BS - Phototransduction and Neurotransmission - Week 8

Question 1

What is the difference between most neurons and photoreceptors in the way they depolarise?

Answer 1

Stimulation causes depolarisation in most neurons, by the opening of cation channels, and the release of neurotransmitter.
Normal state of photoreceptors is depolarised. Activation causes hyperpolarisation, via the closing of cation channels, and stopping the release of neurotransmitter.

Question 2

What involvement of response are found in most neurons vs photoreceptors?

Answer 2

Most neurons are all or nothing

Photoreceptors are graded

Question 3

What kind of cascade is phototransduction an example of?

Answer 3

G-protein signalling cascade.

Question 4

Phototransduction is the transduction of what?

Answer 4

Light into a neural signal.

Question 5

Briefly decribe the start and end of phototransduction.

Answer 5

Start - photon capture, leading to hyperpolarisation

End - slowing of neurotransmitter release at the synapse

Question 6

Does phototransduction differ between the rod and cone system?

Answer 6

Overall mechanisms are very similar.

Question 7

Describe how neuronal signalling occurs in most neurons.

Answer 7

An action potential causes an all or nothing opening of the neuronal cation channels, allowing cation influx into the cell.

Question 8

Describe what is meant by graded hyperpolarisation in relation to phototransduction.

Answer 8

The absorption of light by photoreceptors results in graded closure of cation channels, meaning few or many channels can close depending on the magnitude of phototransduction.

Question 9

During graded hyperpolarisation, in what location are the cation channels affected?

Answer 9

Closure of the cation channels occurs in the outer segment.

Question 10

Changes in what molecule influences the photoreceptor response?

Answer 10

Ca2+ concentration

Question 11

What molecule is implicated in being associated with cation channel closure? How does this change with concentration, and how many binding sites per channel?

Answer 11

cGMP was found to be responsible for cation channels remaining open.
There are 4 binding sites per channel.
≥3 cGMP are needed for the channel to open.
Probability of closing therefore increases with decreased cGMP.

Question 12

What happens to most cation channels in the dark? What does this state generate, and between what two structures? What is this value at rest andd at hyperpolarisation?

Answer 12

Most are open.
Produces a potential difference between the outer and inner segment.
Resting dark adapted rod is -40mV, and -70mV when hyperpolarised.

Question 13

Describe the 4 components of dark current phototransduction. Name the location for each one.

Answer 13

1. Na+/K+/ATPase pump in the inner segment - pumps out 3Na+ in exchange for 2K+ in.
2. K+ leak channel in the inner segment- allows exit of K+, rebalancing extracellular K+
3. cGMP gated cation channels in the outer segment - allows 1Na+ and 1Ca2+ in.
4. Na+:Ca2+/K+ exchanger in the outer segment - allows 4Na+ in, allows 1Ca2+ and 1K+ out.

Question 14

What enzyme maintains Na+/K+ balance? What does it require?

Answer 14

Maintained by Na+/K+/ATPase, and uses 1/3rd of total ATP.

Question 15

Describe what happens to rhodopsin when light hits it.

Name the molecule responsible for the phototransduction cascade.

Answer 15

It isomerises from 11-cis to all-trans.
Metarhodopsin II begins the cascade.
All-trans is the final form.

Question 16

Describe in 8 steps how the activated isomer of rhodopsin begins the cascade.

Answer 16

1. In the dark, cGMP gated channels are open - dark current
2. Light hits rhopdopsin, forming metarhodopsin II.
3. MII binds to transducin, a G-protein.
4. Transducin α-subunit phosphorylates GDP to GTP
5. Transducin α-subunit + GTP binds to phosphodiesterase.
6. Phosphodiesterase is activated, and hydrolyses cGMP to GMP.
7. Cation channels close when cGMP is depleted.
8. Dark current ends.

Question 17

Define the two amplification steps of the transduction cascade.

Answer 17

• MII binding to transducin - many transducins bind to MII

- Activated phosphodiesterase hydrolysing cGMP to GMP - thousands of cGMP hydrolysed per PDE.

