Expression and channel properties of alpha-bungarotoxin-sensitive acetylcholine receptors on chick ciliary and choroid neurons.

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: alpha3*-AChRs recognized by mAb35, that contain alpha3, alpha5, beta4, and occasionally beta2 subunits, and alpha-bungarotoxin (alphaBgt)-AChRs recognized and blocked by alphaBgt, that contain alpha7 subunits. We found that maximal whole cell current densities (I/C(m)) mediated by alphaBgt-AChRs were threefold larger for choroid compared with ciliary neurons, while alpha3*-AChR current densities were similar in the two populations. Different densities of total cell-surface alphaBgt-AChRs could not explain the distinct alphaBgt-AChR response densities associated with ciliary and choroid neurons. Ciliary ganglion neurons display abundant [(125)I]-alphaBgt binding ( approximately 10(6) sites/neuron), but digital fluorescence measurements revealed equivalent site densities on both populations. AChR channel classes having single-channel conductances of approximately 30, 40, 60, and 80 pS were present in patches excised from both ciliary and choroid neurons. Treating the neurons with alphaBgt selectively abolished the 60- and 80-pS events, identifying them as arising from alphaBgt-AChRs. Kinetic measurements revealed brief open and long closed durations for alphaBgt-AChR channel currents, predicting a very low probability of being open (p(o)) when compared with 30- or 40-pS alpha3*-AChR channels. None of the channel parameters associated with the 60- and 80-pS alphaBgt-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 alphaBgt-AChRs (N(F)) per unit area than do ciliary neurons. Comparison with total surface [(125)I]-alphaBgt-AChR sites (N(T)), reveals that N(F)/N(T) << 1 for both neuron populations, suggesting that "silent" alphaBgt-AChRs predominate. Choroid neurons may therefore express a higher density of functional alphaBgt-AChRs by recruiting a larger fraction of receptors from the silent pool than do ciliary neurons.