saxitoxin and Epilepsy

Scn1b-null mice have a severe neurological and cardiac phenotype. Human mutations in SCN1B result in epilepsy and cardiac arrhythmia. SCN1B is expressed as two developmentally regulated splice variants, β1 and β1B, that are each expressed in brain and heart in rodents and humans. Here, we studied the structure and function of β1B and investigated a novel human SCN1B epilepsy-related mutation (p.G257R) unique to β1B. We show that wild-type β1B is not a transmembrane protein, but a soluble protein expressed predominantly during embryonic development that promotes neurite outgrowth. Association of β1B with voltage-gated Na+ channels Na(v)1.1 or Na(v)1.3 is not detectable by immunoprecipitation and β1B does not affect Na(v)1.3 cell surface expression as measured by [(3)H]saxitoxin binding. However, β1B coexpression results in subtle alteration of Na(v)1.3 currents in transfected cells, suggesting that β1B may modulate Na+ current in brain. Similar to the previously characterized p.R125C mutation, p.G257R results in intracellular retention of β1B, generating a functional null allele. In contrast, two other SCN1B mutations associated with epilepsy, p.C121W and p.R85H, are expressed at the cell surface. We propose that β1B p.G257R may contribute to epilepsy through a mechanism that includes intracellular retention resulting in aberrant neuronal pathfinding.

Topics: Amino Acid Sequence; Animals; Animals, Newborn; Arginine; Biotinylation; Cell Adhesion Molecules; Cells, Cultured; Cerebellum; Cricetinae; Cricetulus; Epilepsy; Female; Gene Expression Regulation, Developmental; Genotype; Glycine; Humans; Immunoprecipitation; Male; Membrane Potentials; Mice; Mice, Inbred C57BL; Mice, Knockout; Mutation; NAV1.3 Voltage-Gated Sodium Channel; Neurites; Neurons; Patch-Clamp Techniques; Protein Isoforms; RNA, Messenger; Saxitoxin; Sodium Channels; Statistics, Nonparametric; Transfection; Tritium; Voltage-Gated Sodium Channel beta-1 Subunit

We investigated the effect of in vivo administration of an antiepileptic drug, phenytoin, on the saxitoxin binding capacity of receptor site 1 of the Na+ channel alpha-subunit, and the expression activity of the channel messenger RNA in epileptic El mouse brains, as compared with parental ddY mice. Subchronic treatment with phenytoin (25 mg/kg per day) for 14 days increased the [3H]saxitoxin binding to brain-derived synaptic membranes of both El and control ddY mice in a time dependent manner. This increase plateaued at 21 +/- 4% in El mice and 28 +/- 3% in ddY control mice after administration of phenytoin for seven days. After cessation of treatment with phenytoin, [3H]saxitoxin binding capacity returned to the basal level within two weeks in both ddY and El brains. Scatchard plot analysis revealed that the phenytoin treatment caused a 20-30% increase in maximum binding capacity of [3H]saxitoxin binding without any change in equilibrium dissociation constant in the brain cortical synaptic membranes of both epileptic El and control ddY mice. A single injection of phenytoin (25 mg/kg) elevated the level of Na+ channel messenger RNA within 1 h in ddY mouse brains. The increase in Na+ channel messenger RNA reached a peak (about 80% increase) after 5 h of phenytoin administration in a concentration-dependent manner (6.25-50 mg/kg). On the other hand, in El mouse brains, Na+ channel messenger RNA was not elevated until more than 5 h after phenytoin injection, and was increased by only about 33%.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Amphibian Proteins; Animals; Carrier Proteins; Cerebral Cortex; Epilepsy; Mice; Mice, Neurologic Mutants; Nerve Tissue Proteins; Phenytoin; RNA, Messenger; Saxitoxin; Sodium Channels; Synaptosomes; Tetrodotoxin; Up-Regulation; Veratridine

Tottering mice, in which a single gene lesion leads to prolonged hyperexcitability and spontaneous epilepsy, were studied to determine whether enhanced electrical activity leads to down regulation of sodium channels in central neurons. The number of sodium channels in synaptosomes, as assessed by saxitoxin binding, was decreased from 5.38 +/- 0.06 pmol/mg protein in coisogenic controls to 3.85 +/- 0.10 pmol/mg protein (P less than 0.001) in tottering mice without a change in the KD for saxitoxin. Neurotoxin-activated 22Na+ influx per sodium channel was increased 80% in tottering mice (P less than 0.001). Evidently, the increased level of electrical excitability characteristic of the tottering phenotype causes down regulation of the sodium-channel number and alteration of channel function in the nerve terminals of central neurons.

Topics: Amphibian Proteins; Animals; Carrier Proteins; Epilepsy; Ion Channels; Kinetics; Mice; Mice, Neurologic Mutants; Nerve Endings; Saxitoxin; Sodium; Synaptosomes