Dopamine D1 Receptors Contribute Critically to the Apomorphine-Induced Inhibition of Form-Deprivation Myopia in Mice

Purpose: To determine the roles of dopamine D2 receptors (D2Rs) and dopamine D1 receptors (D1Rs) in the inhibition of form-deprivation myopia (FDM) by the nonselective dopamine agonist apomorphine (APO) in D2R-knockout (D2R-KO) and D1R-KO mice. Methods: Retinal layer thicknesses and electroretinograms (ERGs) were analyzed in KO mice and in D2R and D1R antagonist-treated mice. D2R-KO or D1R-KO mice and wild-type (WT) littermates were subjected to form deprivation during postnatal weeks 5 to 8. Both groups were intraperitoneally injected daily with either APO (5 μg/g body weight) dissolved in 1 μg/μL ascorbic acid or vehicle alone. Refraction, vitreous chamber depth (VCD), and axial length (AL), among other parameters, were measured prior to and at the end of the treatment period. Results: The retinal layer thicknesses and ERGs in KO mice were similar to those treated with D2R and D1R antagonists. APO administration in WT mice inhibited the development of FDM by approximately 80%. FDM in D2R-KO mice was inhibited approximately 50% compared with WT mice and was further inhibited by APO to a level similar to that in APO-treated WT mice. FDM development in D1R-KO mice was similar to that in WT mice and was not affected by APO administration. The changes in VCD and AL were consistent with refraction data. Conclusions: In mice, APO-mediated FDM inhibition was abolished by D1R KO but not D2R KO. This indicates the specificity of D1Rs for the pharmacologic inhibitory effect of APO on FDM and a nonessential role of D2Rs in this process in mice.