Lecture 2 Light regulation of the circadian system Flashcards
phase response curve in hamsters and light intensity.
dim light little effect, strong big.
Circadian photoentrainment relies upon accurate quantitative measurement of environmental light.
do rods and cones fully explain light measurement by circadian system?
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.
extra retinal birds photoentrainment?
still entrain to LD without eyes (DD period delays, LD normal again)
injecting black dye under scalp prevents photoentrainment (so brain)
(all non mammalian vertebrates, but interspecific variation)
rodless coneless mice?
Still show normal entrainment response to DD, LD.
SCN input from?
how do we know?
RHT retinohypothalamic tract
Retrograde tracing from SCN: injected fluorescent 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
RGC of RHT showed to …
…intrinsically photosensitive, removed cell so isolated and still show response.
rod/cone phototransduction.
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
rods and cones in humans?
1 rod, 3 cones RBY, (melanopsin in RGCs)
areas expressing melanopsin found how?
remember- genetic manipulation
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–> blue cell expresses melanopsin
Expressing retinal ganglion cells formed 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.
melanopsin KO mice response to light
Unresponsive to light
adding melanopsin to cells to make light sensitive.
Neuro2A cells from mouse neuroblastoma (not light sensitive) –> 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.
humans with damage to RHT vs blind (rod/cone damage but eye otherwise intact)
entrainment can survive loss of rods and/or cones but not RHT damage
melanopsin KO mice photoentrainment capabilities
Still retain photoentrainment –> all RGCs receive input from rods and cones
combinations of melanopsin/rod/cones which can’t entrain
cones ONLY –> can’t entrain
what covers the full range of light intensities relevant to the clock?
rods and melanopsin, complementary function at different intensities.
mRGCs lack the membranous discs to make them sensitive, so respond to higher light intensities.
Appropriate timing under natural photoperiods requires photoreception across a wide range
