© The Authors Journal compilation © 2011 Biochemical Society
Essays Biochem. (2011) 49, 1–17; doi:10.1042/BSE0490001
1
The neural circadian system
of mammals
Hugh D. Piggins1 and Clare Guilding
Faculty of Life Sciences, AV Hill Building, University of Manchester,
Oxford Road, Manchester M13 9PT, U.K.
Abstract
Humans and other mammals exhibit a remarkable array of cyclical changes
in physiology and behaviour. These are often synchronized to the changing
environmental light–dark cycle and persist in constant conditions. Such
circadian rhythms are controlled by an endogenous clock, located in the
suprachiasmatic nuclei of the hypothalamus. This structure and its cells have
unique properties, and some of these are reviewed to highlight how this central
clock controls and sculpts our daily activities.
Introduction
Early researchers in the field of biological rhythms recognized that the
daily behavioural and hormonal rhythms of laboratory rodents were readily
inluenced by environmental lighting regimes. Since many of these rhythms
were sustained in the absence of cyclical changes in lighting, but could be
altered by brief exposure to light, it was reasoned that the clock or pacemaker
responsible for these rhythms should be found within the visual system of
the brain (for a review, see [1]). The development in the 1970s of new ways
to visualize neural connections enabled researchers to map novel pathways
from sense organs to the brain. Injection of such anatomical tracers into the
1To whom correspondence should be addressed (email hugh.d.piggins@manchester.
ac.uk).
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Essays in Biochemistry volume 49 2011
eye revealed that the tracer was transported from the retina to an area of the
hypothalamus called the SCN (suprachiasmatic nuclei, meaning ‘nuclei above
the optic chiasm’) [2]. Studies were then initiated to experimentally destroy
(‘lesion’) this part of the brain to determine whether and how circadian
rhythms in behaviour and physiology were affected. Lesions of the SCN, but
not other hypothalamic structures, abolished rhythms in plasma corticosterone
[3] in addition to drinking and general locomotor activity [4], thereby
implicating the SCN as a possible site of a circadian pacemaker in mammals.
Further studies revealed that grafts of fetal SCN tissue into the hypothalamus
of SCN-lesioned arrhythmic adult rodents restored rhythms in behaviour (but
not endocrine function). Grafts of other brain structures do not have these
rhythm-promoting actions [5]. Hence, a combination of lesion and transplant
approaches has led to the widely accepted view that the SCN function as the
master circadian pacemaker in mammals.
The SCN are composed of bilateral lobes adjacent to the ventral loor of
the third ventricle, one of the luid-illed structures of the brain (Figure 1).
Each rugby-ball-shaped lobe contains ~8000 neurons in addition to ~1000–
2000 non-neuronal glial cells. SCN neurons are very small (~8–12 μm in diameter, approx. half the size of neurons found in other hypothalamic sites, such
as the paraventricular nuclei) and are so densely packed that the SCN lobes are
readily distinguishable in brain sections stained for standard histological markers, such as Nissl substance. SCN cells have a number of remarkable properties. (i) Metabolic measurements in vivo demonstrate a pronounced day–night
rhythm in SCN metabolic activity, with 2-deoxyglucose uptake being much
higher during the day than during the night [6]. (ii) In vivo, rodent SCN neurons show a pronounced rhythm in spontaneous electrical activity, with high
levels of AP (action potential) discharge during the day [peak at ~CT6 (circadian time 6)] and lower frequencies at night [7]. This in vivo rhythm is sustained
when the SCN is isolated from the surrounding brain by ine knife cuts that
sever its neural connections. Remarkably, in vitro, neurons in brain slices containing the SCN also express a similarly phased circadian rhythm in spontaneous AP production [8,9]. The peak in this rhythm is used as a marker of the
mid-phase (~CT6) of the SCN circadian pacemaker in vitro. (iii) Neonatal and
fetal SCN neurons that are dissociated and cultured on glass plates embedded
with MEAs (multi-electrode arrays) can sustain spontaneous rhythms in electrical discharge [10]. The period of these rhythms is determined genetically by
the intracellular molecular clock. These and other studies (see the chapter by
Herzog [10a] and below) establish that individual SCN neurons can function
as cell-autonomous circadian pacemakers.
