Effects of Quisqualic Acid on Retinal ZENK Expression Induced by Imposed Defocus in the Chick Eye

Mechanisms of emmetropization and what might go wrong in myopia

Studies in animal models and humans have shown that refractive state is optimized during postnatal development by a closed-loop negative feedback system that uses retinal image defocus as an error signal, a mechanism called emmetropization. The sensor to detect defocus and its sign resides in the retina itself. The retina and/or the retinal pigment epithelium (RPE) presumably releases biochemical messengers to change choroidal thickness and modulate the growth rates of the underlying sclera. A central question arises: if emmetropization operates as a closed-loop system, why does it not stop myopia development? Recent experiments in young human subjects have shown that (1) the emmetropic retina can perfectly distinguish between real positive defocus and simulated defocus, and trigger transient axial eye shortening or elongation, respectively. (2) Strikingly, the myopic retina has reduced ability to inhibit eye growth when positive defocus is imposed. (3) The bi-directional response of the emmetropic retina is elicited with low spatial frequency information below 8 cyc/deg, which makes it unlikely that optical higher-order aberrations play a role. (4) The retinal mechanism for the detection of the sign of defocus involves a comparison of defocus blur in the blue (S-cone) and red end of the spectrum (L + M-cones) but, again, the myopic retina is not responsive, at least not in short-term experiments. This suggests that it cannot fully trigger the inhibitory arm of the emmetropization feedback loop. As a result, with an open feedback loop, myopia development becomes "open-loop".

Studies on retinal mechanisms possibly related to myopia inhibition by atropine in the chicken

PURPOSE

While low-dose atropine eye drops are currently widely used to inhibit myopia development in children, the underlying mechanisms are poorly understood. Therefore, we studied possible retinal mechanisms and receptors that are potentially involved in myopia inhibition by atropine.

METHODS

A total of 250 μg atropine were intravitreally injected into one eye of 19 chickens, while the fellow eyes received saline and served as controls. After 1 h, 1.5 h, 2 h, 3 h, and 4 h, eyes were prepared for vitreal dopamine (DA) measurements, using high-pressure liquid chromatography with electrochemical detection. Twenty-four animals were kept either in bright light (8500 lx) or standard light (500 lx) after atropine injection for 1.5 h before DA was measured. In 10 chickens, the α_2A-adrenoreceptor (α_2A-ADR) agonists brimonidine and clonidine were intravitreally injected into one eye, the fellow eye served as control, and vitreal DA content was measured after 1.5 h. In 6 chickens, immunohistochemical analyses were performed 1.5 h after atropine injection.

RESULTS

Vitreal DA levels increased after a single intravitreal atropine injection, with a peak difference between both eyes after 1.97 h. DA was also enhanced in fellow eyes, suggesting a systemic action of intravitreally administered atropine. Bright light and atropine (which both inhibit myopia) had additive effects on DA release. Quantitative immunolabelling showed that atropine heavily stimulated retinal activity markers ZENK and c-Fos in cells of the inner nuclear layer. Since atropine was recently found to also bind to α_2A-ADRs at doses where it can inhibit myopia, their retinal localization was studied. In amacrine cells, α_2A-ADRs were colocalized with tyrosine hydroxylase (TH), glucagon, and nitric oxide synthase, peptides known to play a role in myopia development in chickens. Intravitreal atropine injection reduced the number of neurons that were double-labelled for TH and α_2A-ADR. α_2A-ADR agonists clonidine and brimonidine (which were also found by other authors to inhibit myopia) severely reduced vitreal DA content in both injected and fellow eyes, compared to eyes of untreated chicks.

CONCLUSIONS

Merging our results with published data, it can be concluded that both muscarinic and α_2A-adrenergic receptors are expressed on dopaminergic neurons and both atropine and α_2A-ADR antagonists stimulate DA release whereas α_2A-ADR agonists strongly suppress its release. Stimulation of DA by atropine was enhanced by bright light. Results are in line with the hypothesis that inhibition of deprivation myopia is correlated with DA stimulation, as long as no toxicity is involved.

