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