tetrodotoxin and Syndrome

Two mutations that cause generalized epilepsy with febrile seizures plus (GEFS+) have been identified previously in the SCN1A gene encoding the alpha subunit of the Na(v)1.1 voltage-gated sodium channel (Escayg et al., 2000). Both mutations change conserved residues in putative voltage-sensing S4 segments, T875M in domain II and R1648H in domain IV. Each mutation was cloned into the orthologous rat channel rNa(v)1.1, and the properties of the mutant channels were determined in the absence and presence of the beta1 subunit in Xenopus oocytes. Neither mutation significantly altered the voltage dependence of either activation or inactivation in the presence of the beta1 subunit. The most prominent effect of the T875M mutation was to enhance slow inactivation in the presence of beta1, with small effects on the kinetics of recovery from inactivation and use-dependent activity of the channel in both the presence and absence of the beta1 subunit. The most prominent effects of the R1648H mutation were to accelerate recovery from inactivation and decrease the use dependence of channel activity with and without the beta1 subunit. The DIV mutation would cause a phenotype of sodium channel hyperexcitability, whereas the DII mutation would cause a phenotype of sodium channel hypoexcitability, suggesting that either an increase or decrease in sodium channel activity can result in seizures.

Topics: Amino Acid Substitution; Animals; Cells, Cultured; Epilepsy, Generalized; Gene Expression; Membrane Potentials; Models, Biological; Mutagenesis, Site-Directed; Mutation; NAV1.1 Voltage-Gated Sodium Channel; Nerve Tissue Proteins; Oocytes; Patch-Clamp Techniques; Phenotype; Protein Subunits; Rats; Seizures, Febrile; Sodium; Sodium Channels; Structure-Activity Relationship; Syndrome; Tetrodotoxin; Transfection

A neuromuscular blocking factor has been described in the serum of patients with Miller-Fisher syndrome (MFS). We here examined the effect of immunoglobulins (Ig) on neuromuscular transmission in mice recording quantal endplate currents by means of a perfused macro-patch-clamp electrode. Ig and IgM- and IgG-fractions from an anti-GQ1b-positive patient with typical MFS were highly purified. After application of MFS-IgG, quantal release decreased 1000-fold within 2 min. Returning to control solution the average release came back to the baseline level within 4 min. In contrast, control-IgG and MFS-IgM did not cause any blocking effect. The very fast and fully reversible presynaptic blockade of release caused by the highly purified IgG-fraction may be one factor producing muscle weakness in MFS.

Topics: Animals; Demyelinating Diseases; Diaphragm; Humans; Immunoglobulin G; Immunoglobulin M; In Vitro Techniques; Mice; Mice, Inbred BALB C; Motor Endplate; Neuromuscular Junction; Peripheral Nervous System Diseases; Syndrome; Tetrodotoxin

Lambert-Eaton syndrome, an autoimmune disorder frequently associated with small-cell carcinoma of the lung, is characterized by impaired evoked release of acetylcholine from the motor nerve terminal. Immunoglobulin G (IgG) antibodies from patients with the syndrome, applied to bovine adrenal chromaffin cells, reduced the voltage-dependent calcium channel currents by about 40 percent. When calcium was administered directly into the cytoplasm, however, the IgG-treated cells exhibited normal exocytotic secretion, as assayed by membrane capacitance measurement. Measurement with the fluorescent calcium indicator fura-2 indicated that the IgG treatment reduced potassium-stimulated increase in free intracellular calcium concentration. The pathogenic IgG modified neither kinetics of calcium channel activation nor elementary channel activity, suggesting that a reduction in the number of functional calcium channels underlies the IgG-induced effect. Therefore, Lambert-Eaton syndrome IgG reacts with voltage-dependent calcium channels and blocks their function, a phenomenon that can account for the presynaptic impairment characteristic of this disorder.

Topics: Adrenal Glands; Autoantibodies; Autoimmune Diseases; Benzofurans; Calcium; Carcinoma, Small Cell; Cell Membrane; Chromaffin System; Electric Conductivity; Exocytosis; Fluorescent Dyes; Fura-2; Humans; Immunoglobulin G; Ion Channels; Lung Neoplasms; Neuromuscular Diseases; Sodium; Synapses; Syndrome; Tetrodotoxin

Immunoglobulin G (IgG) from Lambert-Eaton myasthenic syndrome (LEMS) patients acts at motor nerve terminal Ca2+ channels. It was injected into mice to investigate effects on cardiac Ca2+ channels. Intracellular recordings were made of slow action potentials in right ventricular muscle cells in the presence of high K+ concentrations and isoprenaline (1 microM). Reduction in Ca2+ concentration reduced the rate of rise and amplitude, but not the duration, of slow action potentials whereas verapamil (1 microM) blocked them. They were not blocked by tetrodotoxin (10 microM), and 4-aminopyridine (1 mM) prolonged the decay phase without affecting the rate of rise and amplitude. The rate of rise, amplitude and duration of slow action potentials were not affected by LEMS IgG. These results show that LEMS IgG does not act on Ca2+ channel currents that underlie slow action potentials in mouse ventricles, suggesting antigenic differences between Ca2+ channels at motor nerve terminals and heart.

Topics: 4-Aminopyridine; Action Potentials; Aminopyridines; Animals; Calcium Channels; Heart; Heart Ventricles; Humans; Immunoglobulin G; In Vitro Techniques; Mice; Neuromuscular Diseases; Reference Values; Syndrome; Tetrodotoxin; Ventricular Function; Verapamil