Novel method using 3-dimensional segmentation in spectral domain-optical coherence tomography imaging in the chick reveals defocus-induced regional and time-sensitive asymmetries in the choroidal thickness

Studies into the mechanisms underlying the active emmetropization process by which neonatal refractive errors are corrected, have described rapid, compensatory changes in the thickness of the choroidal layer in response to imposed optical defocus. While high frequency A-scan ultrasonography, as traditionally used to characterize such changes, offers good resolution of central (on-axis) changes, evidence of local retinal control mechanisms make it imperative that more peripheral, off-axis changes also be tracked. In this study, we used in vivo high resolution spectral domain-optical coherence tomography (SD-OCT) imaging in combination with the Iowa Reference Algorithms for 3-dimensional segmentation, to more fully characterize these changes, both spatially and temporally, in young, 7-day old chicks (n = 15), which were fitted with monocular +15 D defocusing lenses to induce choroidal thickening. With these tools, we were also able to localize the retinal area centralis, which was used as a landmark along with the ocular pectin in standardizing the location of scans and aligning them for subsequent analyses of choroidal thickness (CT) changes across time and between eyes. Values were derived for each of four quadrants, centered on the area centralis, and global CT values were also derived for all eyes. Data were compared with on-axis changes measured using ultrasonography. There were significant on-axis choroidal thickening that was detected after just one day of lens wear (∼190 µm), and regional (quadrant-related) differences in choroidal responses were also found, as well as global thickness changes 1 day after treatment. The ratio of global to on-axis choroidal thicknesses, used as an index of regional variability in responses, was also found to change significantly, reflecting the significant central changes. In summary, we demonstrated in vivo high resolution SD-OCT imaging, used in combination with segmentation algorithms, to be a viable and informative approach for characterizing regional (spatial), time-sensitive changes in CT in small animals such as the chick.

Chicks use changes in luminance and chromatic contrast as indicators of the sign of defocus

As the eye changes focus, the resulting changes in cone contrast are associated with changes in color and luminance. Color fluctuations should simulate the eye being hyperopic and make the eye grow in the myopic direction, while luminance fluctuations should simulate myopia and make the eye grow in the hyperopic direction. Chicks without lenses were exposed daily (9 a.m. to 5 p.m.) for three days on two consecutive weeks to 2 Hz sinusoidally modulated illumination (mean illuminance of 680 lux) to one of the following: in-phase modulated luminance flicker (LUM), counterphase-modulated red/green (R/G Color) or blue/yellow flicker (B/Y Color), combined color and luminance flicker (Color + LUM), reduced amplitude luminance flicker (Low LUM), or no flicker. After the three-day exposure to flicker, chicks were kept in a brooder under normal diurnal lighting for four days. Changes in the ocular components were measured with ultrasound and with a Hartinger Coincidence Refractometer (aus Jena, Jena, East Germany. After the first three-day exposure, luminance flicker produced more hyperopic refractions (LUM: 2.27 D) than did color flicker (R/G Color: 0.09 D; B/Y Color: -0.25 D). Changes in refraction were mainly due to changes in eye length, with color flicker producing much greater changes in eye length than luminance flicker (R/G Color: 102 μm; B/Y Color: 98 μm; LUM: 66 μm). Our results support the hypothesis that the eye can differentiate between hyperopic and myopic defocus on the basis of the effects of change in luminance or color contrast.

Transforming growth factor-beta in the chicken fundal layers: an immunohistochemical study

