Abstracts

Rationale: Our recent results showed (Jung et al. (2007), J Neurosci. ;27(47):13012-21) that pilocarpine-induced status epilepticus (SE) produced an acute loss of hyperpolarization-activated cation (HCN) channel protein expression, and the onset of spontaneous seizures caused downregulation in the voltage-dependent activation of HCN channel-mediated current (Ih) in CA1 hippocampal pyramidal neuron dendrites. Both of these decreases in HCN channel function produced hyperexcitability in hippocampal pyramidal neurons. However, the specific mechanisms by which spontaneous seizures cause modification of the voltage-dependent activation of HCN channels are yet unknown. It has been shown that calcineurin (CaN, a phosphatase) activity is increased in the CA1 area of the hippocampus after pilocarpine treatment. Thus, the main hypothesis of this study is that activation of phosphatases such as CaN in epileptic animals might downregulate Ih voltage-dependent activation. To investigate this hypothesis, we first asked whether CaN activity affects Ih voltage-dependent activation and neuronal excitability under normal conditions. Methods: We preincubated hippocampal slices from naive animals using the CaN inhibitior FK506 (1 μM), then performed dendritic recordings in CA1 hippocampal pyramidal neurons to measure changes in Ih properties under CaN inhibition. Further, to test the role of CaN with an in vitro model of seizures, we first perfused the slices for one hour in artificial cerebrospinal fluid (ACSF) with 0 Mg2+ and 20 μM bicucullineand, and monitored ictal and interictal events extracellularly. Results: Dendritic recordings in CA1 hippocampal pyramidal neurons showed that CaN inhibition caused a depolarized shift in Ih voltage-dependent activation (half-activation voltage, V1/2; control: -91 + 1.8 mV, n = 18; FK506-treated: -78 + 3.3 mV, n = 8; p < 0.01), increasing Ih active at resting potential. Further, input resistance (IR), temporal summation (TS), and action potential (AP) firing, excitability parameters influenced by Ih, were found to be decreased 35%, 20%, and 30-65%, respectively, indicating that CaN inhibition decreased neuronal excitability. In addition, the in vitro seizure condition caused spontaneous seizure-like events in slices and hyperpolarized V1/2 (-100 + 2.7 mV, n = 20; p < 0.05), similar to that seen in pilocarpine-treated animals experiencing spontaneous seizures. The Ih activation V1/2 following FK506 (-88 + 3.6 mV, n = 8) was similar to control slices (-89 + 1.2 mV, n = 22), thus FK506 prevented the seizure-induced hyperpolarized shift in Ih activation V1/2.