HCN4 is expressed in PKD2L1 but not TRPM5 cells. A, HCN4 is expressed in a population of CV taste cells by IHC. Inset shows expression of HCN4 in FG taste cells. B, HCN4 peptide blocks taste cell labelling. C-E and I-K, Double label IHC for TRPM5 (C, I) and HCN4 (D, J) demonstrating absence of HCN4 immunoreactivity in TRPM5 cells. TRPM5 and HCN4 signals label distinct cells in the merged images (E, K). F-H and L-N, Double label IHC for PKD2L1 (F, L) and HCN4 (G, M) demonstrating HCN4 immunoreactivity in PKD2L1 cells. PKD2L1 and HCN4 signals label the same cells in the merged images (H, N). O, Magnification of apical taste bud region from boxed area in N. PKD2L1 immunoreactivity extends to the apical taste pore whereas HCN4 immunoreactivity terminates beneath the PKD2L1 signal. P-R, Double label IHC for ZO-1 (P) and HCN4 (Q) demonstrating HCN4 localization below tight junctions in the merged image (R). Quantitation of HCN4 expression in TRPM5 cells (S) and PKD2L1 cells (T). Images are from CV papilla and are longitudinal sections except for C-E which are transverse sections. Images in A-H illustrate multiple taste buds; images in I-N and P-R illustrate one to two taste buds with the taste pore at the top. Scale bar is 25 μm in B and represents scale for A-H, 15 μm in I and represents scale for I-N and P-R, and 5 μm in O.