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N-methyl-D-aspartate-induced excitotoxicity causes myopia in hatched chicks
A J Fischer1, R L Seltner, W K Stell
1Department of Anatomy, Lions' Sight Centre, Faculty of Medicine, University of Calgary, Alta. ajfische@acs.ucalgary.ca
Objective:
To characterize the effect of the amacrine cell-selective toxin N-methyl-D-aspartate (NMDA) on ocular growth in chicks.
Design:
Single injections of NMDA in doses ranging from 20 to 2000 nmol in 20 microL of sterile saline were made into the vitreous chamber of one eye of 7-day-old white leghorn chicks (six chicks per group); the contralateral (control) eye was injected with saline. Six NMDA-treated eyes were also deprived of form vision by applying a translucent goggle 7 days after treatment, to determine whether myopia could still be induced or enhanced in NMDA-treated eyes.
Outcome Measures:
Axial length, equatorial diameter and refractive error, measured immediately after and 7, 14, 21, 28 and 35 days after treatment.
Results:
NMDA-treated eyes became myopic within 7 days of treatment and remained so until at least 35 days after treatment. During this time the eyes continued to grow but could not be induced to become more myopic by depriving them of patterned images. The half-maximal effective dose of NMDA was calculated to be 670 nmol, 7 days after treatment.
Conclusions:
NMDA-induced excitotoxicity destroys retinal pathways by which patterned visual stimuli restrain ocular growth in the chick.