Unexpected cone position in 6‐hydroxydopamine‐treated dark‐adapted goldfish retina

Abstract

Dopamine (DA) is an important modulator of light‐adaptation in the vertebrate retina. A known source of DA in the retina is amacrine or interplexiform cells, but another potential source is the retinal pigmented epithelium (RPE). The neurotoxin 6‐hydroxydopamine (6‐OHDA) has been used extensively to ablate DAergic retinal neurons and could be used to distinguish between neuronal and RPE sources of DA. We have, therefore, examined first the impact of 6‐OHDA on light‐induced cone contraction, a phenomenon mediated by D2 DA receptors. Under MS‐222 anesthesia, adult goldfish (C. auratus) received two 50 μg intraocular injections of 6‐OHDA on consecutive days. After 10 days, control (uninjected) or 6‐OHDA‐treated fish were placed in the light (>100 lux) or in complete darkness for 1 hr and then sacrificed, eyes removed and fixed in glutaraldehyde. Retinas were then isolated, cones labeled with anti‐zrp1 (ZIRC, Eugene, OR) and cone length determined from confocal z‐stack reconstruction. In control light‐adapted retinas, cones were contracted, with a length of 55 ± 3 µm (mean ± SD), compared to cones from dark‐adapted retinas (115 ± 7 µm). In light‐adapted retinas, treated with 6‐OHDA, cones were contracted (55 ± 1 µm) but in dark‐adapted 6‐OHDA‐treated retinas cones were partially contracted (82 ± 5 µm). A previous study reported similar results and suggested that the partially contracted cone position in dark‐adapted 6‐OHDA‐treated retina indicated a role for DA in dark‐adaptation. Another possibility is that DAergic neurons damaged, but not destroyed by 6‐OHDA, have increased tonic DA release. To investigate this further, we are currently determining to what extent cone position in 6‐OHDA‐treated retinas is affected by treatment with D2 DA receptor agonists and antagonists. Funded by the NSERC.