The hypothesis is advanced that the circadian pacemaker in the mammalian suprachiasmatic nucleus (SCN) is composed at the molecular level of a nonredundant double complex of circadian genes (per1, cry1, and per2, cry2). Each one of these sets would be sufficient for the maintenance of endogenous rhythmicity and thus constitute an oscillator. Each would have slightly different temporal dynamics and light responses. The per1/cry1 oscillator is accelerated by light and decelerated by darkness and thereby tracks dawn when day length changes. The per2 /cry2 oscillator is decelerated by light and accelerated by darkness and thereby tracks dusk. These M (morning) and E (evening) oscillators would give rise to the SCN's neuronal activity in an M and an E component. Suppression of behavioral activity by SCN activity in nocturnal mammals would give rise to adaptive tuning of the endogenous behavioral program to day length. The proposition-which is a specification of Pittendrigh and Daan's E-M oscillator model-yields specific nonintuitive predictions amenable to experimental testing in animals with mutations of circadian genes.

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Rhythmicity of the rat suprachiasmatic nucleus (SCN), a site of the circadian clock, develops prenatally. A molecular clockwork responsible for the rhythmicity consists of clock genes and their negative and positive transcriptional-translational feedback loops. The aim of the present study was to discover the development of the clockwork during ontogenesis. Daily profiles of Per1, Per2, Cry1, Bmal1, and Clock mRNA in the SCN of fetuses at the embryonic day (E)19 and of newborn rats at the postnatal day (P)3 and P10 were assessed by the in situ hybridization method. In addition, daily profiles of PER1, PER2, and CRY1 proteins at E19 were assessed by immunohistochemistry. As early as at E19, all the studied clock genes were already expressed in the SCN. However, no SCN rhythm in their expression was detected; Per1, Cry1, and Clock mRNA levels were low, whereas Bmal1 mRNA levels were high and Per2 mRNA levels were medium. Moreover, no rhythms of PER1, PER2, and CRY1 were detectable, as no immunoreactive cells were present at E19. At P3, rhythms in Per1, Per2, Cry1, and Bmal1, but not in Clock mRNA, were expressed in the SCN. The rhythm matured gradually; at P10, the amplitude of Per1, Per2, and Bmal1 mRNA rhythms was more pronounced than at P3. Altogether, the data show a gradual development of both the positive and negative elements of the molecular clockwork, from no detectable rhythmicity at E19 to highly developed rhythms at P10. All mammals exhibit an array of daily behavioral, physiological, hormonal, biochemical, and molecular rhythms. The circadian rhythms persist even in a nonperiodic environment with a period close, but not equal to, 24 h (1). Under natural conditions, the rhythms are entrained to the 24-h day by the light-dark (LD) cycle, mostly by the light period of the day. The circadian rhythms are controlled by a pacemaker located in two suprachiasmatic nuclei (SCN) of the hypothalamus (2). The SCN themselves exhibit rhythms in the uptake of 2-deoxyglucose, a marker of metabolic activity (3), in electrical activity (4), in spontaneous as well as light-induced expression of immediate early genes, namely c-fos, a marker of neural activity (5-9), in the production of many peptides, e.g., of arginine vasopressin (10, 11), and other rhythms.The SCN rhythmicity is due to the SCN molecular clockwork (for review see refs. 12 and 13). Eight mammalian, mostly mouse (m) clock genes cloned so far, namely three period genes (mPer1, -2, and -3), two cryptochrome genes (mCry1 and -2), Clock, Bmal1, and casein kinase 1 epsilon (CK1), are thought to be involved in the clockwork by forming interacting transcriptionaltranslational feedback loops. Briefly, the protein products CLOCK and BMAL1 heterodimers positively activate rhythmic expression of Per and Cry genes. In the cytoplasm, the PER and CRY proteins form complexes important for nuclear translocation of both proteins; the phosphorylation state of PER protein monomers by CK1 may also regulate their cellular location and stability. The nuclear localized PER⅐CRY c...

The pineal hormone melatonin exhibits a circadian rhythm in body fluids. No data are available on melatonin in human milk. The present study was undertaken to determine whether melatonin is detectable in human milk and, if so, whether it exhibits a daily rhythm. Blood and milk were sampled between 1400-1700 h and again between 0200-0400 h from 10 mothers 3-4 days after delivery. Melatonin in both fluids was beyond the limit of detection during the day, whereas during the night, its concentration was 280 +/- 34 pmol/L in serum and 99 +/- 26 pmol/L in milk. Six mothers collected milk after each feeding throughout 1 24-h period within 3 months after delivery. Melatonin in the milk of all subjects exhibited a pronounced daily rhythm, with high levels during the night and undetectable levels during the day. The presence of the rhythm in milk suggests that melatonin fluctuations in milk might communicate time of day information to breast-fed infants.