Letter to the Editor Zinc release at the synaptic terminals of rod photoreceptors Stephen Redenti a,b , Harris Ripps a,c , Richard L. Chappell a,d, * a Marine Biological Laboratory, 7 MBL Street, Woods Hole, MA 02543, USA b Schepens Eye Research Institute, Department of Ophthalmology, Harvard Medical School, 20 Staniford Street, Boston, MA 02114, USA c Department of Ophthalmology and Visual Science, University of Illinois at Chicago, College of Medicine, 1855 West Taylor Street, Chicago, IL 60612, USA d Department of Biological Sciences, Hunter College and Ph.D. Program in Biology, The Graduate Center, CUNY, 695 Park Avenue, New York, NY 10021, USA Received 30 June 2007; accepted in revised form 23 July 2007 Available online 3 August 2007 Abstract The presence of reactive zinc (Zn 2þ ) 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 Mu ¨ller cells. Using membrane permeant and membrane impermeant forms of a fluorescent zinc indicator (Newport green), we show both the intracellular distribution of Zn 2þ 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 þ . 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. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: zinc; depolarization-induced exocytosis; zebrafish rod photoreceptors; Newport green; a-latrotoxin The co-release of zinc with glutamate from vertebrate reti- nal photoreceptors was suggested over a decade ago (Wu et al., 1993), and it was shown subsequently that exogenous zinc affects electrical responses of cells in the distal retina (Wu et al., 1993; Chappell and Redenti, 2001; Redenti and Chappell, 2002). In addition, zinc release was visualized in the region of the outer synaptic layer of a retinal slice pre- paration (Redenti and Chappell, 2005), but owing to its wide- spread distribution within the photoreceptors and neighboring glia, there was uncertainty as to the source(s) from which zinc was released. To provide more definitive evidence as to the in- tracellular localization of zinc, and the site from which it is discharged under conditions that promote glutamate release, we examined isolated zebrafish rods bathed in membrane per- meable or membrane impermeable forms of the zinc-specific fluorescent dye, Newport green; measurements of fluorescence intensity were made from confocal images of dark-adapted rod photoreceptors, and from light-adapted cells after exposure to high K þ or a-latrotoxin. All procedures were conducted in accordance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research, and the experimental protocols were re- viewed and approved by the Animal Care Committee of the Marine Biological Laboratory. Adult zebrafish (Danio rerio) were obtained from a colony maintained at the Marine Biolog- ical Laboratory, Woods Hole, MA, and were sacrificed by decapitation under ambient light. After enucleation, the lens and cornea were removed, and the retina was detached from the posterior eyecup and immersed in a modified Leibovitz * Corresponding author. Department of Biological Sciences, Hunter College, CUNY, 695 Park Avenue, New York, NY 10021, USA. Tel.: þ1 212 772 5294. E-mail address: rchappell@gc.cuny.edu (R.L. Chappell). 0014-4835/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.exer.2007.07.017 Experimental Eye Research 85 (2007) 580e584 www.elsevier.com/locate/yexer