Immunohistochemical localization of monoclonal antibodies to the nicotinic acetylcholine receptor in chick midbrain.

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Abstract
The indirect immunofluorescence method was used to determine the crossreactivity of a library of 57 monoclonal antibodies (mAb) against each of the subunits of the nicotinic acetylcholine receptor (nAcChoR) isolated from Torpedo and Electrophorus electric organs or from fetal calf and human muscle, with specific neural elements in the midbrain of the chick. Out of 17 mAb and recognized motor end plates on chick muscle, 14 produced a similar pattern of labeling in the midbrain: the neuronal perikarya and dendrites in the lateral spiriform nucleus (SpL) were intensely labeled, and there was moderate labeling of fibers in certain of the deeper layers of the optic tectum, which disappeared after the SpL was destroyed electrolytically. Two lines of evidence suggest that the mAb may be crossreacting with nAcChoR in the midbrain. First, all of the mAb that stained the SpL also stained neuromuscular junctions in skeletal muscle, whereas none of the 40 mAb that failed to stain end plates crossreacted with the SpL; 2nd, in vitro immunological studies and blocking experiments on tissue sections (in which unlabeled mAb were used to block the staining of a directly fluorescein-treated mAb) indicated the presence of mAb specific for unique antigenic determinants on all 4 of the subunits (.alpha., .beta. .gamma. and .delta.) the Torpedo nAcChoR in chick midbrain and muscle. The distribution of mAb staining in the optic tectum does not closely parallel that of either acetylcholinesterase staining or of 125I-labeled .alpha.-bungarotoxin binding; no toxin binding was observed autoradiographically in the SpL, but the nucleus does contain moderately dense acetylcholinesterase staining. There may be a cholinergic input to the SpL and that the projection fibers from the SpL to the optic tectum (which are also stained with an antiserum to [Leu]enkephalin) may contain presynaptic nAcChoR. The distribution of the putative nAcChoR, .alpha.-bungarotoxin binding sites and acetylcholinesterase staining in the avian midbrain are quite different, although they do overlap to some degree in the deeper layers of the optic tectum.