It is estimated that most, if not all, SCN neurons synthesize the inhibitory neurotransmitter GABA (γ-aminobutyric acid), and functional GABAA
receptors are present throughout the SCN. This indicates that the SCN is
composed of a network of inhibitory neurons and it would be tempting to
speculate that the SCN neurons are homogeneous. However, within this
© The Authors Journal compilation © 2011 Biochemical Society
H.D. Piggins and C. Guilding
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Figure 1. Per1 clock gene and VIP are key components of SCN timekeeping
(A) Coronal photomicrograph of the SCN in mouse, showing Per1::GFP luorescence (green)
and VIP immunoreactivity (red) in ixed tissue. VIP immunoreactive cell bodies are found in the
ventral SCN with ibres projecting to the dorsal SCN. Note that not all SCN neurons express
clock genes. Image courtesy of Dr Alun Hughes. (B) Schematic diagram demonstrating the effect
of loss of VIP signalling in the SCN. Green circles represent clock-gene-expressing cells and blue
circles represent non-clock-gene-expressing cells. Red lines indicate VIP connections (right-hand
panel), which synchronize individual cellular oscillators, resulting in a co-ordinated high-amplitude
rhythm in single cells and consequently from the SCN as a whole. The left-hand panel indicates
loss of VIP connections, which uncouples individual cellular oscillators, leading to low-amplitude
desynchronized single-cell rhythms and consequently a low-amplitude output from the SCN as a
whole.
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GABAergic network, the location of SCN neurons can be partly distinguished and differentiated by their neuropeptide content. The neuropeptide
AVP (arginine vasopressin) is produced by neurons in the medial aspect of
the SCN, whereas VIP (vasoactive intestinal polypeptide) is mostly found in
neurons in the ventral part of the SCN (Figure 1). Other neuropeptides such
as GRP (gastrin-releasing peptide) and SP (substance P) are predominantly
found in neurons of the central part of the SCN. In the rat and hamster, the
RHT (retinohypothalamic tract; see below), which conveys light information
from the retina to the SCN, densely innervates VIP and GRP neurons. This
led some researchers to schematize the SCN as having a ‘core’ area where
non-rhythmic cells integrate environmental light input and a ‘shell’ area
where the actual master pacemaker cells are contained [11]. Both GABAergic
and peptidergic neurotransmission is then used to link activity between and
within these functional subregions of the SCN. However, this model does not
appear universally applicable between different species (for a detailed review,
see [12]). What is apparent is that within the rodent SCN, VIP signalling is
very important for synchronizing SCN neurons (see below). Further research
is necessary to determine whether the other neurochemical phenotypes of
SCN neurons have distinct roles in circadian timekeeping.
Photic and non-photic resetting
There are two basic types of stimuli that reset the SCN circadian pacemaker
and hence behavioural rhythms. These stimuli alter the SCN at different
phases of the SCN circadian cycle and communicate with the SCN via
different pathways. Many nocturnal laboratory rodents, such as rats,
hamsters and mice, exercise voluntarily in running-wheels, and this rhythm
in locomotor activity shows a very readily measurable day–night profile
under LD (light–dark) conditions that is sustained with near-24h periodicity
in DD (constant dark) or LL (constant light; see Figure 2). Typically, the
onset of this behavioural rhythm is used as a marker of the beginning of
subjective night (CT12). Steady-state changes in the phase of these onsets are
used to interpret how a stimulus resets the behavioural rhythm and hence
the phase of the underlying SCN circadian pacemaker. Exposure to light or
photic stimuli characteristically shifts the nocturnal rodent SCN only during
the subjective night or active phase of the circadian cycle; exposure to light
during the day has minimal phase-resetting actions. This pattern of resetting
[PRC (phase–response curve)] is called a photic PRC and is mediated via the
RHT (Figures 2 and 3). In contrast, stimuli that promote arousal phase-shift
the SCN pacemaker during the middle of the day or inactive phase and
have minimal resetting actions during the active subjective night (Figure 2;
see also the chapter by Mistlberger and Antle [12a]). This temporal pattern of
sensitivity is very different to that of photic stimuli and hence has been named
as the ‘non-photic’ PRC. Examples of the non-photic stimuli include sleep
© The Authors Journal compilation © 2011 Biochemical Society
H.D. Piggins and C. Guilding
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Figure 2. Resetting effects of photic and non-photic stimuli on wheel-running
rhythms of mice
(A) Blue illed bars depict running-wheel activity under LD and DD conditions. The animal initially entrains to lights off (illustrated by the black illed bar) and then free-runs with a tau-value
of <24 h when released into DD. Note phase-dependent phase-shifting effects of 15 min light
pulses (red asterisks) delivered at late subjective night, early subjective night or middle subjective day phases respectively. These differential responses to external stimuli can be used to plot
a phase–response curve. (B) Phase–response curves for photic (solid green line) and non-photic
stimuli (broken red line) with phase-advances and phase-delays plotted as positive and negative
values respectively. The magnitude of the shift is plotted according to the time of the circadian
cycle at which the stimulus was delivered.
