Effects of Zn2+ on wild and mutant neuronal α7 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 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 α₇ subunits. In oocytes expressing wild-type receptors, Zn²⁺ alone did not elicit appreciable membrane currents. Acetylcholine (AcCho) elicited large currents (I(AcCho)) that were reduced by Zn²⁺ in a reversible and dose-dependent manner, with an IC₅₀ of 27 μM 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 blockade mechanism. In sharp contrast, in oocytes expressing a receptor mutant, with a threonine-for-leucine 247 substitution (L247Tα₇), subnanomolar concentrations of Zn²⁺ 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 Zn²⁺ opened channels that had a mean open time of 5 ms and a conductance of 48 pS. At millimolar concentrations Zn²⁺ reduced I(AcCho) and the block became stronger with cell hyperpolarization. Thus, Zn²⁺ is a reversible blocker of wild-type α₇ 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 Zn²⁺ is a modulator as well as an activator of homomeric nicotinic α₇ receptors.