α7 and α8 Nicotinic Receptor Subtypes Immunopurified from Chick Retina have Different Immunological, Pharmacological and Functional Properties

Nicotinic receptors are present in the chick retina, but their structure and functional characteristics are still unclear. Using anti-α7 and anti-α8 subunit-specific antibodies, we immunopurified the α7 and α8 subtypes of chick retina neuronal nicotinic receptors. When analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, the two purified subtypes consistently showed a similar peptide composition characterized by the presence of two major peptides of M_r 58 ± 1 and 54 ± 1 kDa, and two minor peptides of 67 and 61 ± 1 kDa. In the α7 subtype, the 58 kDa peptide was recognized by anti-α7 but not by anti-α8 antibodies; in the α8 subtype, the 58 kDa peptide was recognized only by anti-α8 antibodies. The α7 subtype had a single class of [¹²⁵]a-bungarotoxin binding sites with a KD value of 1.2 nM, whereas the purified α8 subtype had two classes of binding sites, one with a K _D of 5.5 nM and the other with very high affinity ( K _D 52 pM), but present in only 8% of the receptors. Competition binding experiments also showed the presence on the α8 subtype of highand low-affinity classes of binding sites; the affinity for cholinergic drugs of the former was greater than that of the single class present on the α7 subtype. When reconstituted in planar lipid bilayers, both subtypes formed ligand-gated cation channels with major conductance levels of 42 and 52 pS but with different lifetimes; the two channels were activated by agonists and blocked by d-tubocurarine and the glycinergic antagonist strychnine. In line with the binding data, the reconstituted α8 subtype had greater agonist sensitivity than the α7 subtype.