Downregulation of dendritic HCN channel gating in epilepsy is mediated by altered phosphorylation signaling.
The onset of spontaneous seizures in the pilocarpine model of epilepsy causes a hyperpolarized shift in the voltage-dependent activation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated current (Ih) in CA1 hippocampal pyramidal neuron dendrites, contributing to neuronal hyperexcitability and possibly to epileptogenesis. However, the specific mechanisms by which spontaneous seizures cause downregulation of HCN channel gating are yet unknown. We asked whether the seizure-dependent downregulation of HCN channel gating was due to altered phosphorylation signaling mediated by the phosphatase calcineurin (CaN) or the kinase p38 mitogen-activated protein kinase (p38 MAPK). We first found that CaN inhibition upregulated HCN channel gating and reduced neuronal excitability under normal conditions, showing that CaN is a strong modulator of HCN channels. We then found that an in vitro model of seizures (1 h in 0 Mg2+ and 50 microM bicuculline at 35-37 degrees C) reproduced the HCN channel gating change seen in vivo. Pharmacological inhibition of CaN or activation of p38 MAPK partially reversed the in vitro seizure-induced hyperpolarized shift in HCN channel gating, and the shift was fully reversed by the combination of CaN inhibition and p38 MAPK activation. We then demonstrated enhanced CaN activity as well as reduced p38 MAPK activity in vivo in the CA1 hippocampal area of chronically epileptic animals. Pharmacological reversal of these phosphorylation changes restored HCN channel gating downregulation and neuronal hyperexcitability in epileptic tissue to control levels. Together, these results suggest that alteration of two different phosphorylation pathways in epilepsy contributes to the downregulation of HCN channel gating, which consequently produces neuronal hyperexcitability and thus may be a target for novel antiepileptic therapies.
Jung
Sangwook
S
Department of Neurology, University of Washington, Seattle, Washington 98104, USA.
Bullis
James B
JB
Lau
Ignatius H
IH
Jones
Terrance D
TD
Warner
Lindsay N
LN
Poolos
Nicholas P
NP
D000818:Animals; D001640:Bicuculline; D056547:CA1 Region, Hippocampal / Q000187:drug effects / Q000503:physiopathology*; D019703:Calcineurin / Q000378:metabolism; D065095:Calcineurin Inhibitors; D002908:Chronic Disease; D054815:Cyclic Nucleotide-Gated Cation Channels / Q000378:metabolism*; D003712:Dendrites / Q000187:drug effects / Q000502:physiology*; D004195:Disease Models, Animal; D015536:Down-Regulation / Q000187:drug effects; D004827:Epilepsy / Q000139:chemically induced / Q000503:physiopathology*; D066298:In Vitro Techniques; D017616:Magnesium Compounds; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D010766:Phosphorylation / Q000187:drug effects; D017966:Pyramidal Cells / Q000187:drug effects / Q000503:physiopathology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012640:Seizures / Q000139:chemically induced / Q000503:physiopathology; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism
D065095:Calcineurin Inhibitors; D054815:Cyclic Nucleotide-Gated Cation Channels; D017616:Magnesium Compounds; D048051:p38 Mitogen-Activated Protein Kinases; D019703:Calcineurin; D001640:Bicuculline
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