Alpha2 -adrenoceptor agonists inhibit form-deprivation myopia in the chick

Topical review: Potential mechanisms of atropine for myopia control

SIGNIFICANCE

Atropine is effective at slowing myopia progression in children, but the mechanism of action by which it controls myopia remains unclear. This article is an evidenced-based review of potential receptor-based mechanisms by which atropine may act to slow the progression of myopia.The rising number of individuals with myopia worldwide and the association between myopia and vision-threatening ocular pathologies have made myopia control treatments one of the fastest growing areas of ophthalmic research. High-concentration atropine (1%) is the most effective treatment for slowing myopia progression to date; low concentrations of atropine (≤0.05%) appear partially effective and are currently being used to slow myopia progression in children. While significant progress has been made in the past few decades in understanding fundamental mechanisms by which atropine may control myopia, the precise characterization of how atropine works for myopia control remains incomplete. It is plausible that atropine slows myopia via its affinity to muscarinic receptors and influence on accommodation, but animal studies suggest that this is likely not the case. Other studies have shown that, in addition to muscarinic receptors, atropine can also bind, or affect the action of, dopamine, alpha-2-adrenergic, gamma-aminobutyric acid, and cytokine receptors in slowing myopia progression. This review summarizes atropine's effects on different receptor pathways of ocular tissues and discusses how these effects may or may not contribute to slowing myopia progression. Given the relatively broad array of receptor-based mechanisms implicated in atropine control of myopia, a single mode of action of atropine is unlikely; rather atropine may be exerting its myopia control effects directly or indirectly via several mechanisms at multiple levels of ocular tissues, all of which likely trigger the response in the same direction to inhibit eye growth and myopia progression.

The potential of brimonidine for myopia treatment: Targeting MMP-2 to regulate choroidal thickness and control eye growth

Drug treatment studies are a focal point for identifying novel approaches to reduce myopia progression through basic science research. Here, we investigated the effects of various brimonidine administration routes and concentrations on form-deprivation myopia (FDM) progression, matrix metalloproteinase-2 (MMP-2), and collagen alpha1 chain of type I (COL1A1) expression in the retinal pigment epithelial (RPE)-choroid complex and sclera of guinea pigs. They demonstrate that brimonidine has the capacity to impede choroidal thinning induced by FDM, potentially through the induction of choroidal vasodilation. Additionally, we observed that brimonidine effectively counteracts FDM-induced downregulation of choroidal and scleral MMP-2 expression. Suppression of MMP-2 expression may reduce disruption of scleral and choroidal structural integrity which reduces declines in choroidal blood circulation and mitigates increases in ocular elongation. This research elucidates the effects of brimonidine on myopia progression, offering potential insights into therapeutic interventions for myopia.

Brimonidine as a possible treatment for myopia

BACKGROUND

Myopia is becoming a huge burden on the world's public health systems. The purpose of this study was to explore the effect of brimonidine in the treatment of form-deprivation myopia (FDM) and the relationship between intraocular pressure (IOP) and myopia development.

METHODS

Monocular form deprivation myopia (FDM) was induced in three-week-old pigmented male guinea pigs. They were treated with 3 different methods of brimonidine administration (eye drops, and subconjunctival or intravitreal injections). Four different concentrations of brimonidine were tested for each method (2µg/µL, 4µg/µL, 20µg/µL, and 40µg/µL). All treatments continued for a period of 21 days. Tonometry, retinoscopy, and A-scan ultrasonography were used to monitor intraocular pressure, refractive error and axial length (AL), respectively.

RESULTS

Treatment with subconjunctival brimonidine at 40µg/µL, and intravitreal brimonidine at 2µg/µL and 4µg/µL, inhibited the development of FDM. The myopic refraction, excessive axial length, and elevation of IOP were significantly decreased. Brimonidine in eye drops was ineffective.

