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more depolarized potential than in normal ringer solution (figure 4.1 A). Compare this with
the simulation of a normal light response in figure 4.2 A. The depolarization in darkness is
consistent with a reduction in the outward potassium current (due to Iχx and other un-
characterized potassium conductances) that normally counteract the inward dark current.
A small transient “nose” in the light response is seen due to the presence of Ih (figure 4.1 A).
When Ih currents are then blocked with ZD7288, the light response is seen to increase
in magnitude and the transient “nose” is abolished (figure 4.2 B). This demonstrates that 7⅛
can play a role light response recovery even when Iχx is blocked. In both the solutions with
and without ZD7288, an overshoot is seen following the recovery phase of the light response
(figures 4.1 A and B). This is a known effect of TEA on the rod light response first observed
by Fain et al., but unlike in other studies, the overshoots we observed failed to generate
regenerative spikes due to the block of calcium currents with Co [73, 42]. The ionic current
that causes this overshoot is not completely clear. Although our model could account for
much of the shape of the waveforms in figure 4.1 A and B when Kx and !!-conductances
were blocked (data not shown), it failed to account for this overshoot. One potential source
of the overshoot could be an uncharacterized effect of TEA and/or Co on the photocurrent,
which the model did not include.
Although others have noted the complementary conductance changes by ∕⅛ and IKx
during a light response, the magnitude and time courses of these changes are unknown. To
evaluate the simultaneous contributions of ∕⅛ and Iχx to the rod light response, we simu-
lated the rod light response by solving differential equations describing voltage gated chan-