Gates and oscillators: a network model of the brain clock

The suprachiasmatic nuclei (SCN) control circadian oscillations of physiology and behavior. Measurements of electrical activity and of gene expression indicate that these heterogeneous structures are composed of both rhythmic and nonrhythmic cells. A fundamental question with regard to the organization of the circadian system is how the SCN achieve a coherent output while their constituent independent cellular oscillators express a wide range of periods. Previously, the consensus output of individual oscillators had been attributed to coupling among cells. The authors propose a model that incorporates nonrhythmic "gate" cells and rhythmic oscillator cells with a wide range of periods, that neither requires nor excludes a role for interoscillator coupling. The gate provides daily input to oscillator cells and is in turn regulated (directly or indirectly) by the oscillator cells. In the authors' model, individual oscillators with initial random phases are able to self-assemble so as to maintain cohesive rhythmic output. In this view, SCN circuits are important for self-sustained oscillation, and their network properties distinguish these nuclei from other tissues that rhythmically express clock genes. The model explains how individual SCN cells oscillate independently and yet work together to produce a coherent rhythm.     

Circadian expression of clock genes in the rat eye and brain

The light sensing system in the eye directly affects the circadian oscillator in the mammalian suprachiasmatic nucleus (SCN). To investigate this relationship in the rat, we examined the circadian expression of clock genes in the SCN and eye tissue during a 24 h day/night cycle. In the SCN, rPer1 and rPer2 mRNAs were expressed in a clear circadian rhythm like rCry1 and rCry2 mRNAs, whereas the level of BMAL1 and CLOCK mRNAs decreased during the day and increased during the night with a relatively low amplitude. It seems that the clock genes of the SCN may function in response to a master clock oscillation in the rat. In the eye, the rCry1 and rCry2 were expressed in a circadian rhythm with an increase during subjective day and a decrease during subjective night. However, the expression of Opn4 mRNA did not exhibit a clear circadian pattern, although its expression was higher in daytime than at night. This suggests that cryptochromes located in the eye, rather than melanopsin, are the major photoreceptive system for synchronizing the circadian rhythm of the SCN in the rat.     

Gates and oscillators II: zeitgebers and the network model of the brain clock

Circadian rhythms in physiology and behavior are regulated by the SCN. When assessed by expression of clock genes, at least 2 distinct functional cell types are discernible within the SCN: nonrhythmic, light-inducible, retinorecipient cells and rhythmic autonomous oscillator cells that are not directly retinorecipient. To predict the responses of the circadian system, the authors have proposed a model based on these biological properties. In this model, output of rhythmic oscillator cells regulates the activity of the gate cells. The gate cells provide a daily organizing signal that maintains phase coherence among the oscillator cells. In the absence of external stimuli, this arrangement yields a multicomponent system capable of producing a self-sustained consensus rhythm. This follow-up study considers how the system responds when the gate cells are activated by an external stimulus, simulating a response to an entraining (or phase-setting) signal. In this model, the authors find that the system can be entrained to periods within the circadian range, that the free-running system can be phase shifted by timed activation of the gate, and that the phase response curve for activation is similar to that observed when animals are exposed to a light pulse. Finally, exogenous triggering of the gate over a number of days can organize an arrhythmic system, simulating the light-dependent reappearance of rhythmicity in a population of disorganized, independent oscillators. The model demonstrates that a single mechanism (i.e., the output of gate cells) can account for not only free-running and entrained rhythmicity but also other circadian phenomena, including limits of entrainment, a PRC with both delay and advance zones, and the light-dependent reappearance of rhythmicity in an arrhythmic animal.     

The development of molecular compartmentation in the cerebellar cortex

The cerebellum is subdivided into hundreds of discrete modules defined by their connectivity and molecular signatures. Cerebellar compartmentation arises very early in development through the formation of multiple populations of chemically distinct Purkinje cells that migrate in a coordinated fashion to form parasagittal bands of cells. Different Purkinje cell bands are then innervated by discrete subpopulations of cerebellar afferents. Because of its stereotyped and strikingly beautiful organization the cerebellum is an excellent model in which to explore genetic/epigenetic aspects of pattern formation in the central nervous system.     

Circadian disruption alters mouse lung clock gene expression and lung mechanics

Most aspects of human physiology and behavior exhibit 24-h rhythms driven by a master circadian clock in the brain, which synchronizes peripheral clocks. Lung function and ventilation are subject to circadian regulation and exhibit circadian oscillations. Sleep disruption, which causes circadian disruption, is common in those with chronic lung disease, and in the general population; however, little is known about the effect on the lung of circadian disruption. We tested the hypothesis circadian disruption alters expression of clock genes in the lung and that this is associated with altered lung mechanics. Female and male mice were maintained on a 12:12-h light/dark cycle (control) or exposed for 4 wk to a shifting light regimen mimicking chronic jet lag (CJL). Airway resistance (Rn), tissue damping (G), and tissue elastance (H) did not differ between control and CJL females. Rn at positive end-expiratory pressure (PEEP) of 2 and 3 cmH(2)O was lower in CJL males compared with controls. G, H, and G/H did not differ between CJL and control males. Among CJL females, expression of clock genes, Bmal1 and Rev-erb alpha, was decreased; expression of their repressors, Per2 and Cry 2, was increased. Among CJL males, expression of Clock was decreased; Per 2 and Rev-erb alpha expression was increased. We conclude circadian disruption alters lung mechanics and clock gene expression and does so in a sexually dimorphic manner.     

