Upregulation of inward rectifier K+ (Kir2) channels in dentate gyrus granule cells in temporal lobe epilepsy

In humans, temporal lobe epilepsy (TLE) is often associated with Ammon's horn sclerosis (AHS) characterized by hippocampal cell death, gliosis and granule cell dispersion (GCD) in the dentate gyrus. Granule cells surviving TLE have been proposed to be hyperexcitable and to play an important role in seizure generation. However, it is unclear whether this applies to conditions of AHS. We studied granule cells using the intrahippocampal kainate injection mouse model of TLE, brain slice patch-clamp recordings, morphological reconstructions and immunocytochemistry. With progressing AHS and GCD, 'epileptic' granule cells of the injected hippocampus displayed a decreased input resistance, a decreased membrane time constant and an increased rheobase. The resting leak conductance was doubled in epileptic granule cells and roughly 70–80% of this difference were sensitive to K⁺ replacement. Of the increased K⁺ leak, about 50% were sensitive to 1 m m Ba²⁺. Approximately 20–30% of the pathological leak was mediated by a bicuculline-sensitive GABA_A conductance. Epileptic granule cells had strongly enlarged inwardly rectifying currents with a low micromolar Ba²⁺ IC₅₀, reminiscent of classic inward rectifier K⁺ channels (Irk/Kir2). Indeed, protein expression of Kir2 subunits (Kir2.1, Kir2.2, Kir2.3, Kir2.4) was upregulated in epileptic granule cells. Immunolabelling for two-pore weak inward rectifier K⁺ channels (Twik1/K2P1.1, Twik2/K2P6.1) was also increased. We conclude that the excitability of granule cells in the sclerotic focus of TLE is reduced due to an increased resting conductance mainly due to upregulated K⁺ channel expression. These results point to a local adaptive mechanism that could counterbalance hyperexcitability in epilepsy.     

Properties of single voltage-dependent K+ channels in dendrites of CA1 pyramidal neurones of rat hippocampus

Voltage-dependent K⁺ channels in the apical dendrites of CA1 pyramidal neurones play important roles in regulating dendritic excitability, synaptic integration, and synaptic plasticity. Using cell-attached, voltage-clamp recordings, we found a large variability in the waveforms of macroscopic K⁺ currents in the dendrites. With single-channel analysis, however, we were able to identify four types of voltage-dependent K⁺ channels and we categorized them as belonging to delayed-rectifier, M-, D-, or A-type K⁺ channels previously described from whole-cell recordings. Delayed-rectifier-type K⁺ channels had a single-channel conductance of 19 ± 0.5 pS, and made up the majority of the sustained K⁺ current uniformly distributed along the apical dendrites. The M-type K⁺ channels had a single-channel conductance of 11 ± 0.8 pS, did not inactivate with prolonged membrane depolarization, deactivated with slow kinetics (time constant 100 ± 6 ms at −40 mV), and were inhibited by bath-applied muscarinic agonist carbachol (10 μ m ). The D-type K⁺ channels had a single-channel conductance of around 18 pS, and inactivated with a time constant of 98 ± 4 ms at +54 mV. The A-type K⁺ channels had a single-channel conductance of 6 ± 0.6 pS, inactivated with a time constant of 23 ± 2 ms at +54 mV, and contributed to the majority of the transient K⁺ current previously described. These results suggest both functional and molecular complexity for K⁺ channels in dendrites of CA1 pyramidal neurones.     

Small conductance Ca2+-activated K+ channels and calmodulin

Small conductance Ca²⁺-activated K⁺ channels (SK channels) contribute to the long lasting afterhyperpolarization (AHP) that follows an action potential in many central neurones. The biophysical and pharmacological attributes of cloned SK channels strongly suggest that one or more of them underlie the medium component of the AHP that regulates interspike interval and plays an important role in setting tonic firing frequency. The cloned SK channels comprise a distinct subfamily of K⁺ channels. Heterologously expressed SK channels recapitulate the biophysical and pharmacological hallmarks of native SK channels, being gated solely by intracellular Ca²⁺ ions with no voltage dependence to their gating, small unitary conductance values and sensitivity to the bee venom peptide toxin, apamin. Molecular, biochemical and electrophysiological studies have revealed that Ca²⁺ gating in SK channels is due to heteromeric assembly of the SK α pore-forming subunits with calmodulin (CaM). Ca²⁺ binding to the N-terminal E–F hands of CaM is responsible for SK channel gating. Crystallographic studies suggest that SK channels gate as a dimer-of-dimers, and that the physical gate of SK channels resides at or near the selectivity filter of the channels. In addition, Ca²⁺-independent interactions between the SK channel α subunits and CaM are necessary for proper membrane trafficking.