Fumagalli_1978_Brain.Res_153_87

alpha-Bungarotoxin (alpha-BuTX) binds in a saturable and practically irreversible fashion to membrane-associated receptors in the ciliary ganglion of the adult chick. The binding of toxin to receptors is competitively inhibited by nicotinic cholinergic ligands, and for these properties the receptors are regarded as acetylcholine receptors of the nicotinic type (alpha-buTX-AChRs). The rate constant of association (K1) and dissociation (K-1) of the toxin-receptor reaction has been estimated to be K1 = 7.4 x 104 M(-1) sec(-1) and K-1 = 9.6 X 10(-6) sec(-1), respectively. Light autoradiography shows that most, if not all, the receptors are related to surface membrane, probably to synaptic areas of both choroid and ciliary neurons. The choroid neurons contain more receptors than the ciliary ones. A single chick ciliary ganglion binds specifically 47 fmole of alpha-BuTX in situ corresponding to 2.83 x 1010 alpha-BuTX-AChRs/ganglion. No changes in number and distribution of the toxin receptors occur following preganglionic denervation. Conversely, postganglionic axotomy causes a rapid disappearance of the receptors in situ. Since binding experiments in vitro revealed a partial, instead of a total, loss of the receptors, it is suggested that the disappearance of the receptors in situ includes both a partial loss of the original receptors and the masking of the residual ones.