A muscarinic cholinergic antagonist and a dopamine agonist rapidly increase ZENK mRNA expression in the form-deprived chicken retina

Effects of atropine on choroidal hemodynamics and VEGFA and HIF-1α expression in form-deprivation myopia guinea pigs

We investigated the mechanism of action of atropine in myopia control by examining its effect on choroidal hemodynamics. Blood flow was evaluated using indocyanine green (ICG) fluorescence and molecular variation during the development of form-deprivation myopia (FDM) and atropine treatment in guinea pigs. Guinea pigs were divided randomly into the normal control (NC), FDM, and FDM + 1% atropine (ATR) groups, and evaluated by spherical equivalent refractive error (SE) and axial length (AL). Choroidal hemodynamic parameters were measured via ICG fluorescence imaging including the maximal ICG fluorescence intensity (Imax), rising time (Trising), blood flow index (BFI), and mean transit time (MTT). Additionally, the expression in the choroid-RPE complex of choroidal vascular endothelial growth factor A (VEGFA) and HIF-1 α were assessed via Western blotting. Atropine inhibited the development of FDM, with effects of FD on both SE and AL being reduced. ICG fluorescence hemodynamic wide-field maps and time-series curves revealed that the atropine significantly accelerated choroidal blood flow, with reduced Trising and MTT, while increasing Imax, BFI and the number of lobulated choriocapillaris structures compared with the FDM group. In terms of molecular markers, atropine inhibited the effect of FDM, increasing VEGFA levels and reducing HIF-1α expression. These findings suggest that atropine improved choroidal hemodynamics and changed vascular markers, potentially contributing to its role in inhibiting the progression of myopia in the FDM model.

Advances in myopia control strategies for children

Myopia has long been a global threat to public health. Timely interventions are likely to reduce the risk of vision-threatening complications. There are both established and rapidly evolving therapeutic approaches to slow myopia progression and/or delay its onset. The effective methods for slowing myopia progression include atropine eye-drops, defocus incorporated multiple segments (DIMS) spectacle lenses, spectacle lenses with highly aspherical lenslets target (HALT), diffusion optics technology (DOT) spectacle lenses, red light therapy (RLT), multifocal soft contact lenses and orthokeratology. Among these, 0.05% atropine, HALT lenses, RLT and +3.00 peripheral addition soft contact lenses yield over 60% reduction in myopia progression, whereas DIMS, DOT and MiSight contact lenses demonstrate at least 50% myopia control efficacy. 0.05% atropine demonstrates a more optimal balance of efficacy and safety than 0.01%. The efficacy of 0.01% atropine has not been consistent and requires further validation across diverse ethnicities. Combining atropine 0.01% with orthokeratology or DIMS spectacles yields better outcomes than using these interventions as monotherapies. Increased outdoor time is an effective public health strategy for myopia prevention while recent studies suggest that 0.05% low-concentration atropine and RLT therapy have promising potential as clinical myopia prevention interventions for high-risk groups. Myopia control spectacle lenses, being the least invasive, are safe for long-term use. However, when considering other approaches, it is essential to ensure proper instruction and regular follow-ups to maintain safety and monitor any potential complications. Ultimately, significant advances have been made in myopia control strategies, many of which have shown meaningful clinical outcomes. However, regular use and adequate safety monitoring over extended durations are imperative to foster confidence that can only come from extensive clinical experience.

Low-concentration atropine for controlling myopia onset and progression in East Asia

PURPOSE

Over the past few years, there has been a rapid accumulation of data on the use of low-concentration atropine for myopia control, especially in East Asian children, with its effectiveness varying in different studies. This review aims to evaluate the current evidence surrounding the efficacy and safety of low-concentration atropine in the management of myopia onset and progression in East Asia.

METHODS

Clinical trials involving atropine for myopia control in East Asia were reviewed.

RESULTS

Atropine has been shown to reduce myopia progression in East Asian children, compared with placebo. Its efficacy is concentration-dependent, with 1 % atropine yielding the greatest effect in slowing myopia progression by over 70 %, but it is associated with significant rebound and side effects. Lower concentrations also confer significant myopia-control effects while maintaining a more acceptable safety profile, with relative reductions of 67 % and 43 % reported for 0.05 % and 0.025 % atropine, respectively. While 0.01 % atropine showed the least effect compared to 0.05 % and 0.025 %, it still yielded a significant efficacy in slowing myopic refraction. Over two years, 0.05 % atropine reduced the incidence of myopia by nearly 50 %, demonstrating greater effectiveness than 0.01 % atropine. This effect was particularly notable in children with low hyperopic reserves (< +0.75 D), but not in those with higher reserves.

CONCLUSIONS

The current evidence shows that low-concentration atropine plays a crucial role in managing myopia in East Asian children and demonstrates satisfactory safety profiles. Timely administration of the most effective and safest concentration can potentially prevent sight-threatening complications and subsequent vision loss.

The association between pupillary responses and axial length in children differs as a function of season

The association between pupillary responses to repeated stimuli and adult refractive error has been previously demonstrated. This study evaluated whether this association exists in children and if it varies by season. Fifty children aged 8-17 years (average: 11.55 ± 2.75 years, 31 females) with refractive error between + 1.51 and - 5.69 diopters (non-cycloplegic) participated (n = 27 in summer, and n = 23 in winter). The RAPDx pupilometer measured pupil sizes while stimuli oscillated between colored light and dark at 0.1 Hz in three sequences: (1) alternating red and blue, (2) red-only, and (3) blue-only. The primary outcome was the difference in pupillary responses between the blue-only and red-only sequences. Pupillary constriction was greater in response to blue light than to red for those with shorter eyes in summer (β = - 9.42, P = 0.034) but not in winter (β = 3.42, P = 0.54). Greater constriction comprised faster pupillary escape following red light onset and slower redilation following stimulus offset of both colors (P = 0.017, 0.036, 0.035 respectively). The association between axial length and children's pupillary responses in summer, but not winter may be explained by greater light-associated release of retinal dopamine in summer. Shorter eyes' more robust responses are consistent with greater light exposure inhibiting axial elongation and reducing myopia risk.

Glucagon Increases Retinal Rod Bipolar Cell Inhibition Through a D1 Dopamine Receptor-Dependent Pathway That Is Altered After Lens-Defocus Treatment in Mice

Purpose

Members of the secretin/glucagon family have diverse roles in retinal physiological and pathological conditions. Out of them, glucagon has been associated with eye growth regulation and image defocus signaling in the eye, both processes central in myopia induction. On the other hand, dopamine is perhaps the most studied molecule in myopia and has been proposed as fundamental in myopia pathogenesis. However, glucagonergic activity in the mammalian retina and its possible link with dopaminergic signaling remain unknown.

Methods

To corroborate whether glucagon and dopamine participate together in the modulation of synaptic activity in the retina, inhibitory post-synaptic currents were measured in rod bipolar cells from retinal slices of wild type and negative lens-exposed mice, using whole cell patch-clamp recordings and selective pharmacology.

Results

Glucagon produced an increase of inhibitory post-synaptic current frequency in rod bipolar cells, which was also dependent on dopaminergic activity, as it was abolished by dopamine type 1 receptor antagonism and under scotopic conditions. The effect was also abolished after 3-week negative lens-exposure but could be recovered using dopamine type 1 receptor agonism.

Conclusions

Altogether, these results support a possible neuromodulatory role of glucagon in the retina of mammals as part of a dopaminergic activity-dependent synaptic pathway that is affected under myopia-inducing conditions.

Opsin 5 mediates violet light-induced early growth response-1 expression in the mouse retina

Myopia is an abnormal vision condition characterized by difficulties in seeing distant objects. Myopia has become a public health issue not only in Asian countries but also in Western countries. Previously, we found that violet light (VL, 360-400 nm wavelength) exposure effectively suppressed myopia progression in experimental chick and mice models of myopia. The inhibitory effects of VL on myopia progression are reduced in retina-specific opsin 5 (Opn5) knockout (KO) mice. Furthermore, VL exposure upregulated early growth response-1 (Egr-1) expression in the chorioretinal tissues of chicks. However, the expression of EGR-1 and role of OPN5 in mice following VL exposure remain unclear. In this study, we examined whether VL exposure-induced EGR-1 upregulation depends on Opn5 expression in the mouse retina. EGR-1 mRNA and protein expressions increased in the mouse retina and mouse retinal 661W cells following VL exposure. These increases were consistently reduced in retina specific Opn5 conditional KO mice and Opn5 KO 661W cells. Our results suggest that OPN5 mediates VL-induced EGR-1 upregulation in mice. These molecular targets could be considered for the prevention and treatment of myopia.

