The eyes of primates, including humans, undergo an extensive period of postnatal growth before they reach adult size. The relationship between the length of the globe and the focal length of the optics of the eye is so finely tuned that the image of distant objects fall on the plane of the photoreceptors of the retina, and normal vision results. The exact mechanisms responsible for this coordinated eye growth are still unknown. We have developed two monkey models to study eye growth. (1) We found that depriving newborn monkeys of visual experience by occluding one eye with opaque contact lenses results in excessive elongation of the occluded eye. (2) In contrast, exposing newborn monkeys to blurred vision by surgical removal of the natural lens from one eye results in retarded elongation of the aphakic eye. Thus, degradation of retinal image quality must be an important factor in eye growth regulation. A neurochemical link in a pathway connecting retinal activity to the regulation of eye growth is implied by our finding of a reduction in the retinal dopamine (DA) system in occluded eyes and by the effect of apomorphine, a DA receptor agonist, in the prevention of occlusion-induced excessive eye elongation. A combination of neurochemical and neuroanatomical approaches is planned in this project to further investigate this link. We will focus on the role of D1 and D2 receptors in eye growth control, extend our investigation to the muscarinic cholinergic system and determine whether certain growth factors may be altered in occluded and/or aphakic eyes. These experiments should elucidate whether the same or different mechanisms control excessive and retarded eye growth. Drugs that interact with specific retinal neurochemicals and can prevent deprivation-induced excessive eye elongation may be relevant clinically. Excessive axial elongation in humans causes axial myopia, a common vision disorder affecting a large percentage of the U.S. population. In extreme cases, myopia can result in the loss of vision. The etiology of myopia is not well understood. The visual systems of infant rhesus monkeys and human infants are highly similar in structural and functional development. Therefore, information about eye growth regulatory mechanisms obtained from the monkey model may be useful in developing therapeutic strategies for ocular growth abnormalities in humans.
