The presence of an endogenous circadian clock within most mammalian cells is associated with the amazing observation that within a given tissue, these clocks are largely in synchrony with each other. Different tissues use a variety of systemic or environmental cues to precisely coordinate the phase of these clocks. The cornea is a unique tissue in that it is largely isolated from the direct blood supply that most tissues experience, it is transparent to visible light, and it is exposed directly to environmental light and temperature. Melatonin is a hormone that has been implicated in regulation of the cornea’s circadian clocks. Here, we analyze the ability of rhythmic melatonin to entrain corneas ex vivo, and analyze the phase of corneal circadian clocks in vivo both in light: dark cycles and in constant darkness. We find that the presence of a retina from a melatonin-proficient mouse strain, C3Sn, can photoentrain the circadian clocks of a co-cultured mouse cornea, but a retina from a melatonin-deficient strain, C57Bl/6, cannot. Furthermore, pharmacologic blockade of melatonin or use of a retina with advanced retinal degeneration, Pde6b^rd1, blocks the photoentraining effect. Corneal circadian clocks in vivo adopt an advanced phase in C3Sn mice compared with C57Bl/6, but the circadian clocks in the liver are unaffected. This observation is not attributable to a shorter endogenous period of the cornea or behavior between the strains. Some transcripts of circadian genes in the corneas of C3Sn mice also show an advanced phase of expression in a light: dark cycle, while the transcript of Per2 exhibits a light-dependent transient induction at the onset of darkness. We conclude that melatonin acts as a phase modifying factor in a rhythmic manner for the circadian clocks of the cornea.

大多数哺乳动物细胞中存在内源性生物钟与令人惊奇的观察有关，即在给定的组织中，这些生物钟在很大程度上彼此同步。不同的组织使用各种系统或环境线索来精确协调这些时钟的相位。角膜是一种独特的组织，因为它在很大程度上与大多数组织经历的直接血液供应隔离，它对可见光是透明的，并且直接暴露在环境光和温度下。褪黑激素是一种与角膜生物钟调节有关的激素。在这里，我们分析了有节奏的褪黑激素离体携带角膜的能力，并分析了体内角膜生物钟在光照下的相位：黑暗周期和持续黑暗。我们发现，来自褪黑素精通小鼠品系 C3Sn 的视网膜的存在可以对共同培养的小鼠角膜的生物钟进行光诱导，但来自褪黑素缺乏品系 C57Bl/6 的视网膜则不能。此外，药物阻断褪黑激素或使用患有晚期视网膜变性的视网膜，Pde6b ^rd1，阻止光夹带效应。与 C57Bl/6 相比，C3Sn 小鼠体内的角膜生物钟采用晚期阶段，但肝脏中的生物钟不受影响。这种观察不能归因于较短的角膜内源期或菌株之间的行为。C3Sn 小鼠角膜中的一些昼夜节律基因转录本也显示出在光：暗循环中的高级表达阶段，而Per2的转录本在黑暗开始时表现出光依赖性瞬时诱导。我们得出结论，褪黑激素以有节奏的方式作为角膜生物钟的相位调节因子。