Phototransduction (F) Flashcards
Where is the biochemical cascade of phototransduction restricted to?
outer segment of photoreceptors
Where is protein production and oxidative metabolism is restricted to?
inner segment of photoreceptors
Where are the outer and inner segments of the photoreceptor connected?
cilium
What is the microtubule arrangement of the cilium?
9+0
What is the resting membrane potential of rods in the dark?
-40mV
Where is the localized Na+-K+-ATPase in the photoreceptor?
inner segment membrane
Where does the potassium channel allows the flow of K+ ions back out of the photoreceptor cell along its concentration gradient?
inner segment membrane
What is the cGMP-gated channel also called? 1. What passes through the channel? 2. Where is it located? 3
- CNG channel (cyclic nucleotide-gated)
- Non-discriminate: Na+, K+, Ca2+ (mostly Na+) into cell
- outer segment membrane of photorecptor
What is the cGMP-gated channel also called? 1. What passes through the channel? 2. Where is it located? 3
- CNG channel (cyclic nucleotide-gated)
- Non-discriminate: Na+, K+, Ca2+ (mostly Na+) into cell
- outer segment membrane of photorecptor
Where is the NCKX channel located? 1. What does it exchange? 2
- outer segment membrane
2. 1 molecule of K+ and Ca2+ leave cell while 4 Na+ enter the cell
Why is there a low basal level of calcium in dark-adapted rods?
there is an inward leak of Ca2+ (via CNG) preventing an extremely low intracellular [Ca2+]
What are the regulators of cGMP (and what do each of them do)?
- guanylate cyclase (GC) (Synthesizes cGMP)
2. cGMP phosphodiesterase (PDE6) (degrades cGMP to GMP)
What are the regulators of cGMP (and what do each of them do)? 1-2. Are these more active in the light or the dark? 3
- guanylate cyclase (GC) (Synthesizes cGMP)
- cGMP phosphodiesterase (PDE6) (degrades cGMP to GMP)
- light
What is a transmembrane G protein coupled receptor (GPCR) photon receptor for the dark? 1. When is it inactive?
- Rhodopsin
2. dark
What does Rhodopsin do when activated?
catalyzes the activation of a heterotrimeric G-protein (Transducin)
What does guanylyl cyclase and phosphodiesterase do?
turns GTP to cGMP
What does guanylyl cyclase and phosphodiesterase do?
turns GTP to cGMP
What is the inverse agonist that rhodopsin is bound to while inactive?
11-cis retinal
When in dark-state what neurotransmitter is consistently in the vesicles?
glutamate
When a photon is absorbed by rhodopsin what happens to 11-cis retinal?
all become trans retinal
What does trans retinal do to rhodopsin?
converts it to Metarhodopsin II (R*)
Does Metarhodopsin II have a higher or lower affinity than rhodopsin?
higher
Does Metarhodopsin II have a higher or lower affinity than rhodopsin?
higher
What decreases GDP affinity for Metarhodopsin II and dissociates from it?
Gtα-GTP
What does Gtα-GTP have an increased affinity for? 1. WHat does this do? 2
- PDE6
2. converts cGMP to GMP
What does a small reduction in cGMP do to channels?
large effect on channel closure
What does channel closure in due to the reduction in cGMP lead to?
- reduces the flow of Na+ ions into the cell
- reduces the dark current and makes the membrane potential more negative and hyperpolarized
- flux of Ca2+ into the cell is also reduced and NCKX
What are the mechanisms to block Metarhodopsin II?
- Phosphorylation by Rhodopsin kinase (RK1)
- Arrestin binding to MII
- Schiff’s base hydrolysis
- cGMP restoration
What increases with lowering of cytoplasmic [Ca2+] in recovery?
Guanylate cyclase
Even after MII quenching PDE6 will remain active until what happens?
GTP unbinds from G-protein which makes PDE6 inactivated
Even after MII quenching PDE6 will remain active until what happens?
GTP unbinds from G-protein which makes PDE6 inactivated
What is a GTPase accelerating (activating) protein and will cause hydrolysis of the GTP to GDP associated with activated transducin?
RGS9-1
What is a GTPase accelerating (activating) protein and will cause hydrolysis of the GTP to GDP associated with activated transducin?
RGS9-1
What can occur if enough photons (bright light) are absorbed such that all channels are closed?
saturation
The sensitivity of the phototransduction system leads to a high potential for what?
“noise amplification”
What are the types of opsin in cones?
S-Cones: S-opsin
M-Cones: M-opsin
L-Cone: L-opsin
Are rods or cones more sensitive?
rods
Are spontaneous isomerization rates of cone opsins less than or greater than Rhodopsin? 1. Do rods or cones have lower spatial and temporal resolution? 2
- greater
2. rods
Do rods or cones have a greater convergence of photoreceptors to a single retinal ganglion cell?
rods
Do rods or cones have faster phototransduction cascade kinetics? 1. What does this mean for saturation? 2
- cones
2. cones saturate less
Where on the retina are there only cones?
fovea
How many rod bipolar cell types are there? 1. Cone cell types? 2.
- 1
2. 10
What are the two types of bipolar cells?
- On-center or depolarizing bipolar cells (DBC)
2. Off-center or hyperpolarizing bipolar cells (HBC)
What are the two types of bipolar cells?
- On-center or depolarizing bipolar cells (DBC)
2. Off-center or hyperpolarizing bipolar cells (HBC)
What is the major difference of HBC cells and DBC cells?
When stimulated HBC cation channels close and the cell becomes hyperpolarized. When stimulated DBC cation channels open, and the cell becomes depolarized
What is the major difference of HBC cells and DBC cells?
When stimulated HBC cation channels close and the cell becomes hyperpolarized. When stimulated DBC cation channels open, and the cell becomes depolarized
What are the glutamate receptors in HBC cells? 1. In DBC cells? 2
- AMPA and KA
2. mGlur6 glutamate receptor
When bipolar cells (HBC or DBC) are depolarized where do they release glutamate to?
ganglion cells
Do hyperpolarizing bipolar cells increase or decrease their release of glutamate to its synapsed ganglion cell? 1. Does this decrease or increases its firing rate? 2
- decrease
2. decrease
Do depolarizing bipolar cells increase or decrease their release of glutamate to its synapsed ganglion cell? 1. Does this decrease or increases its firing rate? 2
- increase
2. increase
