The period (per) gene, controlling circadian rhythms in Drosophila, is expressed throughout the body in a circadian manner. A homolog of Drosophila per was isolated from rat and designated as rPer2. The rPER2 protein showed 39 and 95% amino acid identity with mPER1 and mPER2 (mouse homologs of per) proteins, respectively. A robust circadian fluctuation of rPer2 mRNA expression was discovered not only in the suprachiasmatic nucleus (SCN) of the hypothalamus but also in other tissues including eye, brain, heart, lung, spleen, liver, and kidney. Furthermore, the peripheral circadian expression of rPer2 mRNA was abolished in SCNlesioned rats that showed behavioral arrhythmicity. These findings suggest that the multitissue circadian expression of rPer2 mRNA was governed by the mammalian brain clock SCN and also suggest that the rPer2 gene was involved in the circadian rhythm of locomotor behavior in mammals.Circadian rhythms in physiology and behavior are governed by the endogenous clock (1, 2). Many circadian rhythms have been described in a diverse range of species, from bacteria to human (3). However, the common molecular mechanism of the circadian clock in diverse species is totally unknown. In mammals, the suprachiasmatic nucleus (SCN) 1 of the anterior hypothalamus has been shown to be the circadian pacemaker (1, 2). Much effort is being directed to identify the master genes that control the circadian rhythm in the SCN. One of the strong candidates is the clock gene, because a mutation in the clock gene results in arrhythmic locomotor behavior (4, 5). The period ( per) gene in Drosophila, which is expressed throughout the body in a circadian manner, regulates the circadian locomotor rhythm (6, 7). Recently two different homologs of Drosophila per gene were reported for mouse and human (8 -11). Though the two mammalian per homologs show circadian mRNA oscillation in the mouse SCN, their functional involvement in the circadian locomotor activity has not yet been reported.To examine whether a mammalian per homolog is involved in the circadian rhythm of locomotor behavior, we cloned a rat per homolog and monitored its circadian expression rhythms in peripheral tissues of SCN-lesioned rats that showed arrhythmic locomotor activity. EXPERIMENTAL PROCEDURESAnimals-Adult male Wistar rats (10 weeks old; 300 -350 g) were obtained from Clea Japan, Inc. (Tokyo) and were housed in a 12 h light-12 h dark cycle (LD12:12; lights on at zeitgeber time (ZT) 0) for at least 1 week before the day of the experiment. A white fluorescent lamp was used as a source of light during the day (150 -200 lux at the level of the cages). In this study, we killed rats in accordance with institutional guidelines.In Situ Hybridization-Animals used for in situ analysis were anesthetized with pentobarbital and were perfused from the left ventricle with 4% paraformaldehyde in phosphate-buffered saline (pH 7.4). Tissues were fixed with 4% paraformaldehyde in phosphate-buffered saline (pH 7.4) for 1 h at room temperature. Then the tissues were embedded...

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