On the mechanism of acetylcholine receptor accumulation at newly formed synapses on chick myotubes.

We have examined the specificity and the mechanism of acetylcholine receptor (AChR) accumulation at embryonic chick nerve-muscle contacts that form in culture. Spinal cord motoneurons were identified in vitro after labeling them in vivo with Lucifer Yellow-wheat germ agglutinin conjugates. All of their processes induced receptor clusters on contacted myotubes; after 24 to 48 hr of co-culture, the incidence of neurite-associated receptor patches (NARPs) was approximately 1.2/100 microns of contact. In contrast, NARPs were rarely associated with spinal cord interneurons (approximately 0.1/100 microns of contact). Neurons dissociated from ciliary ganglia induce NARPS to the same extent as motoneurons. The relative contribution to NARPs of AChRs present in the membrane prior to plating ciliary ganglion neurons and of "new" AChRs inserted 8, 11, or 17 hr after addition of neurons was assessed with two fluorescent receptor probes. Rhodamine-conjugated alpha-bungarotoxin was used to label either old or new receptors; a monoclonal, anti-receptor antibody visualized with fluorescein-second antibody was used to label all (new and old) receptors. Analysis of digitized fluorescence images showed that NARPs contained both new and old receptors but that within the first 24 hr of co-culture the majority (60 to 80%) were new. We estimate that cholinergic neurites increase the rate of receptor insertion 4- to 5-fold during the first 8 hr of NARP formation. The contribution of new receptors to NARPs declines with time. After 3 days of co-culture, receptors inserted over an 8-hr interval comprised only 20% of the total NARP complement.(ABSTRACT TRUNCATED AT 250 WORDS)