The name is absent

50

3.4.2 ConductanceofsingleHCNlchannels

By using NSFA, we provide the first known estimate of a single channel conductance in the
retina. We show that photoreceptor HCN channels have a conductance of approximately 663
fS (figure 3.3), comparable to HCN1 conductance in rat cortical neurons [66]. This similarity
is in line evidence demonstrating that HCN channels are highly conserved across species
[59]. We find that HCN channels are expressed in the inner segments of rods and cones,
with approximately 2000 channels per cell. The density of these channels in the salamander
rod and cone inner segment appears to be similar to the density of HCN channels near
the soma of rat pyramidal cells, but less than the density near the distal ends of dendrites
(see Results) [66, 74]. Of the many channels present, only a small number are active at
the rod dark membrane potential (figure 3.4). With only a few channels open in dim light,
the extremely low single channel conductance of HCN channels may be advantageous by
minimizing membrane noise.

3.4.3 HCN1 channels create a bandpass filter in photoreceptors

To investigate how HCN channels affect the electrical properties of rods and cones, we ex-
amined the effect of HCN channels on their responses to extrinsic electrical stimuli. Using
frequency-chirped stimuli, we demonstrate that HCN channels turn the low-pass filter char-
acteristic of the rod and cone membrane into a band-pass filter, with a peak of approximately
4 Hz for both cells (figure 3.5). This is similar to observations of the frequency response in
the dendrites of rat hippocampal neurons [84]. Although the bandpass filtering effect has

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