Flurazepam: effects on sodium and potassium currents in myelinated nerve fibres

Connexin-43 reduction prevents muscle defects in a mouse model of manifesting Duchenne muscular dystrophy female carriers

Duchenne muscular dystrophy (DMD) is a severe X-linked neuromuscular disorder that affects males. However, 8% of female carriers are symptomatic and underrepresented in research due to the lack of animal models. We generated a symptomatic mouse model of DMD carriers via injection of mdx (murine DMD) embryonic stem cells (ESCs) into wild-type (WT) blastocysts (mdx/WT chimera). mdx/WT chimeras developed cardiomyopathic features and dystrophic skeletal muscle phenotypes including elevated mononuclear invasion, central nucleation, fibrosis and declined forelimb grip strength. The disease was accompanied by connexin-43 (Cx43) aberrantly enhanced in both cardiac and skeletal muscles and remodeled in the heart. Genetic reduction of Cx43-copy number in mdx/WT-Cx43(+/-) chimeras protected them from both cardiac and skeletal muscle fiber damage. In dystrophic skeletal muscle, Cx43 expression was not seen in the fibers but in adjacent F4/80+ mononuclear cells. Ethidium Bromide uptake in purified F4/80+/CD11b+ mdx macrophages revealed functional activity of Cx43, which was inhibited by administration of Gap19 peptide mimetic, a Cx43 hemichannel-specific inhibitor. Thus, we suggest that Cx43 reduction in symptomatic DMD carrier mice leads to prevention of Cx43 remodeling in the heart and prevention of aberrant Cx43 hemichannel activity in the skeletal muscle macrophages neighboring Cx43 non-expressing fibers.

[Long-term efficacy and tolerance of stiripentaol in severe myoclonic epilepsy of infancy (Dravet's syndrome)]

OBJECTIVE

To describe the long term efficacy and tolerance of stiripentol associated with valproate and clobazam in an exhaustive cohort of patients with severe myoclonic epilepsy of infancy (Dravet's syndrome), in which short term efficacy of such a treatment has recently been demonstrated in a placebo-controlled trial.

RESULTS

In 46 patients the frequency and the duration of seizures was significantly reduced (p < 0.001) as well as the number of convulsive status at a median of three-year follow-up. Ten patients were dramatically improved (seizures significantly decreased in number [p = 0.002] and duration [p = 0.002] and status epilepticus disappeared), 20 were moderately improved (seizures significantly decreased in duration [p = 0.001] and status were less numerous), four had no response and efficacy was non evaluable in 12 mainly because of adverse events. Efficacy was better in the youngest patients. The most frequent adverse events were loss of appetite and loss of weight. They could be so severe in patients over 12 years of age that the stiripentol dosage could not be increased to 50 mg kg-1 j-1.

CONCLUSION

This follow-up study shows that stiripentol added to valproate and clobazam maintains its efficacy at long term in children with severe myoclonic epilepsy of infancy and suggests that the tritherapy should be introduced as early as possible in these patients in order to suppress the convulsive status epilepticus.

The benzodiazepine midazolam preferentially blocks inactivated Na channels in skeletal muscle fibre

The effects of the benzodiazepine midazolam were studied on frog skeletal muscle fibres held under current- or voltage-clamp conditions. Midazolam induced a concentration-dependent (10(-5) mol/l to 10(-3) mol/l) block of the action potential and of the underlying Na current. Block of the Na current occurred without any changes in its voltage dependence or in its activation and inactivation kinetics. An apparent dissociation constant of 223 mumol/l was determined for midazolam from the rested Na channels of well polarized fibres. The blocking effect of a threshold concentration (10(-5) mol/l) could be greatly enhanced (up to the complete suppression of the current) by predepolarizations, positive holding potentials or high stimulation frequencies. This apparent voltage- and frequency-dependent block (no use dependence, i.e., no activation block) could be ascribed to a blockade of inactivated Na channels. From the apparent shift towards negative potentials of the steady-state inactivation curve, a dissociation constant of 6.0 mumol/l was calculated for midazolam from the inactivated Na channels, according to the modulated-receptor model. These results show that midazolam preferentially blocks inactivated rather than rested Na channels, and suggest that this mechanism of action might contribute to the well-known myorelaxant effect of the benzodiazepines.

Differential inhibition of transient and long-lasting calcium channel currents by benzodiazepines in neuroblastoma cells

The effects of diazepam, nitrazepam, clonazepam, and Ro5-4864 on transient (type I) and long-lasting (type II) calcium channels associated with low-affinity benzodiazepine receptors were investigated using the whole-cell patch-clamp technique. Clonazepam (100 microM), a specific agonist for the central-type benzodiazepine receptor, reduced transient currents through the type I calcium channel by 40% without affecting long-lasting currents through the type II calcium channel. Diazepam and nitrazepam (100 microM), non-specific agonists for both the central- and peripheral-type benzodiazepine receptors, reduced both transient and long-lasting currents equally by 25-30%. A similar non-selective inhibition was observed by Ro5-4864 (1-10 microM), a specific agonist for the peripheral-type benzodiazepine receptor. It is concluded that the two calcium channel types are regulated differentially by two different kinds of benzodiazepines; central-type for type I channel and peripheral-type for both type I and type II channels.

Anticalmodulin drugs block the sodium gating current of squid giant axons

The effects of calmodulin (CaM) antagonists (W-7, W-5, trifluoperazine, chlorpromazine, quinacrine, diazepam, propericyazine and carmidazolium) on the sodium and potassium channels were studied on the intracellularly perfused and voltage-clamped giant axon of the squid. It was found that the drugs are more potent blockers of the sodium current than of the potassium current. The drugs also reduce the sodium gating current. The blockage of the sodium and gating current can be explained by assuming that the drugs interact with the sodium gating subunit in one of its closed states. The site of action is probably the intracellular surface of the axolemma where presumably a Ca(2+)-calmodulin complex can be formed.

