Mechanism of Phototransduction in Retinal Rods 
and Cones 
King- Wat Yau, Ph.D. — Investigator 
Dr. Yau is also Professor of Neuroscience at the Johns Hopkins University School of Medicine. He received 
an A.B. degree in physics from Princeton University and a Ph.D. degree in neurobiology from Harvard 
University. He did postdoctoral research at Stanford University with Denis Baylor and at Cambridge 
University, England, with Alan Hodgkin. For six years thereafter, he was on the faculty at the Department 
of Physiology and Biophysics of the University of Texas Medical Branch at Galveston. He has received the 
Rank Prize in Optoelectronics from the Rank Prize Funds, England. 
VISION begins in the rods and cones of the 
retina, where light is absorbed and trans- 
duced into a neural signal consisting of an elec- 
trical hyperpolarization at the photoreceptor 
membrane. This signal is relayed to second-order 
neurons in the retina through a modulation of the 
release of synaptic transmitter at the photorecep- 
tor's terminal. In darkness the transmitter is re- 
leased at a high rate, and in light the membrane 
hyperpolarization reduces the release in a graded 
fashion. This modulation of synaptic transmitter 
release can lead to a hyperpolarizing or depolar- 
izing response to light in a second-order neuron, 
depending on the polarity of a given synapse. 
The phototransduction process — the way the 
hyperpolarizing response to light is generated in 
the receptors — is as follows. In darkness an ionic 
conductance in the plasma membrane of the re- 
ceptor's outer segment (the part of the cell that 
contains the visual pigment) is kept open by the 
cyclic nucleotide guanosine 3':5'-cyclic mono- 
phosphate (cGMP), letting both Na^ and Ca^"^ 
into the cell. This "dark" current depolarizes the 
cell and causes the steady release of synaptic 
transmitter described above. 
Light activates the following reaction cascade: 
light photoisomerization of visual pigment -»• 
G protein activation -> cGMP phosphodiesterase 
stimulation cGMP hydrolysis. As a result, the 
cGMP level falls in the outer segment, causing 
the ionic conductance to close and leading se- 
quentially to membrane hyperpolarization and 
the reduction of synaptic transmitter release. This 
phototransduction scheme applies to both rods 
and cones, with only quantitative differences be- 
tween the two types of receptors. 
One consequence of the conductance closure 
in the light is that the Ca^^ influx stops. The re- 
sulting imbalance between influx and efflux 
leads to a decrease in the intracellular free Ca^* 
concentration. This Ca^"*^ decrease reduces a tonic 
inhibition exerted by Ca^+ on the cGMP-synthe- 
sizing enzyme guanylate cyclase and causes an 
increase in the synthesis of cGMP in the light. 
Thus Ca^"^ mediates a negative feedback control 
on the light-activated cGMP hydrolysis, and this 
feedback should be a candidate mechanism un- 
derlying the well-known phenomenon of back- 
ground light adaptation in photoreceptors. In- 
deed, we have found that this adaptation 
essentially disappears upon removing the feed- 
back experimentally by eliminating the Ca^"^ in- 
flux and efflux. 
The effect of Ca^"^ on rod guanylate cyclase has 
been studied by others in biochemical experi- 
ments in vitro. This effect is now known to in- 
volve recoverin, a novel Ca^^-binding protein 
that activates guanylate cyclase at low Ca^^ con- 
centrations but loses this ability when Ca^"^ is 
bound to it. One drawback of the biochemical 
experiments is that unphysiological ionic con- 
centrations (e.g., very high Mg^"^) were used to 
measure the cyclase activity. We have now stud- 
ied this Ca^^ modulation of the cyclase in more 
physiological conditions, by recording the 
cGMP-activated current from an isolated, open- 
ended rod outer segment with a suction pipette 
while dialyzing the interior of the outer segment 
with different Ca'^^ concentrations. 
The Ca^^ effect on the guanylate cyclase could 
be derived from the magnitude of the cGMP- 
activated current. We have found that the cyclase 
activity is very sensitive to the free Ca^^ concen- 
tration, with its maximum activity being approxi- 
mately halved at 100 nM Ca^^. This is similar to 
biochemical measurements. The cyclase depen- 
dence on Ca^^ shows a Hill coefficient of approxi- 
mately 1.5, which is lower than the value of 
around 4 in biochemical measurements. The cy- 
clase activity becomes relatively insensitive to 
Ca^^ at a GTP concentration of greater than 1 mM, 
suggesting that the effect of Ca^"^, acting through 
recoverin, may primarily be to reduce the affinity 
of the enzyme for its substrate. Further experi- 
ments on this problem are in progress. 
Another problem we are working on is a molec- 
ular characterization of the cGMP-activated con- 
ductance mediating phototransduction. The con- 
ductance now appears to belong to a family of 
cyclic nucleotide-gated channels that includes 
the cGMP-gated channels in retinal rods and 
cones, as well as the cGMP/cAMP-gated channel 
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