Circadian Rhythm

Question 1

circadian rhythm

Answer 1

biological rhythm with a 24hr period that persists in constant conditions
present in all organisms

Question 2

benefit of the CR

Answer 2

survival advantage because organisms can anticipate rather than respond to environmental changes

Question 3

CR functions

Answer 3

anticipates regular changes in the environment - tunes internal physiology to the external world
internal synchronisation (temporal organisation) - internal processes in different organs are coordinated
allow synchrony (temporal organisation) between species

Question 4

examples of CRs

Answer 4

behavioural - sleep/wake, drinking, food
biological - glucose uptake, metabolic rate, alcohol degradation
physiological - bp, HR, pain threshold

Question 5

amplitude

Answer 5

measures robustness of circadian period (highest to lowest period)

Question 6

period

Answer 6

duration of one complete cycle in rhythmic variation

Question 7

free running (tau)

Answer 7

rhythm free runs according to circadian clock H>24hrs mouse<24hrs
constant conditions, no external cues

Question 8

entrainment

Answer 8

synchronisation of internal biological rhythms by external cues

Question 9

zeitgeber

Answer 9

external cue
light is the primary zeitgeber
e.g. food intake/temperature

Question 10

actogram

Answer 10

graph in CR research
vertical line = activity

Question 11

exogenous vs endogenous daily rhythms

Answer 11

exogenous - response to change in environment by external/environmental rhythms (not internally generated)
endogenous - generated internally within the organism by a self-sustaining oscillator/biological clock (true CR)

Question 12

what is the endogenous master clock

Answer 12

suprachiasmatic nucleus (SCN)

located at base of hypothalamus (above optic chiasm)

Question 13

light perception

Answer 13

light detected by retina
impulses sent to SCN to entrain clock
rod and cones send signal to RGC via bipolar cells

Question 14

how do RGCs detect light

Answer 14

melanopsin (opsin photopigment)
found in intrisically photosensitive RGCs (ipRGCs)
endoded by Opn4 gene

Question 15

roles of melanopsin

Answer 15

light modulation of sleep
entrainment of CR
pupillary light reflex
exacerbates migraines

Question 16

3 SCN inputs

Answer 16

1)input light pathway: retina-SCN via RHT
2)intergeniculate leaflet (IGL) innervation IGL-SCN conveys photic and non-photic info from dorsal raphe nucleus
3)DRN activated and MRN mediate entraining of arousal (non-photic)

Question 17

role of CLOCK/BMAL1 heterodimers

Answer 17

produce Cry1/Cry2/Per1/Per2 genes in early circadian days - inhibited by nuclear accumulation of Per/Cry complex (late circadian days)
oscillatory feedback loop
core controlled genes (CCGs)
24 hrs to transcribe/translate genes - next cycle when Per/Cry degrades

Question 18

rev-erb + per&cry

Answer 18

TF/represses Bmal1
negative regulator of Bmal1 (anti-phase to Per/cry) enhances core oscillations

Question 19

mPer1 gene expression

Answer 19

highest expression during the mid circadian day (low proteins levels)
low mPer1 mRNA at the end of the circadian day (highest protein levels)
mPer1 mRNA expression occurs only when nuclear mPer1 protein cleared at the end of the circadian night

Question 20

light entraining

Answer 20

RHT releases glutamate & PACAP
increases Ca2+ in SCN
activates MAPK/CaMK/PKA
CREB phosphorylation
Per regulation light resets cycle by increasing Per1/Per2 (clock genes)

Question 21

hamster CR

Answer 21

> 24hrs
Tau mutant hamster has a shorter CR
Tau encodes protein kinase which phosphorylates Per1 and controls entry into nucleus
Casein Kinase I epsilon - degrades Per1/PTM of cry/per/changes length of TTFL/controls ability to go back to nucleus

Question 22

in vitro CR monitoring

Answer 22

obtain single SCN neurons expressing bio-luminescence reporter gene per-1 luciferase
measure: individual cell oscillations and population
individuals SCN neurons retain CR activity and different period lengths (light up at different time) - cell-cell communication is important

Question 23

structure of SCN neurons

Answer 23

shell: AVP/GABA
core: GRP/GABA/VIP
axon projects from core to shell, GABA is an activator and acts on shell, core synchronises shell, shell generates most SCN output, VIP excitatory action (binds VPAC2) like PACAP

Question 24

VIP KO mice

Welsh et al., 2010

Answer 24

decreased transcription of per
desynchronised firing of SCN
weak behavioural rhythms

Question 25

AVP mediated communication between SCN neurons

Answer 25

resistance to pertubation
AVP V1a-/V1b- normal CR but resistant to jet lag/ when L&D delayed = resynchronisation
AVP role = confers resistance to perturbation

Question 26

which clock genes are present outside SCN

Answer 26

per/cry/bmal
otuside SCN
ovary and kidney
in vivo rhythm maintain by SCN signals/in vitro amplitude & precision declines

