Optical causes of experimental myopia

Experiments in which chicks are reared wearing a translucent goggle, or some similar device designed to degrade the retinal image, usually result in the induction of a significant degree of myopia. The induced myopia is due to enlargement of the vitreous chamber of the eye. Despite extensive change in the size, shape and refractive index distribution of the crystalline lens, its refractive power is static in the embryonic and early chick eye. The contribution to myopia of the cornea is uncertain; some studies have indicated either an increase or a decrease in corneal radius of curvature while others found no change. The fact that experimental myopia can be produced in chicks even when the optic nerve has been cut or when the degraded retinal image is restricted to one sector of the eye suggests that accommodation is not involved. Nevertheless, when the retinal image is degraded by being defocused with convex or concave lenses, myopia (concave lenses) or hyperopia (convex lenses) results. Chicks wearing concave or convex soft contact lenses from the day of hatching develop refractive states equal to the lens power (+8 and -10 diopters) within one week. The ability of the eye to vary its refractive development according to the sign of defocus suggests a role for accommodation. However, study of the ciliary muscle and accommodative apparatus of myopic and emmetropic chick eyes does not reveal any morphological differences that might indicate that the myopic eye had experienced an increased level of accommodation.

Opiate and N-methyl-D-aspartate receptors in form-deprivation myopia

Pharmacological studies have implicated retinal opiate pathways in the visual regulation of ocular growth. However, the effects of opiate receptor subtype-specific compounds on form-deprivation myopia (FDM) are inconsistent (Seltner et al., 1997), and may be mediated by non-opiate receptors. The purpose of this study was to test whether opiate receptor-inactive (D-) enantiomers elicit the same FDM-suppressing effect as their opiate receptor-active (L-) counterparts. Since some opiates are thought to act at NMDA receptors, we also tested whether NMDA receptor agonists and antagonists influence ocular growth or FDM. We found that both L- and D- enantiomers of morphine-like compounds (dextrorphanol and levorphanol, and D- and L-naloxone) were equally effective in blocking FDM. The NMDA receptor antagonists dextromethorphan, MK801, and AP5 also suppressed FDM. A single toxic dose of NMDA, that destroys many subtypes of amacrine cells (including those that synthesize the opioid peptide enkephalin), induced myopia and ocular enlargement in ungoggled eyes, and eliminated the ability of form-deprivation to enhance ocular growth. The NR-1 subunit of the NMDA receptor was localized to a narrowly stratified, intense stratum at approximately 50% depth in the inner plexiform layer, diffusely throughout the proximal inner plexiform layer, and to many somata in the amacrine and ganglion cell layers. These observations suggest that most effects of opiate receptor ligands on FDM in the chick are mediated by non-opiate receptors, which are likely to include NMDA receptors. NMDA as an excitotoxin transiently enhances ocular growth, but thereafter disables retinal mechanisms that promote emmetropization and FDM. These observations are consistent with a prominent role for pathways utilizing NMDA receptors in FDM and ocular growth-control.

Immunocytochemical characterization of quisqualic acid- and N-methyl-D-aspartate-induced excitotoxicity in the retina of chicks

A single, large dose of N-methyl-D-aspartate (NMDA) or quisqualic acid (QA) injected into the chick eye has been shown previously to destroy many retinal amacrine cells and to induce excessive ocular growth accompanied by myopia. The purpose of this study was to identify distinct populations of retinal cells, particularly those believed to be involved in regulating ocular growth, that are sensitive to NMDA or QA. Two pmol of NMDA or 0.2 micromol of QA were injected unilaterally into eyes of 7-day-old chicks, and retinas were prepared for observation 1, 3, or 7 days later. Retinal neurons were identified by using immunocytochemistry, and cells containing fragmented DNA were identified by 3'-nick-end labelling in frozen sections. NMDA and QA destroyed many amacrine cells, including those immunoreactive for vasoactive intestinal polypeptide, Met-enkephalin, and choline acetyltransferase, but they had little effect upon tyrosine hydroxylase-immunoreactive cells. Other cells affected by both QA and NMDA included those immunoreactive for glutamic acid decarboxylase, gamma-aminobutyric acid, parvalbumin, serotonin, and aminohydroxy methylisoxazole propionic acid (AMPA) receptor subunits GluR1 and GluR2/3. Cells largely unaffected by QA or NMDA included bipolar cells immunoreactive for protein kinase C (alpha and beta isoforms) and amacrine cells immunoreactive for glucagon. DNA fragmentation was detected maximally in many amacrine cells and in some bipolar cells 1 day after exposure to QA or NMDA. We propose that excitotoxicity caused by QA and NMDA induces apoptosis in specific populations of amacrine cells and that these actions are responsible for the ocular growth-specific effects of QA and NMDA reported elsewhere.

