Bio‐avalibale zinc in the outer segments of short wavelength cones

Abstract

Abstract Purpose: Zinc has been postulated to play a role in both normal and degenerative processes in the retina. Often these processes require zinc that is tightly bound to enzymes. However, new evidences indicate that the presence of bio‐available zinc is a better indicator for the involvemnt of zinc in retinal fuctions. Using autometallography on sectioned tissues suggested that bio‐available zinc is absent in photoreceptor outer segments. The purpose of this study was to re‐examine this observation using a zinc selective fluorescence probe and freshly dissected, flat mounted retinae . Methods: Following deep anesthesia and decapitation, dark‐adapted retinae were immediately dissected from rat, goldfish and chicken eyes and flat mounted with the photoreceptor up. The retinae were immediately labeled with the zinc‐specific fluorescent probe ZP1 (Neurobiotex, USA). Readily releasable zinc was visualized using fluorescent and confocal microscopy. Results: Only a subset of photoreceptor outer segments were labeled by ZP1 in rat retinae (>0.5 %). This labelling was localized to a subset of the cones. The cone localization of this labeling was confirmed by using goldfish retinae where the cones are readily distinguishable from rods morphologically. To identify the subtype of zinc postive cones we labeled chicken retinae where the characteristic oil droplets within the cones identify their sub‐type. Here zinc appeared to be localized to short wavelength cone outer segments. Conclusions: This is the first demonstration that bio‐available zinc is present in photoreceptor outer segments. The specific localization to short wavelength cones suggest that zinc plays a selective role in visual processing of short wavelength light.