Direct observation of the rapid aggregation of acetylcholine receptors on identified cultured myotubes after exposure to embryonic brain extract.

We have directly observed the redistribution of acetylcholine receptors (AChR) on the surface of cultured myotubes, induced by a soluble brain extract. The AChR were fluorescently labeled with rhodamine-conjugated alpha-bungarotoxin and viewed under low incident illumination with a video image intensification system. The results of our sequential observations indicate that AChR aggregates can be assembled rapidly (30-120 min) from mobile, diffuse AChR. This assembly was characterized by the initial formation of microaggregates (less than 1 micron diameter) that increased in number and coalesced or grew to form larger aggregates. The redistribution of fluorescently labeled AChR was completely inhibited by illumination of cells at levels used for conventional fluorescence micrography and could be observed only by using low light levels.