Myopia-inhibiting concentrations of muscarinic receptor antagonists block activation of alpha 2a -adrenoceptors in vitro
Investigative Ophthalmology and Visual Science
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© 2018 The Authors. 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 α-adrenoceptors at concentrations ≥0.1 mM, and another potent myopia-inhibiting ligand, mamba toxin-3 (MT3), binds equally well to human mAChR M 4 and α 1A -and α 2A -adrenoceptors. We hypothesized that mAChR antagonists could inhibit myopia via α 2A -adrenoceptors, rather than mAChR M 4 . METHODS. Human mAChR M 4 (M 4 ), chicken mAChR M 4 (cM 4 ), or human α 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 Dual-Glo 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 56☓ 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 α 2 -adrenoceptors, instead of through the mAChR M 4 /cM 4 receptor subtype.