Zinc release at the synaptic terminals of rod photoreceptors.

The presence of reactive zinc (Zn2+) within photoreceptor terminals, and evidence that exogenous zinc affects the electrophysiological activity of the distal retina, led to the suggestion that its co-release with glutamate could play an essential role in the modulation of information at the first synapse in the visual pathway. Although we had shown previously that zinc release could be visualized in the region of the outer synaptic layer of a retinal slice preparation, it could not be ascertained with certainty that the release sites were at the presynaptic terminal rather than from the mitochondria-rich inner segment or from zinc within the distal processes of photoreceptors and Müller cells. Using membrane permeant and membrane impermeant forms of a fluorescent zinc indicator (Newport green), we show both the intracellular distribution of Zn2+ and its depolarization-dependent discharge from the terminals of isolated zebrafish photoreceptors in culture. Zinc release could be detected in the dark-adapted preparation, and was further enhanced by brief exposures to black widow spider venom or high K+. Synaptically released zinc may significantly influence neural processing in the vertebrate retina by modulating the activity of excitatory and/or inhibitory receptors as well as intracellular signaling proteins.