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

Abstract

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 to 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.

Cytoprotection by endogenous zinc in the vertebrate retina

“…This notion was based on results by Wu and co-workers (Wu et al 1993), showing decreased calcium entry into photoreceptor terminals when exogenous zinc was applied to the salamander retina preparation. We have since confirmed these results, and in addition have shown that using chelators to remove endogenous zinc leads to a marked increase in both calcium entry (Anastassov et al 2013) and in the photoreceptor's dark current (Chappell et al 2008). These findings indicate that a reduced zinc concentration results in a concomitant increase in the discharge of glutamate, and led us to suggest that if endogenous zinc can suppress transmitter release, it may serve to protect the retina from the toxic effects of glutamate.…”

“…We have since confirmed these results, and in addition have shown that using chelators to remove endogenous zinc leads to a marked increase in both calcium entry (Anastassov et al . ) and in the photoreceptor's dark current (Chappell et al . ).…”

Zinc Nutrition and Inflammation in the Aging Retina

“…Depolarization of the retinal neurons can induce zinc release at the plexiform layers, which provided the first evidence for the hypothesized neuro modulator role for zinc in the retina . Endogenous zinc is co‐released with glutamate from synaptic terminals of photoreceptors and, by negative feedback, reduces calcium entry and concomitant vesicular release of glutamate . This is, therefore, likely to protect the retina from glutamate excitotoxicity …”

“…[68,131,132] Endogenous zinc is co-released with glutamate from synaptic terminals of photoreceptors and, by negative feedback, reduces calcium entry and concomitant vesicular release of glutamate. [81,83] This is, therefore, likely to protect the retina from glutamate excitotoxicity. [83] Zinc can be released from the cellular compartments or stores by molecules such as MTs, by external stimuli.…”

“…Role of extracellular zinc in regulation of AMPA mediates postsynaptic signals in skate horizontal cells and bipolar cells. [70][71][72] Inhibition of extracellular zinc on hemi-gap-junction channel currents on retinal horizontal cells in bass [73] Zinc release by depolarization of retinal cells in rat [74] Modulation of A-type potassium currents by zinc in retinal horizontal cells [75] Amacrine cells Differential modulation of signal from cones to horizontal cells by extracellular zinc [76] Zn(2+) modulates light responses of bipolar and amacrine cells in carp retina [77] Modulatory role of endogenous zinc upon glycinergic response in amacrine cells in rat retina [78] Suppression by zinc of transient OFF responses of carp amacrine cells on GABA receptors [79] Photoreceptors Synaptically released zinc from rod photoreceptors in zebrafish [80] Modulatory role of extracellular zinc upon calcium activity the photoreceptor terminal [81] Role of Yin Yang 1 in melatoninergic function of retinal photoreceptors. [82] Zinc co-release with glutamate by photoreceptors; cytoprotective role in skate retina [81,83] Translocation of zinc within photoreceptors in light stimulation [17] Stabilization of disc membranes in bovine and mice retina and in vitro model modelling [84][85][86][87][88] Role of KLF15 in repression of photoreceptor-specific gene expression in non-photoreceptor cells in vitro molecular model system [89] RPE Zinc decrease in RPE layer in disease in vitro [9] Regulatory role of melanin synthesis and melanosome formation in RPE cells in vitro and in vivo in rats [90][91][92] Protective role of zinc against oxidative stress in vitro RPE [32,93,94] Role of zinc in phagocytic and lysosomal activity in vitro RPE [95][96][97]…”