Abstract
Purpose: :
Dopamine has been identified as a candidate retinal messenger to prevent excessive axial elongation and myopia in chicken eyes. This study investigated effects of local administration of apomorphine (apo, a subtype-nonspecific dopamine-receptor agonist) on form deprivation and defocus-induced myopia in guinea pigs.
Methods: :
One hundred guinea pigs (aged 3 weeks) were randomly assigned to monocularly-deprived facemask (MDF), defocus (induced by a -4.00D lens) and control groups. A subconjunctival injection of 100µl apomorphine at doses of log 10 steps was performed daily for 11 days for the MDF (2.5 to 250ng) and defocus groups (250 to 25000ng). The control groups included normal control (free of any treatment), 250ng apo-only and vehicle-only (0.1% ascorbic acid) with the latter 2 groups matching the experimental groups in volume and frequency of the local injection. Refraction, corneal curvature and axial components of the eye were measured prior to and at the end of the experiment.
Results: :
After 11 days of treatment, the myopia induced by MDF and hyperopic defocus was -4.00D and -3.77D, respectively. The corresponding increase in vitreous length was 0.12mm in the MDF-only group and 0.10mm in the defocus-only group (p ≤ 0.001 between 2 eyes of same animals in each group). When compared to eyes treated with MDF-only, the amount of myopia decreased by 19%, 22%, 54% and 84% respectively in eyes treated with vehicle-MDF, 2.5ng apo-MDF, 25ng apo-MDF and 250ng apo-MDF. Accordingly, axial elongation of the vitreous chamber decreased by 12%, 22%, 80% and 99% respectively in the same group order from vehicle-MDF to 250ng apo-MDF. All defocused eyes treated with various doses of apomorphine still developed myopia with an increased vitreous length. Other biometric results were not affected by the local injection of apomorphine in eyes treated with MDF or defocus.
Conclusions: :
Apomorphine can inhibit form-deprivation myopia in guinea pigs when locally administered and the degree of myopia inhibited by apomorphine is dose-dependent. This suggests that a dopaminergic mechanism plays some role in visually-regulated emmetropization of guinea pig eyes. However, the efficacy of apomorphine on minus lens-induced myopia in guinea pigs is not sure and requires a further confirmation based on a more specific design in sample size, time points and titration of the drug concentrations.
Keywords: myopia • dopamine • receptors: pharmacology/physiology