Melatonin Entrains PER2::LUC Bioluminescence Circadian Rhythm in the Mouse Cornea

PURPOSE. Previous studies have reported the presence of a circadian rhythm in PERIOD2::-LUCIFERASE (PER2::LUC) bioluminescence in mouse photoreceptors, retina, RPE, and cornea. Melatonin (MLT) modulates many physiological functions in the eye and it is believed to be one of the key circadian signals within the eye. The aim of the present study was to investigate the regulation of the PER2:: LUC circadian rhythm in mouse cornea and to determine the role played by MLT. METHODS. Corneas were obtained from PER2:: LUC mice and cultured to measure bioluminescence rhythmicity in isolated tissue using a Lumicycle or CCD camera. To determine the time-dependent resetting of the corneal circadian clocks in response to MLT or IIK7 (a melatonin type 2 receptor, MT2, agonist) was added to the cultured corneas at different times of the day. We also defined the location of the MT2 receptor within different corneal layers using immunohistochemistry. RESULTS. A long-lasting bioluminescence rhythm was recorded from cultured PER2:: LUC cornea and PER2:: LUC signal was localized to the corneal epithelium and endothelium. MLT administration in the early night delayed the cornea rhythm, whereas administration of MLT at late night to early morning advanced the cornea rhythm. Treatment with IIK7 mimicked the MLT phase-shifting effect. Consistent with these results, MT2 immunoreactivity was localized to the corneal epithelium and endothelium. CONCLUSIONS. Our work demonstrates that MLT entrains the PER2:: LUC bioluminescence rhythm in the cornea. Our data indicate that the cornea may represent a model to study the molecular mechanisms by which MLT affects the circadian clock.