Agonist activation and alpha-bungarotoxin inhibition of wild type and mutant alpha 7 nicotinic acetylcholine receptors

The properties of wild type and mutant rat nicotinic alpha 7 receptors expr essed in Xenopus oocytes were investigated using electrophysiology and site -directed mutagenesis. When compared at individual agonist concentrations, neither the normalised nicotinic, nor acetylcholine, responses of the wild type receptors were significantly different from the corresponding response s obtained from a first extracellular domain mutant, phenylalanine(189) tyr osine (P > 0.05). The dissociation constants (K-D) of the wild type (4.7 nM ) and Phe(189)Tyr mutant (5.2 nM) receptors for alpha-bungarotoxin were est imated by an electrophysiological approach. The similarity of the results s uggests that the mutation did not lead to a widespread disruption of struct ure-function relationships, although a slight change in nicotine sensitivit y may have occurred. In contrast, the mutations (Tyr(190)Gln, first extrace llular domain), (Glu(261)Ala, M2 region) severely compromised receptor func tion. An additional mutation was made in a negatively charged motif of the second extracellular domain which is conserved in homomeric nicotinic recep tors. This mutation, Asp(268)Ala, also caused a loss of function. Thus the structure-function relationships in nicotinic alpha 7 receptors have parall els with heteromeric nicotinic receptors, but there may also be some marked differences. (C) 1999 Elsevier Science B.V. All rights reserved.