Question 27

SCN is the master clock and slaves clocks in tissues

SCN communication with peripheral clocks

Answer 27

SCN entrains the peripheral clocks:
* hormonal signals - melatonin rhythms regulates CRs in pars tuberalis of pituitary
* cortisol/corticosteroid
* TSH
* Autonomic NS signals
* behavioural signals
* metabolic signals: restricted feeding can entrain liver enzyme rhythms

Question 28

SCN output

Answer 28

subparaventricular zone (SPZ) - dSPZ controls body temp and projects to MPO region vSPZ relays to DMH (corticosteroid production)
dorsomedial hypothalamus (DMH) GABAergic to ventrolateral preoptic nucleus (VLPO) (arousal)
orexin neurons in lateral hypothalamus (LHA) - wakefulness and feeding
MCH neurons - GABAergic - active during sleep

Question 29

melatonin

Answer 29

secreted by pineal gland - pinealocytes (which have a dense capillary network)
secreted during darkness (high in CSF/blood/saliva)
short t1/2 15-20 mins - fall at dawn/low during daytime
sends info about time of day to tissue which need it
humans - facilitates sleep and lowers body temperature

Question 30

input pathway

Answer 30

PVN -multisynaptic pathway - superior cervical gangia in SC
sympathetic (adrenergic) fibres from SCG - innervate pineal gland
adrenergic fibres end in varicosities (no synapses) release NA close to pinealocytes (where sympathetic neuron fires)

light - SCN stimulation (GABA) - PVN inhibition - suppression of melatonin

Question 31

melatonin location

Answer 31

SCN intensely in pars tuberalis of pituitary
melatonin onset - natural dim light
Receptors: DMH/VMH/MPO/PVTN/hippocampus

Question 32

control of sleep

sleep = reduction of synaptic strength

Answer 32

controlled by 2 pathways: homeostatic/circadian
increased sleep = increased energy consumption duration
accumulation of metabolic byproduct - adenosine - activates sleep promoting neurons
caffeine blocks adenosine binding via R’s

Question 33

wake vs sleep

Answer 33

waking = synaptic potentiation
sleep = synaptic downscaling, removes irrelavant info during memory consolidation/recovery of learning potential
synaptic proteins required for memory consolidation downregulated during sleep

Question 34

sleep removes cellular by products

Answer 34

CSF diffuses into extracellular space to clear waste (sleep increases extracellular space by 60% vs 5% of sleep during awake
AB cleared 2x faster in sleep vs awake (Xie et al., 2013)
sleep improves both types of memory: declarative and non-declarative

Question 35

cost of learning

Answer 35

increased energy consumption/signal:noise ration
reduced selectivity of firing
saturation of plasticity potential

Question 36

nREM sleep

non REM sleep reactivates neural circuits implicated in information enco

Answer 36

hippocampus (temporary store/sharp wave ripples) to…
neocortex (long term store/slow wave oscillations)

thalamocortical spindles (coincide with slow wave prime neocortex for LT memory storage)
slow wave oscillations: synchronise spindles and sharp wave ripples/facilitates reactivation of hippocampal memories and redistribution to neocortex/allow plasticity at cortical synapses

Question 37

circadian disruptors

Answer 37

change in period - high fat
change in phase - shift work
change in amplitude - night eating, insulin resistance
peripheral clocks: liver/muscle/fat/pancreas

Question 38

when do rhythm disturbances occur

Answer 38

when clock is not synced with the environment
* shift work (outside 8am-6pm)
* jet lag
* old age
several days needed to resynchronise to new LD cycle (1day/1hr shift)
desynchronisation brings about: poor sleep/poor cognition/mood swings/GI probelms (risk of disease and cancer)

Question 39

evidence of CR disturbances having an effect on health

Answer 39

increased risk of breast cancer in female shift workers and cabin staff for airlines
circadian disruption in carcinogenic
jones et al., 2019 - serial questionnaire (significant trend with number of hours per week but not night) (melatonin suppression/CR disruption?)

Question 40

faster tumour growth in mice with disrupted CR

Filipski et al., 2004

Answer 40

mice injected with metastatic cells - tumour weight increased
decreased survival

Question 41

timing of food intake contributes to weight gain

Arble et al., 2009

Answer 41

mice are nocturnal ~80% calorie intake at night
high fat food provided only during day/night
faster weight gain when food is consumed at the wrong circadian time (during day)

Question 42

mutant animals with shorter circadian period (22hrs) under a 24hr light cycle

Martino et al., 2008

Answer 42

cardiac & renal disease when internal clock in SCN is in conflict with external environment - usually in shift workers
tau mutant (encodes Ck1e) affects TTFL - shorter endogenous period (22hr) in hamsters
tau under 24hrs: heart disease (cardiomyopathy/fibrosis/kidney disease/early death)
tau under 22 hrs: normal cardiac and renal structure and function