina. Cells isolated from papain-treated retinas retained complex morphologies and were readily identifiable with cell classes (photoreceptors, Muller cells, multipolar and bipolar cells). Levels of α-BTX receptors, measured by filtration assays, averaged 120 fmol/mg of protein whether or not retinas were treated with papain. Light microscope autoradiographs of isolated cells showed that only bipolar neurons had receptors for α-BTX. In the presence of d-tubocurarine, no cells were labeled above background. Receptors were localized both close to and far from known sites of synaptic contacts. Highly labeled bipolar cells had grains over Landolt clubs, cell bodies, and axons as well as over dendrites, but lightly labeled cells had few grains over axons and cell bodies. Overall, most receptors occurred on dendrites, suggesting the activity of cellular mechanisms capable of directing receptor molecules to specific subcellular positions. The receptor phenotype was not expressed all or none, as autoradiographs of individual bipolar cells showed a wide spectrum of receptor levels. A maximum level of about 30,000 receptors/cell was seen. Unlabeled bipolar cells also were prevalent (70%), and this group included virtually all 'displaced' type bipolar cells. The data show molecular heterogeneity both between and within retina cell classes.