Effects of intravitreally and intraperitoneally injected atropine on two types of experimental myopia in chicken.

Atropine, a non-selective muscarinic receptor antagonist, is currently the most potent agent used to prevent myopia in animal models and children. However, the ocular target tissues are not well defined. To learn more about the effect of atropine on experimental myopia, atropine was applied both intravitreally and systemically (intraperitoneally) to chickens wearing either negative lenses or light diffusers. Furthermore, the effect of ipsilateral intravitreal atropine on myopia development in the saline-treated fellow eye was studied. Monocular intravitreal injections of atropine were performed daily for a period of 4 successive days, starting at day 8 post-hatching. Fellow eyes received saline injections. Chicks were fitted with -7D lenses, either over the atropine-injected eyes only (unilateral "lens-induced myopia (LIM)"), or over both eyes (bilateral LIM). Other groups of chicks were fitted with translucent diffusers over the atropine-injected eyes (unilateral "form deprivation myopia (FDM)"). Finally, atropine was intraperitoneally injected for 4 days in chicks that wore monocularly -7D lenses. Refractive errors (RE) were measured with infrared photoretinoscopy and axial length (AL) with A-scan ultrasonography. Atropine prevented development of myopia in both unilateral LIM and FDM in a dose-dependent fashion. Fifty percent inhibition of myopia was observed at a dose of 25 microg (unilateral LIM) or 90 microg atropine (bilateral LIM) and complete inhibition at 750 microg; in unilateral FDM, 50% inhibition occurred at 2.5 microg and almost 100% inhibition at 250 microg. Interestingly, at the highest dose of atropine (2500 microg), the treated eyes became even more hyperopic compared to the saline-injected contralateral eyes with normal visual experience. In the bilateral LIM model, atropine suppressed development of myopia in both the treated and the saline-injected control eye. However, about 8.3 times higher doses were necessary to achieve comparable contralateral suppression. Since this ratio is lower than the vitreous volume to blood volume ratio (about 1:23 in young chicks), it seems unlikely that systemic dilution of the intravitreally injected drug can fully account for the contralateral suppression. Intraperitoneal injection inhibited myopia development only at the highest dose (2500 microg) but, strikingly, this inhibition was still less when the same dose was provided through the vitreous of the fellow eye. Both eyes seem to be coupled by a yet unknown, perhaps neuronal pathway. Estimations of the scleral concentrations of atropine after intravitreal injection are compatible with the assumption that the suppression of myopia by atropine occurs by direct inhibition of scleral chondrocytes.

Identification of apolipoprotein A-I as a "STOP" signal for myopia.

Good visual acuity requires that the axial length of the ocular globe is matched to the refractive power of the cornea and lens to focus the images of distant objects onto the retina. During the growth of the juvenile eye, this is achieved through the emmetropization process that adjusts the ocular axial length to compensate for the refractive changes that occur in the anterior segment. A failure of the emmetropization process can result in either excessive or insufficient axial growth, leading to myopia or hyperopia, respectively. Emmetropization is mainly regulated by the retina, which generates two opposite signals: "GO/GROW" signals to increase axial growth and "STOP" signals to block it. The presence of GO/GROW and STOP signals was investigated by a proteomics analysis of the retinas from chicken with experimental myopia and hyperopia. Of 18 differentially expressed proteins that were identified, five displayed an expression profile corresponding to GO/GROW signals, and two corresponded to STOP signals. Western blotting confirmed that apolipoprotein A-I (apoA-I) has the characteristics of a STOP signal both in the retina as well as in the fibrous sclera. In accordance with this, intraocular application of the peroxisome proliferator-activated receptor alpha agonist GW7647 resulted in up-regulation of apoA-I levels and in a significant reduction of experimental myopia. In conclusion, using a comprehensive functional proteomics analysis of chicken ocular growth models we identified targets for ocular growth control. The correlation of elevated apoA-I levels with reduced ocular axial growth points toward a functional relationship with the observed morphological changes of the eye.

Stimulus requirements for the decoding of myopic and hyperopic defocus under single and competing defocus conditions in the chicken.

PURPOSE: The bidirectional nature of emmetropization, as observed in young chicks, implies that eyes are able to distinguish between myopic and hyperopic focusing errors. In the current study the spatial frequency and contrast dependence of this process were investigated in an experimental paradigm that allowed strict control over both parameters of the retinal image. Also investigated was the influence of accommodation. METHODS: Defocusing stimuli were presented through lens-cone devices with attached targets. These devices were monocularly applied to 5-day-old chickens for 4 days. Defocus conditions included: (1) 7 D of myopic defocus, (2) 7 D of hyperopic defocus, and (3) a combination of the two. Two high contrast target designs, a spatially rich, striped Maltese cross (target 1) and a standard Maltese cross (target 2) were used, except in some experiments where target contrast or spatial frequency content was further manipulated. To test the role of accommodation, the treated eye of some chicks underwent ciliary nerve section before attachment of the device. Refractive error (RE) was measured by retinoscopy and axial ocular dimensions measured by A-scan ultrasonography, both in chicks under anesthesia. RESULTS: With imposed myopic defocus and high contrast, target 1 elicited significantly better compensation than did target 2. With imposed hyperopic defocus, both targets elicited near normal compensatory responses. Reducing image contrast to 32% for target 2 and to 16% for target 1 precluded compensation for myopic defocus, inducing myopia instead. The low-pass-filtered target also induced myopia, irrespective of the sign of imposed defocus. With competing defocus and intact accommodation, target 1 induced a transient hyperopic growth response, whereas myopia was consistently observed with target 2. When accommodation was rendered inactive, both targets induced myopia under these competitive conditions. CONCLUSIONS: Compensation to myopic defocus is critically dependent on the inclusion of middle to high spatial frequencies in the stimulus and has a spatial frequency-dependent threshold contrast requirement. With competing myopic and hyperopic defocus, the former transiently dominates the latter as a determinant of ocular growth, provided that the stimulus conditions include sufficient middle to high spatial frequency information and that accommodation cues are available.