The Role of Retinal Dysfunction in Myopia Development

Myopia is a refractive disorder arising from a mismatch between refractive power and relatively long axial length of the eye. With its dramatically increasing prevalence, myopia has become a pervasive social problem. It is commonly accepted that abnormal visual input acts as an initiating factor of myopia. As the first station to perceive visual signals, the retina plays an important role in myopia etiology. The retina is a fine-layered structure with multitudinous cells, processing intricate visual signals via numerous molecular pathways. Accordingly, dopaminergic mechanisms, contributions of rod and cone photoreceptors, myopic structural changes of retinal pigment epithelium (RPE) and neuro-retinal layers have all suggested a vital role of retinal dysfunction in myopia development. Herein, we separately discuss myopia-related retinal dysfunction and current dilemmas by different levels, from molecules to cells, with the hope that the comprehensive delineation could contribute to a better understanding of myopia etiology, indicate novel therapeutic targets, and inspire future studies.

Experimentally induced myopia and myopic astigmatism alter retinal electrophysiology in chickens

Myopia (or "short-sightedness") and astigmatism are major causes of visual impairment worldwide. Significant amounts of astigmatism are frequently observed in infants and have been associated with myopia development. Although it is well established that both myopia and astigmatism are associated with ocular structural changes from anterior to posterior segments, very little is known on how these refractive errors alter retinal functions. This study investigated the effects of experimentally induced myopia and myopic-astigmatism on retinal electrophysiology by using an image-guided, multifocal global flash stimulation in chickens, a widely used animal model for refractive error development. Myopia and myopic-astigmatism were experimentally induced, respectively, by wearing spherical (- 10 D, n = 12) and sphero-cylindrical lenses (- 6.00 DS/- 8.00 DCx90: Hyperopic With-The Rule, H-WTR, n = 15; - 6.00 DS/- 8.00 DCx180: Hyperopic Against-The-Rule, H-ATR, n = 11) monocularly for a week (post-hatching day 5 to 12). An aged-matched control group without any lens treatment provided normal data (n = 12). Multifocal electrophysiological results revealed significant regional variation in the amplitude of induced component (IC) (central greater than peripheral; both p < 0.05) in the normal and H-ATR groups, but not in the - 10 D and H-WTR groups. Most importantly, for the first time, our results showed that both H-WTR and H-ATR groups exhibited a significantly longer implicit time of the inner retinal response at the central region when compared to the normal and - 10 D groups, highlighting a significant role of astigmatism in retinal physiology.

Candidate pathways for retina to scleral signaling in refractive eye growth

The global prevalence of myopia, or nearsightedness, has increased at an alarming rate over the last few decades. An eye is myopic if incoming light focuses prior to reaching the retinal photoreceptors, which indicates a mismatch in its shape and optical power. This mismatch commonly results from excessive axial elongation. Important drivers of the myopia epidemic include environmental factors, genetic factors, and their interactions, e.g., genetic factors influencing the effects of environmental factors. One factor often hypothesized to be a driver of the myopia epidemic is environmental light, which has changed drastically and rapidly on a global scale. In support of this, it is well established that eye size is regulated by a homeostatic process that incorporates visual cues (emmetropization). This process allows the eye to detect and minimize refractive errors quite accurately and locally over time by modulating the rate of elongation of the eye via remodeling its outermost coat, the sclera. Critically, emmetropization is not dependent on post-retinal processing. Thus, visual cues appear to influence axial elongation through a retina-to-sclera, or retinoscleral, signaling cascade, capable of transmitting information from the innermost layer of the eye to the outermost layer. Despite significant global research interest, the specifics of retinoscleral signaling pathways remain elusive. While a few pharmacological treatments have proven to be effective in slowing axial elongation (most notably topical atropine), the mechanisms behind these treatments are still not fully understood. Additionally, several retinal neuromodulators, neurotransmitters, and other small molecules have been found to influence axial length and/or refractive error or be influenced by myopigenic cues, yet little progress has been made explaining how the signal that originates in the retina crosses the highly vascular choroid to affect the sclera. Here, we compile and synthesize the evidence surrounding three of the major candidate pathways receiving significant research attention - dopamine, retinoic acid, and adenosine. All three candidates have both correlational and causal evidence backing their involvement in axial elongation and have been implicated by multiple independent research groups across diverse species. Two hypothesized mechanisms are presented for how a retina-originating signal crosses the choroid - via 1) all-trans retinoic acid or 2) choroidal blood flow influencing scleral oxygenation. Evidence of crosstalk between the pathways is discussed in the context of these two mechanisms.

Retinal Dopamine D2 Receptors Participate in the Development of Myopia in Mice

Purpose

To learn more about the locations of dopamine D2 receptors (D2Rs) that regulate form-deprivation myopia (FDM), using different transgenic mouse models.

Methods

One eye of D2R-knockout (KO) mice and wild-type littermates was subjected to four weeks of monocular FDM, whereas the fellow eye served as control. Mice in both groups received daily intraperitoneal injections of either the D2R antagonist sulpiride (8 µg/g) or vehicle alone. FDM was also induced in retina- (Six3^creD2R^fl/fl) or fibroblast-specific (S100a4^creD2R^fl/fl) D2R-KO mice. A subset of retina-specific D2R-KO mice and D2R^fl/fl littermates were also given sulpiride or vehicle injections. Refraction was measured with an eccentric infrared photorefractor, and other biometric parameters were measured by optical coherence tomography (n ≈ 20 for each group).

Results

FDM development was attenuated in wild-type littermates treated with sulpiride. However, this inhibitory effect disappeared in the D2R-KO mice, suggesting that antagonizing D2Rs suppressed myopia development. Similarly, the development of myopia was partially inhibited by retina-specific (deletion efficiency: 94.7%) but not fibroblast-specific (66.9%) D2R-KO. The sulpiride-mediated inhibitory effects on FDM also disappeared with retinal D2R-KO, suggesting that antagonizing D2Rs outside the retina may not attenuate myopia. Changes in axial length were less marked than changes in refraction, but in general the two were correlated.

Conclusions

This study demonstrates that D2Rs located in the retina participate in dopaminergic regulation of FDM in mice. These findings provide an important and fundamental basis for further exploring the retinal mechanism(s) involved in dopamine signaling and myopia development.

Orthokeratology Lenses Versus Administration of 0.01% Atropine Eye Drops for Axial Length Elongation in Children With Myopic Anisometropia

OBJECTIVE

To investigate the effect of orthokeratology (OK) lenses and that of 0.01% atropine eye drops on axial length (AL) elongation in children with myopic anisometropia.

METHODS

Ninety-five children with myopic anisometropia who used OK lenses (N=49) or 0.01% atropine eye drops (N=46) were enrolled in this retrospective 1-year study. For all children, the eyes with higher spherical equivalent refractive error (SER) were assigned to the H-eye subgroup, whereas the fellow eyes with lower SER were assigned to the L-eye subgroup.

RESULTS

After 1-year treatment, the mean change in the AL of H eyes and L eyes in the OK lenses group was 0.18±0.16 mm and 0.24±0.15 mm, respectively (P=0.15), and 0.28±0.20 mm and 0.25±0.18 mm, respectively (P=0.48), in the 0.01% atropine group. Multivariate regression analyses showed significant differences in AL change between H and L eyes after treatment with OK lens (P=0.03), whereas no significant difference in the 0.01% atropine (P=0.22). The change in the AL in the H-eye group was less with OK lenses than with 0.01% atropine (P=0.04), whereas there was no significant difference between the change in AL in the L-eye group between treatment with OK lens and 0.01% atropine (P=0.89).

CONCLUSIONS

In myopic anisometropic children, AL differences between 2 eyes decrease by wearing OK lenses but do not change after administration of 0.01% atropine eye drops. The increased effect of OK lenses, but not 0.01% atropine, in reducing axial elongation at 1 year in the eye with higher SER in anisometropic children warrants further investigation.

