4. Circadian Rhythms Flashcards
what is the circadian clock
an endogenous molecular pacemaker that drives daily rhythms affecting physiology, biochemistry and behaviour
what species were the first clock mutants isolated in?
drosophila
what is the most conserved mechanism of the circadian clock
the negative feedback loop
what is the main way to measure the circadian clocks in drosophila
locomotor activity
what does free run mean
the biological rhythm exists without any external input or cues
what does entrainment mean
the biological rhythm is synchronised to an external oscillations
give an example of an external oscillation that entrains biological rhythms
the light/ dark cycle
at what point in the day are flies the most active
at dawn - they anticipate the light
Flies are also active at dusk
what was the first gene identified that could affect circadian rhythms
period
what happens in Per0 mutants
produces arrhythmicity
what happens in PerL mutants
produced long period rhythms of 28 hours
what happens in PerS mutants
produced short period rhythms of 19 hours
what 4 properties define circadian rhythms
- oscillations free run under constant conditions, indicating the presence of a self-sustaining clock
- the clock driven events recur approximately every 24 hours
- rhythms are entrained by sun-driven changes in light or temperature
- the period of the clock is remarkably stable over a wide temperature range.
circadian systems are organised into 3 main parts, what are the three parts?
the core clock
the input pathways
the output pathwyas
what is the function of the input pathways
synchronise the clock to its environment
what is the function of output pathways
transmit information to temporally organise behaviour and physiology
what is the function of the core clock
keeps the time
what degrades period, how?
double-time
via phosphorylation
period and timeless are under the control of a promotor, what is the name of the element of this promotor
E-box element
what happens when clock and cycle bind to the E-box
drives the production of per and Tim mRNA
name the 5 things that modify period and timeless proteins before it dimerises
doubletime
casein kinase 2
shaggy
phosphates: PP2A and PP1
what happens when period and timeless dimerise
they can enter the nucleus where they are able to inhibit clock and cycle proteins from binding to the E-box = suppressing their own expression
= negative feedback loop
describe the processes occurring in a clock cell: in the morning
- all timeless has been degraded
- period is no longer protected in the dimer so is also degraded by hyperphosphorylation by double-time
describe the processes occurring in a clock cell: at mid-day
no inhibition of per/Tim transcription
Clk and Cyc bind to the E-box and mRNA content rises
NO PROTEIN
why is no protein content of period or timeless produced at mid-day
timeless protein is degraded by light
and no period as it is not protected by timeless
describe the processes occurring in a clock cell: at dusk
timeless is no longer degraded by light
mRNA = proteins
these proteins dimerise after being modified by a host of factors.
describe the processes occurring in a clock cell: at midnight
dimers enter the nucleus
period associated with clock = double-time phosphorylates clock
phosphorylation prevents clock from binding to the E-box
= inhibits the transcription of per/Tim
all remaining protein content is degraded = repression is released
define exogenous Zeitgeber
external cues that help regulate the internal biological clock
name 2 exogenous zeitgebers
light
temperature
how does light reach clock neurons
via photoreceptive organs containing rhodopsins
what organs contain rhodopsin in flies
ocelli and HB-eyelets
what is cryptochrome
a blue light photopigment that is activated by light
what does activated cryptochrome degrade
timeless
what happens to cyrptochrome after timeless has been degraded
Jet Lag binds to Cry = degradation
how does cryptochrome degrade timeless
binds to timeless and recruits F box protein Jet Lag = targets Tim for ubiquitination and degradation
what happens in flies with a mutation to jet lag
cant adapt to different light/ dark cycles
which gene and protein is evolutionarily conserved across mice and flies
clock genes / proteins
what do clock neurons regulate in a fly
locomotion
how many neurons in the fly brain regulate locomotion
20-30
what neurons promote morning activity
S-lateral neurons
what neurons promote evening activity
LNds
s-LNv
what neuron peaks at dusk
LNd
what neuron peaks at dawn
s-LNv
what neurons promote both morning and evening activity
DN1s
what happens to DN1 neurons in the evening when there is high light
they have low output
- similarly when there is low light in the evening = they have strong output
what does PDF stand for
pigment dispersing factor
what is the role of PDF neurons
synchronise clock neurons in the absence of light
what are PDF neurons termed
internal zeitgebers
name three things that sleep is important for
cognition
immune function
physiological and neural homeostasis
what human gene underpins ‘night owls’
CRY1
describe sleep in tribes
tribes exhibit natural variation in chronotype (genes which dictate sleeping patterns) - this is driven by selective forces so that the entire tribe does not sleep at the same time
= variation in sleep schedules can be beneficial in preventing predation.
name 3 tools used to measure sleep in drosophila
infrared detection (to measure locomotion)
neurophysiology (using electrodes)
beam crossings
how do beam crossings work to monitor sleep in drosophila
an infrared beam is placed in a tube with a fly
the number of times the fly crosses the beam is counted by the DAM (drosophila activity monitor)
- able to distinguish periods of sleep from periods of activity
what happens when light activates I-vLNs?
they release PDF onto other s-vLNs that then project to other clock neurons or brain regions to increase motor activity
name one of these brain regions that s-vLNs project to?
ellipsoid bodies
what is the role of ellipsoid bodies
motor control
what happens to these neural pathways at sleep
GABAergic sleep-promoting neurons inhibit these pathways via GABA activity which reduce alertness and arousal
what are the 3 neuronal sleep centres in flies
Kenyon cells (MB)
fan-shaped bodies
ellipsoid bodies (EB)
name 2 neuromodulatory neurons that regulate sleep centres
DA neuorns
dorsal paired medial neurons
what is the response to sleep deprivation caused by caffeine
sleep rebound = an increase in sleep to make up for the lost sleep
what is the response to sleep deprivation caused by failed copulation or starvation
no sleep rebound = overrides homeostatic mechanisms to feed and breed which are essential to survival - a likely evolutionary process
what happens to the activity of starved flies? why?
increased locomotor activity - which is likely to enable them to discover new food sources.
ER: what is evidence for the role of the SCN?
DeCoursey et al. 2000
destroyed the SCN connections in 30 chipmunks - they were returned to their natural habitat and observed for 80 days
results: their sleep wake cycle disappeared and a significant proportion were killed by predators by the end of the study.
in humans what is the endogenous pacemaker and where is it found
the suprachiasmatic nucleus of the hypothalamus
how many groups of clock neurons in flies and what are they named after
7 major groups named after their anatomical position
what also expresses clock proteins in fly brains
a few hundred glial cells.
ER: temperature can entrain flies, a change of how many degrees is sufficient to adapt a flies circadian clock
3 deg c change is sufficient to change the circadian rhythm
what is the mechanism of how temperature entrains the clock
nobody knows - it has not yet been discovered
temperature can entrain the clock under constant light conditions, what does this mean?
suggests there is an override of light dependent degradation of timeless
where is temperature perceived, what is the evidence
in the tissues and not in the CNS
evidence:
- isolated body parts are able to perceive temperature = temperature reception is tissue- autonomous
-isolated brain are not able to synchronise to temperature cycles
what gene is involved in circadian temperature reception
norpA gene
what does norpA encode
phospholipase C
how do we know norpA is involved in temperature reception
animals with mutated norpA are not able to synchronise to temperature cycles.
describe Siffre (1962) case study
spent 6 months in a cave - with no exposure to light (an exogenous zeitgeber)
= his circadian rhythm remained consistent at 24 hours and 30 minutes
- supports the fact that circadian rhythms free run under constant conditions
