57
Organization of living matter
Spatial organization has been an key concept from the earliest studies of biology, and particularly physiology.
Temporal organization is more subtle and usually overlooked, but equally important.
The fitness of an organism depends on its physical structure and its relationship with the environment. The time domain of this physical structure, and of the relationship with the environment, is as important as the structural domain. Time as niche variable.
Sleep wake cycle in a human being
Days
Wake
Sleep
Days
Wake
Sleep
The persistence of rhythms in the absence of environmental cycles is taken as evidence of an innate self-sustained timekeeping biological mechanism, that is a Biological Clock
EnvironmentalCycle
(e.g.. light-dark cycle)
Clock(suprachiasmatic nucleus)
Photoreceptors(retina)
Input pathwa
ys
Overt circadian rhythms(e.g. sleep-wake cycle)
III
VI
IX
IOutput
pathways
Components of a Generic Biological Timing System
Virtually all known hormones show a daily pattern of
secretion
LD cyclePeriod = 24 h
Free running in constant conditions(DD or LL)Period circa-24 h
0 24
Clock time
Days
Analysis of circadian rhythmslocomotor activity in nocturnal rodents
0 2448
EnvironmentalCycle
(e.g.. light-dark cycle)
Clock(suprachiasmatic nucleus)
Photoreceptors(retina)
Input pathwa
ys
Overt circadian rhythms(e.g. sleep-wake cycle)
III
VI
IX
IOutput
pathways
Components of a Generic Biological Timing System
Fitzgerald and Zucker (1976) PNAS 73: 2923-2927
The same circadian oscillator appears to control locomotor activity and estrous
Looking for the Circadian Clock
Lesions of suspected site should abolish rhythmicity Transplants of suspected site should restore
circadian rhythmicity in lesioned animals. Intrinsic properties of the suspected clock should be
rhythmic: in vivo and in vitro Identification of molecular and cellular mechanisms
that generate rhythmicity Mutations of specific genes should affect circadian
rhythms There should be a pathway communicating the
synchronizer with the clock
Lehman (1987) J. Neurosci. 7:1626-38
The SCN is necessary and sufficient for expression of locomotor activity circadian
rhythmicity
Ralph & Lehman (1991) Trends Neurosci. 14:362
0 24
?
Lesions of the SCN abolish all endocrine circadian rhythms
SCN lesions abolish the rhythm of corticosterone release
Moore (1972) Brain Research 42: 201
Everett and Sawer (1950) Endocrinol. 47:198
The release of hormones is initiated in the brain:Daily gating of the LH surge as an example
The release of hormones is initiated in the brain:Daily gating of the LH surge as an example
% of women showing an LH surge, as determined by urinary LH
Edwards (1981) Nature 293:253-256
SCN transplants that restore locomotor activity rhythms do not restore endocrine rhythms
Meyer-Bernstein et al. (1999) Endocrinol 140: 207-218
Identification of molecular components of the clockThe first mammalian clock mutant is found
•It is a semi-dominant mutation•Heterozygous mice in constant darkness have a longer period than wildtypes
•Homozygous mice in constant darkness have initially a very long period and then become arrhythmic
Vitaterna et al. (1994) Science 264:719-725
Time
mR
NA
or
Pro
tein
le
vel
Inhibition
Delay (~24 h)
-
Transcription
Clock gene 2
+mRNA
Translation Protein(negative element)
mRNATranscription
Clock gene 1
Translation
Protein(positive element)
Transcription and translation feedback loops constitute the core clock mechanism
Per1expression
Light
Dark
This unparalleled localization of function within the SCN makes the circadian system unique for the
study of the neural and molecular bases of behavior and physiology
De la Iglesia et al. (2000) Science 290:799-801
Splitting in the hamster
haPer1
RN
A level
in situ(SCN)
Locomotoractivity
UNSPLIT HAMSTER
~24 hours
Left
SC
NR
ight
SC
N
Time
L R
haPer1
RN
A level
Left
SC
NR
ight
SC
N
Locomotoractivity
SPLIT HAMSTER
~24 hours
Time
in situ(SCN)
Dual Circadian Oscillators in the SCNThe ensemble of oscillators is reflected in overt behavior
Mechanism and “sticking” force?
Clock outputs?
Split female hamsters present a circa-12-h LH surge.
Swann and Turek (1985)
The HPG Axis
The SCN could have input the neuroendocrine hypothalamus
•SCN efferents project to LHRH cells and these projections are believed to mediate the release of LHRH that triggers the LH surge.
•The projections are predominantly ipsilateral.
de la Iglesia et al. (2003) J. Neurosci., 23:7412-7412
.
. ..
. .. ..
.. .
..
..
.
..
.. .
.... .
...
..
.
.
. .
Neural and diffusible outputs of the clock.de la Iglesia and Schwartz Endocrinology (2006) 147:1148-1153
SCN lesions abolish the LH surgeIntracerebroventricular vasopressin induces a surge in the SCN-lesioned animals
van der Beek et al. (1999) Neuroscience 93: 659-666
Endocrinology (2012) 153: 2839–2850
Smarr et al.
Synchronization of peripheral clocks
Input pathways
III
VI
IX
I
Other entraining inputs
Overt physiological and behavioral rhythms (activity, feeding, core body temperature, sleep-wake)
Kidney Liver Skeletal muscle
SCN
GnRH cells and the pituitary show autonomous oscillations ov clock gene expression
Chappell et al. (2003) J. Neurosci. 23: 11202-11213
Yoo et al. (2003) PNAS 101-5339-5346
Sellix et al. (2003) Current Biol. 20:R266-R267
GnRH neurons(slave oscillators?)
Median eminence
Anterior pituitary(Peripheral clock?)
GnRH
LH
Ovary (Peripheral clock?)
E2
SCN neurons
ERER
Portal system
?
ER
?AVP
VIP
AVP
de la Iglesia and Schwartz Endocrinology (2006) 147:1148-1153
The HPG Axis
Miller et al.
The HPA Axis
Ishida et al. (2005) Cell Metabolism 2:297-307
Light induces expression of clock genes in the adrenal
Light induction of corticosterone release is ACTH-independent
Light induction of Per2 expression and of corticosterone release depends on a n intact SCN
SCN control of glucocorticoid release
Melatonin tracks night length in humans
Pineal Melatonin
Czeisler (1995)N.E.J.Medicine
Light acutely inhibits melatonin production
When humans are given light during the dark phase or in complete darkness when melatonin is high, melatonin levels in the blood quickly drop.
Czeisler (1995)N.E.J.Medicine
When humans are given light during the dark phase or in complete darkness when melatonin is high, melatonin levels in the blood quickly drop. This is even observed in some blind patients.
Light acutely inhibits melatonin production
Melatonin and artificial light in humans
Melatonin is a night time signal
Humans are seasonal breeders
The reproductive axis a prominent output of the circanual system
Entrainment and photoperiod measurement
Photoperiodic measurement in most species is not based on an hourglass mechanism
But on a circadian oscillation of photoinducibility
Clearly cannot be explained by hourglass mechanism
How is it explained?
Resonance cycles show an entrainment-based photoperiod measurement
T cycles show an entrainment-based photoperiod measurement
Gestational photoperiod measurement
Schematic design:
Pregnancy
After birth
12:12
Small testis large testis
