Homeostatic regulation of synaptic excitability: tonic GABA(A) receptor currents replace I(h) in cortical pyramidal neurons of HCN1 knock-out mice

In Vivo Dynamic Clamp Study of Ih in the Mouse Inferior Colliculus

Approximately half of the cells in the mouse inferior colliculus have the hyperpolarization-activated mixed cation current Ih, yet little is known about its functional relevance in vivo. We therefore studied its contribution to the processing of sound information in single cells by making in vivo whole cell recordings from the inferior colliculus (IC) of young-adult anesthetized C57Bl/6 mice. Following pharmacological block of the endogenous channels, a dynamic clamp approach allowed us to study the responses to current injections or auditory stimuli in the presence and absence of Ih within the same neuron, thus avoiding network or developmental effects. The presence of Ih changed basic cellular properties, including depolarizing the resting membrane potential and decreasing resting membrane resistance. Sound-evoked excitatory postsynaptic potentials were smaller but at the same time reached a more positive membrane potential when Ih was present. With Ih, a subset of cells showed rebound spiking following hyperpolarizing current injection. Its presence also changed more complex cellular properties. It decreased temporal summation in response to both hyperpolarizing and depolarizing repetitive current stimuli, and resulted in small changes in the cycle-averaged membrane potential during sinusoidal amplitude modulated (SAM) tones. Furthermore, Ih minimally decreased the response to a tone following a depolarization, an effect that may make a small contribution to forward masking. Our results thus suggest that previously observed differences in IC cells are a mixture of direct effects of Ih and indirect effects due to the change in membrane potential or effects due to the co-expression with other channels.

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.