Melanopsin modulates refractive development and myopia

Myopia, or nearsightedness, is the most common form of refractive abnormality and is characterized by excessive ocular elongation in relation to ocular power. Retinal neurotransmitter signaling, including dopamine, is implicated in myopic ocular growth, but the visual pathways that initiate and sustain myopia remain unclear. Melanopsin-expressing retinal ganglion cells (mRGCs), which detect light, are important for visual function, and have connections with retinal dopamine cells. Here, we investigated how mRGCs influence normal and myopic refractive development using two mutant mouse models: Opn4-/- mice that lack functional melanopsin photopigments and intrinsic mRGC responses but still receive other photoreceptor-mediated input to these cells; and Opn4DTA/DTA mice that lack intrinsic and photoreceptor-mediated mRGC responses due to mRGC cell death. In mice with intact vision or form-deprivation, we measured refractive error, ocular properties including axial length and corneal curvature, and the levels of retinal dopamine and its primary metabolite, L-3,4-dihydroxyphenylalanine (DOPAC). Myopia was measured as a myopic shift, or the difference in refractive error between the form-deprived and contralateral eyes. We found that Opn4-/- mice had altered normal refractive development compared to Opn4+/+ wildtype mice, starting ∼4D more myopic but developing ∼2D greater hyperopia by 16 weeks of age. Consistent with hyperopia at older ages, 16 week-old Opn4-/- mice also had shorter eyes compared to Opn4+/+ mice (3.34 vs 3.42 mm). Opn4DTA/DTA mice, however, were more hyperopic than both Opn4+/+ and Opn4-/- mice across development ending with even shorter axial lengths. Despite these differences, both Opn4-/- and Opn4DTA/DTA mice had ∼2D greater myopic shifts in response to form-deprivation compared to Opn4+/+ mice. Furthermore, when vision was intact, dopamine and DOPAC levels were similar between Opn4-/- and Opn4+/+ mice, but higher in Opn4DTA/DTA mice, which differed with age. However, form-deprivation reduced retinal dopamine and DOAPC by ∼20% in Opn4-/- compared to Opn4+/+ mice but did not affect retinal dopamine and DOPAC in Opn4DTA/DTA mice. Lastly, systemically treating Opn4-/- mice with the dopamine precursor L-DOPA reduced their form-deprivation myopia by half compared to non-treated mice. Collectively our findings show that disruption of retinal melanopsin signaling alters the rate and magnitude of normal refractive development, yields greater susceptibility to form-deprivation myopia, and changes dopamine signaling. Our results suggest that mRGCs participate in the eye's response to myopigenic stimuli, acting partly through dopaminergic mechanisms, and provide a potential therapeutic target underling myopia progression. We conclude that proper mRGC function is necessary for correct refractive development and protection from myopia progression.

Correlation between small-scale methylation changes and gene expression during the development of myopia

Visually induced changes in the expression of early growth response-1 (EGR1), FBJ osteosarcoma oncogene (FOS), and NGFI-A binding protein-2 (NAB2) appear to form a part of a retinal network fundamental to ocular growth regulation, and thus, the development of myopia (short-sightedness). However, it is unclear how environmental (visual) cues are translated into these molecular changes. One possibility is through epigenetic modifications such as DNA methylation, a known regulator of such processes. By sequencing bisulfite-converted DNA amplicons, this study examined whether changes in DNA methylation occur within specific regulatory and promoter regions of EGR1, FOS, and NAB2 during the periods of increased and decreased ocular growth in chicks. Visually induced changes in ocular growth rates were associated with single-point, but not large-scale, shifts in methylation levels within the investigated regions. Analysis of methylation pattern variability (entropy) demonstrated that the observed methylation changes are occurring within small subpopulations of retinal cells. This concurs with previous observations that EGR1 and FOS are differentially regulated at the peptide level within specific retinal cell types. Together, the findings of this study support a potential role for DNA methylation in the translation of external visual cues into molecular changes critical for ocular growth regulation and myopia development.

Ocular Penetrance and Safety of the Dopaminergic Prodrug Etilevodopa

Purpose

Animal models have demonstrated the role of dopamine in regulating axial elongation, the critical feature of myopia. Because frequent delivery of dopaminergic agents via peribulbar, intravitreal, or intraperitoneal injections is not clinically viable, we sought to evaluate ocular penetration and safety of the topically applied dopaminergic prodrug etilevodopa.

Methods

The ocular penetration of dopamine and dopaminergic prodrugs (levodopa and etilevodopa) were quantified using an enzyme-linked immunosorbent assay in enucleated porcine eyes after a single topical administration. The pharmacokinetic profile of the etilevodopa was then assessed in rats. A four-week once-daily application of etilevodopa as a topical eye drop was conducted to establish its safety profile.

Results

At 24 hours, the studied prodrugs showed increased dopaminergic derivatives in the vitreous of porcine eyes. Dopamine 0.5% (P = 0.0123) and etilevodopa 10% (p = 0.370) achieved significant vitreous concentrations. Etilevodopa 10% was able to enter the posterior segment of the eye after topical administration in rats with an intravitreal half-life of eight hours after single topical administration. Monthly application of topical etilevodopa showed no alterations in retinal ocular coherence tomography, electroretinography, caspase staining, or TUNEL staining.

Conclusions

At similar concentrations, no difference in ocular penetration of levodopa and etilevodopa was observed. However, etilevodopa was highly soluble and able to be applied at higher topical concentrations. Dopamine exhibited both high solubility and enhanced penetration into the vitreous as compared to other dopaminergic prodrugs.

Translational Relevance

These findings indicate the potential of topical etilevodopa and dopamine for further study as a therapeutic treatment for myopia.

Effect of methylphenidate as a dopaminergic agent on myopia: Pilot study

Background Methylphenidate (MPH) hydrochloride is used as a first-line treatment for attention deficit hyperactivity disorder (ADHD). However, there is concern that this treatment may be associated with increased risk of refractive disorder. The aim of this study was to investigate the effect of MPH therapy on myopic shifts in refraction in children diagnosed with ADHD. Methods This study, children with ADHD and meeting inclusion criteria were examined before the initiation of MPH treatment and 3, 6 and 12 months after the initiation of treatment. Twenty age-gender-matched participants who applied to the outpatient ophthalmology clinic with various complaints were included in the study as a control group. Cycloplegic refraction examination and detailed eye measurements were performed at each visit. Results Nineteen patients were included in this study and the group consisted of 11 (57.9%) females and 8 (42.1%) males. The mean age of patients was 11.3 ± 2. (range: 8-18) years. During 12 months of use of MPH, the spherical equivalent changed from -0.36 ± 1.08 to -0.39 ± 1.05, and this difference was not statistically significant (P = .187). Axial length ranged from 22.92 ± 0.66. There was a change to 22.93 ± 0.62, and this difference was not statistically significant (P = .076). In the control group, the spherical equivalent changed from -0.43 ± 0.62 to -0.56 ± 0.84, and this difference was statistically significant. (P = .012) There was a change in the axial length from 22.97 ± 0.78 to 22.99 ± 0.62, and this difference was statistically significant (P = .015). Conclusions No significant changes spherical equivalent and axial length were detected during 12-month MPH use, but the increased spherical equivalent and axial length in the control group in the similar age group may indicate that MPH may reduce myopic shifts in refraction progression through dopamine, similar to in vivo studies. What's known Myopia is spreading rapidly in technologically advanced societies. There is strong evidence that myopia develops as the axial length of the eye increases as a result of spending more time indoors and working in close distances in parallel with the increase in education level. Animal studies have shown that decreased dopamine release plays an important role in the development of myopia. What's new The effect of dopamine in slowing or stopping myopia in experimental studies has also been demonstrated in human studies. No significant change in spherical equivalent and axial length was observed in methylphenidate users compared with control patients of similar age group. A significant increase in spherical equivalent and axial length was detected in the control group. This pilot study will shed light on future studies on the safe use of dopamine in the treatment of myopic shifts.

Transcriptome-based insights into gene networks controlling myopia prevention

Myopia (short-sightedness), usually caused by excessive elongation of the eye during development, has reached epidemic proportions worldwide. In animal systems including the chicken model, several treatments have been shown to inhibit ocular elongation and experimental myopia. Although diverse in their apparent mechanism of action, each one leads to a reduction in the rate of ocular growth. We hypothesize that a defined set of retinal molecular changes may underlie growth inhibition, irrespective of the treatment agent used. Accordingly, across five well-established but diverse methods of inhibiting myopia, significant overlap is seen in the retinal transcriptome profile (transcript levels and alternative splicing events) in chicks when analyzed by RNA-seq. Within the two major pathway networks enriched during growth inhibition, that of cell signaling and circadian entrainment, transcription factors form the largest functional grouping. Importantly, a large percentage of those genes forming the defined retinal response are downstream targets of the transcription factor EGR1 which itself shows a universal response to all five growth-inhibitory treatments. This supports EGR1's previously implicated role in ocular growth regulation. Finally, by contrasting our data with human linkage and GWAS studies on refractive error, we confirm the applicability of our study to the human condition. Together, these findings suggest that a universal set of transcriptome changes, which sit within a well-defined retinal network that cannot be bypassed, is fundamental to growth regulation, thus paving a way for designing novel targets for myopia therapies.

Insights into the mechanism of atropine's anti-myopia effects: evidence against cholinergic hyperactivity and modulation of dopamine release

Background and Purpose

The ability of the muscarinic cholinergic antagonist atropine to inhibit myopia development in humans and animal models would suggest that cholinergic hyperactivity may underlie myopic growth. To test this, we investigated whether cholinergic agonists accelerate ocular growth rates in chickens. Furthermore, we investigated whether atropine alters ocular growth by downstream modulation of dopamine levels, a mechanism postulated to underlie its antimyopic effects.

Experimental Approach

Muscarinic (muscarine and pilocarpine), nicotinic (nicotine) and non‐specific (oxotremorine and carbachol) cholinergic agonists were administered to chicks developing form‐deprivation myopia (FDM) or chicks that were otherwise untreated. Vitreal levels of dopamine and its primary metabolite 3,4‐dihydroxyphenylacetic acid (DOPAC) were examined using mass spectrometry MS in form‐deprived chicks treated with atropine (360, 15 or 0.15 nmol). Further, we investigated whether dopamine antagonists block atropine's antimyopic effects.

