Blockade of nicotinic responses in rat retinal ganglion cells by neuronal bungarotoxin

The effects of neuronal bungarotoxin (NBT) on nicotinic acetylcholine responses recorded from rat retinal ganglion cells in culture were studied with patch electrodes. We observed that the concentration of this toxin needed to induce a total blockade of nicotinic currents varied according to the method of toxin application utilized. Rapid addition of 20 microM NBT by pressure ejection from micropipettes produced total blockade of 50-microM acetylcholine-induced currents. In contrast, when added slowly via the physiological solution which continuously superfused the cells, NBT was able to produce a complete blockade of the response at 200 nM. The IC50 determined for NBT by the superfusion method was 55 nM. Recovery from block was slow and incomplete with both drug application methods, although some differences were found. The results are discussed with reference to a scheme in which NBT binds with both low and high affinity to functional nicotinic receptors in rat retinal ganglion cells in culture.