non-visual photoreceptors

Question 1

mimosa plant

Answer 1

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

Question 2

circadian rhythm

Answer 2

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

Question 3

what is the most reliable signal of time of day and why?

Answer 3

day-night variation in light intensity

ambient illumination can be 10^9 times higher at midday than midnight

Question 4

monitoring rodent locomotor activity results

Answer 4

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

Question 5

conclusion monitoring rodent locomotor activity

Answer 5

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

Question 6

what are 24hr variations in behaviour and physiology caused by?

Answer 6

endogenous circadian clocks synchronised to the light:dark cycle

Question 7

where does the circadian clock originate in mammals

Answer 7

hypothalamus: suprachiasmatic nuclei

located just above the optic chiasm

Question 8

what is the suprachiasmatic nuclei innervated by?

Answer 8

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

Question 9

photons/cm/s night

Answer 9

10^6

Question 10

photons/cm/s day

Answer 10

10^16

Question 11

what happens to rods at twilight

Answer 11

they go from fully depolarised at night to fully hyperpolarised at twilight

Question 12

what light are rods sensitive to?

Answer 12

dim light

Question 13

what light are cones sensitive to?

Answer 13

bright light

Question 14

when do cone cells stop responding?

Answer 14

when they reach a point of full hyperpolarisation

Question 15

over what range of light intensity do cone cells go from fully depolarised to fully hyperpolarised?

Answer 15

1000x times

Question 16

what is photoreceptor adaption?

Answer 16

photoreceptor activity is adapted based on the background light intensity

Question 17

GCAP

Answer 17

guanylyl cyclase activating protein

Question 18

what is GCAP sensitive to?

Answer 18

the amount of Ca2+ in the cytosol

Question 19

what is calcium’s impact on GCAP?

Answer 19

inhibits it

Question 20

what does activated guanylyl cyclase do?

Answer 20

converts GTP to cGMP

Question 21

cGMP production in dark

Answer 21

is low

Question 22

GCAP in light

Answer 22

closure of cGMP-gated channels
reduced intracellular Ca2+
increased cGMP production
cGMP-gated channels re-open

Question 23

sensitivity of our photoreceptors changes dependent on…

Answer 23

how much light they are being exposed to and how active their phototransduction mechanism has been

Question 24

what does photoreceptor adaption allow?

Answer 24

allows us to have sensitivity to the large range of light intensities that we do

Question 25

changes in the relationship between light intensity and photoreceptor polarisation

under dark adapted conditions…
under light adapted conditions…

Answer 25

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

Question 26

how does receptor adaption impact cones?

Answer 26

allows them to be responsive at all background levels but makes them bad at distinguishing ambient light

Question 27

photoreceptors in non mammals

Answer 27

extra retinal photoreceptors and in eyes

Question 28

bilateral enucleation

Answer 28

the removal of the eye that leaves the eye muscles and remaining orbital contents intact

Question 29

sparrows experiment

Answer 29

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)

Question 30

birdy frizardish

Answer 30

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

Question 31

effect of bilateral enucleation in mammals and conclusions

Answer 31

abolishes all responses to light in mammals

so time of day response must originate in the retina

Question 32

experiment with rodless and coneless wild type mice

Answer 32

• 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

Question 33

conclusion of wild type mice experiment

Answer 33

a photoreceptor other than rods and cones must drive nonvisual responses (responses to ambient light)

Question 34

experiment to see if there is another photoreceptor in the retina than rods and cones

Answer 34

• 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

Question 35

Physiological responses of retinal ganglion cells projecting to the hypothalamus

Answer 35

these ganglion cells depolarise in response to light, therefore are ON type

Question 36

what happens when a retinal ganglion cell that projects to the thalamus is isolated?

Answer 36

it still depolarises
-they can respond directly to light signals

Question 37

what photoreceptors exist that aren’t rods and cones?

Answer 37

retinal ganglion cells that project to the hypothalamus

Question 38

what does the amino acid sequence of ‘opsin’ like proteins in vertebrate genomes indicate?

Answer 38

shared evolutionary history with rod and cone opsins
membrane associated
GPCRs
bind chromophore

Question 39

what opsin is responsible for non visual photoreception?

Answer 39

melanopsin

Question 40

neurons in mice that have been labelled to express melanopsin with blue dye?

Answer 40

small number of retinal ganglion cells labelled, these go to the optic chiasm and then to the hypothalamus

Question 41

isolated retinal ganglion cells from melanopsin knockout mice

Answer 41

no longer respond to light

Question 42

what happens when you insert melanopsin into a ganglion cell that is not light sensitive?

Answer 42

it causes depolarising inwards currents in response to light

Question 43

melanopsin phototransduction g protein

Answer 43

Gq/11

Question 44

melanopsin phototransduction effector enzyme

Answer 44

phospholipase C

Question 45

melanopsin phototransduction second messengers

Answer 45

DAG

Question 46

melanopsin phototransduction effector channel

Answer 46

TRPC