Expression of Melatonin Receptors in Sclera and Cornea of the Vertebrate Eye

Abstract

Abstract: : Purpose: The hormone melatonin is a circadian signal that may act as an endocrine and/or paracrine neurohormone by binding to specific receptors in the eye. The goal of this study was to determine the presence of melatonin receptor expression in the sclera and cornea of the eye, and to identify the cells that express the Mel1a and Mel1c receptor proteins. Methods: Sections of adult Xenopus laevis (African clawed frog) were analyzed by immunocytochemistry and confocal microscopy, using antibodies prepared against specific peptide sequences of the frog Mel1a and Mel1c receptor proteins. Additionally, the distribution of the Mel1c receptor was evaluated in two-week-old chick eyes. Antibodies were pre-incubated with their appropriate antigenic peptides to control for non-specific labeling. Results: Analysis of the distribution of Mel1a and Mel1c receptor subtype immunoreactivity in the frog eye demonstrated that both the Mel1a and Mel1c receptors were located in the outer fibrous layer (OFL) of the sclera, with Mel1c labeling being the most prominent. Mel1a and Mel 1c (Mel1c mostly) were also located in cells of the inner fibrous layer (IFL). The chondrocytes of the cartilaginous layer also appeared to express Mel1a, Mel1c, or both receptors. The OFL of the chick sclera demonstrated Mel1c immunoreactivity which was abolished following pre-incubation of the tissue section with Mel1c peptide. In the frog cornea, both Mel1a and Mel1c receptor immunolabeling was observed in the corneal epithelium and endothelium. Whereas the Mel1a antibody labeled the entire corneal epithelial layer, the Mel1c antibody labeled only the most superficial layer of epithelial cells. Cell processes of fibrocytes of the corneal stroma were immunoreactive for either Mel1a or Mel1c receptors. In the chick retina, Mel1c receptor labeling was observed on the apical and basal surfaces of the corneal epithelium and in the endothelium. Conclusions: The Mel1a and Mel1c receptor proteins are present in the frog sclera and cornea, and show a differential distribution of expression. The Mel1c receptor is expressed in the cells of the OFL of the chick sclera and in the corneal epithelium and endothelium. The presence of melatonin receptors on the sclera and cornea suggest that cells in these tissues may be target sites for melatonin action, and that some cellular activities in these tissues may be influenced by the circadian exposure to melatonin.

Keywords: melatonin • extracellular matrix • sclera