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The Journal of Neuroscience, February 15, 2006, 26(7):2053-2059; doi:10.1523/JNEUROSCI.1575-05.2006

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Cellular/Molecular
A Spontaneous Mutation Involving Kcnq2 (Kv7.2) Reduces M-Current Density and Spike Frequency Adaptation in Mouse CA1 Neurons

James F. Otto,¹ Yan Yang,² Wayne N. Frankel,² H. Steve White,¹ and Karen S. Wilcox¹

¹Anticonvulsant Drug Development Program, Department of Pharmacology and Toxicology, University of Utah, Salt Lake City, Utah 84112, and ²The Jackson Laboratory, Bar Harbor, Maine 04609

Correspondence should be addressed to Dr. Karen S. Wilcox, Anticonvulsant Drug Development Program, Department of Pharmacology and Toxicology, University of Utah, 20 South 2030 East, Room 408, Salt Lake City, UT 84112. Email: karen.wilcox{at}hsc.utah.edu

The M-type K⁺ current [I_K(M)] activates in response to membrane depolarization and regulates neuronal excitability. Mutations in two subunits (KCNQ2 and KCNQ3; Kv7.2 and Kv7.3) that underlie the M-channel cause the human seizure disorder benign familial neonatal convulsions (BFNC), presumably by reducing I_K(M) function. In mice, the Szt1 mutation, which deletes the genomic DNA encoding the KCNQ2 C terminus and all of CHRNA4 (nicotinic acetylcholine receptor 4 subunit) and ARFGAP-1 (GTPase-activating protein that inactivates ADP-ribosylation factor 1), reduces seizure threshold, and alters M-channel pharmacosensitivity. Genomic deletions affecting the C terminus of KCNQ2 have been identified in human families with BFNC, and truncation of the C terminus prevents proper KCNQ2/KCNQ3 channel assembly in Xenopus oocytes. We showed previously that Szt1 mice have a reduced baseline seizure threshold and altered sensitivity to drugs that act at the M-channel. Specifically, the proconvulsant M-channel blocker linopirdine and anticonvulsant enhancer retigabine display increased and decreased potency, respectively, in Szt1 mice. To investigate the effects of the Szt1 mutation on I_K(M) function explicitly, perforated-patch electrophysiology was performed in CA1 pyramidal neurons of the hippocampus in brain slices prepared from C57BL/6J-Szt1/+ and control C57BL/6J+/+ mice. Our results show that Szt1 reduces both I_K(M) amplitude and current density, inhibits spike frequency adaptation, and alters many aspects of M-channel pharmacology. This is the first evidence that a naturally occurring Kcnq2 mutation diminishes the amplitude and function of the native neuronal I_K(M), resulting in significantly increased neuronal excitability. Finally, the changes in single-cell biophysical properties likely underlie the altered seizure threshold and pharmacosensitivity reported previously in Szt1 mice.

Key words: M-current; KCNQ2; C-terminal deletion; spike frequency adaptation; BFNC; retigabine

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Received Dec. 10, 2004; revised Dec. 13, 2005; accepted Jan. 5, 2006.

Correspondence should be addressed to Dr. Karen S. Wilcox, Anticonvulsant Drug Development Program, Department of Pharmacology and Toxicology, University of Utah, 20 South 2030 East, Room 408, Salt Lake City, UT 84112. Email: karen.wilcox{at}hsc.utah.edu

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