subunits of the same type to produce four different isoforms of HCN channels, HCNl-4.
Each of these isoforms has characteristic properties, including different gating kinetics, ac-
tivation curve, and sensitivity to cyclic nucleotides. However, subunits can also combine to
form heteromeric HCN channels with properties that are intermediate to those of the purely
homomeric channels [3].
Meanwhile retinal researchers had also discovered a hyperpolarization activated current
in photoreceptors in the early 1980s. Detwiler and Hodgkin found that such a current was
important in causing signals propagating in the rod network to peak earlier in more distant
rods than in the source rod, an effect they called ’’negative propagation delay” [33, 34]. At-
twell and Wilson dubbed this hyperpolarization activated current Ia, and studied its prop-
erties in a model of the salamander rod network [5]. They were unable to find a reversal
potential for Ia, and as a result, were the first to suggest that the observed current may be
a sum of two or more currents with similar kinetics. Owen and Torre studied this current’s
role in high pass filtering of signals in the rod network, and discovered that the current was
composed of two parts: a Cesium sensitive component that caused a conductance increase
on hyperpolarization, and a separate TEA sensitive potassium component that caused a
conductance decrease on hyperpolarization [87]. Bader and Bertrand first measured the
reversal potential for the cesium sensitive component, and showed that it is between the
reversal potentials for sodium and potassium, naming component 7∕l [9]. Hestrin was the
first to study the kinetics of ∕⅛ in the retina in detail, and showed that it is different from
the inward rectification that is present in muscle and oocytes. Barnes and Hille and Maricq