Dopamine Receptor Subtypes Mediate Opposing Effects on Form Deprivation Myopia in Pigmented Guinea Pigs

We reported previously that changes in dopamine receptor (DR) subtype activation modulate spontaneous myopia progression in albino guinea pigs. To determine if DR control of refractive error development is different than in its normal counterpart, we evaluated the contribution of dopaminergic pathways to emmetropization and form deprivation myopia (FDM) progression in pigmented guinea pigs.

Monocular myopia was induced by unilateral form-deprivation (FD). The effects of agonists of D1R (SKF38393) and D2R (quinpirole), the corresponding antagonists (SCH23390 and sulpiride), and vehicle were tested by peribulbar injection around FD or untreated control eyes. High-performance liquid chromatography with electrochemical detection quantified retinal and vitreous dopamine (DA) and 4-dihydroxyphenylacetic acid (DOPAC) levels. Ocular refraction and axial dimensions were measured using eccentric infrared photoretinoscopy (EIR) and A-scan ultrasonography, respectively, initially and after 2 or 4 weeks of treatment.

After treatment with any of these four agents for 2 weeks, retinal and vitreal DA and DOPAC levels were not significantly different in drug- and vehicle-treated eyes. Neither agonism nor antagonism of D1R or D2R activity affected emmetropization. In contrast, D1R activation by SKF38393 inhibited FDM progression, while D2R activation by quinpirole augmented this response. On the other hand, D2R antagonism with sulpiride slowed FDM progression while D1R antagonism with SCH23390 had no effect.

In pigmented guinea pigs, D1R activation inhibited, whereas D2R activation enhanced, FDM. These results closely mirror previous findings in albino animals and offer further evidence that DA and its cognate receptors affect refractive error regulation in guinea pigs.