Circadian Gating Research Articles

Neurochemistry of Mammalian Entrainment: Signal Transduction Pathways in the Suprachiasmatic Nuclei

Neurochemical events leading to photic entrainment of circadian rhythms are reviewed. This entrainment pathway includes the retinohypothalamic tract and a glutamate-NMDA receptor (among others) interaction in the suprachiasmatic nuclei (SCN). The model we propose involves an increase in intracellular calcium levels and the activation of specific proteins in SCN neurons, including the Ca 2+ /calmodulin dependent protein kinase (CaM kinase) and phosphatase (calcineurin), other kinases (such as the cGMP-dependent protein kinase, PKG) and enzymes (nitric oxide synthase, NOS), which in turn activate specific transcription factors, in a cascade of events that is controlled both by light and by the circadian clock itself. Although the step at which the circadian gating of this process occurs is unknown, we propose it occurs downstream of glutamate binding, calcium entrance, and NOS activation. We conclude that a promising way of studying the function of the circadian pacemaker is to investigate the signal transduction pathway(s) leading to changes in the SCN, including the biochemical activity of its components.

Temporal and spatial expression of the period gene in the reproductive system of the codling moth.

The authors examined patterns of spatial and temporal expression of Drosophila per gene homologue in the codling moth, Cydia pomonella. Since sperm release in moths is regulated in a circadian manner by an autonomous clock that is independent from the brain, the authors investigated per expression in male reproductive system along with its expression in moth heads. per mRNA is rhythmically expressed with the same phase and amplitude in both tissues under light-dark (LD) conditions. The levels of per mRNA are low during the day, start to increase before lights-off, reach the peak in dark, and decrease after lights-on. In constant darkness (DD), cycling of per mRNA continued in heads with severely blunted amplitude. No cycling of per mRNA was detected in testis in DD. In situ hybridization and immunocytochemistry revealed distinct spatial patterns of per expression in the moth reproductive system. There is no expression of per in cells forming the wall of testes or in sperm bundles. However, per mRNA and protein are rhythmically expressed in the epithelial cells forming the wall of the upper vas deferens (UVD) and in the cells of the terminal epithelium, which are involved in the circadian gating of sperm release. Increase in per mRNA in the UVD coincides with sperm accumulation in this part of the insect reproductive system.

Period Protein Is Necessary for Circadian Control of Egg Hatching Behavior in the Silkmoth Antheraea pernyi

We examined the molecular basis of the circadian control of egg hatching behavior in the silkmoth Antheraea pernyi. Egg hatching is rhythmically gated, persists under constant darkness, and can be entrained by light by midembryogenesis. The time of appearance of photic entrainment by the silkmoth embryo coincides with the appearance of Period (PER) and Timeless (TIM) proteins in eight cells in embryonic brain. Although daily rhythms in PER and/or TIM immunoreactivity in embryonic brain were not detected, a robust circadian oscillation of PER immunoreactivity is present in the nuclei of midgut epithelium. per antisense oligodeoxynucleotide treatment of pharate larvae on the day before hatching consistently abolishes the circadian gate of egg hatching behavior. per antisense treatment also causes a dramatic decrease in PER immunoreactivity in newly hatched larvae. The results provide direct evidence that PER is a necessary element of a circadian system in the silkmoth.

Circadian gating of cell division in cyanobacteria growing with average doubling times of less than 24 hours.

To ascertain whether the circadian oscillator in the prokaryotic cyanobacterium Synechococcus PCC 7942 regulates the timing of cell division in rapidly growing cultures, we measured the rate of cell division, DNA content, cell size, and gene expression (monitored by luminescence of the PpsbAI::luxAB reporter) in cultures that were continuously diluted to maintain an approximately equal cell density. We found that populations dividing at rates as rapid as once per 10 h manifest circadian gating of cell division, since phases in which cell division slows or stops recur with a circadian periodicity. The data clearly show that Synechococcus cells growing with doubling times that are considerably faster than once per 24 h nonetheless express robust circadian rhythms of cell division and gene expression. Apparently Synechococcus cells are able to simultaneously sustain two timing circuits that express significantly different periods.

Chapter 10 Light entrainment and activation of signal transduction pathways in the SCN

The molecular mechanisms by which the circadian clock is entrained are largely unknown; there is considerable and growing evidence that gene expression and macromolecular synthesis are essential for both the function and entrainment of the circadian clock in a wide range of organisms. For this reason, the identification of intracellular signaling pathways and subsequent changes in gene expression in the suprachiasmatic nucleus (SCN) that are activated by light may lead to the elucidation of entrainment mechanisms. The photic and circadian regulation of c-fos gene expression in the SCN is, therefore, important for several reasons. This chapter focuses on the second utility of c-fos induction—that is, the insight it can provide about how light and the circadian clock regulate signal transduction pathways in the SCN. The chapter investigates the signaling pathway involving mitogen-activated protein (MAP) kinase and the serum response element (SRE) on the c-fos promoter to determine how it is regulated by light in the SCN. Using immunoblot analysis of SCN extracts with MAP kinase antibodies, the chapter finds that MAP kinase in the SCN is rapidly phosphorylated on specific threonine and tyrosine residues in response to light, indicating that this signaling pathway is also activated. The further elucidation of the mechanisms of the circadian gating of photic induction of c-fos and jun-B may lead to broader insights about how gene expression is controlled by a mammalian circadian pacemaker.

