Expression and Channel Properties of a-Bungarotoxin-Sensitive Acetylcholine Receptors on Chick Ciliary and Choroid Neurons

McNerney, Mary Ellen, Desiree Pardi, Phyllis C. Pugh, Qiang Nai, and Joseph F. Margiotta. Expression and channel properties of a-bungarotoxin-sensitive acetylcholine receptors on chick ciliary and choroid neurons. J Neurophysiol 84: 1314–1329, 2000. Cell-specific expression of nicotinic acetylcholine receptors (AChRs) was examined using ciliary and choroid neurons isolated from chick ciliary ganglia. At embryonic days 13 and 14 (E13,14) the neurons can be distinguished by size, with ciliary neuron soma diameters exceeding those of choroid neurons by about twofold. Both neuronal populations are known to express two major AChR types: a3*-AChRs recognized by mAb35, that contain a3, a5, b4, and occasionally b2 subunits, and a-bungarotoxin (aBgt)-AChRs recognized and blocked by aBgt, that contain a7 subunits. We found that maximal whole cell current densities (I/Cm) mediated by aBgt-AChRs were threefold larger for choroid compared with ciliary neurons, while a3*-AChR current densities were similar in the two populations. Different densities of total cell-surface aBgt-AChRs could not explain the distinct aBgtAChR response densities associated with ciliary and choroid neurons. Ciliary ganglion neurons display abundant [I]-aBgt binding ('10 sites/neuron), but digital fluorescence measurements revealed equivalent site densities on both populations. AChR channel classes having single-channel conductances of '30, 40, 60, and 80 pS were present in patches excised from both ciliary and choroid neurons. Treating the neurons with aBgt selectively abolished the 60and 80-pS events, identifying them as arising from aBgt-AChRs. Kinetic measurements revealed brief open and long closed durations for aBgt-AChR channel currents, predicting a very low probability of being open (po) when compared with 30or 40-pS a3*-AChR channels. None of the channel parameters associated with the 60and 80-pS aBgt-AChRs differed detectably, however, between choroid and ciliary neurons. Instead calculations based on the combined whole cell and singlechannel results indicate that choroid neurons express approximately threefold larger numbers of functional aBgt-AChRs (NF) per unit area than do ciliary neurons. Comparison with total surface [I]-aBgtAChR sites (NT), reveals that NF/NT ,, 1 for both neuron populations, suggesting that “silent” aBgt-AChRs predominate. Choroid neurons may therefore express a higher density of functional aBgtAChRs by recruiting a larger fraction of receptors from the silent pool than do ciliary neurons.