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Objective The dopamine (DA) system in the retina is critical to normal visual development as lack of retinal DA signalling may contribute to myopic development.The involvement of DA in myopic development is complex and may be different between form deprivation and hyperopic defocus.This study aims to explore the possible different roles of DA signaling between the FDM and defocus-induced myopia using guinea pigs.Methods This study evaluated effects of a non-selective DA receptor agonist, apomorphine (APO) on refractive development in guinea pigs treated with form deprivation or hyperopic defocus (from age of 21 to 32 days).Changes in retinal concentration of DA and its metabolites (DOPAC) were also measured in the 2 animal models in order to assess the level of DA involvement in each of the models (the less the change, the lower the involvement).Results (1) Similar myopic degree was induced in both the deprived and defocused eyes (-4.06 D vs.-3.64 D) at 11 days of the experiment.(2)DA and DOPAC levels were reduced in the deprived eyes but did not change significantly in the defocused eyes compared to the fellow and normal control eyes.(3) A subconjunctival injection of APO (from day 1 to 11) at concentrations from 2.5 ng to 250 ng/eye reduced axial myopia treated by form deprivation in a concentration-dependent manner.(4) By contrast, the effectiveness of APO treatment (at concentration from 2.5 ng up to 25 mg/eye) was significantly lower in inhibition of the defocus-induced myopia and the associated axial elongation.Conclusion DA signaling may play a more critical role in form deprivation myopia than in defocus-induced myopia, raising a question whether the mechanisms of DA signaling are different under these two types of experimental myopia.