Key Results

Unexpectedly, administration of each cholinergic agonist inhibited FDM but did not affect normal ocular development. Atropine only affected dopamine and DOPAC levels at its highest dose. Dopamine antagonists did not alter the antimyopia effects of atropine.

Conclusion and Implications

Muscarinic, nicotinic and non‐specific cholinergic agonists inhibited FDM development. This indicates that cholinergic hyperactivity does not underlie myopic growth and questions whether atropine inhibits myopia via cholinergic antagonism. This study also demonstrates that changes in retinal dopamine release are not required for atropine's antimyopic effects. Finally, nicotinic agonists may represent a novel and more targeted approach for the cholinergic control of myopia as they are unlikely to cause the anterior segment side effects associated with muscarinic treatment.

Form-Deprivation and Lens-Induced Myopia Are Similarly Affected by Pharmacological Manipulation of the Dopaminergic System in Chicks

Purpose

Animal models have demonstrated a link between decreases in retinal dopamine levels and the development of form-deprivation myopia (FDM). However, the consistency of dopamine's role in the other major form of experimental myopia, that of lens-induced myopia (LIM), is less clear, raising the question as to what extent dopamine plays a role in human myopia. Therefore, to better define the role of dopamine in both forms of experimental myopia, we examined how consistent the protection afforded by dopamine and the dopamine agonist 6-amino-5,6,7,8-tetrahydronaphthalene-2,3-diol hydrobromide (ADTN) is between FDM and LIM.

Methods

Intravitreal injections of dopamine (0.002, 0.015, 0.150, 1.500 µmol) or ADTN (0.001, 0.010, 0.100, 1.000 µmol) were administered daily to chicks developing FDM or LIM. Axial length and refraction were measured following 4 days of treatment. To determine the receptor subtype by which dopamine and ADTN inhibit FDM and LIM, both compounds were coadministered with either the dopamine D2-like antagonist spiperone (0.005 µmol) or the D1-like antagonist SCH-23390 (0.005 µmol).

Results

Intravitreal administration of dopamine or ADTN inhibited the development of FDM (ED₅₀ = 0.003 µmol and ED₅₀ = 0.011 µmol, respectively) and LIM (ED₅₀ = 0.002 µmol and ED₅₀ = 0.010 µmol, respectively) in a dose-dependent manner, with a similar degree of protection observed in both paradigms (P = 0.471 and P = 0.969, respectively). Coadministration with spiperone, but not SCH-23390, inhibited the protective effects of dopamine and ADTN against the development of both FDM (P = 0.214 and P = 0.138, respectively) and LIM (P = 0.116 and P = 0.100, respectively).

Conclusions

pharmacological targeting of the retinal dopamine system inhibits FDM and LIM in a similar dose-dependent manner through a D2-like mechanism.

Coadministration With Carbidopa Enhances the Antimyopic Effects of Levodopa in Chickens

Purpose

Topical application of levodopa inhibits the development of form-deprivation myopia (FDM) and lens-induced myopia (LIM) in chicks. Here we examine whether coadministration with carbidopa enhances this protection and compare the effectiveness of topical versus systemic administration. We also investigate the degree to which topical and systemic administration of these compounds alters retinal dopamine release and examine whether this is the mechanism by which they inhibit experimental myopia.

Methods

Levodopa and levodopa:carbidopa (at a 4:1 ratio) were administered as twice-daily eye drops or once-daily intraperitoneal injections to chicks developing FDM or LIM over an ascending dose range. Axial length and refraction were measured following 4 days of treatment. Dopamine levels in the vitreous and blood were analyzed using liquid chromatography-mass spectrometry following topical or systemic administration of levodopa or levodopa:carbidopa. Finally, chicks receiving topical or systemic levodopa or levodopa:carbidopa were cotreated with the dopamine antagonist spiperone.

Results

Levodopa:carbidopa inhibited the development of FDM and LIM to a greater extent than levodopa alone (P < 0.05). Topical application was more effective than systemic administration (P < 0.001). Vitreal dopamine levels were increased to the greatest extent by topical application of levodopa:carbidopa (P < 0.001). Systemic but not topical administration significantly increased dopamine levels within the blood (P < 0.01). Cotreatment with spiperone inhibited the antimyopic effects (P < 0.05) of levodopa and levodopa:carbidopa.

Conclusions

The presence of carbidopa increases the bioavailability of levodopa within the eye, enhancing its antimyopic effects, with topical application showing the greatest efficacy. Thus levodopa:carbidopa may be a promising treatment for controlling the progression of human myopia.

Effects of Single and Repeated Intravitreal Applications of Atropine on Choroidal Thickness in Alert Chickens

INTRODUCTION

Atropine, a muscarinic antagonist, is known since the 19th century to inhibit myopia development in children. One of its effects is that it stimulates choroidal thickening. Thicker choroids, in turn, have been linked to myopia inhibition. We used the atropine-stimulated choroidal response in the chicken to learn more about the time courses and amplitudes of the effects of atropine, as well as whether repeated applications lead to accumulation or desensitization.

METHODS

Intravitreal injections containing 250 µg atropine sulfate were performed in 1 eye around 10:00 in the morning, the fellow eye received vehicle. Chickens with bilateral vehicle injections served as controls. Choroidal thickness was measured over the day for every 2-3 h in alert animals, using spectral domain optical coherence tomography, with 3-5 independent measurements in each eye. Three experiments were done - (1) single injection and time course measured over 1 day, (2) single injection and time course measured over 4 days, and (3) daily injections and time course measured over 4 days for measuring the effects of atropine on vitreal, retinal, and choroidal dopamine, and 3,4-dihydroxyphenylacetic acid levels by using high-performance liquid chromatography with electrochemical detection.

RESULTS

Atropine induced an increase in choroidal thickness by about 60 percent, with a peak amplitude after about 2 h. The effect persisted only for a few hours and had nearly disappeared by evening. Initially, similar amounts of choroidal thickening were observed in vehicle-injected fellow eyes but recovery to baseline was faster. When atropine was injected daily for 4 days, choroids thickened every day with similar amplitudes and time courses, with no signs of either accumulation or desensitization effects. Interestingly, while dopamine release from the retina was stimulated by atropine and followed approximately, the time course of choroidal thickening, its tissue concentration dropped in the choroid.

CONCLUSIONS

Even at relatively high intravitreal doses, effects of atropine on choroidal thickness remained transient, similar to its effects on retinal dopamine. With repeated application every day, the diurnal patterns of choroidal thickening could be reproduced for 4 days with similar amplitudes and time courses. The transient nature of the effects of atropine on the choroid may be relevant for application protocols of atropine against myopia.

Comparison of Administration of 0.02% Atropine and Orthokeratology for Myopia Control

OBJECTIVE

To compare the efficacies of 0.02% atropine eye drops and orthokeratology to control axial length (AL) elongation in children with myopia.

METHODS

In this historical control study, 247 children with myopia whose administration of 0.02% atropine (n=142) or underwent orthokeratology from an earlier study (n=105, control group) were enrolled. Data on AL and other baseline parameters were recorded at baseline and after 1 and 2 years of treatment.

RESULTS

The mean changes in AL in the first and second years of treatment were 0.30±0.21 and 0.28±0.20 mm, respectively, in the 0.02% atropine group and 0.16±0.20 and 0.20±0.16 mm, respectively, in the orthokeratology group. Axial length elongations after 2 years of treatment were 0.58±0.35 and 0.36±0.30 mm (P=0.007) in the 0.02% atropine and orthokeratology groups, respectively. Multivariate regression analyses showed that the AL elongation was significantly faster in the 0.02% atropine group than in the orthokeratology group (β=0.18, P=0.009). In multivariate regression analyses, younger age and shorter baseline AL were associated with a rapid AL elongation in the 0.02% atropine group (βage=-0.04, P=0.01; βAL=-0.17, P=0.03), while younger age, lower baseline spherical equivalent refractive error (SER), and shorter baseline AL were associated with a greater increase in AL in the orthokeratology group (βage=-0.03, P=0.04; βSER=0.06, P=0.03; βAL=-0.11, P=0.009). Faster AL elongation was found in the 0.02% atropine group compared with the orthokeratology group at higher baseline SER (P=0.04, interaction test).

CONCLUSION

Within the limits of this study design, orthokeratology seems to be a better method for controlling AL elongation compared with administration of 0.02% atropine in children with higher myopia over a treatment period of 2 years.

Pharmacotherapeutic candidates for myopia: A review

This review provides insights into the mechanism underlying the pathogenesis of myopia and potential targets for clinical intervention. Although the etiology of myopia involves both environmental and genetic factors, recent evidence has suggested that the prevalence and severity of myopia appears to be affected more by environmental factors. Current pharmacotherapeutics are aimed at inhibiting environmentally induced changes in visual input and subsequent changes in signaling pathways during myopia pathogenesis and progression. Recent studies on animal models of myopia have revealed specific molecules potentially involved in the regulation of eye development. Among them, the dopamine receptor plays a critical role in controlling myopia. Subsequent studies have reported pharmacotherapeutic treatments to control myopia progression. In particular, atropine treatment yielded favorable outcomes and has been extensively used; however, current studies are aimed at optimizing its efficacy and confirming its safety. Furthermore, future studies are required to assess the efficacy of combinatorial use of low-dose atropine and contact lenses or orthokeratology.

