Circadian clock regulates tear secretion in the lacrimal gland

Although diurnal variations have been observed in tear film parameters in various species, the molecular mechanisms that control circadian tear secretion remain unclear. The aim of our study was to evaluate the role of clock genes in the lacrimal gland (LG) in regulation of tear secretion. Tear volume was measured by cotton thread test in core clock genes deficient (Cry1^−/− Cry2^−/−-) mice which are behaviorally arrhythmic. Real-time quantitative RT-PCR was used to examine expression profiles of core clock genes in the LG including Per1, Per2, Per3, Clock, Bmal1. All experiments were performed under a 12 h of light and 12 h of darkness (LD) and constant dark (DD) conditions. Under both LD and DD conditions, diurnal and circadian rhythms were observed in tear secretion of wild-type mice with tear volume increased in the objective and subjective night while disruption in diurnal and circadian variations of tear secretion were found in Cry1^−/− Cry2^−/−-mice. In wild-type mice, the expression level of major clock genes in the LG showed oscillatory patterns under both LD and DD conditions. In contrast, expression clock genes in the lacrimal gland of Cry1^−/− Cry2^−/−- mice showed complete loss of oscillation regardless of environmental light conditions. These findings confirmed the presence of diurnal and circadian rhythms of tear secretion and provided evidences supporting a critical role for the clock in the control of tear secretion.