PHARMACOLOGICAL CHARACTERIZATION OF ALPHA-BUNGAROTOXIN-SENSITIVE ACETYLCHOLINE-RECEPTORS IMMUNOISOLATED FROM CHICK RETINA - CONTRASTING PROPERTIES OF ALPHA-7-SUBUNIT-CONTAINING AND ALPHA-8-SUBUNIT-CONTAINING SUBTYPES

At least three subtypes of alpha-bungarotoxin-sensitive acetylcholine receptors (alphaBgt-sensitive AChRs) exist in chick brain and retina. All may contain previously unknown structural subunits. One subtype co ntains alpha7 subunits. Another contains alpha8 subunits. A third cont ains both alpha7 and alpha8 subunits. In this article, we describe, fo r the first time, the pharmacological characterization of alpha7 AChRs and alpha8 AChRs immunoisolated from chick retina. Pharmacologically, the alpha8 AChRs exhibit two classes of binding sites, the high affin ity of which have higher affinity for most cholinergic ligands than do alpha7 AChRs. These differences are most accentuated for ACh (approxi mately 5400-fold), decamethonium (approximately 1400-fold), 1,1,-dimet hyl-4 phenylpiperazinium (approximately 200-fold), atropine (approxima tely 200-fold), nicotine (approximately 100-fold), and tetramethylammo nium (approximately 100-fold). The alpha8 AChR low affinity sites exhi bit affinities that are similar but not identical to that of alpha7 AC hRs. Many of the pharmacological differences between the alpha7 AChRs and alpha8 AChRs can be attributed to the limited differences between the amino acid sequences of the N-terminal region of the alpha7 and al pha8 subunits because expressed alpha7 homomers and alpha8 homomers al so exhibit these characteristic differences.1