We conclude that this de novo mutation can contribute to the cause of "sporadic" childhood absence epilepsy by a loss of function and haploinsufficiency of the GABA(A) receptor alpha(1)-subunit, and that GABRA1 mutations rarely are associated with idiopathic generalized epilepsy.

The GluR2 flop subunit of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors greatly determines calcium permeability and kinetic properties of heteromeric AMPA subunit assemblies. Post-transcriptional editing of this subunit at the Q/R/N site controls calcium permeability whereas editing at the R/G site is involved in the regulation of biophysical properties. We used patch-clamp techniques with ultrafast solution exchange to examine the kinetics of recombinant human homomeric GluR2 flop channels transiently expressed in HEK293 cells [edited at the R/G site and Q/R/N site (GR), and unedited (RN) and edited (GN) at the R/G site both with asparagine (N) at the Q/R/N site]. The time constant of desensitization after application of 10 mm glutamate was 1.38 +/- 0.05 ms (n = 10), 5.53 +/- 0.57 ms (n = 7) and 1.33 +/- 0.06 ms (n = 12) for the GluR2 flop GR, RN and GN channels, respectively. The time constant of resensitization was 75 ms for the GluR2 flop RN and 30 ms for the GN channels. The dose-dependence of the peak current amplitude, kinetics of activation and deactivation, and peak open probability did not differ between RN and GN channels. The study shows that desensitization and resensitization kinetics of homomeric GluR2 flop channels are controlled by a single amino acid exchange (glycine by arginine) at the R/G site. Quantitative analysis by computer simulation using a circular kinetic scheme allows the prediction of the main experimental results.

AMPA-type glutamate receptors (AMPAR) display a high variability in functional properties, which determine the time course of excitatory postsynaptic potentials. They are assembled as tetramers of GluR subunits 1-4 of different splice variants and nuclear edited isoforms. Presently, the kinetics of activation, desensitization and recovery from desensitization of human AMPARs (GluR1, 3 and 4 flip and flop, and GluR2 flip and flop in R and G edited forms, respectively) transiently expressed in HEK293 cells were studied with patch-clamp techniques and ultra fast agonist application. Activation time constants were identical for all receptors (0.13 ms). The GluR2 flip G variant showed the slowest desensitization (10.8 ms), GluR4 flip the fastest (1.6 ms). Recovery from desensitization varied between 3.1 ms (GluR4 flip) and 178 ms (GluR1 flip). To determine functional interactions between subunits in heteromeric receptors the GluR1 flip and the GluR2 flip R were coexpressed. The time constant of desensitization increased linearly from 2.5 ms (GluR1 flip homomers) to 6.8 ms (GluR2 flip R homomers) with the amount of GluR2 flip R cDNA transfected. Recovery followed a monoexponential time course and had a time constant of 178 ms in GluR1 flip homomeric expression. In all GluR1 flip/GluR2 flip heteromers and in GluR2 flip R homomers desensitization recovered with a time constant of approximately 50 ms. Thus, subunit interaction seems likely during recovery but not desensitization. Both parameters might influence the ability of AMPA receptors to mediate glutamate induced chronic excitotoxicity in neurodegenerative diseases.

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Adverse 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) effects are usually ascribed to neurotransmitter release in the central nervous system. Since clinical features such as fasciculations, muscle cramps, rapidly progressing hyperthermia, hyperkalemia, and rhabdomyolysis point to the skeletal muscle as additional target, we studied the effects of MDMA on native and cultured skeletal muscle. We addressed the question whether malignant hyperthermia (MH)-susceptible (MHS) muscle is predisposed to adverse MDMA reactions. Force measurements on muscle strips showed that 100 M MDMA, a concentration close to that determined in some MDMA users, regularly enhanced the sensitivity of skeletal muscle to caffeine-induced contractures but did not cause contractures on its own. The left-shift of the dose-response curve induced by MDMA was greater in normal than in MHS muscle. Furthermore, MDMA did not release Ca 2ϩ from isolated sarcoplasmic reticulum vesicles. These findings do not support the view of an MH-triggering effect on muscle. However, MDMA induced Ca 2ϩ

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Background: Blockade of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors is a promising pharmacological strategy in the treatment of neurodegenerative diseases. The aim of the study is to elucidate if there are direct interactions of riluzole and phenobarbital with AMPA-type receptor channels and to determinethe molecular pharmacological mechanisms. Methods: The patch-clamp technique was used combining an ultrafast solution exchange system to investigate the interaction of riluzole and phenobarbital with recombinant AMPA-type glutamate receptor channels (homomeric GluR2flipGQ or nondesensitizing GluR2L504Y). Results: A dose-dependent decrease in the relative peak current amplitude (rAmp) and the relative area-under-the-current curve (rAUC) were found after preincubation with 0.1 mmol/l or higher concentrations of riluzole. Furthermore, in coapplication experiments with GluR2L504Y, the application of 1 or 3 mmol/l riluzole showed a decrease in the current decay time constant, and a reopening current was observed at 3 mmol/l riluzole. Phenobarbital blocks AMPA receptor channels dose-dependently in the coapplication experiments, and reopening currents after removing glutamate and blocker were observed. A slight block effect after preincubation should indicate an additional competitive block effect. Conclusion: Riluzole and phenobarbital modulate AMPA-type receptor channels separately, which could be both characterized as a combination of open-channel block and competitive-block mechanism.