Lecture 12 - Retinal Physiology

What are the main functions of the retinal pigment epithelium (RPE)?

Blood-retinal barrier, phagocytosis of photoreceptor discs, absorption of light, interphotoreceptor matrix formation, vitamin A metabolism/storage, and growth factor production

What role does melanin in the RPE play?

It absorbs UV and visible light

What growth factors are produced by the RPE?

Vascular endothelial growth factor (VEGF) and pigment epithelial-derived factor (PEDF)

What is the function of the RPE in the blood-retinal barrier?

Selective absorption of nutrients from the choriocapillaris and excretion of waste/metabolic byproducts

Where does exchange occur between RPE and retina?

Through the interphotoreceptor matrix (IPM)

Is RPE fluid transport isotonic?

Yes, there is no difference in osmolarity between the cell and extracellular space

Does isotonic mean equal charge in RPE physiology?

No, isotonic does not mean equal charge

What is the transepithelial potential difference in RPE?

Approximately 30 mV

Which side of the RPE is more positive?

The apical side is more positive than the basal side

What determines ion permeability in RPE membranes?

The location of transporters

What mediates fluid and nutrient transport in the RPE?

Na+/K+ ATPase pumps, cotransporters, exchangers, Cl- channels, aquaporins, and glucose transporters

What cotransporters are involved in RPE transport?

Na+/K+/Cl-, Na+/2HCO3-, and H+/lactate

What is the role of H+/lactate transport?

Removal of metabolic byproducts

What exchangers are involved in RPE transport?

Na+/H+ and Cl-/HCO3- exchangers

What is the direction of net Na+ transport in RPE?

Toward the choroid

What determines movement of other ions into the RPE cell?

The Na+ gradient

Which ions are influenced by the Na+ gradient?

HCO3-, K+, and Cl-

Can RPE use lactate metabolically?

Yes

What is the direction of net Cl- transport in RPE?

Toward the choroid

Which ion is more abundant, Cl- or HCO3-?

Cl-

What is the main driver of water transport in the RPE?

Cl- transport

What reaction is catalyzed by carbonic anhydrase in the RPE?

CO2 + H2O → HCO3- + H+

What is the function of carbonic anhydrase in the RPE?

Buffering and continuous production of HCO3-

How is HCO3- used in RPE transport?

In HCO3- exchangers (bringing Cl- into cell and secreting HCO3- toward choroid) and Na+/HCO3- cotransporters (moving both ions toward choroid)

What is the direction of net HCO3- transport?

Toward the choroid

What role does HCO3- play in water movement?

Helps determine direction of water movement

What is the direction of net glucose transport in the RPE?

Toward the retina

Who consumes more glucose, retina or RPE?

Retina

Who releases more glucose, retina or RPE?

RPE

What is the direction of net lactate transport in the RPE?

Toward the choroid

Who releases more lactate, retina or RPE?

Retina

Who consumes more lactate, retina or RPE?

RPE

Why does the RPE use lactate?

For metabolism to conserve glucose for the retina

Which cells can also use lactate as an energy source?

Müller cells and ganglion cells

Why are RPE phagocytosis and vitamin A metabolism important?

They maintain photoreceptor health and function

What are photoreceptor outer segments composed of?

Stacks of membranous discs containing photopigments

Where are new discs formed?

In the inner segment

What is the lifespan of photoreceptor discs?

Approximately 10 days

What happens to older discs?

They are displaced and shed

What is the first step of the photoreceptor renewal system?

New discs are formed in the inner segment

What is the second step of photoreceptor renewal?

Discs move through the cilium to the outer segment

What is the third step of photoreceptor renewal?

Older discs are displaced by new discs

What is the fourth step of photoreceptor renewal?

Older discs are shed from the tip of the outer segment

Are discs shed individually?

No, usually in groups

What is required for disc removal?

Assistance from the RPE

What is the fifth step of photoreceptor renewal?

