The name is absent

27

the traces in figure 3.1 Al and Bl. The similarity in the activation curves and time constants
in rods and cones provides strong evidence that the kinetics, and therefore identity of the
channels that carry rod and cone 7⅛ is the same.

Most striking, however, is how closely the activation curves and activation time constants
for rod and cone I_h resemble the activation curve and time constants for the HCNl channel
isoform. Colored lines in figures 3.1 A3 and B3 show the activation curves for an allosteric
model of HCN 1, 2, and 4 channels. These allosteric models were developed from voltage
clamp data of homogenous populations of HCN isoforms expressed in human embryonic
kidney cells [2]. HCN3 kinetics are known to be intermediate between HCN2 and 4 [ 15]. We
estimated the parameters for HCN3 channels from another set of data [98]. The activation
curves for rod and cone I_h appear to be similar to HCNl, HCN3 and HCN4 channels (figure
3.1 A3 and B3), however, the voltage dependence of the time constants closely resembles the
time constants for HCNl channels, and is significantly different from HCN2, 3 and 4 (figure
3.1 A4 and B4). The similarity in the activation curves and activation time constants for rod
and cone I_h to the HCN1 isoform provides electrophysiological evidence that HCN channels
carry the I_h current, and that HCNl is the predominant HCN isoform present in both rods
and cones.

3.2.2 Distribution and Molecular Identification

We used antibodies against the two known neuronal HCN isoforms (HCNl and HCN3) to
label HCN channels in the salamander retina. See methods for a description of the antibod-

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