Artur W. Wamil

Inhibition of aminophylline-induced convulsions in mice by antiepileptic drugs and other agents

Common antiepileptic drugs and agents affecting different neurotransmitter systems were studied against aminophylline (280 mg/kg i.p.)-induced convulsions in mice. All drugs and agents were administered i.p. Diazepam and phenobarbital antagonized the whole seizure pattern and the respective ED50 values for the clonic phase were 3.5 and 62 mg/kg. Valproate at 500 mg/kg protected fewer than 50% of mice against the clonic phase. The remaining antiepileptics (acetazolamide, up to 1,000 mg/kg; carbamazepine and diphenylhydantoin, up to 50 mg/kg; ethosuximide, 500 mg/kg and trimethadione, 400 mg/kg) were totally ineffective in this respect. Propranolol (up to 20 mg/kg), baclofen (20 mg/kg), gamma-hydroxybutyric acid (300 mg/kg), aminooxyacetic acid (20 mg/kg), clonidine (up to 0.2 mg/kg), ketamine (30 mg/kg), atropine (20 mg/kg), papaverine (50 mg/kg) and L-phenylisopropyladenosine (2 mg/kg) did not affect the clonic phase either. Only antagonists of N-methyl-D-aspartic acid excitation, 2-amino-5-phosphonopentanoic acid and 2-amino-7-phosphonoheptanoic acid afforded protection against aminophylline-induced clonic seizure activity. The results show that aminophylline convulsions are relatively resistant to antiepileptic drugs and suggest that antagonists of excitatory transmission are potential antiaminophylline drugs.

Acetazolamide for the treatment of chronic paroxysmal hemicrania.

SYNOPSIS

Anesthetic-induced alteration of Ca2+ homeostasis in neural cells: a temperature-sensitive process that is enhanced by blockade of plasma membrane Ca2+-ATPase isoforms.

Background Many inhalation anesthetics at clinically relevant concentrations inhibit plasma membrane Ca2+‐adenosine triphosphatase (PMCA) ion pumping in brain synaptic membranes and in cultured cells of neural origin. In this study, the authors investigated the effect of inhalation anesthetics on cytosolic calcium homeostasis in cortical neurons maintained at physiologic and room temperatures and on cortical neurons and pheochromocytoma cells with antisense blockade of specific PMCA isoforms. Methods Using Ca2+‐specific confocal microfluorimetry, the anesthetic effects on Ca2+ dynamics were examined in mouse embryonic cortical neurons in association with ligand‐stimulated Ca2+ influx. Studies were done at 21 [degree sign]C and 37 [degree sign]C. Mouse embryonic cortical neurons with oligodeoxyribonucleotide blockade of PMCA2 expression and transfected rat pheochromocytoma cells with blocked expression of PMCA1 were also examined. Results Baseline and poststimulation peak cytosolic calcium concentrations ([Ca2+]i) were increased, and Ca2+ clearance was delayed in cells exposed at 37 [degree sign]C, but not at 21 [degree sign]C, to concentrations <or= to 1 minimum alveolar concentration (MAC)‐equivalent of halothane, isoflurane, and sevoflurane. Neurons exposed to xenon solutions <or= to 0.4, 0.6, and 0.8 MAC showed dose‐related perturbations of cytosolic Ca2+. Calcium dynamics were altered in neural cells with blocked PMCA isoform production, but at much lower halothane concentrations: 0.5 MAC for cortical neurons and 0.1 MAC for pheochromocytoma cells. Conclusions By extruding Ca2+ through the plasma membrane, PMCA maintains resting neuronal [Ca2+]i at low levels and clears physiologic loads of Ca2+ after influx through calcium channels. Inhalation anesthetics perturb this process and thus may interfere with neurotransmitter release, altering interneuronal signaling.

Effect of aminophylline upon the protective activity of common antiepileptic drugs and their plasma levels in mice

Aminophylline (50 mg/kg) decreased the protective efficacy of carbamazepine (20 mg/kg), diphenylhydantoin (8-12 mg/kg), phenobarbital (20 and 25 mg/kg), and valproate (250 and 300 mg/kg) against electroconvulsions in mice. On the other hand, aminophylline (5 mg/kg) was devoid of such activity. Plasma levels of antiepileptic drugs were measured with the help of the Abbott TDx analyzer and after administration of carbamazepine (20 mg/kg), diphenylhydantoin (10 mg/kg), phenobarbital (25 mg/kg), and valproate (250 mg/kg) were as follows: 8.61, 6.48, 24.3 and 329 micrograms/ml, respectively. Aminophylline (50 mg/kg) remained without any significant influence upon these plasma levels. This may lead to the conclusion that aminophylline-induced reversal of antiepileptic drug activity is not dependent upon a pharmacokinetic mechanism and probably occurs at the neuronal level.

Effect of temperature on limitation by MK-801 of firing of action potentials by spinal cord neurons in cell culture

The anticonvulsant, MK-801, limited sustained high frequency repetitive firing of sodium-dependent action potentials by mouse spinal cord neurons in monolayer dissociated cell culture. Limitation was voltage- and temperature-dependent and was accompanied by decreasing rate of rise of action potentials until firing ceased during the 400 ms depolarizations. The IC50 for limitation was 2 x 10(-7) M at 37 degrees C, 6.4 x 10(-7) M at 35 degrees C, and 4 x 10(-5) M at 23 degrees C. The relationship between the percentage of neurons capable of sustained repetitive firing and MK-801 concentration at 33 degrees C was biphasic. The first phase (about 50%) of limitation had IC50a = 1.5 x 10(-7) M, and the second had IC50b = 2 x 10(-4) M; the midpoint of the connecting plateau was 10(-5) M. At temperatures below 37 degrees C, the current needed to achieve maximal firing increased. The maximal rate of rise, maximal firing frequency and sensitivity to MK-801 of action potentials elicited by 1 ms stimuli decreased at temperatures below 37 degrees C. Passive membrane properties were unchanged. Slow firing and a temperature-sensitive conformational change in voltage-activated sodium channels could account for the higher concentrations of MK-801 required to block sodium-dependent action potentials at temperatures below 37 degrees C.

Halothane alters electrical activity and calcium dynamics in cultured mouse cortical, spinal cord, and dorsal root ganglion neurons

Halothane inhibits neural plasma membrane Ca(2+)-ATPase, a pump that ejects Ca2+ from the cell after influx through voltage- or ligand-activated channels. Intracellular microelectrode recordings in mouse embryonic cortical and spinal cord neurons showed that halothane and eosin, a pump inhibitor, prolonged repolarization associated with spontaneous bursts of depolarization. These agents also prolonged the repolarization phases of electrically induced action potentials and of capsaicin-mediated Ca(2+)-dependent depolarization in mouse adult dorsal root ganglion neurons. In keeping with these findings, confocal microfluorimetry showed that halothane delayed clearance of intracellular Ca2+ accumulated by N-methyl-D-aspartate stimulation of single neurons.