Shed discs are taken up by RPE cells

What is the sixth step of photoreceptor renewal?

RPE cells degrade discs via phagocytosis

What are the steps of RPE phagocytosis?

Recognition/binding, internalization, and digestion

When does photoreceptor shedding occur?

When the photoreceptor is less active

When do cones shed discs?

At night

When do rods shed discs?

Early morning

What is the purpose of the photoreceptor renewal system?

To combat damage from light absorption over time

Why is this necessary?

The retina is exposed to high levels of light and oxidative stress (free radicals and oxidized lipids)

What are photopigments?

Photosensitive pigments that undergo chemical changes when absorbing photons of light

What are the two components of photopigments?

Opsin and chromophore

What is opsin?

A membrane protein interlaced into disc membranes that determines absorption profile

How many times does opsin cross the membrane?

Seven times

What is the chromophore?

A molecule within opsin that absorbs photons of light

What is the typical chromophore in photoreceptors?

11-cis-retinal

Which photopigment is in rods?

Rhodopsin (498 nm)

Which photopigment is in L-cones?

Erythrolabe (~588 nm)

Which photopigment is in M-cones?

Chlorolabe (~532 nm)

Which photopigment is in S-cones?

Cyanolabe (~420 nm)

What is 11-cis-retinal derived from?

Vitamin A

Why is vitamin A important for vision?

It is required for photopigment chromophore function

What is the state of photopigments without light?

Inactive (opsin + 11-cis-retinal)

What happens when light is present?

11-cis-retinal is converted to all-trans-retinal

What causes this transformation?

Breakage of a double bond in 11-cis-retinal

What is the photopigment state after activation?

Opsin + all-trans-retinal

What is the resting membrane potential of photoreceptors in the dark?

Approximately -40 mV

How does this compare to typical neurons?

It is more depolarized

What happens in photoreceptors at rest?

They continuously release neurotransmitter

What is this process called?

Dark current

What is the outer segment permeable to?

Na+

What is located in the inner segment?

Na+/K+ ATPase pumps

Which ion is the main contributor to dark current?

Na+ (Ca2+ plays a smaller role)

What opens Na+ and Ca2+ channels in photoreceptors?

cGMP

What increases PDE6 activity?

Light stimulation via transducin

What does transducin do?

Stimulates PDE6

What is the first step of dark current?

Na+/K+ ATPase pumps remove Na+ from inner segment (active transport)

What is the second step of dark current?

Na+ and Ca2+ leak into the outer segment

What is the third step of dark current?

Na+ and Ca2+ move through the cilium to the inner segment

What is the fourth step of dark current?

Photoreceptor remains depolarized and releases glutamate

What happens to dark current when light appears?

It is inactivated

What is phototransduction?

The process of converting light into an electrical signal

How fast does phototransduction occur?

Within milliseconds

What happens to photoreceptors in light?

They hyperpolarize (approach -70 mV)

How does this compare to normal neurons?

It is opposite (they hyperpolarize instead of depolarize)

What happens to neurotransmitter release in light?

It decreases or stops

What determines the degree of hyperpolarization?

Stimulus intensity (graded potential)

What is the first step of phototransduction?

Photon absorbed by chromophore

What is the second step of phototransduction?

11-cis-retinal → all-trans-retinal

What is the third step of phototransduction?

Activation of transducin

What is the fourth step of phototransduction?

Decrease in cGMP concentration

What is the fifth step of phototransduction?

Closure of Na+ and Ca2+ channels

What is the effect of channel closure?

Reduced positive ion influx and increased negativity inside the cell

What is the sixth step of phototransduction?

Reduced or stopped glutamate release

What happens to phototransduction after glutamate release decreases?

Signal is passed to bipolar cells leading to downstream retinal processing

What type of potentials do photoreceptors produce?

Graded potentials (not action potentials)

Why are photoreceptor responses graded?

The degree of hyperpolarization depends on light intensity