Ligand responses of alpha-bungarotoxin binding sites from skeletal muscle and optic lobe of the chick.

Binding properties of detergent-solubilized receptors for alpha-bungarotoxin from skeletal muscle of the 13th day chick embryo and from optic lobe of the hatching chick were compared. It was found that both types of receptor are nicotinic, although they differ in their affinities for individual ligands and in the rank order of ligands. In contrast to the muscle receptor, the neuronal receptor binds the toxin in a reversible fashion (KD = 2.1 X 10(-10) M at 23 degrees C). Small ligands inhibit brain equilibrium procedures. Toxin and ligands compete for a single type of noninteracting site, and the ratio of toxin binding sites to ligand-binding sites is unity. The inhibitory potency of ligands parallels their ability, at higher concentrations, to accelerate receptor . toxin by interaction with the same site on the receptor derived from the optic lobe.