Lecture 8 Light regulation of the circadian system

Question 1

phase response curve in hamsters and light intensity.

Answer 1

dim light little effect, strong big.

Circadian photoentrainment relies upon accurate quantitative measurement of environmental light.

Question 2

rods and cones and light entrainment

Answer 2

Sensitivity of established retinal photoreceptors (rods cones) can not fully explain light measurement by the circadian system.

rods - starlight/moonlight.
cones - relative light intensity/contrast, so not a reliable indicator of brightness.

Question 3

extra retinal birds photoentrainment?

Answer 3

still entrain to LD without eyes.
DD period delays, LD normal again.
so photoreceptors capable of entraining clock can’t be in eyes.

injected black dye under scalp, if the photoreceptors are in the brain it will block, if in skin it won’t.
lost entrainment so in the brain.

true for all non mammalian vertabrates, vary between species though

Question 4

rodless coneless mice?

Answer 4

cyclic GMP phosphodiesterase mutation which fucks rods, also no cones (why?)
still show normal entrainment response to DD, LD.

Question 5

SCN input from?

how do we know?

Answer 5

RHT retinohypothalamic tract

therefore either RGC themselves are photoreceptive or they relay a signal.

injected fluourescent beads into SCN of rats, taken up by axon terminals of retinal ganglion cells and transported back to retina and into cell body.
can identify retinal cells projecting to SCN

Question 6

RGC showed to be what?

Answer 6

intrinsically photosensitive, removed cell so isolated and still show response.

Question 7

red/cone phototransduction.

Answer 7

rods:
outer segment has stacked rows of membranous discs, full of light sensitive proteins, allow rods to pack photopigment into small space, increases absorbtion of photons.
ret gang cells don’t have this.

membranous discs contain rhodopsin in rods, codopsin in cones. all animal use opsin proteins

opsins - GPCRs, ligand is 11-cis retinal.
binding of ligand keeps it in an inactive state, it’s an inverse agonist.

when light hits the retinal/ligand, it causes it to isomerise, changes to trans, released and opsin then signals

new type of opsin?

Question 8

rods and cones in h0omans?

Answer 8

1 rod, 3 cones RBY, melanopsin

Question 9

melanopsin found how?

Answer 9

micey mice.
made transgenic mice expressing tau::lacZ melanopsin (motor region normal, add taulac coding seq)

LacZ encodes beta-galactosidase,
digests X-gal a colourless sugar –> blue precipitate.

Tau targets proteins to microtubules.

therefore if a cell expresses blue that cell expresses melanopsin.

then saw melanopsin expressing retinal ganglion cells fromed the retinohypothalamic tract.

axons of cell bodies towards optic disk.

underside of the brain showed blue optic nerves towards the chiasm, SCN has a dark blue stain.

Question 10

melanopsin KO mice

Answer 10

don’t respond anymore. no depolarisation in response to light.

Question 11

adding melanopsin to cells to make light sensitive.

Answer 11

Neuro2A cells from mouse neuroblastoma not light sensitive, make express melanopsin, light sensitive only if also given 11-cis retinal cofactor.

melanopsin is therefore sufficient for photosensitivity, and light detection in melanopsin is consistent with that of other known opsins.

Question 12

rodless coneless humans

Answer 12

disease that causes degeneration.

look at their melatonin production - still normal and synced to LD.

Question 13

humans with lost eyes or damage to RHT

Answer 13

maintain normal sleep wake cycles with alarm clocks etc but melatonin free runs.

therefore a non rod/cone photoreceptor in eyes of humans regulates melatonin similar to melanopsin

Question 14

melanopsin KO mice

Answer 14

more severe free running but retain photoentrainment.

DD followed by light pulse, show robust phase delay.

Question 15

where do ganglion cells get info from?

Answer 15

rods and cones through other cells

Question 16

combinations of melanopsin/rod/cones which can’t entrain

Answer 16

only if cones only can’t support circadian entrainment.

cones only measure relative light intensity

Question 17

what covers the full range of light intensities relevant to the clock?

Answer 17

rods and melanopsin, complementary function at different intensities.

mel lack the membranous disks to make them sensitive, so respond to higher light intensities.

Appropriate timing under natural photoperiods requires photoreception across a wide range

Question 18

lil summary

Answer 18

Appropriate circadian timing relies upon accurate quantitative measurement of environmental light

Question 19

colour discrimination?

Answer 19

requires 2 or more cones to compare.

blue/yellow humans???

Question 20

colour and circadian clock?

Answer 20

Dawn and dusk are associated with pronounced changes in the spectral composition of light.

when suns below horison lots of green/yellow, light has to travel through more ozone layer and gets filtered out.

The mammalian visual system can detect changes in blue-yellow colour around twilight.

Colour changes are unaffected by the weather, unlike light intensity, ie big ass clouds. can adjust as a result.

Question 21

mice SCN neurons and colour?

Answer 21

can detect changes in blue yellow around twilight.

multielectrode recordings from SCN of mice.

Selectively manipulated activity of short/long sensitive cones.
yellow increases M/L cones

Question 22

white light and clock?

Answer 22

we can’t detect differences between various white light types, but only sunlight activates melanopsin.