Transgenic mouse lines expressing synaptopHluorin in hippocampus and cerebellar cortex

We generated six transgenic mouse lines in which synaptopHluorin (SpH), one of green fluorescent protein-based sensors of vesicular exocytosis, was expressed under the control of neuron-specific Thy-1.2 promoter. In situ hybridization study revealed that SpH mRNA was expressed in a broad spectrum of brain regions in four of them, whereas in others it was expressed in the specific regions of the hippocampus. In one particular line, SpH immunoreactivity was specifically observed in the mossy fiber presynaptic terminals of both hippocampus and cerebellar cortex. The fluorescence intensity of these presynaptic terminals was somewhat decreased by acidic buffer superfusion and greatly increased by vesicular neutralization of pH, indicating that the SpH molecules are mainly distributed in the synaptic vesicles. The exocytosis-dependent fluorescence increment was measured upon activation of a single presynaptic terminal. These transgenic lines are expected to facilitate morphological and physiological studies of presynaptic terminals in a variety of regions of the brain.     

Circadian rhythms: mop up the clock!

All circadian clock genes discovered in Drosophila have mammalian counterparts with extensive sequence homology. Similarities and differences have been identified between insect and mammalian oscillators. Recent studies have shed new light on two mammalian clock components: Mop3 and Per2.     

Circadian clock and the concept of homeostasis

Comment on: Mercier Zuber A, et al. Proc Natl Acad Sci U S A 2009; In press.     

Circadian rhythms: per2bations in the liver clock

A master circadian clock resides in the brain and is required to synchronize the clocks in peripheral tissues such as the liver. Until now, it has been unclear how the central clock synchronizes the peripheral ones. New work points to one of the core clock genes, mPer2, as an essential link in this chain.     

Circadian rhythm in adenosine A1 receptor of mouse cerebral cortex

In order to investigate diurnal variation in adenosine A1 receptors binding parameters, Bmax and Kd values of specifically bound N6 - cyclohexyl-[3H]adenosine were determined in the cerebral cortex of mice that had been housed under controlled light-dark cycles for 4 weeks (light on from 7.00 to 19.00 h). Significant differences were found for Bmax values measured at 3-hr intervals across a 24-h period, with low Bmax values during the light period and high Bmax values during the dark period. The amplitude between 03.00 and 18.00 hr was 33%. No substantial rhythm was found in the Kd values. It is suggested that the changes in the density of A1 receptors could reflect a physiologically-relevant mechanism by which adenosine exerts its modulatory role in the central nervous system.     

Species variation in the localisation of esterases in the cerebellar cortex of mouse and bat

A comparative study of the distribution of a simple esterase and acetylcholinesterase in the cerebellar cortex of mouse and bat has been made. The Purkinje layer is intensely positive for simple esterase in both species. The granular and molecular layers showed mild to moderate activity in mouse and intense activity in bat. Acetylcholinesterase in cerebellar layers of bat is more intense than in mouse. In bat cerebellum, acetylcholinesterase is observed in the dendrites of Purkinje cells, but not in their cell bodies. Acetylcholinesterase was not found in Purkinje cells of mouse.     

Circadian clock gene expression in brain regions of Alzheimer 's disease patients and control subjects

Circadian oscillators have been observed throughout the rodent brain. In the human brain, rhythmic expression of clock genes has been reported only in the pineal gland, and little is known about their expression in other regions. The investigators sought to determine whether clock gene expression could be detected and whether it varies as a function of time of day in the bed nucleus of the stria terminalis (BNST) and cingulate cortex, areas known to be involved in decision making and motivated behaviors, as well as in the pineal gland, in the brains of Alzheimer's disease (AD) patients and aged controls. Relative expression levels of PERIOD1 (PER1 ), PERIOD2 (PER2), and Brain and muscle Arnt-like protein-1 (BMAL1) were detected by quantitative PCR in all 3 brain regions. A harmonic regression model revealed significant 24-h rhythms of PER1 in the BNST of AD subjects. A significant rhythm of PER2 was found in the cingulate cortex and BNST of control subjects and in all 3 regions of AD patients. In controls, BMAL1 did not show a diurnal rhythm in the cingulate cortex but significantly varied with time of death in the pineal and BNST and in all 3 regions for AD patients. Notable differences in the phase of clock gene rhythms and phase relationships between genes and regions were observed in the brains of AD compared to those of controls. These results indicate the presence of multiple circadian oscillators in the human brain and suggest altered synchronization among these oscillators in the brain of AD patients.