Retina - Biochemistry and Physiology - Part 2 - Flashcards
Briefly discuss the inactivation mechanisms of phototransduction: role of rhodopsin kinase, arrestin, recoverin, guanylate cyclase and calcium levels.
R* is phosphorylated by a rhodopsin kinase (RK), partially blocking the activity of R*
♣ Recoverin
a. A calcium-binding protein
b. Regulates rhodopsin kinase (RK) and phosphorylation of rhodopsin
♣ Guanylate cyclase in photoreceptor
a. Guanylate cyclase activity is regulated by a calcium-feedback mechanism
1) Inhibited by physiologic concentrations of calcium (dark, unstimulated state of photoreceptor)
2) Activated when calcium levels drop during phototransduction (light, stimulated state)
Discuss the mechanisms of light adaptation involving the role of calcium:
1) regulating guanylate cyclase activity and cGMP levels (most important mechanism in regulating photoreceptor response to steady light)
Role of calcium
(1) Guanylate cyclase
- Cytoplasmic Ca++ has a powerful negative-feedback action on synthesis of cGMP
- Modulates photoreceptor sensitivity
- Regulates activity of guanylate cyclase (guanylyl cyclase)
Cytoplasmic calcium levels fall with activation of visual pigment
°Light ultimately results in activating PDE, promoting cGMP hydrolysis and closing cGMP-gated channels.
°Ca++ (and Na+) influx stops, but Ca++ efflux continues due to the Na+/Ca++ exchanger (in outer segment plasma membrane.
°Cytosolic Ca++ levels decrease, activating guanylate cyclase
°Partial recovery in cGMP level, causing adaptation to light
Discuss the mechanisms of light adaptation involving the role of calcium:
2) effect on recoverin.
(2) Recoverin – under high calcium levels recoverin binds to rhodopsin kinase. As calcium levels fall, recoverin detaches from rhodopsin kinase, and RK binds to R*.
What is light-induced translocation of proteins; how does it affect transducin & arrestin?
Movements of proteins within photoreceptor occur at high (saturating) light intensities
°Continuous bright light causes majority of transducin in rods (but not cones) to move from the outer segment to inner segment. Reduces transducin activation rate
°Intense light causes translocation of arrestin in both rods and cones, moving from inner to outer segment.
Describe the 2 phases of dark adaptation.
Fast phase =- cones (<10 min)
Cones have rapid recovery from light exposure compared to rods
1) Inactivation of phototransduction is much more rapid in cones than rods
2) Cones regenerate 11-cis-retinal from both RPE + Muller cells.
Slow phase = rods (20-30 min) More sensitive to a single photon than cones, but slower kinetics (shutting off phototransduction response) and rods saturate with bright light.
How do cone responses differ from rods?
Cones have faster kinetics than rods. Thus, cones operate over a wide range of bright light intensities without saturating.
What is the rate limiting step of dark adaptation?
Rate limited kinetics due to rate-limited delivery of 11-cis-retinal from the RPE to the opsin in the outer segments
Electroretinography: measures
an extracellular potential arising from currents that flow through the retina as a result of neuronal signaling. Early receptor potential: no latency, reflects the bleaching of visual pigment within 1.5 milliseconds (e.g., rhodopsin to metarhodopsin II).
Concept of parallel processing
There is simultaneous processing of visual information through independent circuits (pathways).
a. Representation of the visual scene is reduced to a limited number of specialized parallel circuits.
b. The photoreceptor signal is processed and integrated within the retina in the terms of center-surround receptive fields.
- Retinal processing can be described in the context of a receptive field.
c. Retinal neurons enhance and segregate different sensory cues of the visual stimulus
Convergence:
126 million photoreceptors converge onto 1 million ganglion cells. Signals from many rod photoreceptors can converge onto several bipolar cells, then to one ganglion cell. Rods pool signals to provide high sensitivity for dark-adapted vision. The lower convergence for cones preserves higher spatial discrimination & acuity.
Divergence:
a cone can transmit a signal to several bipolar cells of different types, then onto several ganglion cell classes.
List the vertical connections for processing the photoreceptor signal; what neurotransmitter do these neurons utilize?
- vertical pathway: photoreceptors to bipolar/ganglion cells
- photorecptors and bipolar cells use glutamate as the neurotransmitter to encode retinal signals
- glutamate provides fast vertical channel signaling
Which cells of the vertical pathway are multipolar neurons?
- ganglion cells
Which of these cells give a graded response?
- photoreceptors & bipolar cells
- glutamate release varies with changes in illumination
An action potential?
- ganglion cells generate action potentials, sending info to the visual cortex about how the light is distributed in space and time
Realize that photoreceptors and bipolar cells have ribbon synapses - what is the general advantage of ribbon release?
- use ribbon synapses for high rates of tonic neurotransmitter release
List the neurons responsible for lateral interactions.
- lateral interactions: horizontal cell connections in outer plexiform layer (OPL) & amacrine cell connections in the inner plexiform layer (IPL)
- lateral pathways emphasize differences in signals between neighboring photoreceptors
Realize the general concept that photoreceptor glutamate has a different effect on bipolar cells depending
on the receptor on the bipolar dendrites.
Realize that the type of glutamate receptor defines the type of bipolar cell:
(Bipolar cells are classified as OFF or ON- cells based on the light conditions when the cell is depolarized).
OFF bipolar cells:
ionotropic receptors, depolarized when light is OFF
ON bipolar cells:
metabotropic receptors, hyperpolarized in response to steady release of glutamate (photoreceptor resting state) and depolarized when the light is ON.
At what level of the retina are cone signals split into on- and off- bipolar cell pathways?
- Cone signals are split into off and on channels at the outer plexiform layer (synapse with off and on bipolar cells)
Describe the pathway by which rods send their signals to ganglion cells
rod —> rod bipolar cell (ON cell) —> all amacrine cell —> cone bipolar cell (ON and OFF pathways) —> ganglion cell
At what level of retina do rod signals split into on- and off- pathways?
- INNER plexiform layer
