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of rods and cones differs in accordance with the distinct function of these two cell types.
Applying 50 μM of the HCN blocker ZD 7288 greatly increased the amplitude of the
light response to a bright flash for both dark adapted rods and cones (figure 3.8 Al and Bl).
In rods, blocking HCN channels abolished the transient “nose” present in the light response
(figure 3.8 Al), while in cones, blocking it increased the response magnitude and appeared
to eliminate the voltage overshoot present in the recovery phase (figure 3.8 Bl). With HCN
channels blocked, the light response magnitude and time course for rods and cones are both
increased for flash stimuli. This means that HCN channels play a role in aiding recovery of
from bright stimuli in both cell types from a dark-adapted baseline.
To measure the contribution of HCN channels to the frequency response of photorecep-
tors in response to light, their natural stimulus, chirped sine wave-modulated light stimuli
were generated that ranged from .5 to 5 Hz for a rod over the course of 20 seconds (equa-
tion 2.1). These stimuli were similar to the electrical stimuli described previously, but in the
form of light rather than current. Background illumination for 2 seconds adapted the retina
to the average light intensity of the stimulus. In a normal rod the light stimulus produced
a response of consistent amplitude from .5 to 1 Hz at 1 ■ 10~2 lux (figure 3.8 A2)., falling off
rapidly at higher frequencies (figure 3.8 A3). However, when HCN channels were blocked
with ZD 7288, the response was much greater in magnitude, especially at low frequencies
(figure 3.8 A2, red trace). The frequency components of the HCN blocked response were
seen to decay beginning at .5 Hz, similar to a low pass filter (figure 3.8 A3). The decline at
frequencies higher than 1 Hz was much more rapid than the falloff of the low-pass filter of