Two types of receptors for alpha-bungarotoxin in the synaptic layers of the pigeon retina.

Pigeon retinae were analyzed for binding [125I]alpha-bungarotoxin (alphaBgt) by radioautographic and biochemical methods. Toxin binding, localized to the outer and inner plexiform layers (OPL and IPL), was inhibited by micromolar concentrations of native alphaBgt and D-tubocurarine and by 1 mM acetyl- and butyrylcholine in both synaptic layers. Nicotine, at comparable concentrations affected only the IPL. In vitro drug competition experiments showed that the pigeon retina contains two types of receptors for alphaBgt which differ in sensitivity to inhibition by nicotine by 4 orders of magnitued. The results suggest that: (1) the receptor for alphaBgt in the IPL is a nicotinic receptor, (2) the receptor for alphaBgt in the OPL may be involved in an unusual cholinergic system, and (3) ability to bind alphaBgt is not a sufficient criterion for identifying nicotinic-cholinergic receptors.