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Figure 3. Depiction of the major pathways and their neurotransmitters in the
mammalian circadian system
PACAP and glutamate relay photic information, while NPY, GABA and serotonin convey
non-photic information. Activation of receptors for these neurochemicals impacts on core
clock-gene expression (e.g. per, cry and bmal1) in SCN neurons to inluence the timing of outputs
from the circadian clock, including AVP, PK2 and TGFα, CLC (cardiotropin-like cytokine),
glutamate and GABA.
deprivation, forced exercise, exposure to a novel environment etc. There are
two main pathways that communicate non-photic information to the SCN.
The first is the GHT (geniculohypothalamic tract), which conveys neural
signals from the IGL (intergeniculate lealet) of the thalamus. The second
pathway originates in the MR (median raphe) of the brain stem (Figure 3). It is
the daily resetting actions of photic and non-photic stimuli that are responsible
for sculpting and shaping our daily rhythms.
The neurochemical signals of the photic and non-photic input pathways
are also different. Glutamate is the principal neurochemical of the RHT,
whereas NPY (neuropeptide Y) and 5-HT (5-hydroxytryptamine; also
known as serotonin) are key neurochemicals of the GHT and MR pathways
respectively. Glutamate is the brain’s main excitatory transmitter, whereas
NPY and 5-HT are known to be predominantly inhibitory in the mammalian central nervous system. In situ hybridization and immunohistochemical studies show that both ionotropic and metabotropic glutamate receptors
are expressed in the SCN, and a variety of NPY and serotonergic receptors are present in this structure. Consistent with this, electrophysiological
studies conirm that retinal illumination increases the electrical activity of
rodent SCN neurons in vivo, and electrical stimulation of the optic nerve in
SCN brain slices excites SCN neurons in vitro. These actions are blocked
by pre-treatment with glutamate receptor antagonists [13]. In particular,
activation of the ionotropic glutamate receptors are implicated in photic
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H.D. Piggins and C. Guilding
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entrainment. Complementary studies of NPY and 5-HT indicate that these
neurochemicals predominantly suppress/inhibit SCN neuronal activity. The
precise roles of the receptors mediating these inhibitory actions are dificult to
elucidate, as a number of NPY and 5-HT receptors are located both pre- and
post-synaptically in the SCN. Overall, at the level of the electrical activity of
an SCN neuron, photic and non-photic stimuli differ by their excitatory and
inhibitory actions respectively.
Our knowledge of the neural substrates of photic and non-photic resetting
is constantly changing and expanding. In the 1980s and 1990s, the prevailing
view was that circadian light information was captured by rods and cones, but
over the past decade, there has been a revolution in our understanding of photoreception in vertebrates. One of the main classes of neurons in this structure,
the RGCs (retinal ganglion cells), were thought to be light-insensitive such
that they relied on rods and cones to sense and capture photic information.
However, ipRGCs (intrinsically photosensitive RGCs) were discovered and
subsequently determined to express a novel photopigment, melanopsin [14,15].
