EFFECTS OF ZN2+ ON WILD AND MUTANT NEURONAL ALPHA(7) NICOTINIC RECEPTORS

Zn2+ is a key structural/functional component of many proteins and is present at high concentrations in the brain and retina, where it modul ates ligand-gated receptors. Therefore, a study was made of the effect s of zinc on homomeric neuronal nicotinic receptors expressed in Xenop us oocytes after injection of cDNAs encoding the chicken wild or mutan t alpha(7) subunits. In oocytes expressing wild-type receptors, Zn2+ a lone did not elicit appreciable membrane currents. Acetylcholine (AcCh o) elicited large currents (I-AcCho) that were reduced by Zn2+ in a re versible and dose-dependent manner, with an IC50 of 27 mu M and a Hill coefficient of 0.4. The inhibition of I-AcCho by Zn2+ was competitive and voltage-independent, a behavior incompatible with a channel block ade mechanism. In sharp contrast, in oocytes expressing a receptor mut ant, with a threonine-for-leucine 247 substitution ((L247T)alpha(7)), subnanomolar concentrations of Zn2+ elicited membrane currents (IZ,) t hat were reversibly inhibited by the nicotinic receptor blockers methy llycaconitine and alpha-bungarotoxin. Cell-attached single-channel rec ordings showed that Zn2+ opened channels that had a mean open time of 5 ms and a conductance of 48 pS. At millimolar concentrations Zn2+ red uced I-AcCho and the block became stronger with cell hyperpolarization . Thus, Zn2+ is a reversible blocker of wildtype alpha(7) receptors, b ut becomes an agonist, as well as an antagonist, following mutation of the highly conserved leucine residue 247 located in the M2 channel do main. We conclude that Zn2+ is a modulator as well as an activator of homomeric nicotinic alpha(7) receptors.