Visual conditions affecting eye growth alter diurnal levels of vitreous DOPAC

In chicks, the diurnal patterns of retinal dopamine synthesis and release are associated with refractive development. To assess the within-day patterns of dopamine release, we assayed vitreal levels of DOPAC (3,4-dihydroxyphenylacetic acid) using high performance liquid chromatography with electrochemical detection, at 4-h intervals over 24 h in eyes with experimental manipulations that change ocular growth rates. Chicks were reared under a 12 h light/12 h dark cycle; experiments began at 12 days of age. Output was assessed by modelling using the robust variance structure of Generalized Estimating Equations. Continuous spectacle lensdefocus or form deprivation: One group experienced non-restricted visual input to both eyes and served as untreated "normal" controls. Three experimental cohorts underwent monocular visual alterations known to alter eye growth and refraction: wearing a diffuser, a negative lens or a positive lens. After one full day of device-wear, chicks were euthanized at 4-h intervals over 24 h (8 birds per time/condition). Brief hyperopic defocus: Chicks wore negative lenses for only 2 daily hours either in the morning (starting at ZT 0; n = 16) or mid-day (starting at ZT 4; n = 8) for 3 days. Vitreal DOPAC was assayed. In chicks with bilateral non-restricted vision, or with continuous defocus or form-deprivation, there was a diurnal variation in vitreal DOPAC levels for all eyes (p < 0.001 for each). In normal controls, DOPAC was highest during the daytime, lowest at night, and equivalent for both eyes. In experimental groups, regardless of whether experiencing a growth stimulatory input (diffuser; negative lens) or growth inhibitory input (positive lens), DOPAC levels were reduced compared both to fellow eyes and to those of normal controls (p < 0.001 for each). These diurnal variations in vitreous DOPAC levels under different visual conditions indicate a complexity for dopaminergic mechanisms in refractive development that requires further study.

Levodopa inhibits the development of lens-induced myopia in chicks

Animal models have demonstrated a link between dysregulation of the retinal dopamine system and the development of myopia (short-sightedness). We have previously demonstrated that topical application of levodopa in chicks can inhibit the development of form-deprivation myopia (FDM) in a dose-dependent manner. Here, we examine whether this same protection is observed in lens-induced myopia (LIM), and whether levodopa’s protection against FDM and LIM occurs through a dopamine D1- or D2-like receptor mechanism. To do this, levodopa was first administered daily as an intravitreal injection or topical eye drop, at one of four ascending doses, to chicks developing LIM. Levodopa’s mechanism of action was then examined by co-administration of levodopa injections with D1-like (SCH-23390) or D2-like (spiperone) dopamine antagonists in chicks developing FDM or LIM. For both experiments, levodopa’s effectiveness was examined by measuring axial length and refraction after 4 days of treatment. Levodopa inhibited the development of LIM in a dose-dependent manner similar to its inhibition of FDM when administered via intravitreal injections or topical eye drops. In both FDM and LIM, levodopa injections remained protective against myopia when co-administered with SCH-23390, but not spiperone, indicating that levodopa elicits its protection through a dopamine D2-like receptor mechanism in both paradigms.

Protective effects of increased outdoor time against myopia: a review

Myopia has become a major cause for concern globally, particularly in East Asian countries. The increasing prevalence of myopia has been associated with a high socioeconomic burden owing to severe ocular complications that may occur with progressive myopia. There is an urgent need to identify effective and safe measures to address the growing number of people with myopia in the general population. Among the numerous strategies implemented to slow the progression of myopia, longer time spent outdoors has come to be recognized as a protective factor against this disorder. Although our understanding of the protective effects of outdoor time has increased in the past decade, considerably more research is needed to understand the mechanisms of action. Here, we summarize the main potential factors associated with the protective effects against myopia of increased outdoor time, namely, exposure to elevated levels and shorter wavelengths of light, and increased dopamine and vitamin D levels. In this review, we aimed to identify safe and effective therapeutic interventions to prevent myopia-related complications and vision loss.

Increased endogenous dopamine prevents myopia in mice

Experimental evidence suggests that dopamine (DA) modulates refractive eye growth. We evaluated whether increasing endogenous DA activity using pharmacological or genetic approaches decreased myopia susceptibility in mice. First, we assessed the effects of systemic L-3,4-dihydroxyphenylalanine (L-DOPA) injections on form deprivation myopia (FDM) in C57BL/6 J (WT^C57) mice. WT^C57 mice received daily systemic injections of L-DOPA (n = 11), L-DOPA + ascorbic acid (AA, n = 22), AA (n = 20), or Saline (n = 16). Second, we tested transgenic mice with increased or decreased expression of vesicular monoamine transporter 2 (VMAT2^HI, n = 22; WT^HI, n = 18; VMAT2^LO, n = 18; or WT^LO, n = 9) under normal and form deprivation conditions. VMAT2 packages DA into vesicles, affecting DA release. At post-natal day 28 (P28), monocular FD was induced in each genotype. Weekly measurements of refractive error, corneal curvature, and ocular biometry were performed until P42 or P49. WT^C57 mice exposed to FD developed a significant myopic shift (treated-contralateral eye) in AA (-3.27 ± 0.73D) or saline (-3.71 ± 0.80D) treated groups that was significantly attenuated by L-DOPA (-0.73 ± 0.90D, p = 0.0002) or L-DOPA + AA (-0.11 ± 0.46D, p = 0.0103). The VMAT2^LO mice, with under-expression of VMAT2, were most susceptible to FDM. VMAT2^LO mice developed significant myopic shifts to FD after one week compared to VMAT2^HI and WT mice (VMAT2^LO: -5.48 ± 0.54D; VMAT2^HI: -0.52 ± 0.92D, p < 0.05; WT: -2.13 ± 0.78D, p < 0.05; ungoggled control: -0.22 ± 0.24D, p < 0.001). These results indicate that endogenously increasing DA synthesis and release by genetic and pharmacological methods prevents FDM in mice.

The Increasing Burden of Myopia in Israel among Young Adults over a Generation: Analysis of Predisposing Factors

PURPOSE

To determine the trends in prevalence of myopia in Israeli young adults over approximately a generation, as well as associated factors and variation in the impact of these factors on myopia prevalence in this region over time.

DESIGN

Retrospective, cross-sectional study.

PARTICIPANTS

One hundred four thousand six hundred eighty-nine consecutive persons 16 to 19 years of age born between 1971 and 1994 who had not yet enlisted in the Israeli Army but had completed the medical profiling process.

METHODS

Using data collected at a north Israel recruitment center, the prevalence of myopia over time was estimated, and a polynomial regression analysis was performed to assess significance of nonlinear trends. Associations of demographic and socioeconomic factors with myopia were assessed, and trends over time were analyzed using a factorial logistic regression.

MAIN OUTCOME MEASURES

The primary outcome measure was factors associated with the prevalence of myopia over time. The secondary outcome measure was a description of the change in prevalence of myopia over time.

RESULTS

The prevalence of myopia increased 1.284-fold over 24 years from 20.4% among participants born between 1971 and 1982 to 26.2% among participants born between 1983 and 1994. A quite similar increase was observed among males (from 17.9% to 22.7%, respectively) and females (from 23.9% to 30.8%, respectively). The factors found to be associated with myopia were as follows: more recent date of birth, female gender, more years of education, being the eldest child, non-Israeli ethnic origin, and urban residence. However, there were significant trends over time in the effects of some of these factors, most notably an attenuation of the difference between participants of different religions in the recent birth-years period. Most of these associations and trends were observed in both males and females separately, with some gender-specific variations. Immigrants from Ethiopia who were raised in Israel were highly more likely to demonstrate myopia than those who arrived at an older age.

CONCLUSIONS

This study demonstrated an increase in the prevalence of myopia and the possible associations of urbanization- and higher education-related factors among several subpopulations and the risk for myopia developing.

IMI - Report on Experimental Models of Emmetropization and Myopia

The results of many studies in a variety of species have significantly advanced our understanding of the role of visual experience and the mechanisms of postnatal eye growth, and the development of myopia. This paper surveys and reviews the major contributions that experimental studies using animal models have made to our thinking about emmetropization and development of myopia. These studies established important concepts informing our knowledge of the visual regulation of eye growth and refractive development and have transformed treatment strategies for myopia. Several major findings have come from studies of experimental animal models. These include the eye's ability to detect the sign of retinal defocus and undergo compensatory growth, the local retinal control of eye growth, regulatory changes in choroidal thickness, and the identification of components in the biochemistry of eye growth leading to the characterization of signal cascades regulating eye growth and refractive state. Several of these findings provided the proofs of concepts that form the scientific basis of new and effective clinical treatments for controlling myopia progression in humans. Experimental animal models continue to provide new insights into the cellular and molecular mechanisms of eye growth control, including the identification of potential new targets for drug development and future treatments needed to stem the increasing prevalence of myopia and the vision-threatening conditions associated with this disease.

