Retinal dopamine in the recovery from experimental myopia

To address further a possible role for retinal dopamine in postnatal eye growth, we studied the response of retinal dopamine in eyes of chicks recovering from myopia.

Newborn chicks either received a unilateral translucent goggle to induce form deprivation myopia or were reared with unimpaired visual input. The goggle was removed from half of the chicks on day 7. Myopic, recovering and control never-goggled chicks were studied on days 7, 9 and 14. Eyes were enucleated postmortem and measured in axial and equatorial dimensions with calipers. Retinal levels of dopamine and its principal metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were assayed by high performance liquid chromatography with electrochemical detection.

Compared to contralateral and control eyes, retinas of goggled eyes at each time point had reduced levels of dopamine and DOPAC and a lowered calculated DOPAC/dopamine ratio, an index of dopamine metabolism. In eyes recovering from myopia, all biochemical parameters showed prominent increases by 2 days after goggle removal and had reached the level of both contralateral eyes and control eyes by one week after goggle removal. As evidence of a contralateral effect, the retinas of open eyes of chicks wearing a unilateral goggle demonstrated equal dopamine levels but reduced DOPAC compared to eyes of never-goggled chicks.

An early rise and eventual normalization of retinal dopamine, DOPAC and the DOPAC/dopamine ratio correlate with recovery from myopia. Combined with recent results from lens rearing experiments, these findings suggest that dopaminergic amacrine cells may participate in visually guided eye growth regulation and not just in the myopia response to visual form deprivation. The retinal biochemical alteration in eyes contralateral to a goggle identifies a previously unappreciated binocular interaction in the chick.