Regulation of CREB Phosphorylation in the Suprachiasmatic Nucleus by Light and a Circadian Clock

Mammalian circadian rhythms are regulated by a pacemaker within the suprachiasmatic nuclei (SCN) of the hypothalamus. The molecular mechanisms controlling the synchronization of the circadian pacemaker are unknown; however, immediate early gene (IEG) expression in the SCN is tightly correlated with entrainment of SCN-regulated rhythms. Antibodies were isolated that recognize the activated, phosphorylated form of the transcription factor cyclic adenosine monophosphate response element binding protein (CREB). Within minutes after exposure of hamsters to light, CREB in the SCN became phosphorylated on the transcriptional regulatory site, Ser133. CREB phosphorylation was dependent on circadian time: CREB became phosphorylated only at times during the circadian cycle when light induced IEG expression and caused phase shifts of circadian rhythms. These results implicate CREB in neuronal signaling in the hypothalamus and suggest that circadian clock gating of light-regulated molecular responses in the SCN occurs upstream of phosphorylation of CREB.

Effects of photoperiod on the timing of larval wandering in Drosophila melanogaster

ABSTRACT. A rearing method is described that will allow the use of well‐timed larvae of Drosophila melanogaster (Canton S) in physiological investigations. Using a LD 16:8 h cycle and uncrowded rearing conditions, it was shown that photoperiod influences the departure of third instar larvae from the food (i.e. post‐feeding larvae or wandering behaviour). Larval wandering occurs during the scotophase, suggesting a gated event, and does not occur in larvae raised under constant light or in overcrowded cultures reared in a LD 1623 h cycle. Although the data suggest, but do not prove, the existence of circadian gating, the rearing regimen described will provide well‐timed post‐feeding larvae for future studies.

THE CIRCADIAN-GATED TIMING OF BIRTH IN RATS: DISRUPTION BY MATERNAL SCN LESIONS OR BY REMOVAL OF THE FETAL BRAIN

In rats, the hour of birth is gated over a 36-hr temporal window by the phase of the daily light-dark cycle during pregnancy. We have previously shown that the suprachiasmatic nuclei (SCN), the site of a known circadian pacemaker, are oscillating in phase with the prevailing light-dark cycle in the fetus. Since the onset of parturition is governed by the fetal brain in some species, we have speculated that a possible role of a functioning and entrainable circadian clock during fetal life is that it might be involved in the circadian-gated initiation of parturition. First, we showed the circadian gating of birth in our animals by exposing different groups of dams to lighting cycles of opposite phase during pregnancy. Regardless of the phase of the prenatal lighting cycle, the time of birth was gated over a 36-hr temporal window so that most births occurred during the daytime hours. Next, we found that destruction of the maternal SCN (on day 7 of gestation) eliminated the circadian gating; births occurred in a single distribution that peaked in the middle of the 36-hr window. Finally, removal of all the fetal brains from each litter also disrupted the circadian gating of birth; dams of brain-aspirated fetuses no longer exhibited a daytime preference for births. These results show that the maternal SCN are necessary for the normal circadian gating of birth and are also consistent with a role for the fetal brain (and possibly the fetal SCN) in this process. Support by HD14427.

The circadian-gated timing of birth in rats: disruption by maternal SCN lesions or by removal of the fetal brain

The roles of the maternal suprachiasmatic nuclei (SCN) and fetal brain in the circadian-gated timing of birth were studied in rats. The circadian gating of birth was shown by exposing different groups of dams to lighting cycles of opposite phase during pregnancy. Regardless of the phase of the prenatal lighting cycle, the time of birth was gated over a 36-h temporal window so that most births occurred during subjective day. Destruction of the maternal SCN eliminated the circadian gating; births occurred in a single distribution that peaked in the middle of the 36-h window. Removal of the fetal brain also disrupted the circadian gating of birth; dams of brain-aspirated fetuses no longer exhibited a daytime preference for births. These results show that the maternal SCN are necessary for the normal circadian gating of birth and are also consistent with a role for the fetal brain in this process.

Timing of parturition and postpartum mating in norway rats: Interaction of an interval timer and a circadian gate

Two temporal variables have been implicated in the timing of postpartum heat in Norway rats: time of day and time since parturition. The nature of the interaction between these variables is not clearly understood. We extend previous observations on this problem to freely behaving animals continuously videotaped during parturition and postpartum mating in a seminatural habitat. The timing of postpartum mating is best described by a model in which an interval timer is coupled to a circadian gating mechanism. Interactions between circadian and interval timing mechanisms may be widespread in the neuroendocrine control of behavior.

Moth mating periodicity: temperature regulates the circadian gate.

Les lepidopteres modifient leur rythme endogene d'activite sexuelle selon le regime de temperature du jour. Les fluctuations de temperature dans certaines parties du cycle photoperiodique servent de signal pour determiner l'heure de l'accouplement.