Interactions of choroid and sclera in the myopia model of the chicken

Changes in choroidal thickness are currently used to predict future refractive error development but there is incomplete knowledge about the communication between choroid and sclera. We studied how choroidal thickness changes interact with scleral thickness changes and how the abundance of dopamine (DA)- and all-trans retinoic acid (atRA)-synthetizing choroidal cells varies when choroidal thickness is altered by drugs.

Changes in choroidal thickness were induced by a single intravitreal injection in the morning of the muscarinic antagonist atropine, the DA agonist apomorphine or the DA antagonist spiperone. Thickness of the choroid and the scleral layers was measured by spectral domain optical coherence tomography (SD-OCT). Immunocytochemistry was used to study the distribution of dopamine-synthetizing structures in the choroid and their colocalisation with retinaldehyde dehydrogenase 2 (RADLH2), the key synthetizing enzyme of atRA.

(1) Both atropine and apomorphine increased choroidal thickness over the day while spiperone resulted in a decrease. (2) For apomorphine and spiperone, choroidal thickness changes were positively correlated with thickness changes in both the cartilaginous and fibrous layers of the sclera. With atropine, only the cartilaginous layer thickened. (3) DA was co-localized with RALDH2 in stromal cells in the choroid in a few cases but the numbers of double-stained cells increased massively after drug injections. (4) RALDH2-immunoreactivity (indicating atRA activity) increased, no matter whether the choroid and the sclera thickened or thinned.

Following drug injections, thickness changes of choroid and sclera were correlated and occurred without phase delay. Numbers of DA and RALDH2 co-expressing cells in the choroid increased. Choroidal dopaminergic cells that synthesize atRA appear to act as activators of scleral metabolic activity during both scleral growth stimulation and inhibition.