Oral crocetin administration suppressed refractive shift and axial elongation in a murine model of lens-induced myopia

Increased global incidence of myopia necessitates establishment of therapeutic approaches against its progression. To explore agents which may control myopia, we screened 207 types of natural compounds and chemical reagents based on an activity of a myopia suppressive factor, early growth response protein 1 (Egr-1) in vitro. Among the candidates, crocetin showed the highest and dose-dependent activation of Egr-1. For in vivo analysis, experimental myopia was induced in 3-week-old C57BL/6 J mice with −30 diopter (D) lenses for 3 weeks. Animals were fed with normal or mixed chow containing 0.003% (n = 19) and 0.03% (n = 7) of crocetin during myopia induction. Refraction and axial length were measured at 3-week-old and the 6-week-old with an infrared photorefractor and a SD-OCT system. Compared to controls (n = 14), crocetin administration showed a significant smaller change of refractive errors (−13.62 ± 8.14 vs +0.82 ± 5.81 D for 0.003%, p < 0.01, −2.00 ± 4.52 D for 0.03%, p < 0.01) and axial elongation (0.27 ± 0.03 vs 0.22 ± 0.04 mm for 0.003%, p < 0.01, 0.23 ± 0.05 mm for 0.03%, p < 0.05). These results suggest that a dietary factor crocetin may have a preventive effect against myopia progression.

Prevention of Progression in Myopia: A Systematic Review

The prevalence of myopia has increased worldwide in recent decades and now is endemic over the entire industrial world. This increase is mainly caused by changes in lifestyle and behavior. In particular, the amount of outdoor activities and near work would display an important role in the pathogenesis of the disease. Several strategies have been reported as effective. Spectacles and contact lenses have shown only slight results in the prevention of myopia and similarly ortokerathology should not be considered as a first-line strategy, given the high risk of infectious keratitis and the relatively low compliance for the patients. Thus, to date, atropine ophthalmic drops seem to be the most effective treatment for slowing the progression of myopia, although the exact mechanism of the effect of treatment is still uncertain. In particular, low-dose atropine (0.01%) was proven to be an effective and safe treatment in the long term due to the lowest rebound effect with negligible side effects.

Genome-wide association meta-analysis highlights light-induced signaling as a driver for refractive error

Refractive errors, including myopia, are the most frequent eye disorders worldwide and an increasingly common cause of blindness. This genome-wide association meta-analysis in 160,420 participants and replication in 95,505 participants increased the number of established independent signals from 37 to 161 and showed high genetic correlation between Europeans and Asians (>0.78). Expression experiments and comprehensive in silico analyses identified retinal cell physiology and light processing as prominent mechanisms, and also identified functional contributions to refractive-error development in all cell types of the neurosensory retina, retinal pigment epithelium, vascular endothelium and extracellular matrix. Newly identified genes implicate novel mechanisms such as rod-and-cone bipolar synaptic neurotransmission, anterior-segment morphology and angiogenesis. Thirty-one loci resided in or near regions transcribing small RNAs, thus suggesting a role for post-transcriptional regulation. Our results support the notion that refractive errors are caused by a light-dependent retina-to-sclera signaling cascade and delineate potential pathobiological molecular drivers.

Dopamine signaling and myopia development: What are the key challenges

In the face of an "epidemic" increase in myopia over the last decades and myopia prevalence predicted to reach 2.5 billion people by the end of this decade, there is an urgent need to develop effective and safe therapeutic interventions to slow down this "myopia booming" and prevent myopia-related complications and vision loss. Dopamine (DA) is an important neurotransmitter in the retina and mediates diverse functions including retina development, visual signaling, and refractive development. Inspired by the convergence of epidemiological and animal studies in support of the inverse relationship between outdoor activity and risk of developing myopia and by the close biological relationship between light exposure and dopamine release/signaling, we felt it is timely and important to critically review the role of DA in myopia development. This review will revisit several key points of evidence for and against DA mediating light control of myopia: 1) the causal role of extracellular retinal DA levels, 2) the mechanism and action of dopamine D1 and D2 receptors and 3) the roles of cellular/circuit retinal pathways. We examine the experiments that show causation by altering DA, DA receptors and visual pathways using pharmacological, transgenic, or visual environment approaches. Furthermore, we critically evaluate the safety issues of a DA-based treatment strategy and some approaches to address these issues. The review identifies the key questions and challenges in translating basic knowledge on DA signaling and myopia from animal studies into effective pharmacological treatments for myopia in children.

Bio-environmental factors associated with myopia: An updated review

Experimental studies in animals, as well as observational and intervention studies in humans, seem to support the premise that the development of juvenile myopia is promoted by a combination of the effect of genetic and environmental factors, with a complex interaction between them. The very rapid increase in myopia rates in some parts of the world, such as Southeast Asia, supports a significant environmental effect. Several lines of evidence suggest that humans might respond to various external factors, such as increased activity in near vision, increased educational pressure, decreased exposure to sunlight outdoors, dietary changes (including increased intake of carbohydrates), as well as low light levels indoors. All these factors could be associated with a higher prevalence of myopia.

Genetics of Refraction and Myopia

Both genetic and environmental factors play roles in the development of refractive errors. Identification of genes involved in refractive errors may help in elucidating the underlying molecular mechanism related to both genetic defects and environmental pressure. Recent development of techniques for genome wide analysis provides unique opportunity in dissecting the genetic basis related to refractive errors. This chapter tries to give a brief overview on the recent progress of genetic study of refractive errors, especially myopia.

Pharmaceutical intervention for myopia control

Myopia is the result of a mismatch between the optical power and the length of the eye, with the latter being too long. Driving the research in this field is the need to develop myopia treatments that can limit axial elongation. When axial elongation is excessive, as in high myopia, there is an increased risk of visual impairment and blindness due to ensuing pathologies such as retinal detachments. This article covers both clinical studies involving myopic children, and studies involving animal models for myopia. Atropine, a nonselective muscarinic antagonist, has been studied most extensively in both contexts. Because it remains the only drug used in a clinical setting, it is a major focus of the first part of this article, which also covers the many shortcomings of topical ophthalmic atropine. The second part of this article focuses on in vitro and animal-based drug studies, which encompass a range of drug targets including the retina, retinal pigment epithelium and sclera. While the latter studies have contributed to a better understanding of how eye growth is regulated, no new antimyopia drug treatments have reached the clinical setting. Less conservative approaches in research, and in particular, the exploration of new bioengineering approaches for drug delivery, are needed to advance this field.

Form deprivation and lens-induced myopia: are they different?

In the following point-counterpoint article, internationally-acclaimed myopia researchers were challenged to defend the two opposing sides of the topic defined by the title; their contributions, which appear in the order Point followed by Counterpoint, were peer-reviewed by both the editorial team and an external reviewer. Independently of the invited authors, the named member of the editorial team provided an Introduction and Summary, both of which were reviewed by the other members of the editorial team. By their nature, views expressed in each section of the Point-Counterpoint article are those of the author concerned and may not reflect the views of all of the authors.

Expanding the phenotypic spectrum of ECEL1-related congenital contracture syndromes

Using a combination of homozygosity mapping and whole-exome sequencing (WES), we identified a novel missense c.1819G>A mutation (G607S) in the endothelin-converting enzyme-like 1 (ECEL1) gene in a consanguineous pedigree of Turkish origin presenting with a syndrome of camptodactyly, scoliosis, limited knee flexion, significant refractive errors and ophthalmoplegia. ECEL1 mutations were recently reported to cause recessive forms of distal arthrogryposis. This report expands on the molecular basis and the phenotypic spectrum of ECEL1-associated congenital contracture syndromes.