Although this photopigment is expressed by a comparatively small proportion
of RGCs, these ipRGCs are intrinsically photosensitive, and electrophysiological recordings in vitro show that they depolarize in response to exposure
to light [16]. The temporal proile of this depolarization is a slow onset and
offset, and resembles the temporal proile of SCN neuron responses to retinal
illumination. Studies in rodless coneless mice establish that ipRGCs alone are
suficient for entrainment of circadian rhythms to environmental light [17].
IpRGCs are particularly sensitive to blue light wavelengths. More recently,
ipRGCs have been subdivided into a number of different classes, and it is now
apparent that the projections of ipRGCs are much more extensive than previously thought, raising the possibility that ipRGCs convey particular kinds of
photic information to many other brain areas [18]. Future studies are necessary
to determine whether and how neuronal activity in these areas is inluenced by
the activation of ipRGCs. A key implication here is that the RHT is unlikely
to be a speciic projection to the hypothalamus and that the name, RHT,
is a misnomer. Furthermore, many ipRGCs also contain the neuropeptide
PACAP (pituitary adenylate cyclase-activating polypeptide) [19], and PACAP
modulates the actions of glutamate in the SCN [20,21]. Thus we are indeed in
our infancy in our understanding of the communication of light information to
the neural circadian system.
Similarly, knowledge of arousal and non-photic neural mechanisms continues to expand. In the late 1990s, a new group of neuropeptides, called the
orexins (also named the hypocretins) were identiied and found to be synthesized exclusively by neurons in the lateral hypothalamus [22,23]. Orexin
neurons are crucial for sustaining wakefulness; mice lacking orexin neurons
or orexin receptors cannot maintain wakefulness and show rapid transitions
in brain state. Orexin neurons are activated by arousal-promoting stimuli,
including those that reset the SCN pacemaker [24]. Orexin neurons innervate
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many neural structures of the circadian system, including the IGL and MR in
addition to the SCN region [25]. Consistent with the observation that orexin
receptors are present in the SCN region, the electrical activity of SCN neurons is inluenced by orexin [26] and can be reset by exogenous applications
of orexins given during the subjective day [27]. Hence, orexin neurons may
transpire to be major players in relaying non-photic information throughout
the neural circadian system.
Molecular basis of the SCN clock
The molecular basis of circadian timekeeping in mammals is remarkably similar
to that of insects (see the chapter by Glossop [27a]), with many of the key
molecular components evolutionarily conserved. This intracellular oscillator
is composed of both positive and negative feedback/feedforward loops. To
summarize briefly, the transcription factors CLOCK or NPAS2 dimerize
through their PAS domains to BMAL1 and positively drive the rhythmic
transcription of the per1-2 (period) and cry1-2 (cryptochrome) genes. Following
translation, PER and CRY proteins accumulate in the cytoplasm, where they are
phosphorylated by CK1ε (casein kinase 1ε) and CK1δ, and translocate back into
the nucleus. Here, they exert their negative drive to the system by inhibiting
CLOCK–BMAL1 transcriptional activity, essentially inhibiting their own
transcription. Over several hours, the phosphorylated PER–CRY complexes are
broken down and the negative inluence on their own transcription is removed,
restarting the cycle. Several other auxiliary loops (e.g. Dec1-2 and Reverbα)
stabilize the system. The speed at which PER–CRY complexes are degraded
sets the speed of the molecular clock, which usually cycles with a periodicity
of ~24 h. Mutations that affect the interactions between PER proteins and CK1
accelerate the clock [28] and underpin familial advanced sleep-phase syndrome
in humans [29], whereas the after-hours and overtime mutations, which slow
the degradation of CRY, decelerate the clock [30,31]. More recent results
have indicated an increased complexity to this model of the molecular clock.
Biochemical (e.g. changes in intracellular Ca2+ and cAMP) and epigenetic factors
(e.g. alteration of chromatin structure) are both under control of the clock and
regulate the transcription of core clock genes [32,33].
