ALPHA-7 AND ALPHA-8 NICOTINIC RECEPTOR SUBTYPES IMMUNOPURIFIED FROM CHICK RETINA HAVE DIFFERENT IMMUNOLOGICAL, PHARMACOLOGICAL AND FUNCTIONAL-PROPERTIES

Nicotinic receptors are present in the chick retina, but their structu re and functional characteristics are still unclear. Using anti-alpha 7 and anti-alpha 8 subunit-specific antibodies, we immunopurified the alpha 7 and alpha 8 subtypes of chick retina neuronal nicotinic recept ors. When analysed by sodium dodecyl sulphate-polyacrylamide gel elect rophoresis, the two purified subtypes consistently showed a similar pe ptide 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 alpha 7 subtype, the 58 kDa peptide was recognized b y anti-alpha 7 but not by anti-alpha 8 antibodies; in the alpha 8 subt ype, the 58 kDa peptide was recognized only by anti-alpha 8 antibodies . The alpha 7 subtype had a single class of [I-125]alpha-bungarotoxin binding sites with a K-D value of 1.2 nM, whereas the purified alpha 8 subtype had two classes of binding sites, one with a K-D of 5.5 nM an d the other with very high affinity (K-D 52 pM), but present in only 8 % of the receptors. Competition binding experiments also showed the pr esence on the alpha 8 subtype of high- and low-affinity classes of bin ding sites; the affinity for cholinergic drugs of the former was great er than that of the single class present on the alpha 7 subtype. When reconstituted in planar lipid bilayers, both subtypes formed ligand-ga ted 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 strychnin e. In line with the binding data, the reconstituted alpha 8 subtype ha d greater agonist sensitivity than the alpha 7 subtype.