Melatonin receptor expression in the cornea and sclera

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 nonspecific labelling. Analysis of the distribution of Mel(1a) and Mel(1c) receptor immunoreactivity in the Xenopus eye revealed that both the Mel(1a) and Mellc 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 Mellc being most abundant. The chondrocytes of the cartilaginous layer also appeared to express Mell, Mel1c, or both receptors. Both Mel(1a) and Mellc receptor immunoreactivity were observed in the corneal epithelium and endothelium. Whereas the Mel(1a) antibody labelled the entire corneal epithelial layer, the Mellc 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. (C) 2003 Elsevier Ltd. All rights reserved.