The name is absent

22

Chapter 3

Properties and Function of HCN Channels in Photoreceptors

3.1 Introduction

Hyperpolarization-activated cyclic nucleotide gated (HCN) ion channels are activated by
membrane hyperpolarization, and depending on the channel isoform, modulated by cyclic
nucleotides such as cAMP to a varying degree [39, 15, 96]. HCN channels serve important
functions in many systems of the body. For example, they act as a cardiac pacemaker in
the SA node and other parts of the heart [39] and contribute to oscillatory potentials in
the central nervous system (CNS) [96]. In the retina, immunocytochemical studies have
shown that various isoforms of HCN channels are expressed in retinal neurons [83], but the
functional specificity of these expression patterns are not clear.

In rod and cone photoreceptors, hyperpolarization-activated currents (∕⅛) have been
characterized with whole-cell or microelectrode recording techniques [54, 13, 78, 30, 31].
It is not clear, however, whether Ih in rods and cones is mediated by HCN channels and
whether these two photoreceptors use the same isoform of channels. Although current-
voltage relations and activation properties of whole-cell ∕⅛ currents in rods have been de-
scribed [54, 30, 31], the single channel conductance and density of I_h or HCN channels
in photoreceptors are unknown. Here we present a systematic study of the single channel

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