Lecture 18. Sight and Blue-Tinged Vision

Question 1

What are rod cells responsible for?

Answer 1

Non-colour vision at low light

Question 2

What makes up the outer segment of rods?

Answer 2

~1000 discs not connected to the plasma membrane
Each is a closed sac of membrane with embedded photosensitive rhodopsin molecules

Question 3

What is rhodopsin?

Answer 3

A visual pigment, is a specialised GPCR made of opsin (the GPCR protein component), linked to 11-cis-retinal (a prosthetic group that is the chromophore or light-absorbing group)

Question 4

What happens to retinal during light capture?

Answer 4

1. Alternating single and double bonds form a ‘polyene’ with a long unsaturated network of electrons that can absorb light energy
2. Light absorption causes cis-trans isomerisation around the C12 and C13 bond
3. The N of the key lysine moves 5 Å (0.5 nm)

Question 5

How long does it take for light to be converted into atomic motion?

Answer 5

A few picoseconds

Question 6

What does light absorption by retinal alter?

Answer 6

The conformation of the GPCR (inactive rhodopsin becomes activated metarhodopsin II)

Question 7

What does metarhodopsin do?

Answer 7

Stimulates nucleotide exchange on the α-subunit of a specific heterotrimeric G protein called transducin (Gt)

Question 8

What is the process of transducin Gαt activating cGMP phosphodiesterase?

Answer 8

Light activates rhodopsin which activated the Gt transducin (Gαt,Gβt,Gγt)
Gαt (GTP) stimulates cGMP phosphodiesterase (cGMP PDE) which removes cGMP from cGMP-gated ion channels

Question 9

What is mammalian rhodopsin’s peak absorbance?

Answer 9

500nm

Question 10

What does light close?

Answer 10

cGMP gated ion channels, reducing influx of Ca⁺⁺

Question 11

How is rhodopsin terminated in very high light intensities?

Answer 11

Light activated rhodopsin
Light-activated rhodopsin can be phosphorylated by rhodopsin kinase

Question 12

What does arrestin do?

Answer 12

Binds to fully phosphorylated rhodopsin: and this rhodopsin stops activation of transducin

Question 13

What are the three mechanism to make rods insensitive to light?

Answer 13

1. Prolonged cGMP-gated channel closure
2. Phosphorylation of opsin reduces transducin activation
3. Arrestin binding to phosphorylated opsin stops transducin activation

Question 14

How long does it take for the rods to reset?

Answer 14

20-30 minutes

Question 15

What are the three visual pigments human colour relies on?

Answer 15

‘Blue’ - peak absorbance at 416-426nm
‘Green’ - peak absorbance at 530-532nm
‘Red’ - peak absorbance at 560-563nm

Question 16

How many individual pigments do each cone cell express?

Answer 16

One

Question 17

What is the photoreceptor comprised of?

Answer 17

An opsin (a modified GPCR) with 11-cis-retinal as the chromophore and a (different) transducin

Question 18

How are cephalopod eyes different from human eyes?

Answer 18

Light strikes the retina directly
There is no blind spot
The retina has only rod cells

Question 19

Why do cephalopod have oddly shaped pupils?

Answer 19

Allow light to enter the eye through the lens from many directions at the same time, rather than just straight into the retina

Question 20

What do dichromats have difficulty distinguishing?

Answer 20

Similarly sized objects where lightness varies in an unpredictable manner

Question 21

What causes protanopia?

Answer 21

Mutation in L-cone (red)

Question 22

What causes deuteranopia?

Answer 22

Mutation in M-cone (green)

Question 23

What causes tritanopia?

Answer 23

Mutation in S-cone (blue)

Question 24

What is sildenafil citrate?

Answer 24

A potent inhibitor of cGMP phosphodiesterase. It is most active against phosphodiesterase type 5 (PDE-5)
Sildenafil citrate also inhibits PDE-6. PDE-6 regulates blue-green colour discrimination in the retina

Question 25

What is a side effect of using sildenafil?

Answer 25

A side-effect of sildenafil citrate can be blue-tinged vision