Inhibitory Effects of Apomorphine and Atropine and Their Combination on Myopia in Chicks

Purpose: The inhibitory effect of apomorphine on form-deprivation myopia implies a role for dopaminergic pathways in eye growth; however, the effect of apomorphine on lens-induced changes has not been studied. Our study filled this deficiency. After establishing that apomorphine inhibited lens-induced myopia, we investigated whether apomorphine and atropine acted sequentially via the same control pathway or via different parallel pathways.

Methods: This study, conducted in 8-day-old chicks, was comprised of two parts: (1) a comparative study of apomorphine's effect on lens-induced myopia (-15 D), form-deprivation myopia (diffusers), and lens-induced hyperopia (+15 D) and (2) a study of the interacting effects of apomorphine and atropine on lens-induced myopia and form-deprivation myopia. In the first part, dH2O and six apomorphine doses (8 pmole to 800 nmole in log10 steps) were given as 10-microL intravitreal injections in combination with the above visual treatments. Apomorphine was used alone or given with atropine in the second part, which included four drug treatment groups: (1) control (dH2O); (2) 80 pmole of apomorphine; (3) 18 nmole of atropine; and (4) apomorphine + atropine. Additional dH2O injections were used to equalize the number of injections across groups. After 4.5 days of treatment, refractive errors and axial ocular dimensions were measured.

Results: The myopic shifts and axial elongation typical of lens-induced myopia (-15 D lens wear) were inhibited to maxima of 43% (4.5 D) and 52% (0.17 mm) by apomorphine, which, in contrast, enhanced lens-induced hyperopia (refractive error: 114%, 1.55 D; axial length: 134%, 0.16 mm). Inhibitory effects of apomorphine on lens-induced myopia were observed at doses > or = 80 pmole, whereas the doses required to enhance lens-induced hyperopia were 2 log10 units higher. Only a weak inhibitory effect of apomorphine on form-deprivation myopia was observed. Although both apomorphine and atropine inhibited lens-induced myopia, atropine was slightly more effective for the doses compared (refractive error, 53% cf. 32%), and the effect of the combination was not significantly greater than that of atropine alone (refractive error, 59% cf. 53%).

Conclusions: Apomorphine inhibits both types of experimental myopia, which implies the involvement of dopaminergic mechanisms in both phenomena; likewise, cholinergic mechanisms are indicated by the inhibitory effects of atropine on both lens-induced myopia and form-deprivation myopia. We speculate that apomorphine and atropine act at different sites on a common control pathway because the combined effect of apomorphine and atropine was no more than atropine alone.