Ganglionic nAChRs and high-affinity nicotinic binding sites are not equivalent

High-affinity (K_d {reversed tilde equals} 10 nM) binding sites for nicotine and acetylcholine (ACh) have recently been identified in vertebrate brain. It has been suggested that these sites are desensitized ganglionic (C6) nicotinic acetylcholine receptors (nAChRs). We have tested the pheochromocytoma cell line PC 12, which is known to contain well-expressed C6 nAChRs, to determine if these nAChRs are associated with high-affinity [³H]ACh-binding sites. We found that the high-affinity nicotinic [³H]ACh-binding site is absent in PC 12 cells. We also found that the concentration of nicotine or ACh necessary to desensitize carbamylcholine-stimulated Na⁺ flux was at least two orders of magnitude greater than the concentrations used in binding experiments. We conclude that high-affinity nicotinic binding sites are not equivalent to C6 ganglionic receptors.