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The Big PICTURE: influence of the SCN timing system
on various functions
From Zigmond et al Fundamental Neuroscience, AP 1999
Individual neurons dissociated from rat suprachiasmatic nucleus
express independently phased circadian firing rhythms
Neuron 14: 697-706, 1995 DK Welsh, DE Logothetis, M Meister and SM Reppert
• Within the mammalian hypothalamus, the suprachiasmatic nucleus
(SCN) contains a circadian clock for timing of diverse neuronal,
endocrine, and behavioral rhythms.
• By culturing cells from neonatal rat SCN on fixed microelectrode
arrays, we have been able to record spontaneous action potentials
from individual SCN neurons for days or weeks, revealing prominent
circadian rhythms in firing rate.
• Despite abundant functional synapses, circadian rhythms expressed
by neurons in the same culture are not synchronized.
• After reversible blockade of neuronal firing lasting 2.5 days,
circadian firing rhythms re-emerge with unaltered phases.
• These data suggest that the SCN contains a large population of
autonomous, single-cell circadian oscillators, and that synapses
formed in vitro are neither necessary for operation of these
oscillators nor sufficient for synchronizing them.
Electrical synapses coordinate activity in the
suprachiasmatic nucleus
MA Long, MJ Jutras, BW Connors, RD Burwell
Nature Neuroscience 8, 61 - 66 (2004)
• In the suprachiasmatic nucleus (SCN), the master circadian pacemaker,
neurons show circadian variations in firing frequency. There is also
considerable synchrony of spiking across SCN neurons on a scale of
milliseconds, but the mechanisms are poorly understood.
• Using paired whole-cell recordings, we have found that many neurons in
the rat SCN communicate via electrical synapses. Spontaneous spiking was
often synchronized in pairs of electrically coupled neurons, and the degree
of this synchrony could be predicted from the magnitude of coupling.
• In wild-type mice, as in rats, the SCN contained electrical synapses, but
electrical synapses were absent in connexin36-knockout mice. The
knockout mice also showed dampened circadian activity rhythms and a
delayed onset of activity during transition to constant darkness.
• We suggest that electrical synapses in the SCN help to synchronize its
spiking activity, and that such synchrony is necessary for normal circadian
behavior.
Electrical coupling in SCN- the data
•
(a) In a well-coupled pair (coupling
coefficient = 0.13), hyperpolarizing
current steps resulted in symmetric
membrane-potential deflections in the
coupled cell. In the left panel, we injected
current into cell 1 and recording voltage
responses in that cell and its pair. In the
right panel, current was injected into cell
2. Traces are averaged from 50 trials. (b)
Single action-potential response in a
coupled pair in the presence of APV,
DNQX and picrotoxin. Shown here is the
mean postsynaptic response to 25
spontaneously occurring action potentials
(dashed lines = s.e.m.). (c) Scatterplot of
the coupling strength of SCN cell pairs as
a function of the time of day recorded.
Note the greater incidence of coupling in
the middle of the subjective day (ZT 4−8)
as compared the end of the subjective
day (ZT 8−12).
Coupled vs non-coupled SCN cells
•
All experiments were conducted in the
presence of APV, DNQX and picrotoxin. (a)
Examples of intracellular recordings from a
pair showing direct electrical coupling (left)
and a pair lacking such a connection (right).
Below are spike cross-correlograms
describing long epochs (>200 s) of spiking
data, including the above traces. The time
scale for the cross-correlograms is in
milliseconds. Correlogram values are
normalized by the total spike count. Baseline
correlations are subtracted from these
analyses. (b) Examples of cell-attached
recordings (top) from a coupled (left) and
noncoupled (right) pair, and the
corresponding spike cross-correlograms
(below). (c) The spiking correlation
coefficient of electrically coupled pairs is
directly related to the strength of coupling.
This scatterplot shows data from cellattached recordings ( ) as well as
intracellular recordings (
Electrophysiology and circadian behavior in
wild-type (WT) and Cx36-knockout (KO) mice.
Bridging the gap: coupling single-cell oscillators in the
suprachiasmatic nucleus
CS Colwell Nature Neuroscience 8, 10 - 12 (2005)
•
Top, schematic of pairs of SCN neurons (blue)
from wild-type (WT) and Cx36-/- mice.
