56
and cones, and demonstrated that this increases the amplitude and duration of rod and cone
light responses. When considered in the frequency domain, HCN block reveals the low-
pass filter characteristic of the photocurrent [14]. This low-pass characteristic comes from
the slowness of the photocurrent, whose gating depends on a complex cascade of molecular
interactions. In contrast, the opening of voltage gated channels such as HCN channels de-
pends on the motion of charged voltage sensors and their interactions with the pore, which
is generally a much faster process [77, 76]. In normal physiological conditions, HCN chan-
nels reduce response amplitudes at low frequencies, flattening the frequency response of the
cell to light stimuli [14]. This flattening allows the voltage response to be frequency indepen-
dent over a wider range of stimulus frequencies, enabling the synapse to avoid saturation at
low frequencies while still passing higher frequency signals. From a signal processing stand-
point, this compensatory effect by HCN channels is analogous to high-pass filtering. Other
studies previously described high-pass filtering in the rod network and postulated that it
could be a way for the network to increase the signal to noise ratio for transient signals by
spreading them over a larger area [33,87,103]. By dividing transient signals into networked
rods, multiple parallel rod to bipolar cell synapses can be used, increasing the signal-to-noise
ratio at the bipolar cell layer. In addition to their role in the rod network, other studies have
also shown how high-pass filtering due to 7⅛ is important in increasing the speed of the the
individual photoreceptor light response [49, 30].
Although HCNl channels appear to be major players in the rod and cone light response,
they are not the only source of filtering by voltage-gated ion channels. Another current called