Loss of circadian rhythm and light‐induced suppression of pineal melatonin levels in Cry1 and Cry2 double‐deficient mice

Cryptochrome 1 and 2 (Cry1 and Cry2) are considered essential for generating circadian rhythms in mammals. The role of Cry1 and Cry2 in circadian rhythm expression and acute light‐induced suppression of pineal melatonin was assessed using Cry1 and Cry2 double‐deficient mice (Cry1^?/?/Cry2^?/?) developed from the C3H strain that synthesizes melatonin. We examined the circadian variation of pineal melatonin under a 12:12‐h light–dark (LD) cycle and constant darkness (DD). Light suppression of pineal melatonin concentration was analyzed by subjecting a 30‐min light pulse at the peak phase of melatonin concentration. Wild‐type mice showed significant rhythmicity in pineal melatonin concentration with the highest level at Zeitgeber time 22 (ZT22, where time of light on was defined as ZT0) under LD or ZT18 on the first day of DD. In contrast, Cry1^?/?/Cry2^?/? mice did not show significant circadian rhythmicity, with only a small peak observed at ZT22 in LD. Nevertheless, a significant daily variation could be observed under DD, with a small increase at ZT6 and ZT18 h. Melatonin concentration was significantly suppressed by acute light pulse at ZT22 in wild‐type mice but not in Cry1^?/?/Cry2^?/? mice. The present results suggest that Cry genes are required for regulating pineal melatonin synthesis via circadian and photic signals from the suprachiasmatic nucleus of the hypothalamus (SCN).