Iarizing bipolar cells, amacrine cells, and retinal ganglion cells. However, salamanders, like
all non-primates, do not have the dense central collection of cones and associated bipolar
cells that comprise the fovea.
While genetic manipulation has made the mouse the model organism of choice for most
biologists, its photoreceptors are too small to make practical recordings from. However, it is
possible to record from other cell types in the mouse retina such as bipolar cells, horizontal
cells or ganglion cells. Because the focus of this thesis is photoreceptor electrophysiology,
the salamander is the model organism of choice for our studies.
1.2.1 Ih and HCN Channels
The study of the ∕⅛ current and HCN channels has a very diverse history. It was first recog-
nized as a hyperpolarization gated conductance that regulates pacemaking in the sino-atrial
(SA) node of the heart [23]. In cardiac tissue the current was called ʃʃ, or ’’funny cur-
rent”, because it was gated during hyperpolarization and contributed to depolarization of
myocytes. It was found that the activity of this current could be increased by ∕3-adrenergic
input to the heart [22], which accelerated heart rate. If activity can be decreased by cholin-
ergic input to the heart through the vagus nerve [37, 36]. Both of these effects were found
to be mediated by cAMP internal to the cell. Increasing cAMP concentrations cause a neg-
ative shift in the activation curve, and decreasing levels a rightward shift. In the late 1990s,
Molecular cloning revealed the genes encoding the protein subunits (HCN1 -4) that form the
channels for the Ih and If currents [92, 93]. These subunits normally combine with other