In the chicken model of myopia, it has first been shown that imposing defocus to the retina results in active remodelling of the sclera which, in turn, results in axial length changes of the eye. Transforming growth factor-beta (TGF-beta) is one of the scleral growth modulators but its cellular localization in the fundal layers, colocalization and function are not well known. The aim of the current study was to investigate the cellular distribution of the three isoforms TGF-beta1, 2 and 3 by immunohistochemical labelling. Furthermore, the effects of visual experience that induces refractive errors on TGF-beta2 labelling were examined. Transversal cryostat sections of the fundal layers were analyzed by indirect immunofluorescent labelling and cell counts. Visual experience was changed by having the chicks wear either diffusers, or positive or negative lenses of 7D power in front of the right eyes for various periods of time. Left eyes served as uncovered controls. All TGF-beta isoforms were localized in both scleral layers. In choroid, diffuse labelling of all isoforms was found. In retina, TGF-beta1 and 3 were detected in bipolar, amacrine and ganglion cells and TGF-beta2 in amacrine and ganglion cells. To further characterize these cells, double-labelling with known amacrine and bipolar cell markers was performed (calbindin, cellular retinoic acid binding protein (CRABP), Islet1, Lim3 and protein kinase C (PKC)). TGF-beta1, 2 and 3 could be colocalized with calbindin and CRABP in single amacrine cells. TGF-beta1-positive bipolar cells were immunoreactive to Lim3. TGF-beta1 and 3 were never colocalized with PKC in bipolar cells. Also, colocalization with peptides known to be involved in myopia development in chicks, such as glucagon, or vasointestinal polypeptide and the key enzyme for dopamine synthesis, tyrosine hydroxylase, was not observed. Lenses or diffusers, worn by the chicks for various periods of time, had no effect on TGF-beta2 immunoreactivity in choroid or sclera, or on the number of TGF-beta2 (active and latent form) expressing amacrine cells. This result did not change when the two identified populations of TGF-beta2 expressing amacrine cells (one calbindin-positive and the other CRABP-positive) were separately considered. Also no modulation was seen in choroid, although an earlier study had found changes in TGF-beta2 mRNA after lens treatment. The lack of any visually-induced changes in retina or choroid suggests that TGF-beta may not represent a key molecule in the retino-choroidal signalling cascade although it has previously been shown to have a primary role in scleral remodelling.

Spatial resolution, contrast sensitivity, and sensitivity to defocus of chicken retinal ganglion cells in vitro

The chicken has been extensively studied as an animal model for myopia because its eye growth is tightly controlled by visual experience. It has been found that the retina controls the axial eye growth rates depending on the amount and the sign of defocus imposed in the projected image. Glucagonergic amacrine cells were discovered that appear to encode for the sign of imposed defocus. It is not clear whether the downstream neurons, the retinal ganglion cells, still have access to this information—and whether it ultimately reaches the brain. We have analyzed the spike rates of chicken retinal ganglion cells in vitro using a microelectrode array. For this purpose, we initially defined spatial resolution and contrast sensitivity in vitro. Two classes of chicken retinal ganglions were found, depending on the linearity of their responses with increasing contrast. Responses generally declined with increasing defocus of the visual stimulus. These responses were well predicted by the modulation transfer function for a diffraction-limited defocused optical system, the first Bessel function. Thus, the studied retinal ganglion cells did not distinguish between a loss of contrast at a given spatial frequency due to reduced contrast of the stimulus pattern or because the pattern was presented out of focus. Furthermore, there was no indication that the retinal ganglion cells responded differently to defocus of either sign, at least for the cells that were recorded in this study.

Evaluation of inner retinal function in myopia using oscillatory potentials of the multifocal electroretinogram

PURPOSE

Oscillatory potentials have been suggested to arise from the inner retina at the level of amacrine cells and inner plexiform layer and they are thought to provide a non-invasive assessment of inner retinal function. We sought to investigate the response dynamics of the inner retina of adult emmetropes and myopes by analysing the oscillatory potentials of the multifocal electroretinogram (mfERG) in these groups.

METHODS

Eleven emmetropes and 18 myopes underwent mfERG testing using VERIS 5.1.5X. Myopes were further separated based on whether their myopia was stable (n=9) or progressing (n=9). Oscillatory potentials were recorded using a modified mfERG stimulation technique, the slow flash paradigm, and they were extracted using band-pass filtering from 100 to 300 Hz. The slow flash mfERG stimulus array consisted of 103-scaled hexagons and flickered according to a pseudorandom binary m-sequence (2(13)-1). Amplitudes and implicit times of the first-order oscillatory potentials were analysed.

RESULTS

There were significant differences in the implicit time of the oscillatory potentials of the emmetropes, stable myopes and progressing myopes (F(2,25)=3.663, p=0.043). Progressing myopes had significantly shorter implicit times compared to emmetropes (p=0.026 by 1.0-4.7 ms) and stable myopes (p=0.043 by 0.8-1.3 ms), whereas implicit times of stable myopes and emmetropes were similar. There were no statistically significant differences in amplitude of the oscillatory potentials between the groups (F(2,25)=0.890, p=0.426).

CONCLUSIONS

Significant differences in multifocal oscillatory potentials between stable and progressing myopes were found. This finding is further evidence of an inner retinal involvement in human myopia progression and may suggest an underlying alteration to dopaminergic or GABAergic retinal systems.