CONCLUSION

Brimonidine at appropriate doses significantly reduced the development of FD myopia in guinea pigs. The IOP may change with FD myopia.

Ocular Posterior Segment Distribution and Pharmacokinetics of Brimonidine After Intravitreal Administration in Guinea Pigs

Purpose: Brimonidine is a highly alpha-2 adrenergic agonist, which provides a potential myopia control effect. This study aimed to examine the pharmacokinetics and concentration of brimonidine in the posterior segment tissue of eyes in guinea pigs. Methods: A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was successfully used for brimonidine pharmacokinetics and tissue distribution research in guinea pigs following intravitreal administration (20 μg/eye). Results: Brimonidine concentrations in the retina and sclera were maintained at a high level (>60 ng/g) at 96 h postdosing. Brimonidine concentration peaked in the retina (377.86 ng/g) at 2.41 h and sclera (306.18 ng/g) at 6.98 h. The area under curve (AUC0-∞) was 27,179.99 ng h/g in the retina and 39,529.03 ng h/g in the sclera. The elimination half-life (T1/2e) was 62.43 h in the retina and 67.94 h in the sclera. Conclusions: The results indicated that brimonidine was rapidly absorbed and diffused to the retina and sclera. Meanwhile, it maintained higher posterior tissue concentrations, which can effectively activate the alpha-2 adrenergic receptor. This may provide pharmacokinetic evidence for the inhibition of myopia progression by brimonidine in animal experiments.

RNA-sequencing analysis reveals the long noncoding RNA profile in the mouse myopic retina

Aim: Myopia is a prevalent public health problem. The long noncoding RNA (lncRNA) mechanisms for dysregulated retinal signaling in the myopic eye have remained elusive. The aim of this study was to analyze the expression profiles and possible pathogenic roles of lncRNAs in mouse form-deprived myopia (FDM) retinas. Methods: A mouse FDM model was induced and retinas from the FDM right eyes and the contralateral eyes were collected for RNA sequencing. Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment, and lncRNA-mRNA coexpression network analyses were conducted to explore the biological functions of the differentially expressed lncRNAs. In addition, the levels of differentially expressed lncRNAs in the myopic retinas were validated by quantitative real-time PCR (qRT-PCR). Fluorescence in situ hybridization (FISH) was used to detect the localization of lncRNAs in mouse retinas. Results: FDM eyes exhibited reduced refraction and increased ocular axial length compared to control fellow eyes. RNA sequencing revealed that there were 655 differentially expressed lncRNAs between the FDM and control retinas. Functional enrichment analysis indicated that the differentially expressed RNAs were mostly enriched in cellular processes, cytokine-cytokine receptor interactions, retinol metabolism, and rhythmic processes. Differentially expressed lncRNAs were validated by qRT-PCR. Additionally, RNA FISH showed that XR_384718.4 (Gm35369) localized in the ganglion cell (GCL) and inner nuclear layers (INL). Conclusion: This study identified the differential expression profiles of lncRNAs in myopic mouse retinas. Our results provide scientific evidence for investigations of myopia and the development of putative interventions in the future.

Intravitreal brimonidine inhibits form-deprivation myopia in guinea pigs

Background

The use of ocular hypotensive drugs has been reported to attenuate myopia progression. This study explores whether brimonidine can slow myopia progression in the guinea pig form-deprivation (FD) model.

Methods

Three-week-old pigmented male guinea pigs (Cavia porcellus) underwent monocular FD and were treated with 3 different methods of brimonidine administration (eye drops, subconjunctival or intravitreal injections). Four different concentrations of brimonidine were tested for intravitreal injection (2 μg/μL, 4 μg/μL, 20 μg/μL, 40 μg/μL). All treatments continued for a period of 21 days. Tonometry, retinoscopy, and A-scan ultrasonography were used to monitor intraocular pressure (IOP), refractive error and axial length (AL), respectively. On day 21, guinea pigs were sacrificed for RNA sequencing (RNA-seq) to screen for associated transcriptomic changes.

