N-METHYL-D-ASPARTATE-INDUCED EXCITOTOXICITY CAUSES MYOPIA IN HATCHED CHICKS

Objective: To characterize the effect of the amacrine cell-selective t oxin N-methyl-D-aspartate (NMDA) on ocular growth in chicks, Design: S ingle injections of NMDA in doses ranging from 20 to 2000 nmol in 20 m u L of sterile saline were made into the vitreous chamber of one eye o f 7-day-old white leghorn chicks (six chicks per group); the contralat eral (control) eye was injected with saline, Six NMDA-treated eyes wer e also deprived of form vision by applying a translucent goggle 7 days after treatment, to determine whether myopia could still be induced o r enhanced in NMDA-treated eyes. Outcome measures: Axial length, equat orial diameter and refractive error, measured immediately after and 7, 14, 21, 28 and 35 days after treatment. Results: NMDA-treated eyes be came myopic within 7 days of treatment and remained so until at least 35 days after treatment. During this time the eyes continued to grow b ut could not be induced to become more myopic by depriving them of pat terned images. The half-maximal effective dose of NMDA was calculated to be 670 nmol, 7 days after treatment. Conclusions: NMDA-induced exci totoxicity destroys retinal pathways by which patterned visual stimuli restrain ocular growth in the chick.