Individual SCN neurons contain the molecular
machinery necessary to generate circadian
oscillations. One gap in our knowledge is the
lack of understanding of how these single-cell
oscillators are coupled. The new study3
demonstrates that SCN neurons are coupled
through direct electrical connections. This
coupling is lost in mice deficient in Cx36.
Bottom, schematics of wheel-running activity
records from WT and Cx36-deficient mice.
Animals maintained in constant darkness show
rhythms driven by the endogenous timing
system. Each horizontal row represents the
activity record for a 24-hour day. Successive
days are plotted from top to bottom. The
colored bars represent activity. The WT mice
express robust circadian rhythms of locomotor
activity with period shorter then 24 h. The onset
of activity is typically under precise control. In
contrast, the Cx36-deficient mice showed
rhythms that were weaker and less coherent
than controls. Without the Cx36, the circadian
clock still keeps time but lacks the temporal
precision that typically characterizes the
behavioral output.
A CLOCKWORK WEB: CIRCADIAN
TIMING IN BRAIN AND PERIPHERY,
IN HEALTH AND DISEASE
Michael H. Hastings, Akhilesh B. Reddy &
Elizabeth S. Maywood
Nature Neuroscience Reviews
August 2003
Time after time: inputs to and
outputs from the mammalian
circadian oscillators
David Morsea, Paolo Sassone-Corsi
TINS 25: 632-637, 2002
•
•
•
•
Fig. 1: Rhythmic oscillation of clock gene
expression.
(a) The mammalian circadian system has two
negative feedback loops, one involving
inhibition of bmal1 transcription by a
heterodimer of BMAL1 (brain–muscle Arnt-like
protein 1) and CLOCK (left), the other involving
inhibition of period ( per) and cryptochrome (
cry) transcription by PER and CRY (right). The
two loops are linked by the positive action of
PER on bmal1 transcription and the positive
action of the CLOCK–BMAL1 heterodimer
(indicated by the yellow box) on E-boxcontaining promoters. Grey parts of the genes
represent promoters; wavy lines indicate that
levels of bmal1, per and cry transcripts
oscillate.
(b) In the mouse suprachiasmatic nucleus
(SCN), per and bmal1 transcripts oscillate
almost 12 h out of phase, with per levels at a
maximum around midday. PER and BMAL1
proteins also oscillate, but PER lags behind per
RNA.
(c) In peripheral tissues, such as the liver, the
same clock gene components oscillate as in
the SCN, but with peaks occurring later. The
black and white bars above the graphs indicate
night and day, respectively.
Nature Reviews Neuroscience 2, 521-526 (2001);
HYPOTHALAMIC INTEGRATION OF CENTRAL AND
PERIPHERAL CLOCKS
Ruud M. Buijs & Andries Kalsbeek
• During sleep, our biological clock prepares us for the forthcoming
period of activity by controlling the release of hormones and the
activity of the autonomic nervous system. Here, we review the
history of the study of circadian rhythms and highlight recent
observations indicating that the same mechanisms that govern our
central clock might be at work in the cells of peripheral organs.
Peripheral clocks are proposed to synchronize the activity of the
organ, enhancing the functional message of the central clock. We
speculate that peripheral visceral information is then fed back to
the same brain areas that are directly controlled by the central
clock. Both clock mechanisms are proposed to have a
complementary function in the organization of behaviour and
hormone secretion.
Interaction between peripheral and central clocks
The Circadian Gene Period2 Plays an Important Role in
Tumor Suppression and DNA Damage Response In Vivo
Fu, L et al. Cell 111: 41-50, 2002
• The Period2 gene plays a key role in controlling circadian
rhythm in mice. We report here that mice deficient in the
mPer2 gene are cancer prone. After γ radiation, these mice
show a marked increase in tumor development and reduced
apoptosis in thymocytes. The core circadian genes are induced
by γ radiation in wild-type mice but not in mPer2 mutant mice.
Temporal expression of genes involved in cell cycle regulation
and tumor suppression, such as Cyclin D1, Cyclin A, Mdm-2,
and Gadd45α, is deregulated in mPer2 mutant mice. In
particular, the transcription of c-myc is controlled directly by
circadian regulators and is deregulated in the mPer2 mutant.
Our studies suggest that the mPer2 gene functions in tumor
suppression by regulating DNA damage-responsive pathways.
Drosophila melanogaster