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The Journal of Neuroscience, June 2, 2004, 24(22):5239-5248; doi:10.1523/JNEUROSCI.0992-04.2004

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Neurobiology of Disease
Elevated Thalamic Low-Voltage-Activated Currents Precede the Onset of Absence Epilepsy in the SNAP25-Deficient Mouse Mutant Coloboma

Yi Zhang, Alexander P. Vilaythong, Daniel Yoshor, and Jeffrey L. Noebels

Developmental Neurogenetics Laboratory, Department of Neurology, Baylor College of Medicine, Houston, Texas 77030

Recessive mutations in genes encoding voltage-gated Ca²⁺ channel subunits alter high-voltage-activated (HVA) calcium currents, impair neurotransmitter release, and stimulate thalamic low-voltage-activated (LVA) currents that contribute to a cortical spike-wave epilepsy phenotype in mice. We now report thalamic LVA current elevations in a non-Ca²⁺ channel mutant. EEG analysis of Coloboma (Cm/+), an autosomal dominant mutant mouse lacking one copy of the gene for a synaptosomal-associated protein (SNAP25) that interacts with HVA channels, reveals abnormal spike-wave discharges (SWDs) in the behaving animal. We compared the biophysical properties of both LVA and HVA currents in Cm/+ and wild-type thalamic neurons and observed a 54% increase in peak current density of LVA currents evoked at –50 mV from –110 mV in Cm/+ before the developmental onset of seizures relative to control. The midpoint voltage for steady-state inactivation of LVA currents in Cm/+ was shifted in a depolarized direction by 8 mV before epilepsy onset, and the mean time constant for decay of LVA Ca²⁺ currents at –50 mV was also prolonged. No significant differences were found in recovery from inactivation of LVA currents or in HVA current densities and kinetics. Our data demonstrate that a non-Ca²⁺ channel subunit gene mutation leads to potentiated thalamic LVA currents that precede the appearance of SWDs and that altered somatodendritic HVA currents are not required for abnormal thalamocortical oscillations. We suggest that presynaptic release defects shared by these mutants lead to postsynaptic LVA excitability increases in thalamic pacemaker neurons that favor rebound bursting and absence epilepsy.

Key words: T-type calcium channels; thalamocortical relay cells; calcium channelopathy; stargazer; tottering; lethargic

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Received March 17, 2004; revised April 20, 2004; accepted April 21, 2004.

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