Novel subpopulation of neuronal acetylcholine receptors among those binding alpha-bungarotoxin

Neuronal acetylcholine receptors (AChRs) that bind alpha-bungarotoxin (alpha Bgt) (alpha Bgt-AChRs) have previously been found to contain at least one of the alpha 7-alpha 9 gene products. No other gene products of the 11 neuronal AChR genes cloned to date from rat and/or chick have been identified in such receptors. Chick ciliary ganglia have about 20 fmol of alpha Bgt-AChRs that contain alpha 7 subunits and 5 fmol of synaptic-type AChRs that bind the monoclonal antibody (mAb) 35 and collectively contain alpha 3, beta 4, alpha 5, and, to a lesser extent, beta 2 subunits. Using a sensitive solid-phase immunoprecipitation assay, we show here that ciliary ganglia have about 1 fmol of novel putative AChRs that bind both alpha Bgt and mAb 35 but appear to lack all of the known neuronal AChR gene products in ciliary ganglia, including alpha 3, alpha 5, alpha 7, beta 2, and beta 4. The putative receptors are also unlikely to contain either alpha 8 or alpha 9 gene products, because of the known expression patterns of these gene products. Nonetheless, the component sediments at 10 S, as expected for neuronal AChRs, and has a nicotinic pharmacology similar but not identical to that of alpha 7-containing alpha Bgt-AChRs. The AChR alpha 1 gene product expressed in muscle is known to bind both alpha Bgt and mAb 35, and we show here that ciliary ganglia contain small amounts of alpha 1 transcript. The putative ciliary ganglion AChR defined by joint alpha Bgt and mAb 35 binding, however, does not appear to contain alpha 1 subunits. A similar component binding both mAb 35 and alpha Bgt can be detected in sympathetic ganglia and dorsal root ganglia but not in brain, spinal cord, or retina. The developmental time course of the component in ciliary ganglia is comparable to that of the alpha 7-containing alpha Bgt-AChRs. If the component is a functional AChR on ciliary ganglion neurons, as seems likely, it would represent the fourth AChR subtype produced by this population of cells. Our inability to identify subunits comprising the putative receptors raises the possibility that additional AChR genes remain to be cloned.