Localization of α-bungarotoxin binding sites to the goldfish retinotectal projection

The optic tectum of the goldfishCarassius auratus is a rich source of α-bungarotoxin (α-Btx) binding protein. In order to determine whether some fraction of these receptors is present at retinotectal synapses, we have compared the histological distribution of receptors revealed by the use of ¹²⁵Iα-Btx radioautography to the distribution of optic nerve terminals revealed by the use of cobalt and horseradish peroxidase (HRP) techniques. The majority of α-Btx binding is concentrated in those tectal layers containing primary retinotectal synapses. The same layers contain high concentrations of acetylcholinesterase (AChE), revealed histochemically. Following enucleation of one eye, there is a loss of α-Btx binding in the contralateral tectum, observed both by radioautography and by a quantitative binding assay of α-Btx binding. Approximately 40% of the α-Btx binding sites are lost within two weeks following enucleation. By contrast, no significant change in AChE activity could be demonstrated up to 6 months enucleation. These results are discussed in light of recent studies which show that the α-Btx binding protein and the nicotinic acetylcholine receptor are probably identical in goldfish tectum. We conclude that the 3 main classes of retinal ganglion cells projecting to the goldfish tectum are nicotinic cholinergic and that little or no postdenervation hypersensitivity due to receptor proliferation occurs in tectal neurons following denervation of the retinal input.