title: High-conductance states and A-type K+ channels are potential regulators of the conductance-current balance triggered by HCN channels.
abstract: An increase in the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel conductance reduces input resistance, whereas the consequent increase in the inward h current depolarizes the membrane. This results in a delicate and unique conductance-current balance triggered by the expression of HCN channels. In this study, we employ experimentally constrained, morphologically realistic, conductance-based models of hippocampal neurons to explore certain aspects of this conductance-current balance. First, we found that the inclusion of an experimentally determined gradient in A-type K(+) conductance, but not in M-type K(+) conductance, tilts the HCN conductance-current balance heavily in favor of conductance, thereby exerting an overall restorative influence on neural excitability. Next, motivated by the well-established modulation of neuronal excitability by synaptically driven high-conductance states observed under in vivo conditions, we inserted thousands of excitatory and inhibitory synapses with different somatodendritic distributions. We measured the efficacy of HCN channels, independently and in conjunction with other channels, in altering resting membrane potential (RMP) and input resistance (Rin) when the neuron received randomized or rhythmic synaptic bombardments through variable numbers of synaptic inputs. We found that the impact of HCN channels on average RMP, Rin, firing frequency, and peak-to-peak voltage response was severely weakened under high-conductance states, with the impinging synaptic drive playing a dominant role in regulating these measurements. Our results suggest that the debate on the role of HCN channels in altering excitability should encompass physiological and pathophysiological neuronal states under in vivo conditions and the spatiotemporal interactions of HCN channels with other channels.
authors.affiliation: Cellular Neurophysiology Laboratory, Molecular Biophysics Unit, Indian Institute of Science, Bangalore, India.
authors.forename: Poonam
authors.initials: P
authors.lastname: Mishra
authors.affiliation: Cellular Neurophysiology Laboratory, Molecular Biophysics Unit, Indian Institute of Science, Bangalore, India rishi@mbu.iisc.ernet.in.
authors.forename: Rishikesh
authors.identifier: http://orcid.org/0000-0002-1362-4635
authors.initials: R
authors.lastname: Narayanan
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D056547:CA1 Region, Hippocampal / Q000502:physiology; D003198:Computer Simulation; D017097:Electric Impedance; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology; D008959:Models, Neurological; D009433:Neural Inhibition / Q000502:physiology; D010507:Periodicity; D024642:Potassium Channels, Voltage-Gated / Q000378:metabolism*; D017966:Pyramidal Cells / Q000502:physiology*; D051381:Rats
chemical_list: D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D024642:Potassium Channels, Voltage-Gated
keywords: HCN channel; computational model; conductance-current balance; high-conductance state; hippocampal pyramidal neuron; transient potassium channel