Ligand- and voltage-gated ion channels are fundamental build- ing blocks of excitable nerve cells. The molecular diversity of these channels contributes to their functional heterogeneity 1 . However, not only the molecular structure but also the precise subcellular location and the density of channels are crucial in neuronal com- munication and integration 2–8 . The organizational principles of the cell-surface expression of ligand-gated ion channels have been studied. For example, high-resolution immunolocalization showed that AMPA-type 9–11 and NMDA-type 11–13 glutamate receptor subunits and GABA A receptor subunits 12,14,15 are selectively tar- geted to functionally different synapses of a single cell, establish- ing a presynaptic input-selective distribution of postsynaptic receptors. The amount and density of postsynaptic GABA A and glutamate receptors are also regulated in a presynaptic input-spe- cific manner 10,11,16,17 . Furthermore, distinct GABA A receptor sub- types are segregated to synaptic versus extrasynaptic sites, underlying distinct forms of inhibition in certain cell types 18 . Much less is known about the cell-surface distribution of voltage-gated channels, although they are important in neuronal integration 5,8 . This is mainly due to the scarcity of high-resolu- tion immunogold localization studies of these channels. Elec- tron microscopic (EM) immunogold localization of voltage- and Ca 2+ -activated K + channels (BK) showed their enrichment in presynaptic active zones of glutamatergic terminals and the lack of labeling on postsynaptic dendrites 19 . Voltage-gated channels on the axo-somato-dendritic surface of nerve cells have been mapped with patch-clamp recordings. This technique is extreme- Polarized and compartment- dependent distribution of HCN1 in pyramidal cell dendrites Andrea Lörincz 1,2 , Takuya Notomi 3 , Gábor Tamás 2 , Ryuichi Shigemoto 3 and Zoltan Nusser 1 1 Laboratory of Cellular Neurophysiology, Institute of Experimental Medicine, Hungarian Academy of Sciences Szigony Street 43, 1083 Budapest, Hungary 2 Department of Comparative Physiology, University of Szeged, Közép fasor 52, 6726 Szeged, Hungary 3 Division of Cerebral Structure, National Institute for Physiological Sciences, The Graduate University for Advanced Studies, School of Life Science, Okazaki, CREST Japan Science and Technology Corporation, Kawaguchi,Okazaki 444-8585, Japan Correspondence should be addressed to Z.N. (nusser@koki.hu) Published online 21 October 2002; doi:10.1038/nn962 An ion channel’s function depends largely on its location and density on neurons. Here we used high-resolution immunolocalization to determine the subcellular distribution of the hyperpolarization-activated and cyclic-nucleotide-gated channel subunit 1 (HCN1) in rat brain. Light microscopy revealed graded HCN1 immunoreactivity in apical dendrites of hippocampal, subicular and neocortical layer-5 pyramidal cells. Quantitative comparison of immunogold densi- ties showed a 60-fold increase from somatic to distal apical dendritic membranes. Distal dendritic shafts had 16 times more HCN1 labeling than proximal dendrites of similar diameters. At the same distance from the soma, the density of HCN1 was significantly higher in dendritic shafts than in spines. Our results reveal the complex cell surface distribution of voltage-gated ion-channels, and predict its role in increasing the computational power of single neurons via subcellular domain and input-specific mechanisms. ly useful because it reveals the location of functional channels. However, small subcellular compartments remain inaccessible with this approach, and a differential current density does not necessarily mean distinct densities of channels. Patch-clamp studies suggested uneven subcellular distribution of transient A-type K + channels (I A ) 20 , N-type Ca 2+ channels 21,22 , Na + chan- nels 23,24 and hyperpolarization-activated channels in nerve cells 25–31 , but a uniform density of Na + channels in the axo- somato-dendritic domains of some other cells was also report- ed 32,33 . The differential somato-dendritic distribution of I A may not necessarily reflect a differential channel distribution, as these channels are differentially regulated by PKA and PKC across the somato-dendritic surface of hippocampal pyramidal cells 34 . Fur- thermore, all of these channels/currents have been recorded in large-diameter apical dendrites, but their density in small-diam- eter secondary dendrites and in spines remains elusive. Here we used electron microscopy with immunogold localiza- tion of HCN1 to reveal its cell-surface distribution. HCN1 is one of the four known subunits (HCN1–4) of the hyperpolarization-acti- vated and cyclic-nucleotide-gated nonselective cation channels 35–38 . The homo- or heteromeric assemblies of these subunits are main- ly responsible for the functional diversities of H current (I h ) 39,40 . Our results demonstrate that this channel is unevenly distributed in the axo-somato-dendritic surface of pyramidal cells. The distal dendritic shafts had ∼60-fold larger immunoparticle densities than the somata, whereas pyramidal cell axons were immunonegative. There was also a ∼16-fold difference in the density of HCN1 label- articles nature neuroscience • volume 5 no 11 • november 2002 1185 © 2002 Nature Publishing Group http://www.nature.com/natureneuroscience