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Sophie Thapa, William H Baldridge; The role of tyrosinase in the time course of light-induced cone contraction in the goldfish retina. Invest. Ophthalmol. Vis. Sci. 2019;60(9):5265.
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© ARVO (1962-2015); The Authors (2016-present)
Many teleosts regulate the amount of light incident to the retina through retinomotor movements (RMMs), that include changes in cone and rod length. It is thought that RMMs are regulated by the release of dopamine in the light from tyrosine hydroxylase-containing interplexiform cells. However, RMMs are not lost when 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 0.2 mM PTU for 48 hours. Control and fish treated with PTU were placed in the dark for 1 hr, or in the light (>100 lux) for 1 hr, or after 1 hr darkness then exposed to varying durations of light (1.5-30 min). Another set of fish were given intraocular injections of 50 μμg 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-zrp1 (ZIRC, Eugene, OR) and cone length determined using confocal microscopy.
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, cone length at 7.5 min 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 (PTU) or loss of neuronal dopamine (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.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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