N-methyl-D-aspartate-induced excitotoxicity causes myopia in hatched chicks

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.

Endogenous opiates in the chick retina and their role in form-deprivation myopia

In this study, the possible role of the retinal enkephalin system in form-deprivation myopia (FDM) in the chick eye was investigated. Daily intravitreal injection of the nonspecific opiate antagonist naloxone blocked development of FDM in a dose-dependent manner, while injection of the opiate agonist morphine had no effect at any dose tested. The ED50 for naloxone (calculated maximum concentration in the vitreous) was found to be in the low picomolar range. The results using receptor-subtype-specific drugs were contradictory. Drugs specific for mu and delta receptors had no effect on FDM. The kappa-specific antagonist nor-binaltorphimine (nor-BNI) reduced FDM by about 50% at maximum daily retinal doses ranging between 4 x 10(-10) and 4 x 10(-7) M, while the kappa-specific agonist U50488 blocked FDM in a dose-dependent manner with an ED50 between 5 x 10(-8) and 5 x 10(-7) M. Met-enkephalin immunoreactivity (ME-IR) was localized immunocytochemically to a subset of amacrine cells (ENSLI cells) and their neurites in the inner plexiform layer (IPL). As reported previously, ENSLI cells from untreated chick retinas showed a cyclical pattern of immunoreactivity, with increased immunoreactivity in the light compared to the dark. Form-deprivation did not appear to change this pattern. Amounts of preproenkephalin mRNA from normal or form-deprived eyes were approximately the same under all conditions. Daily injection of naloxone, however, did increase ME-IR in the dark. These results suggest that naloxone may affect release of enkephalin from the ENSLI cells. The results as presented are inconclusive with regards to the role of the enkephalin system in FDM. While the kappa receptor may participate, there is no conclusive evidence here for a direct effect of opiate receptors. The effect of naloxone on form-deprived eyes may be due to its effect on release of peptides from the ENSLI cells.

The effect of vasoactive intestinal peptide on development of form deprivation myopia in the chick: a pharmacological and immunocytochemical study

The role of vasoactive intestinal peptide (VIP) in the development of form deprivation myopia (FDM) was examined. Daily intravitreal injection of porcine VIP reduced, but did not eliminate FDM at a maximal daily dose of 1 x 10(-5) mol/injection. A VIP analogue reported to be relatively hydrolysis-resistant in vivo, had no effect on development of FDM at any dose tested. Two VIP antagonists completely abolished FDM. The one reported to be selective for central nervous system VIP receptors was 100 times more potent than one reported to be selective for peripheral nervous system receptors (ED50 = 2 x 10(-10) and 2 x 10(-8) mol/injection respectively). By immunofluorescence using antiserum to porcine VIP, VIP-like immunoreactivity was localized to a subset of amacrine cells (AC) and in three parallel layers in the inner plexiform layer (IPL) (10%, 40% and 70% of IPL thickness from the AC layer). Immunoreactive nerve fibres were also seen in the choroid, the ciliary body and the iris. These results suggest that VIP may play a role in both normal development of the refractive properties of the eye, and in the development of FDM.

Role of the lens and vitreous humor in the refractive properties of the eyes of three strains of goldfish

Certain strains of goldfish are characterized by large bulging eyes which have been shown to be extremely myopic. Such myopia has been hypothesized to have its origin in enlargement of the posterior chamber of the eye. The focal length of the lens was found to be similar for paraxial and marginal lens zones in common, Chinese Black Moor and Celestial goldfish, thus providing indirect support of that hypothesis. Freeze-sectioning study of intraocular dimensions shows that the axial length of the vitreous chamber of the common goldfish eye is about 37% of total axial length. In contrast, it becomes more than 70% of axial length in Chinese Black Moors and Celestials. An examination of vitreous protein concentration indicates that ocular enlargement in Chinese Black Moors and in Celestials is related to an increase in vitreous quantity, and is accompanied by vitreous dilution. The fact that similar findings are reported in the case of experimental myopia in chicks, suggests the existence of a fundamental mechanism of refractive development for the vertebrate eye.

A role for the vitreous humor in experimentally-induced myopia

The refractive error observed in experimentally-induced myopia in chicks appears to occur as a result of expansion of the vitreous chamber. Fourteen days after the application of a goggle to one eye, the eye was myopic and had a greater volume of liquid vitreous than the non-myopic eye. The relative proportion of liquid to gel vitreous was also higher in the myopic eye than in the non-myopic eye. The results suggest that the vitreous humor plays a key role in the development of experimentally-induced myopia in chicks.