The name is absent

33

to the number predicted with whole cell NSFA (N = 2214). Due to the small size and the
geometry of cones, long-term stable recordings (necessary for NSFA) of cone HCN currents
were very challenging to obtain. However, recorded traces from cones gave an estimate of
526 fS, with 183 channels present per patch (figure 3.3 D), which is similar to the single
channel conductance in rods. From this and the cone whole cell conductance (1.34 ± .48
nS), we estimate there are 2021 ± 725 channels per cone. The comparable conductance and
number of HCN channels in rods and cones is not surprising, because from analysis of the
whole cell currents, we show the identity of the channels is the same, and the magnitude of
the whole cell currents are similar (figures 3.1).

Our estimates of photoreceptor HCN conductance are comparable to the conductance
reported for single HCN channels in rat cortical pyramidal neurons [66], which were also
believed to be the HCNl isoform. The significance of the similarity in conductance of HCN
channels in photoreceptors and in pyramidal cells is twofold. First, this supports our whole
cell and immunohistochemical evidence that the HCNl isoform is the dominant isoform
expressed in photoreceptors. Second, it indicates that the conductance of HCNl channels
is similar in different species, which is consistent with genetic evidence showing that HCN
channels are highly conserved across species [59].

Comparison of the whole-cell and cell attached NSFA allows us to make estimates of
HCN channel density and distribution. HCN currents could only be observed with cell-
attached patches from the inner segment and cell bodies of rod and cone photoreceptors.
This distribution is consistent with the proposed role of HCN channels as modulating the

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