Results

The myopia model was successfully established in FD animals (control eye vs. FD eye, respectively: refraction at day 20, 0.97 ± 0.18 D vs. − 0.13 ± 0.38 D, F = 6.921, P = 0.02; AL difference between day 0 and day 21, 0.29 ± 0.04 mm vs. 0.45 ± 0.03 mm, F = 11.655, P = 0.004). Among the 3 different brimonidine administration methods, intravitreal injection was the most effective in slowing myopia progression, and 4 μg/μL was the most effective among the four different concentrations of brimonidine intravitreal injection tested. The AL and the refraction of the brimonidine intravitreal injection group was significantly shorter or more hyperopic than those of other 2 groups. Four μg/μL produced the smallest difference in AL and spherical equivalent difference values. FD treatment significantly increased the IOP. IOP was significantly lower at 1 day after intravitreal injections which was the lowest in FD eye of intravitreal injection of brimonidine. At day 21, gene expression analyses using RNA-seq showed upregulation of Col1a1 and Mmp2 expression levels by intravitreal brimonidine.

Conclusions

Among the 3 different administration methods, intravitreal injection of brimonidine was the most effective in slowing myopia progression in the FD guinea pig model. Intravitreal brimonidine at 4 μg/μL significantly reduced the development of FD myopia in guinea pigs. Expression levels of the Col1a1 and Mmp2 genes were significantly increased in the retinal tissues of the FD-Inj-Br group.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40662-021-00248-0.

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.

Update and guidance on management of myopia. European Society of Ophthalmology in cooperation with International Myopia Institute

The prevalence of myopia is increasing extensively worldwide. The number of people with myopia in 2020 is predicted to be 2.6 billion globally, which is expected to rise up to 4.9 billion by 2050, unless preventive actions and interventions are taken. The number of individuals with high myopia is also increasing substantially and pathological myopia is predicted to become the most common cause of irreversible vision impairment and blindness worldwide and also in Europe. These prevalence estimates indicate the importance of reducing the burden of myopia by means of myopia control interventions to prevent myopia onset and to slow down myopia progression. Due to the urgency of the situation, the European Society of Ophthalmology decided to publish this update of the current information and guidance on management of myopia. The pathogenesis and genetics of myopia are also summarized and epidemiology, risk factors, preventive and treatment options are discussed in details.

IMI Risk Factors for Myopia

Risk factor analysis provides an important basis for developing interventions for any condition. In the case of myopia, evidence for a large number of risk factors has been presented, but they have not been systematically tested for confounding. To be useful for designing preventive interventions, risk factor analysis ideally needs to be carried through to demonstration of a causal connection, with a defined mechanism. Statistical analysis is often complicated by covariation of variables, and demonstration of a causal relationship between a factor and myopia using Mendelian randomization or in a randomized clinical trial should be aimed for. When strict analysis of this kind is applied, associations between various measures of educational pressure and myopia are consistently observed. However, associations between more nearwork and more myopia are generally weak and inconsistent, but have been supported by meta-analysis. Associations between time outdoors and less myopia are stronger and more consistently observed, including by meta-analysis. Measurement of nearwork and time outdoors has traditionally been performed with questionnaires, but is increasingly being pursued with wearable objective devices. A causal link between increased years of education and more myopia has been confirmed by Mendelian randomization, whereas the protective effect of increased time outdoors from the development of myopia has been confirmed in randomized clinical trials. Other proposed risk factors need to be tested to see if they modulate these variables. The evidence linking increased screen time to myopia is weak and inconsistent, although limitations on screen time are increasingly under consideration as interventions to control the epidemic of myopia.

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.

Lens-induced myopization and intraocular pressure in young guinea pigs

Background

Intraocular pressure (IOP) is an important physiological measure of the eye and is associated with some ocular disorders. We aimed to assess the influence of topical beta blocker-induced IOP reduction on lens-induced axial elongation in young guinea pigs.

