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Hong Liu, Frank Schaeffel, Klaus Trier, Marita P Feldkaemper; Effect of 7-methylxanthine (7-MX) on Deprivation Myopia and Retinal Dopamine Release in Chickens. Invest. Ophthalmol. Vis. Sci. 2019;60(9):5878.
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Intake of 7-MX has been shown to inhibit school myopia in children and deprivation myopia (DM) in rhesus monkeys. Different from mammals, chicks have both a fibrous and a cartilaginous sclera. We have studied (1) whether 7-MX can also inhibit DM in chicks and (2) whether the effects of 7-MX may involve the retinal dopamine system.
7-MX was kindly provided by one of the authors (KT). It was applied by either tube-feeding (100 mg/kg body weight, twice a day) or binocular intravitreal injection (0.1%, every other day). Fourty-eight 2-week old chicks wore unilateral diffusers and then were randomly assigned to either tube-feeding (7-MX, n=10; only the carrier xanthan gum, n=10; no feeding, n=5) or intravitreal injections (7-MX, n=12; only vehicle DMSO, n=11). Tube feeding was continued for 13 days; intravitral injection was continued for 8 days (4 injections). Refractions (RE), ocular biometry (AL, VCD) and scleral and choroidal thickness (ChT) were measured before and after treatments. Dopamine and DOPAC content were determined in retina and vitreous by HPLC at the end of the treatment.
No matter how 7-MX was applied, it did not seem to inhibit the development of DM in chicks. No significant differences were observed in RE, VCD, AL, ChT and fibrous layer thickness after its application. In fact, the deprived eyes of 7-MX-fed chicks showed greater decrease in RE and ChT, although the difference was not significant. Also, DOPAC and dopamine in vitreous or retina did not change with 7-MX. Only vitreal dopamine was decreased in deprived eyes in the groups fed with xanthan gum (p<0.01), or injected with DMSO (p<0.01). Although a similar trend was found in 7-MX treated eyes, the differences were not significant (0.31±0.09 vs 0.50±0.08 ng/100mg in tube-fed chicks and 0.23±0.10 vs 0.29±0.03 ng/100mg in intravitreally injected chicks), perhaps indicating as small effect of 7-MX on dopamine.
In our study, 7-MX had no effect on DM in chicks and perhaps a minor effect on retinal dopamine. It remains unclear whether 7-MX inhibits myopia through a retinal mechanism or acts directly on sclera. In the latter case, it would not be surprising that myopia in mammals and birds responds differently to 7-MX since they have differently structured scleras. Considering the receptor adaptation, our much shorter treating period may be responsible for the different results from mammals.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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