Fig. 1. Expression patterns of HCN1 and HCN2 immunoreactivity in the hippocampal formation of the immature rat.A–C, HCN1; D–F, HCN2. A, Low-magnification overview of HCN1 protein expression using immunocytochemistry, demonstrating this HCN isoform in the pyramidal cell layers of both CA1 and CA3. B, C, Detail of CA1 and CA3, respectively, demonstrating that the HCN1 isoform is expressed at the protein level in both pyramidal cells [arrows and insets; note the thick cytoplasmic band (boxed areas in A) of signal; see also Fig. 2] and interneurons (arrowheads). Note that in general, immunoreactivity in pyramidal cell somata is weaker than in interneurons, consistent with the distribution of channel proteins to the pyramidal cell dendritic fields (asterisks in A). D, Low-magnification view of HCN2 protein, revealing generally lower expression levels of this isoform. E, F, Detail of the CA1 and CA3 pyramidal cell layer (boxed areas inD), respectively, showing expression of the HCN2 isoform in individual eccentric, strongly labeled, presumed interneurons (arrowheads) as well as the much weaker expression in pyramidal cells (arrows and insets). Scale bars: A, D, 600 μm; B, C, E, F, 200 μm; insets, ∼60 μm.