Localization of alpha-bungarotoxin binding sites in synapses of the developing chick retina

Alpha-bungarotoxin binding sites in chick retina from 12 days in ovo to hatching, were visualized at the ultrastructural level by use of a 1:1 conjugate of horseradish peroxidase with alpha-bungarotoxin. At all stages binding sites in the inner plexiform layer were localized in synapses, predominantly on or near the postsynaptic membrane. Localization of binding sites was found in bipolar and amacrine-cell synapses which appeared morphologically immature as well as in more well developed synapses. The results suggest that alpha-bungarotoxin-binding synapses, tentatively considered to be nicotinic cholinergic, are formed throughout the course of synaptogenesis, and that the aggregation of nicotinic receptors occurs early in the formation of these synapses.