Question 18

What happens to dark current levels with cGMP decreasing concentration?

Answer 18

It decreases

Question 19

What happens to PDE activation levels with increasing transducin levels?

Answer 19

It increases

Question 20

Define the a-wave in phototransduction and what it is caused by. What resembles this wave?

Answer 20

The a-wave is the initial dip seen in an electroretinogram. It is due to the closure of cationic channels.
A single cell recording exposed to light and allowed to recover resembes an a-wave.

Question 21

Describe the changes in intracellular Ca2+ before and after closure of cationic channels, and how this affects neurotransmitter release.

Answer 21

When channels are open, Ca2+ levels are high, and this allows release of the neurotransmitter glutamate in the dark.
When closed, Ca2+ levels deplete, and glutamate release is reduced in light.

Question 22

What happens to glutamate release in the presence of light?

Answer 22

It decreases.

Question 23

What is the minimum number of photons that is required to stimulate a rod cell, and how many rod cells must be activated to elicit a visual response?

Answer 23

They respond to 1 photon minimum.

5-14 must be stimulated to elicit a response.

Question 24

What 3 mechanisms are present to increase the likelihood that a rod will respond to a photon of light?

Answer 24

• Large number of rods with high concentrations of rhodopsin.
• High probability of undergoing a chemical reaction.
• Amplification of cascades by transducins and cGMP.

Question 25

What is the goal of the recovery phase?

Answer 25

Deactivating metarhodopsin II to stop it activating transducin.

Question 26

What protein is responsible for the first step of deactivation, what does it bind to, and what happens when this occurs?

Answer 26

GTPase Activating Protein (GAP) binds to a-transducin, causing it to convert GTP to GDP, deactivating PDE.

Question 27

What is a consequence of low Ca2+? What two proteins does it result in the activation of?

Answer 27

Causes activation of guanylate cyclase activating protein (GCAP), which activates GC.

Question 28

What is the role of GC?

Answer 28

GC converts GTP to cGMP, resulting in channel OPENING.

Question 29

Low Ca2+ causes what molecule to dissociate into what two components?

Answer 29

Recoverin dissociates from rhodopsin kinase.

Question 30

What is the role of rhodopsin kinase? What protein binds after its action, and to what?

Answer 30

Rhodopsin kinase phosphorylates MII, allowing arrestin to bind to it.

Question 31

How is MII prevented from binding to transducin?

Answer 31

It can no longer bind to transducin when bound to arrestin.

Question 32

What happens to guanylate cyclase activity with increasing calcium concentration?

Answer 32

It decreases, sigmoidal curve.

Question 33

Describe the prevalence of retinitis pogmentosa, the age it begins, and when visual impairment occurs.
Describe 4 symptoms.

Answer 33

1:4000 prevalence
Begins in teenage years, visual impairment begins by middle age.
Symptoms are:
-Night blindness
-Decreasing visual field
-Tunnel vision
-Blindness

Question 34

What is one of the earliest signs of retinitis pigmentosa, and what does it mostly affect?

Answer 34

Abnormal light evoked in ERGs.

Mostly affecs rods.

Question 35

How does the retina appear in retinitis pigmentosa (3)?

Answer 35

Bone spicule pigmentary changes
Loss of vessel definition
Optic disc pallor

Question 36

Retinitis pigmentosa is a mutation to what protein, and what does this result in?

Answer 36

Mutation to PDE.

Channels never close, and phototransduction never deactivates.

Question 37

Describe melanopsin, where its found, and whether it polarises or depolarises.

Answer 37

It is a specialised photopigment found in RGCs.

They depolarise in response to light.

Question 38

Does melanopsin activation depend on photoreceptor activation, or are they independent?

Answer 38

Activates independently of photoreceptors.

Question 39

What 2 roles does melanopsin have?

Answer 39

Circadian rhythms

Light induced sleep regulation

Question 40

Describe 3 differences between cone and rod phototransduction.

Answer 40

• Thermal isomerisation rates of cone pigments are higher than those for rhodopsin.
• Cones are less sensitive.
• Cones have much faster kinetics.