An updated view on the role of dopamine in myopia

A large body of data is available to support the hypothesis that dopamine (DA) is one of the retinal neurotransmitters involved in the signaling cascade that controls eye growth by vision. Initially, reduced retinal DA levels were observed in eyes deprived of sharp vision by either diffusers ("deprivation myopia", DM) or negative lenses ("lens induced myopia", LIM). Simulating high retinal DA levels by intravitreal application of a DA agonist can suppress the development of both DM and LIM. Also more recent studies using knock-out mouse models of DA receptors support the idea of an association between decreased DA levels and DM. There seem to be differences in the magnitude of the effects of DA on DM and LIM, with larger changes in DM but the degrees of image degradation by both treatments need to be matched to support this conclusion. Although a number of studies have shown that the inhibitory effects of dopamine agonists on DM and LIM are mediated through stimulation of the D2-receptor, there is also recent evidence that the balance of D2- and D1-receptor activation is important. Inhibition of D2-receptors can also slow the development of spontaneous myopia in albino guinea pigs. Retinal DA content displays a distinct endogenous diurnal, and partially circadian rhythm. In addition, retinal DA is regulated by a number of visual stimuli like retinal illuminance, spatial frequency content of the image, temporal contrast and, in chicks, by the light input from the pineal organ. A close interaction was found between muscarinergic and dopaminergic systems, and between nitric oxide and dopaminergic pathways, and there is evidence for crosstalk between the different pathways, perhaps multiple binding of the ligands to different receptors. It was shown that DA agonists interact with the immediate early signaling molecule ZENK which triggers the first steps in eye growth regulation. However, since long treatment periods were often needed to induce significant changes in retinal dopamine synthesis and release, the role of dopamine in the early steps is unclear. The wide spatial distribution of dopaminergic amacrine cells in the retina and the observation that changes in dopamine levels can be locally induced by local retinal deprivation is in line with the assumption that dopaminergic mechanisms control both central and peripheral eye growth. The protective effect of outdoor activity on myopia development in children seems to be partly mediated by the stimulatory effect of light on retinal dopamine production and release. However, the dose-response function linking light exposure to dopamine and to the suppression of myopia is not known and requires further studies.

Pharmacology of myopia and potential role for intrinsic retinal circadian rhythms

Despite the high prevalence and public health impact of refractive errors, the mechanisms responsible for ametropias are poorly understood. Much evidence now supports the concept that the retina is central to the mechanism(s) regulating emmetropization and underlying refractive errors. Using a variety of pharmacologic methods and well-defined experimental eye growth models in laboratory animals, many retinal neurotransmitters and neuromodulators have been implicated in this process. Nonetheless, an accepted framework for understanding the molecular and/or cellular pathways that govern postnatal eye development is lacking. Here, we review two extensively studied signaling pathways whose general roles in refractive development are supported by both experimental and clinical data: acetylcholine signaling through muscarinic and/or nicotinic acetylcholine receptors and retinal dopamine pharmacology. The muscarinic acetylcholine receptor antagonist atropine was first studied as an anti-myopia drug some two centuries ago, and much subsequent work has continued to connect muscarinic receptors to eye growth regulation. Recent research implicates a potential role of nicotinic acetylcholine receptors; and the refractive effects in population surveys of passive exposure to cigarette smoke, of which nicotine is a constituent, support clinical relevance. Reviewed here, many puzzling results inhibit formulating a mechanistic framework that explains acetylcholine's role in refractive development. How cholinergic receptor mechanisms might be used to develop acceptable approaches to normalize refractive development remains a challenge. Retinal dopamine signaling not only has a putative role in refractive development, its upregulation by light comprises an important component of the retinal clock network and contributes to the regulation of retinal circadian physiology. During postnatal development, the ocular dimensions undergo circadian and/or diurnal fluctuations in magnitude; these rhythms shift in eyes developing experimental ametropia. Long-standing clinical ideas about myopia in particular have postulated a role for ambient lighting, although molecular or cellular mechanisms for these speculations have remained obscure. Experimental myopia induced by the wearing of a concave spectacle lens alters the retinal expression of a significant proportion of intrinsic circadian clock genes, as well as genes encoding a melatonin receptor and the photopigment melanopsin. Together this evidence suggests a hypothesis that the retinal clock and intrinsic retinal circadian rhythms may be fundamental to the mechanism(s) regulating refractive development, and that disruptions in circadian signals may produce refractive errors. Here we review the potential role of biological rhythms in refractive development. While much future research is needed, this hypothesis could unify many of the disparate clinical and laboratory observations addressing the pathogenesis of refractive errors.

Scleral changes induced by atropine in chicks as an experimental model of myopia

PURPOSE

To determine the effects of intravitreal atropine on scleral growth in the form-deprived chick as an experimental model of myopia.

METHODS

Five groups of five chicks were studied from day 0-12 post-hatching. One group remained untreated (C), and four were form-deprived by monocular light diffusers to induce myopia. Two groups (RL and A) wore diffusers for 9 days, and the other two groups (D and D + A) wore diffusers throughout the study. Group D received no further treatment (myopia positive control). Groups A and D + A received intravitreal injections of atropine for days 9-12. Measurements of refractive error and axial length were performed on days 0, 9, and 12. Sclera changes were assessed in cartilaginous and fibrous layers by histological analysis.

RESULTS

All form-deprived eyes had a myopic refractive error on day 9. All atropine-treated groups were hyperopic on day 12. The effect of atropine was most evident in Group D + A in which diffusers were maintained throughout treatment and changes in refractive error were statistically significant. The observed changes in axial length were in line with the changes in refractive error. The scleral fibrous layer thickness increased, and the sceral cartilaginous layer underwent a slight thinning compared to Group D, the myopia positive control.

CONCLUSIONS

If the signals that induce growth remain during atropine treatment, morphological changes in sclera are produced: the scleral fibrous layer thickened, and the sceral cartilaginous layer thinned. These changes resulted in refractive error recovery, and the ocular growth was stopped. The data suggested the atropine was acting throughout the scleral fibrous layer.

The complex interactions of retinal, optical and environmental factors in myopia aetiology

Myopia is the commonest ocular abnormality but as a research topic remains at the margins of mainstream ophthalmology. The concept that most myopes fall into the category of 'physiological myopia' undoubtedly contributes to this position. Yet detailed analysis of epidemiological data linking myopia with a range of ocular pathologies from glaucoma to retinal detachment demonstrates statistically significant disease association in the 0 to -6 D range of 'physiological myopia'. The calculated risks from myopia are comparable to those between hypertension, smoking and cardiovascular disease. In the case of myopic maculopathy and retinal detachment the risks are an order of magnitude greater. This finding highlights the potential benefits of interventions that can limit or prevent myopia progression. Our understanding of the regulatory processes that guide an eye to emmetropia and, conversely how the failure of such mechanisms can lead to refractive errors, is certainly incomplete but has grown enormously in the last few decades. Animal studies, observational clinical studies and more recently randomized clinical trials have demonstrated that the retinal image can influence the eye's growth. To date human intervention trials in myopia progression using optical means have had limited success but have been designed on the basis of simple hypotheses regarding the amount of defocus at the fovea. Recent animal studies, backed by observational clinical studies, have revealed that the mechanisms of optically guided eye growth are influenced by the retinal image across a wide area of the retina and not solely the fovea. Such results necessitate a fundamental shift in how refractive errors are defined. In the context of understanding eye growth a single sphero-cylindrical definition of foveal refraction is insufficient. Instead refractive error must be considered across the curved surface of the retina. This carries the consequence that local retinal image defocus can only be determined once the 3D structure of the viewed scene, off axis performance of the eye and eye shape has been accurately defined. This, in turn, introduces an under-appreciated level of complexity and interaction between the environment, ocular optics and eye shape that needs to be considered when planning and interpreting the results of clinical trials on myopia prevention.

DNA methylation changes elicited by social stimuli in the brains of worker honey bees

Social environments are notoriously multifactorial, yet studies in rodents have suggested that single variables such as maternal care can in fact be disentangled and correlated with specific DNA methylation changes. This study assesses whether non-detrimental social environmental variation in a highly plastic social insect is correlated with epigenomic modifications at the DNA methylation level. Honey bee workers perform tasks such as nursing and foraging in response to the social environment in the hive, in an age-linked but not age-dependent manner. In this study, the methylation levels of 83 cytosine-phosphate-guanosine dinucleotides over eight genomic regions were compared between the brains of age-matched bees performing nursing or foraging tasks. The results reveal more changes correlated with task than with chronological age, and also hive-associated methylation at some sites. One methylation site from a gene encoding Protein Kinase C binding protein 1 was consistently more methylated in foragers than nurses, which is suggested to lead to production of task-specific protein isoforms via alternative splicing. This study illustrates the ability of the neural epigenome to dynamically respond to complex social stimuli.

Image defocus and altered retinal gene expression in chick: clues to the pathogenesis of ametropia

PURPOSE

Because of the retina's role in refractive development, this study was conducted to analyze the retinal transcriptome in chicks wearing a spectacle lens, a well-established means of inducing refractive errors, to identify gene expression alterations and to develop novel mechanistic hypotheses about refractive development.

METHODS

One-week-old white Leghorn chicks wore a unilateral spectacle lens of +15 or -15 D for 6 hours or 3 days. With total RNA from the retina/(retinal pigment epithelium, RPE), chicken gene microarrays were used to compare gene expression levels between lens-wearing and contralateral control eyes (n = 6 chicks for each condition). Normalized microarray signal intensities were evaluated by analysis of variance, using a false discovery rate of <10% as the statistical criterion. Selected differentially expressed genes were validated by qPCR.

RESULTS

Very few retina/RPE transcripts were differentially expressed after plus lens wear. In contrast, approximately 1300 transcripts were differentially expressed under each of the minus lens conditions, with minimal overlap. For each condition, low fold-changes typified the altered transcriptome. Differentially regulated genes under the minus lens conditions included many potentially informative signaling molecules and genes whose protein products have roles in intrinsic retinal circadian rhythms.