Retinal adaptation responses revealed by global flash multifocal electroretinogram are dependent on the degree of myopic refractive error

PURPOSE

There is evidence that the inner retina is involved in eye growth control processes and the development of myopia. We sought to investigate the response dynamics of the inner retina of adult emmetropes and myopes using the global flash multifocal electroretinogram (mfERG) paradigm.

METHODS

Fourteen myopes and 10 emmetropic subjects (mean age: 21.0+/-2.8 years) underwent mfERG testing using VERIS 5.1.5X. The global flash stimulus array consisted of 103-scaled hexagons which flickered according to a pseudorandom binary m-sequence (2(13)-1). The stimulation sequence was modified by inserting three frames, a dark frame, a global (full screen) flash, and another dark frame. The amplitude and implicit time of the two distinct waveform features, an early direct component (DC) and a later induced component (IC) of the first-order kernel were analyzed. Retinal responses were averaged over rings of increasing eccentricity, or into nasal and temporal hemifields.

RESULTS

There was a significant correlation between the DC and IC response amplitude and myopic refractive error, i.e., the greater the myopic error, the greater the response amplitude. However, when comparing between the two refractive error groups, DC and IC response amplitudes of emmetropes and myopes were similar, even after compensating for the effect of axial length. There were no significant differences in implicit times of the DC and IC in emmetropes and myopes.

CONCLUSIONS

Global flash mfERG responses of emmetropes and myopes were not found to be significantly different. The measured retinal adaptation response however varied according to the degree of myopia. We hypothesize that dopamine, a neurotransmitter that plays a key role in retinal adaptation and is known to be reduced in myopic eyes, may be involved in the retinal adaptation effect observed.

Effects of muscarinic antagonists on ZENK expression in the chicken retina

Muscarinic antagonists, particularly atropine, can inhibit myopia development in several animal models and also in children. However, the biochemical basis of the inhibition of axial eye growth remains obscure, and there are doubts whether muscarinic receptors are involved at all. Experiments in chickens and monkeys have shown that the synthesis of the transcription factor ZENK, also named Egr-1, in retinal glucagon amacrine cells is strongly associated with inhibition of axial eye growth (assumed to create a STOP signal). We have tested whether the muscarinic antagonists atropine, pirenzepine, oxyphenonium, gallamine, MT-3, himbacine, and 4-DAMP can stimulate ZENK expression so that the drugs' inhibitory effect on myopia development could be explained by an enhanced STOP signal. Because it is known that intravitreal quisqualic acid (QA) eliminates most cholinergic neurons in the retina within 6 or 7 days, in a second set of experiments, we tested whether these antagonists could still stimulate ZENK production, 6 days after QA was applied. Muscarinic antagonists, injected intravitreally at various concentrations, affected ZENK synthesis in various and unpredictable ways. Pirenzepine, oxyphenonium, and MT-3 increased the proportion of glucagon cells that were ZENK-immunoreactive, whereas himbacine decreased that proportion, and gallamine and 4-DAMP had no significant effect. Atropine caused an upregulation of ZENK only if all positive amacrine and bipolar cells were counted and therefore appeared to affect primarily cells other than glucagon amacrines. The pattern of results remained unchanged after ablation of most cholinergic neurons by QA. Our results suggest that at least some muscarinic antagonists do not activate cells that synthesize ZENK when they inhibit axial eye growth. Therefore, in line with other studies they also cast doubt on the assumption that muscarinic transmission is crucial, and they suggest that muscarinic antagonists may inhibit myopia through extraretinal target sites or through non-cholinergic retinal actions.

Homeostasis of eye growth and the question of myopia

As with other organs, the eye's growth is regulated by homeostatic control mechanisms. Unlike other organs, the eye relies on vision as a principal input to guide growth. In this review, we consider several implications of this visual guidance. First, we compare the regulation of eye growth to that of other organs. Second, we ask how the visual system derives signals that distinguish the blur of an eye too large from one too small. Third, we ask what cascade of chemical signals constitutes this growth control system. Finally, if the match between the length and optics of the eye is under homeostatic control, why do children so commonly develop myopia, and why does the myopia not limit itself? Long-neglected studies may provide an answer to this last question.