Four genes encode the related protein subunits that assemble to form the nicotinic acetylcholine receptor (nAChR) at the motor endplate of vertebrates. We have isolated from the chicken genome four additional members of the same gene family whose protein products, termed alpha 2, alpha 3, alpha 4 and n alpha (non‐alpha) probably define three distinct neuronal nAChR subtypes. The neuronal nAChR genes have identical structures consisting of six protein‐coding exons and specify proteins that are best aligned with the chicken endplate alpha subunit, whose gene we have also characterized. mRNA transcripts encoding alpha 4 and n alpha are abundant in embryonic and in adult avian brain, whereas alpha 2 and alpha 3 transcripts are much scarcer. The same set of neuronal genes probably exists in all vertebrates since their counterparts have also been identified in the rat genome.

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The nicotinic acetylcholine receptor ( AChR) present at the neuromuscular junction of vertebrates is the best characterized of neurotransmitter-gated ion channels and a very attractive model system in molecular neurobiology. By 1983, several groups had succeeded in cloning and sequencing the cDNAs encoding the four receptor subunits in the electric fish Torpedo. Examination of the derived protein sequences confirmed that the subunits were related throughout their entire primary structures, and thus that the corresponding genes were certainly derived from a unique ancestral sequence by a process involving gene duplications. The four genes encoding the AChR at the neuromuscular junction can thus be said to constitute a monofunctional gene family. Using appropriate AChR cDNA probes derived from Torpedo, we screened avian genomic and cDNA libraries in an attempt to isolate and characterize the corresponding genes and gene products. In addition, we thought that this approach might yield other, related receptor genes perhaps encoding the as yet poorly characterized nicotinic receptors known to be present in the central and peripheral nervous systems. In this report we show that this approach resulted in the complete characterization of the a, y, and 6 genes of chicken AChR. Moreover, we show that several receptor-related genes could be isolated. These genes, termed a2, a3, and 72, differ significantly from their namesakes at the neuromuscular junction. The fact that none of them are expressed in muscle but that a3 and y2 are respectively expressed in the adrenal medulla and in the brain supports the view that they encode nervous-system-specific receptors. In addition, their isolation establishes that the family of AChR-related genes includes numerous sibs. RESULTS AChR Genes Expressed at the Avian Neuromuscular JunctionScreening of a chicken genomic library with Torpedo cDNA probes encoding subunit-specific sequences in conditions of moderate stringency yielded numerous overlapping clones that were ordered, subcloned, and sequenced by conventional "This work was supported by Swiss National Foundation for Scientific Research Grant 3.154.0.85. 141

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