To understand the molecular basis for resetting of the clock, the effects
of photic/non-photic stimuli and/or their neurochemical correlates on the
expression of clock genes in the SCN have been studied. Consistent with
the observation that NPY acts to suppress SCN neuronal activity, giving
NPY or non-photic stimuli in vivo at the time that the SCN clock is reset
by non-photic stimuli suppresses SCN expression of per1/per2 [34]. In vivo,
peripheral injections of a serotonin agonist suppresses per gene expression at
the same phase (CT9–CT10) that this compound resets behavioural rhythms
[35]. Similarly for photic stimuli given during the subjective night, exposure
to light in vivo or glutamate agonists in vitro increases per1 expression in
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H.D. Piggins and C. Guilding
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the SCN [36,37]. Hence, similar to their actions on SCN neuronal activity,
non-photic and photic stimuli have very different actions on the core molecular clock in the SCN.
To further understand intra- and extra-SCN inluences on clock-gene
regulation, transgenic mice in which bioluminescent [luc (luciferase)] or luorescent [EGFP (enhanced destabilized green luorescent protein] reporters are
driven by clock-gene promoters have been generated (Figure 1A). Transgenic
animals in which the promoter of the gene is fused to the luc reporter gene
have been generated to monitor circadian rhythms in transcription. These
animals include c-fos::luc mice, per1::luc rats and per1::luc mice [38]. Here,
when the promoter of the gene is activated, luc is transcribed. The resulting
luc enzyme acts on its substrate, luciferin, which is included in the culture
medium, and this reaction produces photons of light that can be measured
with photosensitive devices. These devices include photomultiplier tube
assemblies or highly sensitive luminescence-imaging microscopy systems.
More recently, a transgenic animal has been generated with two different luc
genes expressed under the promoters of per2 and bmal1 [39]. The luc proteins
induce differently coloured emission spectra that can be imaged simultaneously using optic ilters to separate the spectra, thus allowing concurrent measurement of two different clock-reporter constructs. To assess the clock protein as
opposed to clock mRNA activity, an mPer2Luc knockin mouse was produced
that accurately reports the level of PER2 protein [40].
Per1 promoter activity has been tracked in mice using EGFP [41]. This
construct, unlike normal GFP constructs, which are relatively stable, has a
half-life of ~2.1 h and thus dynamic changes in promoter level can be assessed.
Visualization of per1-promoter-driven GFP activity requires luorescent excitation of the GFP protein. Fluorescence microscopy produces high-quality
images; however, over time the reporter is bleached, and cells and tissues suffer phototoxic damage, so this approach is not suitable for long-term imaging.
Conversely, bioluminescence imaging requires no external energy source for
visualization and SCN explants have been maintained in culture for over 600
days [38].
Electrical and chemical communication
At one time it was thought that the maintenance of timekeeping in the SCN
did not depend on conventional forms of intercellular communication.
Schwartz [41a] showed that impairment of AP-dependent communication
through infusion of the sodium-channel blocker TTX (tetrodotoxin) into the
SCN in vivo suppressed the expression of circadian rhythms in behaviour.
However, on termination of the TTX infusion, behavioural rhythms returned
at a time of day that was consistent and predictable from their phasing
pre-TTX treatment. This indicates that the SCN had maintained its phase
during the blockade of AP-mediated synaptic communication. Hence, it was
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the circadian control of behaviour and not SCN clock phase that had been
impaired by the TTX treatment. However, in vitro studies of the actions of
TTX on per1::luc expression in SCN brain slices are not consistent with this.
Yamaguchi and colleagues [41b] found that TTX greatly suppresses per1::luc
expression and seems to disrupt the synchrony between SCN neurons. This
indicates that the in vitro SCN is much more sensitive to TTX than the
SCN in vivo or that TTX infusions in vivo may have been acting outside
the SCN rather than on the SCN itself. Other TTX-independent forms of
neural communication are possible via gap junctions, and connexin proteins,
key constituents of gap junctions, are present in the SCN [42]. Mice lacking
connexin32 have imprecise behavioural rhythms and their SCN neurons
show diminished electrical coupling [43,44]. Perhaps the in vitro SCN slice
culture has reduced levels of connexins, thereby making it more sensitive to
perturbations in AP production.
