Three subtypes of α-bungarotoxin-sensitive nicotinic acetylcholine receptors are expressed in chick retina

Abstract

A recent report described the isolation of cDNA clones encoding α7 and α8 subunits of α-bungarotoxin-sensitive nicotinic ACh receptors (αBgtAChRs) from chick brain and demonstrated that they were related to, but distinct from, the α subunits of nicotinic ACh receptors (nAChRs) from muscles and neurons. Monoclonal antibodies against the two αBgtAChR subunits were used to demonstrate that at least two subtypes are present in embryonic day 18 chicken brain. The predominant brain subtype contains α7 subunits, while a minor subtype contains both α7 and α8 subunits. Both subtypes may also contain other subunits. Here we report the results of immune precipitation studies and immunohistochemical studies of αBgtAChRs in the chick retina. In addition to the two subtypes found in brain, a new αBgtAChR subtype that contains α8 subunits, but not α7 subunits, was identified and was found to be the major subtype in chick retina. This subtype has a lower affinity for α-bungarotoxin (αBgt) than does the subtype containing only α7 subunits. Small amounts of this α8 subtype were also detected in brain by labeling with higher concentrations of 125I-αBgt been used previously. The subtype containing only α7 subunits comprised 14% of the αBgtAChRs in hatchling chick retina. The subtype containing α8 subunits (but no α7 subunits) accounted for 69%, and the α7α8 subtype accounted for 17%. Amacrine, bipolar, and ganglion cells displayed α8 subunit immunoreactivity, and a complex pattern of labeling was evident in both the inner and outer plexiform layers. In contrast, only amacrine and ganglion cells exhibited α7 subunit immunoreactivity, and the pattern of α7 subunit labeling in the inner plexiform layer differed from that of α8 subunit labeling. These disparities suggest that the αBgtAChR sub- units are differentially expressed by different populations of retinal neurons. In addition, the distribution of αBgtAChR subunit immunoreactivity was found to differ from that of α-Bgt-insensitive nAChR subunits.