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J Appl Physiol 92: 401–408, 2002;
10.1152/japplphysiol.00836.2001.
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Functional Genomics of Sleep and Circadian Rhythm
Invited Review: A neural clockwork for encoding circadian time
ERIK D. HERZOG1 AND WILLIAM J. SCHWARTZ2
Department of Biology, Washington University, St. Louis, Missouri 63130; and 2Department of
Neurology, University of Massachusetts Medical School, Worcester, Massachusetts 01655
1
suprachiasmatic nucleus; period; oscillator; pacemaker; photoperiod
THE EARTH’S DAILY ROTATION about its axis has imposed
potent selective pressures on organisms. The fundamental adaptation to the environmental day-night cycle is an endogenous 24-h clock that regulates biological
processes in the temporal domain. This clock coordinates
physiological events around local (geophysical) time, optimizing the economy of biological systems and allowing
for a predictive, rather than purely reactive, homeostatic
control. Circadian clocks contribute to the regulation of
sleep and reproductive rhythms, seasonal behaviors,
and celestial navigation. The practical importance of
human circadian rhythmicity, as well as its consequences for health and disease, is now being realized.
INVESTIGATING CIRCADIAN RHYTHMICITY AND
LOCALIZING A PACEMAKER TO MAMMALIAN BRAIN
Many biological activities are restricted to specific
times of day. In the absence of external timing cues,
Address for reprint requests and other correspondence: W. J.
Schwartz, Dept. of Neurology, Univ. of Massachusetts Medical
School, 55 Lake Ave. North, Worcester, MA 01655 (E-mail:
[email protected]).
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some of these processes remain rhythmic (“free run”)
with ⬃24-h (circadian) periods. The features of these
self-sustaining oscillations have suggested the existence of an endogenous timekeeping mechanism. The
circadian pacemaker receives input (afferent) pathways for synchronization (entrainment) to light-dark
cycles and expresses its rhythmicity through output
(efferent) pathways. The pacemaker works as a clock
because its endogenous period is adjusted to the external 24-h period, primarily by light-induced phase shifts
that reset the pacemaker’s oscillation. Advances or
delays occur because the pacemaker is differentially
sensitive to light exposure at different times in its
free-running circadian cycle; this rhythm of light sensitivity can be quantified as a “phase-response curve”
(PRC). Variations in photic sensitivity, in concert with
changes in the pacemaker’s endogenous period and
amplitude, can dramatically affect the temporal sequencing of clock-controlled events.
In mammals, a circadian pacemaker has been localized to the suprachiasmatic nucleus (SCN). The body of
evidence identifying the SCN as the master circadian
pacemaker in mammals is so multidisciplinary in na-
8750-7587/02 $5.00 Copyright © 2002 the American Physiological Society
401
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Herzog, Erik D., and William J. Schwartz. Invited Review: A
neural clockwork for encoding circadian time. J Appl Physiol 92:
401–408, 2002; 10.1152/japplphysiol.00836.2001.—Many daily biological
rhythms are governed by an innate timekeeping mechanism or clock.
Endogenous, temperature-compensated circadian clocks have been localized to discrete sites within the nervous systems of a number of organisms. In mammals, the master circadian pacemaker is the bilaterally
paired suprachiasmatic nucleus (SCN) in the anterior hypothalamus.
The SCN is composed of multiple single cell oscillators that must synchronize to each other and the environmental light schedule. Other
tissues, including those outside the nervous system, have also been
shown to express autonomous circadian periodicities. This review examines 1) how intracellular regulatory molecules function in the oscillatory
mechanism and in its entrainment to environmental cycles; 2) how
individual SCN cells interact to create an integrated tissue pacemaker
with coherent metabolic, electrical, and secretory rhythms; and 3) how
such clock outputs are converted into temporal programs for the whole
organism.
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INVITED REVIEW
ASSAYING SCN CIRCADIAN RHYTHMICITY
To begin to understand the clockwork’s mechanisms,
reliable indicators are needed as markers for the presence, phase, and period of the circadian pacemaker’s
oscillation. The use of a variety of observable clockcontrolled outputs allows greater insights. For example, a manipulation that eliminates an overt rhythm
might act by inactivating the pacemaker or, equally
J Appl Physiol • VOL
likely, by uncoupling the measured output from the
still-oscillating pacemaker (loss of the clock’s “hands”
rather than damage to its “gears”). Given that monitoring the entire pacemaking mechanism is currently
impossible, distinguishing between an arrested or uncoupled clock is best assayed from multiple outputs. On
the other hand, alterations in the free-running period
of a rhythm must reflect changes in pacemaker behavior, either directly or indirectly via an input pathway.
