Figure 4. Distinct HCN1 and HCN2 distribution in juvenile and adult gerbil MSO. A–F, Before the onset of hearing (P9), scattered synaptophysin-positive presynaptic terminals (SYN; blue) and low levels of HCN1 (A, B, green) and HCN2 (C, D, green), and HCN4 (E, F, green) expression were observed in the somata of principal neurons (A, C, E, open asterisks) and dendrites (B, D, F). G–L, In contrast, ∼1 week after onset of discriminatory hearing (P22), distinct patterns of HCN1, HCN2, and HCN4 expression were observed in MSO neurons. Specifically, along with diffuse staining of the cytoplasm, HCN1 immunoreactivity (G, H, green) was enriched at the membrane of both somata (G, open asterisks) and dendrites (H), often juxtaposing SYN-positive presynaptic terminals (blue). Interestingly, HCN2 immunoreactivity was not observed in somata or dendrites of principal neurons, but was present in smaller cells (solid asterisks) and neuropil in the MSO (I, J). HCN4 immunoreactivity was observed in both the soma and dendrites but at lower levels than that for HCN1 (K, L). Scale bar, 5 μm.