Abstracts

RATIONALE: Carbonic anhydrase inhibitors (CAI) are known to have antiepileptic potency, especially sulthiame is used in treatment of benign and symptomatic focal epilepsy in children. Typical side effects are hyperpnoea and acroparethesia. Studies show an inconstant mild metabolic acidosis in patients after treatment with CAI. A proposed mechanism of CAI is an acidification of brain tissue. It is unclear whether CAI might also induce intracellular acidification. Aim of our study was to investigate the effects on neuronal activity and intracellular pH (pHi) of CA3 neurones of guinea pigs. METHODS: Intracellular recordings from CA3 neurones in the stratum pyramidale of guinea pigs (slice preparation)were obtained with sharp glass microelectrodes filled with 2M potassium methylsulfate. Epileptiform activity was induced by 1mM caffeine or 50?M 4-aminopyridine. Changes of the pHi were analysed by fluorescence imaging after loading hippocampal slices with 0.5-1.0?M 2,7-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-acetoxymetyl ester (BCECF). Superficially located principal neurones of the stratum pyramidale of CA3 region were identified by stained apical dendrites. RESULTS: Frequency of epileptiform burst activity were reversibly suppressed after 15 minutes when slices were exposed to 0.6-2.5mM sulthiame (n=5). 1.0-1.5mM sulthiame reversibly reduced the steady-state pHi by up to 0.20 pH-units after 10 minutes (n=5). 0.3-1.0mM acetazolamide reversibly suppressed neuronal activity within 25 minutes (n=7) and reduced the steady-state pHi by up to 0.25 pH-units after 10 minutes (n=5). With each drug in six neurones no changes of neuronal activity or pHi could be observed. CONCLUSIONS: Both CAI reduced the pHi. It can be assumed that the proposed inhibition of the carbonic anhydrase isoenzym II of glial cells by CAI increases extracellular and secondarily intracellular CO2 of neurones. Increased intracellular CO2 causes an acidification of neurones which is known to dampen neuronal activity. We conclude that the powerful action of CAI was caused by intracellular acidification of neurones.