COEXPRESSION OF THE NEURONAL ALPHA(7) AND L247T ALPHA(7) MUTANT SUBUNITS YIELDS HYBRID NICOTINIC RECEPTORS WITH PROPERTIES OF BOTH WILD-TYPE ALPHA(7) AND ALPHA(7) MUTANT HOMOMERIC RECEPTORS

Injection of cDNA encoding the neuronal alpha(7) subunit into Xenopus oocytes yields homomeric receptors showing responses to AcCho that hav e low affinity, fast desensitization, nonlinear current-voltage (I-V) relation, and sensitivity to alpha-bungarotoxin (alpha-BTX) and 5-hydr oxytryptamine (5HT), both substances acting as antagonists, Mutation o f the Leu-247, located in the channel domain, changes 5HT from an anta gonist to an agonist, slows the rate of desensitization, renders the I -V relation linear, and increases the affinity for acetylcholine (AcCh o), A study was made of receptors expressed after injecting Xenopus oo cytes with mixtures of cDNAs encoding the wild-type alpha(7) (WT alpha (7)) and the L247T alpha(7) mutated nicotinic AcCho receptors (nAcChoR s), The receptors expressed were again blocked by alpha-bungarotoxin ( 100 nM) but exhibited both WT alpha(7) and alpha(7) mutant functional characteristics. Out of eight different types of hybrid receptors iden tified, most were inhibited by 5HT (1 mM) and showed low sensitivity t o AcCho, like the WT alpha(7) receptors, but exhibited a slow rate of desensitization and an I-V relation similar to those of alpha(7) mutan t receptors, Together, these findings indicate that the increased nAcC hoR affinity and the decreased nAcChoR desensitization after Leu-247 m utation are uncoupled events, We propose that receptor diversity is pr edicted by permutations of WT alpha(7) and L247T alpha(7) subunits in a pentameric symmetrical model and that even partial replacement of Le u-247 with a polar residue within the leucine ring in the channel doma in considerably influences the properties of neuronal alpha(7) nAcChoR s.