Lecture 12 - Photosensation and Vision Flashcards
What is phototransduction?
‘The transduction of light into a sense’
- light hits neurons (or equivulent) and an action potential is induced
- many species respond to light/it’s absense (plants, bacteria, animals)
- can be occular or extraoccular
What is light required for in an organism?
- circadian rhythm
- vitamin production
- metabolism
- escape responses
What is vision?
‘The faculty or state of being able to see’
- includes colour, shape, movement, and dimension
- usually occular and a decidcated structure is required (e.g. lens)
What animals use basic photosensation and what does this involve?
Barnacles
- if a shadow is cast over they retract (predator evasion)
- vision confined to the perception of this shade
- involves a primitive eye
What are the features of a primitive eye?
- simple
- often respond to the strength of light for a simple reflex response or phototascic responses
- light response is mediated through a reticular cillia structure
What is the reticular cillia structure?
structure through with a light response is mediated in a primitive eye
-through a membrane rich cillium or rhobdmere, i.e. the light collecting point of a villa (with neurons extending below)
What is the structure of photoreceptors?
-similar to neuroepithelial cells
-2 major differences (based on the orientation of polarity):
Rhabdomeric or Cilliary
-drosophila are rhabdomeric
-normally embedded within larger structure in multiples to give complex vision
What is the conserved gene between humans and insects for initiating eye development?
Pax6(human)/eyeless(insects)
How was the conserved natures of Pax6 and eyelesss exhibited experimentally?
- Pax6 expressed in the imaginal disks (discs of epithelial tissue that generate all external structures of the adult drosphila in metamorphesis) of drosophila larvae
- RESULT
- all external structures of the fly contained ectopic eye related structures
Concl. = Pax6 and eyeless are developmentally conserved and all eyes have been derived from similar primitive evolutionary tract
What anatomical structures of the eye are likely to have been conserved between fly and human? And what differences are there in the structure?
- cornea, iris cell and cone cells
- photoreceptor cells
- both have optic nerves at the back
Differences
Insect-light is channeled down through the eye
Vertberates- light focused at the back
As organisms have evolved to be more complex, what is shown by photoreceptor anatomy?
Photo receptors show an increasing development of sophistication
- can see in different dimensions
- convex eyes can harvest more light than concave (but most eyes are concave - essential to deal with intensity, to gain control and saliency of a signal)
What is Pax6 and what is its mutant phenotype?
homeobox gene in mammals
- mutated pax6=> no iris or lens
- eyeless = homolog in insects
What was the experiment on drosophila that exhibited rescue of sight in organisms with mutated eyeless?
When pax6 was expressed when a drosophila’s eyeless gene was mutated this could rescue the eye
-shows ancientness (sorry) - both eyes arose from same simple structure, with an ancester gene of eyeless/pax6
What is rhodopsin?
AG protein coupled receptor
-light sensing molecule (ancient)
What are the two types of opsin?
cilliary opsins
rhabdomeric opsins
What are melanopsins?
light harvesting sensory proteins
-for the maintenance of the circadian clock not for vision
What type of opsins do cridona use?
C-opsins
What is are some of the main differences in vertebrate and insect vision?
Vertebrate
-rods hyperpolarise to light
-this inhibts the activity of the assicated neurons
Insects
-depolarise to light, turning the neuron ON
-rhodopsins are tuned to different frequencies and are expressed in different receptors
What are the features of rods and cones in vertebrates?
Rods
-majority of cells in the vertberate eye
-detect the degree of light
-bleached by light
-hyperpolaraise to light (turns neuron off)
-rod sensitivity is determined by the amount of rhodopsin
-generally low sensitivity, used by vertebrates in dim situations
Cones
-sensitive to light but maintain function in high illumination (use pigment iodopsins - in iris)
-iris filters out light to see ambient light
What are the features of -R and -C opsins?
- g protein coupled recptors
- r opsins act through phospholipase C
- C opsins act through phosphodiesterase
- both phopho- are downstream of the rhodopsins (are receptors themselves)
- both contain arrestins (2, 3 in r-opsins; 1 in c-opsins) which have inhibitory functions - block cycle of vision
What are the features of drosophila phototransduction?
- TRP channels
- has lots of pigment to protect it from light
- photoreceptors cluster in groups of 7 (with one responding to UV) and are sensitive to different wavelengths
- contains rhadbomeres and submircrovillus C beneath (where Ca2+ released from to depolarise the photoreceptor)
What experiment showed the involvement of DAG/glyerol and TRP channels in drosophila phototransduction?
- light sent to the photoreceptor under a cell ptch clamp to record the output
- 1 photon and 1 Ca2+ molcule was released and measured as a ‘bump’ - 20ms in duration and 10pA in amplitutde
This is the time for DAG/glycerol to accumulate and activate TRP channels
What are the features of TRP channels involvedin drosophila phototransduction?
- normally present on the villi
- in light, TRPs shuttle down microtubules into submicrovilli (towards the cell body)
- TRPs in the form of heterodimers (TRPγ and TRPL)
- tightly associated with PKC (anchors)
- results in the initiation of a closely coupled molecular cascade
What is the molecular casecade of phototransduction in drosophila?
1-photoisomerisation of rhodopsin to melarhodopsin activates heteromeric Gq - Gαq is released
2-Gαq activates phospholipase C, generating IP3 and DAG from PIP2. DAG also releases PUFAS by activation of DAG lipase
3-TRP and TRPL are activated by PUFAS and/or DAG. TRPs, PKC, PLC (+NINAC/inaS) form a signalplex
4-SMC Ca2+ stores are activated and an action potential is generated
5-DAG is converted to PA via DAG kinases and CDP-DAG by CD synthesis.
6-PI regenerated and transported back to microvillar membrane by PI transfer protein and converted to PIP2 which silences DAG.
7-Arrestin is then activated to silence Rhodopsin
This results in the cessation of signal