Neuronal firing rate. Circadian rhythms of single- or
multiple-unit electrical activities in the SCN have been
recorded in vivo and in vitro in hypothalamic slices,
slice cultures, and dissociated cell cultures (see Ref. 98
for further references). In nocturnal and diurnal rodents, the firing rate is high during the subjective day
and low during the subjective night. Recording electrical activity has been an extremely useful assay of SCN
rhythmicity, especially for in vitro studies, but it is
important to note that Na⫹-dependent action potentials are not a part of the pacemaker’s actual oscillatory apparatus; the internal timekeeping mechanism
continues to run unperturbed in the presence of tetrodotoxin, which blocks these action potentials (75, 88).
Because electrical discharge represents one of the
pacemaker’s outputs and because the nature of the
coupling between the circadian pacemaker and firing
rate is not known, there may be some experimental
conditions in which the patterns of SCN electrical
activity might not reflect the state of the pacemaker’s
oscillation. Importantly, it is not known how firing rate
relates to the coding of efferent signals sent by the SCN
to other brain regions.
Energy metabolism. SCN metabolic activity was the
first property of the nucleus reported to exhibit circadian rhythmicity (72). These experiments utilized an
autoradiographic method for the in vivo determination
of the rates of glucose utilization of individual structures within the brain by using tracer amounts of
2-deoxy-D-[1-14C]glucose. This assay is useful because
regional brain functional activity is closely coupled to
regional brain energy utilization, which is dependent
on the continuous provision of glucose (80). Like electrical activity, glucose utilization is high during the
subjective day and low during the subjective night in
both nocturnal and diurnal animals, and the rhythm
persists in vitro. The deoxyglucose method has been a
helpful assay of SCN rhythmicity in vivo, e.g., in fetal
animals, but it too has limitations. A measured level of
glucose utilization is not a unitary, indivisible quantity
but a dynamic summation of all the individual metabolic costs entailed by an assortment of cellular tasks.
Autoradiographic images fail to reflect this underlying
complexity, and the spatial resolution of the technique
ordinarily does not extend to the level of individual
cells.
Neuropeptide levels. Rhythms of neuroactive peptides synthesized in the SCN have been measured in
the cerebrospinal fluid and in tissues (32). The circadian rhythm of AVP levels has been extensively studied. Levels in the cerebrospinal fluid are highest during
the subjective day of both nocturnal and diurnal ani-
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ture that the strength of this functional localization is
unsurpassed by that of any other structure in the
vertebrate brain (40). The SCN also appears to function as a seasonal clock underlying photoperiodic time
measurement (74). SCN neurons (about 16,000/paired
nucleus) are among the smallest in the brain and are
very densely packed. In the rat, as a result of a combination of various anatomic techniques, the nucleus is
shown to be composed of distinct dorsomedial (“shell”)
and ventrolateral (“core”) subdivisions (44). Arginine
vasopressin (AVP) or somatostatin is synthesized
within some of the dorsal neurons, whereas some ventral neurons synthesize vasoactive intestinal polypeptide (VIP) or gastrin-releasing peptide and are innervated by serotonergic and visual inputs. In Syrian
hamsters, a cluster of cells expressing calbindin seems
to be crucial for the expression of locomotor rhythmicity (45). Most SCN axons terminate locally within the
SCN amid myriad synaptic interactions. ␥-Aminobutyric acid (GABA) appears to be the most plentiful
substance and is probably a neurotransmitter in all
SCN neurons (55).