CONCLUSIONS

Plus or minus lens wear induce markedly different, not opposite, alterations in retina/RPE gene expression. The initial retinal responses to defocus are quite different from those when the eye growth patterns are well established, suggesting that different mechanisms govern the initiation and persistence or progression of refractive errors. The gene lists identify promising signaling candidates and regulatory pathways for future study, including a potential role for circadian rhythms in refractive development.

Study on retinal dopamine transporter in form deprivation myopia using the radiopharmaceutical tracer 99mTc-TRODAT-1

PURPOSE

To investigate the distributions and changes in dopamine transporters (DATs) using Tc-TRODAT-1 ([Tc-(2((2-(((3-(4-chlorophenyl)-8-methyl-8-azabicyclo(3,2,1)-oct-2-yl)-methyl)(2-mercaptoethyl)amino)ethyl)amino)ethane-thiolato(3-)-N2,N2',S2,S2)oxo-(1R-(exo-exo)))]) in form deprivation myopia retina.

METHODS

Pigmented guinea pigs aged 3 weeks were randomly assigned into two groups: form-deprivation myopia and normal control group. The test group wore a translucent goggle covering randomly for 4 weeks, and both groups underwent biometric measurement before and after the experiment. Both Micro-single-photon emission computed tomography (SPECT) imaging and ex-vivo autoradiography were performed with the injection of Tc-TRODAT-1 on the 4th week for all the guinea pigs.

RESULTS

The retinas were clearly resolved with Tc-TRODAT-1 in both Micro-SPECT imaging and ex-vivo autoradiography. In Micro-SPECT imaging, the ratio of Tc-TRODAT-1 uptake in the myopic retinas (11.55+/-2.80) was 3.64+/-1.40 lower than that in the normal control eyes (15.20+/-1.98, P=0.026, F=2.94, t=2.605), and 2.35+/-1.05 lower than that in the fellow eyes (13.90+/-2.04, P=0.003, t=5.476). In ex-vivo autoradiography, the ratio of Tc-TRODAT-1 uptake in the myopic retina (95.52+/-12.04) was 18.54+/-5.86 lower than in the normal control eyes (114.06+/-7.81, P=0.01, F=0.331, t=3.164), and was 16.95+/-5.78 lower than in the fellow eyes (112.47+/-15.67, P=0.001, t=7.179).

CONCLUSION

Tc-TRODAT-1 can be used to trace the distributions and changes in DAT in the retina. DATs in the myopic retinas were lower than that in the fellow and normal control eyes. Radionuclide tracing may provide a new approach in vivo for further studies on the dopamine system in myopia.

Levodopa inhibits the development of form-deprivation myopia in guinea pigs

PURPOSE

It has been shown that visual deprivation leads to a myopic refractive error and also reduces the retinal concentration of dopamine. Exogenously 3,4-dihydroxy-L-phenylalanine (levodopa, L-DOPA) can be converted into dopamine in vivo, which safely and effectively treats Parkinson disease. Moreover, L-DOPA was also used in the treatment of amblyopia in clinical studies. However, the effect of L-DOPA on the development of myopia has not been studied. The aim of this study was to investigate whether intraperitoneal injection of L-DOPA could inhibit form-deprivation myopia in guinea pigs and to explore a new strategy for drug treatment of myopia.

METHODS

Sixty guinea pigs, at age of 4 weeks, were randomly divided into six groups: normal control, L-DOPA group, saline group, deprived group, deprived plus L-DOPA group, and deprived plus saline group. Form deprivation was induced with translucent eye shields on the right eye and lasted for 10 days. L-DOPA was injected intraperitoneally into the guinea pig once a day. The corneal radius of curvature, refraction, and axial length were measured in all animals. Subsequently, retinal dopamine content was evaluated by high-performance liquid chromatography with electrochemical detection.

RESULTS

Ten days of eye occlusion caused the form-deprived eyes to elongate and become myopic, and retinal dopamine content to decrease, but the corneal radius of curvature was not affected. Repeated intraperitoneal injection of L-DOPA could inhibit the myopic shift (from -3.62 +/- 0.98 D to -1.50 +/- 0.38 D; p < 0.001) due to goggles occluding and compensate retinal dopamine (from 0.65 +/- 0.10 ng to 1.33 +/- 0.23 ng; p < 0.001). Administration of L-DOPA to the unoccluded animals had no effect on its ocular refraction. There was no effect of intraperitoneal saline on the ocular refractive state and retinal dopamine.

CONCLUSIONS

Systemic L-DOPA was partly effective in this guinea pig model and, therefore, is worth testing for effectiveness in progressing human myopes.

Changes in retinal alphaB-crystallin (cryab) RNA transcript levels during periods of altered ocular growth in chickens

Changes in retinal crystallin gene expression have been implicated in the development of myopia in animal models. We therefore investigated the expression of alphaB-crystallin (cryab) in the chicken retina during periods of increased ocular growth induced by form-deprivation and negative lens-wear, and during periods of decreased ocular growth induced by diffuser removal from previously form-deprived eyes, and plus lens-wear. Cryab RNA transcript levels in the chicken retina were measured using semi-quantitative real-time RT-PCR, at times between 1 h and 10 days after the fitting of diffusers or negative lenses, and at times between 1 h and 3 days following the removal of diffusers from previously form-deprived eyes, or the addition of plus lenses. Changes in expression for each condition at each time-point are analysed relative to expression in retinas from age-matched untreated control birds. No change in relative expression of cryab RNA transcript was detected 1 h after fitting diffusers to induce form-deprivation myopia. A transient increase in cryab RNA transcript expression was detected around 1 day later (p = 0.02), but expression returned to control levels after three days. After 7 (p = 0.005) and 10 (p = 0.001) days, retinal cryab RNA transcript expression progressively increased relative to controls. After removal of the diffusers, to initiate recovery, cryab RNA transcript expression remained elevated, with only a slight return to control levels. During the development of lens-induced myopia, no changes in cryab RNA transcript expression relative to controls were seen on day 1, but increases were seen at 10 days (p = 0.004). No significant changes in retinal cryab RNA transcript expression were seen in response to plus lenses compared to either contralateral control values (MANOVA; F = 0.60, p = 0.48) or age-matched untreated values (MANOVA; F = 4.10, p = 0.08). Changes in retinal cryab RNA transcript expression were not systematically related to changes in the rate of eye growth. The role of the transient increase in cryab expression observed after 1 day of form-deprivation, which was not seen after fitting negative lenses, is unclear. The later increases in relative cryab expression seen during the development of form-deprivation and lens-induced myopia occur too late to have a major role in the differential regulation of eye growth between experimental and control eyes. Given that cryab is a member of the small heat shock protein family, the later increases may reflect the emergence of cell damage related to high myopic pathology in the experimentally enlarged eyes and retina.

Colchicine attenuates compensation to negative but not to positive lenses in young chicks

Optic nerve-sectioned (ONS) chick eyes are capable of emmetropisation, but these eyes also exhibit increased hyperopia without any visual manipulations, which suggests altered eye growth regulation. These altered growth changes may be related to the loss of retinal ganglion cells that follows nerve lesioning. Colchicine, which also destroys retinal ganglion cells in chicks, was used to further examine the effects of retinal ganglion cell loss on emmetropisation. Growth responses of +10D and -10D lens-wearing colchicine-injected eyes were compared to those of +10D and -10D lens-wearing saline-injected eyes, respectively. Changes after removal of lenses were also analysed. Prior to lens-wear, colchicine-injected eyes exhibited longer optical axial lengths (OL; distance from cornea to retina; p=0.0185) but no differences in refractive error (RE; p=0.6588). Although myopic shifts were not significant for -10D lens-wearing colchicine-injected eyes (p=0.5913), but were for the saline-injected eyes (p=0.0034), these changes were not different (p=0.1646). However, -10D lens-induced OL changes in colchicine-injected eyes showed insignificant (p=0.2214) and reduced (p=0.0102) changes compared to those of saline-injected eyes. +10D lens-treated colchicine-injected eyes showed significant hyperopic shifts (p<0.0001) and significant reductions in OL (p<0.0001) that were similar to those of saline-injected eyes (p=0.7990 and p=0.1495, respectively). Growth responses in eyes recovering from -10D lenses were minimal, with REs unaffected (p=0.3325), but OL reductions affected (p=0.0199) by colchicine. Colchicine-injected eyes recovering from +10D lenses showed significant myopic shifts (p=0.0003) and OL elongations (p<0.0001) that were similar to those of saline-injected eyes (p=0.3999 and p=0.4731, respectively). The results showing that colchicine suppresses the ability to respond to negative lenses but leaves compensation to positive lenses relatively unchanged, are opposite to those of optic nerve sectioned eyes. We speculate that the differences are probably related to the way retinal cells are lost.