Effects of Zn(2+) on wild and mutant neuronal α(7) nicotinic receptors

Zn(2+) is a key structural/functional component of many proteins and is present at high concentrations in the brain and retina, where it modulates ligand-gated receptors. Therefore, a study was made of the effects of zinc on homomeric neuronal nicotinic receptors expressed in Xenopus oocytes after injection of cDNAs encoding the chicken wild or mutant α(7) subunits. In oocytes expressing wild-type receptors, Zn(2+) alone did not elicit appreciable membrane currents. Acetylcholine (AcCho) elicited large currents (I(AcCho)) that were reduced by Zn(2+) in a reversible and dose-dependent manner, with an IC(50) of 27 μM and a Hill coefficient of 0.4. The inhibition of I(AcCho) by Zn(2+) was competitive and voltage-independent, a behavior incompatible with a channel blockade mechanism. In sharp contrast, in oocytes expressing a receptor mutant, with a threonine-for-leucine 247 substitution ((L247T)α(7)), subnanomolar concentrations of Zn(2+) elicited membrane currents (I(Zn)) that were reversibly inhibited by the nicotinic receptor blockers methyllycaconitine and α-bungarotoxin. Cell-attached single-channel recordings showed that Zn(2+) opened channels that had a mean open time of 5 ms and a conductance of 48 pS. At millimolar concentrations Zn(2+) reduced I(AcCho) and the block became stronger with cell hyperpolarization. Thus, Zn(2+) is a reversible blocker of wild-type α(7) receptors, but becomes an agonist, as well as an antagonist, following mutation of the highly conserved leucine residue 247 located in the M2 channel domain. We conclude that Zn(2+) is a modulator as well as an activator of homomeric nicotinic α(7) receptors.