Over the past few years, analyses of induced and
spontaneous mutations, gene sequence homologies,
and protein-protein interactions have identified molecular processes within SCN cells that are likely to constitute the actual oscillatory mechanism of the pacemaker (50, 67). It is believed that genes at the clock’s
core are functioning as autoregulatory feedback loops,
with oscillating levels of nuclear proteins negatively
regulating the transcription of their own mRNAs. To
date, mutations in the Doubletime (or tau), BMAL1 (or
MOP3), Clock, Per1, Per2, and the two Cryptochrome
genes have been shown to alter or abolish mammalian
circadian rhythmicity [see Albrecht (1a)]. Implicit in
the emerging molecular models is the assumption that
individual SCN cells contain their own circadian oscillators. Cell autonomous clocks have been definitively
demonstrated in fully isolated invertebrate neurons
(53) and implicated in cells from the retinas of amphibians and birds (7) and the pineals of birds, reptiles, and
fish (16). SCN neurons, when cultured at low density,
express firing rate rhythms with different periods on
multielectrode arrays (1, 26, 27, 46, 48, 88). This result
shows that the SCN is a multioscillator system and
strongly suggests that individual SCN cells can act as
competent circadian pacemakers. Furthermore, SCN
neurons from Doubletime (48) and Clock (26) mutant
animals display altered periodicities, supporting a role
for these molecules in an intracellular timekeeping
mechanism.
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INVITED REVIEW
TRANSLATING PRCS TO THE PHYSIOLOGY OF
PACEMAKER INPUTS
Because the 24-h alternation of light and darkness is
the most obvious indication of the Earth’s daily rotation, it is not surprising that evolution has selected
J Appl Physiol • VOL
ambient light intensity as the preeminent signal for
entraining circadian rhythms to local geophysical time.
In mammals, the retina is required for photic entrainment, but the responsible visual system is anatomically and physiologically distinct from the visual systems for oculomotor function and image formation (56).
The “circadian” system includes a specialized photoreceptive mechanism that may rely on a novel nonrod,
noncone opsin; a subset of ganglion cells that forms a
monosynaptic pathway to the SCN [retinohypothalamic tract (RHT)]; and a population of SCN neurons
that appears to function as “luminance” detectors, with
electrophysiological responses to light that are sustained, proportional to light intensity, and elicited from
large receptive fields lacking a retinotopic organization
(99). These unique features account for the preserved
circadian responses of some apparently “sightless” animals, including the blind mole rat, retinally degenerate mouse mutants, and even some blind people who
lack pupillary light reflexes and conscious light perception.
Multiple lines of evidence suggest that the excitatory
amino acid glutamate is the primary RHT neurotransmitter responsible for mediating the circadian actions
of light, with NMDA, AMPA/kainate, and metabotropic
receptors all seeming to play some role in mediating
glutamate’s postsynaptic effects. Intracellular elevation of both Ca2⫹ and cAMP leads to the phosphorylation of the Ca2⫹/cAMP response-element binding protein (CREB, a transcription factor), and, in the SCN,
CREB becomes phosphorylated after photic or glutamatergic stimulation during the subjective night (but
not during the subjective day). These data have raised
the working hypothesis that light induces a cascade
involving the release of glutamate, stimulation of ionotropic receptors, influx of Ca2⫹, activation of nitric
oxide synthase and serine/threonine protein kinases,
phosphorylation of CREB, and regulation of target
gene transcription, e.g., fos, jun, and per (18, 97). As
shown at least for c-fos, the threshold, magnitude, and
phase dependence of gene activation correlate with
light-induced phase shifts of overt circadian rhythmicity. For some of these steps, however, there is a paucity
of data on their relationship to one another or to phase
shifting. Moreover, at least in rats and hamsters
(where SCN subdivisions are clearly defined), the photic and circadian regulation of fos, jun, and per genes
appears to occur in separate cell populations (20, 73,
97), implying that the function of these genes may also
be cell specific.
Further complications to the simplicity of an idealized linear input pathway have been discovered. There
is evidence that light-induced phase advances and delays may be mediated by different sets of neurotransmitters and second-messenger networks (18). The
pacemaker’s photic input appears to be gated by a
rhythm of visual sensitivity in the eyes and modulated
by the secretion of RHT peptides (substance P, pituitary adenylate cyclase-activating peptide; Refs. 21, 22)
and of serotonin from the midbrain raphe (5-HT1B;
Refs. 61, 64). Finally, nonphotic stimuli (e.g., benzodi-
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mals, and the rhythm persists in hypothalamic explants and dissociated cell cultures. Identifying such
peptide rhythms is an important step in analyzing
SCN rhythmicity because they provide windows on
clock-controlled gene expression (transcription and
protein translation and degradation). If the biochemistry of a circadian-regulated process is known, then the
molecules involved in its control should have a clear
relationship to the oscillatory mechanism of the pacemaker itself. The demonstration that two peptide
rhythms can be simultaneously measured in slice cultures (AVP and VIP) suggests the potential power of
this approach (77).
