The name is absent

12

light sources were eliminated. Night vision scopes mounted to the microscope eyepieces
converted infra-red light to visible light in order to view the preparation. An MP-285 micro-
manipulator (Sutter Instruments, Novato, CA) provided fine control of the pipette electrode
for recordings.

2.2 Solutions

Normal Ringer’s Solution was used for recordings of cell light response, and consisted of
(in mM) 108 NaCl, 2.5 KCl, 1.2 MgCl₂, 2 CaCl₂, and 5 HEPES. Solution pH was titrated to
7.7 with NaOH. For recordings of HCN gated currents, 20 TEA, 5 CoCl₂ and 5 BaCl₂ were
added to the solution to block BK and Ca-dependent potassium channels, calcium channels,
and Ikx potassium channels, respectively. Intracellular solutions for whole cell recordings
consisted of (in mM) 106 K gluconate, 5 NaCl, 2 MgCl₂, 5 EGTA and 5 HEPES, with pH 7.4
titrated with KOH. Pipette solutions for cell attached recordings of HCN channels consisted
of 106 KCl, 167 NaCl, 1.8 CaCl₂, 1 MgCl₂, and 5 HEPES, and were titrated to pH 7.7 with
NaOH. The purpose of this non-physiologic solution was to increase the driving force of
the HCN channels inside the patch. The reversal potential for the HCN channels with this
solution was approximately +9 mV assuming a [Na]∕[K] permeability ratio of 0.33 [ 108,54].
In chirped current and current step injection experiments, K+ and Ca++ channels were
blocked, and the HCN channel blocker ZD 7288 was perfused at a concentration of 100 μM
to selectively block the Ih current. Although ZD 7288 is known to be a specific blocker of
HCN channels [15], in light response experiments, normal saline solution was used, and the

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