The suprachiasmatic nucleus (SCN) is the neuroanatomical locus of the mammalian circadian pacemaker. Here we demonstrate that an abrupt shift in the light/dark (LD) cycle disrupts the synchronous oscillation of circadian components in the rat SCN. The phases of the RNA cycles of the period genes Per1 and Per2 and the cryptochrome gene Cry1 shifted rapidly in the ventrolateral, photoreceptive region of the SCN, but were relatively slow to shift in the dorsomedial region. During the period of desynchrony, the animals displayed increased nighttime rest, the timing of which was inversely correlated with the expression of Per1 mRNA in the dorsomedial SCN. Molecular resynchrony required approximately 6 d after a 10 hr delay and 9 approximately 13 d after a 6 hr advance of the LD cycle and was accompanied by the reemergence of normal rest-activity patterns. This dissociation and slow resynchronization of endogenous oscillators within the SCN after an LD cycle shift suggests a mechanism for the physiological symptoms that constitute jet lag.

Serotonergic innervation is believed to inhibit the effects of light on the mammalian circadian timing system. Two anatomical components of this system, the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL), receive serotonergic input from midbrain raphe nuclei. The present studies use retrograde and anterograde tracing as well as neurotoxic lesion techniques to demonstrate that serotonergic cells in the median raphe nucleus (MR) project to the SCN and that serotonergic cells in the dorsal raphe nucleus (DR) project to the IGL. Neurotoxic lesions were also used to investigate the effects of selective serotonin (5-HT) neuron loss in the MR or DR on circadian rhythm parameters of animals entrained to a light/dark cycle or housed in constant light. 5-HT depletion in the MR, but not in the DR, induces an advance in onset, a delay in offset, and a longer duration of the nocturnal running-wheel activity phase. Circadian rhythm disruption in constant light is also more frequent in hamsters with MR lesions. A second experiment was designed to investigate the relationship between lesion location, 5-HT-immunoreactive (5-HT-IR) fiber loss, and behavioral changes. Destruction of 5-HT neurons in the MR causes 5-HT-IR fiber loss in the SCN, which may account for the observed changes in circadian parameters. DR lesions result in 5-HT-IR fiber depletion of the IGL, with no associated changes in the entrained rhythm. The anatomical and behavioral results support the view that a 5-HT projection from the MR mediates 5-HT effects on circadian rhythm regulation in hamsters.

Oscillations of the period (per) and timeless (tim) gene products are an integral part of the feedback loop that underlies circadian behavioral rhythms in Drosophila melanogaster. Resetting this loop in response to light requires the putative circadian photoreceptor cryptochrome (CRY). We dissected the early events in photic resetting by determining the mechanisms underlying the CRY response to light and by investigating the relationship between CRY and the light-induced ubiquitination of the TIM protein. In response to light, CRY is degraded by the proteasome through a mechanism that requires electron transport. Various CRY mutant proteins are not degraded, and this suggests that an intramolecular conversion is required for this light response. Light-induced TIM ubiquitination precedes CRY degradation and is increased when electron transport is blocked. Thus, inhibition of electron transport may "lock" CRY in an active state by preventing signaling required either to degrade CRY or to convert it to an inactive form. High levels of CRY block TIM ubiquitination, suggesting a mechanism by which light-driven changes in CRY could control TIM ubiquitination.In all organisms examined thus far, a feedback loop comprising cycling gene products that negatively regulate their own synthesis lies at the heart of the circadian clock (7). In Drosophila melanogaster, two of the genes that autoregulate in this fashion are period (per) and timeless (tim). The per and tim mRNA levels cycle with a circadian rhythm, such that RNA levels are high at the end of the day (or beginning of the night) and low at the end of the night (13, 36, 37). The two encoded proteins (PER and TIM) also cycle, with protein accumulation starting in the early evening and peaking in the middle of the night (8,15,26,47). As they accumulate in the cytoplasm PER and TIM associate to form heterodimers (20,47). This association stabilizes PER and permits nuclear entry of both proteins (30,31,34). Thus, while TIM does not require PER for stability, it is dependent on PER for nuclear transport. In the nucleus, either one or both proteins repress the synthesis of the per and tim mRNAs. Turnover of the two proteins, TIM in the late night and PER in the early morning, allows RNA levels to rise once again and the cycle continues.Light acts as the primary zeitgeber, or timegiver, to synchronize an organism to its environment. At a molecular level the effect of light is to reduce levels of the TIM protein, an effect that appears to mediate entrainment of the molecular loop and thereby that of behavioral rhythms (15,20,26,47). In fact, in all organisms examined (Neurospora crassa, Drosophila, and mammals), light changes levels of a clock component, indicating that this is a conserved general mechanism (7, 32, 41). The photoreceptors used by the circadian clock are still a subject of considerable debate, but in Drosophila it is clear that the visual system is not required although it is a redundant pathway that can mediate entrainment (14,40,46). The dedicated circadian...