Transcript levels. Within the SCN, AVP mRNA and
peptide levels exhibit circadian rhythmicity, and nuclear run-on analysis has demonstrated that mRNA
abundance is regulated by transcription (9). Circadian
regulation of the AVP gene is likely mediated by the
binding of two transcription factors, CLOCK and
BMAL1, to an E box within its promoter (35). Posttranscriptional mechanisms may also contribute to the
temporal pattern of SCN AVP expression (8, 69). Another clock-controlled gene within the SCN encodes the
bZIP transcription factor albumin D-element binding
protein (DBP). Like AVP, its transcription is regulated
by the binding of CLOCK to an E-box enhancer (68,
95). Animals lacking AVP or DBP exhibit electrophysiological or behavioral circadian rhythms of decreased
amplitude (5, 17, 31).
It is clear that a significant percentage of the genome is
modulated by the circadian clock. In plants and flies, an
estimated ⬃6% of transcripts oscillate on a circadian
basis (24, 85). Such a survey has recently been reported
for the mouse liver, where roughly 3% of the screened
genes were modulated on a circadian basis (41). Several
laboratories have now taken advantage of rhythms in
transcription to introduce reporter gene constructs for
real-time monitoring of gene activity (42, 94, 96). Transgenic SCN carrying the promoter for mouse Per1 linked
to the gene for luciferase or enhanced green fluorescent
protein (GFP) show robust circadian rhythms in light
emission in vitro and in vivo. Because the Per1 gene is
ubiquitously expressed in the body, these constructs have
been used to test for circadian regulation of Per1 in
cultured tissues. Isolated explants from the lung, liver,
and kidney have been shown to express a circadian
rhythm in Per-driven bioluminescence. The kinetics of
such reporters allow for monitoring changes in gene activity that are slower than ⬃1.5 h. Fluorescence from the
enhanced GFP reporter is bright enough to be measured
in individual cells but requires repeated illumination
with potentially damaging ultraviolet radiation. Bioluminescence from luciferase is much dimmer and requires
delivery of substrate (luciferin) to the region under study.
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INVITED REVIEW
azepines, cage changes, social interactions) generate a
PRC essentially opposite (180° out of phase) to the
photic PRC (57). Nonphotic effects appear to be
strongly species specific, and the critical physiological
variable responsible for their mediation remains unidentified. Elucidating the substrates of nonphotic input pathway(s) should help to clarify pacemaker mechanisms, especially because nonphotic stimuli depress
the normally high level of SCN Per1 expressed during
the subjective day (29, 51). Much research has focused
on the likely roles of serotonin-containing (5-HT1A/7)
and neuropeptide Y-containing afferents to the SCN
(the latter from the intergeniculate leaflet of the thalamus) (54).
ASSEMBLING A MULTICELLULAR CIRCADIAN
PACEMAKER IN THE SCN
J Appl Physiol • VOL
CONVERTING MULTIPLE PACEMAKER OUTPUT
MECHANISMS INTO TEMPORAL PROGRAMS
The SCN governs a wide array of rhythms, from
biosynthetic to behavioral, exhibiting a range of waveforms and phases different from the central oscillation
that drives them. Because the pacemaker’s outputs are
coupled to effector cells via synaptic transmission and
hormonal secretion and indirectly through the rhythmic regulation of behavior, the fidelity of the circadian
signal might be altered and overt rhythms could differ
substantially from their underlying cellular and molecular oscillations. The timing and waveforms of manifest rhythms are also shaped by noncircadian factors,
as exemplified by the regulation of sleep (in which
consolidated sleep results from the nonadditive interaction of a circadian process with a homeostatic factor
related to the duration of preceding wakefulness) (14a,
15, 92). Further complicating the conceptual and experimental analysis of this problem is the existence of
nested feedback loops, by which some of the pacemaker’s rhythmic outputs may modulate the behavior
of the pacemaker itself. As examples, rhythmic locomotion on a running wheel appears to modify the pacemaker’s free-running circadian period, whereas pineal
melatonin secretion (also driven by the SCN) has
phase-resetting and entraining effects on overt rhythmicity via melatonin receptors on SCN neurons
(30, 47).
