57
IKx has also been shown to be involved in high-pass filtering of rod light responses, espe-
cially responses to dim stimuli [19]. Iχx is similar to the М-current in neurons in that it is a
potassium conductance that is partially activated at resting potential, is further activated by
depolarization, and is largely non-inactivating [67] . Kx channels also appear to be similar
to EAG and Kcnv2 potassium channels, but its exact molecular origin is presently unknown
[47,27]. While both IKx and 7⅛ are known to mediate high-pass filtering in rods, an impor-
tant difference between the two is that during a light response (in which the rod membrane
hyperpolarizes), Kx conductance decreases, while h conductance increases. Due to their
different reversal potentials (-30 mV for Ih and -75 mV for Iκx)> the net current change
caused by Ih and IKx gating is inward during a light response, tending to counteract the ini-
tial hyperpolarization phase of the response [54,19]. This reactive depolarizing effect leads
to high-pass filtering of the input signal.
4.1.2 Results
To extend our previous studies of the h-current, we examined the contribution of Ih to the
rod light response when the Kx conductance, calcium conductance, and other potassium
conductances are blocked. To do this, we recorded the rod light response with 5mM Co, 5
mM Ba and 20 mM TEA present in the bath [10, 43, 54]. Five flashes of light of increasing
intensity were delivered to a rod and the voltage response was recorded. Then Ih was blocked
with 50 μM ZD7288 and light responses were recorded again.
In the presence of Co, Ba, and TEA, the rod’s light response is smaller and occurs from a