Another aspect of intercellular communication is revealed through investigations of neuropeptidergic transmission. Studies with mice lacking VIP or
its cognate receptor, VPAC2, (VIP−/− and Vipr2−/− mice respectively) establish that intercellular VIP–VPAC2 signalling is important for normal circadian timekeeping. Both VIP−/− and Vipr2−/− mice show disrupted behavioural
rhythms and diminished levels of core clock-gene expression and immediate-early gene expression in the SCN [45–47]. Electrophysiological studies
indicate that SCN neurons in brain slices from VIP−/− and Vipr2−/− mice have
diminished synchrony, with a reduced proportion of cells showing detectable
rhythms in electrical activity [47–50]. Patch-clamp studies reveal that Vipr2−/−
SCN neurons tend to hyperpolarized (i.e. less electrically excited) than their
wild-type counterparts and this is consistent with the reduced frequency
of AP production observed in extracellular recordings from these neurons
in vitro. In Vipr2−/− mice crossed with per1::luc or per1::EGFP mice, some
Vipr2−/− SCN neurons sustain a degree of molecular rhythmicity, but many do
not and there is diminished synchrony and co-ordination of molecular activities between these neurons (Figure 1) [51,52]. In contrast, mice lacking GRP
receptors, which are normally present in the SCN, have a diminished response
to light, but sustain behavioural rhythms, indicating that loss of neuropeptide
signalling does not generally drastically affect the SCN timekeeping function.
Hence, intercellular communication via VIP–VPAC2 signalling is very important for normal SCN timekeeping and the circadian control of behaviour.
VIP also resets the wild-type rodent SCN circadian pacemaker.
Microinjection of VIP into the SCN of hamsters free-running in constant conditions resets their behavioural rhythms in a pattern that partly resembles the
phase-shifting effects of light [53]. Similarly, in vitro, exogenous VIP resets the
iring-rate rhythm of rat SCN neurons, with a temporal pattern of sensitivity
mimicking that of the RHT transmitter, glutamate [47]. These resetting actions
of VIP seem to recruit cAMP-dependent mechanisms, as blockade of adenylate
cyclase alters the resetting actions of VIP [54]. Since VIP shifts the SCN in a
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H.D. Piggins and C. Guilding
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light-like manner, it is perhaps not surprising that mice lacking VIP–VPAC2
signalling do not synchronize normally to the LD cycle. Indeed, recent evidence indicates that a non-photic stimulus is more effective at organizing
behavioural rhythms in these mice [55].
SCN control of behaviour
Lesion and transplant studies have irmly established that the SCN controls
the circadian proile of behaviour and physiology [5]. Precisely how it does
this is less clear. Encasing a fetal SCN graft in material that prevents the
graft forming synaptic connections with the host brain does not prevent
the restoration of behavioural rhythms [56]. This implicates the rhythmic
secretion of neurochemicals as a paracrine signal from the graft in the circadian
control of behaviour. Whether such mechanisms are also the norm in the
SCN-intact adult animal is unknown.
The identity of this neurochemical signal has thus far eluded researchers. There are at least six candidates: VIP, PK2 (prokineticin 2), TGFα
(transforming growth factor α), cardiotropin-like cytokine, GABA and glutamate (Figure 3). The central concept here is that these SCN molecules are
rhythmically synthesized and released. They could be communicated locally
from SCN efferents, secretion into the cerebrospinal luid of the ventricles or
via passive diffusion in the extracellular space between neurons (‘volume transmission’). Whatever the release mechanism, these signals are postulated to alter
the activity of brain centres controlling behaviour and physiology. For example, based mostly on anatomical studies demonstrating that PK2-containing
SCN neurons project throughout the hypothalamus and thalamus to areas
where expression of PK2 receptor is expressed [57], it is suggested that PK2
signals from the SCN are very important for conveying circadian information
throughout the brain [58].
Unfortunately, the demonstration of rhythmic release of such neurochemicals is technically dificult, and hence precise knowledge in this domain
is limited. One exception is the assessment of glutamate and GABA. Using the
implantation of microdialysis probes into the PVN (paraventricular nuclei),
Kalsbeek, Buijs and others have shown that glutamate and GABA release from
SCN efferents modulates PVN neuronal activity and inluence the timing of
the autonomic nervous system (see the chapter by Kalsbeek [58a]).
