Myopia is the most common childhood refractive disorder. Atropine inhibits myopia progression, but its mechanism is unknown. Here, we show that myopia-prevention by atropine requires production of nitric oxide (NO). Form-deprivation myopia (FDM) was induced in week-old chicks by diffusers over the right eye (OD); the left eye (OS) remained ungoggled. On post-goggling days 1, 3, and 5, OD received intravitreally 20 µL of phosphate-buffered saline (vehicle), or vehicle plus: NO source: L-arginine (L-Arg, 60–6,000 nmol) or sodium nitroprusside (SNP, 10–1,000 nmol); atropine (240 nmol); NO inhibitors: L-NIO or L-NMMA (6 nmol); negative controls: D-Arg (10 µmol) or D-NMMA (6 nmol); or atropine plus L-NIO, L-NMMA, or D-NMMA; OS received vehicle. On day 6 post-goggling, refractive error, axial length, equatorial diameter, and wet weight were measured. Vehicle-injected goggled eyes developed significant FDM. This was inhibited by L-Arg (ED50 = 400 nmol) or SNP (ED50 = 20 nmol), but not D-Arg. Higher-dose SNP, but not L-Arg, was toxic to retina/RPE. Atropine inhibited FDM as expected; adding NOS-inhibitors (L-NIO, L-NMMA) to atropine inhibited this effect dose-dependently, but adding D-NMMA did not. Equatorial diameter, wet weight, and metrics of control eyes were not affected by any treatment. In summary, intraocular NO inhibits myopia dose-dependently and is obligatory for inhibition of myopia by atropine.

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.

Citation: Carr BJ, Mihara K, Ramachandran R, et al. Myopia-inhibiting concentrations of muscarinic receptor antagonists block activation of alpha 2A -adrenoceptors in vitro. Invest Ophthalmol Vis Sci. 2018;59:2778-2791. https://doi.org/10.1167 PURPOSE. Myopia is a refractive disorder that degrades vision. It can be treated with atropine, a muscarinic acetylcholine receptor (mAChR) antagonist, but the mechanism is unknown. Atropine may block a-adrenoceptors at concentrations ‡0.1 mM, and another potent myopiainhibiting ligand, mamba toxin-3 (MT3), binds equally well to human mAChR M 4 and a 1A -and a 2A -adrenoceptors. We hypothesized that mAChR antagonists could inhibit myopia via a 2A -adrenoceptors, rather than mAChR M 4 . METHODS.Human mAChR M 4 (M 4 ), chicken mAChR M 4 (cM 4 ), or human a 2A -adrenergic receptor (hADRA2A) clones were cotransfected with CRE/promoter-luciferase (CRE-Luc; agonist-induced luminescence) and Renilla luciferase (RLuc; normalizing control) into human cells. Inhibition of normalized agonist-induced luminescence by antagonists (ATR: atropine; MT3; HIM: himbacine; PRZ: pirenzepine; TRP: tropicamide; OXY: oxyphenonium; QNB: 3-quinuclidinyl benzilate; DIC: dicyclomine; MEP: mepenzolate) was measured using the DualGlo Luciferase Assay System. RESULTS.Relative inhibitory potencies of mAChR antagonists at mAChR M 4 /cM 4 , from most to least potent, were QNB > OXY ‡ ATR > MEP > HIM > DIC > PRZ > TRP. MT3 was 563 less potent at cM 4 than at M 4 . Relative potencies of mAChR antagonists at hADRA2A, from most to least potent, were MT3 > HIM > ATR > OXY > PRZ > TRP > QNB > MEP; DIC did not antagonize.CONCLUSIONS. Muscarinic antagonists block hADRA2A signaling at concentrations comparable to those used to inhibit chick myopia ( ‡0.1 mM) in vivo. Relative potencies at hADRA2A, but not M 4 /cM 4 , correlate with reported abilities to inhibit chick form-deprivation myopia. mAChR antagonists might inhibit myopia via a 2 -adrenoceptors, instead of through the mAChR M 4 /cM 4 receptor subtype.

Mutations in prominin-1 (prom1) and photoreceptor cadherin (cdhr1) are associated with inherited retinal degenerative disorders but their functions remain unknown. Here, we used CRISPR-Cas9 to generate prom1-null, cdhr1-null, and prom1 plus cdhr1 double-null Xenopuslaevis and then documented the effects of these mutations on photoreceptor structure and function. Prom1-null mutations resulted in severely dysmorphic photoreceptors comprising overgrown and disorganized disc membranes. Cone outer segments were more severely affected than rods and had an impaired electroretinogram response. Cdhr1-null photoreceptors did not appear grossly dysmorphic, but ultrastructural analysis revealed that some disc membranes were overgrown or oriented vertically within the plasma membrane. Double-null mutants did not differ significantly from prom1-null mutants. Our results indicate that neither prom1 nor cdhr1 are necessary for outer segment disc membrane evagination or the fusion event that controls disc sealing. Rather, they are necessary for the higher-order organization of the outer segment. Prom1 may align and reinforce interactions between nascent disc leading edges, a function more critical in cones for structural support. Cdhr1 may secure discs in a horizontal orientation prior to fusion and regulate cone lamellae size.This article has an associated First Person interview with the first author of the paper.