Project Summary Significance: Myopia (short-sightedness) results from a failure of emmetropization in which eyes grow too long for their optical power. Myopia’s association with sight-threatening complications and projected near-epidemic levels world-wide (50% by 2050) make it critical that effective treatments for slowing myopia progression be developed. As an animal model for myopia, the mammalian guinea pig model shares many of the structural features of the human eyes, including a human-like fibrous sclera. Our proposal takes advantage of our recently developed contact lens-wearing guinea pig model and related choroidal thickness/vascularity and retinal pigment epithelium (RPE)-RNA Seq data to explore pharmacological avenues for improved myopia control under 3 aims: (1) Topical levodopa-carbidoa combination therapy in guinea pig: This combination of a precursor to dopamine, and an inhibitor of its breakdown will be investigated with respect to its ability to inhibit defocus-induced myopia, with underlying mechanisms for their anti-myopia actions (retina &/or choroid- based) explored at the same time. (2) Topical atropine-levodopa-carbidopa combination versus atropine only therapy in guinea pig; (3) Increased accommodative tone is encoded by RPE & inhibited by atropine. The goals for aims 2 and 3 are similar to those in (1) of understanding mechanisms. Planned studies will use advanced ocular imaging and functional testing, as well as routine ocular biometric measurements to track and compare treatment outcomes. Treatment efficacy will be compared across all 3 treatment arms, in terms of both interocular differences and changes over time. Comparing data from experiments in Aims 1 and 2 will provide additional insight into their sites of action, with efficacy data for the combination vs. single ingredient treatments having important translational value. Functional testing also has potential translational significance, given the recognized role of dopamine in retinal processing, connections with circuits involving ipRGCs and circadian rhythms and hints of altered visual processing in myopic young adults. Molecular biology tools applied to timely sampling of RPE from treated and control eyes of the three treatment groups will be used to obtain additional insight into treatment mechanisms and interactions. Established RPE gene expression signatures along with choroidal thickness and morphological changes will be used to evaluate retinal versus choroidal contributions to recorded therapeutic effects (Aims 1 & 2). Aim 3 will provide additional insight into the mechanism of action of topical atropine.