Action of antiepileptic and anaesthetic drugs on Na- and Ca-spikes in mammalian non-myelinated axons

The actions of the antiepileptic drugs phenytoin, carbamazepine and phenobarbitone, and the local anaesthetic drugs lignocaine and procaine on sodium-dependent and calcium-dependent compound action potentials (Na- and Ca-spikes) have been compared in rat preganglionic cervical sympathetic nerves, using extracellular recording techniques. There was no discernible difference in the frequency dependence of Na-spike block over the frequency range 0.2-20 Hz between these two groups of drugs. However the antiepileptic drugs were more potent blockers of the Ca-spike, whereas the local anaesthetics were more potent on the Na-spike. It is proposed that a dual action of antiepileptics, a frequency-dependent block of sodium currents combined with a block of calcium currents, may explain their efficacy in the treatment of seizures.

Effects of medazepam on voltage-gated ion currents of cultured chick sensory neurons

The effects of the benzodiazepine, medazepam, were investigated in current and voltage-clamped cultured chick dorsal root ganglion neurons. Under current clamp, micromolar concentrations initially elevated the action potential threshold and blocked both the sodium and calcium components of the spike. In voltage clamp, low (I(Ca.T)) and high (I(CA.N/L)) threshold calcium, sodium (I(Na)) and the delayed rectifier potassium (I(K)) currents were isolated by the use of appropriate solutions and voltage command protocols. Medazepam depressed both subtypes of I(Ca) equally well with calculated half-maximal depression at 77 microM. At a fixed concentration of 200 microM, medazepam depressed I(Na) (70 +/- 9%) and I(K) (73 +/- 6%) to a degree comparable to I(Ca) (75 +/- 3%). The results show that benzodiazepines can modulate the activity of several voltage-gated ion currents in chick dorsal root ganglion neurons.

Benzodiazepines produce contrasting effects on the active membrane properties of an invertebrate neuron

A perturbation of excitable membranes mediated by non-receptor (non-specific) mechanisms might be predicted from the hydrophobic nature of 1,4-benzodiazepines. Since correlations between membrane properties and neuronal effects have not been described for benzodiazepines, the effects of flurazepam, oxazepam and the benzodiazepine antagonist flumazepil (Ro 15-1788) were examined on both passive and active electrical properties of the membrane and neuronal discharge frequency. In this study, the isolated sensory neuron of the crayfish has been utilized as a neuronal model system. Flurazepam and flumazepil both enhanced the discharge frequency, in contrast to the depression produced by oxazepam. Discharge frequency was directly correlated with the maximum rate of rise of membrane potential during the threshold phase and was inversely correlated with spike threshold. In addition, the discharge frequency appeared to exhibit little dependence on peak amplitude, duration and the maximum rate of depolarization of the action potential. These findings are discussed in relation to non-specific mechanism(s) of action for benzodiazepines. It is suggested that, in the absence of a specific drug-receptor interaction, benzodiazepines in larger concentrations (greater than or equal to 50 mumol/l) exhibit selective membrane perturbations.

Benzodiazepine receptor ligand actions on GABA responses. Benzodiazepines, CL 218872, zopiclone

The effects on GABA (4-aminobutyric acid) responses of several benzodiazepine and nonbenzodiazepine benzodiazepine receptor ligands were examined using mouse spinal cord neurons in dissociated cell culture. Diazepam, clonazepam and nitrazepam enhanced GABA responses potently at low nanomolar concentrations. Diazepam and clonazepam were most potent with significant enhancement at 1 nM and peak enhancement of 80.7 and 50.2% at 10 nM respectively. Nitrazepam was least potent with no significant enhancement at 1 nM and enhancement of only 20.7% at 10 nM. The benzodiazepine antagonist, Ro 15-1788, blocked enhancement by diazepam but also weakly enhanced GABA responses at low micromolar concentrations, suggesting partial agonist activity. The convulsant benzodiazepine, Ro 5-4864, did not enhance GABA responses at any concentration tested but antagonized GABA responses at 1 microM and above. Diazepam shifted GABA dose-response curves to the left by decreasing the apparent KD but without altering the apparent Vmax (Lineweaver-Burk analysis). Two nonbenzodiazepine anxiolytic/anticonvulsants, CL 218872 and zopiclone, were weak enhancers of GABA responses at high nanomolar concentrations. These results with benzodiazepines, CL 218872 and zopiclone are consistent with their anxiolytic and anticonvulsant profile in vivo and with studies of their effects upon low affinity GABA binding in vitro.

Evidence for excitatory and depressant non-receptor-mediated membrane effects of benzodiazepines in the crayfish

Oxazepam produced a significant reversible monophasic concentration-dependent (50-500 mumol/l) depression of stretch-induced discharge frequency of the isolated crayfish sensory neuron. In total contrast, flurazepam (10-200 mumol/l) evoked reversible excitation of the neuronal firing rate but at concentrations greater than 300 mumol/l it induced transient excitation followed by secondary total depression. The benzodiazepine antagonist flumazepil (less than or equal to 1 mmol/l) also produced an inherent increase in neuronal discharge frequency, though respective concentrations of 50 and 100 mumol/l failed to block flurazepam-excitation or oxazepam-depression. It is suggested that these high concentration qualitatively divergent neuronal effects are not mediated through specific benzodiazepine receptors.