Methods

The experimental study included 20 pigmented guinea pigs (age: 2–3 weeks). Myopia was induced in the right eyes for 5 weeks with − 10 diopter lenses. The right eyes additionally received either one drop of carteolol 2% (study group, n = 10) or one drop of artificial tears daily (control group, n = 10), while the contralateral eyes of all animals remained untouched. The outcome parameter was axial elongation during the follow-up period. The mean of all IOP measurements taken during the study was referred to as mean IOP.

Results

Greater axial elongation was associated with a shorter axial length at baseline (P < 0.001; standardized regression coefficient beta: − 0.54) and lens-induced myopization (P < 0.001; beta: 0.55). In the multivariable model, axial elongation was not significantly correlated with the IOP at study end (P = 0.59), the mean IOP during the study period (P = 0.12), the mean of all IOP measurements (P = 0.17), the difference between the IOP at study end and baseline IOP (P = 0.38), the difference between the mean IOP during the study period and the baseline IOP (P = 0.11), or the application of carteolol eye drops versus artificial tears eye drops (P = 0.07). The univariate analysis of the relationships between axial elongation and the IOP parameters yielded similar results. The inter-eye difference between the right eye and the left eye in axial elongation was significantly associated with the inter-eye difference in baseline axial length (P = 0.001; beta:-0.67) but not significantly correlated with the inter-eye difference in any of the IOP-related parameters (all P > 0.25).

Conclusions

In young guinea pigs with or without lens-induced axial elongation, neither the physiological IOP nor the IOP reduced by carteolol, a topical beta-blocker, was associated with the magnitude of axial elongation. These results suggest that IOP, regardless of whether it is influenced by carteolol, does not play a major role in axial elongation in young guinea pigs.

Efficacy of combined orthokeratology and 0.01% atropine solution for slowing axial elongation in children with myopia: a 2-year randomised trial

Eighty Japanese children, aged 8–12 years, with a spherical equivalent refraction (SER) of − 1.00 to − 6.00 dioptres (D) were randomly allocated into two groups to receive either a combination of orthokeratology (OK) and 0.01% atropine solution (combination group) or monotherapy with OK (monotherapy group). Seventy-three subjects completed the 2-year study. Over the 2 years, axial length increased by 0.29 ± 0.20 mm (n = 38) and 0.40 ± 0.23 mm (n = 35) in the combination and monotherapy groups, respectively (P = 0.03). Interactions between combination treatment and age or SER did not reach significance level (age, P = 0.18; SER, P = 0.06). In the subgroup of subjects with an initial SER of − 1.00 to − 3.00 D, axial length increased by 0.30 ± 0.22 mm (n = 27) and 0.48 ± 0.22 mm (n = 23) in the combination and monotherapy groups, respectively (P = 0.005). In the − 3.01 to − 6.00 D subgroup, axial length increased by 0.27 ± 0.15 mm (n = 11) and 0.25 ± 0.17 mm (n = 12) in the combination and monotherapy groups, respectively (P = 0.74). The combination therapy may be effective for slowing axial elongation, especially in children with low initial myopia.

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.

Pharmacogenomic Approach to Antimyopia Drug Development: Pathways Lead the Way

Myopia is the most common eye disorder in the world which is caused by a mismatch between the optical power of the eye and its excessively long axial length. Recent studies revealed that the regulation of the axial length of the eye occurs via a complex signaling cascade, which originates in the retina and propagates across all ocular tissues to the sclera. The complexity of this regulatory cascade has made it particularly difficult to develop effective antimyopia drugs. The current pharmacological treatment options for myopia are limited to atropine and 7-methylxanthine, which have either significant adverse effects or low efficacy. In this review, we focus on the recent advances in genome-wide studies of the signaling pathways underlying myopia development and discuss the potential of systems genetics and pharmacogenomic approaches for the development of antimyopia drugs.