Of the multisynaptic pathways from the SCN to
distal targets, the best studied is the one regulating
pineal melatonin synthesis. The paraventricular nucleus of the hypothalamus receives extensive SCN projections, and the neurotransmitter GABA may form a
necessary link in this pathway (36, 37). The circuit is
completed by paraventricular nucleus projections to
preganglionic neurons of the sympathetic nervous sys-
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In vivo and in cultured explants, SCN cells synchronize their circadian rhythms to one another. The genotype-specific free-running period characteristically
expressed by whole animals may represent the mean
period arising from the coupling of these multiple SCN
cellular oscillators (26, 27, 48, 49); this contrasts with
many other rhythmic tissues (e.g., the heart), in which
the faster cells set the rate. The SCN’s intercellular
coupling mechanism is unknown. Interest has focused
on GABA (19, 46, 86), nonsynaptic/dendritic exocytosis
(10), and chemical and electrical low-resistance interneuronal junctions (11, 28, 34, 76, 78). Glial cells are
also likely to be an important part of any synchronizing
mechanism; intercellular Ca2⫹ waves can travel long
distances across cultured SCN astrocytes, presumably
due to gap junctions (84, 89).
Based initially on behavioral data, the rodent circadian clock has been modeled as a complex pacemaker
consisting of two mutually coupled oscillators (62).
Variability in the phase relationship between these
two oscillators could account for several circadian properties, including the ability to measure seasonal
changes of day length, with a morning oscillator (M)
accelerated by light and synchronized to dawn, and an
evening oscillator (E) decelerated by light and synchronized to dusk. There is evidence to suggest that the
circadian system is functionally organized into two
such oscillating components. Light pulses have been
reported to cause independent phase shifts of the
evening onset and morning offset of two circadian
rhythms dependent on the SCN, one biochemical [pineal N-acetyltransferase activity (NAT)] and the other
behavioral (locomotion) (for references, see Refs. 12
and 74). Recordings from cultured SCN explants have
demonstrated that the vast majority of neurons reach
their peak of firing together, with a smaller population
that is ⬃12 h out of phase (25, 58). Recording of
Per1-driven GFP fluorescence has shown similar results but with cells assuming perhaps more phase
relationships (65). Recently, dual circadian oscillations
have been demonstrated in SCN tissue isolated in vitro
(33). In slices from Syrian hamsters, the circadian
rhythm of SCN multiunit neuronal activity exhibits
distinct morning and evening peaks when the slices are
cut in the horizontal plane. The morning peak follows
projected dawn, whereas the evening peak occurs
around projected dusk. The two peaks are differentially affected by changes in the antecedent photoperiod, and they are shifted independently in a phasedependent manner after stimulation with glutamate.
Assuming that M and E oscillators really do exist, it
is not known whether they are a property of individual
SCN cells or instead emerge from an intercellular
(network) interaction within SCN tissue. The latter
mechanism accounts for the phenomenon known as
“splitting” (seen especially, but not exclusively, in Syrian hamsters maintained in constant light), in which
an animal’s single daily bout of locomotor activity dissociates into two components that each free run with
different periods until they become stably coupled at
180° (⬃12 h) apart. This behavior appears to be a
consequence of a paired SCN that has become reorganized into two oppositely phased, left- and right-sided
circadian pacemakers (14). However, most current
data suggest that these “split” oscillators are unlikely
to represent M and E.
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INVITED REVIEW
J Appl Physiol • VOL
Fig. 1. Model of the organization of the mammalian circadian system
from the master pacemaker [suprachiasmatic nucleus (SCN)] to its
various targets in the brain and body. The SCN, in the hypothalamus, contains cells that are competent, sustained circadian oscillators. In vivo, they are synchronized to one another, with the ensemble showing maximum electrical and metabolic activity during the
day. Efferent pathways, both neural and humoral, terminate on
structures that are either weakly rhythmic (bottom) or arrhythmic
(top). The arrhythmic structures most likely receive inhibitory input
from the SCN and are thus driven to produce rhythms in anti-phase
to the SCN. The weakly rhythmic structures studied thus far have
all shown activity that is high during the night. It is not clear
whether their damped rhythmicity is intrinsic to single cells or the
result of rhythmic cells drifting out of phase from one another. This
multioscillator system can drive rhythms at various phases and with
different amplitudes, providing a temporal control of physiology and
behavior that anticipates daily events and adapts to changes in
environmental cycles.
