Postnatal control of ocular growth: dopaminergic mechanisms.

A vision-dependent feedback mechanism contributes to the regulation of postnatal eye growth and refraction; this mechanism is located at least in part in the retina. In chicks and rhesus monkeys, visual deprivation leads to ocular enlargement and a myopic refractive error, and it also reduces the retinal concentration of dopamine. In neonatal chicks, local application of the dopamine agonist apomorphine limits the excessive axial elongation that is associated with visual deprivation. Both D1 and D2 dopamine receptor mechanisms may participate. Remarkably, apomorphine is geometrically selective, not limiting the exaggerated equatorial growth that occurs during visual deprivation. Preliminary results in neonatal rhesus monkeys indicate that apomorphine eye drops also suppress exaggerated axial growth and myopic refractive error during visual deprivation; equatorial diameters were not measured. Our observations are consistent with a retinal hypothesis for emmetropization and suggest that retinal dopamine participates in a mechanism linking ocular growth control to vision. In addition, we conclude that axial and equatorial dimensions are independently regulated in the chick.