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...

No abstract

Daily rhythms are a ubiquitous feature of living systems. Generally, these rhythms are not just passive consequences of cyclic fluctuations in the environment, but instead originate within the organism. In mammals, including humans, the master pacemaker controlling 24-hour rhythms is localized in the suprachiasmatic nuclei of the hypothalamus. This circadian clock is responsible for the temporal organization of a wide variety of functions, ranging from sleep and food intake, to physiological measures such as body temperature, heart rate and hormone release. The retinal circadian clock was the first extra-SCN circadian oscillator to be discovered in mammals and several studies have now demonstrated that many of the physiological, cellular, and molecular rhythms that are present within the retina are under the control of a retinal circadian clock, or more likely a network of hierarchically organized circadian clocks that are present within this tissue.

In mammals a subpopulation of retinal ganglion cells are intrinsically photosensitive (ipRGCs), express the photopigment melanopsin, and play an important role in the regulation of the nonimage-forming visual system. We have recently reported that melanopsin mRNA and protein levels in the rat retina are under photic and circadian control. The aim of the present work was to investigate the mechanisms that control melanopsin expression in the rat retina. We discovered that dopamine (DA) is involved in the regulation of melanopsin mRNA, possibly via dopamine D2 receptors that are located on these ipRGCs. Interestingly, we also discovered that pituitary adenylate cyclase-activating peptide (PACAP) mRNA levels are affected by DA. Dopamine synthesis and release in the retina are regulated by the rod and the cone photoreceptors via retinal circuitry; our new data indicate that DA controls melanopsin expression, indicating that classical photoreceptors may modulate the transcription of this new photopigment. Our study also suggests that DA may have an important role in mediating the light signals that are used for circadian entrainment and for other responses that are mediated by the nonimage-forming visual system.