Zinc modulation of calcium activity at the photoreceptor terminal: A calcium imaging study

• Exogenous zinc produced a dose-dependent reduction in calcium entry.
• Zinc's effect on calcium entry is by suppression of synaptic L-type calcium channels.
• Chelating endogenous zinc increased calcium entry in response to depolarization.
• L-type Ca2+ channel blockers abolished the enhancement produced by zinc chelation.
• Co-release of zinc provides a feedback mechanism that reduces glutamate release.

There is abundant experimental evidence that zinc ions (Zn2+) are present in the synaptic vesicles of vertebrate photoreceptors, and that they are co-released with glutamate. Here we show that increasing the concentration of extracellular zinc (2 μM–2 mM) suppresses the entry of calcium into the synaptic terminals of isolated salamander double cones. The resultant dose-dependent curve was fit by an inverse Hill equation having an IC50 of 38 μM, and Hill coefficient of 1.1. Because there is currently no reliable way to measure the concentration of extracellular zinc, it is not known whether the zinc released under normal circumstances is of physiological significance. In an attempt to circumvent this problem we used zinc chelators to reduce the available pool of endogenous zinc. This enabled us to determine how the absence of zinc affected calcium entry. We found that when intra- or extra-cellular zinc was chelated by 250 μM of membrane-permeable TPEN or 500 μM of membrane-impermeable histidine, there was a significant rise in the depolarization-induced intracellular calcium level within photoreceptor terminals. This increase in internal [Ca2+] will undoubtedly lead to a concomitant increase in glutamate release. In addition, we found that blocking the L-type calcium channels that are expressed on the synaptic terminals of photoreceptors with 50 μM nicardipine or 100 μM verapamil abolished the effects of zinc chelation. These findings are a good indication that, when released in vivo, the zinc concentration is sufficient to suppress voltage-gated calcium channels, and reduce the rate of glutamate release from photoreceptor terminals.