The hyperpolarization-activated and cyclic nucleotide-gated (HCN) channel isoforms HCN1, HCN2, and HCN4 were localized by immunofluorescence in the rat retina. Double labeling with the vesicular glutamate transporter (VGLUT1) was used to identify bipolar cell axon terminals in the inner retina. The HCN1 channel was localized to two cell types with differing intracellular distributions, insofar as staining was seen in the dendrites of a putative OFF-type cone bipolar cell and in the axon terminals of an ON-type bipolar that ramifies in stratum 3 (s3) of the inner plexiform layer (IPL). Staining for HCN4 was seen in two sets of bipolar axon terminals located in s2 and s3 and positioned between the two bands of choline acetyltransferase (ChAT) staining. The cells that ramify in s2 were identified as type 3 cone bipolar cells and the cells that ramify in s3 cells as a subclass of type 5 cone bipolars. The latter group, designated here as type 5b, exhibit diffuse axon terminals and can be distinguished from the narrowly stratifying type 5a cells. Double labeling showed that type 5b cone bipolar cells express both HCN1 and HCN4 as well as HCN2. Superposition of HCN channel labeling with VGLUT1 staining confirmed the presence of a cone bipolar cell whose terminals ramify in the same stratum of the IPL as type 5b cells but that do not express these HCN channels.
(c) 2007 Wiley-Liss, Inc.