CHICK CILIARY GANGLION NEURONS CONTAIN TRANSCRIPTS CODING FOR ACETYLCHOLINE RECEPTOR-ASSOCIATED PROTEIN AT SYNAPSES (RAPSYN)

A peripheral membrane protein of approximate to 43 kDa (rapsyn) cluste rs muscle nicotinic acetylcholine receptors (AChRs), but molecules rel evant to clustering neuronal AChRs have not been identified. Here, we have detected rapsyn transcripts in the chick nervous system, localize d rapsyn mRNA in ciliary ganglion (CG) neurons, which are known to clu ster AChRs, and identified three rapsyn cDNAs derived from the ganglio n. Our initial Northern blots, performed using a mouse probe, revealed rapsyn-like transcripts in chick muscle and brain. To develop species -specific probes, we prepared a chick rapsyn cDNA construct, Ch43K.1, that encodes a protein having extensive homology to mouse rapsyn. Usin g primers designed to anneal near the 5' and 3' boundaries of Ch43K.1, three prominent cDNAs were amplified from chick muscle templates by r everse transcriptase based-PCR. Products of similar size were also amp lified using cDNA prepared from neuronal tissues expected to contain c lustered AChRs (CG and brain), whereas none were detected using templa tes from tissues not displaying clustered AChRs (sensory ganglia and l iver). In situ hybridization confirmed that rapsyn mRNA is expressed b oth in chide muscle fibers and in CG neurons. Sequencing the three cDN As amplified from CG templates revealed the largest to be Ch43K.1, whe reas the smaller two may represent splice variants. These findings sug gest that multiple rapsyn-like molecules are involved in clustering th e distinct AChRs expressed by muscle fibers and neurons.