Melatonin receptor expression in the cornea and sclera

Abstract

The scornea and sclera have been shown to exhibit circadian rhythms in cellular proliferation, wound healing and extracellular matrix synthesis. The distribution of melatonin Mel 1a and Mel 1c receptors was examined in the cornea and sclera of the Xenopus laevis eye in order to determine whether melatonin may potentially influence the growth and/or development of these ocular tissues. Sections of adult X. laevis eyes were analyzed by immunocytochemistry and confocal microscopy, using antibodies prepared against specific peptide sequences of the Xenopus Mel 1a and Mel 1c receptor proteins. Antibodies were pre- incubated with their appropriate antigenic peptides to control for non-specific labelling. Analysis of the distribution of Mel 1a and Mel 1c receptor immunoreactivity in the Xenopus eye revealed that both the Mel 1a and Mel 1c receptors were located in the outer fibrous layer (OFL) of the sclera, with Mel 1c labelling being the most prominent. Similarly, Mel 1a and Mel 1c (Mel 1c mostly) were also located in cells of the inner fibrous layer (IFL) with Mel 1c being most abundant. The chondrocytes of the cartilaginous layer also appeared to express Mel 1a, Mel 1c, or both receptors. Both Mel 1a and Mel 1c receptor immunoreactivity were observed in the corneal epithelium and endothelium. Whereas the Mel 1a antibody labelled the entire corneal epithelial layer, the Mel 1c antibody labelled only the most superficial layer of epithelial cells. Cell processes of fibroblasts of the corneal stroma were immunoreactive for either Mel 1a or Mel 1c receptors. The identification of Mel 1a and Mel 1c receptors in restricted distributions in the cornea and sclera suggests that melatonin may play a role in the cellular physiology of these ocular tissues.