(with an ⬃100-fold nocturnal increase in rat) and, with
the recent cloning of the NAT gene, the best understood biochemical rhythm in mammals (39). In rodents,
a mechanism involving protein kinase A and the phosphorylation of CREB induces NAT gene transcription
and the consequent elevation of mRNA content, protein level, and enzymatic activity. On the other hand, a
posttranscriptional mechanism, proteosomal proteolysis, appears to be the dominant NAT regulatory mechanism in ungulates, in which a modest nocturnal increase of NAT mRNA levels (⬃1.5-fold) does not
account for the ⬃10-fold increase of NAT enzymatic
activity. Another well-analyzed cellular rhythm is in
the rat liver, in which DBP mRNA and protein levels
exhibit a circadian rhythm, with a ⬃100-fold magnitude protein oscillation that peaks ⬃2 h after lights off
in a light-dark cycle (91). Rhythmic DBP levels appear
responsible for circadian transcription of the cholesterol 7-␣-hydroxylase gene (43), which encodes the
rate-limiting enzyme for the conversion of cholesterol
to bile acids [maximal during the dark (feeding) phase
in nocturnal rats].
CODA
The circadian timekeeping system is a unique and
powerful model for investigating the cellular and molecular mechanisms that underlie behavioral state con-
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tem (in the intermediolateral column of the spinal
cord) and postganglionic cell bodies in the superior
cervical ganglion that noradrenergically synapse in the
pineal gland. Other SCN efferent fibers contain AVP or
VIP and have been implicated in the circadian regulation of reproductive (59, 60) and adrenal (6, 38) function.
On the other hand, the rhythm of locomotor activity
appears to depend on an SCN-secreted, diffusible substance; locomotor rhythmicity can be reestablished in
SCN-lesioned hamsters by transplants of SCN tissue
isolated within semipermeable polymeric capsules that
prevent neural outgrowth but allow diffusion of substances with mass ⬍500 kDa (79). The soluble signal
released by the encapsulated tissue is unknown.
Although extra-SCN oscillators have been suspected
in mammals for some time, a direct demonstration was
first provided by measuring a circadian rhythm of
melatonin synthesis in hamster retinas in vitro (82).
Since then, a variety of cultured mammalian tissues
(96) and even immortalized fibroblasts (3, 93) have
been shown to exhibit damped oscillations of “clock”
gene expression. The sustained cycling of these peripheral oscillators probably depends on the SCN (71), and
ongoing work is investigating the possible coupling
mechanisms between such brain and body rhythms. A
phase shift of the light-dark cycle immediately shifts
SCN Per1 gene transcription, but peripheral tissues
require up to several days to reestablish their normal
phase relationships to the SCN and each other (96). On
the other hand, restricted feeding entrains circadian
oscillations in the liver (13, 23, 81) and in the cerebral
cortex and hippocampus (87) but not in the SCN. One
surprising finding is that redox state (e.g., the ratio of
NAD⫹ to NADH within a cell) can directly affect the
activity of circadian transcription factors (70). This
raises the exciting possibility that the phase and, perhaps, the generation of circadian rhythms are directly
linked to factors that sense the metabolic fluctuations
within the cell (66). Both humoral (2, 52) and neural
(83) signals are likely to be involved in tissue synchronization, probably via multiple signaling pathways (4).
This contrasts with the situation in semitransparent
animals (like flies and zebrafish), where the peripheral
oscillators appear to be directly photosensitive (63, 90).
These results raise important questions about whether
some of the transitions seen in circadian behavior
following shifts of the light schedule (e.g., with jet lag
after transmeridian travel) may be the result of a
“supra-SCN” multioscillator organization. Output
pathways are thus likely to provide an important substrate for plasticity in the circadian timing system. The
SCN may not only drive, but also entrain, rhythmicity
in downstream structures (Fig. 1). The functional implications and adaptive significance of these organismal differences in circadian organization should be an
exciting area for future investigation.
Ultimately, clock regulation of physiology and behavior must rely in some way on changes in cellular
biochemistry. The circadian rhythm of the enzymatic
activity of pineal NAT is arguably the most robust
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INVITED REVIEW
trol. A natural stimulus (light) can be presented in a
controlled fashion, resulting in a long-lasting neural
change (in the pacemaker’s oscillation), which can be
measured as an adaptive behavioral response (a permanent phase shift of overt rhythmicity). As a result,
there are remarkable opportunities to begin to understand behavior at multiple levels of biological organization, ranging from intracellular regulatory molecules
and gene expression, to intercellular networks and
multisynaptic pathways, and finally to integrated patterns of physiology and performance.
18.
19.
20.
21.
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