What are the implications for the existence of subthreshold ion channels, their localization profiles, and plasticity on local field potentials (LFPs)? Here, we assessed the role of hyperpolarizationactivated cyclic-nucleotide-gated (HCN) channels in altering hippocampal theta-frequency LFPs and the associated spike phase. We presented spatiotemporally randomized, balanced theta-modulated excitatory and inhibitory inputs to somatically aligned, morphologically realistic pyramidal neuron models spread across a cylindrical neuropil. We computed LFPs from seven electrode sites and found that the insertion of an experimentally constrained HCN-conductance gradient into these neurons introduced a location-dependent lead in the LFP phase without significantly altering its amplitude. Further, neurons fired action potentials at a specific theta phase of the LFP, and the insertion of HCN channels introduced large lags in this spike phase and a striking enhancement in neuronal spike-phase coherence. Importantly, graded changes in either HCN conductance or its half-maximal activation voltage resulted in graded changes in LFP and spike phases. Our conclusions on the impact of HCN channels on LFPs and spike phase were invariant to changes in neuropil size, to morphological heterogeneity, to excitatory or inhibitory synaptic scaling, and to shifts in the onset phase of inhibitory inputs. Finally, we selectively abolished the inductive lead in the impedance phase introduced by HCN channels without altering neuronal excitability and found that this inductive phase lead contributed significantly to changes in LFP and spike phase. Our results uncover specific roles for HCN channels and their plasticity in phase-coding schemas and in the formation and dynamic reconfiguration of neuronal cell assemblies.L ocal field potentials (LFPs) have been largely believed to be a reflection of the synaptic drive that impinges on a neuron. In recent experimental and modeling studies, there has been a lot of debate on the source and spatial extent of LFPs (1-9). However, most of these studies have used neurons with passive dendrites in their models and/or have largely focused on the contribution of spike-generating conductances to LFPs (7,8,10,11). Despite the widely acknowledged regulatory roles of subthreshold-activated ion channels and their somatodendritic gradients in the physiology and pathophysiology of synapses and neurons (12-17), the implications for their existence on LFPs and neuronal spike phase have surprisingly remained unexplored. This lacuna in LFP analysis is especially striking because local and widespread plasticity of these channels has been observed across several physiological and pathological conditions, translating to putative roles for these channels in neural coding, homeostasis, disease etiology and remedies, learning, and memory (16,(18)(19)(20)(21)(22)(23).In this study, we focus on the role of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels that mediate the h current (I h ) in regulating LFPs and ...