An intriguing complexity to this problem of circadian control of behaviour
is that the SCN electrical activity in diurnal rodents also peaks during the day
and not at night as might be expected [59]. Similarly, per-gene expression in the
SCN of such day-active rodents is higher during the day than night and thus
does not differ greatly from that seen in nocturnal rodents [60]. This suggests
that a combination of SCN output and the interpretation of this efferent signal
by downstream substrates determines the phase of an animal’s behaviourally
active phase.
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In addition to direct SCN signals, the SCN can also inluence the brain
and body by indirectly regulating the release of the pineal hormone, melatonin. Melatonin-binding sites are present in many brain regions and peripheral tissues. The SCN efferents act through the PVN and spinal cord to
ultimately regulate the activity of the pineal gland, and it is via this regulation
that the SCN can communicate circadian and daylength information to the
pineal gland. The SCN itself expresses melatonin-binding sites and its phase
can be reset by exogenous melatonin [61]. Hence, the SCN relays circadian-clock-phase information to the brain and body both directly and indirectly,
and it is possible that some of these signals feed back on the SCN itself.
Relationship between the intracellular clock
and cellular activity
The relationship between clock-gene expression and SCN cellular activity
remains a key question in chronobiology. Since the peak iring rate of SCN
neuronal activity follows when per1 expression is approaching maximal levels,
it is tempting to speculate that high levels of per1 transcript lead to elevated
electrical activity. Indeed, studies of mouse per1::EGFP SCN neurons show
that these neurons become more excitable and produce more APs, as the level
of EGFP luorescence is at approx. maximal levels [62]. However, a more
recent study suggests that per1::EGFP neurons become so excitable during
the day that they cannot ire APs and that it is the non-per1::EGFP neurons
that are iring APs at maximal rates during the middle of the day [62a]. This
naturally occurring highly excited state of SCN per1 neurons during the
day may explain why SCN neurons are highly resistant to the effects of
excitotoxins that kill most central mammalian neurons. Future studies are
needed to clarify the functional relationship between per1 expression and
SCN cellular excitability.
Studies of brain and tissues from PER2::LUC/per1::luc rodents demonstrate that, at least in culture, many brain regions can display circadian
rhythms in clock gene/protein expression, raising the possibility that circadian
oscillators are present in a variety of tissues [63,64]. Among the best investigated are the olfactory bulbs, hippocampus, mediobasal hypothalamus and
habenula [65–68]. These studies also show that it is not only neurons that
rhythmically express per genes/proteins; non-neuronal glial cells and, in particular, the ependymal cells lining the ventricular walls of the brain can also
exhibit rhythms in PER2::LUC [66,67].
Conclusions
The ubiquitous importance of circadian-clock genes in controlling a range of
fundamental biological processes is an emerging theme in biology, highlighting
the wide-ranging scope of this ield [69,70]. However, although we have seen
a recent explosion in our understanding of circadian rhythmicity, not least
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H.D. Piggins and C. Guilding
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following the discovery of core clock genes, there are a range of questions
that still need addressing. How does the molecular clockwork affect the
electrical properties of SCN neurons and vice versa? How does the SCN
communicate its phase information to control downstream oscillators? What is
the role and function of these downstream extra-SCN oscillators in controlling
tissue-speciic and physiological functions? We predict that the resolution of
such questions will not only further basic scientiic knowledge, but will also
impact on many areas concerning human health and disease.
Summary
•
•
•
•
•
Daily circadian rhythms in physiology and behaviour are controlled by
a master pacemaker based in the SCN of the hypothalamus.
The molecular basis of this pacemaker consists of interlocking feedforward and feedback loops that oscillate with a period of approx.
24 hours.
The phase of this pacemaker can be adjusted by light input from the
retina and by a variety of non-photic input pathways.
Neurochemical signalling and electrical activity are important in maintaining synchronized rhythmicity within the SCN.
Outputs from the SCN synchronize oscillators in downstream tissues to
co-ordinate physiology and behaviour.
H.D.P. and C.G. are supported by grants from the Biotechnology and Biological
Sciences Research Council (U.K.) and the Wellcome Trust.
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