The Role of Tyrosinase in the Time Course of Light‐Induced Cone Contraction in the Goldfish Retina

Abstract

Dopamine is an important modulator of light‐adaptation in the vertebrate retina. Many teleosts regulate the amount of light incident on the retina through retinomotor movements (RMMs), that include changes in cone and rod length. In the dark, cones elongate and rods contract. In the light, rods elongate and cones contract. It is thought that RMMs are regulated by the release of dopamine in the light from tyrosine hydoxylase‐containing interplexiform cells. However, RMMs are not lost when these dopaminergic retinal neurons are ablated by injection of the neurotoxin 6‐hydroxydopamine (6‐OHDA). A potential alternate source of dopamine in the retina is the retinal pigmented epithelium, due to the activity of tyrosinase.Adult goldfish (C. auratus) were treated with a specific inhibitor of tyrosinase, phenylthiourea (PTU), by placing them in tank water containing 0.2mM PTU for 48 hours. Control and fish treated with PTU were placed in dark for 1hr or light (>100 lux) for 1 hr, or after 1hr darkness then exposed to varying durations of light (1.5 – 30 min). Another set of fish were given intraocular injections of 50μg 6‐OHDA twice on the first 2 days of a 10‐day time course and then subjected to dark/light treatments in the presence or absence of PTU. Fish were sacrificed, eyes removed and fixed in glutaraldehyde. Retinas were isolated and sliced using a razor blade. Cones were labeled by anti‐zpr1 (ZIRC, Eugene, OR) and cone length determined using confocal microscopy. Differences between means (in all case n=10 eyes) were assessed for statistical significance using ANOVA and, when applicable, Tukey's multiple comparison test.In fully light‐ or dark‐adapted conditions, retinas treated with PTU, 6‐OHDA or 6‐OHDA + PTU were indistinguishable from controls, However, both PTU and 6‐OHDA treatment slowed light‐induced cone contraction over the first 15 min of light. This delay was even greater when 6‐OHDA and PTU were combined. In the dark cones were elongated (115 ± 7 μm) and, under control conditions (no PTU, no 6‐OHDA) reached 80% (66 ± 2 μm) of maximum contraction (55 ± 3 μm) after 7.5 min light exposure. In the presence of either PTU or after 6‐OHDA treatment, cone length at 7.5min was significantly (p<0.001) longer (75 ± 3 μm and 79 ± 2 μm, respectively) and was longer still when PTU and 6‐OHDA were combined (85 ± 1 μm).These results suggest that inhibition of tyrosinase (via PTU) or loss of neuronal dopamine (via 6‐OHDA) affects the early phase of light‐induced cone contraction. That neither treatment prevented full contraction may indicate that these treatments did not fully eliminate dopamine signaling in the retina, or that light‐induced cone contraction may also involve other mechanisms independent of dopamine.Support or Funding InformationThis work was supported by NSERC Discovery Grant awarded to WHB.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.