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

Abstract

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: α3*-AChRs recognized by mAb35, that contain α3, α5, β4, and occasionally β2 subunits, and α-bungarotoxin (αBgt)-AChRs recognized and blocked by αBgt, that contain α7 subunits. We found that maximal whole cell current densities ( I/ C m) mediated by αBgt-AChRs were threefold larger for choroid compared with ciliary neurons, while α3*-AChR current densities were similar in the two populations. Different densities of total cell-surface αBgt-AChRs could not explain the distinct αBgt-AChR response densities associated with ciliary and choroid neurons. Ciliary ganglion neurons display abundant [125I]-αBgt binding (≈106 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 αBgt selectively abolished the 60- and 80-pS events, identifying them as arising from αBgt-AChRs. Kinetic measurements revealed brief open and long closed durations for αBgt-AChR channel currents, predicting a very low probability of being open ( p o) when compared with 30- or 40-pS α3*-AChR channels. None of the channel parameters associated with the 60- and 80-pS αBgt-AChRs differed detectably, however, between choroid and ciliary neurons. Instead calculations based on the combined whole cell and single-channel results indicate that choroid neurons express approximately threefold larger numbers of functional αBgt-AChRs ( N F) per unit area than do ciliary neurons. Comparison with total surface [125I]-αBgt-AChR sites ( N T), reveals that N F/ N T≪ 1 for both neuron populations, suggesting that “silent” αBgt-AChRs predominate. Choroid neurons may therefore express a higher density of functional αBgt-AChRs by recruiting a larger fraction of receptors from the silent pool than do ciliary neurons.