Dose-dependent effects of 6-hydroxy dopamine on deprivation myopia, electroretinograms, and dopaminergic amacrine cells in chickens

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Abstract
We found that a single intravitreal injection of 6-hydroxy dopamine (6-OHDA) is highly efficient in blocking the development of deprivation-induced myopia in young chickens. To investigate the effects of 6-OHDA on retinal function, we studied electroretinograms (ERGs) in chickens aged 15-25 days, 4 days subsequent to the injection. Both spectral sensitivity and oscillatory potentials were tested. In addition, a histological examination was performed of dopaminergic amacrine cells labeled by a monoclonal antibody against tyrosine hydroxylase. We found that, at doses of 6-OHDA sufficient to suppress deprivation myopia entirely, no effect could be detected on either the ERGs or on the density and appearance of dopaminergic amacrine cells. For higher doses, spectral sensitivity and the number of dopaminergic amacrine cells declined gradually. In contrast, as doses increased, oscillatory potentials 1 and 2 grew in amplitude only to decline at the highest doses. The results indicate that (1) development of deprivation myopia requires normal retinal function and that (2) slight changes in the gains of dopaminergic pathways are sufficient to block the development of deprivation myopia.