non-visual photoreceptors Flashcards
mimosa plant
an internal circadian clock drives rhythm in leaf
leaves opened and closed in rhythm even when the plant was placed in an environment with no light
circadian rhythm
24 hour variations in physiology and behaviour
persist in the absence of any cyclic cue from the environment
must somehow retain synchrony with external time
what is the most reliable signal of time of day and why?
day-night variation in light intensity
ambient illumination can be 10^9 times higher at midday than midnight
monitoring rodent locomotor activity results
1) mouse starts running reliably at 8 every day, this is when the light goes off
2) light now goes off at midday, mouse adjusts its cycle to start running on the wheel at this time
3) lights are always off, and the mouse relies on its circadian rhythm only, the mouse’s circadian rhythm is shorter than 24hrs, so the time the mouse gets up shifts to earlier each day
conclusion monitoring rodent locomotor activity
sleep/ awake time produced by an endogenous timing clock as there is still activity with no cues
endogenous timing clock is synchronised to the light dark cycle
what are 24hr variations in behaviour and physiology caused by?
endogenous circadian clocks synchronised to the light:dark cycle
where does the circadian clock originate in mammals
hypothalamus: suprachiasmatic nuclei
located just above the optic chiasm
what is the suprachiasmatic nuclei innervated by?
the optic nerve, relies on retina to measure light
a small fraction of neurons stop at the optic chiasm and go into the suprachiasmatic nuclei
photons/cm/s night
10^6
photons/cm/s day
10^16
what happens to rods at twilight
they go from fully depolarised at night to fully hyperpolarised at twilight
what light are rods sensitive to?
dim light
what light are cones sensitive to?
bright light
when do cone cells stop responding?
when they reach a point of full hyperpolarisation
over what range of light intensity do cone cells go from fully depolarised to fully hyperpolarised?
1000x times
what is photoreceptor adaption?
photoreceptor activity is adapted based on the background light intensity
GCAP
guanylyl cyclase activating protein
what is GCAP sensitive to?
the amount of Ca2+ in the cytosol
what is calcium’s impact on GCAP?
inhibits it
what does activated guanylyl cyclase do?
converts GTP to cGMP
cGMP production in dark
is low
GCAP in light
closure of cGMP-gated channels
reduced intracellular Ca2+
increased cGMP production
cGMP-gated channels re-open
sensitivity of our photoreceptors changes dependent on…
how much light they are being exposed to and how active their phototransduction mechanism has been
what does photoreceptor adaption allow?
allows us to have sensitivity to the large range of light intensities that we do
changes in the relationship between light intensity and photoreceptor polarisation
under dark adapted conditions…
under light adapted conditions…
under dark adapted conditions, even relatively dim light drives full hyperpolarisation
under light adapted conditions, photoreceptor can be partially hyperpolarised even under much brighter condition light
how does receptor adaption impact cones?
allows them to be responsive at all background levels but makes them bad at distinguishing ambient light
photoreceptors in non mammals
extra retinal photoreceptors and in eyes
bilateral enucleation
the removal of the eye that leaves the eye muscles and remaining orbital contents intact
sparrows experiment
sparrows that had gone under bilateral eye enucleation still synchronised their sleep wake activity to light in experimental conditions
however
black ink injected (C.B injection) under the scalp, prevents light reaching the brain
-stops the sparrow synchronising its light hopping ability to the light
photoreceptors synchronising the clock must be in brain (possible as tissue is permeable to light)
birdy frizardish
photo receptors are found all over the body
each area that has its own photoreceptors can set its own circadian clock
family of opsins that are specialised to be sensitive extra-retinally
effect of bilateral enucleation in mammals and conclusions
abolishes all responses to light in mammals
so time of day response must originate in the retina
experiment with rodless and coneless wild type mice
- Rodless and coneless mice
-would predict there is not light responses - Measure the diameter/radius of the pupil in different light intensities
the brighter the light the smaller the pupil in transgenic mice also - Measure wheel running behaviours
mice also adjust their behaviours when lights on and removed
conclusion of wild type mice experiment
a photoreceptor other than rods and cones must drive nonvisual responses (responses to ambient light)
experiment to see if there is another photoreceptor in the retina than rods and cones
- Inject dyes into the suprachiasmatic nucleus which is then transported (retrograde) down the axons of retinal ganglion cells
- This labelled a small number of retinal ganglion cells that send information to the circadian clock
->1% of all ganglion cells
Physiological responses of retinal ganglion cells projecting to the hypothalamus
these ganglion cells depolarise in response to light, therefore are ON type
what happens when a retinal ganglion cell that projects to the thalamus is isolated?
it still depolarises
-they can respond directly to light signals
what photoreceptors exist that aren’t rods and cones?
retinal ganglion cells that project to the hypothalamus
what does the amino acid sequence of ‘opsin’ like proteins in vertebrate genomes indicate?
shared evolutionary history with rod and cone opsins
membrane associated
GPCRs
bind chromophore
what opsin is responsible for non visual photoreception?
melanopsin
neurons in mice that have been labelled to express melanopsin with blue dye?
small number of retinal ganglion cells labelled, these go to the optic chiasm and then to the hypothalamus
isolated retinal ganglion cells from melanopsin knockout mice
no longer respond to light
what happens when you insert melanopsin into a ganglion cell that is not light sensitive?
it causes depolarising inwards currents in response to light
melanopsin phototransduction g protein
Gq/11
melanopsin phototransduction effector enzyme
phospholipase C
melanopsin phototransduction second messengers
DAG
melanopsin phototransduction effector channel
TRPC
